CRYSTAL-STRUCTURES AND PHASE-TRANSFORMATION IN IN2SE3 COMPOUND SEMICONDUCTOR

Crystal structures and phase transformation in In2Se3 compound semicon ductor have been studied by electron diffraction, high resolution elec tron microscopy and X-ray diffraction (XRD) together with optical abso rption measurements. The time-temperature-transformation (TTT) diagram reveals that there exist only two phases in In2Se3 and the transforma tion temperature is 853 K. The transformation from the high temperatur e phase to the low requires a long incubation time for crystal nucleat ion and a relatively high temperature for crystal growth. The low and high temperature phases are the vacancy ordered in screw form (VOSF) p hase and the layer structure phase, respectively. Both phases possess semiconducting optical properties and are constructed on the basis of a tetrahedral bonding structure. The VOSF phase is of a defect wurtzit e structure, in which vacancies on 1/3 of the cation sites are ordered in screw form along the c-axis. The space group is P6(1) or P6(5) wit h a = 7.14 Angstrom, c = 19.38 Angstrom, Z = 6. The layer structure is constructed of five-layer Se-In-Se-In-Se sets and the sets are linked by weak van der Waals' force with slacking sequence of ABC. The space group is R3m with N = 4.00 Angstrom, c = 28.80 Angstrom, Z = 3 (index ed in hexagonal system). In the layer structure, structure vacancies o n 1/3 of the cation sites aggregate to film a vacancy plane for every three In-planes. The structural difference between the two phases is m ost clearly characterized by the difference in coordination numbers of the Se atoms.