Specific Contact Resistance of GaSb Ohmic Contacts Daniel ...

Specific Contact Resistance of GaSb Ohmic Contacts
Daniel Herrera
Graduate Mentor: Nassim Rahimi
Faculty Mentor: Dr. Luke Lester

Activities
Introduction
My name is Daniel Herrera, and I’m a junior studying electrical engineering. After being
accepted into the CHTM REU program, I was placed into Dr. Luke Lester’s research group,
which focuses on the research on solar cells and other optoelectronic devices. I was to follow
instructions given by my graduate mentor, Nassim Rahimi and otherwise assist the group in any
other way. My work was focused on the research of creating high-efficiency gallium antimonide
(GaSb) ohmic contact structures. The vast majority of my work was done in the cleanroom at
CHTM, where I processed and tested the ohmic contacts.
Background
I am currently a junior studying electrical engineering, therefore my background prepared
me fairly well for this program. During the fall semester, I was enrolled in ECE 371, which is a
materials and devices course for electrical engineers. While enrolled in that class, I learned about
semiconductor physics while simultaneously doing research at CHTM. I feel that I would have
been better prepared had I taken the course before participating in this program, but the program
also accelerated my learning and understanding of the material. As a result of my work at
CHTM, I have decided to build upon what I’ve learned by focusing in the optoelectronics track
in the ECE program at UNM.
Research Objective
The objective of my research is to test the specific contact resistance of GaSb and
compare the results to that of gallium arsenide (GaAs). An ohmic contact is a non-rectifying
junction between a series of metals and a semiconductor. Ohmic contacts are used when current
needs to be transferred from one semiconductor to another in many electronic devices. The
current-voltage (IV) characteristics of an ohmic contact are linear, as opposed to rectifying
metal-semiconductor barriers (Schottky barriers), which don’t conduct any current until a
threshold voltage has been reached.
To create an ohmic contact, a metal structure is joined to a semiconductor that has a
similar, low energy band-gap. The junction of these two materials creates an energy barrier that
stops the flow of electrons. To minimize the energy barrier, a low band-gap material is usually
placed between the metal and semiconductor. This is usually another type of semiconductor with
a higher doping concentration. Below are the band diagrams for n and p doped ohmic contacts.

n-doped Ohmic contact
p-doped ohmic contact
As shown above in the figures, there is a small energy barrier between the conduction
bands of the metal and semiconductor. The goal of ohmic contact research is to minimize that
barrier, so that electrons can quantum mechanically tunnel easily through to the other side. The
main reason my research was done on GaSb is because very little research has been done on it,
while plenty of research and manufacturing has already been done on gallium arsenide (GaAs).
Many different methods have already proven to be successful for GaAs, while GaSb has the
potential to be a better ohmic contact material, since it has a lower energy band-gap.

The main characterizations of high quality ohmic contacts are cleanliness, morphology,
thermal stability, and contact resistance. For an ohmic contact to have high quality IV traces, the
surface must be free of any dirt or scum that can either impede current transfer or cause a short
circuit. The sample also must be completely smooth and level, so that the current is more easily
isolated into one path. The ohmic contact should not degrade at higher temperatures or react with
oxygen. Finally, lower contact resistances are vital, with the target contact resistance being close
to 5x10 -6 Ω·cm 2 or lower.
Methodology
Being that I was to fabricate ohmic contacts on semiconductor materials, 100 percent of
my work was done within the cleanroom at CHTM. Before any fabrication was to be done, I was
to first be trained to gain access into the cleanroom. This included watching several safety
videos, followed by a comprehensive test on the material. After gaining access to the cleanroom,
I needed to be trained and tested on the equipment that I would be using.
The major parts of the fabrication to be done were photolithography, inductively couple
plasma etching (ICP), and metallization. Photolithography is the process of creating a nano-scale
pattern on a semiconductor wafer by using photoresist, which has special characteristics when
exposed to ultra-violet light. In my research, I used AZ-5214-E_IR photoresist, which becomes
soluble in a developer solution after being exposed to UV light. To utilize the photoresist, I
exposed the samples using specific patterned masks which only cover some parts of the sample.
After exposure and development, a photoresist pattern on the sample will match the pattern on
the mask.
The process for photolithography on a GaAs sample is listed below:
1. Soak the substrate for 5 minutes each in acetone, methanol, and isopropyl alcohol
(IPA) before rinsing with deionized (DI) water and blowing dry with nitrogen.
2. Remove the native oxide on the sample by soaking in NH 4 OH for 30 seconds,
then blow dry with nitrogen.
3. Bake the sample for 10 minutes at 150°C
4. Spin hexamethyldisilazane (HMDS) onto the sample at 4000 rpm for 30 seconds
5. Bake the sample for 3 minutes at 150°C after spinning on the HMDS.
6. Spin AZ-5214-E_IR photoresist onto the sample at 4000 rpm for 30 seconds.
7. Soft bake the sample at 90°C for 2 minutes.
8. Place sample onto Karl Suss Mask Aligner and adjust height of stage until the
sample comes into contact with the mask being used (mesa etching or metal
deposition)
9. Expose the sample to 405nm wavelength ultraviolet light for 2 seconds using the
mask aligner.

