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·cm2 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 NH4OH 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

NH4OH 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.

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 LT and the transfer resistance RT can

be found by solving for the intercepts of that line, as shown below.

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

-1

-0.9

10

00

00

26

-0.8

19

99

99

93

-0.7

30

00

00

19

-0.6

39

99

99

86

-0.5

50

00

00

12

-0.4

60

00

00

08

-0.3

70

00

00

05

-0.2

80

00

00

01

-0.1

89

99

99

98

-0.1

00

00

00

02

-0.0

1

0.0

79

99

99

98

0.1

70

00

00

02

0.2

59

99

99

91

0.3

49

99

99

94

0.4

39

99

99

98

0.5

29

99

99

71

0.6

20

00

00

05

0.7

09

99

99

79

0.8

00

00

00

12

0.8

89

99

99

86

0.9

80

00

00

19

Voltage

n-GaSb 40 μm

Current

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, RC is the contact resistance, and LT 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

required 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 NH4OH 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.

ρc =0.74493 x 10-6

Ω·cm2 RT = 1.64285 Ω LT = 4.36 µm

ρc =2.1324 x 10-6

Ω·cm2 RT = 0.7490 Ω LT = 28.4772 µm

y = 0.3768x + 3.2857

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0 20 40 60 80

n-GaAs Substrate

Resistance

Linear(Resistance)

y = 0.0263x + 1.4979

0

0.5

1

1.5

2

2.5

3

3.5

4

0 20 40 60 80

n-GaSb Substrate

Resistance

Linear(Resistance)

Linear(Resistance)

ρc =10.2900 x 10-6

Ω·cm2 RT = 0.8392 Ω LT = 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.

y = 0.0684x + 1.6783

0

1

2

3

4

5

6

7

0 20 40 60 80

n-GaSb, 300°C Anneal

Resistance

Linear(Resistance)

Linear(Resistance)