Structure and orientation of As precipitates in GaAs grown at low temperature by molecular beam epitaxy

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Abstract
The structure and orientation relationships of the precipitates found in arsenicrich GaAs layers grown at low temperature by molecular beam epitaxy after annealing have been elucidated by high resolution electron microscopy, transmission electron diffraction and computer simulation of the diffraction patterns. These small precipitates are hexagonal arsenic with an orientation relationship which is of high symmetry (3m). Therefore, symmetry group theory predicts the existence of four orientation variants which are indeed experimentally observed. This orientation relationship usually provides the lowest interfacial and elastic energies obtainable by the precipitates within this matrix. The absence of well-defined facets in the interfacial regions seems to indicate that the hexagonal structure may not be the initial nucleation structure. Other orientation relationships are found in some precipitates but they are identified as ‘accidental’ and then thought to be subject to experimental (growth and annealing) conditions. We have also used the moiré fringe technique to obtain evidence of strain distribution within precipitates. It has been found that the strain within precipitates is strongly anisotropic, leading to lattice spacing compression in some directions and expansion in other. Lattice spacings vary by up to 3% near the interfacial regions. The resulting matrix stress in the surrounding regions can be easily relaxed from one variant to the other through a 90[ddot] rotation along a GaAs direction.