Lead-tin telluride double-heterojunction laser diodes: Theory and experiment

Abstract

Calculations of threshold current densities for lead-tin telluride double-heterojunction (DH) laser diodes are presented, in which the spatial distribution of the injected minority carriers and the critical level of minority carrier injection necessary for population inversion are considered. Excellent agreement between the predictions of this model and experimental data is obtained, which suggests that credible predictions of high-temperature performance can be made.The need to inject a critical level of minority carriers is shown to be the principal contribution to the threshold current density except at very low temperature and to lead to a rapid increase in threshold with increasing temperature. It is found experimentally that below 40 K the threshold no longer decreases with temperature; two possible explanations for this behavior are suggested. INTRODUCTIONT HE IMPORTANCE of the double-heterojunction (DH) device geometry, see Fig. 1, in reducing semiconductor diode laser threshold current densities has been well documented in the (Ga, A1)As alloy system [ l ] . Heterojunction geometries have also very recently been used in Pb-salt diode lasers, specifically in the Pb(S, Se) [Z] and (Pb, Sn)Te [3] -[5] alloy systems. Significant systematic reduction of threshold current densities in DH (Pb, Sn)Te laser diodes has recently been demonstrated by Tomasetta and Fonstad [ 3 ] and continuous operation of (Pb, Sn)Te laser diodes at 77 K has been achieved by Groves et al. with the aid of the DH geometry and careful heat sinking [SI .While experimental progress in Pb-salt laser diodes has been dramatic, theoretical calculations of threshold currents have been less successful. Published calculations [ 6 ] predict threshold current densities much lower than those observed, and furthermore predict a much more gradual increase of threshold with temperature than one in fact observes. These discrepancies are important to resolve because without a credible model it is impossible to understand the limiting device and materials parameters, or to predict ultimate device performance, i.e., threshold current density and efficiency.In this paper we will first look at the limitations of the simple model and point out where it is invalid for DH (Pb, Sn)TeDistance, x 2 2 Fig. 1. An ideal, symmetrical DH lead-tin telluride laser diode and the variation of energy gap and refractive index perpendicular to the junction plane. The numbers indicated are for 77 K and typical of those used in this work. The improvement in performance of the DH geometry in comparison with the simple p-n junction device is a result of optical and minority carrier confinement that occurs at the heterojunctions. The three-layer dielectric waveguide structure helps to keep the optical field confined to the higher index active region where the optical field aids stimulated emission. The p-p heterojunction creates an electrical barrier to minority carriers thereby requiring their recombination in the center higher index active region where the optical field and min...