Carrier Recombination Dynamics of Al x Ga 1−x N Epilayers Grown by MOCVD

We present a comprehensive study of the optical characteristics of Al x Ga 1−x N epilayers by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. All Al x Ga 1−x N epilayers were grown by metalorganic chemical vapor deposition and the Al mole fraction ( x) was varied from 0 to 0.6. We observed that (i) the full width at half maximum of the PL emission, (ii) the energy difference between the PL emission peak energy and the PLE absorption edge, and (iii) the effective lifetime increase with increasing x . These facts indicate that degree of band-gap fluctuation due to a spatially inhomogeneous Al alloy content distribution increases with increasing x . We observed anomalous temperature-induced emission shift behavior for Al x Ga 1−x N epilayers, specifically, an S-shaped (decrease-increase-decrease) temperature dependence of the peak energy with increasing temperature. This anomalous temperature-dependent emission behavior was enhanced as the Al mole fraction was increased. Since the band-gap fluctuation in Al x Ga 1−x N epilayers due to inhomogeneous spatial variations of the Al content increases with increasing Al content, we believe that band-gap fluctuation causes the PL peak energy to deviate from the typical temperature dependence of the energy gap shrinkage. Therefore, the anomalous temperature-induced emission shift can be attributed to energy tail states due to alloy potential inhomogeneities in the Al x Ga 1−x N epilayers with large Al content.