Abstract

Photodiodes made of III-V materials are ubiquitous with applications for telecommunications, photonics, consumer electronics, and spectroscopy. The III-V solar cell, specifically, is a large-area photodiode that is used by the satellite industry for power conversion due to its unrivaled efficiency and wide range of available materials. As a device driven by its minority carrier diffusion length (MCDL), the performance of a photodiode is sensitive to crystallographic defects that create states in the forbidden energy gap. Defects commonly arise during growth of the crystal and during device fabrication, and they accumulate slowly over time when deployed into the damaging environment of space. Defect-assisted carrier recombination leads to lower MCDL, higher dark current, reduced sensitivity and signal-to-noise ratio, and, in the case of solar cells, reduced power conversion efficiency. Consequently, the development of photodiode technology requires techniques for detection, characterization, and mitigation of defects and the inter-bandgap states they create. In this work, III-V material defects are addressed across a variety of materials and devices.

The first half of the work makes use of deep-level transient spectroscopy (DLTS) to deduce the energy level, cross-section, and density of traps the InAlAs, InAlAsSb, and InGaAs lattice-matched to InP. An in situ DLTS system that can monitor defects immediately after irradiation was developed and applied to InGaAs photodiodes irradiated by protons. Evidence of trap annealing was found to occur as low as 150 K. The second half begins with development of GaSb solar cells grown by molecular beam epitaxy on GaAs substrate intended for use in lower-cost monolithic multi-junction cells. Defect analysis by microscopy, dark lock-in thermography, and dark current measurement, among others, was performed. The best GaSb-on-GaAs cell achieved state-of-the-art 4.5% efficiency under concentrated solar spectrum. Finally, light management in III-V photodiodes was explored as a possible route for defect mitigation. Textures, diffraction gratings, metallic mirrors, and Bragg reflectors were simulated by finite difference time domain for single- and multi-junction GaAs-based cells with the aim of reducing the amount of absorber material required and to simultaneously reduce MCDL requirements by generating carriers closer to the junction. The results were inputted into a device simulator to predict efficiency. A backside reflective pyramidal-textured grating was simulated to allow a GaAs cell to be thinned by a factor of >30 compared to a conventional cell.

Publication Date

3-1-2019

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Seth M. Hubbard

Advisor/Committee Member

Sean Rommel

Advisor/Committee Member

Michael Pierce

Campus

RIT – Main Campus

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