Recombination Dynamics in III-Nitride Quantum Wells of Various Crystal Orientations

Abstract

The present research focuses on the recombination dynamics in III-nitride quantum wells, which represent the key structures for modern solid state lighting. In particular, the recombination processes in GaInN/GaN quantum wells of various crystal orientations are measured by time-resolved photoluminescence spectroscopy, which allows insight into basic material properties and loss mechanisms. On the one hand, the results emphasize the effects of strain and well width on the radiative recombination in quantum wells in nonpolar orientations. Compared to c-plane structures, where the radiative recombination is dominated by the quantum-confined Stark effect, nonpolar quantum wells show significantly increased radiative emission. This can be explained by a combination of the vanishing piezoelectric polarization, an enhanced exciton binding energy, and, beyond that, the anisotropic in-plane strain that leads to higher effective hole masses. Introducing a partially relaxed AlInN buffer layer reduces the impact of strain, but still yields enhanced radiative recombination compared to c-plane structures. Additionally, the radiative emission of nonpolar quantum wells is decreasing significantly for larger well widths, which is related to forbidden dipole transitions in the field-free quantum well. Furthermore, a drastic reduction of the radiative rate is found towards higher temperatures for quantum wells of all orientations, related to the loss of charge carriers into the barriers, or even exciton dissociation. On the other hand, strain is found to be related to defect-related nonradiative recombination in semipolar quantum wells. Previously, a similar effect was observed for c-plane quantum wells. By introducing a strain-reducing, partially relaxed AlInN buffer layer, the nonradiative recombination can be reduced significantly, which opens a perspective to overcome the green gap. Finally, a reliable determination of the internal quantum efficiency is introduced. Modeling the characteristic temperature dependence of radiative and nonradiative lifetimes over the whole temperature range, a safe indicator for the absence of nonradiative recombination at cryogenic temperatures is developed. Namely, a synchronous rise of the effective and radiative carrier lifetimes with temperature indicates an internal quantum efficiency of 100%. Thereby, the measurement of absolute internal quantum efficiencies becomes possible.