10. Bake the sample at 112°C for 1 minute.
11. Expose the sample to 205 nm wavelength light again for 30 seconds, but without
using a mask on the aligner (flood exposure)
12. Develop the sample by soaking in a 1:4 ratio of AZ400K developer: DI water
solution for 20 seconds.
13. Remove sample from solution and immediately blow dry with nitrogen gun.
14. After inspecting the pattern definition on the sample, clean sample with the
oxygen reactive ion etcher (RIE) for 1 minute with a pressure of 90 mTorr and a
power of 50 W.
There were a few necessary adjustments in the above process for GaSb. Rather than using
NH 4 OH to remove the native oxide, I used a 1:3 ratio of HCl and DI water for 30 seconds. Also,
the sample’s contact with water had to be limited, so instead of soaking the sample in each
solvent for 5 minutes, I simply washed it with each solvent and rinsed with DI water before
immediately blow drying the sample.
After photolithography, ICP etching was done on the samples. This process etched into
the substrate only where the photoresist pattern did not exist. Following the etching, I rinsed the
excess photoresist off of the sample, which left a pattern of mesa-like structures on the surface.
Then, the same photolithography process was done, but using a mask specific for the metal
structures of the contacts. The pattern of these structures was aligned so that the ohmic contacts
were within the mesa structures. After photolithography, the oxide on top the sample was
removed again before evaporating the metal on the sample. Below is a picture of a sample after
mesa photolithography and ICP etching.

To perform metal evaporation, the sample is first placed into a chamber which is pumped
down to about 2 x 10 -6 Torr or lower. Then, a high-intensity electron beam steered by magnetic
coils is projected down onto a metal source. Once the temperature of the source rises enough,
metal particles then begin evaporate and rise up and stick onto the sample, which is directly
above the source. The metal sequence most commonly done for the GaSb samples was
Ge/Au/Ni/Ti/Au. Below is a photo taken from the microscope of the TLM (Transmission Line
Method) pattern on the sample. The distance between two consecutive contacts increases from
10 microns to 70 microns.
After metal deposition, the sample would go through an annealing process, which quickly
rises the temperature of the sample, before cooling down. This process alters many
characteristics of the sample, including ductility, hardness, and internal stresses. Each sample
was annealed at a different temperature to observe the changes in specific contact resistance, or
not annealed at all.
Finally, to measure the specific contact resistance of the ohmic contacts, a 2-probe IV
(current-voltage) curve tracer was used. The two probes were placed on two consecutive ohmic
contacts. Then, the machine applies a voltage across the two probes. The curve tracer measures
the current passing from one probe to the other and plots the current as a function of voltage.
This process was repeated several times for each distance between two contacts. The following
figure gives the IV curve for one distance.

-1
-0.910000026
-0.819999993
-0.730000019
-0.639999986
-0.550000012
-0.460000008
-0.370000005
-0.280000001
-0.189999998
-0.100000002
-0.01
0.079999998
0.170000002
0.259999991
0.349999994
0.439999998
0.529999971
0.620000005
0.709999979
0.800000012
0.889999986
0.980000019
0.0015
n-GaSb 40 μm
0.001
0.0005
0
Current
-0.0005
-0.001
-0.0015
Voltage
As shown in the figure, the contact displays a linear or ohmic relationship. After
repeating for all distances, the resistance is found by Ohm’s Law and plotted as a function of
distance. After the resistance is plotted, the transfer length L T and the transfer resistance R T can
be found by solving for the intercepts of that line, as shown below.

