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COMPOUNDSEMICONDUCTOR

September 2006 Volume 12 Number 8

C O N N E C T I N G T H E C O M P O U N D S E M I C O N D U C T O R C O M M U N I T Y

Australians orderhalf a million cellsfrom Spectrolab p10

SOLAR POWER TECHNOLOGY INTERVIEW

Super-brightSemiLEDs show how toboost efficacy with metalalloy substrates. p16

Moving on upIQE chief Drew Nelson on the Welsh company’s latest acquisition andplans for the future. p14

WIDE-BANDGAP DEVICES

SiC in vogue asfour new fabsopen for business

SiC SubstratesSiC EpitaxyGaN SubstratesGaN EpitaxyIII Nitride Epitaxy

Cree Zero Micropipe SiC substrates.The Revolution Starts Now.

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S E P T E M B E R 2 0 0 6

V O L U M E 1 2

N U M B E R 8

C O N N E C T I N G T H E

C O M P O U N D

S E M I C O N D U C T O R

C O M M U N I T Y

Compound Semiconductor September 2006 compoundsemiconductor.net 1

TECHNOLOGY

16 Sapphire-free vertical design boosts LED performance:Poor current handling and thermal managementare suppressing the performance of LEDs for solid-state lighting applications. These problems canbe avoided, however, by switching to a low-costvertical design and a metal alloy substrate, saysTrung Doan from SemiLEDs Corporation.

21 Application Focus: Portable DNA analyzer to use GaN LEDsForget about men in white coats. Soon police officerscould be using LEDs to analyze and identify DNAevidence at the crime scene, discovers Jon Cartwright.

22 Skyworks favors hybrid BiFET design: Skyworks believesthat its hybrid design for BiFETs, which includes aquicker and lower-cost processing route, outweighs thegreater versatility of a monolithic design. RichardStevenson investigates.

25 Etching and regrowth technique increases bipolar diodestability: The lack of forward voltage stability in SiCbipolar devices is hampering their deployment inelectrical power transformers. However, this problem canbe overcome with an etching and regrowth process, sayJoe Sumakeris, Brett Hull and Dave Grider from US chipmanufacturer Cree.

29 TDI cracks AIN template trouble: GaN HEMTmanufacturers and ultraviolet LED developers are usinglarge crack-free AlN-based templates thanks to a newdeposition process, say TDI’s Vladimir Dmitriev andAlexander Usikov.

31 Product Showcase

32 Research Review: US Air Force makes diamondadvance...Introducing thin SiN layer cuts GaNdefects...Photodiodes produce highest optical gain.

Innovation gameChipmakers must innovate to remain

profitable in today’s market. Skyworks

has launched a range of BiFET chips with

advanced modules that can command a

higher price tag. p21

Main cover image: Aclose-up of an SiC wafer with gold metallizationmanufactured at SemiSouth’s new facility in Starkville, Mississippi.

Compound Semiconductor’s circulation figures are audited by BPA International

INDUSTRY

5 Headline News: All eyes on SiC as four new fabs openup...IQE prepared for future profit as revenues rocket.

6 The Month in RFICs: Usual suspects maintain GaAs device dominance...Filtronic scales back facilityplans...GaN developers are ‘chasing wrongmarket’...Sprint gives WiMAX green light...Motorolamakes gains as market nears 1 bn.

8 The Month in HB-LEDs: Optimism returns as Aixtronorders grow...Toyoda Gosei warns firms about patents...Memory company launches LED venture.

10 The Month in Optoelectronics: III-V cells power outbackhomes...Solar start-up firm nets $25 million fund...$6 million pledged to research silicon lasers.

12 Portfolio: Transitional Cree seeks next big thing Stillmaking the bulk of its revenue from sales of LED chips forcell-phone applications, Cree’s finances are taking a hit asthe company looks to move into a new growth phase.

14 Interview: IQE slots in the final piece of the puzzle With theelectronic materials division formerly belonging toEmcore now under its wing, IQE is the world’s biggestindependent supplier of III-V epiwafers to the compoundsemiconductor industry. Michael Hatcher asks DrewNelson,the CEO at IQE, about the company’s latest move.

Solar breakthrough Remote homes in the Australian outback

are becoming the first in the world to use

triple-junction solar cells. p10

Fab fourCree, SemiSouth Laboratories, II-VI and

Norstel each cut the ribbon on new fabs

during August. p5

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E D I T O R I A L

Going down underAustralia isn’t generally regarded as a hotbed of compoundsemiconductor action, but some recent developments haveshown how the III-V industry is expanding across the globe.

Somewhat perversely, it is the outback communities ofAustralia’s Northern Territory and Queensland regions – someof the oldest, most remote human settlements in the world – that

have become the first to benefit from cutting-edge solar cell technology.Developed with military and commercial satellite applications in mind,

triple-junction cells represent extreme high-tech. The devices are based ongermanium and a variety of GaInAs and GaInP compounds, which convertthe sunlight into electricity far more efficiently than any rival approach.

Until earlier this year, however, the very high cost of the semiconductorelement meant that it was only satellite applications that had felt the benefitof the technology. But now Hermannsburg, a community of indigenousAustralians located 125 km from Alice Springs, has an eight-dish solarpower station that provides half of its electricity. Only one of those dishes

features compound semiconductor cells, butSolar Systems, the Australian energy firmpioneering deployment of the technology, hasplans to extend that and provide competitivelypriced solar power on a much larger scale.

With its own band “The Concentrators” helping to spread the word, SolarSystems has also put its money where its mouth is and ordered half amillion multijunction cells from Spectrolab. The company is hoping tobuild a 154 MW solar power station in its home state of Victoria, aninstallation that would represent the largest photovoltaic project in theworld by an order of magnitude.

Nor is Australia likely to be simply a consumer of III-V chips. Epitactixand BluGlass are two companies in the Sydney area developing novelsemiconductors, and the latter is working on a potentially game-changingtype of LED that uses a low-cost glass substrate and low-temperaturedeposition method. BluGlass recently issued a public share offering tosupport its development and, if successful, it could become a key part of thenascent compounds industry in Australia.

Michael Hatcher Editor

“Triple-junction cellsrepresent extremehigh-tech.”

Air Products and Chemicals 6Bandwidth Semiconductor 8BOC Edwards 24Cree Inc IFCDowa International Corporation 27Freescale Semiconductor Inc 19Honeywell Electronic Materials CA IBCIII/V Reclaim 15Indium Corporation of America 27Instrument Systems GmbH 24

KLA-Tencor 13Materials Research Society 20Nitronex Inc 11NuSil 9ORS Ltd 27Raboutet 10Riber 3Seoul Semiconductor Co Ltd 20Shiva Technologies 24Veeco Instruments Inc OBC

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Editor Michael [email protected]: +44 117 930 1013. Fax: +44 117 925 1942

Features editor Richard [email protected]: +44 117 930 1192

Reporter Jon [email protected]: +44 117 930 1014

Consulting editor Tim [email protected]: +44 117 930 1233

Senior sales executive David [email protected]: +44 117 930 1032. Fax: +44 117 920 0977

Business development manager Rosemarie [email protected]: +1 215 627 0880. Fax: +1 215 627 0879

Circulation manager Claire [email protected]: +44 117 930 1252. Fax +44 117 920 0742

Publisher Sarah [email protected]: +44 117 930 1020

Production editor Ruth HarwoodAd production Joanne Derrick, Mark TrimnellArt director Andrew GiaquintoTechnical illustrator Alison Tovey

SubscriptionsAvailable free of charge to qualifying individualsworking at compound semiconductor fabs andfoundries. For further information visitcompoundsemiconductor.net/subscribe. Subscriptionsfor individuals not meeting qualifying criteria:individual £86/$155 US/7125; library £193/$348US/7280. Orders to Compound Semiconductor, WDIS, Units 12 & 13, Cranleigh Gardens IndustrialEstate, Southall, Middlesex UB1 2DB, UK. Tel: +44 208 606 7518; Fax: +44 208 606 7303. General enquiries: [email protected].

9314 average total qualified circulation**June 2006 BPA audit statement

Editorial boardMayank Bulsara Atlas Technology (USA); Andrew Carter Bookham Technology (UK); Jacob TarnOCP/Gigacomm (Taiwan); Ian Ferguson GeorgiaInstitute of Technology (USA); Toby Strite JDSU(USA); Mark Wilson Motorola (USA); Dwight StreitNorthrop Grumman (USA); Joseph Smart Crystal IS(USA); Colombo Bolognesi Simon Fraser University(Canada); Shuji Nakamura University of California atSanta Barbara (USA)

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COMPOUND SEMICONDUCTORWEEK 2006

November 12–15, 2006San Antonio,Texas, USA

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THE KEY CONFERENCENOVEMBER 13–14, 2006PART OF COMPOUND SEMICONDUCTOR WEEK 2006

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• multi-junction solar cells;

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I NDUSTRY H E A D L I N E N E W S

With no less than four new facilities openingfor business during the past few weeks, August2006 could go down in history as the monthwhen SiC power electronics came of age.

US firms Cree, SemiSouth Laboratories andII-VI each cut the ribbon on new fabs, while inSweden, Norstel did likewise. MicroSemi’sadvanced power division is also in the processof finalizing its new plant.

Cree’s 230,000 ft2 production facility inResearch Triangle Park was the first to be inau-gurated officially. John Palmour, the Durham,NC, company’s vice-president of advanceddevices, said: “The new Cree site houses oneof the first commercial SiC and GaN produc-tion facilities in the world devoted to the powerand wireless infrastructure markets.”

GaN and SiC electronic devices areexpected to find use as higher-efficiencyreplacements for silicon technology in appli-cations such as high-end power supplies, motordrives and wireless communications.

Next to open was SemiSouth Laboratories,a spin-off company from Mississippi StateUniversity (MSU) that specializes in SiCpower devices. According to SemiSouth, itsfab in Starkville, Mississippi, does not just rep-resent the dawn of the firm as a volume manu-facturer, it also marks the emergence of thesemiconductor industry in this area of the US.

“The celebration signifies the beginning ofhigh-tech economic development in the heartof Mississippi,” said SemiSouth. As well asthe device fab, SemiSouth’s new buildinghouses a cleanroom dedicated to wafer pro-cessing that is operated by SiC substrate sup-plier II-VI. The materials company recentlyreceived an order worth $1 million for 3 inchSiC substrates from a large US customer.

Aptly named US Congressman ChipPickering was among the dignitaries takingpart in the opening ceremony, along withSemiSouth CEO Jeff Casady and II-VI coun-

terpart Carl Johnson. Spun out from MSU backin 2001, SemiSouth now boasts 45 employeesand is actively hiring. It expects the semicon-ductor fab to provide jobs for more than 250people within five years as the market forenergy-efficient high-power electronic com-ponents gathers pace.

SemiSouth is aiming to generate annual rev-enue in excess of $100 million from the pro-duction of more than 50 million SiC-basedcomponents in the same time-frame. “Weanticipate production will begin in the fourthquarter of 2006,” said the company. “[We are]developing proprietary products and havesecured contracts with both governmental andcommercial customers.”

Norstel, the spin-off from Finland-basedsilicon materials specialist Okmetic Oyj,opened its new SiC wafer manufacturing facil-

ity near Norrköping, Sweden, on August 29,with Sweden’s Minister for Industry and TradeThomas Östros cutting the ribbon.

Construction of the fab began in February2005, and, according to the company, it iskitted out with the very latest in process toolsand characterization equipment. Norstel usesa technique called high-temperature chemicalvapor deposition (HTCVD) to produce itsmaterial, a manufacturing method that was pio-neered at nearby Linköping University.

“We are now taking a major step towardsestablishing Norstel as a significant supplierof SiC materials,” said Asko Vehanen,Norstel’s CEO. “Making HTCVD truly indus-trial will enable Norstel to produce high-qual-ity, large-diameter SiC crystals and waferscost-efficiently, thereby opening new marketsand applications.”

All eyes on SiC as four new fabs open up

Boosted by strengthening markets across allapplications, independent epiwafer supplierIQE reported a 51% increase in sales revenuein the opening half of 2006.

In the six months up to June 30, IQE postedsales of £14.6 million ($27.8 million), com-pared with £11.2 million in the opening half of2005. That upwards trend is set to continueafter shareholders approved a £12 millionshare issue and the $16 million acquisition of

Emcore’s electronic materials division (EMD)at an extraordinary general meeting in August.

The Cardiff, UK, company reported anoperating loss of £1.5 million in the latestfinancial period, but is on track to post a profitnext year. Under Emcore’s ownership, theEMD business – now known as IQE RF – wasrunning close to profitability, and synergiesbetween it and the rest of the IQE group com-panies, such as enhanced materials buying

power, ought to help improve the financial pic-ture considerably.

IQE has also negotiated a two-year exten-sion of its “effective exclusivity” deal relatingto the firm’s largest outsource contract, andsays that all areas of the business are growing,with wireless products showing real strength.

● See interview with IQE’s CEO DrewNelson on page 14.

IQE prepared for future profit as revenues rocket

W I D E - B A N D G A P S E M I C O N D U C T O R S

E P I W A F E R S

Swedish success: Norstel’s state-of-the-art SiC wafer facility was one of four SiC fabs to be inaugurated in August.

