Characterization of Ohmic Contacts on n-GaSb and n-GaAs Grown on GaAs
Nathan Traynor
Undergraduate Researcher-UNM REU in Nanophotonics
Center for High Technology Materials-Department of Electrical and Computer Engineering, University of New Mexico
ACTIVITIES
INTRODUCTION
As energy demands increase in the modern era it is evident that development of novel technologies for energy production will grow in importance. One extremely viable option for this energy production is photovoltaic devices. Further, one material that will almost certainly become a key component of many these devices is Gallium Antimonide (GaSb). As a result, study of Gallium Antimonide to better understand its fundamental properties will lead to the fabrication of more efficient and cost effective photovoltaic cells that will be more viable in the marketplace.
BACKGROUND
Currently the marketplace for photovoltaic devices is dominated by Silicon based devices. This is due to the relatively low cost of Silicon in comparison to other materials from which these devices can be made. However, one of the disadvantages of Silicon is the limited portion of the electromagnetic spectrum it absorbs and converts to energy. One solution to this is the fabrication of multilayer photovoltaic cells that contain layers of multiple materials that absorb different portions of the electromagnetic spectrum. Through fabrication of these devices, photovoltaic cells can be made that more efficiently absorb and convert a much broader portion of the electromagnetic spectrum to electrical energy.
As previously stated, Gallium Antimonide is one of the materials that have a high potential for future use in these multilayer structures. This is due to the fact that GaSb absorbs a lower energy portion of the electromagnetic spectrum. Also, due to the fact that it absorbs these lower energy photons, it is optimal to have the layer of GaSb as the first growth layer in a multi-junction photovoltaic device. As such, the interaction between the grown GaSb and the substrate on which it is grown will be crucial to the functioning of the device. An example of a potential structure can be found in Figure 1 that clearly shows why this study of GaSb grown on GaAs is particularly relevant.
In order to achieve the highest material quality in the thin-film Gallium Antimonide, it should be grown on a GaSb substrate to ensure that there is exact lattice matching. However, a commercial GaSb substrate is not currently available in semi-insulating form and is also relatively expensive compared to GaAs or Silicon. For our work here on metal contacts to n-GaSb, a semi-insulating substrate is desirable because we want to assess the quality of the metallization to the thin-film layer only. A conductive substrate would complicate this goal.
As a result various methods of growing GaSb on SI-GaAs have been attempted, as having a semi-insulating substrate is an extremely desirable characteristic of many semiconductor devices. In general, these attempts result in a high number of dislocations that propagate into the grown GaSb layer, degrading its electrical qualities and making it not viable for device fabrication as shown in figure 2. However, the Interfacial Misfit Technique developed at the Center for High Technology Materials by Dr. Ganesh Balakrishnan is a novel growth technique that reduces the number of dislocations when GaSb is grown on GaAs. As this is a novel material, study into its basic properties is necessary in order to understand it as well as determine if it will be a viable option for device fabrication. The differences between GaSb grown on GaAs using and not using the IMF technique can be visually in TEM pictures seen in Figure 2.
One property specifically researched will be the nature of ohmic contact formation on this novel material. The goal of an ohmic contact is to allow charge to flow both into and out of the material it is formed on while obeying a linear current-voltage relationship. This is known as Ohm’s Law. Further, a high quality ohmic contact should have a very low resistance in order to allow for extremely efficient devices to be created that have minimal energy loss. However, formation of an ohmic contact on most semiconductor materials, including GaSb, is not a simple task. In general, to form an ohmic contact to GaSb the material must be extremely highly doped with intentional impurities. This challenge can be overcome through the judicious choice of alloyed metal layers that are in intimate contact with the material and contain elements that can act as dopants to the semiconductor. By having this type of structure the alloyed contacts allow for highly localized doping at the interface between the metal and the semiconductor, thus allowing for the formation of an ohmic contact.
