Nano Hall BarsNano Hall Bars

Daniel BrunskiDaniel Brunski

2008 Fall2008 Fall

Advisors:Advisors:

Dr. Matthew JohnsonDr. Matthew Johnson

Dr. Joel KeayDr. Joel Keay

Special Thanks:Special Thanks:

Ruwan Dedigama Ruwan Dedigama

the quest for single defect scatteringthe quest for single defect scattering

OutlineOutlineIntroductionIntroduction

MotivationMotivation

BackgroundBackground Band GapBand Gap Quantum WellsQuantum Wells The Hall EffectThe Hall Effect

Single Defect MeasurementsSingle Defect Measurements

Microfabrication TechniquesMicrofabrication Techniques PhotolithographyPhotolithography Electron Beam LithographyElectron Beam Lithography EtchingEtching

Hall Bar PlanHall Bar Plan Top-Down ViewTop-Down View Cross-sectional ViewCross-sectional View

Progress to DateProgress to Date PhotolithographyPhotolithography Reactive-Ion EtchingReactive-Ion Etching Ohmic ContactsOhmic Contacts

Current IssuesCurrent Issues

The FutureThe Future

Scanning electron microscope (SEM) Scanning electron microscope (SEM) image showing several defects (circled) image showing several defects (circled) near a devicenear a device

10μm10μm

IntroductionIntroductionDefects in semiconductor devices act as scattering centers, effectively Defects in semiconductor devices act as scattering centers, effectively increasing resistanceincreasing resistance

As devices become smaller, single particle interactions with defects become As devices become smaller, single particle interactions with defects become very significantvery significant

Effects may include tunneling or other unexpected phenomenaEffects may include tunneling or other unexpected phenomena

A Hall bar will be used to investigate the effects of a single defect on charge A Hall bar will be used to investigate the effects of a single defect on charge carrierscarriers

InSb semiconductors used are grown with molecular-beam epitaxy (MBE)InSb semiconductors used are grown with molecular-beam epitaxy (MBE)

50nm50nm

9%AlInSb9%AlInSb

InSb Quantum WellInSb Quantum Well

DefectDefect

1μm1μm

GaAsGaAs

GaSbGaSb

AlSbAlSb

9%AlInSb9%AlInSb

9%AlInSb9%AlInSb

QWQW

Cross-section transmission electron microscope (TEM) images of Cross-section transmission electron microscope (TEM) images of InSb/AlInSb on GaAs substrateInSb/AlInSb on GaAs substrate

MotivationMotivationBetter understanding of defect Better understanding of defect scatteringscattering

Improving semiconductor Improving semiconductor qualityquality

Quantum wells are integral to Quantum wells are integral to high-speed transistors such as high-speed transistors such as MODFETs, used in low noise MODFETs, used in low noise devices:devices:

Satellite receiversSatellite receivers Low power amplifiersLow power amplifiers Cell phonesCell phones

More efficient semiconductor More efficient semiconductor laserslasers

Blue diode lasers employ InGaN Blue diode lasers employ InGaN quantum wellsquantum wells

Semiconductor laserSemiconductor laser

Band GapBand GapAvailable electron energies in materials form bandsAvailable electron energies in materials form bands

Band gap is the gap in energy between valence band and conduction bandBand gap is the gap in energy between valence band and conduction band Forbidden region, no allowed energies in gapForbidden region, no allowed energies in gap

In conductors, valence electrons are essentially free, represented by In conductors, valence electrons are essentially free, represented by overlap in bandsoverlap in bands

Quantum WellsQuantum WellsFormed when a thin layer of Formed when a thin layer of narrow band-gap InSb is narrow band-gap InSb is sandwiched between wider band-sandwiched between wider band-gap AlInSbgap AlInSb

Quantum well confines charges, Quantum well confines charges, wavefunctions become quantizedwavefunctions become quantized

Electrons are confined to discrete Electrons are confined to discrete energy levelsenergy levels

For lasers, more electrons are For lasers, more electrons are confined to energies above the confined to energies above the lasing thresholdlasing threshold

Leads to semiconductor lasers Leads to semiconductor lasers that require less current to operatethat require less current to operate

Micro-twin defects change Micro-twin defects change quantum well geometryquantum well geometry

Smaller well, higher energy Smaller well, higher energy confinementconfinement

Acts as potential barrier, Acts as potential barrier, scattering chargesscattering charges

