Phase Diagram of the CdTe-CdSe Pseudobinary System

Abstract

The liquidus and solidus curves of the CdTe-CdSe pseudobinary system have been determined by thermal analysis of cooling and heating curves, respectively, for homogenized liquid and solid alloy samples. The phase diagram has a eutectie point at 1091 ~ • 1~ near 20 m/o (mole per cent) CdSe. Above the eutectic composition both liquidus and solidus temperatures increase monotonically and sublinearly with increasing CdSe content. In this region the experimental values of the liquidus-solidus gap do not exceed iI m/o and are in excellent agreement with the ideal thermodynamic liquidus-solidus relationship. The phase diagram below the solidus has been investigated by x-ray diffraction measurements on alloy powders annealed between 770 ~ and 1050~ and quenched. It consists of two broad single-phase regions, one of alloys with the zinc blende structure of CdTe and the other of alloys with the wurtzite structure of CdSe, separated by a two-phase region only about 3 m/o wide, whose boundaries shift toward increasing CdSe content with decreasing temperature. For compositions between 30 and 45 m/o CdSe, either structure could be obtained at room temperature, depending on annealing temperature and rate of cooling. In addition, a polytype of unknown structure, which appears to be a metastable intermediate in the wurtzite-to-zine blende transformation, was observed in two melt-grown furnace-cooled ingots containing 45-50 m/o CdSe.The II-VI compounds CdTe and CdSe have the cubic zinc blende and hexagonal wurtzite structures, respectively, when prepared by solidification of stoichiometric melts or nonstoichiometric Cd-Te or Cd-Se solutions.[Other methods of preparation can be used to obtain CdTe with wurtzite structure (1) or CdSe with zinc blende structure (2).] The phase diagram of the CdTeCdSe system has not been determined previously but x-ray diffraction studies (3, 4) on a limited number of samples prepared by solidifying stoichiometric melts indicate that pseudobinary solid solutions are formed over the entire composition range. According to these studies, undoped alloys containing less than about 40 m/o CdSe have the zinc blende structure, those containing at least 70 m/o CdSe have the wurtzite structure, and those with intermediate compositions may have either structure.In this investigation, the liquidus and solidus curves in the CdTe-CdSe pseudobinary system have been determined by thermal analysis measurements on homogenized liquid and solid alloy samples. The cubic-hexagonal transition in the solid phase has been studied by x-ray diffraction analysis of annealed and quenched alloy powders. The x-ray data show that the cubic and hexagonal phase fields are separated by a two-phase region about 3 m/o wide, whose boundaries shift toward increasing CdSe content with decreasing temperature. Therefore, within a certain composition range, alloys can be obtained with either cubic or hexagonal structure, as reported earlier (3, 4), depending on the annealing temperature and rate of cooling. In addition to these two phases, a p...

Thermal Properties

“…The experimental data are taken from Steininger et al [18]. This type of x versus T variation suggests small and nearly comparable positive deviations from ideality in the solid and liquid phases and is similar to those observed in other II-VI systems, such as ZnSe-ZnTe [19], CdSe-CdTe [20] and HgSe-HgTe (Figure 2.9). The solidus and liquidus curves in Figure 2.6 represent monotonic and sublinear variations with x from the melting point of CdTe to that of ZnTe.…”

A Comparative Study of the Effects of Nontoxic Chloride Treatments on CdTe Solar Cell Microstructure and Stoichiometry

“…where I hkl is the intensity of a refl ection from the {hkl} family of planes, I r,hkl is the relative intensity of this family of planes for a powder sample, and n p is the number of refl ections present in the diffractogram (note that the I r,hkl values used here were averages of XRD data for CdTe powder from six entries to the ICSD database [32][33][34][35][36][37] and the spread in the relative intensities reported by these studies was less than 4% for all refl ections). Then, the standard deviation, σ , of these texture coeffi cients as given by [ 30 ] 1/ 1…”

Electrodeposited Quantum Dots. 6. Epitaxial Size Control in Cd(Se, Te) Nanocrystals on {111} Gold

“…is known to form solid solutions, namely the lattice parameters vary linearly with composition according to Vegard's law, without causing a structural phase transformation (no transformation to a different crystal structure) in the region x < 0.5.12. 13 Perfect lattice match (zero mismatch) in epitaxial growth of Cd(Se 1_ xTex) on Au (in the {111}Aull{00.1}cd(se. Te) ; (110)Aull(11.0)Cd(Se.…”

“…or by temperature variation.'? Lattice matching in the epitaxial growth of CdSI_ySeyon {111} InP was achieved for y =0.08,11 Studies of homogeneous alloys of hexagonal CdSel_xTe x showed a behavior conforming to Vegard's law (i.e., the lattice parameters vary linearly with composition) for 0 < x < 0.3, with different slopes for a o and for C O • 12,13 The phase diagram of the CdSe l_x Te, binary system indicated a wurtzite structure for alloys with x < 0.5 and a zinc-blende structure when x > 0.5, with a narrrow two-phase region when x ... 0.5.J3Raman spectroscopy of Cd(Se,Te) single-crystals indicated a two-mode behavior, namely the presence of CdSe-like and CdTe-like optical phonons in the Raman spectrum.lv"…”