Carrier diffusion length in GaN revisited

04.04.2022

A figure of merit for the potential performance of any bipolar semiconductor device is the carrier diffusion length. For the technologically secondmost relevant semiconductor GaN, popular ways to determine this quantity using cathodoluminescence (CL) have been found to be based on incomplete physical models. This background motivates us to take a fresh, in-depth look on how to determine the carrier diffusion length in GaN using CL spectroscopy.

A prerequisite for carrying out such a quantitative CL study is an accurate knowledge on the spatial extent of the generation volume, which determines the spatial resolution. Contrary to common belief and theoretical predictions, we find experimentally that the generation volume strongly depends on temperature. We identify electron-phonon scattering during the energy relaxation of hot carriers as the underlying physical mechanism. These results are of importance also for other techniques relying on the interaction of high-energy electrons with solids used in imaging, analysis, and lithography.

This newly established knowledge on the generation volume allows a reliable analysis of carrier diffusion processes in semiconductors by CL. We can thus establish a complete physical understanding of carrier and exciton diffusion in GaN and obtain quantitative values for the diffusion length and diffusivity across the full temperature range from 10-300 K. The methodology we introduce in this work is suitable to investigate carrier diffusion in a wide range of materials and structures, thus facilitating the comprehension of the mechanisms governing the performance of future devices based on nanostructures.

In a final part of the work, we establish a complementary method for measuring the carrier diffusion length at the outcrops of threading dislocations that validate our previous results. As devices based on GaN are plagued by a high density of threading dislocations, elucidating their role for light emission in layers and heterostructures is another crucial issue. Our work also addresses the fact that little experimental evidence exists for the nonradiative activity of threading dislocations in the bulk as opposed to at the surface. In this context, we demonstrate that the mechanisms governing the impact of threading dislocations on carrier recombination depend strongly on depth.