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Nano Hall Bars

Nano Hall Bars. the quest for single defect scattering. Daniel Brunski 2008 Fall Advisors: Dr. Matthew Johnson Dr. Joel Keay Special Thanks: Ruwan Dedigama. Outline. 10μm. Introduction Motivation Background Band Gap Quantum Wells The Hall Effect Single Defect Measurements

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Nano Hall Bars

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  1. Nano Hall Bars the quest for single defect scattering Daniel Brunski 2008 Fall Advisors: Dr. Matthew Johnson Dr. Joel Keay Special Thanks: Ruwan Dedigama

  2. Outline 10μm • Introduction • Motivation • Background • Band Gap • Quantum Wells • The Hall Effect • Single Defect Measurements • Microfabrication Techniques • Photolithography • Electron Beam Lithography • Etching • Hall Bar Plan • Top-Down View • Cross-sectional View • Progress to Date • Photolithography • Reactive-Ion Etching • Ohmic Contacts • Current Issues • The Future Scanning electron microscope (SEM) image showing several defects (circled) near a device

  3. Introduction QW Defect 9%AlInSb InSb Quantum Well GaSb 9%AlInSb 9%AlInSb AlSb 1μm 50nm GaAs • Defects in semiconductor devices act as scattering centers, effectively increasing resistance • As devices become smaller, single particle interactions with defects become very significant • Effects may include tunneling or other unexpected phenomena • A Hall bar will be used to investigate the effects of a single defect on charge carriers • InSb semiconductors used are grown with molecular-beam epitaxy (MBE) Cross-section transmission electron microscope (TEM) images of InSb/AlInSb on GaAs substrate

  4. Motivation • Better understanding of defect scattering • Improving semiconductor quality • Quantum wells are integral to high-speed transistors such as MODFETs, used in low noise devices: • Satellite receivers • Low power amplifiers • Cell phones • More efficient semiconductor lasers • Blue diode lasers employ InGaN quantum wells Semiconductor laser

  5. Band Gap • Available electron energies in materials form bands • Band gap is the gap in energy between valence band and conduction band • Forbidden region, no allowed energies in gap • In conductors, valence electrons are essentially free, represented by overlap in bands

  6. Quantum Wells Micro- twin ~ 16° InSb Quantum Well 9%AlInSb 10 nm TEM image showing a Micro-twin defect 15.8° Micro-twin • Formed when a thin layer of narrow band-gap InSb is sandwiched between wider band-gap AlInSb • Quantum well confines charges, wavefunctions become quantized • Electrons are confined to discrete energy levels • For lasers, more electrons are confined to energies above the lasing threshold • Leads to semiconductor lasers that require less current to operate • Micro-twin defects change quantum well geometry • Smaller well, higher energy confinement • Acts as potential barrier, scattering charges

  7. The Hall Effect B F v Hall current sensor • A magnetic field is applied to a conductor, perpendicular to current flow • Moving charge carriers experience a Lorentz force • Charges accumulate on one side of the conductor, equal but opposite charge left on other side • Separation of charges creates an electric potential, the Hall voltage • Hall effect has numerous applications: • Non-contact current sensors • Solid-state position and motion sensors • At low temperatures Hall conductivity becomes quantized, leads to a standard of resistance (h/e2 = 25812.8ohms)

  8. Single Defect Measurements 100μm • Use photolithography to create a Hall bar over an area containing defects • Electron-beam lithography used to isolate a single defect • Defects not isolated act as effective resistance • Apply magnetic field to induce Hall effect • Contact points allow voltage measurements before and after the defect • Current is plotted against voltage difference • Nonlinearities may be signs of scattering or tunneling I Hall bar optical image, ~1mm x 1.5mm

  9. Photolithography UV light Mask Resist Deposited Film Substrate Film Deposition Photoresist application Exposure Development Etching Resist removal • Parallel process • Sample coated with a photo-reactive resist • Mask is placed on sample and then exposed to UV light • Exposed resist reacts to UV • Developer removes unstable resist • Resolution limited by diffraction of light • Current commercial processes produce down to 45nm structures

  10. Electron Beam Lithography Electron beam Electron gun Anode Magnetic Lens Output Scanning coil Backscattered Electron Detector Secondary electron detector Stage Sample Standard SEM column • Electron beam instead of UV light • Smaller scale structures • Down to 25nm • SEM can perform electron beam lithography (EBL) • Electron beam is computer controlled • Serial process • Not suited for high volume production • Resolution limited by • Electron scattering in photoresist • Proximity effect • Acoustic noise • 100nm line widths possible on our Zeiss 960A