Using the transfer resistance and transfer length, the specific contact resistance can be
bound by using the following equation,
where Z is the width of the channel, R C is the contact resistance, and L T is the transfer length.
The total resistance of two ohmic contacts is shown in the following figure. There is a contact
resistance from each ohmic contact, along with a sheet resistance coming from the substrate. The
mesa etching better isolates the current so that it travels laterally from one contact to the other,
rather than spreading throughout the substrate.
Description of Experiments
To start my research, I processed a GaAs sample first, both to become familiar with the
process of making ohmic contacts, and to get a reference contact resistance for the GaSb samples
I would be processing later. After the GaAs, I started processing the GaSb samples, with
variations on each sample. I varied the annealing temperature or the metallization for each
sample to find the ideal temperature. Not all GaSb were successful; I encountered several issues
with processing.
The biggest issue was being able to fully clean the GaSb samples. This happened because
DI water actually etches into the polished surface a GaSb sample. Since cleaning the samples

equired using DI water, some samples became severely scratched, and therefore unreliable for
any further research. The following picture shows a GaSb sample after an attempted cleaning.
Annealing the GaSb samples also proved to be difficult. Upon annealing a GaSb, Ge/Au/Ni/Au
sample at 350°, the gold on top of the metal structure actually diffused down through the other
metals, leaving a blistered, unusable contact, as shown in the following photos.

It was discovered that this happened because the NH 4 OH did not successfully remove the oxide
before metallization. To prevent this problem, I began removing the oxide with an HCl solution
instead. Also after varying the temperature, 300°C was found to produce optimal results, while
260°C gave a very high resistance.

Findings
Results
The following results are for three different samples: a GaAs contact (Ge/Au/Ni/Au) for
reference, a GaSb contact (Pd/Ge/Au/Pt/Au) annealed at 300°C, and a GaSb contact
(Ge/Au/Ni/Pt/Au) that was not annealed. A GaSb substrate annealed at 260°C was also
measured, but the results proved to be far too unreliable. Another process would need to be done
to find a more valid set of results.
n-GaAs Substrate
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
0 20 40 60 80
y = 0.3768x + 3.2857
Resistance
Linear
(Resistance)
ρ c =0.74493 x 10 -6 Ω·cm 2 R T = 1.64285 Ω L T = 4.36 µm
n-GaSb Substrate
4
3.5
3
2.5
2
1.5
1
0.5
0
0 20 40 60 80
y = 0.0263x + 1.4979
Resistance
Linear
(Resistance)
Linear
(Resistance)
ρ c =2.1324 x 10 -6 Ω·cm 2 R T = 0.7490 Ω L T = 28.4772 µm

7
6
5
4
3
2
1
0
n-GaSb, 300°C Anneal
0 20 40 60 80
y = 0.0684x + 1.6783
Resistance
Linear
(Resistance)
Linear
(Resistance)
ρ c =10.2900 x 10 -6 Ω·cm 2 R T = 0.8392 Ω L T = 12.2683 µm
Conclusions
As shown in my results, the specific contact resistance of GaSb ohmic contacts is
comparable with that of GaAs contacts, but none of the resistances were lower. With further
research in this area, a higher quality ohmic contact can still be found. More reliable results
could have also been found by using a 4-probe curve tracer rather than a 2-probe tracer. Also, the
results could have been affected by the cleanliness sample, along with the success of the
photolithography and metallization processes.
Future Work
Many different approaches can be taken to improve upon this research. One method is to
use different metallization. Another approach is to change the doping concentrations of the GaSb
wafer, so that it is n-doped GaSb on top of p-doped GaSb, or vice-versa. Also, another type of
small band-gap semiconductor material can be grown on top of the n-doped GaSb to provide
another layer of diffusion for a current to pass through. One last adjustment that could be done is
using a mesa mask with narrower patterns. This would further limit the current to lateral
movement, which would minimize the error involved in testing the samples.
My work at CHTM has given me very valuable experience for my future career.
Immediately after the spring semester is over, I will be doing a summer co-op for Toyota
Technical Center. I will be a part of their Materials Research Department at their headquarters in
Ann Arbor, Michigan. While there, I will be continuing similar research on semiconductor
materials that I started in the REU program. In fact, the research experience with CHTM on my
resume is what led them to offer me the co-op. I am very grateful for the opportunity to
participate in this REU program. I thank my faculty mentor, Luke Lester, my graduate mentor,
Nassim Rahimi, and the program coordinator, Linda Bugge for giving me this opportunity.