ACTIVITIES

GASB FOR USE IN TPV’S AND HIGH EFFICIENCY HETEROJUNCTION SOLAR CELLS

REU Student: Matthew Erdman

Graduate Student Mentor: Andrew Aragon

Faculty Mentor: Luke Lester and Sayan Mukherjee

A. Introduction

PV and applications

Intro into TPV and applications

The first use of light to create electricity was first demonstrated in 1839 by Edmund

Becquerel when he demonstrated that a galvanic cell can produce more electricity when it is

exposed to light. The mechanics behind this demonstration of light’s interaction with a metal was

not correctly described until Albert Einstein described the photoelectric effect in 1904. Einstein’s

description of the photoelectric effect states that light has quantized energy that is correlated to

its wavelength. Therefore, certain wavelengths of light have enough energy to excite electrons in

a material to contribute a current [1]. The interactions between light and different materials, such

as semiconductors, could correctly be explained and studied once Einstein was able to properly

explain the wave-particle duality of light that has quantized energy. This revolution led to the

development of many devices such as light emitting diodes and photovoltaic (PV) solar cells.

Research on PV cells has led to the use of different materials and structures for solar cells

in order to better increase the overall efficiency and decrease the cost of manufacturing. This has

led to the production of different types of solar cells such as monocrystalline, polycrystalline,

thin film, and multi-junction solar cells with and without the use of Silicon as their substrate

material. Monocrystalline PV cells are characterized as a PV cell made out of one material, with

the material grown as one continuous, uniform crystal. Polycrystalline PV cells are characterized

as a PV cell made of one material, but with a substrate that has grain boundaries. These grain

boundaries are areas of non-continuous uniformity within the crystal. This makes the material

less efficient for devices, but also less expensive to manufacture. Thin film material is typically

made with amorphous material, which has no inherent uniformities in the crystal. This makes it a

very poor material for the use in devices, but it has a very low manufacturing cost. Lastly,

heterojunction or multijunction solar cells are made by combining two or more different

materials to greatly increase the efficiency of a solar cell by effectively widening the spectrum of

light that is absorbed by the cell. Even though this creates the most efficient solar cell, this is the

most expensive to create [1, 4].

B. Background

I. Thermal Photovoltaic Device

A thermal Photovoltaic device or TPV is simply a photovoltaic device that is most sensitive

to light in the infrared spectrum. This corresponds to wavelengths between 700nm and 1mm.

Therefore, TPVs are most sensitive to wavelengths of light that are created by thermal radiation.

One particular material of interest for TPV’s is Gallium Antimonide (GaSb). The bandgap

energy of GaSb (0.726eV) corresponds to an optimal excitation wavelength of approximately

1.7nm from the equation below [2].

( )

( )

The relatively narrow bandgap of GaSb makes it a great candidate to make rudimentary TPV

cells for radiation collection from large heat producers such as power plants and other sources of

extreme heat [3].

II. GaSb

Gallium Antimonide (GaSb) has been used primarily in infrared detectors due to its small

band gap of 0.726eV. This band gap correlates to a peak sensitivity of light wavelengths around

1700nm. This allows for a great application to infrared detectors used in infrared cameras and

other similar detectors. This also allows for a great application to TPV’s. GaSb can be used in a

monocrystalline, single junction TPV which can be used to harvest thermal radiation from steam

stacks from power plants or other large sources of infrared radiation. GaSb also has the potential

to be used in a very efficient solar cell by making a heterojunction solar cell with GaAs or

possibly Ge. If a heterojunction solar cell can be efficiently made with GaSb and GaAs or Ge, a

very large portion of the solar spectrum can be absorbed. However, more information about the

characteristics of the materials needs to be studied in order to make high efficiency devices [3].

High quality contacts on GaSb must be made in order to make high quality heterojunction

solar cells and TPV’s. One of the main concepts in order to understand the formation of ohmic

contacts on GaSb and to understand the most prominent defects that occur during substrate and

device growth is the mobility.

