GaN as Optoelectronic and Photonic Devices

Mainul Hossain*, Ahmed Fazle Rabbi** and Zahid Hasan Mahmood Department of Applied Physics, Electronics and Communication Engineering, University of Dhaka,

Dhaka-1000, Bangladesh. Email: mhnafeez@yahoo.com*, rabbi.ahmed@gmail.com**

Abstract

Gallium Nitride (GaN) has already established itself as an important material for the fabrication of optoelectronic and photonic devices. This review article discusses the current status and the future prospect of this highly emerging wide band semiconductor material and its application as optoelectronic and photonic devices. It also intends to explain the choice of GaN over other semiconductor materials such as GaAs or SiC. Compared to SiC or GaAs materials, GaN technology offers a much higher frequency of operation. Wide bandgap GaN has long been sought for its applications to blue and UV emitters and high power or high temperature devices1. Heterostructures based on GaN and its alloys with AlN and InN find important applications as commercial blue light. Blue (450nm), blue/green, violet and ultraviolet (UV) Light Emitting Diodes (LEDs) based on InGaN/AlGaN have already been developed. Blue, green and white LEDs based on GaN are used in many applications: traffic lights, outdoor displays, car headlights, lighting applications, medical applications etc. GaN based laser diodes now have over 10,000 hours lifetime and will find many applications, e.g. in new generation DVD data storage and movie distribution. Blue (violet) GaN based semiconductor lasers allow to store information with approximately 4 times the higher density and therefore are the basis for the next generation DVDs 2. Recent progress in n-doping of GaN with much reduced background concentrations, p-doping of GaN and its alloys with AlN and InN have smoothened the way for the design, fabrication and characterization of devices such as LEDs and optically pumped lasers. In recent times, progress in micro fabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films. Emerging next generation photonics materials and devices include GaN quantum dots. The -V nitrides have the potential for operating as high efficiency emitters and detectors in the blue-green, blue and ultraviolet region of the optical spectrum3. Extremely bright GaN light emitting diodes combined with the relatively mature red and yellow emitters comprise the three primary colors of the visible spectrum enabling full color displays to be fabricated from semiconductor technology. These semiconductor materials show very promising applications as electroluminescent devices when they are doped with rare earth ions4. 1. INTRODUCTION Existing semiconductor devices of Si and GaAs cannot tolerate high temperatures and chemically hostile environments. The wide bandgap semiconductor GaN and, probably diamond in the future, with their excellent thermal conductivities, large breakdown fields, and resistance to chemical attack, will be the materials of choice for future optoelectronic and photonic applications. Breakthroughs in materials growth have enabled extremely high efficiency blue and green G aN light em itting diodes (L E D ’s) to be achieved. B lue L E D ’s w ith external quantum efficiency exceeding 9% have enabled hybrid G aN /phosphor w hite lam p L E D ’s. G aN L E D ’s com plete the primary color spectrum and have enabled bright and reliable full-color solid-state displays to be realized. Recently, room-temperature operation of continuous wave current-injection blue-violet lasers emitting at 417 nm has further increased the possible applications for GaN-based optoelectronic devices5. Short-wavelength emitters would potentially have numerous commercial applications for displays, optical storage, energy-efficient lighting, communications, printing, and even surgery. Of these applications, the UV/blue laser has been sought after as having potentially the biggest commercial impact. Extremely high-density optical storage devices will be possible if efficient semiconductor laser diodes can be synthesized with these materials. Because the storage density of optical com pact discs (C D ’s) is inversely proportional to the square of the laser w avelength, a four- to eightfold increase in capacity could be realized with short-wavelength lasers5. Full-color LED television (TV) displays can now be made entirely with the metal organic chemical vapor deposition (MOCVD) technique when com bining the blue and green G aN L E D ’s w ith the very high - brightness yellow and orange em itting L E D ’s5. Much progress has recently been made in the growth of III-V nitrides, in determining the properties of these m aterials, and in em ploying them for L E D ’s and high -speed devices. Unlike SiC, one notable advantage of III-V nitrides is that they form direct bandgap heterostructures, have better ohmic contacts and heterostructures. Because of this, it is anticipated that III-V nitrides may eventually prove to be more promising than SiC for high power applications in addition to optoelectronic devices. While the recent developments in GaN and related compounds have been truly breathtaking, further progress depends on the reduction of defects, which is closely tied to substrates1.

2. Gallium Nitride Material Properties Room temperature energy bandgap of wurtzitic GaN on sapphire is 3.42 eV1. The wide bandgap materials possess low dielectric constants with high thermal conductivity pathways. As shown in Table 1, group III nitrides exhibit fairly high bond strengths and very high melting temperatures5.

