Transcript Demonstration PowerPoint - The University of Oklahoma

Nano Hall Bars
the quest for single defect scattering
Daniel Brunski
2008 Fall
Advisors:
Dr. Matthew Johnson
Dr. Joel Keay
Special Thanks:
Ruwan Dedigama
Introduction
Motivation
Background
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Outline
Band Gap
Quantum Wells
The Hall Effect
Single Defect Measurements
Microfabrication Techniques
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Photolithography
Electron Beam Lithography
Etching
Hall Bar Plan
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Top-Down View
Cross-sectional View
Progress to Date
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Photolithography
Reactive-Ion Etching
Ohmic Contacts
Current Issues
The Future
10μm
Scanning electron microscope (SEM)
image showing several defects (circled)
near a device
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)
QW
Defect
9%AlInSb
InSb Quantum Well
GaSb 9%AlInSb
AlSb
9%AlInSb
1μm
50nm
GaAs
Cross-section transmission electron microscope (TEM) images of
InSb/AlInSb on GaAs substrate
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:
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Satellite receivers
Low power amplifiers
Cell phones
More efficient semiconductor
lasers
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Blue diode lasers employ InGaN
quantum wells
Semiconductor laser
Band Gap
Available electron energies in materials form bands
Band gap is the gap in energy between valence band and conduction band
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Forbidden region, no allowed energies in gap
In conductors, valence electrons are essentially free, represented by
overlap in bands
Quantum Wells
Formed when a thin layer of
narrow band-gap InSb is
sandwiched between wider bandgap AlInSb
Quantum well confines charges,
wavefunctions become quantized
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Electrons are confined to discrete
energy levels
For lasers, more electrons are
confined to energies above the
lasing threshold
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Leads to semiconductor lasers
that require less current to operate
Micro-twin defects change
quantum well geometry
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Smaller well, higher energy
confinement
Acts as potential barrier,
scattering charges
Microtwin
~ 16°
InSb
Quantum Well
9%AlInSb
10 nm
TEM image showing a Micro-twin defect
15.8°
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:
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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)
B
F
v
Fe  q(E  v  B)
Hall current sensor
Single Defect Measurements
Use photolithography to create a Hall bar
over an area containing defects
Electron-beam lithography used to
isolate a single defect
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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
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Nonlinearities may be signs of scattering
or tunneling
I
100μm
Hall bar optical image,
~1mm x 1.5mm
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
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Current commercial processes produce down to 45nm structures
UV light
Mask
Resist
Deposited Film
Substrate
Film Deposition
Photoresist
application
Development
Etching
Exposure
Resist removal
Electron Beam Lithography
Electron beam instead of UV light
Smaller scale structures
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Electron
beam
Down to 25nm
SEM can perform electron beam
lithography (EBL)
Electron beam is computer controlled
Serial process
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Anode
Magnetic
Lens
Not suited for high volume production
Resolution limited by
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Electron gun
Electron scattering in photoresist
Proximity effect
Acoustic noise
100nm line widths possible on our
Zeiss 960A
Output
Scanning
coil
Backscattered
Electron
Detector
Stage
Secondary
electron
detector
Sample
Standard SEM column
Etching
Process in which resist pattern is
transferred to material surface
Wet etching
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Chemical solution
Typically produces rounded
isotropic profile
Etch can undercut resist layer
Dry etching
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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
Trenches
A Hall bar featuring EBL and RIE
produced trenches
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
Substrate and
buffer layers
+
VH1
+
+
+
Gate
VG1
VH2
+
Applied magnetic field
+
Defect
e
-
-
Hall bar mesa
-
VH1
IS
-
-
Etched trench
VG2
VH2-
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
180nm
Hall bar mesa
30nm
4μm
GaAs substrate
AlInSb barrier,
AlInSb/AlSb buffer
layers, SLS
Defect
>4.3μm
InSb quantum well
AlInSb barrier,
InSb cap
Hall Bar Photolithography
Produced a series of resolution tests to
obtain a method for good
photolithography results
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Consisted of lines and grids
Procedure for aligning Hall bars on
defects tedious but possible
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Random placement not reliable
Resist thickness 2 to 2.2μm
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Nominal value for S1818 – 1.8μm
10μm
Optical zoom of photoresist on a
quantum well InSb sample, two
potentially usable features
present
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
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
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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
Final Etch
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
6mm x 6mm
10μm
Ohmic Contacts
Contacts need to be modified to ensure
good electrical conduction, linear I-V
behavior
Hall bars coated with resist
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Contact pads exposed, developed
Deposited
indium
Indium deposited onto sample, resist
removed
Sample annealed at 230°C for 5 minutes
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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
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9-11kOhm resistance between contact
pads
Infinite resistance between substrate and
contact pads
After
annealing
100μm
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
High defect density at
layer interfaces in InSb
quantum well sample
Broken contacts
50μm
The Future
Find out what’s causing terminals to break off, possibilities:
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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
Sources
Images:
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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:
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“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