RESEARCH OBJECTIVE
All told, the goal of this research is to contribute to the overall study of Gallium Antimonide as it has great potential for integration into photovoltaic devices. Further, great interest is placed on GaSb that has been grown on semi-insulating Gallium Arsenide (SI-GaAs) using the Interfacial Misfit Technique (IMF), a growth method developed here at the Center for High Technology Materials by Dr. Ganesh Balakrishnan. Specifically, this research looks to investigate and characterize the nature of ohmic contact formation on GaSb grown on SI-GaAs using IMF. This characterization will be done through analysis of current vs. voltage measurements that will allow us to tell if our formed contacts are ohmic, and if so, their resistance as well as the sheet resistivity of the Gallium Antimonide grown using IMF. Also, this data will be compared to that of GaAs grown on SI-GaAs for the purpose of comparison.
METHODOLOGY
The materials of interest in this study was n-GaSb of various doping concentrations grown using Molecular Beam Epitaxy at CHTM on commercially-obtained SI-GaAs substrates. A summary of the materials studied is in Figure 3. Further, fellow CHTM undergraduate researcher Daniel Kim found the doping concentrations in Figure 3 using Hall Measurements on different portions of the corresponding samples used in this study.
Growth Material (all on SI-GaAs) / Wafer Number / Thickness / Doping Concentration (cm-3)n-GaSb / R12-56 / 1.00 μm / 9.25x1016
R12-55 / 1.00 μm / 2.16x1017
R12-54 / .500 μm / 6.95x1017
R12-53 / .300 μm / 9.69x1017
Figure 3
After the grown samples were obtained, the processing required to form the contacts was begun with photolithography. After the first step of photolithography was completed to define a isolating mesa, the sample was etched and photolithography was again used to define the metallization pattern. Next, metal evaporation was used to deposit thin films of metals on the entire surface of the processed sample. Then, using liftoff the photoresist as well as unwanted metal that was deposited on the photoresist is removed. At this point, the sample has the desired pattern of layered metal on the surface of the epitaxial growth layer. Finally, the sample is now heated in a procedure called annealing to allow the thin metal films to create alloys as well as penetrate into the surface of the semiconductor.
After the processing is completed the sample is ready to be tested using the transmission line method (TLM)(3). TLM testing involves supplying current between the metal contacts on the sample and measuring the resulting voltage. These measurements are taken using the four-point probe method on a probe station.
After measurement, the gathered data can be examined and used to determine if the contacts formed on the sample are ohmic rather than a diode or some other undesirable electrical behavior. Further, if the contacts are found to be ohmic their resistance as well as the sheet resistivity of the material they are formed on can be determined.
DESCRIPTION OF METHODS
Photolithography and Mesa Definition
Photolithography involves the use of photoresist, which is a substance that changes its solubility after it has been exposed to ultraviolet light. In order to achieve the desired patterns of metals on the sample as well as an isolated mesa on which the contacts sit, two photolithographic processes are used. In the first processing step of processing a sample positive photoresist is used to create a pattern of photoresist on the sample. Then the sample is bombarded with ions in a process called plasma etching in order to remove the portions of the grown GaSb layer not protected by the remaining photoresist. After this is completed, the remaining photoresist is stripped and the sample is left with a pattern of isolated mesas of GaSb that are sitting on the GaAs substrate. This process is visually shown in Figure 4. At this point the sample is ready to undergo the second photolithographic process.
Figure 4 (4)
The photoresist used in the second photolithography technique of the experiment is an image reversal photoresist, which upon exposure to UV light and a subsequent bake becomes insoluble in a chemical developer. This type of photoresist results in undercut sidewalls in the developed pattern, a key factor in allowing for the liftoff process to be successful. The process of using this photoresist is shown in Figure 5.
