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Substrate reuse of acoustically spalled gallium arsenide for photovoltaics
Neumann, Anica N.
Neumann, Anica N.
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2024
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2026-04-04
Abstract
High cost has long been a limiting factor to widespread III-V photovoltaic applications, with the material cost of the substrate being a significant portion of this cost. My work has focused on the investigation of a substrate reuse technology - Sonic Liftoff (SLO), also known as acoustic spalling - with the goal of III-V photovoltaic cost reduction. Utilizing acoustic energy, SLO allows for lower stress removal compared to traditional methods, and the exfoliated devices have no associated performance losses. This has been evaluated by comparing the performance of inverted and upright solar cells processed via Sonic Liftoff and in traditional (no exfoliation used) processing. The cell efficiencies remained the same in both cases, showing that device damage is not inherent to the Sonic Liftoff process.
Following SLO, the resulting GaAs substrate surface has roughness that requires smoothing for subsequent cell growth. Wet chemical etching was initially investigated to smooth large area roughness of the acoustically spalled substrates with the goal of increasing cell performance and yield when compared to devices grown on an acoustically substrate without etching applied to it. Three etching systems were evaluated, with an 8:1:1 H2SO4 : H2O2 : H2O etchant identified as the etchant that provided the most smoothing for the least material loss. This etchant was then further optimized through an exploration of temerature and convection (stir rate), and subsiquently used to etch a 2-inch acoustically spalled GaAs substrate prior to cell growth. The devices grown on the etched substrate showed an increased average efficiency (12.8% over 2.0%) and yield (89% over 68%) from that of an unetched substrate.
In-situ smoothing via MOVPE growth was also investigated as an alternative smoothing method that did not remove substrate material. Se:InGaP and C:GaAs were evaluated for their ability to planarize the various surface features caused by acoustic spalling. C:GaAs was identified to smooth over these features on a full 2-inch spalled substrate, and was applied to an etched (via the 8:1:1 H2SO4 : H2O2 : H2O etchant developed previously) and unetched 2-inch acoustically spalled GaAs wafer to investigate the effect of this smoothing on cell performance in relation to devcies grown on a spalled substrate without a smoothing buffer or etching. The unetched substrate with a 5  m C:GaAs buffer showed an increase in average efficiency from 2.0% to 4.9%, and the etched substrate showed a further increase in efficiency to 8.1%.
The work outlined in this thesis paves the way for substrate reuse of GaAs for III-V photovoltaic applications using acoustic spalling. It has been shown that acosutic spalling does not damage the surface, and various smoothing methods to produces highly efficient solar cells have beem developed.
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