Fabrication and Characterization of Si Substrate-Free InGaN Light-Emitting Diodes and Their Application in Visible Light Communications
Visible light communications with InGaN-based light-emitting diodes (LEDs) grown on large-diameter (6-inch) and cost-effective Si (111) substrates are investigated experimentally. During epitaxial growth, the transition layers consisted of the step-graded AlGaN buffers incorporated with three low-temperature-grown (∼900 °C) AlN interlayers on AlN/Si substrates that are used to compensate for thermally induced tensile stress and to maintain a reasonable crystalline quality of GaN-on-Si LEDs. Strong light absorption from Si can be prevented by fabricating a Si substrate-free InGaN LED with a composite metal coating of Al/Ag/Al multilayer, providing improved adhesive strength and reflectivity comparable to the unitary Ag film. In comparison with GaN-on-Si LEDs, stripping Si substrates combined with the use of a highly reflective bottom mirror (Al/Ag/Al multilayer) reflected a more intense emission pattern corresponding to a 2.2 times (@ 190 mA) increase in light output power in thin-film LEDs. In addition, a 1.8 times (@ 160 mA) increase in optical channel bandwidth is achieved by using thin-film LEDs as optical transmitters. A direct line-of-sight optical link using the proposed thin-film LEDs achieved data transmission rates of up to 100 Mb/s over a distance of 100 cm, indicating that the proposed LEDs have potential for use as optical transmitters in indoor visible light communications.
An approach to determine the chemical composition in InGaN/GaN multiple quantum wells
In this paper, an approach to determine the average In composition in InGaN/GaN multiple quantum wells (MQWs) was suggested. The period thickness and the average lattice constants (〈aepi〉 and 〈cepi〉) of MQWs were calculated from high-resolution X-ray diffraction (HRXRD). Then by applying the Vegard's law, the average In composition 〈x〉 was simultaneously determined by considering the relationship between the lattice constants under strain. At the same time, by using the period thickness, the average In composition was obtained from the simulation of the Rutherford backscattering spectroscopy (RBS). The results obtained from the two techniques are agreeable. So the approach of combining both HRXRD and RBS can tell reliable chemical composition in InGaN/GaN MQWs.
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 photoluminescence peak of 850 °C 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 °C. In Raman scattering spectra, InGaN/GaN MQWs film without sapphire substrate has a lower E2 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. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)