Comparative optical studies on violet InGaN quantum well laser diode structures grown on sapphire and GaN substrates

We have investigated the optical properties of violet InxGa1-xN/InyGa1-yN multiple quantum well laser diode structures grown on sapphire and GaN substrates by means of photoluminescence (PL), PL excitation, and time-resolved PL spectroscopy. The PL result shows a red-shift behavior of the emissions from GaN capping layer and the 10-nm-thick InyGa1-yN barriers on the GaN substrate due to the reduction of residual compressive strain. On the other hand, we have observed a blue shift of the PL emission from the 4-nm-thick InxGa1-xN wells for the samples grown on GaN substrates. This can be attributed to the reduction of strain-induced piezoelectric field between InxGa1-xN wells and InyGa1-yN barriers due to the direct growth on GaN substrates, which is confirmed by a reduction of the Stokes-like shift between InxGa1-xN PL and its absorption edge and a decrease in overall decay lifetimes for the samples grown on GaN substrates. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)