Publications

Optical Impedence Spectroscopy of GaN/AlGaN high electron mobility transistor structures (HEMTs) using photoreflectance exhibit a photoreflectance lags - the component of the modulated reflectance out of phase with the chopper - ranging from 0.1 to 0.5. Photoreflectance was performed using below gap pumping on various samples. Samples that do exhibit appreciable photoreflectance lag for above gap pumping show significantly enhanced photoreflectance signals for below gap pumping. Yet, samples that do not exhibit appreciable photoreflectance lag for above gap pumping do not exhibit a signal for below gap pumping. This implies that the photoreflectance phase lag is due to mid-gap trap states. At least 2 types of traps are found, above and below about 2.5 eV. This result means that that photoreflectance can be used as a probe of HEMT device quality.

High-temperature (high-T) illumination-induced metastability in undoped semi-insulating (SI) GaN grown on a-plane sapphire by metalorganic vapor phase epitaxy has been studied using thermally stimulated current (TSC) spectroscopy, photocurrent (PC) and persistent photocurrent (PPC) measurements. The metastability can be induced by illumination at 390>T>300K (using either white or 360-nm light), followed by cooling the sample to 83 K in the dark. Without high-T illumination, illumination, the SI-GaN sample stays in its normal state ("off" state), and shows at least six TSC traps, B (0.63 eV), Bx (0.51 eV), C1 (0.44 eV), C (0.32 eV), D (0.23 eV), and E (0.16 eV). However, after high-T illumination the sample goes into a metastable state ("on" state), and shows a strong increase in both the PC at 83 K and the TSC of traps D, C, and E, accompanied by significant change in their relative densities. PPC at 83 K in the "on" state lasts much longer than that in the "off" state. Association of possible point defects and dislocations with the metastability behavior will be discussed.