Mobility is a parameter that describes the linear relationship between carrier velocity in a

semiconductor and an applied electric field of relatively low magnitude. The ideal way to

characterize the mobility of an electron is to sum up all the forces applied on it and to average

those forces over time. These are due to forces from all other free electrons, forces from all the

atoms in the material, and any external forces that are felt by each free electron. It is extremely

unrealistic to count up all of these forces on one electron let alone all the free electrons in the

material because there are over 1022

atoms within one square centimeter of GaSb. Also,

depending on the doping there can be well over 1017

free electrons within a square centimeter.

Therefore, it is much more realistic to approximate the sum of all of the forces felt by a free

electron by saying the mass of the electron is altered when it moves through a material. If this

description is applied, mobility can be explained by how easy or hard a free electron or hole can

move through the material [4]. One factor that can greatly affect the mobility is the presence of

defects due to the fact that they add recombination sites.

Defects in or on a substrate greatly affect the devices that are grown on that material. Though

there are several different types of defects, one of the more relevant defects that arose during the

growth of GaSb observed during the writing of this paper was oval defects. A theory of what

causes oval defects is spitting of a group III element, which is thought to be caused by either a

shutter or effusion cell being dirty in the MBE machine (see section B.III and Figure 2 for more

information on MBE growth). This contamination or dirty component is thought to cause a

droplet of a group III material to be implanted on the substrate in a ‘spitting’ pattern. This greatly

affects devices that are grown on a substrate because it can act as a recombination site or it can

cause shorts within the device [5].

Figure 1 illustrates the theory of how ‘spitting’ of a group III element will propagate as a defect

on the sample. In the case of GaSb this droplet will be Gallium (Ga). The droplet of Ga will form

on the substrate and cause a dissolution pit. The pit will form around the droplet causing the

defect to further propagate due to the different growth rate of the dissolution pit from the rest of

the material [5]. This propagation can create a large-area device killing defect making an entire

solar cell or TPV useless.

III. Importance of Ohmic Contacts

An operable definition of an ideal ohmic contact is one that freely injects electrons in either

direction across its boundary with a semiconductor. Therefore, an ohmic contact introduces

electrons or holes as they are needed in an electrical system [4]. For PV cells this is especially

important because all the current created by the electron hole pairs must be collected. Therefore,

a large portion of current will be lost if a solar cell is made with poor contacts.

Studying ohmic contacts is an essential step in the process when developing new devices

with materials that are not well known. Ohmic contacts need to be optimized not only for each

individual material, but also for both n-doped material and p-doped material. This means that

there needs to be metallization studies done to optimize the metal to semiconductor interface.

The specific resistance of the different metal contacts must be measured using the transmission

line method (TLM) in order to determine the quality of the ohmic contact.

IV. Growth of GaSb by MBE

Molecular beam epitaxy crystal growth (MBE) is a procedure used to grow material very

precisely by adding one monolayer of material on a substrate at a time. This is done by exposing

the substrate to a molecular beam. See Figure 2 for a simplified diagram of an MBE machine.

Figure 1. Diagram of oval defects [5]

As seen by Figure 2, the shutters can open and expose the substrate to the element of desired

growth. This is usually done with two materials at the same time because MBE growth is most

relevant in growth of III-V materials. These materials are made from two elements. One element

is used from column III and one element from column V of the periodic table. This procedure

can also be used to develop more advanced materials such as II-VI and III-Nitride materials.

MBE growth can be applied to pure materials such as silicon, but this procedure is typically not

applied to silicon due to high cost [5].