US companies Cree, II-VI and SemiSouth also held grand openings during the month with MicroSemi to follow suit.

NO

RS

TEL


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I NDUSTRY T H E M O N T H I N R F I C S

Usual suspects maintainGaAs device dominance

D E V I C E M A N U F A C T U R I N G

Strong growth at fabless RF componentsupplier Hittite Microwave has propelled theChelmsford, MA, firm into the world’s top-tenGaAs device manufacturers for the first time.Hittite just scraped into 10th place in StrategyAnalytics’new list, and is the only fabless com-pany to be represented in the top ten.

According to the survey of the industry’sbiggest hitters in 2005, there have also beenmajor changes among the rankings of JapaneseGaAs chipmakers. Eudyna Devices, Sony andNEC all fell out of the top ten, while MitsubishiElectric and Toshiba gained market share.

Eudyna’s fall is perhaps the most surpris-ing. The firm, created in early 2004 by merg-ing Fujitsu Quantum Devices and SumitomoElectric Industries’electronic devices division,had been tipped by Strategy Analytics to breakinto the top five shortly after it formed.

Reasons for the failure to challenge the toptier of US suppliers are unclear, but it is possi-ble that Eudyna has failed to capitalize on thetrend towards multimode handsets, while the

likes of RFMD have concentrated on trans-ceiver modules and amplifier-switch integra-tion. Mitsubishi has gained ground thanks tostrong growth in its GaAs MMIC business. “Itled supply of GaAs power amplifiers to theJapanese 3G cellular handset market,” saidStrategy Analytics.

The US trio of RF Micro Devices,Skyworks Solutions and TriQuint Semicon-ductor continue to dominate the industry,accounting for 52% of the merchant market.“We believe the North American players willcontinue to cement their dominance,” con-cluded the report.

While no European companies made it intothe top ten for 2005, Asif Anwar from StrategyAnalytics reckons that UK-based Filtronic willfeature in 2006. Foundries in the Asia-Pacificregion are also set for continued growth asmany of the major GaAs device companieslook to outsource excess production require-ments rather than invest in their own expen-sive upgrades.

Filtronic, the chipmaker with a GaAs fab inNewton Aycliffe, UK, has cut the projectedspend on its capacity expansion program from£45 million ($83 million) to £15 million.

In June, Filtronic announced plans to sell itswireless infrastructure division, which accountsfor nearly 80% of its current revenue, alongwith details of a huge investment in the fab.

However, after analyzing the requirementsof its customers in detail, Filtronic has nowconcluded that the average die size of many ofthe GaAs-based products needed in the futurewill be smaller than previously envisaged. Asa result, the manufacturing demands will notbe so great. Following the £15 million cashinvestment in the fab, the division shouldbecome self-financing, however.

Filtronic finance director Charles Hindsonconfirmed that the company’s compoundsemiconductor division should be “at or near”its break-even point during fiscal 2007. In fis-cal 2006, which ended on 31 May, the divisionposted an operating loss of £5.1 million aftersales rose sharply to £20.8 million, more thandouble the 2005 revenue figure.

Hindson added that Filtronic is now sup-

plying a second customer with PHEMT die,and that a third would begin receiving ship-ments around the end of this calendar year.

North Carolina-based component supplierRF Micro Devices, which remains FiltronicCompound Semiconductor’s primary custo-mer, is in the process of expanding its ownfacility to make GaAs PHEMT switches,although with demand for its products grow-ing fast, it may still use supplies from Filtronic.

Hindson added that it is mostly back-endfab equipment that is required for the £15 mil-lion expansion, with the majority of therequired epitaxy capacity already in place.Filtronic will continue to use a mixture of in-house and external supplies of epitaxial mater-ial in the future.

Expecting to see market adoption ofPHEMT switches in 80% of mobile handsetsover the next three years, the company nowforecasts sequential revenue growth of morethan 25% in the coming six months. Asidefrom the PHEMT market, Filtronic believesthat there will be strong growth in non-switchGaAs-based products for filter and back-haulradio applications.

Filtronic scales back facility plansF A B E X P A N S I O N

Rajiv K.Agarwal,Ph.D.LeadResearchEngineer

Higher background levels ofn type dopants in GaAs andAlGaAs structures are causedby trace levels of germanium,

silicone and sulfur species present in the arsine.As customer applications evolve, the purityrequirements for arsine must as well. Untilrecently, background doping levels of 1015/cm3

were considered acceptable in most applica-tions. In general, most currently available high-purity grades of arsine can satisfy these require-ments. However, process changes and demandfor more sophisticated devices have resultedin the need for lower background doping levels.To address these more stringent requirements,Air Products is introducing MegabitTM IIIarsine, our purest grade available. Our newMegabit III arsine has significantly reduced theamounts of germanium, silicon and sulfur spe-cies. Testing done at an independent laboratoryproved the effectiveness of Megabit III on thickgallium arsenide films, with excellent results.In all cases, the background doping level was<<1014/cm3. Our research has shown thatAir Products’ Megabit III arsine will performmore consistently and produce fewer defectsin our customers’ finished products.

For more information or to submit aquestion for "Ask the Expert," visit us atwww.airproducts.com/AsktheExpert or callus at (800) 654-4567 or (610) 706-4730 andmention code #144.

© Air Products and Chemicals, Inc., 2006 (24061) LCS-1

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Ask the Expert

I am experiencing highn type backgrounddoping levels in myMOCVD process when Igrow GaAs and AlGaAsdevices. I think theproblem is in the arsineI use. I’m buying the bestgrade available. Whatcan I do?

A

Q

6

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INDUSTRY T H E M O N T H I N R F I C S


Developers of high-power RF semiconductorsbased on GaN and SiC materials will have littlesuccess if they continue to focus on the cellu-lar infrastructure market.

That’s according to a new study carried outby analysts at ABI Research that focuses onthe six key markets for RF semiconductorsoperating at above 5 W and below 3.8 GHz.

It suggests that the overall market for thesedevices should grow to almost $1 billion in2011, driven by applications in cellular infra-structure, defense, broadcasting, commercialavionics, non-cellular communications andscience, medicine and industry.

ABI research director Lance Wilson, a 30-year veteran of the wireless communicationsbusiness who previously worked at Motorola,said: “Everybody gets seduced by wirelessinfrastructure, but getting into [this market] isextremely difficult. To get products qualifiedcan be more difficult than getting into the mil-itary [market].”

While GaN and SiC-based RF transistorsdo show some impressive performance, hefirmly believes that silicon LDMOS will con-tinue to be “the elephant in the room” as far ascellular infrastructure applications are con-cerned. “In my opinion, the chase for [cellu-lar] infrastructure has retarded the growth ofthe GaN business,” said Wilson. “They havegone after the wrong market.”

Instead, he says, the key RF applicationareas for wide-bandgap materials are the onesthat need higher powers and, crucially, thatoperate at high frequencies. This means thatmilitary and satellite communications offerthe best chances for the high-performancesemiconductors in the sub-3.8 GHz range.

However, the area where GaN could reallycome into play is at frequencies of more than3.8 GHz, claims Wilson. “Here, the big ele-phant has gone,” explained the analyst, point-ing out that silicon LDMOS does not work atthese high frequencies and that GaAs-basedsolutions tend to struggle to deliver the highpowers that will be necessary for some emer-ging applications.

Aside from further military uses in thisrange, the medical world could also offer asizable future market for these devices, Wilsonadded. The magnetrons and traveling-wavetubes that are now used as microwave gener-ators could eventually be replaced by solid-state digital technologies.

From our Web pages...visit compoundsemiconductor.net for daily news updates

AXT adds to 6 inch GaAsSubstrate vendor AXT has seen a sharp upturnin sales revenue thanks to the buoyantcellphone handset market and returningcustomers. With strong market conditions andorder visibility from its returning customer base,AXT now plans to add an extra 40% to itsexisting 6 inch semi-insulating GaAs substratecapacity within the next nine months.

NEC touts GaN amplifierJapanese electronics giant NEC electronicsclaims to have developed the world’s mostpowerful transistor amplifier. The 400 W single-transistor package, which operates at 45 V and2.14 GHz, has been developed through a nine-

company partnership that includes ToyodaGosei. Toyoda made the GaN epiwafer on whichthe transistor was fabricated, and NEC says thatafter more development work it will aim tocommercialize the amplifier for 3G base-stationapplications by the end of 2008.

Mitsubishi rampFrom October, Japan’s leading GaAs devicemanufacturer Mitsubishi Electric will rampfabrication of its new GaAs HEMTs to high-volume production. The high-gain transistors aresaid to be ideal for low-noise amplifiersoperating at 18–20 GHz, and have beenordered for use in satellite broadcast receiversand very small aperture terminal systems.

GaN developersare ‘chasingwrong market’

M A R K E T R E P O R T

Sprint Nextel is to build the first fourth-gen-eration (4G) network for broadband wirelessInternet connectivity across the US. The net-work will be based on the mobile WirelessInteroperability for Microwave Access(WiMAX) technology standard, also knownas IEEE 802.16e-2005, and will operate in the2.5 GHz frequency band.

Makers of semiconductor chips and com-ponents such as RF amplifiers are looking tothe new communications protocol as a key dri-ver for devices based on GaAs, GaN and SiC.

Sprint, which has also pioneered the deploy-ment of digital optical and CDMAcellular net-works in the US, says that it will spend up to$3 billion over the next two years as it deploysthe network in rapid fashion.

Long-time WiMAX supporters Intel,Motorola and Samsung are all involved in the

project. Sprint is aiming to launch the wirelessbroadband service in trial markets by the endof 2007, and plans a full roll-out to as many as100 million people in 2008.

GaAs component specialist Anadigics couldbe one of many III-V companies set to benefitfrom the network build, as it already has astrong relationship with Intel through itsCentrino Wi-Fi chipset. The linearity demandsof mobile WiMAX on the power amplifier areeven more demanding than for Wi-Fi and cel-lular applications, suggesting a key role forGaAs when the technology is rolled out.

Cree, which has developed wide-bandgaptransistors for WiMAX infrastructure appli-cations, has also welcomed Sprint’s move. JimMilligan from the company said: “It is proba-bly the firmest commitment to WiMAX thatI’ve seen, at least in North America.”

Sprint gives WiMAX green lightW I R E L E S S N E T W O R K S

Worldwide sales of mobile phones totalled229 million in the second quarter of 2006,according to market researchers at Gartner.

Although that represents rapid year-on-yeargrowth of more than 18% in terms of unit sales,it does mean that the rate of increase in hand-set shipments has slowed since the first quar-ter of 2006, when the figure was almost 24%.

Gartner is keeping faith with its predictionthat 960 million units will sell in 2006 as a

whole. If it proves to be an accurate forecast,that would mean an 18% annual rise in unitsales from last year’s figure of 816.6 million.

Nokia and Motorola, the two market-lead-ing handset brands, are continuing to tightentheir grip on the sector, and now account formore than half of global sales.

“Motorola is the big winner this quarter,”said Gartner analyst Carolina Milanesi. TheUS company’s market share has risen by morethan 4% year-on-year to reach 21.9%.

Nokia, whose phones primarily feature RFcomponents made by RF Micro Devices,remains at the top with more than 33% of themarket, while Samsung remains in third placewith 11.1%.

Motorola makes gainsas market nears 1 bn

H A N D S E T S




I NDUSTRY T H E M O N T H I N H B - L E D S

Optimism returns as Aixtron orders growMOCVD equipment vendor Aixtron says thatnew applications for high-brightness LEDs arethe reason behind a big increase in orders dur-ing the firm’s most recent financial quarter.

Although the market for GaN-based bluelasers and LEDs used in cell-phone cameraflash applications have yet to take off as pre-dicted, orders of compound semiconductorequipment were up 62%, driven largely by ris-ing demand for liquid-crystal display back-lights featuring LEDs.

“At the end of the [previous] quarter we saidthat the cautious growth in industry confidencefirst witnessed in the latter half of 2005 hadcontinued into the first quarter of 2006,”explained Aixtron CEO Paul Hyland. “I amvery pleased to say that this improvement insentiment has continued and perhaps evenstrengthened in the second quarter.”

Aixtron’s total order backlog as of June 30stood at 781.2 million ($103.9 million), up 55%year-on-year. While that figure includes a strongperformance from the group’s silicon division,new MOCVD equipment has also been ordered

by a raft of LED makers recently, includingPhilips Lumileds, Taiwan-based Epitech,Epivalley in Korea and the Chinese firm DalianMeiming Epitaxy Technology Company.

The Aachen-based equipment supplier hasalso noted an upturn in demand from cus-tomers making components for telecommu-nications applications. Although it is unlikelythat existing capacity for telecommunicationsdevices will be used up in the near term,Aixtron is in general very positive about theend markets that it serves.

It cites an early-generation build-up incapacity for blue lasers, LEDs in backlightingand automotive applications, and SiC devicesfor hybrid car engines as three key areas thatpromise to push MOCVD equipment orders.

However, because of the much weaker orderbook one year ago, Aixtron’s sales in the threemonths up until June 30 were only 735.7 mill-ion, down from 744.4 million in the sameperiod last year. Full-year revenue is still exp-ected to reach 7150 million and Hyland saidthat the firm should break even on that basis.

