Surface mechanics of GaAsP/GaAs epilayers for non-linear optical devices

We report on nanoindentation measurements of hydride vapor phase epitaxy (HVPE) grown Gallium Arsenide Phosphide (GaAs_1-xP_x) epilayers, of interest for generation of mid-infrared wavelengths via frequency conversion. Our results indicate a linear trend in Young's modulus with increasing phosphorus content (P-content), in keeping with Vegard's Law for ternary compounds. The corresponding data for hardness demonstrate significant bowing with a peak of 10.7 GPa occurring at 78 % P-content. Some epilayers exhibit non-negligible spatial variation in their mechanical properties, which is investigated with complementary measurements. Photoluminescence measurements indicate high composition uniformity, while atomic force microscopy reveals a direct correlation between surface roughness and mechanical variability. Preliminary laser-induced damage threshold (LIDT) results are reported, and the implications for material optimization are discussed. With a 1070-nm continuous wave laser, GaAs_0.75P_0.25 and GaAs_0.52P_0.48 epilayers demonstrated LID thresholds of 600 kW/cm² and 300 kW/cm², respectively.