Direct optical analysis of the carrier diffusion in semiconductor wire structures

The influence of the sidewall quality on the ambipolar carrier diffusion in semiconductor mesa and wire structures was studied by highly spatially resolved photoluminescence measurements at room temperature. The samples consisting of a single InGaAs quantum well layer with InP barriers were structured by electron-beam lithography and subsequent dry etching with a mixture of CH4-H-2-Ar. As our luminescence measurement set-up allows high spatial resolution for both the excitation beam and the detection of the luminescence signal, the diffusion of the carriers can be imaged directly by scanning the detection spot independently from the fixed excitation spot. In general, we found decreasing diffusion lengths for decreasing wire widths. This behaviour can be attributed to the increasing influence of sidewall recombination as well as to the introduction of non-radiative recombination centres by the sample preparation, especially the dry etching process for wires of smaller size. By comparison of deep etched and overgrown structures, these two contributions are separated. The saturation of surface states on the sidewalls was studied by varying the excitation intensity. Using a numerical model we describe the measured variation of the diffusion length with wire size and deduce the sidewall recombination velocities from the measured widths of the luminescence intensity distributions.