E Q U I P M E N T O R D E R S


...GaN-on-glass firm goes for IPOBluGlass, a newly formed Australian company,has launched an initial public offering (IPO) ofshares in a bid to commercialize a low-cost(non-MOCVD) method for manufacturing GaN-based light emitters.

The BluGlass management, led by CEO DavidJordan, a veteran of the semiconductor andsolar cell industries, is looking to raise at leastAUS$6 million ($4.6 million) through the sale of30 million shares. If successful, it plans to builda pilot manufacturing facility for its low-temperature deposition technology, which isclaimed to be compatible with glass substratesup to 8 inch in diameter.

In the company’s new prospectus, chairmanMichael Taverner said: “Macquarie Universityresearchers have successfully demonstrated aGaN LED grown at below 700 °C.”

...Evans Analytical gets SIMS expertiseIndependent materials characterization serviceprovider Evans Analytical has increased thescope of its III-V offering through the purchase ofApplied Microanalysis Labs (AML). AML, whichwas founded in 1998 by Yumin Gao and isbased in Santa Clara, CA, specializes insecondary ion mass spectrometry (SIMS).

Gao still runs AML and is regarded by Evans’s

VP of operations Mike Edgell as a world leaderin III-V material characterization, specificallyGaN-based LED structures. SIMS provides a wayof characterizing semiconductor materialsthrough depth profiling analysis.

...Synova bags expansion cashArmed with new debt financing, the Swiss laserwafer-dicing equipment vendor Synova plans toopen up micromachining application centersaround the world in key high-tech locations. Thecompany believes that it will now be able topenetrate the semiconductor wafer dicingmarket further and make inroads into newapplications by providing its services in closerproximity to its potential customer base.

One Swiss bank and one mezzanine fund (afinancial combination comprising debt andequity options) have loaned the company atotal of SwFr10 million ($8.1 million).

...New TV uses LED backlightArizona-based high-end television manufacturerNuVision will begin shipping a new model withan LED backlight this month. The company’sillumination system is based on the PhlatLightLED-based light source developed by US firmLuminus Devices and uses individual red, greenand blue LEDs to replace conventional lamps.




INDUSTRY T H E M O N T H I N H B - L E D S


Long-lasting, reliable lighting. Accessible in themost inaccessible locations imaginable. That’s thepromise of LEDs. And thanks to NuSil, high-poweredversions will soon be available from Kaohsiung toCopenhagen to Kodiak, Alaska.

While our advanced packaging materials are helpinghigh-brightness LEDs fulfill their potential, yourneeds might be very different. From LEDs to fiberoptics, large batches to small, our Lightspan brandof products deliver precise, custom formulationsand the most complete line of high-refractive indexmatching adhesives, encapsulants and thermosetsavailable. All backed by more than 25 years ofengineering materials expertise.

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Japanese GaN LED giant Toyoda Gosei hastaken a leaf out of Nichia’s book by warningcompanies about the patents that protect itslight-emitting semiconductors. The company

says that it has received information that firmsother than licensees have been making and sell-ing white LEDs using silicate phosphors.

The phosphor detail relates to the new yel-low materials developed by Toyoda and threeEuropean collaborators, which form white emis-sions when pumped with a blue LED. Toyodahas now signed cross-licensing agreements withPhilips Lumileds and Nichia, allowing the threeparties to use each other’s LED-related patents.

In a related development, lawyers for

Columbia University academic GertrudeNeumark have claimed victory in a patent dis-pute with the Japanese firm. Declining to givedetails of the settlement, Neumark said: “I ampleased that Toyoda Gosei has acknowledgedthe relevance and importance of my patents. Ibelieve that my patents claim a manufacturingprocess for GaN LEDs that is relevant to theLED industry as a whole.” Neumark’s legalteam added that similar claims against Cree,Lumileds and Osram remain unresolved.

Toyoda Gosei warnsfirms about patents

G A N L E D S

In a bid to make a play in the LED market, twoTaiwanese companies from the mainstreamsilicon semiconductor business have set up anew joint venture company called EpiLED.

ProMOS Technologies, a manufacturer ofdynamic random access memory chips andequipment vendor Hermes-Epitek have raisedNT$550 million ($16.8 million) to get the chipmanufacturing company up and running.

ProMOS chairman M L Chen will overseethe running of the EpiLED, with a trial manu-facturing run planned for the second quarterof 2007 and volume production set to followsoon after that. The company told CompoundSemiconductor that it will focus on manufact-uring unpackaged blue LED chip die.

ProMOS has invested NT$160 million ofits own money into the venture and is expec-ted to provide the MOCVD equipment neededfor volume wafer manufacturing at the jointventure’s fab in the Tainan Science Park. Thememory firm hopes to employ its mainstreamsemiconductor know-how and help to open upnew applications like large-scale LCD back-lights, automotive lamps and indoor lighting.Hermes-Epitek sells a range of chip process-ing equipment, including ion implantation,wafer probe, etching and inspection kit.

Already a big semiconductor company, withrecent quarterly revenue at NT$11.7 billion,ProMOS will have sizable resources and expe-rience with which to support the venture.

However, the market entry of the joint ven-ture could be viewed as a late one. Taiwan’sLED industry has undergone huge restructur-ing in the past couple of years, with much con-solidation among major manufacturers.

For example, the merger between Epistarand United Epitaxy Company last year createdthe world’s largest LED maker in terms ofwafer volumes. However, following two yearsof similar retrenchment, Taiwan’s LED mak-ers have begun to order more MOCVD reac-tors as demand continues to grow. Both VeecoInstruments and Aixtron have witnessed a sig-nificant boost in orders for such equipment.

Memory companylaunches LED venture

T A I W A N

http://www.nusil.com



I NDUSTRY T H E M O N T H I N O P T O E L E C T R O N I C S

III-V cells power outback homesRemote homes lying deep in the Australianoutback are becoming the first in the world touse an electricity supply generated by triple-junction solar cells.

In a multi-million pound deal to support thedeployment of new solar power stations,Spectrolab, the Boeing subsidiary that spe-cializes in multi-junction cells based on com-pound semiconductors and germanium, is setto deliver half a million solar cell assembliesto the Australian firm Solar Systems, whichwill manufacture modules, receivers and opticsfor the concentrator systems.

The power stations being built by SolarSystems are expected to generate more than11 MW of electricity in total – enough to meetthe demands of 3500 homes.

The two companies have been partners forsome time and in April this year they devel-oped a 35 kW solar generator. One of its con-centrator systems soon began operating inHermannsburg – deep in Australia’s NorthernTerritory and 120 km from Alice Springs, theclosest town of any size. Solar Systems pro-ject consultant Julia Birch said: “This instal-lation has been successfully operating sinceApril 2006 and has demonstrated a worldrecord for commercial photovoltaics.”

Both Solar Systems and Spectrolab are exc-ited about the potential of the technology.According to Solar Systems managing directorDave Holland, the latest supply deal could bejust the start of what may become a moreextensive agreement, partly because theAustralian government is supporting thedevelopment of alternative technologies todiesel to supply power to remote communities.It recently released an extra AUS$123 million

($93.9 million) for this effort.“The breakthrough shows the potential for

concentrating photovoltaics to change the eco-nomics of solar power,” said Holland. “Weexpect this to be the first commercial phase ofa very large and valuable relationship.”

The concentrator dishes developed by SolarSystems use a set of curved mirrors that directa concentrated beam of sunlight onto the triple-junction material. Atracker system follows thepath of the Sun throughout the day, maxim-izing the collection of direct sunlight.

In the remote areas that Solar Systems is tar-geting, the solar power stations provide energyduring the day before diesel generators takeover at night.


...Emcore on the upFor the quarter that ended 30 June, Emcoreposted total sales of $42 million, up 26% onthe same period last year and slightly upsequentially. Orders for triple-junction solar cellsin both terrestrial and satellite applicationscould drive annual revenue to more than$200 million next year, say analysts.

...Bookham gets a loanBookham, the San Jose, US, company with anoptoelectronic wafer fab in Caswell, UK,registered a net loss of $27 million in its latestfinancial quarter. Bookham’s managers have

arranged a new three-year revolving $25 millioncredit facility with the Wells Fargo bank, as wellas a $23.5 million share issue to bolster itsbalance sheet.

...Whispering lasersMid-infrared lasers based on the InSb materialsystem are to be developed by an eight-partnerconsortium in the UK, thanks to £1 million($1.9 million) of funding from the UKDepartment of Trade and Industry. The Qinetiq-led effort will employ strained-layer engineeringto develop new types of lasers that will operatein the 3–5 μm atmospheric window.

S O L A R C E L L S

The concentrator dish at Hermannsburg featuring

Spectrolab cells has been operating since April. Inset:

the element that houses the triple-junction cells.

SO

LAR

SYS

TEMS



http://www.raboutet.fr


INDUSTRY T H E M O N T H I N O P T O E L E C T R O N I C S

Compound Semiconductor September 2006 compoundsemiconductor.net

SolFocus, a start-up company based at the PaloAlto Research Center near San Francisco, hasagreed a $25 million equity deal that will sec-ure a supply of 600,000 multi-junction solarcells from fellow Californian firm Spectrolab.

SolFocus says that its triple-junction sup-ply deal is the “largest to date” in the concen-trator photovoltaics industry. It will support aseries of field tests and the company’s firstphase of active deployments through 2007.

The deal with New Enterprise Associates(NEA) and seed investors NGEN Partners andYellowstone Capital forms part of a series Afinancing round that SolFocus is aiming toclose at $32 million.

“The strong financial support and world-class team at NEAwill allow us to rapidly growour 10 MW pilot production line,” explained

SolFocus CEO Gary Conley. Conley’s tech-nology is based on mirrors that concentratesunlight onto tiny triple-junction solar cellsthat are based on compound materials.

As well as guaranteeing a long-term supplyof the high-efficiency cells, which employGaAs alloys and germanium substrates, thecash will be used to expand the SolFocus team,accelerate reliability testing and enable pilotproduction to begin.

That team will include technical expertisefrom Bell Laboratories legend and Nobel lau-reate Arno Penzias, who joins as part of thecompany’s technical advisory board.

Penzias was a key member of the researchteam that discovered the cosmic microwavebackground – a hugely significant break-through that confirmed the Big Bang theory.

V E N T U R E C A P I T A L

ray scanners in airports check for metal objects.The alleged plot to detonate improvised liquidexplosives on transatlantic flights, uncovered byUK police last month,highlighted the inability ofcurrent airport security scanners to check forthese types of explosives.

The QCL-based sensor would work byidentifying the tell-tale gases given off bysubstances that could be used as part of a liquidexplosive. “You could also implement thetechnology into existing X-ray screeners,”claimed Cascade’s chief scientific officer ErwandNormand. “Everything and everyone could bechecked.” He added that the technology hasproved its potential in recent experiments whereit spectroscopically fingerprinted two explosivecompounds in only 10 ms.

11

Solar start-up firm nets $25 million fund

Scientists at Cascade Technologies in Stirling,Scotland, say that within the next two years theycould develop a sensor based on quantumcascade lasers (QCLs) that would be able to“sniff out” explosives as routinely as existing X-

The Microphotonics Center at the MassachusettsInstitute of Technology (MIT) has launched a$3.6million research project into silicon-basedlasers and nanophotonics.

Funded by the US government’s Departmentof Defense under the multi-university researchinitiative program, the project, called ElectricallyPumped Silicon Based Lasers for Chip-ScaleNanophotonic Systems, is headed by LionelKimerling, who is director of MIT’s MaterialsProcessing Center and Microphotonics Center.

Although optically pumped silicon lasershave been produced by Intel among others,electrical pumping has so far proved elusive –largely because of silicon’s indirect bandgap.However, if silicon can be made to lase in thismanner, it could have a significant effect on

III-V optoelectronics.The research partners are considering two

approaches. The first aims to use nanocrys-talline silicon in combination with erbium toproduce a 1550 nm source. This will be basedin a dielectric matrix such as SiO2 or Si3N4.

The second approach is to use a germaniumlayer deposited on silicon as the active lasermaterial. In this case, the germanium is modi-fied to act as a direct bandgap semiconductor,which could create a high-power light sourcein the milliwatt range.

“Either way, these devices will be integratedinto a CMOS process. We want to integratethese optical devices on a microchip; we wantto be able to make millions of them,” said prin-cipal investigator Jürgen Michel.

$6 million pledged to research silicon lasersR E S E A R C H


http://www.nitronex.com/contact


I NDUSTRY P O R T F O L I O

When Cree warned in mid-July that its fourth-quarterprofit would not meet initial expectations, investors werespooked and its stock price closed down 25% on the pre-vious day’s valuation at just under $18.

This is not what we’ve been accustomed to hearingfrom the Durham, NC, company – since 2002 almostevery quarter has brought record-highs in revenue andsolid profits as its fortunes followed in the massiveupswing of the cell-phone handset business.

That upswing is still in full force, with almost a bil-lion phones expected to sell this year – all of which willfeature GaN-based LEDs in the keypad and displaybacklight modules. The problem for Cree is that, unlikein the RF space, where GaAs chipmakers have benefitedfrom the need for more complex, higher-value compo-nents, LED backlighting is now largely commoditized.