C. Research Objective

Characterization of GaSb

The main research objective was to study and collect data that will help characterize GaSb

and metal contacts grown on GaSb samples. The first step was to gain a deep understanding on

the physics of the Hall and TLM measurements. Late in the Spring Semester, a defect study

started within the research group. This change in events made it relevant to add another research

objective which was to gain knowledge on the Earth & Planetary Sciences Department’s

Scanning Electron Microscope at the University of New Mexico (SEM). A secondary research

objective during the REU was to gain knowledge on the physics of MBE growth of GaSb on SI-

GaAs substrates.

Hall and TLM measurements were conducted on samples from different MBE growths of

differently doped GaSb growths on SI-GaAs substrates once a good understanding was

developed on the Hall and TLM setup and theory. The gathering of TLM data was the main

focus between the TLM and Hall data due to the timing of the research.

Late in the Spring Semester during the study on Hall and TLM setups and measurements, it

was concluded that the defects on the substrates were hindering further development on the TPV

project. This led to an introduction of a defect study. Due to involvement on the project, data was

gathered during sessions using Earth & Planetary Sciences’ SEM. High quality photos were

taken with the SEM and the compositions of defects were observed.

Figure 2. Molecular Beam Epitaxy diagram [6]

D. Methodology

I. Characterization of GaSb: Hall Measurements

Hall measurements are used to help characterize a material by calculating the carrier type,

carrier concentration, and the mobility of the material. The basics of the Hall measurement can

be explained by applying the Lorentz Force to semiconductors. The Lorentz Force is described

by the following equation:

Were F is the force on the particle due to the magnetic field B, q is the charge of the particle, v is

the velocity of the particle. This relationship can be represented by using the Right Hand Rule

which is shown in Figure 3 below [8, 9].

By applying the concepts of the Lorentz Force, one can easily quantify what occurs when a

semiconductor has a current passing through it and is introduced into a magnetic field. As seen in

Figure 4, a force is applied on the current carrier (in this case an electron, e-). This will build up

an excess charge density that is perpendicular to both the direction of the magnetic field and

perpendicular to the direction of charge flow. This buildup of charge can be measured and is

labeled as the Hall Voltage, or UHall seen in Figure 4. The polarity of the Hall Voltage specifies if

the majority charge carriers are holes or electrons. This determines if the semiconductor is a P-

type or N-type sample. This is a very standard measurement to help study the effects of different

doping procedures such as ion implantation.

The Hall measurement system is designed to take several measurements to correctly

gather all of the necessary information to determine the Hall mobility and the sheet carrier

density of the sample. The sample has a square geometry with four contacts at each of the

corners. Current is applied to one side of the sample, and voltage is measured on the opposite

side of the sample. This geometry can be applied again to the remaining two sides in order to

calculate the two separate resistance values. See figure 5 for the layout [8, 9].

Figure 3. Right Hand Rule [7]

Figure 4. Hall Measurement Schematic [8]

The sheet resistance Rs can be found analytically with the relationship [8, 9]:

(

) (

)

This leads to the bulk resistance, , where is the thickness of the sample. Next, the

voltage and current is measured diagonally across the sample while a magnetic field is passed

perpendicular to the top of the sample as shown in Figure 4. The Hall voltage can be measured

once this is performed.

Using the above information gained from the Hall measurement, the Hall mobility (µ) and the

carrier sheet density, ns can be calculated using the following equation:

| | ⁄ ⁄

Figure 5. Resistance measurements taken during Hall measurements [9]

Figure 6. Measurement of the Hall voltage VH [9]

Hall measurements are a standard measurement taken on a sample of material that is not well

known or to confirm values on a certain material due to the amount of information gained from

one simple set up. [9].

II. Characterization of GaSb: TLM Measurements

Transmission Line Method (TLM) measurements are used to gain knowledge on resistance

of contacts on a device. The TLM makes a very accurate estimate of the contact resistance

contribution. This is done by making a correlation of the layout of the basic metal contacts to a

semiconductor by applying the model of a transmission line to the sample. The layout in figure 1

can explain the contact resistance and the sheet resistance of the sample if the resistance of a volt

meter to be approximated to be very large ( ) [11, 12, 13, 14]. This is a very similar

model of a transmission line hence the name of the measurement.

