Investigations of disorder in InGaN light-emitting diodes

We have used time-resolved photoluminescence (PL), with 400nn (3.1 eV) excitation, to examine InxGa1-xN/GaN light-emitting diodes (LEDs) before the final stages of processing at room temperature. We have found dramatic differences in the time-resolved kinetics between dim, bright and super bright LED devices. The lifetime of the emission for dim LEDs is quite short, 110 +/- 20 ps at photoluminescence (PL) maximum, and the kinetics are not dependent upon wavelength. This lifetime is short compared to bright and super bright LEDs, which we have examined under similar conditions. The kinetics of bright and super bright LEDs are clearly wavelength dependent, highly non-exponential, and are on the nanosecond time scale (lifetimes are in order of 1 ns for bright and 10 ns for super bright LED at the PL max). The non-exponential PL kinetics can be described by a stretched exponential function, indicating significant disorder in the material. Stretched exponential lifetimes are consistent with a distribution of lifetimes. Typical values for beta, the stretching coefficient, are 0.45 - 0.6 for bright LEDs, at the PL maxima at room temperature. We attribute this disorder to indium alloy fluctuations. From analysis of the stretched exponential kinetics we estimate the potential fluctuations to be approximately 75 meV in the super bright LED. Assuming a tunneling based hopping mechanism, the average distance between indium quantum dots in the super bright LED is estimated to be 20 Angstrom.