Nondegenerate Optical Pump-Probe Spectroscopy of Highly Excited Group III Nitrides

Abstract

We report the results of nondegenerate optical pump-probe absorption experiments performed on GaN and InGaN thin films and quantum wells under the conditions of strong optical band to band excitation. The evolution of the band edge in these materials was monitored as the number of photoexcited free carriers was increased beyond that required to achieve population inversion and observe stimulated emission. The band edge of InGaN is shown to exhibit markedly different high excitation behavior than that of GaN, explaining in part the reduction in stimulated emission threshold that typically accompanies the incorporation of indium into GaN to form InGaN. A comparison of the band edge absorption changes observed in pump-probe experiments to the gain spectra measured in variable-stripe gain experiments is also given.

A Comparison of the Optical Characteristics of AlGaN, GaN, and InGaN Thin Films

“…The decrease in PLE signal above the PLE peak position of Al 0.17 Ga 0.83 N with increasing excitation energy is due to the decrease in the excitation intensity of a Xe lamp source likely a result of carrier confinement at (large) potential fluctuations in the InGaN layers, a result of difficulties in uniform indium incorporation [3 to 6]. It has also been shown that the below-gap induced absorption present in the gain region of highly excited GaN films is absent in high quality InGaN films [13]. This effect, a result of the incorporation of indium into GaN, also aids in the reduction of I th .…”