Greater competition has been accompanied by pro-tective measures from rival chip manufacturers and theresulting drop in average selling prices is now out-weighing any increase in unit sales – and hitting Cree’smargins. In the company’s most recent investor confer-ence call, CEO Chuck Swoboda said that he expectedthis market to remain relatively flat.

But there are other factors that are squeezing Cree –some apparently fleeting, but others are longer-termtrends. The first is production. A quiet period of rela-tively low fab utilization in early summer was followedby a sharp spike that the company failed to predict andhad difficulty coping with. Demand has since tailed offagain and these sudden variations in manufacturing vol-umes make efficient management of a chip fab harder.

Bottom line squeezedThe second squeeze to Cree’s bottom line is intentionaland has come from its rising research and developmentspending. Even after some considerable stock compen-sation expenses are taken into account, this has grownby 25% in only 12 months, coming in at nearly $51 mil-lion or 12% of total fiscal 2006 revenue.

Now in the middle of a strategic transition, this is allpart of Cree’s push to reinvigorate its business and repro-duce the record-busting performance that has been woo-ing investors over the last few years. At the moment, thatmeans absorbing the costs of ramping up the productionof Schottky diodes for applications in power switches,as well as the XLamp packaged LED components.

The power device market is going to be an importantone for Cree. Having now officially opened its new pro-duction facility in Research Triangle Park, NC, whereit will manufacture these chips, the foundations are inplace to exploit the global drive to reduce energy wasteby replacing relatively inefficient components with thosebased on GaN and SiC.

In the latest quarter, sales of high-power electronicdevices were just under $5 million, representing a 29%sequential rise and up from just $1.8million in the equiv-alent quarter of 2005. Swoboda says that it will take timeto build both the Schottky diode product line-up and the

associated power device “brand”. He estimates that, interms of their commercial maturity, the power productsare about one year behind the XLamp.

Also fitting into this future investment category areCree’s “Colorwave” lighting modules for large-scaleliquid-crystal displays. Although some rival productshave been commercialized, this is, for now, a long wayfrom being the volume market for HB-LEDs that hasbeen envisaged. Cree’s own experience probably exp-lains why. Although the backlight development teamhas met every technical target that it has been set, thereis still a problem: cost.

Cree’s LCD-making customer needs the LED back-light at a lower cost than the US company is able to pro-vide it for at the moment and this could hamper itscommercialization. The expected date for initial deploy-ment of Colorwave modules in production LCD TVshas now slipped back into 2007.

In the short term, Cree’s shareholders may have to putup with lower margins, squeezed profits and flat salesfigures (guidance for the current quarter is $106–110mil-lion compared with $106.7 m in the period that endedon June 25). The company even admits that forecastingthe next two quarters is tricky. But if Swoboda and thegang can bring the right products to market at the rightprices, then investors playing the long game will reapthe benefits. “We are in the middle of an exciting tran-sition,” said Swoboda. “We are trying to build a muchlarger business and to deliver real energy savings.”

Quite how large Cree becomes will depend on manyfactors aside from its own execution. Sticking his neckout a little, Swoboda says that the goal is to increase rev-enue by 60–70% over three years. That would mean anannual revenue of $700 million in fiscal 2009 and wouldbe great news for the wider industry.

Not that all of that increase in revenue is likely to bea result of organic growth. Cree now has $376 millionburning a hole in its deep pockets and its acquisition ofzero-micropipe SiC substrate developer Intrinsic thissummer seems likely to be followed by similar deals thatwill broaden its net.

Transitional Cree seeks next big thingStill making the bulk ofits revenue from salesof LED chips for cell-phone applications,Cree’s finances aretaking a hit as thecompany looks tomove into a newgrowth phase.

S T O C K S A N D S H A R E S

+10%

0%

–10%

–20%

–30%

–40%Jun 1 Jun 21 Jul 11 Jul 31

NASDAQ

CRFF

While most technology stocks have been hammered since peaking

in early summer, Cree’s have underperformed the Nasdaq index and

are yet to recover from the 25% drop it suffered in mid-July when it

warned of lower-than-expected profits. Source: Yahoo Finance.

“We are trying tobuild a muchlarger business.”Chuck SwobodaCree CEO


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I NDUSTRY I N T E R V I E W

MMHH:: HHooww ddooeess tthhiiss aaccqquuiissiittiioonn ffiitt tthhee IIQQEE ssttrraatteeggyy??DDNN:: We merged with Quantum Epitaxial Devices(QED) in 1999 principally to give ourselves a positionin the wireless market place with MBE technology. Thatbusiness is focused on PHEMTs and has been very suc-cessful. It has established strong positions with a num-ber of key players, but what was obviously missing wasan MOCVD electronic position, principally for HBTsand upcoming materials like BiFETs, BiHEMTs andGaN. The electronics materials division (EMD) acqui-sition fits perfectly into that strategy.

It was a good time for both IQE and Emcore to makethe transaction. As a company Emcore is focusing moreon systems so the EMD was becoming significantly non-core to their main business. They were keen to find agood home for it and IQE fits that bill perfectly.MMHH:: HHooww wwiillll yyoouurr ccuussttoommeerrss bbeenneeffiitt??DDNN:: It allows our customers to deal with a single com-pany for their complete range of current and future prod-ucts. Consolidation of supply chains is a key trend in theindustry and we can offer more cost-effective products,a complete route to future product roadmaps, and large-capacity. Capacity is an important issue. Companies arefaced with the prospect of either having to install inter-nal epi if they don’t have it – which is obviously very

costly and time consuming – or risk expanding theirexisting capabilities. They can also recall the problemsof 2001 and 2002 quite vividly. At IQE, the entire busi-ness is about outsourcing epi effectively and we’re com-mitted to expansion.MMHH:: HHooww iiss MMOOCCVVDD ccaappaacciittyy aaffffeecctteedd??DDNN:: EMD was coming up against some capacity con-straints. For some of their customers there were someconcerns about future investment in capacity. IQE doeshave some spare MOCVD capacity, as a result of theinvestments that we made two or three years ago. InEurope, our main focus is optoelectronics, but we havedeveloped HBT capability here and we can use some ofthe spare capacity [in Cardiff, UK] to help EMD.MMHH:: WWiillll wwee sseeee mmoorree aaccqquuiissiittiioonnss bbyy IIQQEE??DDNN:: From a technological point of view we’ve got allthe pieces of the puzzle. We’d never rule out more acqui-sitions if they made sense, but it’s not something thatwe’re going all-out for. In general, I think that furtherconsolidation is probably inevitable. As the largest inde-pendent epiwafer supplier, we’re in a position to com-mand keener prices [for materials] than a smallersupplier. We should therefore be able to offer our cus-tomers better, more secure, deals than some of our rivals.

Small epi companies will have to come up with some-thing that’s specific and very attractive to the supplychain to overcome the disadvantages in terms of secu-rity of supply, economies of scale and future roadmaps.MMHH:: HHooww ddooeess EEMMDD ffiitt iinnttoo tthhee oovveerraallll bbuussiinneessss??DDNN:: The new IQE-RF site has a solid managerial andoperational team, who have built the business strongly.We don’t envisage making any fundamental changes andit’ll be run as a stand-alone entity. The big benefit is thatthere can be lots of cross-fertilization of ideas and wecan obviously purchase things like raw materials in bulk.MMHH:: DDoo AAssiiaa--PPaacciiffiicc ssuupppplliieerrss ppoossee aa tthhrreeaatt ttoo IIQQEE??DDNN:: There is MOCVD capability supplying HBTs intothe market place from Taiwan. Through the EMD acqui-sition, we have an answer for that. I think that the FarEast companies are starting from a very low base, evenif they are increasing very rapidly. The scale of our oper-ation will mitigate the competition.MMHH:: HHooww aarree ccuurrrreenntt bbuussiinneessss ccoonnddiittiioonnss??DDNN:: We’re pretty excited by wireless because there area number of drivers in the market place – from Wi-Fi toWiMAX to 3G. The EMD division has a strong BiFETtechnology, which is really beginning to get a lot of inter-est. It also has GaN capability for base station applica-tions, as well as the existing InGaP HBT business forCDMA and W-CDMA protocols. Add to that the prod-ucts from IQE and we can cover the entire wireless space.Market growth is forecast to continue and more GaAswill be used [in handsets] because of multiple radios andfaster communications, so I think we’re in a very strongposition and we’re very bullish about the future.

In optoelectronics, there are lots of new applicationsthat require VCSELs in very large quantities – and that’s

IQE slots in the final piece of the puzzleI Q E

With the electronic materials division formerly belonging to Emcore nowunder its wing, IQE is the world’s biggest independent supplier of III-Vepiwafers to the compound semiconductor industry. Michael Hatcherasks Drew Nelson, the CEO at IQE, about the company’s latest move.

1981: Leads MOVPEgroup at BritishTelecom Research Labs.1988: Co-foundsEpitaxial ProductsInternational (EPI).1992: Becomes CEOand chairman of EPI.1999: Merges EPI with (QED) to form IQE.2000: Awarded silvermedal by RoyalAcademy ofEngineering.2001: Receives OBE inthe Queen’s BirthdayHonours List.2004: Elected fellow ofthe Royal Academy ofEngineering.

Drew Nelson : the CV



INDUSTRY I N T E R V I E W

exciting for us as we are one of only a few suppliers ofVCSELwafers in the world. For example, the new laser[computer] mouse uses VCSELs instead of red LEDs.Other applications include laser printers, photocopiersand short-distance communications.

Revenue from the Cardiff business is growing at about25–30% per year. Selling prices have decreased a littlebit, but not that much. That growth rate illustrates thebuoyancy of the market although, undoubtedly, wire-less is growing more rapidly than optoelectronics.MMHH:: WWhheerree iiss tthhee mmaarrkkeett ffoorr GGaaNN tteecchhnnoollooggyy??DDNN:: It is driven by the needs of 3G base stations andalthough right now it is difficult to predict when basestations featuring GaN will appear, we’re certainly get-ting quite a lot of interest in the technology and EMD isselling the wafers on a commercial basis.MMHH:: HHooww mmuucchh rreevveennuuee wwiillll EEMMDD aadddd??DDNN:: We expect revenues to continue to rise, particularlyas we increase production capacity. Next year, analystsexpect the increase to be around $24 million on an annualbasis. With synergies, we will try to improve on that.MMHH:: IIss IIQQEE cclloossee ttoo bbeeccoommiinngg pprrooffiittaabbllee aaggaaiinn??DDNN:: [As part of Emcore], EMD was already approach-ing profitability. By utilizing existing spare MOCVDcapacity within the IQE group of companies, the scaleof the operation will increase. Our purchasing power inthe supply chain will also be greater than Emcore’swould have been as a stand-alone business. Additionally,there are a number of operational technologies and

implementations that we can exchange with EMD. Twoheads are always better than one and if we get the bestof both worlds then we should be able to improve pro-duction efficiency across the business.

As a group, IQE is approaching profitability and ana-lysts’ reports expect full profitability next year. We’reexcited about the future and being able to supply custom-ers with a full range in the wireless market and the otherangles of the optoelectronics and silicon businesses.MMHH:: IInn wwhhaatt wwaayy wwiillll yyoouu bbee aaddddiinngg ccaappaacciittyy??DDNN:: In the short term, we will be getting more wafersthrough [existing] reactors, but very quickly followingthat up with additional capacity at both EMD and fullutilization of IQE’s MOCVD capabilities in Cardiff.

Eventually, that will mean more reactors, but not now.We expect to begin filling up our unused capacity veryquickly now and that will stimulate the need for addi-tional tools. The other advantage of having MOCVDoperating out of both Cardiff and New Jersey is that wewill have two independent sites for MOCVD. So for anycustomer who is using IQE, they have the security ofknowing that two completely different sites are able tosupply the same product.MMHH:: WWhhaatt eellssee wwiillll tthhee ccaappiittaall rraaiisseedd iinn tthhee ££1122 mmiill--lliioonn sshhaarree iissssuuee bbee ssppeenntt oonn??DDNN:: With a rapidly expanding business, working capi-tal has to be funded. We operate a number of supply-managed inventory agreements with customers, whichgives them a buffer stock, and that also has to be funded.


http://www.35reclaim.de



T ECHNOLOGY G A N O P T O E L E C T R O N I C S

GaN LEDs are widely used in handset keypads, back-lighting units, camera flashes and full-color outdoordisplays, but their output is, as yet, insufficient for sig-nificant penetration into the solid-state lighting mar-ket. This is primarily because the LEDs have relativelypoor thermal management characteristics and cannotoperate at the high injection currents required for super-bright emission. However, these issues can be over-come by producing GaN LEDs on electrically andthermally conducting substrates, and this is an approachthat we have pioneered at SemiLEDs. Our verticalLEDs on metal substrates (VLEDMS), which are builtusing low-cost mass-production processes, use a novelvertical design and feature a metal alloy substrate.These emitters deliver many benefits over conventionaland flip-chip LED designs (see figure 1, p17) and candeliver 75 lm/W at 350 mA drive current, which isamong the highest output efficacies achieved to date.

