A uniformity investigation of undoped, semi-insulating GaAs grown by the vertical Bridgman technique

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Abstract
The authors have carried out a uniformity investigation of undoped, semi-insulating GaAs grown by the vertical Bridgman method. They have employed A/B etching, EL2 mapping and near-bandgap absorption (reverse contrast, RC) mapping to reveal dislocations and non-uniformities in concentrations of two specific point defects: neutral EL2 centres and another defect whose identity is unknown but can be revealed by optical absorption imaging when the sample is cooled below 150 K and viewed with light of energy within 50 meV of the bandgap energy. The RC images were obtained with the sample at about 80 K mounted in a cryostat, using light of 830 nm wavelength obtained with a narrow-bandpass interference filter. The EL2 mapping was performed with the sample at room temperature with light of 900 nm wavelength. In both cases a sample thickness of about 500 mu m was employed. The authors demonstrate that although the dislocation density is over an order of magnitude less than that of conventional LEC GaAs (about 103 cm-2), most dislocations are formed into polygonised structures with cells of the order of 1-2 mm in diameter. In addition, concentrations of both EL2 and the defects that give rise to the RC image are very non-uniform in the vicinity of the cell walls. In contrast to conventional LEC GaAs the EL2 absorption near dislocations is much higher than the RC absorption.