Abstract: An indium phosphate semiconductor substrate is prepared for subsequent growth of epitaxial layers to form a semiconductor device. In the preparation, the substrate is first annealed to promote any tendency for surface accumulation of impurity atoms by diffusion from the substrate and to promote impurity atom removal from the surface of the substrate. The substrate is then surface etched to remove further impurities and to provide a clean, flat surface for subsequent epitaxial layer growth. The final stage of preparation involves growing a semi-insulating buffer layer on the substrate to isolate the device epitaxial layers from the substrate.