  11. Etching Trenches A Hall bar featuring EBL and RIE produced trenches • Process in which resist pattern is transferred to material surface • Wet etching • Chemical solution • Typically produces rounded isotropic profile • Etch can undercut resist layer • Dry etching • Sputtering – energetic ions bombard surface and remove material mechanically • Reactive-ion etching (RIE) – chemically reactive plasma and physical processes remove material • Produces anisotropic etch profile

  12. Hall Bar Top-Down Substrate and buffer layers Gate Applied magnetic field VH1+ VG1 VH2+ + + + + Defect e IS - - - - Etched trench VH1- VH2- VG2 Hall bar mesa • Hall bar defined with photolithography and RIE to produce mesa • Trenches defined with EBL and RIE to isolate defect • Gates allow scanning of charges across defect • Defect could be located anywhere in dashed box with extended trenches

  13. Hall Bar Cross-Section • Shown measurements are approximate • Defect may or may not be localized to a small area in the quantum well • Greater than 4.3μm etch needed to electrically isolate quantum well Gate Hall bar mesa Defect 180nm 30nm >4.3μm 4μm GaAs substrate InSb quantum well AlInSb barrier, InSb cap AlInSb barrier, AlInSb/AlSb buffer layers, SLS

  14. Hall Bar Photolithography 10μm • Produced a series of resolution tests to obtain a method for good photolithography results • Consisted of lines and grids • Procedure for aligning Hall bars on defects tedious but possible • Random placement not reliable • Resist thickness 2 to 2.2μm • Nominal value for S1818 – 1.8μm Optical zoom of photoresist on a quantum well InSb sample, two potentially usable features present

  15. Etching Trials • Etching trials performed on 3μm InSb bulk samples • Need a recipe that has at least 2:1 InSb:Resist etch ratio • Initially tried a 24 minute etch with BCl3 + Ar, 1.5μm etch depth • Next trial was 5 steps of 5 minute BCl3 + Ar, with 30 second Ar sputter phases in between, 1.4μm etch depth • Also tried 10 sets of BCl3 + Ar / Ar, 2.2μm etch depth 24 minutes BCl3 + Ar 55 minutes BCl3 + Ar / Ar

  16. Etching Analysis Cross-section back scatter SEM image of 2.2μm etch, white areas are InSb • Analysis of surface shows there is still InSb left to etch • Possible sources of etching slowdown are redeposition of etched products and formation of InCl on surface (high melting point) • Ar sputter phase added in an attempt to mechanically clean surface, but results were not satisfactory • Tried preheating RIE chamber to combat formation of Cl residues, but etch depth not greatly improved • 1.7μm for a 27.5 minute etch compared to 1.4μm

  17. Final Etch 10μm 6mm x 6mm • What worked – Alternating 5 steps 3 minutes BCl3 + Ar / 5 steps 15 seconds BCl3 + SF6 with higher powers and higher flow rate, 5μm etch depth

  18. Ohmic Contacts 100μm • Contacts need to be modified to ensure good electrical conduction, linear I-V behavior • Hall bars coated with resist • Contact pads exposed, developed • Indium deposited onto sample, resist removed • Sample annealed at 230°C for 5 minutes • Causes indium to diffuse down to quantum well • In melts at 156.6°C • Measurements on several Hall bars using a curve tracer showed linear I-V behavior • 9-11kOhm resistance between contact pads • Infinite resistance between substrate and contact pads Deposited indium After annealing

  19. Current Issues 50μm • Over half the devices damaged sometime between contact pad photolithography and annealing • In most cases, current can be applied through other pathways • Measurements with an optical microscope show the break depth to be about 4 to 5μm • GaAs/AlSb interface around 4.3μm High defect density at layer interfaces in InSb quantum well sample Broken contacts

  20. The Future • Find out what’s causing terminals to break off, possibilities: • Crushed during contact pad photolithography • Moving around due to loose storage • Ultrasonic cleaning • Aligning and performing EBL without damaging sample • Gates introduce effective resistance, electric potential narrows conduction path

  21. Sources • Images: • http://www.memsnet.org/mems/processes/wetetch.jpg • http://en.wikipedia.org/wiki/Hall_effect • http://cnx.org/content/m1037/latest/5.15.png • http://curie.umd.umich.edu/Phys/classes/p150/archive/goodfor/SpinFlip.htm • http://en.wikipedia.org/wiki/File:Bandgap_in_semiconductor.svg • http://www.hitequest.com/Kiss/photolithography.gif • Articles/Presentations: • “TEM Study of InSb/AlInSb Quantum Wells Grown on GaAs (001) Substrates” • http://en.wikipedia.org/wiki/Semiconductor_laser • http://en.wikipedia.org/wiki/2DEG • http://en.wikipedia.org/wiki/Electron_beam_lithography • http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html • Kittel, Charles. Introduction to Solid State Physics

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