Peculiarities in the pressure dependence of photoluminescence in InAlN

Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of In_xAl_1-xN layers are presented. The measured evolution of PL energy (E_PL) with x is characterized by a clear decrease of E_PL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes E_G with x. However, values of E_PL are clearly lower than E_G, for 0<x<0.3. For higher x, the measured E_PL follows well the calculated E_G. The experimentally determined pressure coefficient of PL energy (dE_PL/dp) shows a complicated behavior for alloys with different In-content. We found a strong reduction of dE_PL/dp for 0<x<0.3 and a relatively constant magnitude of this coefficient for higher x. Moreover, for the lower x region, we observed dE_PL/dp that can differ even by a factor two in samples with nominally very similar In-content. The general tendency in dE_PL/dp evolution with x corresponds to lower values than calculated dE_G/dp for alloys with non-uniform indium distribution. We propose two not necessary independent explanations of these experimental findings. First, due to non-uniform In distribution (induced, e.g. by defects or non-homogeneous strain) both E_PL and dE_PL/dp are reduced. Second, a similar behavior results from an involvement of the localized states, whose formation and contribution to PL can be induced by strain and/or native defects. In both hypotheses, the strain/defect density can significantly change around x≈0.18 where In_xAl_1-xN layers are lattice matched to GaN template.