K (W/m °K) Tm (°K) PN2 (KBar) Bond (eV/bond) Dielectric () AlN 320 3500 60 2.88 8.9 GaN 120 2800 45 2.2 12.2 InN - 2150 37 1.93 -

T able1. T herm al C ond uctivities, M elting P oints, Y oung’s M oduli, B ond Strengths, and D ielectric Constants of III–V Nitrides The highest mobility reported for unintentionally doped GaN films amounts to 900cm2/Vs. Room-temperature electron mobilities of about 600cm2 /Vs have been obtained from material grown by metal oxide chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) 5.The electron mobility in GaN is quite high considering the magnitude of the bandgap and is even higher in selectively doped AlGaN/GaN heterostructures where 2-dimensional electron gases (2DEG) may form, producing high sheet carrier densities6. The strong piezoelectric and polarization effects present in the AlGaN/GaN heterostructures enhance these carrier densities. These properties make the nitrides well suited for high-frequency applications. GaN possesses a very high breakdown field, allowing the devices to support large voltages for high power operation6. Table 2 summarizes some of the electrical properties of GaN.

Properties GaN

Bandgap (eV) at 300C 3.4 (direct)

Electron Mobility (Cm2/Vs) at RT 1000(Bulk), 2000(2D-gas)

Hole Mobility (Cm2/Vs) at RT 30

Saturation Velocity (107cm/s) 2.5

Breakdown field (106V/cm) 5

Table2. Electrical properties of GaN.

3. Applications of GaN as Optoelectronic and Photonic Devices The III-V nitrides are potentially capable of increased power and higher temperature operation, due to their larger predicted electron velocities, band gaps, and lower ohmic contact resistances. GaN-based visible light-emitting diodes and laser diodes are already commercialized for a variety of lighting and data storage applications6The interest in the AlGaInN family of materials in recent years has been increased considerably by the application of blue/green/UV light-emitting diodes (LEDs) in full-color displays, traffic lights, automotive lighting and general room lighting using so-called white LEDs 6.In addition, blue/green laser diodes can be used in high storage-capacity digital versatile disks (DVDs) systems6. AlGaN-based photodetectors are also useful for solar-blind UV detection and have applications as flame sensors for detection of missiles or for control of gas turbines 6. There are currently major development programs in the US for the following newer applications for GaN-based materials and devices, namely

UV optical sources capable of operation down to 280 nm for use in airborne chemical and biological sensing systems, allowing direct multi-wavelength spectroscopic identification and monitoring of UV-induced reactions 6.

Spin-polarized light emitters for advanced optical switches, modulators, optical encoding and devices with integrated electronic and optical functionality 6.

3.1. GaN Based UV Optical Sources:

The current generation of visible GaN-based LEDs and laser diodes generally employs InGaN quantum well active regions, with AlGaN cladding layers6. Replacing the InGaN with GaN quantum wells typically produces operating wavelengths in the region of 350-360 nm 6. The use of AlGaN quantum wells can shift the LED emission to below 340 nm 6.In some cases the use of quaternary AlGaInN can produce improved lattice matching to the cladding layer and better UV emission efficiency 6. The UV LEDs or laser diodes can be employed in systems for real-time measurement of fluorescence spectra from airborne biological and chemical particles. This data can then be employed as part of a compact biological or chemical agent, warning sensor with fast response and high detection sensitivity. The UV wavelengths are necessary in order to cause fluorescence in many of the targeted chemicals and biological agents 6. 3.2. GaN and Its Alloys for Blue and Blue-Green LED's GaN based L E D ’s are very reliable, w hich have applications, for exam ple, in displays, lighting, indicator lights, advertisement, traffic signs, and traffic signals, possibly light sources for accelerated photosynthesis, and m edicine for diagnosis and treatm ent. Introduction of bright blue L E D ’s paved the w ay for full color displays and raised the probability of mixing primary colors-red, green, and blue-to obtain white light for illumination. If used in place of incandescent light bulbs, these L E D ’s w ould provide not only com pactness and longer lifetim e (tens of thousands for the blue L E D ’s as opposed to ~ 2000 h), but also would consume about 10-20% of the power for the same luminous flux.1.Commercial LEDs emitting in the red (650 to 660 nm) exhibit conversion efficiencies at 16% with lasers exhibiting figures as high as 75% 1. The band edge emission in GaN occurs at 365 nm which is UV, and by varying the alloy composition of Al(In)GaN, the band gap can be altered and made suitable for emission covering ultraviolet to green. Efficient blue and green emitters used in conjunction with the current bright red and yellow LEDs, allow for a full color visual LED display, highly sought by consumer electronics industry. It is in this area that G aN based L E D ’s are expected to dom inate over other potential material systems such as SiC. The quantum efficiencies of commercially available blue SiC are limited by the indirect bandgap nature of the material and would require high drive currents if it is intended for use in outdoor displays. Furthermore, its emission peak wavelength is technically not saturated blue, thus not sufficient to cover all the hues possible if used w ith the red and green L E D ’s in a color-scrambling scheme. On the other hand, InG aN based L E D ’s have the flexibility of being tailored to em it at 450 nm , have the spectral purity needed and have external quantum efficiencies two order of magnitude higher than their SiC counterparts1. 3.3. GaN and Its Alloys for Detectors Interest in GaN based detectors grows due to the fact that the earth's atmosphere is opaque at a wavelength of about 200 nm7. To take advantage of this optical window earth to space communication will need detectors that are UV sensitive but blind to the am bient visible radiation. A t present, satellite m onitoring of the earth’s atmosphere rely on Si photodetectors that require bulky band filters to block the visible solar radiation background. With the use of UV sensitive, visible blind GaN detectors, the need for these filters will be largely eliminated simplifying the design of the spectroscopic monitoring equipment1. Consequently space-to-space communication can take place without the blinding radiation from earth. An additional application of UV detectors in general and Al(Ga)N detectors in particular is that UV can penetrate water vapor, and this has many important consequences in communication systems7. Another area where GaN based photodetector can find application is in the combustion monitoring of gases where UV emission is a normal byproduct1. The other useful applications are combustion sensing and control for aircraft engines as well as power generation plants7. 3.4. GaN and its Alloys for Lasers During the last several years’ w ide bandgap nitrides have attracted attention in the field of digital inform ation storage. Unlike display and lighting applications, digital information storage and reading require coherent light sources, namely, lasers. Significant gains in recording density can be achieved by using potential GaN-based lasers that operate at shorter wavelength(s) 1. The nitride-based materials system, when adapted to semiconductor lasers in blue and UV wavelengths, offer increased data storage density possibly in excess of 1Gb/cm2 1. Present approaches to data storage rely on wavelength doubling of high power red lasers. They are bulky and expensive, and thus compact coherent sources emitting at the wavelength of interest are preferable. Although II-VI compound semiconductor, such as zinc-selenide (E, = 2.67 eV) and its pseudomorphic ternary and lattice-matched quaternary alloys have led to the demonstration of 490-nm diode lasers, they suffer from several weaknesses including very short lifetimes1.The epitome of GaN optoelectronics device research remains the realization of a GaN laser, which will be the shortest wavelength semiconductor laser ever demonstrated1.Blue

