Fracture Mechanics Evaluation of GaAs Final Report

The purpose of this work was to generate a data base of mechanical and fracture properties data for single crystal GaAs to be used in the design of reiiable GaA_ solar cells and modules. This report contains results of this test program inc uding _ysical property characterization, fracture behavior evaluation and the strength of cells determined as a function of cell processing and material parameters. Elastic moduli of GaAs single crystals were found to be highly anisotropic in terms of crystal orientation. The Young's modulus in<111> is 14.4 X I0II dyne/cm 2 while that in <100> is 8.g3 X I0 II dyne/cm 2. The thermal expansion coefficient in the temperature range -130 to 450°C was measured to be 5.5 X 10-6 m/m °C. No significant variation of the thermal expansion coefficient was found as a function of crystal orientation. GaAs single crystal was found to be extremely fragile. The minimum fracture toughness of GaAs was measured to be 0.31 MN/m 3/_ in cleavage on the 1110} plane as compared with silicon which has a minimum fracture toughness of 0.82 MN/m 3/2 in cleavage nn the _Ii_•I plane. Results suggest that GaAs is susceptible to subcritical crack growth. Significant changc_ in fracture strength were found to result from several important processing steps during wafer-to-cell production. The fracture strength data were described by Weibull statistical analysis and can be interpreted in light of the nature of the flaw distribution of the samples.