Sapphire’s drawbacksThe issues affecting conventional GaN LEDs stem fromthe poor thermal and electrical properties of the sap-phire substrates that they are grown on. Sapphire has athermal conductivity of only 35 W/mK (see figure 2,p17), which restricts the LEDs’operating current. Thematerial is also an insulator and so the n-contact can-not be attached to the back of the substrate, but has tobe formed on top of the n-type layer. This means thatthe active material has to be removed from the chip,which decreases the emission intensity by 20–30%.Having both contacts on the top side of the LED alsoresults in current transport through the n-GaN layer,which produces current crowding and a higher dynamicresistance that increases the device’s temperature.

Conventional GaN LEDs also suffer from non-uniform light emission due to low current spreading inthe p-GaN layer. This can be overcome with eithersemi-transparent contact layers or interdigitated elec-trode arrays that spread the current across the device.However, semi-transparent layers also absorb some ofthe chip emission and can reduce the output power.

These are issues that have caused leading LED man-ufacturers such as Lumileds to turn to designs that usea flip-chip geometry. However, this approach alsorequires material from p-GaN and active layers to be

removed in order to form the n-type contact, whichagain reduces the emitting area. Current transport fromanode to cathode is still routed along the n-GaN layer,which means that the current crowding and higherdynamic resistance problems remain.

One advantage of flip-chip LEDs is an improvementin the heat dissipation over conventional structures.Flip-chips also produce higher extraction efficiencythan the conventional LEDs, partly because of the pat-terned or textured sapphire surface, but these structuresare quite complicated to produce.

SemiLEDs’VLEDMS overcome many issues thatrestrict the performance of conventional and flip-chipLEDs. For example, there is no need to remove anymaterial to form the n-type electrode pad, which boostsemission compared with equivalently-sized GaN-on-

Poor current handling and thermal management aresuppressing the performance of LEDs for solid-state lightingapplications. These problems can be avoided, however, byswitching to a low-cost vertical design and a metal alloysubstrate, says Trung Doan from SemiLEDs Corporation.

Sapphire-free vertical desig

SemiLEDs Corporation designs, develops, manufactures and sells high brightnes

company is headquartered in Milpitas, in Silicon Valley, CA, and has operations in H

SemiLEDs can produce 80 lm/W

1mm2 GaN-based LEDs in a

varierty of colors using its

proprietary metal alloy substrate

technology. The light emission

pattern is highly uniform, thanks

to the improved current

spreading in the n-GaN layer.



TECHNOLOGY G A N O P T O E L E C T R O N I C S

sapphire LEDs. Current crowding is avoided becausethe current passes through the device in a vertical direc-tion, while dynamic resistance is cut significantly.

Sapphire-free benefitsOur chip geometry also improves the current spread-ing in the device. This allows the chips to be scaled tolarger sizes without any loss in performance and cir-cumvents the need for semi-transparent conductivelayers that reduce the output efficiency.

In addition, our VLEDMS dissipate heat more effec-tively than conventional and flip-chip LEDs, thanks tothe higher thermal conductivity of a copper alloy sub-strate. This increases their maximum operating currentand output power and makes them more suitable forsolid-state lighting applications.

The structure of our VLEDMS, which we have man-ufactured as blue, green and ultraviolet 1mm2 LED chips,is shown in figure 3 (p18). Using our patent-pending epi-taxial deposition technology, these LEDs are grown onsapphire along with an additional structure that enables

gn boosts LED performance

ss LEDs using proprietary technologies that are protected with over 20 patents. The

Hsinchu Science Park, Taiwan.

p-electrode transparentcontact layer

conventional LED flip-chip LED

VLEDMS structure

p-GaN

MQWs

n-GaN

n-electrode

sapphire substrate

n-GaN

MQWs

p-electrode

n-electrode

solder

sapphire

sub-mount

n-GaN

MQWs

p-GaN

metal alloyed substrate

p-GaN

Fig. 1. SemiLEDs has tackled several of the problems that limit the performance of conventional (a)

and flip-chip (b) LEDs, such as current crowding and device heating, by employing a vertical structure

and a metal alloy substrate (c) with n- and p-electrodes on opposite sides of the device.

sapphire Ge Si GaN SiC metal alloysubstrate

400

350

300

250

200

150

100

50

0

ther

mal

con

duc

tivity

(W

/m-K

)

thermal conductivity

Fig. 2. Sapphire has the lowest thermal conductivity of all the

common substrates used to manufacture GaN LEDs. SiC, which is

used by Cree, offers a significant improvement, but better still is a

metal alloy employed by SemiLEDs.



TECHNOLOGY G A N O P T O E L E C T R O N I C S

removal of the sapphire. After the LED is formed on themetal alloy substrate, the n-GaN surface is patterned toreduce losses through total internal reflection.

Our VLEDMS have superior current-voltage (I-V)characteristics to conventional LEDs, including a 0.2Vreduction in the forward voltage at 350 mA drivecurrent. These LEDs also have a dynamic resistanceof 0.7 Ω, compared with 1.1 Ω for conventional GaN-on-sapphire LEDs, thanks to the switch to a verticalcurrent path and a larger p-GaN contact area. Theseimprovements increase the output efficiency of ourVLEDMS over conventional designs.

The increased brightness of our LEDs is particularlysignificant at higher injection currents (see figure 4).The output from conventional emitters peaks at around1000 mAand then falls off significantly with increas-ing current. This is due to poor heat dissipation thatleads to device degradation. In contrast, our VLEDMScan handle currents of 3000 mAor more without lightoutput power saturation, thanks to the superior ther-mal conductivity of metal alloy substrates.

Performance independent of sizeWe have demonstrated the excellent scaling propertiesof our VLEDMS by manufacturing a range of chipswith various dimensions and measuring their outputper unit area (see figure 5). While conventional sap-phire-based LEDs suffer from a significant drop in effi-cacy at larger chip sizes, this problem does not appear

to impact on the performance of VLEDMS.Figure 6 shows the results of our reliability tests on

1 mm2 VLEDMS chips, which were packaged using asilicone filling and mounted onto a heat sink. The mea-surements were made at 350mAand 700mAdrive cur-rents, and ambient temperatures of up to 65 ºC, whichled to a range of junction temperatures of up to 120 ºC.Our chips, which produce an output that is equivalentto more than 75 lm/W from a white LED, showed onlya small decline in light output power over time and thischange can be kept below 10% even after a 2000h burn-in test. At room temperature – the temperature at whichthe majority of our customers will use these devices –we observed no degradation in light output.

This proven reliability, in conjunction with the exc-ellent heat dissipation characteristics and output eff-icacies of typically 75 lm/W or more, clearly illustratethe advantages of these devices over conventionalLEDs. These LEDs are already being produced in largevolumes at high yields and they offer a lumen/$ figureof over 100, which makes these emitters the device ofchoice for solid-state lighting. ●

Further readingZ S Luo Y2002 et al. IEEE Photo. Tech. Lett. 14 1440.T Fujii 2004 et al. Appl. Phys. Lett. 84 855.TDoan et al.2006 Proceedings of SPIE613461340G-1.C F Chu et al 2006 ISBLLED.http://www.semileds.com.

80μm–150μm

n-bonding padn-GaN MQW

p-GaN

reflector layerno sideemission

passivation

metal alloy

75μm–145μm

patterenedsurface

thin device passless photon loss

reflector (>90%)

metal alloy hasbest thermalconductivity

Au/Sn foreutectic bonding

0 500 1000 1500 2000 2500 3000current (mA)

0

200

400

600

800

1000

1200

1400

1600

light

out

put

pow

er (

mW

)

VLEDMSconventional LED on sapphire

1.0

0.9

0.8

0.7

0.6

0.50 200 400 600 800 1000

chip size (μm)

VLEDMSconventional LED on sapphire

norm

aliz

e ef

ficie

ncy

Fig. 5. (left) SemiLEDs’ vertical

LEDs can be scaled to larger

sizes without any trade-off in

performance, making them

strong candidates for solid-state

lighting applications. The

efficiency was normalized to a

350 μm chip size.

Fig. 3. (left) SemiLEDs’ vertical

LEDs comprise a mirror directly

deposited on metal alloy

substrate, a 0.2 μm thick

p-GaN/p-AlGaN layer, an

InGaN/GaN multiple quantum

well active region and a 4 μm

thick n-GaN layer.

Fig. 4. (right) SemiLEDs’

devices produce an output

power that is higher than GaN-

on-sapphire LEDs, particularly at

drive currents of over 1000 mA.

0 500 1000 1500 2000time (hours)

RT 350 mART 700 mA45°C 350 mA45°C 700 mA65°C 350 mA65°C 700 mA

120

110

100

90

80

70

rela

tive

light

out

put

pow

er (

%)

Fig 6. (right) These reliability

tests demonstrate the long-term

reliability of SemiLEDs’ devices.

Measurements were carried out

in a closed space at a stable,

ambient temperature.

About the authorTrung Doan ([email protected]) is SemiLEDs’chairman and CEO. Prior tofounding the company he wasvice-president of processdevelopment at MicronTechnology, president and CEOof Jusung Engineering Ltd andvice-president of AGS productsat Applied Materials. He is aninventor of over 200 patents,with more pending.

http://www.semileds.com




Freescale™ and the Freescale logo are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of theirrespective owners. Freescale is an Equal Opportunity/Affirmative Action Employer. We welcome and encourage diversity in our workforce.© Freescale Semiconductor, Inc. 2006

Freescale Semiconductor is looking for great engineering talent. Our Compound

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> The ability to relate device response to process characteristics

> Excellent verbal and written communication skills

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> Extensive application of DOE

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www.mrs.org/fall2006/2006 MRS FALL MEETING

SYMPOSIA MEETING ACTIVITIESSOFT MATTER—ACTIVE MATERIALS,HYBRIDS, AND SENSORS

A: Responsive Soft Matter—Chemistry and Physics for Assemblages, Films, and Forms

B: Structure, Processing, and Properties of Polymer Nanofibers for Emerging Technologies

C: Smart Dielectric Polymer Properties, Characterization,and Their Devices

D: Biosurfaces and BiointerfacesE: Nanofunctional Materials, Nanostructures, and Novel

Devices for Biological and Chemical DetectionF: Integrated NanosensorsG: Fibrillar Aggregates as Materials—Assembly, Properties,

and ApplicationsH: Biofilm-Material Interactions—New Tools, Technologies,

and Opportunities

ELECTRONICS, PHOTONICS, AND MAGNETICS

I: Advances in III-V Nitride Semiconductor Materialsand Devices

J: Diamond Electronics—Fundamentals to ApplicationsK: Zinc Oxide and Related MaterialsL: Group IV Semiconductor NanostructuresM: Quantum Dots—Growth, Behavior, and ApplicationsN: Self Assembly of Nanostructures Aided by Ion- or

Photon-Beam Irradiation—Fundamentals and ApplicationsO: Nanostructured and Patterned Materials for

Information StorageP: Nanoscale Magnets—Synthesis, Self-Assembly,

Properties, and ApplicationsQ: Nanowires and Carbon Nanotubes—Science and

ApplicationsR: Meta-Materials at the Milli-, Micro-, and NanoscaleS: Organic Electronics—Materials, Devices, and ApplicationsT: Ferroelectrics and MultiferroicsU: Advances in In Situ Characterization of Film Growth

and Interface Processes

V: Advanced Electronic PackagingW: Heterogeneous Integration of Materials for

Passive Components and Smart SystemsY: Enabling Technologies for 3-D Integration

ENERGY STORAGE AND UTILIZATION

Z: Hydrogen Storage TechnologiesAA: Solid-State IonicsBB: Mobile EnergyCC: Solar Energy Conversion

MICROSTRUCTURE, MECHANICS, AND MODELING

DD: Mechanics of Biological and Bio-Inspired MaterialsEE: Size Effects in the Deformation of Materials—

Experiments and ModelingFF: Processing-Structure-Mechanical Property Relations in

Composite MaterialsGG: Multiscale Modeling of MaterialsHH: Thermodynamics and Kinetics of Phase Transformations

in Inorganic MaterialsII: Advanced Intermetallic-Based AlloysJJ: Structural and Refractory Materials for Fusion and

Fission Technologies

CHARACTERIZATION TOOLS AND TECHNIQUES

KK: Electron Microscopy Across Hard and Soft MaterialsLL: Focused Ion Beams for Analysis and ProcessingMM: Magnetic Resonance in Material Science

GENERAL INTEREST

X: Frontiers of Materials ResearchNN: Scientific Basis for Nuclear Waste Management XXXOO: Actinides—Basic Science, Applications, and TechnologyPP: Materials Research at High PressureQQ: Solid-State Chemistry of Inorganic Materials VI

SYMPOSIUM TUTORIAL PROGRAM

Available only to meeting registrants, the symposiumtutorials will concentrate on new, rapidly breaking areas ofresearch.

EXHIBIT

A major exhibit encompassing the full spectrum ofequipment, instrumentation, products, software, publications,and services is scheduled for November 28-30 in the HynesConvention Center. Convenient to the technical session roomsand scheduled to complement the program, the MRS FallExhibit offers everything you need all under one roof.

PUBLICATIONS DESK

A full display of over 915 books will be available at the MRSPublications Desk.

