Temperature dependence of charge carrier diffusion in GaN and AlGaN layers

The development of ultraviolet AlGaN-based LEDs and laser diodes focuses on efficiency and lifetime. Efficient light generation and the reduction of nonradiative losses in the active layers are key factors in this optimization. However, recombination in AlGaN is a complex interplay of various factors: carrier localization due to local bandgap fluctuations, high defect densities, strong exciton binding, composition- and strain-dependent switching of the valence band ordering, as well as spontaneous and piezoelectric polarization fields in heterostructures. The diffusion of charge carriers determines the probability that they will reach nonradiative recombination centers in the material and thus become lost for light generation. We have compared the temperature-dependent charge carrier diffusion in GaN and AlxGa1−xN (x ≈ 0.5) layers using photoluminescence and cathodoluminescence measurements. Under conditions of low excitation power, the diffusion length in AlGaN is significantly shorter than 200 nm due to compositional and bandgap fluctuations. It is at least 2 times shorter than in homogeneous GaN. In addition, the diffusion length in GaN decreases from cryogenic temperatures to room temperature, which is typical for a homogeneous semiconductor. Regarding AlGaN, carrier diffusion increases in the same temperature range. The thermal activation of carrier diffusion in the presence of strong bandgap fluctuations seems to be more important in AlGaN layers than diffusion-limiting effects like phonon scattering and shorter effective recombination times at higher temperatures. This behavior contributes to a steep decrease in light-generation efficiency in AlGaN layers around room temperature, which is not similarly present in GaN layers.