Isotropic Wet Etching of Acoustically-Spalled GaAs

While III-V photovoltaics are among the most efficient, they suffer from a high manufacturing cost. One of the methods of reducing this cost is through GaAs substrate reuse, with a promising technology being that of acoustic spalling. Acoustic spalling, however, is a developing technology that leaves a rough, faceted GaAs surface. Chemo-mechanical polishing is a method for preparing the acoustically-spalled surface for photovoltaic growth, processing and subsequent respalling of the substrate, but at substantial cost. A less expensive method of substrate preparation is the application of a smoothing etch. Six etching systems were chosen to explore for this application: HCl:H2O2:H2O (40:4:1 and 80:4:1), H3PO4:H2O2:H2O (1:1:3), and H2SO4:H2O2:H2O (3:1:1, 5:1:1, and 8:1:1). Through testing at room temperature and a moderate stir rate of 600 rpm, the HCl:H2O2:H2O (40:4:1 and 80:4:1), H3PO4:H2O2:H2O (1:1:3) etchants proved to have a negative post-etch surface morphology for subsequent growth, processing, and/or re-spalling. The H2SO4:H2O2:H2O (8:1:1) etchant at 30°C and 600 rpm stirring showed to be the best etchant at a high percent roughness reduction to mass loss. This etchant was then applied to a full 2-inch acoustically-spalled GaAs wafer prior to single-junction GaAs cell growth. The processed cells will be compared to an unetched, spalled substrate, and an epi-ready substrate with the same growth/processing to compare cell performance metrics (external quantum efficiency, open circuit voltage, short circuit current) and yield. The growth and comparison is ongoing, but the substrate with the H2SO4:H2O2:H2O (8:1:1) etchant applied is expected to increase yield and show improvements in average cell performance metrics as compared to the unetched substrate.