Thermal and optical properties of bulk GaN crystals fabricated through hydride vapor phase epitaxy with void-assisted separation

The fundamental material parameters associated with GaN, which are important for the design of devices such as light-emitting diodes and laser diodes, were investigated using large high-quality GaN single crystals fabricated through hydride vapor phase epitaxy using the void-assisted separation method. The thermal-expansion coefficients (298-573 K) along the C[0001], A[11 $(2) over bar $0], and M[10 $(1) over bar $0] axes (alpha(C), alpha(A), and alpha(M)) were measured. Thermal expansion in each direction, approximately proportional to the temperature, was observed throughout the measured temperature range. Although the thermal-expansion coefficients in the high-temperature range, i.e., alpha(C)(573 K) = 7.2 +/- 0.02x10(-6) /K, alpha(A)(573 K) = 5.7 +/- 0.2x10(-6) /K, and alpha(M)(573 K) = 5.8 +/- 0.2x10(-6) /K,were relatively close to the reported values, the thermal-expansion coefficients along the C axis in the low-temperature range, i.e., alpha(C)(298 K) = 5.3 +/- 0.02x10(-6) /K, was significantly larger than the reported values. Thermal conductivities parallel and perpendicular to the C axis were almost the same, i.e., 2.0 +/- 0.1 W cm(-1) K-1. This value is close to the values reported on the low-dislocation density part of epitaxially laterally overgrown GaN. This could indicate that the defect density of our crystals is very low and that the defects are distributed uniformly throughout the volume. The wavelength dependence of the refractive index and the optical-absorption coefficient was also investigated. (c) 2005 American Institute of Physics.