DIRECT OPTICAL ANALYSIS OF THE CARRIER DIFFUSION IN SEMICONDUCTOR WIRE STRUCTURES

The influence of the sidewall quality on the ambipolar carrier diffusi on in semiconductor mesa and wire structures was studied by highly spa tially resolved photoluminescence measurements at room temperature. Th e samples consisting of a single InGaAs quantum well layer with InP ba rriers 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 an d the detection of the luminescence signal, the diffusion of the carri ers can be imaged directly by scanning the detection spot independentl y from the fixed excitation spot. In general, we found decreasing diff usion lengths for decreasing wire widths. This behaviour can be attrib uted to the increasing influence of sidewall recombination as well as to the introduction of non-radiative recombination centres by the samp le preparation, especially the dry etching process for wires of smalle r size. By comparison of deep etched and overgrown structures, these t wo contributions are separated. The saturation of surface states on th e sidewalls was studied by varying the excitation intensity. Using a n umerical model we describe the measured variation of the diffusion len gth with wire size and deduce the sidewall recombination velocities fr om the measured widths of the luminescence intensity distributions.