GaSb is a III-V compound semiconductor substrate material most suitable for the epitaxial growth of antimonide based quantum well (QW) and super-lattice (SL) layer photodetector focal plane array (FPA) structures due to its close lattice parameter with the device layers. For back side illumination devices, the substrate material must have high optical transmission. Unfortunately, GaSb has unusual optical transmission characteristics due to the presence of high density of equilibrium point defects (native defects such as vacancies and antisites) which makes the substrates practically opaque to radiation of below bandgap wavelengths where the devices operate. The GaSb substrate must be thinned after device fabrication complicating the device packaging process and often resulting in the introduction of defects in the device layers. Optically transparent GaSb is highly desirable for these applications as well as other infrared optical technologies. GaSb also has excellent nonlinear optical properties that makes it attractive for optical power limiters (OPLs). When integrated with the FPAs, it could provide protection to the QW and SL sensors from threats posed by laser based weapons. For OPLs, optically transparent, thick (mm-cm scale) GaSb substrate is necessary. In this research, two fundamental crystal growth approaches have been experimented for enhancing the optical transmission of GaSb. In the first method, crystal growth of GaSb from liquid phase at low temperature has been conducted from gallium rich non-stoichiometric solution with growth temperature in the range of 400-600 oC. Lowering growth temperature is expected to reduce the concentration of equilibrium point defects (vacancies and antisites), thus enhancing the optical transmission of GaSb. The effects of temperature gradient, synthesis duration and solution cooling rate on the optical transmission of GaSb has been studied. To eliminate the incorporation of gallium inclusions in the crystals, a continuous solute feeding process has been developed. Using the solute feeding process, 2-3 mm thick crystals of 20 mm diameter GaSb have been successfully grown at temperature as low as 500 oC, which is approximately 200 oC lower than the melt growth temperature of GaSb with growth rates in the range of 0.2-0.5 mm per hour. However, the growth rate used was found to be high and the resultant GaSb wafers exhibited optical scattering from gallium and antimony based metallic inclusions frozen in the bulk matrix. Challenges in GaSb growth at such low temperatures will be discussed along with the mitigating strategies. In the second method, impurity doping with n-type dopant tellurium (Te) and p-type dopant zinc (Zn) was experimented for the first time to compensate for the p-type native defects in GaSb crystals. Past research on n-type doping using Te had limitations. The new co-doping approach using Te and Zn provides the flexibility to alter the Fermi level position in GaSb and hence the optical transmission can be enhanced by avoiding the ionization of the native defect levels (vacancies and antisites).
Undoped GaSb exhibit relatively high below bandgap single photon absorption coefficient, α ≈ 10-20 cm-1 at the wavelengths of interest for OPLs (1.9 - 3 μm), primarily due to high concentrations (~ 1017 cm-3) of electrically active native defects due to Ga antisite (GaSb) and Ga vacancies (VGa). By optimizing the Zn and Te concentrations respectively to 6 x 1017 cm-3 (Zn) and 3 x 1017 cm-3 (Te), this research has demonstrated a significant reduction in below band gap optical absorption coefficient by 8-10 cm-1. This is a 50-80% reduction in the absorption coefficient across the spectra studied. The Zn and Te exhibit shallow energy levels compared to the native defect levels in bandgap. In addition to the reduction of optical absorption, free carrier concentration level also decreased by a factor of 15-20 compared to undoped GaSb. This effectiveness of the co-doping approach for the enhancement of optical and electrical properties of GaSb has been attributed to the lowering of the Fermi level by 21.9 meV from the undoped GaSb Fermi level. The shift in Fermi level has been theoretically postulated as a result of the Zn and Te atoms occupying a significant portion (30-50%) of the GaSb and VGa defects. In addition, the lowering of the Fermi level relative to the valence band resulted in the incomplete ionization of the GaSb and VGa defect energy levels. This novel co-doping approach can be further exploited to engineer the Fermi level and equilibrium defect concentration independently to create optically transparent GaSb and other semiconductor compounds.;
May 2022; School of Engineering
Dept. of Electrical, Computer, and Systems Engineering;
Rensselaer Polytechnic Institute, Troy, NY
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