Effect of Substrate Imperfections on GaAs Injection Lasers Prepared by Liquid-Phase Epitaxy

Abstract

A study was made of the correlation between substrate flaws and the performance of GaAs injection lasers fabricated by liquid-phase epitaxy. These devices are p+/n structures where the p+ region is deposited epitaxially, followed by heat treatment to displace the p-n junction about 1–2 μ into the n-type substrate. The melt-grown substrates were studied with the aid of transmission electron microscopy, infrared transmission, x-ray topography, etching studies, and photoluminescence. It is concluded that the dislocation density of the substrate is not the controlling factor in laser performance unless it exceeds 105 cm−2. A dislocation density of 103–104 cm−2 is not excessive. Precipitates and impurity striations in the substrate are probably the most damaging imperfections. In Te-doped GaAs the presence of Ga2Te3 precipitates is to be avoided. In the case of Si-doped materials, the precipitation problem appears to be less severe. Nevertheless, small clusters have been detected by electron transmission microscopy in the back portion of some melt-grown ingots (furthest from the seed end). Their nature is unknown but they severely degrade laser performance. We show that a good correlation exists between the relative photoluminescence efficiency and laser performance. Thus, photoluminescence measurements are useful in the selection of Si-doped substrates.