Far-infrared studies of central-cell structure of shallow donors in GaAs and InP

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Abstract
Far-infrared laser spectroscopy is used to study the central-cell structure from the shallow donors in GaAs and InP. In the case of GaAs up to five donor species can be found in VPE material and one of these has a negative chemical shift. It is suggested that tellurium is the shallowest residual donor found in LPE material. With InP a variety of VPE, MOCVD and bulk (LEC) samples are studied. It is believed that up to twelve shallow donor species are commonly present in high purity in InP. One of the donors found in bulk-grown material has a substantial negative chemical shift. Samples back-doped with silicon, sulphur, tin and germanium are studied and, on the basis of these back-doping experiments, four of the common residual donors are identified. Hydrostatic pressure is used as an additional variable in certain of these experiments to improve the experimental resolution, both by increasing the separation of the central-cell components and by acting to sharpen the lines. In the case of InP the increase in separation of the individual donors is too great to be accounted for by the increase in mass, indicating either an increase in the dielectric constant or an interaction with donor states resonant with the conduction band.