Time-resolved photoluminescence measurements of InGaN light-emitting diodes

We have used time-resolved photoluminescence (TRPL), with 400 nm (3.1 eV) excitation, to examine InxGa1-xN/GaN light-emitting diodes (LEDs) before the final stages of processing at room temperature (RT). 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 nonexponential PL kinetics can be described by a stretched exponential function, indicating significant disorder in the material. Typical values for beta, the streching coefficient are 0.45 - 0.6 for bright LEDs, at the PL maxima at RT. We have also used TRPL, applying 267 nm (4.65 eV) excitation, to examine carrier diffusion from the p-type GaN layer into the InGaN/GaN multiple quantum wells. The carrier diffusion occurs on the 3-5 ns timescale and can be measured using the risetime of the InGaN emission, the extracted value for diffusion constant is 0.9 cm(2)/sec (mu = 35 cm(2)/Vs). This value of the mobility is consistent with Hall measurements.