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

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

3. Introduction 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) 50nm 9%AlInSb InSb Quantum Well Defect 1μm GaAs GaSb AlSb 9%AlInSb 9%AlInSbQW 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 Satellite receivers Low power amplifiers Low power amplifiers Cell phones Cell phones More efficient semiconductor lasers Blue diode lasers employ InGaN quantum wells 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 Forbidden region, no allowed energies in gap In conductors, valence electrons are essentially free, represented by overlap in bands

6. Quantum Wells 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 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 Leads to semiconductor lasers that require less current to operate Micro-twin defects change quantum well geometry Smaller well, higher energy confinement Smaller well, higher energy confinement Acts as potential barrier, scattering charges Acts as potential barrier, scattering charges 15.8° Micro-twin InSb Quantum Well 9%AlInSb Micro- twin twin ~ 16° 10 nm TEM image showing a Micro-twin defect

7. The Hall Effect 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 Non-contact current sensors Solid-state position and motion sensors Solid-state position and motion sensors At low temperatures Hall conductivity becomes quantized, leads to a standard of resistance (h/e 2 = 25812.8ohms) At low temperatures Hall conductivity becomes quantized, leads to a standard of resistance (h/e 2 = 25812.8ohms) v BF Hall current sensor

8. Single Defect Measurements 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 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 Nonlinearities may be signs of scattering or tunneling Hall bar optical image, ~1mm x 1.5mm 100μm I

9. Photolithography 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 Current commercial processes produce down to 45nm structures Deposited Film Substrate Film Deposition Resist Photoresist application Exposure Mask UV light DevelopmentEtchingResist removal

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

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

12. Hall Bar Top-Down 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 Applied magnetic field e V G1 V G1 V G2 V G2 V H2 + V H2 + V H2 - V H2 - Hall bar mesa ISISISIS Defect Gate ---- ++++ Substrate and buffer layers V H1 + V H1 + V H1 - V H1 - Etched trench

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 InSb quantum well AlInSb barrier, InSb cap Gate GaAs substrate Hall bar mesa 30nm 180nm 4μm >4.3μm Defect AlInSb barrier, AlInSb/AlSb buffer layers, SLS

14. Hall Bar Photolithography Produced a series of resolution tests to obtain a method for good photolithography results Consisted of lines and grids Consisted of lines and grids Procedure for aligning Hall bars on defects tedious but possible Random placement not reliable Random placement not reliable Resist thickness 2 to 2.2μm Nominal value for S1818 – 1.8μm Nominal value for S1818 – 1.8μm 10μm 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 BCl 3 + Ar, 1.5μm etch depth Next trial was 5 steps of 5 minute BCl 3 + Ar, with 30 second Ar sputter phases in between, 1.4μm etch depth Also tried 10 sets of BCl 3 + Ar / Ar, 2.2μm etch depth 24 minutes BCl 3 + Ar 55 minutes BCl 3 + Ar / Ar

16. Etching Analysis 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 1.7μm for a 27.5 minute etch compared to 1.4μm Cross-section back scatter SEM image of 2.2μm etch, white areas are InSb

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

18. Ohmic Contacts Contacts need to be modified to ensure good electrical conduction, linear I-V behavior Hall bars coated with resist Contact pads exposed, developed 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 Causes indium to diffuse down to quantum well In melts at 156.6°C 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 9-11kOhm resistance between contact pads Infinite resistance between substrate and contact pads Infinite resistance between substrate and contact pads 100μm Deposited indium After annealing

19. Current Issues 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 50μm Broken contacts High defect density at layer interfaces in InSb quantum well sample

20. The Future Find out what’s causing terminals to break off, possibilities: Crushed during contact pad photolithography Crushed during contact pad photolithography Moving around due to loose storage Moving around due to loose storage Ultrasonic cleaning 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://www.memsnet.org/mems/processes/wetetch.jpg http://www.memsnet.org/mems/processes/wetetch.jpg http://en.wikipedia.org/wiki/Hall_effect http://en.wikipedia.org/wiki/Hall_effect http://en.wikipedia.org/wiki/Hall_effect http://cnx.org/content/m1037/latest/5.15.png http://cnx.org/content/m1037/latest/5.15.png http://cnx.org/content/m1037/latest/5.15.png http://curie.umd.umich.edu/Phys/classes/p150/archive/goodfor/SpinFlip.htm http://curie.umd.umich.edu/Phys/classes/p150/archive/goodfor/SpinFlip.htm http://curie.umd.umich.edu/Phys/classes/p150/archive/goodfor/SpinFlip.htm http://en.wikipedia.org/wiki/File:Bandgap_in_semiconductor.svg http://en.wikipedia.org/wiki/File:Bandgap_in_semiconductor.svg http://en.wikipedia.org/wiki/File:Bandgap_in_semiconductor.svg http://www.hitequest.com/Kiss/photolithography.gif http://www.hitequest.com/Kiss/photolithography.gif http://www.hitequest.com/Kiss/photolithography.gif 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_laser http://en.wikipedia.org/wiki/Semiconductor_laser http://en.wikipedia.org/wiki/Semiconductor_laser http://en.wikipedia.org/wiki/2DEG http://en.wikipedia.org/wiki/2DEG http://en.wikipedia.org/wiki/2DEG http://en.wikipedia.org/wiki/Electron_beam_lithography http://en.wikipedia.org/wiki/Electron_beam_lithography http://en.wikipedia.org/wiki/Electron_beam_lithography http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/band.html Kittel, Charles. Introduction to Solid State Physics Kittel, Charles. Introduction to Solid State Physics