Stoichiometry related defects in CdTe crystals

Abstract

In this work, we report on the structural analyses of undoped CdTe samples grown by the vapor phase and the Bridgman methods. Different techniques were used for determining the structural defects: wet etching, high resolution X-ray diffraction, double crystal X-ray topography and monochromatic SEMcathodoluminescence mapping. The density and the nature of the structural defects were found to be correlated to the stoichiometry of the samples, as determined by a detailed analysis of the temperature dependence of the partial pressure of the vapors in equilibrium with the solid. 1 Introduction CdTe crystals are employed for X-rays detection and as substrates for epitaxy, especially for the preparation of HgCdTe hetero-structures. In the former case, defects can act as recombination centers, limiting the charge collection efficiency. In the latter case, high structural perfection is required, as defects usually propagate from the substrate to the epi-layers, severely limiting the performance of the final device. For this reason, great efforts were done in the scientific community in order to reduce the defect density in CdTe crystals. In particular, Bridgman grown samples show a high density of dislocations and tellurium inclusions. Dislocations are easily formed in CdTe crystals due to the relatively low hardness of this compound. Tellurium precipitates are very common in CdTe crystals for different reasons. At first, the evaporation of a stoichiometric CdTe charge is strongly incongruent close to the melting point [1]. In fact, the vapor that fills the free volume of the ampoule in equilibrium with stoichiometric CdTe melt is almost made by atomic Cadmium. Moreover, at the congruent melting point, the solid CdTe results to be Te-deviated [1,2]. The inclusion density can be reduced by controlling the Cd vapor pressure during the growth by means of a Cd reservoir [3]. In alternative the growth chamber can be pressurized by inert gas, as for High Pressure Bridgman Growth. Recently, we have shown that good quality CdTe crystals can be obtained also by the Boron Oxide encapsulated Bridgman method [4].In this work, the influence of the stoichiometry on the structural properties of the crystals grown with this method are examined and compared with the properties of vapor phase grown (PVT) crystals.

The effect of chlorine doping concentration on the quality of CdTe single crystals grown by the modified physical vapor transport method

“…This leads to the reduction of the charge collection efficiency and hence energy resolution of CdTe-based detectors [11]. Peculiarities of Cd-Te phase diagram [12] imply that the appearance of tellurium inclusions and precipitates and their concentrations are considerably enhanced in CdTe crystals grown under Te-rich conditions [13,14]. It was found that the concentration of Te precipitates can be reduced by annealing of as-grown wafers in Cd vapor [15,16] or in high-purity Ge melt [17].…”

Detection of "Exotic" Dopant-Related Macrodefects in Vapour Grown CdTe:Cr Single Crystals

“…However, structural quality of CdTe crystals suffers from the presence of a considerable number of second phase particles [2,3]. Due to the peculiar thermodynamic properties of CdTe (higher retrograde solid solubility of Te compared with that of Cd and, inversely, much higher partial pressure of metal constituent at applied growth temperature leading to the Cd loss during growth run [4]) tellurium inclusions and/or precipitates are commonly present in the as-grown crystals [5][6][7][8][9], although cadmium macrodefects were also observed in the material obtained under Cd-rich conditions [10,11]. At the same time, extrinsic second phase separation was more rarely reported in CdTe crystals [12][13][14][15][16].…”

Revealing of defects in CdTe crystals by DSL etching

“…The first group of samples has given evidence of cellular structure in the distribution of pits (see fig.3), also confirmed by SEM images [17], whereas in the second group of crystals the etch pits are randomly distributed. Moreover, high resistivity samples have shown lower defects density than Te-rich low resistivity crystals [19]. Fig.…”