STUDENT OPPORTUNITIES

Graduate students planning to attend the 2006 MRS FallMeeting are encouraged to apply for a Symposium Assistantposition and/or a Graduate Student Award.

CAREER CENTER

A Career Center for MRS members and meeting attendees willbe open Tuesday through Thursday.

Member ServicesMaterials Research Society

506 Keystone DriveWarrendale, PA 15086-7573

Tel 724-779-3003Fax 724-779-8313

E-mail: [email protected]

For additional meeting information,visit the MRS Web site at

www.mrs.org/meetings/

or contact:

Meeting Chairs:Babu R. ChalamalaIndocel Technologies, Inc.Tel 919-244-1040Fax [email protected]

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LaboratoryTel 925-423-7956Fax [email protected]

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The 2006 MRS Fall Meeting will serve as a keyforum for discussion of interdisciplinary leading-edgematerials research from around the world.

Various meeting formats—oral, poster, round-table,forum and workshop sessions—are offered tomaximize participation.

http://www.mrs.org/fall2006




http://www.mrs.org/meetings


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T ECHNOLOGY A P P L I C A T I O N F O C U S

There is little doubt as to the importance of DNAanaly-sis in today’s society. A recent example would be thelong-awaited conviction last month of the two brothersresponsible for killing Damilola Taylor, a 10-year-oldboy from London, UK, in 2000. The key piece of evi-dence – a small blood stain on one of the killer’s shoes– was crucial to the prosecution’s case.

Now, UK researchers are trying to push the bound-aries of DNA analysis through the application of opto-electronics. In a joint project between the University ofHull and the Centre for Integrated Photonics (CIP),together with £721,000 ($1.37 million) of funding fromthe Engineering and Physical Sciences Research Council,they are set to pioneer the use of high-brightness LEDsin integrated modules to optically detect DNAfragments.If the approach works, these portable DNA analyzerscould revolutionize forensic science by giving scene-of-crime officers in situ access to genetic information.

Currently, DNAanalysis is a notoriously inconvenientaffair. Evidence must be taken to a laboratory and scru-tinized, then possible samples must be subjected to alaborious profiling procedure involving fragmentationand polymerase chain reaction amplification. In the finalstep, the amplified fragments are separated using elec-trophoresis, whereby an electric field selectively pullsat the molecules based on their size and shape. It is the

layout of the dispersed fragments left that char-acterizes the DNA“fingerprint” of the sample.

Apart from the obvious time expenditure,transporting samples to the laboratory leavesthem dangerously susceptible to contamination.The device that CIPhopes to produce will com-bine all the stages together in one shoebox-sizedunit – simultaneously circumventing possibil-ities of time-delay and contamination. It willalso require an automated detection stage to out-put the fingerprint without human intervention.

According to Steve Oliver, project leader atCIP, units like this could be cheap enough tohand out to police officers for ruling out sus-pects within minutes. “It won’t tell you their

name and address, but it will tell you if they’re human,Caucasian or not, male or female – that kind of thing.”

The project has been split into two – the biologicalpart will be done by the Hull team. The other part, whichentails electrophoretic separation and optical detectionof the DNA fragments, will fall onto CIP’s shoulders.

“As far as CIPgoes, the tricky part will be the opticaldetection,” explained Oliver. “The idea is that you tagthese pieces of DNAwith different dyes. Depending onthe mix of dye that comes through, you can tell some-thing about where the DNAcame from.”

Once the DNAfragments have been tagged, the dyeswill be illuminated using the light of a wavelength thatthey can absorb. This will stimulate fluorescence, wherelight is re-emitted by the dye at a longer wavelength. The

fluorescence signals then have to be filtered and fed toan optical detector and data processing system.

Sensing the color of these dyes will require compo-nents that satisfy two important criteria: cost and relia-bility. Presuming that it works, the only way that suchan analyzer can be justified is if investigators can rou-tinely carry them to crime scenes. CIP could producebespoke light sources in-house, but these run the risk ofbeing delicate and expensive, so instead the team is look-ing to commercial LEDs for the solution.

“We don’t want to design a system and then build alight source to match it,” explained Oliver. “In the firstinstance at least, we want to see how far we can go withwhat’s commercially available.”

Given that the tagged DNAfragments will be absorb-ing visible wavelengths between 400 and 500 nm, theobvious candidates for the light source are undoubtedlyGaN LEDs. These could either be made into an array,with each LED corresponding to a different color of dye,or – preferably – the analyzer could rely on a single,superluminescent LED combined with several filters tocover all of the necessary wavelengths.

CIP’s strong pedigree in optical telecoms should giveit a firm grounding for developing these filters andsqueezing the design into a manageable package. Butthe DNAanalysis project is outside of its normal scope.

“There’s been a lot of interest in microfluidics, butuntil now the devices have been quite simple,” explainedOliver. “We’re trying to do something with the integra-tion technology and then apply it in that field. This is adeviation from the norm for us – we’ve never done any-thing in the biological sciences before.”

Portable DNA analyzer to use GaN LEDsForget about men inwhite coats. Soonpolice officers couldbe using LEDs toanalyze and identifyDNA evidence at thecrime scene, discoversJon Cartwright.

H B - L E D S

Currently DNA evidence is analyzed using complex and laborious

laboratory processes, but UK researchers hope to use GaN-based

LEDs to develop a portable unit that could screen for basic human

characteristics, such as sex and race, within minutes.

SPL

“It will tell you ifthey’re human,Caucasian or not,male or female” Steve OliverCIP



T ECHNOLOGY G A A S T R A N S I S T O R S

Skyworks favors hybrid BiFET design

It’s a competitive market for the manufacturers ofGaAs-based chips for cell phones. Prices are eroding,and handset designers are demanding smaller compo-nents. So chipmakers must innovate to remain prof-itable, by developing products with a smaller footprintand cheaper material costs, or more advanced moduleswith greater functionality that can command a higherprice tag.

Skyworks has been pursuing both of these strate-gies. According to Steve Machuga, vice-president ofRF front-end development for Skyworks’ MobilePlatforms’business, the Woburn-based outfit initiallyinvested in design approaches that minimized the GaAsfootprint in the phone, and this led to power amplifier(PA) costs that were close to those of their silicon coun-terparts. However, the firm has since shifted directionwith the development of more sophisticated compo-nents, culminating in last year’s launch of a range ofBiFETchips featuring HBTs and FETs on the same die.

“The BiFET technology is an example of wherewe’ve been able to integrate an FET, for use in bias con-

trol circuits, at almost no extra incremental cost to thecore HBT technology,” says Machuga. The bias con-trol circuit manages the PA’s performance more effi-ciently, which leads to longer handset talk times.

BiFETs can be built using either a monolithic or ahybrid design, and Skyworks has evaluated both typesof device (see “Different approaches to BiFETdesign”box). The company rejected the monolithic design andselected a hybrid design for manufacturing all of itsBiFETs, which positions the FET on top of the HBT.

“Our motivation for choosing this approach is verysimple,” explains Ravi Ramanathan, Skyworks’man-ager of compound semiconductors advanced processtechnology. “We want to use a simple process that doesnot increase the epitaxial and processing costs, and alow-performance DC-type switch for bias control.”

Skyworks believes that the benefits of the hybriddesign include quick lot (QL) characterization of theHBTand FET, shared processing steps, a minimal imp-act on total processing time compared with stand-aloneHBTmanufacture, and the need for only two additionalmasking layers to define the FET. This device has itslimitations, though, such as poor RF isolation and acompatibility with only n-type FETs.

The monolithic design, which is used by Skyworks’rival Anadigics, allows independent tuning of the FETand HBT, so that individual device characteristics canbe tuned to the needs of the application. However,Skyworks claims that this design cannot be used forprofitable manufacturing in today’s market. Drawbacksinclude complex QL characterization procedures thatcan require further process steps, complicated elec-trolytic capacitance-voltage (C-V) profiling, and alonger “stabilization bake” step for the FET thatincreases the time to manufacture the final product.

With the monolithic design the BiFET’s emitter andgate are typically separated by a few microns but havemicron-sized heights differences, says Ramanathan,making it awkward to carry out the sub-micron-sizedphotolithographic process used for device manufacture.“You need a very high planarizing resist process, but theresist thickness will increase and reduce the line-widthresolution, or you need to move the FET substantiallyfurther away from the HBT, to eliminate severe topol-ogy effects on the gate process,” explains Ramanathan.

Anadigics, which has been manufacturing BiFET-based chips in volume since 2003, rebuts Skyworks’assessment of the monolithic design, claiming that allof the concerns “are either incorrect or do not apply”.According to the Warren, NJ, company, its InGaP-based technology provides product performance and

Skyworks believes that its hybrid design for BiFETs, which includes a quicker and lower-cost processingroute, outweighs the greater versatility of a monolithic design. Richard Stevenson investigates.

Skyworks makes its BiFET products at its Newbury Park, CA, fab using 4 inch epiwafers from Kopin.

SKYW

OR

KS



TECHNOLOGY G A A S T R A N S I S T O R S

integration improvements with no impact on eitheryield or cycle times. The devices are also more versa-tile than Skyworks’ because PHEMT and MESFETstructures can be constructed below the collector, saysAnadigics, and the BiFETs offer an equivalent perfor-mance to stand-alone InGaP and PHEMT structures.

Anadigics also claims that its BiFET manufacturerequires just two additional mask levels compared witha traditional InGaP HBT process, which is the sameadded complexity as Skyworks’BiFET process. Also,its chips do not need complex characterization proce-dures or a long stabilization bake. “The products arecompetitively priced in the market,” says an Anadigicsspokesperson, and its rising revenue and gross marginalso suggest that a greater proportion of InGaP-basedproducts in the company’s sales mix is actually boost-ing its financial performance.

Skyworks, like Anadigics, does not produce itsmaterial in-house, and outsources growth to Kopin.The epiwafer supplier carries out a series of QL testson large-area devices, which are typically 75 × 75 μm,to determine if the material’s quality is suitable for chipproduction. These measurements reveal the DC gain,offset voltage, base-emitter and base-collector turn-onvoltages, and junction breakdown voltages. However,because FETs are sensitive to process conditions thatcan mask growth variations, Kopin cannot predict thepinch-off voltage, saturation current and transcon-ductance of BiFETs made from these epiwafers.

To overcome these issues associated with measur-ing the FET performance, Kopin and Skyworks haveestablished a QL procedure based on C-V measure-ments that identifies run-to-run and machine-to-machine variations. Profiles of the emitter-basejunctions reveal the FET’s channel thickness and dop-ing concentration, which are related to the character-istics of the fully processed FET.

The epiwafer batches that pass all the QL tests atKopin are shipped to Skyworks and processed intoBiFETs. Ramanathan says that the Ti/Pt/Au/TiSchottky gate contact, which is formed by metal evap-oration onto the channel layer, is the key componentand is extremely sensitive to the gate processing steps.

Skyworks has evaluated gates produced by bothphotolithography and etching. The former approachproduces a gate that is free from cracks and whichallows SiN passivation on the gate metal, but it can alsolead to Schottky contacts with undesired characteris-tics due to “gate sinking” and gold and platinum dif-fusion into the channel. As a result, Skyworks employsan etching process for BiFET manufacture that cir-cumvents these problems, and also allows for gateswith thicker gold layers that reduce contact resistance.

Skyworks has assessed its BiFET manufacturingyield by measuring 40–50 parameters related to theFET, HBTor the passive components. With a yield thatis routinely above 95%, very little material is wastedin the production process. ●

Rival US chipmakers Skyworks and Anadigics are both manufacturing products that incorporate BiFET technology. But,while Skyworks prefers a hybrid approach (figure on left), Anadigics backs a monolithic design (figure on right).

Skyworks’ hybrid epitaxial structures are grown by MOCVD on 4 inch semi-insulating GaAs (100) substrates. The FET layers, like the channel and the etch stop layer, are grown within the HBT’s emitter. Dry etching defines the HBT’semitter (E) and base opening, while wet etching forms the collector opening, and helium ion implantation isolates thedevices. Processing with the same fabriation step is reduced by the FET’s drain (D) and source contacts (S) and theHBT’s emitter. Dry etching and ion implantation provide device-to-device isolation, and metal evaporation forms thegate (G) contact.

Anadigics’ monolithic designs are also grown by MOCVD, but in these structures the HBT is deposited on top of thePHEMT structure. This allows the two devices to be decoupled. The HBT and the PHEMT share a highly doped n-typeGaAs layer, which serves as the PHEMT cap and the HBT’s subcollector. At Anadigics, 6 inch epiwafers are processedwith evaporation and lift-off techniques to form the ohmic emitter. Wet etching of the InGaP and emitter layer, followedby evaporated metal lift-off, form the base contact before selective wet etching with a photoresist mask defines thebase mesa. A nitride passivation layer is added by plasma-enhanced CVD to protect the HBT. Like Skyworks, deviceisolation is produced by helium implantation. After isolation, a single AuGe/Ni metallization step is used to form theHBT collector contact and the PHEMT source and drain contacts, which completes the HBT fabrication process. Metalevaporation and lift-off is used to form the PHEMT gate, and a silicon nitride layer is deposited for device passivation.

