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.

Energy relaxation during hot-exciton transport in quantum wells: Direct observation by spatially resolved phonon-sideband spectroscopy

“…Up to now, this classical diffusion has been assumed to be the dominant process in excitonic transport in QWs. [5,6,7,8,9,10,11,12] In this letter, we will show that generally the second regime, i.e., the transport during relaxation, dominates the excitonic transport. Such a process is composed of the simultaneous evolution of the exciton distributions in energy space and real space.…”

“…[5,6,7,8] In investigations of ZnSe based QWs, diffusion length and diffusivity of excitons were also deduced based on diffusion equations. [9,10,12] In the InGaN/GaN system, the transport within 1 µm was analyzed by neglecting relaxation/thermalization processes. [11] Our experiment shows clearly that in the case of nonresonant excitation, the excitons remain hot and nonthermal on a length scale of several micrometers, thus the transport is not a classical diffusion.…”

Diffusion lengths of excited carriers in CdxZn1−xSe quantum wells

“…For example, Gautron and Lemasson 2 determined roomtemperature carrier diffusion lengths in n-type ZnSe to be 1.2, 0.15, and 0.017 m for free carrier concentrations of 2 ϫ10 15 , 2ϫ10 16 , and 1ϫ10 18 cm 3 , respectively. Recently, the diffusion lengths of excited carriers in ZnMgSSe, used as cladding layers in II-VI based blue-green lasers, have been determined at room temperatures by Snoeks et al 9 Diffusion lengths of 0.21 and 0.03 m were determined for Cl doping levels ͑n type͒ of 1ϫ10 17 and 2ϫ10 18 cm 3 , respectively. In the present study, the diffusion lengths for excited carriers confined in CdZnSe multiple quantum wells ͑MQWs͒ were measured between 8 K and room temperature using cathodoluminescence line ͑CL͒ scan measurements.…”