X-ray scattering studies of compound semiconductors

Abstract

In this thesis, techniques of high resolution x-ray diffraction, topography and grazing incidence reflectivity have been employed in order to gain information on compound semiconductors. A recent growth technique. Vertical Gradient Freeze (VGF), has been investigated for 2" InP wafers, and been found to produce virtually dislocation- free crystals. In the one wafer where dislocations have been imaged, they have a density of ~200cm(^-2), with Burgers vectors lying in the plane of the wafer. This is in contrast to topographs of Liquid Encapsulated Czochralski (LEG) InP, where a dislocation density of up to 6.10'(^4)cm(^-2) was observed at the wafer periphery. No growth striations were observed in VGF samples, implying a planar solid-melt interface. VGF GaAs wafers are seen to be virtually dislocation free when Si doped, have dislocation densities of 900cm(^-2) when undoped and 1200cm(^-2) when Zn doped. Triple axis diffraction measurements showed a variation in tilts between/samples, but no strain variation. The tilt variation was attributed to the polishing process. Asymmetric scans showed a variation in strain at high tilts. These data have been used to form a model for the crystal surface: mosaic blocks of perfect crystal surrounded by low angle boundaries consisting of rows of edge dislocations. During the polishing process, these blocks are physically rotated, evidence for which is supplied from analysis of the specular part of reflectivity scans. Specular and diffuse reflectivity scans on InP substrates have been simulated using the Distorted Wave Born Approximation (DWBA). In all cases a 30Å thick oxide layer was identified on the sample surface. In order to obtain a good simulation for transverse scans at two values of q(_z), it was necessary to include a grading in electron density at the top surface. Epitaxial layers of Hg(_1-x)Mn(_x)Te (MMT) grown by the Interdiffused Multilayer Process (IMP) on GaAs with a CdTe buffer layer have been characterised using double and triple axis diffraction. Although reasonable compositional uniformity was observed across the wafers (from 0.3%mm(^-1)), dynamical simulations of pseudo-triple axis scans showed a grading in composition with depth. It was observed that the crystalline perfection deteriorated with increased Mn fraction. The MMT and CdTe layers were almost fully relaxed, and were found to have dislocation densities of l0(^7)-l0(^9)cm(^-2). In one sample the presence of zinc blende MnTe was established using double axis diffraction. Finally, the high intensity of the European Synchrotron Radiation Facility (ESRF) has been exploited in order to topograph highly absorbing materials. The effect of heater failure in the growth of GaAs in space has been shown to produce high levels of strain and twinning. It has also been shown topographically that contact with the crucible during the growth of GaInSb from the melt leads to increased strain, so de-wetting phenomena improve crystalline growth.