laser diodes based on GaN-based material is another area of intensive device research activity. The achievement of the shortest wavelength (417nm) laser diode using InGaN multiple quantum wells (M Q W ’s) indicates that these materials have a promising future in optical recording and displays5 .Currently, several problems still remain to be overcome to achieve a reliable continuously working (CW) electrically pumped laser diode5. Obtaining a high reflectivity mirror cavity and achieving low p-contact resistance remain two of the biggest obstacles to lasing operation5. 4. Conclusion GaN and its alloys form compact and efficient blue and green sources, which, against an already available red background, have resulted in full color display applications and lighting. In the past, the poor quality of material and lack of p-type doping thwarted engineers and scientists from fabricating useful devices from these materials1. Both of these obstacles have recently been sufficiently overcome to the point where high luminosity blue and blue-green light emitting diodes are now available in the market place1. For optical emitters and detectors GaN based heterostructures have demonstrated operation in the green, blue, or ultraviolet (UV) spectra1. These products should find wide use in applications such as full color displays, traffic signs, automobile and aircraft lighting, and traffic lights the prototypes of which are undergoing tests1. Considering the promising features of these nitrides and of the electrical and optical devices made so far from them, it appears that eventually they may prove to be superior to ZnSe devices and its alloys in optoelectronic applications1. The semiconductor GaN and its alloys appear poised to become the semiconductor of choice for short wavelength lasers in optical reading and other applications, as UV class wavelengths are possible1. In order for further development of these materials to be realized, doping incorporation (especially p-type) and ohmic contact technologies to p-type material have to be improved. Suitable substrate materials, not only in terms of lattice and thermal matches, but also stacking order match, ideally GaN itself, need to be aggressively pursued to further develop the GaN based material system and enable the fabrication of reliable injection lasers1.In any case, it is obvious that in the near future all optical and all photonic systems including optoelectronic communication systems would find immense scope and application for GaN based devices. References 1. S.N.Muhammad, Arnel A. Salvador and Hadis Morkoc, Emerging Gallium Nitride Based Devices, proceedings of IEEE, Vol 83, No.8, (1995). 2. Gerhard Fasol, Blue Laser Report, Euro Technology Japan K.K. (www.eurotechnology.com) version 6.1, (2004). 3 H.Morkoc, GaN and Silicon Carbide as Optoelectronic Materials,. 4. Wu, H., Poitras, C. B., Lipson, M. and Spencer, M. G., Photoluminescence and cathodoluminescence analyses of GaN powder doped with Eu, Appl. Phys. Lett., Vol. 88, 011921, (2006). 5. S.P. Denbaars, Gallium-Nitride-Based Materials for Blue to Ultraviolet Optoelectronics Devices, (1997). 6. S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, A.H. Onstine, B.P. Gila, F. Ren, B. Lou and J. Kim, New Applications for GaN-Based Semiconductors. 7. H.Morkoc, GaN And Sic As Optoelectronic Materials.