15.815.8°°Micro-twinMicro-twin

InSb InSb Quantum WellQuantum Well

9%AlInSb9%AlInSb

Micro- Micro- twintwin ~ 16~ 16°°

10 nm10 nm

TEM image showing a Micro-twin defectTEM image showing a Micro-twin defect

The Hall EffectThe Hall EffectA magnetic field is applied to a A magnetic field is applied to a conductor, perpendicular to conductor, perpendicular to current flowcurrent flow

Moving charge carriers experience Moving charge carriers experience a Lorentz forcea Lorentz force

Charges accumulate on one side Charges accumulate on one side of the conductor, equal but of the conductor, equal but opposite charge left on other sideopposite charge left on other side

Separation of charges creates an Separation of charges creates an electric potential, the Hall voltageelectric potential, the Hall voltage

Hall effect has numerous Hall effect has numerous applications:applications:

Non-contact current sensorsNon-contact current sensors Solid-state position and motion Solid-state position and motion

sensorssensors At low temperatures Hall At low temperatures Hall

conductivity becomes quantized, conductivity becomes quantized, leads to a standard of resistance leads to a standard of resistance (h/e(h/e22 = 25812.8ohms) = 25812.8ohms)

v

B F

)( BvEF qe

Hall current sensorHall current sensor

Single Defect MeasurementsSingle Defect MeasurementsUse photolithography to create a Hall Use photolithography to create a Hall bar over an area containing defectsbar over an area containing defects

Electron-beam lithography used to Electron-beam lithography used to isolate a single defectisolate a single defect

Defects not isolated act as effective Defects not isolated act as effective resistanceresistance

Apply magnetic field to induce Hall effectApply magnetic field to induce Hall effect

Contact points allow voltage Contact points allow voltage measurements before and after the measurements before and after the defectdefect

Current is plotted against voltage Current is plotted against voltage differencedifference

Nonlinearities may be signs of scattering Nonlinearities may be signs of scattering or tunnelingor tunneling

Hall bar optical image, Hall bar optical image, ~1mm x 1.5mm~1mm x 1.5mm

100μm100μm

II

PhotolithographyPhotolithographyParallel processParallel process

Sample coated with a photo-reactive resistSample coated with a photo-reactive resist

Mask is placed on sample and then exposed to UV lightMask is placed on sample and then exposed to UV light

Exposed resist reacts to UVExposed resist reacts to UV

Developer removes unstable resistDeveloper removes unstable resist

Resolution limited by diffraction of lightResolution limited by diffraction of light Current commercial processes produce down to 45nm structuresCurrent commercial processes produce down to 45nm structures

Deposited Film

Substrate

Film Deposition

Resist

Photoresist application

Exposure

MaskUV light

Development Etching Resist removal

Electron Beam LithographyElectron Beam LithographyElectron beam instead of UV lightElectron beam instead of UV light

Smaller scale structuresSmaller scale structures Down to 25nmDown to 25nm

SEM can perform electron beam SEM can perform electron beam lithography (EBL)lithography (EBL)

Electron beam is computer controlledElectron beam is computer controlled

Serial processSerial process Not suited for high volume productionNot suited for high volume production

Resolution limited byResolution limited by Electron scattering in photoresistElectron scattering in photoresist Proximity effectProximity effect Acoustic noiseAcoustic noise

100nm line widths possible on our 100nm line widths possible on our Zeiss 960AZeiss 960A

Standard SEM columnStandard SEM column

Electron beam

Electron gun

Anode

Magnetic Lens

Scanning coil

Backscattered Electron Detector

Stage Sample

Secondary electron detector

Output

EtchingEtchingProcess in which resist pattern is Process in which resist pattern is transferred to material surfacetransferred to material surface

Wet etchingWet etching Chemical solutionChemical solution Typically produces rounded Typically produces rounded

isotropic profileisotropic profile Etch can undercut resist layerEtch can undercut resist layer

Dry etchingDry etching Sputtering – energetic ions Sputtering – energetic ions

bombard surface and remove bombard surface and remove material mechanicallymaterial mechanically

Reactive-ion etching (RIE) – Reactive-ion etching (RIE) – chemically reactive plasma and chemically reactive plasma and physical processes remove materialphysical processes remove material