Using four probes with finite resistances represented as Rp1, Rp2, Rp3, and Rp4 in Figure 7, one

can see the correlation between the resistance from the probe (Rp), the connection between the

probe and contact (Rcp), the contact (Rc), and the sheet resistance of the sample (Rs). A simple

equation can be formulated to find both the sheet resistance and the contact resistance if all the

resistances are taken into account [11, 12, 13, 14].

The separation distance of the contacts is increased so the sheet resistance can be subtracted from

total resistance in order to find the contact resistance. This is done by plotting the total resistance

measured at varying contact distances against the distance between the contacts. This is

represented in Figure 8. Therefore if the input current and voltage are known, the contact

resistance can easily be found.

Figure 7. Transmission Line Method [11]

III. Characterization of GaSb: Substrate Quality and Defects Study

Defects on the surface of a semiconductor material can have devastating effects before and

after the MBE growth and make it nearly impossible to make any functioning device on the

material. Therefore, it is necessary to know what is causing defects and how to prevent or

suppress the amount of defects. Even though defects that occur in well understood compound

semiconductors such as GaAs are well known, each material has defects that may have slightly

different effects than other materials.

One of the main types of defects that can occur during MBE growth is spitting of a material.

This can be compared to a clogged airbrush were the flow is disrupted so there are areas of large

implantation of one element causing disrupted growth of the full compound III-V semiconductor.

The most thorough way to determine the composition of a defect and if it is caused by ‘spitting’

is to diagnose the defect with a Scanning Electron Microscope (SEM) [5].

An SEM works by scanning a sample with a high energy (> 1 keV) electron beam and

generating image contrast by the variable emission of secondary electrons from the sample

surface. See figure 9 for a basic schematic of an SEM. An SEM can achieve much higher

magnification than a basic optical microscope that is limited by the diffraction of visible light.

The electron beam used in a SEM also induces x-ray emission from the sample under

examination. Since these x-rays have characteristic energies of the material, compositional

analysis is also possible [ 15].

Figure 8. Graphical representation of TLM measurements [14]

E. Description of Experiments

I. Hall Measurements

Sample R12-55 was chosen to have Hall characteristics measured on it. This sample was a

SI-GaAs substrate with an IMF layer and a 1000nm thick layer of GaSb epitaxial growth on it.

The sample had ohmic contacts evaporated on it composed of Pd 87Å /Ge 560Å /Au 233Å /Pt

476Å/ Au 2000Å . The sample was prepared for measurement by placing it into the sample

holder with the sample clips securely tightened down on the sample. The sample holder was then

placed in the Hall measurement device. The Labview program designed for the set up was

executed and the Hall data was collected. Figure 10 shows a screenshot of the Hall measurement

Labview interface. The correctness of the data was confirmed by observing that the voltage

levels from each measurement were within 10% of each other. If the voltage levels were above

this percentage, the sample was re-placed on the holder making sure the pins correctly contacted

the metal contacts on the hall sample. The measurement was then taken again.

Figure 9. Diagram of a Scanning Electron Microscope [15]

II. TLM Measurment

The growths that had TLM measurements conducted on them were growths R12-53, R12-54,

R12-55, and R12-56. All four growths were on SI-GaAs with an interfacial misfit dislocation

(IMF) layer between the substrate and the GaSb growth. The GaSb MBE growths had varying

thicknesses with different levels of n-type Tellurium (Te) doping. Table 1 compares each growth

run with its intended carrier concentration with Te cell temperature and desired thickness.

