Nano Hall Bars Daniel Brunski 2008 Fall Advisors: Dr. Matthew Johnson Dr. Joel Keay Special Thanks:...
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Transcript of Nano Hall Bars Daniel Brunski 2008 Fall Advisors: Dr. Matthew Johnson Dr. Joel Keay Special Thanks:...
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