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.