Different approaches to BiFET design

“We want touse a simpleprocess thatdoes notincreaseepitaxial andprocessingcosts.”Ravi RamanathanSkyworks

HBT FET

E

B B

S

G

D

emitteretch stop

base

back gate

C

sub collector

SI GaAs substrate

collector

channel

Skyworks emitter

base base

collectorcollectordraingatesourceISO

GaAs InGaP InGaAs metal

Anadigics


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T ECHNOLOGY S I C D E V I C E S

The improved switching efficiencies of high-voltageSiC discrete power devices have the potential to deli-ver significant energy and space savings in AC-DCpower converters deployed in civilian infrastructureand in military vessels. But to fulfill these applicationsmulti-kV SiC devices are desired to be run in bipolarconductivity mode for increased current density.Unfortunately this can lead to a forward voltage (Vf)drift under conductivity modulation, according to stud-ies carried out by us and other researchers. The problemis so significant that it drove some companies to aban-don pursuing this device technology.

The Vf drift stems from the propagation of basalplane dislocations (BPDs) that exist in the SiC sub-strate into the epilayer. Here, they are converted intoShockley stacking faults when the device is operatedin bipolar conductivity mode. In vertical power devices,where current flow is essentially perpendicular to thebasal plane, these stacking faults trap carriers and dra-matically increase the device’s resistance.

To combat the Vf drift we have developed specifictechniques for substrate preparation, epilayer growthand device fabrication (see figure 1, p26). This enablesus to reduce the number of BPDs in the parts of thedevice that experience conductivity modulation andproduce stable, multi-kV bipolar power devices. Webelieve that this approach, which relies on substratepreparation and epilayer regrowth, represents the great-est recent advance in bipolar SiC device technology.

We started developing methods for reducing the Vf

drift about six years ago. Our first incremental improve-ments in stability came by restricting the electron-holeplasma to regions of devices with fewer BPDs. By 2003we had modified our PIN devices to include a thick,heavily doped n-type buffer layer between the substrateand the drift layer, and a relatively thick anode layeron top of the drift layer. The buffer layer isolates theelectron-hole plasma from the BPDs in the substrate,while the anode layer separates the plasma from theohmic regions and protects the drift layer from any

Cree’s SiC device development, which is funded by the US Office of Naval Research (contracts N00014-02-C-0302 and N00014-05-C-0202) and the Defense Advanced

Research Projects Agency, could allow traditional iron core transformers in military vessels to be replaced with smaller, more efficient solid-state power devices.

The lack of forward voltage stability in SiC bipolar devices is hampering their deployment in electricalpower transformers. However, this problem can be overcome with an etching and regrowth process, say Joe Sumakeris, Brett Hull and Dave Grider from US chip manufacturer Cree.

Etching and regrowth techniqueincreases bipolar diode stability

US

NA

VY



TECHNOLOGY S I C D E V I C E S

mechanical damage that can occur during processing.Re-introduction of BPDs at growth interruptions is pre-vented by growing the modulated portion of the struc-ture in a single uninterrupted step.

Although these steps are beneficial, most of the drift-inducing BPDs come from the substrate and propagateinto and through the epilayers. If these BPDs could beeliminated from the substrates, this would improvedevice stability. However, this is an extremely long-term goal and instead we currently have to contendwith the BPD densities of 104–105 cm–2 that exist incommercial substrates.

An alternative approachfor increasing device stabil-ity involves the growth of astrained layer that can blockand redirect the BPDs. Thisapproach has already beenapplied to the GaAsP/GaAsmaterial system, where itwas used to prevent thepropagation of threadingdislocations. However, it is

not clear whether strained layers will have the sameeffect on BPDs in SiC and any efforts to develop thistechnology will be hampered by the lack of publishedinformation concerning the formation of SiC-com-patible strained layers.

Making better defectsDevice stability can also be improved by convertingthese BPDs into other forms of defects that cause fewerproblems. Thankfully, this process can actually occurboth naturally and efficiently during epitaxial growth,with BPDs being transformed into threading edge dis-locations (TEDs) that have less impact on device per-formance (see figure 2). Normally more than 90% ofthe substrate BPDs will naturally convert to TEDs dur-ing epilayer growth, which cuts the typical epilayer

BPD density to 300cm–2. On its own this natural reduc-tion in BPD density is insufficient for the fabricationof commercially relevant SiC power devices. However,improving the efficiency of the natural BPD–TED con-version appears to offer the most promising solution.

Mark Skowronski from Carnegie Mellon University,PA, has suggested that a reduction in dislocation lengthcan boost the subsequent conversion of BPDs intoTEDs during epilayer growth. TEDs are preferred toBPDs because they are shorter and consequently pro-duce a smaller increase in the system’s overall energy.

In SiC substrates the BPDs exist in many differentdirections, while in epilayers they predominantly occurin one particular direction, which is determined by the

epilayer

substrate

TEDetch pit

BPDetch pit

Fig. 2. Basal plane dislocations, which are largely responsible for

the forward voltage drift that occurs in bipolar SiC power devices,

cannot be eliminated but they can be transformed into less harmful

types of defects. The conversion can be seen in this micrograph of a

potassium-hydroxide-etched epilayer near a substrate slip band.

Along the left side of this image is a line of characteristic, scallop-

shaped, etched pits that occur where the BPDs intercept the

surface. On the right there is a grouping of etch pits associated with

the TEDs. Although the specific dislocations depicted in this image

are associated with a localized slip band in the substrate (see the

diagram just below the image), a similar BPD to TED conversion

occurs frequently across the wafer.

Fig. 1. Cree has developed an etch and regrowth process to reduce the number of basal plane dislocations in the epiwafers. This led to a

significant reduction in the drift in forward voltage in bipolar devices of 10 kV, 20A PIN diodes.

We believe that thisapproach representsthe greatest recentadvance in bipolar SiCdevice technology.

(a) start with substrate

(d) repolish surface (e) grow device epilayers (f) fabricate and test devices

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substrate’s off-axis angle and orientation. For typical8º off-axis material, the length of a substrate BPD thatpropagates into the epilayer without undergoing anytransformation is roughly seven times that of a BPDthat is converted to a TED during epilayer growth.Reducing the off-axis angle of the SiC substrate canincrease this disparity and potentially enhance the dis-location conversion process. However, this approachalso has its drawbacks and leads to poorer quality sur-faces with higher overall defect densities.

Our new etching approachSelective etching can be used to locally reduce the off-axis angle in the immediate vicinity of a BPD whilemaintaining a more favorable off-axis angle for themajority of the substrate (see box “The benefits of sel-ective etching”). In this process, the SiC substrate isselectively etched before a BPD conversion epilayeris deposited (see figure 1, p26). The epilayer surfaceis then repolished to recover a smooth, pit-free surfacefor growth of the actual device structure.

To evaluate the benefits of our new process, we grew

a batch of epiwafers on which we fabricated 10 kV,20 A PIN diodes. Seven wafers were produced withlow-BPD processing, along with one control wafer thatdid not have any low-BPD processing. The Vf stabil-ity of the diodes was evaluated by comparing thechange in Vf after 100 A/cm2 was passed through theon-wafer devices for 30 minutes (see figure 3). Thepass mark was set at a drift of less than 0.1 V.

The results show that 51.3% of the diodes fabricatedon low-BPD wafers had a stable Vf. In contrast, noneof the diodes fabricated on the unprocessed substrateexhibited forward-voltage stability and 80% of thedevices drifted by more than 2 V.

Clearly, this low-BPD technology delivers a sub-stantial improvement in bipolar SiC device’s Vf sta-bility and brings us closer to a commercially viableproduct. However, the low BPD conversion process iscumbersome and costly, the yields need to be improved,and there are several issues to be resolved before thisprocess is ready for production.

First, the current process needs to be shortened andsimplified, but this should be possible because we arecurrently using conservative conditions for pre-etch,regrowth and polish. In fact, we expect that as the qual-ity of each stage is improved the overall process willbecome much more manageable. Second, we must con-tinue to reduce the defect density in low-BPD mater-ial, as this will increase the overall device yield. Evenwith an intermediate repolishing step, the legacy of theselective etch process contributes to higher defect den-sities in the device epilayers. Last, we have to confirmthe long-term reliability of low-BPD material. Whilethe 30 minute stress test provides a convenient metric,it fails to deliver the rigor required to ensure that thedevices are stable throughout a typical service life.

If we are able to address these three issues, we willbe in a position to manufacture stable, high-voltageSiC PIN diodes that can deliver significant energy sav-ings for power conversion. ●

Selective etching can cut the density of BPDs by locallyreducing the off-axis angle close to a BPD while retaining amore favorable off-axis angle for most of the SiC substrate.The benefits of this local etching are shown in the figure,which depicts two adjacent basal plane defects: BPD2,which has an associated pit, and BPD1, which does not.

Point A is positioned at the transition of BPD1 from thesubstrate to the epilayer. If BPD1 were converted into aTED, represented by the dotted line, then the defect linewould have a much shorter length within the epilayer. Thereduction in defect length would be greater, though, ifBPD2 could convert from a BPD to a TED at point B.

The force of the mechanism favoring BPD to TEDconversion is inversely proportional to the distancebetween the dislocation and the surface. So if BPD2 didpropagate into the epilayer as a BPD, it would experiencea large and prolonged force encouraging a conversion toTED character. This is partly because the dislocation line

would only slowly diverge from the surface during theepilayer growth near the etch pit, but it is also aconsequence of the lower growth rates that typically occurwithin trenches and pits.

The differences in growth rate will cause the epilayer togrow more quickly at the top of the etch pit (point C) thanat its bottom (point B). This means that if BPD2 started topropagate into the epilayer as a BPD, it is likely toencounter a thicker portion of the epilayer before it canexit the etch pit, which will block its propagation andencourage conversion into a threading-type defect.

The benefits of selective etching

epilayer

BPD2

C

A

BsubstrateBPD1

50%

40%

30%

20%

10%

0%

die

cou

nt

<0.1

0.1–

0.2

0.2–

0.3

0.3–

0.4

0.4–

0.5

0.5–

0.6

0.6–

0.7

0.7–

0.8

0.8–

0.9

0.9–

1.0

1.0–

2.0

2.0–

3.0

3.0–

5.0

>5.0

VF drift (V)

without BPD conversion processwith BPD converstion process

Fig. 3. Cree’s process for reducing basal plane dislocations has

dramatically reduced the voltage drift resulting from a 30 minute

forward-conduction stress test at 100 A/cm–2.

About the authorsJoe Sumakeris (left) is asenior scientist at Cree. He isresponsible for developing SiCepilayer growth technology forRF and bipolar power devices,and for developing high-temperature implant activationanneal technology.Brett Hull (middle) is aprocess scientist at Cree. He isresponsible for the fabricationof high voltage SiC rectifier andMOSFET devices.David Grider (right) is Cree’smanager for governmentcontract R&D SiC powerprograms and customercommercial SiC power projects.



T ECHNOLOGY S U B S T R A T E S

TDI cracks AlN template troubleGaN HEMT manufacturers and ultraviolet LED developers are using large crack-free AlN-based templatesthanks to a new deposition process, say TDI’s Vladimir Dmitriev and Alexander Usikov.

AlN substrates are well suited to the fabrication of ultra-violet LEDs and could boost the performance high-fre-quency transistors used in base-station infrastructure.The LEDs benefit from AlN’s transparency at wave-lengths greater than 200 nm, while the performance ofRF devices is aided by AlN’s very high thermal con-ductivity, electrical insulation and a crystal lattice thatclosely matches that of AlGaN.

However, despite years of development, it is still verydifficult to grow single crystals of AlN with low enoughdefect densities and their size is insufficient for com-mercial applications. For example, our work atTechnologies and Devices International (TDI), MD,has been restricted to the fabrication of 2 inch AlNwafers using a free-standing approach, while 2 inch sub-strates only became commercially available veryrecently through Crystal IS. Although the availabilityof 2 inch material represents some progress, this size isunable to satisfy the demands of electronic device man-ufacturers who want to use 3 and 4 inch substrates now,and 6 inch substrates in the future. This appetite forlarger substrates has led to various AlGaN-baseddevices being developed on foreign substrates.

One way of accelerating AlGaN-based device devel-opment and commercialization is to use engineeredtemplates, which consist of a native AlN surface forsubsequent device epitaxy and a base made of a differ-ent material, like silicon, sapphire or SiC. An advan-tage of this is that the wafer’s size is then determinedby the dimensions of the base substrate (see figure 1).

Using this technique, templates are produced bydepositing a single-crystal AlN epitaxial layer onto aforeign substrate at a high growth rate to form a thick,low-defect layer. Thick AlN is essential for reducingthe defects that result from growth on a foreign sub-strate because the defect density rapidly decreases withdistance from the AlN/substrate interface.

Template substrates with sufficiently thick AlNlayers can also deliver excellent electrical insulationfor the upper device structure because AlN’s electricalresistivity is higher than 1011 Ω cm at room tempera-ture. In addition, the AlN layer has a thermal con-ductivity of at least 3 W cm–1 K–1, which can boostdevice performance, and a native AlN surface forlattice-matched growth.

