Diffusion lengths of carriers in n- and p-type ZnMgSSe cladding layers of green laser diodes

Abstract

We have used cross sectional cathodoluminescence microscopy as a fast and nondestructive tool to characterize II–VI based green laser diodes. We find evidence for carrier mediated excitation of semiconductor layers that are not directly irradiated by the focused electron beam, from which diffusion lengths of lower mobility carriers (presumably holes) can be estimated. We find that N-doped (p-type) ZnMgSSe exhibits a very low (near) band edge luminescence efficiency. The diffusion length of minority carriers in p-type ZnMgSSe:N [(1–2)×1017 cm−3 net acceptor concentration] was found to be lower than for n-type ZnMgSSe:Cl with roughly equal dopant concentration. The diffusion length of minority carriers in n-type ZnMgSSe:Cl decreases from 0.21 μm for a doping level of (1–2)×1017 cm−3 to <0.05 μm when the n-type doping is increased to (2–4)×1018 cm−3. This decrease in diffusion length is accompanied by an increase of a broad luminescence band around 550 nm, which is attributed to Cl-related defects in the gap. The effective probe size in our cross sectional CL is close to the waist diameter of the focused electron beam. This phenomenon is discussed with regard to the sample geometry and its implication for the determination of carrier diffusion lengths.