Investigation Of The Defects In Cd_1-xZn_xTe Crystals And The Heat Treatment Of Wafers

The II -VI semiconductor Cd1-xZnxTe has a wide variety of applications including X-ray or y -ray detectors, solar cells and electro-optical modulators. But, the most common use of Cd1-xZnxTe (especially for x=0.04-0.05) is as the substrate for the epitaxy of Hg1-xCdxTe infrared (IR) detector wafers. Bulk crystals of Cd1-xZnxTe with high quality are very difficult to obtain. Commercial Cd1-xZnxTe crystals usually contain defects such as twins, sub-grain boundaries, dislocations, and Te precipitates. These defects will seriously deteriorate the optical and electrical properties of the crystals. In order to eliminate the defects, especially Te precipitates, Cd1-xZnxTe slices were annealed in CdZn vapor in the present researches. The variation of Te-rich phases in morphology and sizes during the annealing and its effects on the optical and electrical properties of the crystals were examined in detail. The defect chemistry calculations were made. The optimized annealing parameters are obtained by carefully analyzing the calculated results and the experimental data.A model for the evaluation of the point defect category in CdTe was presented based on the principles of the defect physics and the quasi-chemical equilibrium theory. All of the possibly existing single intrinsic point defects in CdTe crystals, namely neutral or ionized Cdi, Vcd, Tei, VTe and Tecd, were dealt with in the model. The reaction enthalpies and entropies and the equilibrium constants of the related quasi-chemical equations were determined. The parameters and conditions for intrinsic annealing were given concretely by calculating the []-Pcd plots of the free carriers and the point defects. It was showed that, in Cd-rich CdTe crystals, atomic defects formed by the excess Cd mainly existed as Cd interstitials. In Te-rich CdTe crystals, at high temperature Cd vacancies were the dominant point defects, while at low temperature the concentrations of Te anti-sites, Te interstitials and Cd vacancies were all very high.We generalized the above model of the point defect to Cd1-xZnxTe, which was taken as ideal cation substituted pseudobinary alloy. The quasi-chemical equilibrium for CdTe and ZnTe were inspected independently while the influences of the integration crystal lattice were taken into consideration by the united concentration ofvacancies, anti-sites, and the cation interstitials. The calculation determined the main point defect under different Cd pressure and the reaction enthalpies and entropies and the equilibrium constants in (Cd, Zn)Te according to the quasi-chemical equations written for CdTe sublattice. Also the recipe of two-zone annealing process for Cd0. 95Zn0.05Te was derived. It was showed that at temperature close to the melting point, neutral and ionized Cd interstitials and neutral Te vacancies were the dominant point defects in Cd-rich Cd1-xZnxTe (x=0.05) crystals. At temperature lower than 1300K, atomic defects formed by excess Cd mainly existed as ionized Cd interstitials. In Te-rich Cd1-xZnxTe (x=0.05) crystals, the concentration of Cd vacancies was the highest. Under very low Cd partial pressure, Te anti-sites also acted as the dominant point defects.Several Cd1-xZnxTe (with the original concentration of x=0.04) crystals of different sizes were grown by Bridgman methods modified with the accelerated crucible rotation technique (ACRT-B). The microstructure morphology, the concentration, the infrared transmittance, and the X-ray rocking curves measured showed that a long single crystal part and axial steadily distributed zone of the concentration existed in the as-grown crystals. The radial concentration distribution has relatively high uniformity. However, Te-rich phases and point defects with high concentration were found locally.Not only the vapor-solid equilibrium but also the vapor-liquid-solid equilibrium should be considered for the annealing of the crystals with Te-rich phases. By analyzing the phase diagrams of Cd-Te and P-T plot of Cd1-xZnxTe (x=0.04), it was concluded that for the purpose of rem...