In our opinion the only suitable method for pro-ducing such thick epitaxial layers is hydride vaporphase epitaxy (HVPE). MOCVD and MBE have typ-ical growth rates of less than 1–2μm per hour and usingthese methods to deposit 10 μm or more of AlN is tooexpensive and time-consuming. HVPE, however, can

produce low-defect GaN and AlN layers at much lowercosts and at rates that can exceed 1 μm per minute.

Unfortunately, the standard HVPE technique suffersfrom the same major drawback that hampers the MBEand MOCVD approach – severe cracking of the AlNlayer when its thickness exceeds a couple of microns.The cracking results from differences in the thermalexpansion coefficients and crystal lattice dimensionsbetween AlN and its foreign substrate, and can producecrack densities in the range of hundreds per millimeter.

Modifying the HVPE processTo address the problems associated with cracking wehave developed a deposition technique called stress-control HVPE. This process, which we have developedusing our proprietry and patented home-built multiwafermachines that feature a hot-wall quartz tube and a resis-tively heated furnace, is able to produce crack-free AlNlayers up to 75 μm thick. It has also been used to growcrack-free AlN films from 10–30μm thick on 2 inch SiCthat have defect densities in the high 107 cm–2 range,which is an improvement by at least an orderof magnitude over device structures growndirectly on SiC that suffer from a small dis-tance from the SiC substrate.

These templates can even be built usingelectrically conducting substrates, such assilicon or conducting SiC. The key is to growthe electrically insulating AlN layer thickenough to prevent high-frequency signalloss in microwave devices. For GaN-basedHEMTs operating at 2 GHz this thicknessshould be a minimum of 10 μm and forhigher-frequencies devices it should be eventhicker. AlN templates offer important adv-antages to manufacturers of this type of dev-ice because they can be produced fromconducting SiC, which is available in larger

Fig. 1. Technologies and Devices International can produce different templates by HVPE, including

2 inch AlN-on-SiC, 3 inch AlN-on-sapphire, 4 inch GaN-on-sapphire and 6 inch AlN-on-Si.

The cleaved edge of an AlN-on-

SiC epitaxial wafer with a

26 μm-thick AlN layer, showing

that it is possible to grow crack-

free AlN using TDI’s stress-

control HVPE process.

The appetite forlarger substrateshas led to variousAlGaN-baseddevices beingdeveloped onforeign substrates.

10 μm



TECHNOLOGY S U B S T R A T E S

sizes than semi-insulating SiC, and at a significantlylower cost. The benefits have led a customer of ours todevelop a process for manufacturing GaN HEMTs for3G base stations using low-cost AlN-on-SiC templates.

We used stress-control HVPE to manufacture 3 inchdiameter AlN-on-conducting SiC template substrateswith an AlN thickness of 10–25 μm. This product,which was launched last year, features high-crystalline-quality AlN layers (see figure 2). These wafers are alsorelatively flat, and a 20 μm-thick AlN layer producesa bow of less than 40 μm, making this platform idealfor sub-micron processing of microwave device struc-tures. The templates can also be polished to producesubstrates with a surface roughness below 0.5 nm.

More recently, we have expanded our stress-freedeposition process to 4 inch conducting SiC andproduced templates with 10–15 μm-thick crack-freeAlN layers (see figure 3). Preliminary X-ray diffrac-tion data from these wafers, which we plan to releasecommercially by the end of the year, shows that theyhave a similar crystal quality to their 3 inch predec-cessors. They are the first semi-insulating 4 inch AlNsubstrates for high-power microwave devices and willallow manufacturers to use standard 4 inch microwaveprocessing lines for the production of AlGaN HEMTsand amplifiers. They can even be scaled to 6 inch sub-strates when conducting SiC substrates of that sizebecome available.

Improving UV LED outputStress-control HVPE can also be used to deposit thickAlN layers on other substrates and we have used it toproduce crack-free material up to 20 μm-thick on 2 inchsapphire. These AlN-on-sapphire templates are trans-parent at ultraviolet wavelengths and make an idealplatform for the production of AlGaN-based LEDs andultraviolet photodetectors. These devices benefit fromthe high thermal conductivity of the AlN layer and alow defect density, thanks to lattice-matched growthon an AlN surface.

Michael Kneissl and his colleagues at Palo AltoResearch Center (PARC), CA, have used an AlN-on-sapphire variant, AlGaN-on-sapphire, to grow ult-raviolet LEDs. After chemically cleaning the 2 inch

templates, they used MOCVD to grow various LEDstructures featuring magnesium-doped p-type layersand an active region of five 4 nm-thick InAlGaN qua-ntum wells separated by 8 nm-thick InAlGaN barriers.Chemically assisted ion-beam etching formed squareLED chips from the epiwafers with dimensions of100–900 μm. By intentionally varying the quantumwell and barrier compositions, the LEDs’ emissionwavelengths were adjusted from 289 nm to 373 nm.

The largest 330 nm-emitting devices tested on thewafer produced a continuous-wave (cw) output of11 mWat 400 mAdrive current. Output was limited bythermal rollover and would have been higher in pack-aged devices because of improved heat dissipation.When driven in pulsed mode (1μs pulse widths, 10kHzduty cycle) the output of these LEDs rose to 55 mW.100 μm square devices emitting at 330 nm producedexternal quantum efficiencies (EQEs) of 1.5% in cwmode and 2.3% in pulsed mode. These results comparefavorably with LEDs built by other researchers on sap-phire substrates, which have EQEs of typically 1% at350 nm, and 0.1% between 324 nm and 269 nm.

These promising results on ultraviolet LEDs, alliedto the development of GaN HEMTs on AlN-basedtemplates, illustrate some of the benefits that resultfrom the stress-control HVPE process. Our launch ofthe 3 inch AlN-templates for high-power devices is alr-eady offering substantial cost savings and benefits forhigh-power device manufacturers, and we expect ourplanned release of the 4 inch AlN-on-SiC later this yearto speed the development and commercialization ofhigh-power microwave electronic components andsystems and ultraviolet optoelectronic devices. ●

Further readingV Soukhoveev et al. 2006 Phys. Stat. Sol. (c) 3 1653.V Dmitriev et al. 2006 Hydride vapor phase epitaxy ofgroup III nitride materials. In: III-Nitride SemiconductorMaterials pp1–40, (ed. Z C Feng), Imperial CollegePress. ISBN 1-86094-636-4.O Kovalenkov et al. 2005 J. Cryst. Growth 281 87.M Kneissl et al. 2006 Jap. J. Appl. Phys. 45, 5A 3905 VSoukhoveev et al. 2006 Mater. Res. Soc. Symp. Proc.892 743.

30

20

10

0

–10

–20

–30

–30 –20 –10 0 10 20 30

350

336

322

308

294

280

266

252

238

224

210

fwhm (arcec)

(00.2) AIN on 4H-SiC

distance (mm)

Fig. 2. (left) The high degree of

crystalline quality in the AlN

layers is revealed through X-ray

characterization. This image

shows full-width at half-

maximum (FWHM) values from

a ω-scan of the (00.2) AlN

reflection on a 3 inch AlN-on-

sapphire epitaxial wafer with a

15 μm-thick AlN layer. For this

reflex the average FWHM value

is 234 arc sec, while in the

(10.2) reflex the average value

is below 800 arc sec.

Fig. 3. (right) TDI has recently

produced 4 inch AlN-on-SiC

template substrates with 12 μm-

thick crack-free AlN layers on

conducting SiC. The company

plans to launch these templates

by the end of 2006.

About the authorsVladimir Dmitriev (right) isTDI’s president and CEO. Hehas previously developed SiChigh-power devices at the IoffeInstitute and GaN LEDs at Cree.Alex Usikov (left) is TDI’ssenior scientist and R&Dprogram manager.



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T ECHNOLOGY R E S E A R C H R E V I E W

Russell Dupuis’ team from Georgia Instituteof Technology, GA, the US Army ResearchLaboratory and Kyma Technologies, claim tohave produced the highest optical gain yet seenin GaN-based avalanche photodiodes.

The devices deliver a maximum optical gainof more than 1000 when illuminated by 360nmradiation. At 50 μm in diameter, they also havethe largest area yet reported for any III-N

avalanche photodiodes.Despite the gain improvements, the photo-

diode’s sensitivity is still well below that ofphotomultiplier tubes (PMTs) that can producegains as high as 106 at ultraviolet wavelengths.However, the team points out that PMTs needa high-voltage supply and cooled photocath-ode, which makes them quite large, expensive,bulky and fragile.

GaN photodiodes also have advantages overultraviolet-enhanced silicon avalanche photo-diodes, such as a lower dark current, and do notneed complex filters for solar blind detection.

The detectors were grown by MOCVD onn-type GaN substrates with a dislocation den-

sity of 105–106 cm–2. They featured a 2.5 μmthick silicon-doped GaN layer, a 0.3 μm thickunintentionally doped GaN drift region, and a0.12 μm thick magnesium-doped GaN layer.Inductively coupled plasma etching definedthe device geometry, before ohmic contactswere formed by metal evaporation.

The team believes that the detector’s noiseperformance could be reduced with a separateabsorption-multiplication structure featuringimpact ionization engineering.

Engineers at the US Air Force ResearchLaboratory (AFRL) claim to have fabricatedthe first operational GaN-on-diamond high-electron mobility transistor.

Although details from the AFRL team aresketchy, with no mention of output power orfrequency of operation, the team did reveal thefollowing: “Initial transistor results show thatthe AlGaN/GaN material survived all theprocess steps, including high-temperatureohmic contact anneal.”

Because of its very high thermal conduc-tivity, diamond has long been regarded as theideal material on which to base high-powertransistors. However, the lattice mismatchbetween it and GaN alloys has proved to be amajor stumbling block.

To get around that issue, the transistor epi-

layers were first grown on a sacrificial “non-SiC” substrate using MOCVD by IvanEliashevich and colleagues at Emcore’s elec-tronic materials division (EMD). Next, GaN-on-diamond specialists at California-basedGroup4 Labs took the epitaxial structure andcarefully attached it to a chemically vapordeposited (CVD) diamond substrate.

A thin, thermally conductive attachmentlayer is sandwiched between the CVD dia-mond and the epitaxial structure, ensuring thatthe transistor channel is close to the diamond.Critically, this enables almost-instantaneousheat extraction from the device. In theory, thisshould mean that transistors with an extremelyhigh power density could be processed usingthe novel combination of materials.

“We are excited by the promise of this tech-

nology,” said Eliashevich, the director ofresearch and development at EMD. “It com-bines the most robust semiconductor materialwith the best heat spreader.”

Eliashevich expects that transistors basedon the novel material combination will beinitially deployed in high-performance mili-tary applications. However, he adds thatbecause the CVD diamond and epitaxyapproaches are inherently scalable to 4 inch oreven larger wafers, the technology could even-tually penetrate lower-cost, higher-volumecommercial markets.

Apart from RF and power switch appli-cations, the approach may also turn out to beuseful for improving heat dissipation in opto-electronic devices, which could enable brighterLEDs and more powerful laser diodes.

US Air Force makes diamond advance

Non-polar GaN-based devices are attractingconsiderable attention because they do not suf-fer from polarization-related electric fields,but at the moment their performance isrestricted by high defect levels.

However, that could all change now thatSteven DenBaars’ group at the University ofCalifornia, Santa Barbara, has shown that it ispossible to reduce the defect density by insert-ing a very thin SiN layer between GaN epilay-ers. The process cuts the stacking fault densityin a-plane non-polar films from 8 × 105 cm–1 to3 × 105 cm–1, and the dislocation density from8 × 1010 cm–2 to 9 × 109 cm–2.

The Santa Barbara team produced the low-defect density films on r-plane sapphire byMOCVD. According to team member Arpan

Chakraborty, depositing SiN in situ was a rel-atively straightforward process: “Silicon doesnot exhibit a memory effect and the experi-ments were pretty reproducible.”

Chakraborty told Compound Semiconductorthat the team based its optimization of the SiN

layer on previous studies of c-plane GaN, whichhad shown an ideal thickness of about one anda half monolayers. Defect reduction did notoccur if this layer was too thin, explainedChakraborty, but if it was too thick it could hin-der the coalescence of the overgrown GaN film.

The researchers are now growing GaN-based LEDs using this approach. “The initialphotoluminescence data from our multiple-quantum-well calibration samples look verypromising,” remarked Chakraborty, whorevealed that the emission intensity hadincreased by almost one order of magnitudewith the new process.

The group is also studying the microstruc-tural evolution of the overgrown GaN layer byvarious techniques, including transmissionelectron microscopy, which can identify stack-ing faults and dislocations.

Introducing thin SiNlayer cuts GaN defects

Journal referenceA Chakraborty et al. 2006 Appl. Phys. Lett. 89

041903.

G A N T R A N S I S T O R S

O P T O E L E C T R O N I C S

Photodiodes producehighest optical gain

G A N P H O T O D E T E C T O R S

300 nm

Transmission electron microscopy images can reveal

the defect density in non-polar films incorporating a SiN

layer. Plan view images revealing the stacking faults

(left) and threading dislocations (right) were produced

by a University of California team using the diffraction

conditions g= 1100 and g= 0002, respectively.

Journal referenceJ B Limb et al. 2006 Appl. Phys. Lett. 89

011112.


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