Optical functions of layered cadmium dichalcogenide crystals in the energy range 4?20 eV

Abstract

OPTICAL FUNCTIONS OF LAYERED CADMIUM DICHALCOGENIDE CRYSTALS IN THE ENERGY RANGE 4-20 eV Ya. O. Dovgii, I. V. Kityk, Yu. M. Aleksandrov, V. N. Kolobanov, V. N. blakhov, and V. V. Mikhailin UDC 548.0:535 Wide-gap layered cadmium dichalcogenide crystals are promising materials for optoelec- tronics and holographic recording of information [i, 2]. In order to use these materials in practice it is important to know the quantitative parameters of the energy band structure. The basic information about them can be obtained by optical studies in the region of the fundamental absorption. Thus far the measurements of the reflection spectra of these crystals have been performed in a limited energy range -- up to ii eV [3, 4]. Calchlations of the optical constants have been performed only for CdBr2, but because of the limited range of the measurements they cannot be regarded as satisfactory. In this work we present the calculations of the optical functions of CdBr2 (space group D~d) and CdI2 (space group C~v) crystals in the energy range 3-20 eV at T = 77~ On their basis and using also the calculations from first principles by the pseudopotential method in the nonlocal approximation [5], quantitative band schemes for the indicated crystals are constructed. The single crystals were grown by the Bridgman--Stockbarger method. The starting stock was first subjected to 45-fold vertical zonal purification. The crystals were grown in a darkened quartz crucible (in order to prevent photodecomposition). The sample was drawn at a rate of 2 rmm/h. During cooling the crystals were held for 28 h at a temperature 120~ lower than the melting temperature. The crystals grown were 1.2 cm in diameter and 2-2.5 cm high; x-ray structural studies, performed on the DRON-5 diffractometer, showed that CdBr2 crystals belong to the D~d space group (Ramsdell's polytype 6R) and CdI2 crystals belong to the C~ v space group (4H)~ Both crystals have the same layer point group: D3d. The absence of mechanical stresses was monitored by an optical polarization method. The good joining of the layers perpendicular to the optic axis enabled obtaining mirror-smooth surfaces of high quality, not requiring additional treatment. The reflection in the energy range 3-6 eV was measured on a setup mounted on the base of an SF-4 spectrophotometer with the angle of incidence of light =4 ~ The relative reflection