Evaluation of defect density in bulk gallium nitrides by photothermal deflection spectroscopy and steady-state photocapacitance methods

•Characterization of bulk GaN samples by photothermal deflection spectroscopy (PDS)•Evaluation of defect levels and density in the middle of bandgap of bulk GaN.•Quantitative estimation of defect density by PDS from the comparison with the steady-state photocapacitance spectroscopy.•Advantages of PDS for insulative bulk GaN. Bulk GaN samples were characterized by both photothermal deflection spectroscopy (PDS) and steady-state photocapacitance (SSPC) methods. The PDS signal intensity in the bandgap was found to correlate quantitatively with the defect density estimated by the SSPC method. The defect density of GaN bulks fabricated by hydride vapor phase epitaxy (HVPE) was decreased by controlling the incorporation of carbon and silicon impurities. Differences in the reciprocal of the slope near the valence band maximum and the signal intensity in the bandgap among HVPE GaN bulks could be detected by PDS, although they had the same crystalline quality. PDS can be used to evaluate the GaN bulks that have been improved with a highly insulative property caused by Fe- doping or low carbon incorporation.