Evidence of a stoichiometry‐related compensation in undoped high‐resistivity CdTe crystals

Abstract

Nominally undoped CdTe crystals are grown by both vapour phase and Bridgman technique. The stoichiometry of the feed charge was adjusted in order to obtain n-type, p-type and high resistivity crystals. The stoichiometry of CdTe samples is studied by means of a detailed analysis of the temperature dependence of the equilibrium partial pressures of Cd and Te 2 vapours. A strong evidence of a correlation of the stoichiometry and the resistivity of the samples has been found. This suggests that a stoichiometry related defect plays an important role in the compensation mechanism of the nominally undoped crystals. 1 Introduction CdTe crystals have been since long employed as gamma and X-rays detectors due to the large atomic number and high energy gap that make possible elevated stopping power for the high energy radiation and room temperature operation respectively [1]. X-and γ-ray detectors can be employed for example for luggage control, medical applications and non-destructive structural elements testing. Moreover, CdTe presents a high electro-optic figure of merit together with isotropic refractive index and good transparency in the infrared. Thus, it is potentially a good material for realizing amplitude modulation of optical signals in the telecommunication range [2]. However, the poor quality of CdTe crystals severely limits the exploitation for devices. In particular, the presence of undesidered impurities, structural defects and grain boundaries, strongly deteriorates transport and optical properties [3,4]. This results in a non-reproducibility of the characteristics of the CdTe crystals and in a strong reduction of the yield in the preparation of device-quality material.On the other hand, the compensation mechanism that leads to high resistivity in CdTe crystals is not fully understood. At least two mechanisms were proposed: i) the so-called self-compensation mechanism between shallow donors and the acceptor A-center (a donor/Cd-vacancy complex) [5,6], ii) a mechanism involving residual acceptors, probably Cd-vacancies and a not fully identified deep donor, probably related to an intrinsic defect [7,8]. Of course, the interpretation of the experimental results is more difficult as impurities-related defects, stoichiometry-related defects and complexes of these must be considered at the same time. Moreover, in most of the investigations on CdTe compensations mechanisms stoichiometry determination is needed: the crystal stoichiometry is usually deduced according with the growth conditions.Recently a novel technique for the precise determination of the stoichiometry of CdTe samples was developed by the authors [9]. In this work, the stoichiometry and the resistivity of CdTe samples grown in different condition are compared.

Growth and properties of CdTe:Bi-doped crystals

“…The most important requirements for this application are high resistivity material with high photosensitivity, very short response time and large carrier lifetime [5]. High resistivity CdTe is possible with ultra-pure material [7][8][9], but the difficulty to avoid the contamination of the product during the crystal growth process creates difficulty during the manipulation of the material. For this reason, doping processes with donor elements to compensate for impurities and native acceptor defects are largely employed [1][2][3][4].…”

Boron oxide fully encapsulated CdZnTe crystals grown by the vertical Bridgman technique

“…Moreover, the boron does not appear to be electrically active in CdTe: in fact, samples grown with a boron oxide layer showed the same electrical behavior as samples grown by physical vapor transport without boron oxide, i.e. without a source of boron contamination [10,11]. The same conclusion was reached by Blackmore et al [12].…”

“…In the case of the growth of CdTe, the authors found that undoped crystals with a tellurium deviation such as the ones in Fig. 6 presented high resistivity (10 9 O cm) without intentional doping [19]. This does not hold in the case of CZT crystals.…”