Photoluminescence from InGaN/GaN MQWs on sapphire and membranes fabricated by laser lift-off

Photoluminescence and Raman scattering spectra of InGaN/GaN MQWs on sapphire and membranes free of substrate fabricated by laser lift-off have been studied. It is observed that photolummescence peak of 850 degreesC annealed sample red-shifts from that of as grown sample, while in the case of membrane samples, the luminescence peak blue-shifts when annealed at 700 degreesC. In Raman scattering spectra, InGaN/GaN MQWs film without sapphire substrate has a lower E-2 mode frequency (569.3 cm(-1)) than that of the films with substrate (570.8 cm(-1)), which indicates that compressive stress in the films releases partially when the sapphire substrate is taken off. It is believed that the piezoelectric field decrease leads to the blue-shift in luminescence spectra. Compared with the samples with sapphire substrate, the free-standing membranes showed blue-shift of luminescence peak after relatively low temperature annealing, because the piezoelectric field reduced more easily in the films without substrate.