Produces anisotropic etch profileProduces anisotropic etch profile

A Hall bar featuring EBL and RIE A Hall bar featuring EBL and RIE produced trenchesproduced trenches

TrenchesTrenches

Hall Bar Top-DownHall Bar Top-DownHall bar defined with photolithography and RIE to produce mesaHall bar defined with photolithography and RIE to produce mesa

Trenches defined with EBL and RIE to isolate defectTrenches defined with EBL and RIE to isolate defect

Gates allow scanning of charges across defectGates allow scanning of charges across defect

Defect could be located anywhere in dashed box with extended trenchesDefect could be located anywhere in dashed box with extended trenches

Applied magnetic field

ee

VVG1G1

VVG2G2

VVH2H2++

VVH2H2--

Hall bar mesaHall bar mesa

IISS

DefectDefect

GateGate

-- -- -- --

++ ++ ++ ++

Substrate and Substrate and buffer layersbuffer layers VVH1H1

++

VVH1H1--

Etched trenchEtched trench

Hall Bar Cross-SectionHall Bar Cross-SectionShown measurements are approximateShown measurements are approximate

Defect may or may not be localized to a small area in the quantum wellDefect may or may not be localized to a small area in the quantum well

Greater than 4.3μm etch needed to electrically isolate quantum wellGreater than 4.3μm etch needed to electrically isolate quantum well

InSb quantum wellInSb quantum well AlInSb barrier, AlInSb barrier, InSb capInSb cap

GateGate

GaAs substrateGaAs substrate

Hall bar mesaHall bar mesa

30nm30nm180nm180nm

4μm4μm>4.3μm>4.3μm

DefectDefect

AlInSb barrier, AlInSb barrier, AlInSb/AlSb buffer AlInSb/AlSb buffer layers, SLSlayers, SLS

Hall Bar PhotolithographyHall Bar PhotolithographyProduced a series of resolution tests Produced a series of resolution tests to obtain a method for good to obtain a method for good photolithography resultsphotolithography results

Consisted of lines and gridsConsisted of lines and grids

Procedure for aligning Hall bars on Procedure for aligning Hall bars on defects tedious but possibledefects tedious but possible

Random placement not reliableRandom placement not reliable

Resist thickness 2 to 2.2μmResist thickness 2 to 2.2μm Nominal value for S1818 – 1.8μmNominal value for S1818 – 1.8μm

10μm10μm

Optical zoom of photoresist on a Optical zoom of photoresist on a quantum well InSb sample, two quantum well InSb sample, two potentially usable features presentpotentially usable features present

Etching TrialsEtching TrialsEtching trials performed on 3μm InSb bulk samplesEtching trials performed on 3μm InSb bulk samples

Need a recipe that has at least 2:1 InSb:Resist etch ratioNeed a recipe that has at least 2:1 InSb:Resist etch ratio

Initially tried a 24 minute etch with BClInitially tried a 24 minute etch with BCl33 + Ar, 1.5μm etch depth + Ar, 1.5μm etch depth

Next trial was 5 steps of 5 minute BClNext trial was 5 steps of 5 minute BCl33 + Ar, with 30 second Ar sputter + Ar, with 30 second Ar sputter phases in between, 1.4μm etch depthphases in between, 1.4μm etch depth

Also tried 10 sets of BClAlso tried 10 sets of BCl33 + Ar / Ar, 2.2μm etch depth + Ar / Ar, 2.2μm etch depth

24 minutes BCl24 minutes BCl33 + Ar + Ar 55 minutes BCl55 minutes BCl33 + Ar / Ar + Ar / Ar

Etching AnalysisEtching AnalysisAnalysis of surface shows there is still InSb left to etchAnalysis of surface shows there is still InSb left to etch

Possible sources of etching slowdown are redeposition of etched Possible sources of etching slowdown are redeposition of etched products and formation of InCl on surface (high melting point)products and formation of InCl on surface (high melting point)

Ar sputter phase added in an attempt to mechanically clean surface, but Ar sputter phase added in an attempt to mechanically clean surface, but results were not satisfactoryresults were not satisfactory

Tried preheating RIE chamber to combat formation of Cl residues, but Tried preheating RIE chamber to combat formation of Cl residues, but etch depth not greatly improvedetch depth not greatly improved

1.7μm for a 27.5 minute etch compared to 1.4μm1.7μm for a 27.5 minute etch compared to 1.4μm

