Abstract
Due to their wide potential applications, the wide bandgap semiconductors gallium nitride (GaN) and zinc oxide (ZnO) have been intensively researched for decades. Despite the intensive researches, some basic properties of them remain unclear. Here we report utilizing unconventional experimental conditions of below bandgap photon-excitation of photoluminescence (PL) spectroscopy to reveal defect states and nonlinear optical properties of GaN and ZnO. Using just below bandgap photon-excitation, negative thermal quenching (NTQ) phenomenon was observed for yellow luminescence of different GaN samples, that containing InGaN quantum well structure, Si-dopant and unintentionally induced dopants respectively. The NTQ phenomenon revealed existences of the carrier thermal-emission from quantum well, the Si-induced deep defect level in GaN, and the multiple unintentionally induced deep levels in GaN. With two-photon excitation, by changing the PL spot position inner the ZnO crystal, absorption coefficients of near-band-edge PL components and Urbach tail depth were detected. An interesting self-focusing phenomenon of laser propagation in ZnO was observed, which is caused by third-order nonlinear optical susceptibility of ZnO.
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