HARDNESS, INTERNAL-STRESS AND FRACTURE-TOUGHNESS OF EPITAXIAL ALXGA1-XAS FILMS

Vickers microhardness indentations of 10 mu m (001) oriented epilayers of AlxGa1-xAs on GaAs substrates have been utilized to evaluate the h ardness H-v, the internal stress, and the fracture toughness K-lc of t he layers as a function of their composition parameter x. The hardness H-v varies linearly according to: (6.9 - 2.2x) GPa and K-lc increases linearly with x according to: K-lc = (0.44 + 1.30x) MPa m(1/2). The i nfluence of the substrate on these measurements was found to be neglig ible for the layer thickness (10 mu m) and the indentation load (0.25 N) used, disregarding internal stresses. Internal film stresses were e valuated by the bimorph buckling method, and were found to depend on t he composition parameter according to sigma = 0.13x GPa. These stresse s did not notably affect the H-v measurements, but for K-lc correction s as large as similar to 25% had to be made. The radial cracks observe d were of the shallow Palmqvist type. In contradiction to previous rep orts on this type of cracking, it was found to initiate during unloadi ng, not during loading, and a physical explanation for this deviation is given. No deep radial/median cracks were observed. It was found imp ortant to use expressions based on the correct crack geometry in the K -lc evaluation. Also, a simple theory for the influence of internal st resses on the K-lc results has been developed.