VACANCY ORDERED STRUCTURE OF THE III2VI3 COMPOUND SEMICONDUCTOR (GAXIN1-X)(2)SE-3 STUDIED BY ELECTRON-DIFFRACTION AND MICROSCOPY

The crystal structure of (GaxIn1-x)(2)Se-3 (x = 0.08-0.40), as a repre sentative of III2VI3 compound semiconductors grown by a vapour transpo rt technique, has been studied by electron diffraction and high-resolu tion electron microscopy. The crystal structure is a defect wurtzite, in which 1/3 of the cation sites are vacant. Such structure vacancies were ordered to form screws along the c-axis. The space group is P6(1) or P6(5) and the lattice constants are a = root 3 a(w) = 6.92 Angstro m, c = 3 c(w) = 18.87 Angstrom, Z = 6 at the composition x = 0.38, whe re the suffix w denotes the axes referred to the wurtzite structure. T his suggests that the lattice mismatch between the c plane and the (11 1) plane of GaAs becomes zero. The c(w)/a(w) value is equal to 1.573, being much smaller than 1.633, the ideal value of the wurtzite structu re. Systematic displacements of atoms from the atomic positions of the ideal wurtzite structure were simply characterized by the displacemen ts of the 2-coordinated Se atoms around the vacancies: 30 degrees rota tion of the 2-coordinated Se atom toward the [001] direction and 9% sh rinkage of the bond length between the 2-coordinated Se atom and the t wo neighbouring group-III atoms.