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

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1 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 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

2 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 atoms within one square centimeter of GaSb. Also, depending on the doping there can be well over 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].

3 Figure 1. Diagram of oval defects [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.

4 Figure 2. Molecular Beam Epitaxy diagram [6] 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.

5 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]. Figure 3. Right Hand Rule [7] Figure 4. Hall Measurement Schematic [8] 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 U Hall 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].

6 Figure 5. Resistance measurements taken during Hall measurements [9] The sheet resistance R s 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. Figure 6. Measurement of the Hall voltage V H [9] Using the above information gained from the Hall measurement, the Hall mobility (µ) and the carrier sheet density, n s can be calculated using the following equation:

7 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. Figure 7. Transmission Line Method [11] Using four probes with finite resistances represented as R p1, R p2, R p3, and R p4 in Figure 7, one can see the correlation between the resistance from the probe (R p ), the connection between the probe and contact (R cp ), the contact (R c ), and the sheet resistance of the sample (R s ). 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.

8 Figure 8. Graphical representation of TLM measurements [14] 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 9. Diagram of a Scanning Electron Microscope [15] E. Description of Experiments I. Hall Measurements Sample R12-55 was chosen to have Hall characteristics measured on it.

The sample had ohmic contacts evaporated on it composed of Pd 87Å /Ge 560Å /Au 233Å /Pt 476Å/ Au 2000Å.

9 Figure 9. Diagram of a Scanning Electron Microscope [15] 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 10. Sample data from Hall measurement II. TLM Measurment The growths that had TLM measurements conducted on them were growths R12-53, R12-54, R12-55, and R12-56.

10 Figure 10. Sample data from Hall measurement II. TLM Measurment The growths that had TLM measurements conducted on them were growths R12-53, R12-54, R12-55, and R 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

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.

11 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. Figure 11. Sample SEM image 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 12. Sample EDAX of an 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).

12 Figure 12. Sample EDAX of an 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 crosssectional 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. F. References [1] "The History of Solar." U. S. Department of Energy, n.d. Web. 15 Apr < [2] "Physical Properties of Gallium Antimonide (GaSb)." Physical Properties of Gallium Antimonide (GaSb). N.p., n.d. Web. 10 Jan < [3] Nelson, Robert E. "A Brief History of Thermophotovoltaic Development." Semiconductor Science and Technology 18.5 (2003): S Print. [4] Neamen, Donald A. Semiconductor Physics and Devices: Basic Principles. Third ed. Boston: McGraw-Hill, Print. [5] Mahajan, S. "Defects in Semiconductors and Their Effects on Devices." Acta Materialia 48.1 (2000): Science Direct. Elsevier. Web. 16 Apr [6] Lee, Yeonbae. "2007 MXP Project, Graphine." Introduction and Theory. Goldman's Research Group, Web. 16 Apr < [7] Tafur, Camilo, and Dan MacIssac. "Right-Hand Rules: A Guide to Finding the Direction of the MagneticForce." Right-hand Rules. National Science Foundation, U.S. Department of Education, and SUNY-Buffalo State College, n.d. Web. 18 Apr <

13 [8] Föll, H. "1.3.4 The Hall Effect." Lecture The Hall Effect. H. Föll. Web. 16 Apr < [9] Van Der Pauw, L. J. "A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shape." Philips Research Reports 13.1 (1958): 1-9. Print. [10] Korenivski, Vladislav. "The Van Der Pauw Technique." The Van Der Pauw Technique. Vladislav Korenivski, 03 Mar Web. 09 Apr < [11] Chen, K. X., J. K. Kim, F. Mont, and E. F. Schubert. Four-Point TLM Measurement for Specific Contact Resistance Assessment. N.p.: n.p., n.d. PDF. [12] Rahimi, N., A. A. Aragon, O. S. Romero, D. M. Kim, N. B. Traynor, T. J. Rotter, G. Balakrishnan, S. D. Mukherjee, and L. F. Lester. Ohmic Contacts to N-type GaSb Grown on GaAs by the Interfacial Misfit Dislocation Technique. N.D. Article. [13] Rahimi, Nassim, Andrew A. Aragon, Orlando S. Romero, Matthew K. Erdman, Tom J. Rotter, Ganesh Balakrishnan, Sayan D. Mukherjee, and Luke F. Lester. Ultra-low Resistance NiGeAu and PdGeAu Ohmic Contacts On N-GaSb Grown On GaAs. N.D,.Article. [14]Galván-Arellano, M., J. Díaz-Reyes, and R. Peña-Sierra. "Ohmic Contacts with Palladium Diffusion Barrier on III V Semiconductors." Vacuum (2010): Elsevier. Web. 10 Apr [15] Schweitzer, Jim. "Scanning Electron Microscope." Scanning Electron Microscope. Purdue University, Web. 17 Apr <

FINDINGS. A. Results. Hall Measurements

FINDINGS. A. Results. Hall Measurements FINDINGS REU Student: Matthew Erdman Graduate Student Mentor: Andrew Aragon Faulty Mentor: Luke Lester and Sayan Mukherjee A. Results Hall Measurements Figure 1. Sample R1-55 data from Hall measurement