Processing of GaN and SiC epitaxial layers for opto-electronic device applications
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Gallium nitride (GaN) and silicon carbide (SiC) are often used to fabricate sophisticated electronic and photonic devices due to their inherent wide bandgap, high electron saturation velocity, thermal conductivity. Moreover, devices fabricated out of these materials are capable of operating at high voltages, breakdown fields, and frequencies within wide temperature ranges. However, it is hard to grow and manufacture these devices because of their chemically strong natures. This dissertation explores the impact of mesa sidewall passivation by SiO2 on characteristics of III–nitride/silicon tandem solar cells. These dual junction solar cells were fabricated from standard n-type Si (111) substrates with III–nitride epitaxial layers grown by plasma-assisted molecular beam epitaxy (PAMBE). Photovoltaic testing was experimentally carried out before and after the passivation of these solar cell mesa side walls, and it was concluded that the passivation moderately improved the efficiency of these solar cells. After passivation, the open-circuit voltage increased from 1.45 to 1.53 V, the short-circuit current density improved from 0.116 to 0.121 mA/cm2, and the fill factor increased from 39.7 to 41.5% under AM 1.5 illumination. This yielded a conversion efficiency improvement of approximately 13%. Furthermore, the dominant mechanism of carrier transport in the hybrid GaN/AlN/Si solar cells was investigated. Fabrication of 4H-SiC based photoconductive semiconductor switches (PCSS) by inductively coupled plasma reactive ion etching (ICP-RIE) was also studied. Sets of experiments were conducted to develop a plasma etch recipe which enabled moderate etch rate and smooth surface morphology by optimizing system parameters such as the plasma composition, ICP power, RIE power, and process pressure.