An insulator layer for single crystal gallium arsenide substrates in which the insulator layer is compliantly matched with the substrate and the insulator layer is free of defects causing surface roughness and crystalline defect problems which, otherwise, could impair device performance. To accomplish this, the insulator layer is formed on a gallium arsenide substrate as an integral composite or variegated structure including (a) a uniform homogenous film of Group IIa metal atoms attached directly onto a gallium arsenide substrate surface in the form of a monolayer, and (b) a single crystal epitaxial film of a Group IIa metal fluoride deposited on the monolayer.