Crystal Structures and Phase Transformation in In2Se3 Compound Semiconductor

Crystal structures and phase transformation in In 2 Se 3 compound semiconductor have been studied by electron diffraction, high resolution electron microscopy and X-ray diffraction (XRD) together with optical absorption measurements. The time-temperature-transformation (TTT) diagram reveals that there exist only two phases in In 2 Se 3 and the transformation temperature is 853 K. The transformation from the high temperature phase to the low requires a long incubation time for crystal nucleation and a relatively high temperature for crystal growth. The low and high temperature phases are the vacancy ordered in screw form (VOSF) phase 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 wurtzite 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 with a =7.14 Å, c =19.38 Å, 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 stacking sequence of ABC. The space group is R 3 m with a = 4.00 Å, c = 28.80 Å, Z =3 (indexed in hexagonal system). In the layer structure, structure vacancies on 1/3 of the cation sites aggregate to form a vacancy plane for every three In-planes. The structural difference between the two phases is most clearly characterized by the difference in coordination numbers of the Se atoms.