Defect characterizations of γ-LiAlO2 single crystals

LiAlO2 is a potential substrate for growing III-nitride semiconductors since the lattice mismatch between LiAlO2 and GaN is only 1.4%. GaN grown on (1 0 0) LiAlO2 substrates is along the (101¯0)M-plane which does not have the spontaneous polarization. This paper describes the growth of (1 0 0) LiAlO2 single crystals using the Czochralski method. The as-grown crystals revealed a serious chemical decomposition problem in the cone area and some cracks in the body of the boule. Cracks indicate strain in the crystal and also relieve the strain. The crystal's structure was identified as the γ-phase by X-ray diffraction analysis. No phase transition was found. The thermal expansion coefficients of LiAlO2 was found to be 6.50×10−6 °C−1 along (1 0 0) axis, and 14.90×10−6 °C−1 along (0 0 1) axis. Stress distribution was investigated using an optical polarizer and no residual stress was found. Chemical etching and electron microcopy revealed a multi-domain structure along the a-axis. This domain structure is due to the polarity inversion. An unique cross-hatched pattern was also found in (0 0 1) TEM images.