Optical spectroscopy and composition of InGaN

Commercial light emitting devices (LEDs) containing InGaN layers offer unrivalled performance in the violet (approx. 400 nm), blue (approx. 450 nm) and green (approx. 520 nm) spectral regions. Nichia Chemicals Company has also produced amber InGaN LEDs with peak output near 590 nm. Here, we predict, on purely theoretical grounds, a surprisingly high limiting value of 1020 nm (peak) for InGaN intrinsic emission. We partly confirm this prediction by spectroscopic measurements of samples with photoluminescence (PL) peaks between 370 nm and 980 nm. In addition, we have measured the indium content of a range of light-emitting layers, using Rutherford Backscattering Spectrometry (RBS), Extended X-ray Absorption Fine Structure (EXAFS) and Energy Dispersive X-Ray Analysis (EDX). The PL peak energy is found to depend linearly on the indium fraction: violet-emitting layers have an indium content of approx. 8%, blue layers approx. 16% and green layers approx. 25%. A linear extrapolation to the limit set by the Stokes' shift prediction, mentioned earlier, yields a limiting indium concentration of only approx. 52%. The profound impact of these results on future extensions of nitride technology and current theoretical models of InGaN is briefly discussed.