Epitaxial III-V-Bismide Materials for Space Power Generation
Stevens, Margaret.
2020
-
Thesis
(Ph.D.)--Tufts University, 2020.
Submitted to the Dept. of Electrical Engineering.
Advisor: Thomas Vandervelde.
Committee: Kevin Grossklaus, Brian Aull, Matthew Panzer, and Stephanie Tomasulo.
Keywords: Electrical engineering, and Materials Science.
To push beyond the present limitations on space exploration, researchers must develop ... read morelong-lasting, lightweight, and efficient energy sources to support long-term missions and in situ studies. Historically, photovoltaic (PV) and radioisotope power systems (RPS) have supported these missions, but new developments in diode materials are needed to implement these devices in extreme space environments. For PV devices, new band gap and lattice constant combinations are needed to maximize multijunction device efficiency under various solar spectrums. RPS systems need new heat-to-electricity power conversion mechanisms to increase power conversion efficiency. One option, thermophotovoltaic (TPV) devices use the photovoltaic effect to convert heat, or infrared radiation, into electricity. Radioisotope TPVs (RTPVs) have the potential to provide high specific power without the added complication of moving parts that could reduce system reliability. This makes RTPVs attractive for new thermal power technologies, especially in modularized applications and small systems that require low power. This dissertation proposes III-V-Bismides, an underdeveloped group of III-V compounds, as epitaxial materials for PVs and TPVs. Incorporation of bismuth (Bi) has a dramatic effect on the electrical and optical properties of III-V alloys. This leads to new band gap and lattice constant combinations previously unattainable by epitaxial growth on conventional substrates. However, Bi is challenging to incorporate in III-V alloys due to a large size mismatch between Bi and the host group-V element as well as a significant electronegativity difference. As a result, III-V-Bi films are prone to phase separation, surface segregation, droplet formation, and nonradiative recombination. In this dissertation, we explored alternative methods of increasing Bi incorporation in GaAsBi as a model system to be applied to other technologically relevant III-V-Bismides. Strain stabilization was found to be a successful method of reducing Bi rejection and phase separation in films, leading to a relative 24% improvement in Bi incorporation when grown on relaxed InGaAs buffer layers compared to samples grown on GaAs substrates. Choice of n-type dopant was explored for GaAsBi, and tellurium was found to be a more effective n-type dopant than silicon when Ga-droplets are present on the growth surface. GaAsBi alloys were simulated in potential PV and TPV structures and heterojunctions were found to be critical to achieving high open-circuit voltage in devices with small band gaps. Lessons learned from this research project can be applied to GaSbBi, InGaAsBi, and InAsSbBi to develop improved growth conditions and device architectures for bismide PV and TPV diodes.read less - ID:
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