DESIGN, FABRICATION AND CHARACTERIZATION OF EPITAXIAL AND NON-EPITAXIAL THERMO-PHOTOVOLTAIC CELLS
Thermophotovoltaics (TPVs) have significant potential in efficiently converting thermal energy to electrical energy. These applications include conversion from internal combustion engines, small nuclear sources and even portable fuel-based sources. Group-III antimonide semiconductors have been identified as the material of choice for such TPV devices due to the possibility of growing materials with the bandgap energies of 0.51 eV (GaInAsSb quaternary) to 0.72 eV (GaSb binary) that are correspond to commonly available heat sources. The quaternary alloys are grown epitaxially while the binary GaSb devices can be realized through non-epitaxial techniques. In this work, we have pursued fabrication and design methods that will allow us to realize large area GaSb-based diode technology for TPV applications. TPV yield is a serious issue in such large area devices. Functional TPV cells using epitaxial GaSb, epitaxial GaInAsSb, and implanted GaSb with areas up to 1 square cm are realized. The epitaxial cells fabricated in this study allow for the engineering of the bandgap in the structure and also allows for the tailoring of the absorber in the cell to 2.4 μm which is a blackbody wavelength of interest. These cells however are not straightforward to scale in dimension due to the presence of large epitaxy related defects that end up shorting the devices. We have identified and mitigated the effect of such shunt defects that were limiting the yield of the epitaxial TPVs on GaSb. The Non-epitaxial TPV cells are realized using beryllium ion implantation into an n-type GaSb substrate. Through the use of rapid thermal annealing a pn junction is formed. The ion-implanted approach is intended to maximize shunt resistance compared to the epitaxial technique. The presentation will involve in-depth characterization and analysis of the materials from the quality of the semiconductor materials and interfaces to the ohmic contacts. Extensive analysis of the material using transmission electron microscopy, electron dispersive spectroscopy and secondary ion mass spectroscopy will be presented and correlated to electrical characterization results.
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