Electron concentration dependence of exciton localization and freeze-out at local potential fluctuations in InN films

InN films with electron concentration ranging from n similar to 10(17) to 10(20) cm(-3) grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of these InN samples with electron concentration above 10(18) cm(-3) show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions, the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature. The exciton localization energy sigma (loc) is found to follow the n (5/12) power relation, which testifies to the observed strong localization effects in InN with high electron concentrations.

AN ANALYSIS OF CHARGE STATES FOR LOCAL VIBRATIONAL-MODES BY OXYGEN DEFECTS IN SEMI-INSULATING GAAS

The high-resolution spectral measurements for new local vibrational modes near 714 cm-1 due to the oxygen defect in semi-insulating GaAs are analyzed on the basis of a model calculation by self-consistent bond orbital approach. Two charge states of oxygen atom with 1 and 2 extra electrons are assigned to be responsible for these local modes. The observed frequencies are explained by the properties of Ga-O-1 and Ga-O-2 bonds and the calculated cohesive energy indicates that the O-2 state is stable. The results are in good agreement with the kinetic analysis.

RELATION BETWEEN THE METASTABILITY OF EL2 AND THE PHOTOSENSITIVITY OF LOCAL VIBRATIONAL-MODES IN SEMIINSULATING GAAS

Recent infrared spectroscpic observations of local vibrational mode absorptions have revealed a number of photosensitive centers in semi-insulating GaAs. They include (OVAs) center which has three modes at 730 cm(-1) (A), 715 cm(-1) (B), and 714 cm(-1) (C), respectively, a suggested NH center related to a line at 983 cm(-1) (X(1)), and centers related to hydrogen, such as (H-O) or (H-N) bonds, corresponding to a group of peaks in the region of 2900-3500 cm(-1). The photosensitivity of various local vibration centers was observed to have similar time dependence under near-infrared illumination and was suggested to be due to their charge-state interconversion. Mainly described in this work is the effect of the 1.25-eV illumination. It is confirmed that this photoinduced kinetic process results from both electron capture and hole capture, which are closely related to the photoionization behavior and metastability of the EL2 center.