High-temperature strength of III-V nitride crystals

The hardness of AlN and GaN bulk crystals was investigated in the temperature range 20-1400°C by the Vickers indentation technique. The hardnesses of GaN and AlN at room temperature are 10.8 and 17.7 GPa, respectively. With increasing temperature the hardness shows a gradual decrease and then a steep decrease from around 1000°C, which indicates the beginning of macroscopic dislocation motion and plastic deformation. The yield strength of GaN bulk crystals determined directly by means of compressive deformation is around 100-200 MPa in the temperature range 900-1000°C, i.e. similar to that of α-SiC and much higher than those of Si, Ge, GaP, GaAs, InP and InAs. On the basis of the observed temperature dependence of yield stress, the activation energy for dislocation motion at elevated temperatures in GaN is deduced.