Growth Run Carrier concentration (cm-3

) Thickness

R12-53 n-1e19 (Te:503C) 300nm

R12-54 n-5e18 (Te:484C) 500nm

R12-55 n-1e18 (Te:444C) 1000nm

R12-56 n-5e17 (Te:428C) 1000nm Table 1. Desired carrier concentration and thickness for individual growth runs

The TLM samples for each growth were prepared by having the standard TLM metallization

evaporated on them with the contact size being 50µm x 100µm and the spacing between pads

being 10µm to 70µm with an increase of 10µm from one space to the preceding space as

demonstrated in Figure 8. The four probes included two current source probes and two voltmeter

probes. The probes were placed on two contacts in the same fashion as the diagram in Figure 7

from above. The I-V curve was measured, and should be approximately linear from -10 to 10

mV and between -100mA and 100mA. If the I-V curve was grossly non-liner, the measurement

was conducted again insuring the probes were correctly contacting the sample. This information

was sent to a Labview program where I-V characteristics were recorded and the resistance can be

calculated for each measurement. This procedure was done for the remaining metal contacts

leaving the initial two probes on the initial metal contact and moving the other two probes to an

incrementing metal contact. The resistances can be calculated and plotted as a function of

distance. Therefore, the specific contact resistance can be calculated. If the contacts in Figure 8

are referred to as contacts 1, 2, 3, 4, 5, and 6 increasing from right to left, one set of probes

would be left on probe one and the other set of probes would move from 2 to 3, then from 3 to 4

Figure 10. Sample data from Hall measurement

and so on. Growths R12-53, R12-54, R12-55, and R12-56 had an average of 10 TLM samples on

them, therefore on average 70 measurements were taken per growth. There was four different

metallizations that were studied. All four metallizations were annealed at six different

temperatures at 5 different annealing times. The annealing temperatures were 260°C, 280°C,

290°C, 300°C, 310°C, and 320°C and the annealing times were 40sec, 45sec, 50sec, 55sec, and

60sec. See table 2 for a list of the different metalizations and annealing temperatures [12,13]. See

the Findings paper for the outcomes of the resistances measured of the different ohmic contacts.

Metallization Annealing Temperatures Pd 87Å /Ge 560Å /Au 233Å /Pt 476Å

/Au 2000Å 260°C 280°C 290°C 300°C 310°C 320°C

Ni 87Å /Ge 560Å /Au 233Å /Pt 476Å

/Au 2000Å 260°C 280°C 290°C 300°C 310°C 320°C

Ge 560Å /Au 233Å /Pd 87Å /Pt 476Å

/Au 2000Å 260°C 280°C 290°C 300°C 310°C 320°C

Ge 560Å /Au 233Å /Ni 87Å /Pt 476Å

/Au 2000Å 260°C 280°C 290°C 300°C 310°C 320°C

Table 2. List of metallizations and annealing temperatures

III. Defect Study

The defects of different growths were studied using the Earth & Planetary Sciences

Department’s SEM at UNM’s main campus. Once the sample of interest was chosen, the sample

was placed in the SEM and the SEM’s vacuum was initiated. The electron beam was then started

and a defect was chosen to study. First an image was taken of the defect. Next, the composition

of the defect was studied. See Figure 11 for a sample of an image taken by the SEM.

Once a defect of interest was found the composition of the defect was studied. This is done using

the Energy-Dispersed Analysis of X-rays program (EDAX) coupled with the SEM. With an

SEM equipped with EDAX it is possible to determine which elements are present in the surface

layer of the sample. See Figure 12 for a sample view of the EDAX program.

Figure 11. Sample SEM image

From the figure is can be seen the area that was selected (the highlighted red area in the SEM

image in the top right corner) is Gallium (Ga). Several defects on GaSb are being studied in a

similar fashion. Future plans include the use of a Focused Ion Beam (FIB) to cut a thin cross-

sectional area containing a particular defect. Once this is accomplished Transmission Electron

Microscopy (TEM) will be conducted. Doing so will allow us to study in more detail how the

defects affect the epitaxial growth.

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Figure 12. Sample EDAX of an SEM image

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