Modeling of Flux Composition for Thermal CdCl 2 :O 2 Annealing of Polycrystalline CdTe
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Modeling of Flux Composition for Thermal CdCl2:O2 Annealing of Polycrystalline CdTe Jaan Hiie1, Vello Valdna1 and Andres Taklaja Department of Radio and Communication Engineering, 1 Department of Materials Science, Tallinn Technical University, 5 Ehitajate Street, 19086 Tallinn, Estonia ABSTRACT Differential thermal analysis (DTA) measurements were carried out in order to investigate thermal properties of model flux compositions of CdTe-CdCl2 with CdTeO3, the last recognized as a main oxidation product of CdTe. The composition of the flux was chosen to obtain larger quantity of residual flux phases for analyses and the presence of solid CdTe phase (about 20 mol %) at the fusion temperature 520 °C in a vacuum-closed isothermal quartz ampoule. Quantity of CdTeO3 constituted 20 mol % of the whole sample mass. DTA curves taken in the 300-700°C interval reveal considerable lowering of melting and solidification temperatures for the flux with addition of CdTeO3. The fused batches were vacuum annealed at 470 °C in a long narrow quartz tube for separation and deposition of volatile components CdCl2 and Te at the room temperature part of the tube. After vacuum annealing the residual CdTe phase on the batch with tellurite was coated with transparent platelike crystals morphologically typical to the CdTeO3, while oxidefree batch had no alien phases. In the flux, CdCl2 has a role of catalytic agent forming low melting point composition with native oxides and CdTe. After processing, CdCl2 can be separated by vacuum annealing. INTRODUCTION In the CdTe:CdCl2:O2 annealing process optimum structural and optoelectronic properties of the CdTe layer are formed [1,2]. At high concentrations of chloride (up to 10 mol % CdCl2), needed for the optimal CdCl2:O2 treatment, more than 1019 at/cm3 of chlorine can be incorporated into the CdTe lattice resulting in a “semi-insulating” material [3] while it is known that the low resistance p-type conductivity in CdTe can be attributed to the low concentration of chlorine (1016-1018at/cm3) in the CdTe lattice [4]. The process leaves a significant amount of Cd-, Te-, O, and Cl-rich residue on the CdTe surface and in grain boundaries [5,6]. Presence of residuals and precipitates in grain boundaries, on interfaces and in the grains is a frequent bar on the way to high performance chalcogenide solar cell. The inclusions are responsible for surface contamination and over-optimal lattice concentrations of dopants. The chemically and thermally stable residue was not removable by vacuum annealing, only HNO3 solution etch removed chlorine from the surface [4]. It is clear, that not before full removal of the residues the concentration of the chlorine in the telluride lattice can be decreased and regulated. Even in the case HNO3 solution etch removes all detectable traces of chlorine from the surface, solution etch will not be able to remove the traces from the grain-boundaries. In the previous works [7,8], we studied a thermal behaviour of fine-grinded CdTe mixtures with CdCl2, CdI2, TeO2 and CdTeO
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