Observation of strong many-body effects in thin InN films grown on GaN buffer layers

摘要

InN thin films grown on GaN, AlN, or InGaN/AlN buffer layers investigated recently exhibited two fundamentally new features. First of all, the optical bandgap was as narrow as 660 meV [1] or 600 meV [2]. This value is much narrower than 1.9 eV measured by the earlier researchers. Secondly, residual electron densities in these thin films were relatively high, i.e. on the orders of 10{sup}17-10{sup}19 cm{sup}-3 [2-4]. Here, we present our new results following our studies of the temperature- and intensity-dependent photoluminescence (PL) spectra. We grew two samples of the InN thin films by using molecular-beam epitaxy. Both of the films were deposited on the 4-μm-thick semi-insulating GaN buffer layers. These buffer layers were directly grown on the (0001) sapphire substrates. Samples A and B have the film thicknesses of 1270 nm and 700 nm with the electron concentrations of 6.2×10{sup}14 cm{sup}-2 and 2.5×10{sup}14 cm{sup}-2, respectively, measured at room temperature. The PL spectra for each of the two InN thin films were measured under the pumping of a cw laser beam at 532 nm. The PL signal collected by a positive lens was sent through a spectrometer and then measured by using a photomultiplier tube. Fig. 1 shows the PL spectra of sample A and B at different pump intensities, measured by us at 4.3 K. At low pump intensities the dominant emission peaks occurred at the transition energies of around 676 meV and 662 meV for samples A and B, respectively. As the pump intensity was increased, the peak energy was red-shifted, see Fig. 1. At the highest intensity used in our experiment, i.e. ～ 418 W/cm{sup}2, the peak energy was red-shifted by an amount as large as 51.9 meV and 18.8 meV for samples A and B, respectively. After carefully examining the temperature and intensity dependences of the PL spectra measured by us, see Figs. 1 and 2, we would like to point out that the changes of the line shapes on the low-energy side of the PL peaks were quite different between increasing pump intensities and temperatures. Based on the results of the GaAs structures studied in the past, we can attribute these shifts primarily to the bandgap renormalization [5]. Besides the dominant transition peaks, there was a shoulder on the high-energy side which was located at the transition energy of about 691 meV for sample A and B. As the pump intensity was increased, the shoulders became more pronounced. However, as the pump intensity was increased further, the two shoulders were gradually broadened. In fact, at the high pump intensities the shoulders were almost gone. On the other hand, for the fixed pump intensity as the temperature was increased the two shoulders gradually disappeared. Upon the further examination of the PL spectra, a broad shoulder appeared at the transition energy of 587 meV on the low-energy side for sample A and B, as the pump intensity was increased, see Fig. 1. These two shoulders were most obvious at the highest pump intensity used in our experiment. According to Ref. [6], when the electron density became sufficiently high, the degenerate electrons recombined with the holes bound to interface traps, defects, and/or impurities (similar to Fermi edge singularity observed in modulation-doped InGaAs/InP quantum wells [7]). We believe that the shoulders on the high-energy side are primarily due to the recombination of the degenerate electrons and bound holes. This is consistent with our observation that these shoulders were readily modified by increasing either the pump intensity or the temperature. The dominant emission peaks are originated from the recombination of the free electrons near the bottom of the conduction band and free holes near the top of the valence band. Therefore, the bandgaps of our InN films should be somewhat lower than the peak transition energies of 676 meV and 662 meV, determined from our PL spectra at 4.3 K for samples A and B, respectively. These two values are lower than those measured for the Si-doped