Gallium antimonide (GaSb) is a promising low-bandgap binary substrate for the fabrication of various infrared-based optoelectronic devices, particularly thermophotovoltaics (TPV). In order to make GaSb-based technologies like TPV more widely available, non-epitaxial dop- ing methods for GaSb must be pursued. Ion implantation is relatively unexplored for GaSb, and can offer advantages over the more common method of zinc diffusion, including higher flexibility with regards to substrate type and control over the resulting doping profile. Pre- vious work has shown beryllium (Be+) implantation to be a suitable method for fabricating a diode in an n-type GaSb substrate, opening the possibility for other ions to be considered for implanting into both n-type and p-type substrates.
This work identifies sulfur (S+) as another species to investigate for this purpose. To do so, material and electrical characterization was done on S+ and beryllium implanted GaSb films grown onto a semi-insulating gallium arsenide (GaAs) substrate. X-ray Diffraction spectroscopy (XRD) and Atomic Force Microscopy (AFM) indicate that the post-implant anneal of 600 for 10 s repaired the implant damage in the bulk material, but left behind a damaged surface layer composed of coalesced vacancies. While the beryllium implant resulted in moderate doping concentrations corresponding to an activation percentage near 15 %, Hall Effect data showed that implanting S+ ions induced a strongly p-type behavior, with hole concentrations above 1 × 19 cm^3 and sheet hole densities 3.5 times higher than the total implanted dose. This strong p-type behavior is attributed to the remaining lattice damage caused by the implant, which induces a large density of acceptor-like defect states near the valence band edge.
This technique was used on an unintentionally-doped p-type GaSb substrate to create a + /p junction. The implant process succeeded in producing a potential barrier similar to that of a hole-majority camel diode with a thin delta-doped region suitable for collecting diffused carriers from the p-type substrate. A post-fabrication etching process had the effect of strongly increasing the short circuit current density to as high as 41.8 mA/cm^2 and the open circuit voltage as high as 0.21 V by simultaneously removing a high carrier recombination surface layer. This etching process resulted in a broadband spectral response, giving internal quantum efficiencies greater than 90 %. / Doctor of Philosophy / Thermophotovoltaics (TPV) is a technology that converts light and other forms of electromagnetic energy into electrical power, much like a typical solar panel. However, instead of sunlight, the energy source used in a TPV system is a terrestrial heat source at a temperature range of 1250–1750 ◦C, whose radiation is primarily infrared (IR). The IR-absorbing qualities and commercial availability of the compound semiconductor gallium antimonide (GaSb) have made it a key component in the development of absorber devices for TPV-related systems. GaSb-based devices have most often been fabricated using epitaxy, a method in which layer(s) of material are ‘grown’ in a layer-by-layer fashion atop a substrate GaSb wafer to induce an interface between negatively-charged (n-type) and positively-charged (p-type) regions. In order to improve upon the scalability of TPV production, device fabrication methods for GaSb that avoid the use of epitaxy are sought after as a lower-cost alternative. In this work, sulfur ion implantation is examined as one of these methods, in which elemental sulfur ions are injected at a high energy into a p-type GaSb substrate. The implanted ions then alter the charge characteristics at the surface of the material, producing an electric field from which a photovoltaic (PV) device can be fabricated. The results of this study showed that by implanting sulfur ions, an extremely p-type (p++) layer was formed at the surface of the GaSb substrate, which was attributed to residual damage induced by the implant process. The resulting interface between the p++ surface and the moderately p-type GaSb substrate was found to induce an electric field suitable for a PV device. Removing the excess surface damage away from the device’s metal contacts resulted in an improvement in the output electrical currents, with measured values being significantly higher than that of other devices made using more common non-epitaxial fabrication methods. The success of this work demonstrates the advantages of using a p-type GaSb substrate in place of an n-type substrate, and could help diversify the types of TPV-related devices that can be produced.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/93767 |
| Date | 18 September 2019 |
| Creators | Herrera, Daniel |
| Contributors | Electrical Engineering, Lester, Luke F., Guido, Louis J., Jia, Xiaoting, Asryan, Levon V., Zhou, Wei, Hudait, Mantu K. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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