Figure 5 (1)
Metal Evaporation
In order to create very uniform and intimate metal contacts, evaporation is used to deposit thin films of metal on the entire sample. In this technique, samples are loaded into a metal evaporator chamber above crucibles containing the metals to be used in the contacts. Then the chamber is brought under very high vacuum and an electron beam is directed at the crucible containing a desired metal. This bombardment of the metal with electrons causes the metal to heat rapidly, become a vapor, and travel up to and condense on the surface of the sample. After this process, the sample is completely coated with the layers of metal. Further, this method allows for precise control of the thicknesses of the deposited layers, which in turn leads to more control of the contact properties. This study focused on two different metallizations that were deposited on the semiconductor layers. The specific composition of these metallizations was (from the substrate up and with thicknesses in parenthesis) Ge (282 Å)/ Au (547 Å)/ Ni (50 Å)/ Pt (470 Å)/ Au (2000 Å) and Pd(87 Å)/Ge(560 Å)/Au(233 Å)/Pt(476 Å)/Au(2000 Å). Further, the former metallization will be referred to in charts and graphs as Ni as it contains nickel based and the latter as Pd because it contains Palladium.
Liftoff
The liftoff process is the next required step in the processing of a sample. At this point, the thin films of deposited metal are present on the entire sample. Using acetone the remaining photoresist that is present below the thin metal film is removed. This is possible due to the undercut sidewalls that were formed during photolithography allowing the acetone to contact where the photoresist contacts the growth layer and remove the photoresist. This process is shown visually in Figure 6.
Figure 6 (1)
The final step is to anneal the metal-semiconductor system in order to allow the metals to alloy as well as physically penetrate into the GaSb. Placing the sample under high temperature for a period of time such that the metal layers are able to melt and undergo the aforementioned changes does this.
TLM
After the desired metal pattern is created and annealed on a sample, the method used to study the materials properties as well as the properties of the metal contacts formed on them is the Transmission Line Method (TLM). Further, when using the TLM to characterize a material and its metal contacts, two variations may be used, the four-probe method or two-probe method.
In the two-probe method, two probes are placed on adjacent metal contacts. Then, a varying voltage is applied across the two probes and the resulting current through the probes is measured. This is an extremely reliable way to generate TLM data. However, when doing so the resistance of the probes and the setup must be taken into account, as this resistance will affect the resulting current that is being measured.
In order to avoid the possible errors present in the use of the two-probe method, this study used the four-probe method to collect transmission line data. In this variation of TLM, two pairs of probes are placed on adjacent metal contacts. Then two probes on opposite contacts deliver a constant current while the remaining pair of probes senses the resulting voltage since zero current flows between these two. By using this method the resistance of the probes does not become a factor in any of the measurements.
Analysis of Data
After Transmission Line Data is taken between two contacts, a Current vs. Voltage plot and corresponding mathematical relationship can be obtained. If the plot is non-linear, for example having the current-voltage relationship of a diode as displayed in Figure 7, then it is known that this set of contacts is not ohmic and doesn’t have the qualities we wish to study. Some factors that can cause the contacts to be non-ohmic are having a doping concentration in the sample that is too low, not using a correct metallization, or not using a suitable annealing temperature or time.
Figure 7
However, if the plot is found to be linear, showing that the metal contacts are Ohmic, then the resistance between the two contacts can be found. A visual example of an ohmic current vs. voltage relationship plot as well as the correspondence to resistance, labeled RLINEAR, can be found in Figure 8.
Figure 8 (2)
After the current-voltage relationship and resistance between each set of ohmic contacts is found, a plot of these resistances as a function of the distance between them can be generated. From this plot the sheet resistivity as well as the resistance and specific resistivity of the contacts can be determined. An example of this plot and the mathematical relationship between the properties it reveals is found in Figure 9 with Rc being the contact resistance, Rs being the sheet resistivity, Lt being the transfer length, and W being the width of the metal contacts. Further, the reason that the y-intercept of the graph is equal to twice the contact resistance is that this is the resistance present when the distance between the two contacts is zero, so the resistance of one contact is half of this value.