Abstract
We present a comprehensive study of the optical characteristics of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ epilayers $(0l~xl~0.6)$ by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. For ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ with large Al content, we observed an anomalous PL temperature dependence: (i) an ``S-shaped'' PL peak energy shift (decrease-increase-decrease) and (ii) an ``inverted S-shaped'' spectral width broadening (increase-decrease-increase) with increasing temperature. We observed that the thermal decrease in integrated PL intensity was suppressed and the effective lifetime was enhanced in the temperature region showing the anomalous temperature-induced emission behavior, reflecting superior luminescence efficiency by suppressing nonradiative processes. All these features were enhanced as the Al mole fraction was increased. From these results, the anomalous temperature-induced emission shift is attributed to energy tail states due to alloy potential inhomogeneities in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}\mathrm{}$ epilayers with large Al content.
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