Cross-section back scatter SEM image of 2.2μm etch, Cross-section back scatter SEM image of 2.2μm etch, white areas are InSbwhite areas are InSb

Final EtchFinal EtchWhat worked – Alternating 5 steps 3 minutes BClWhat worked – Alternating 5 steps 3 minutes BCl33 + Ar / 5 steps 15 + Ar / 5 steps 15 seconds BClseconds BCl33 + SF + SF6 6 with higher powers and higher flow rate, 5μm with higher powers and higher flow rate, 5μm etch depthetch depth

6mm x 6mm10μm10μm

Ohmic ContactsOhmic ContactsContacts need to be modified to ensure Contacts need to be modified to ensure good electrical conduction, linear I-V good electrical conduction, linear I-V behaviorbehavior

Hall bars coated with resistHall bars coated with resist Contact pads exposed, developedContact pads exposed, developed

Indium deposited onto sample, resist Indium deposited onto sample, resist removedremoved

Sample annealed at 230°C for 5 minutesSample annealed at 230°C for 5 minutes Causes indium to diffuse down to quantum Causes indium to diffuse down to quantum

wellwell In melts at 156.6°CIn melts at 156.6°C

Measurements on several Hall bars Measurements on several Hall bars using a curve tracer showed linear I-V using a curve tracer showed linear I-V behaviorbehavior

9-11kOhm resistance between contact 9-11kOhm resistance between contact padspads

Infinite resistance between substrate and Infinite resistance between substrate and contact padscontact pads

100μm100μm

Deposited Deposited indiumindium

After After annealingannealing

Current IssuesCurrent IssuesOver half the devices damaged sometime Over half the devices damaged sometime between contact pad photolithography and between contact pad photolithography and annealingannealing

In most cases, current can be applied through In most cases, current can be applied through other pathwaysother pathways

Measurements with an optical microscope show Measurements with an optical microscope show the break depth to be about 4 to 5μmthe break depth to be about 4 to 5μm

GaAs/AlSb interface around 4.3μmGaAs/AlSb interface around 4.3μm

50μm50μm

Broken contactsBroken contacts

High defect density at High defect density at layer interfaces in InSb layer interfaces in InSb quantum well samplequantum well sample

The FutureThe FutureFind out what’s causing terminals to break off, possibilities:Find out what’s causing terminals to break off, possibilities:

Crushed during contact pad photolithographyCrushed during contact pad photolithography Moving around due to loose storageMoving around due to loose storage Ultrasonic cleaningUltrasonic cleaning

Aligning and performing EBL without damaging sampleAligning and performing EBL without damaging sample

Gates introduce effective resistance, electric potential narrows Gates introduce effective resistance, electric potential narrows conduction pathconduction path

SourcesSourcesImages:Images:

http://www.memsnet.org/mems/processes/wetetch.jpghttp://www.memsnet.org/mems/processes/wetetch.jpg http://en.wikipedia.org/wiki/Hall_effecthttp://en.wikipedia.org/wiki/Hall_effect http://cnx.org/content/m1037/latest/5.15.pnghttp://cnx.org/content/m1037/latest/5.15.png http://curie.umd.umich.edu/Phys/classes/p150/archive/goodfor/SpinFlip.htmhttp://curie.umd.umich.edu/Phys/classes/p150/archive/goodfor/SpinFlip.htm http://en.wikipedia.org/wiki/File:Bandgap_in_semiconductor.svghttp://en.wikipedia.org/wiki/File:Bandgap_in_semiconductor.svg http://www.hitequest.com/Kiss/photolithography.gifhttp://www.hitequest.com/Kiss/photolithography.gif

Articles/Presentations:Articles/Presentations: ““TEM Study of InSb/AlInSb Quantum Wells Grown on GaAs (001) Substrates”TEM Study of InSb/AlInSb Quantum Wells Grown on GaAs (001) Substrates” http://en.wikipedia.org/wiki/Semiconductor_laserhttp://en.wikipedia.org/wiki/Semiconductor_laser http://en.wikipedia.org/wiki/2DEGhttp://en.wikipedia.org/wiki/2DEG http://en.wikipedia.org/wiki/Electron_beam_lithographyhttp://en.wikipedia.org/wiki/Electron_beam_lithography http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html Kittel, Charles. Kittel, Charles. Introduction to Solid State PhysicsIntroduction to Solid State Physics