Comparison of terahertz emission from N-face and In-face indium nitride thin films

Narrow band gap semiconductors are attractive as emitters of terahertz radiation when optically excited with femtosecond laser pulses. We present a comparison of THz emission from N-face and In-face indium nitride (InN) thin films. The InN samples are optically pumped with a 160 fs laser pulse at 800nm. The subsequent THz radiation is detected by ultrafast electro-optic sampling using a 2mm thick ZnTe crystal. The measured In-face InN films have a Hall mobility of 838 cm$^{2}$/Vs and 2098 cm$^{2}$/Vs, and the measured N-face InN samples have a Hall mobility of 645 cm$^{2}$/Vs and 1460 cm$^{2}$/Vs. For both polarities, we show an increase in THz power from InN with higher mobilities. However, THz radiation from the In-face InN sample with a Hall mobility of 2098 cm$^{2}$/Vs is lower in power than from the N-face InN film with a lower Hall mobility of 1460 cm$^{2}$/Vs. We attribute the lower THz power from In-face InN samples to lower crystalline quality of the In-face material, as determined by x-ray, TEM and photoluminescence measurements. The ratio of the defect density and the PL intensity between the In-face and N-face materials is approximately 3 and 10, respectively.