Boron oxide encapsulated Bridgman growth of high-purity high-resistivity cadmium telluride crystals

Armani

Salviati

et al. 2004

phys. stat. sol. (c)

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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.

Section: Discussionmentioning

confidence: 98%

Section: Methodsmentioning

confidence: 99%

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confidence: 99%

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Stoichiometry related defects in CdTe crystals

Armani

Salviati

et al. 2004

phys. stat. sol. (c)

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“…In this communication we report for the first time on the effect of this etching technique on CdTe crystals. CdTe crystals were grown from the melt by Boron Oxide encapsulated Bridgman method [13]. The feed charge was obtained by high pressure synthesis from 7N pure Cd and Te [14] followed by a thermal treatment [15] in order to obtain the desired composition.…”

Section: Introductionmentioning

confidence: 99%

Revealing of defects in CdTe crystals by DSL etching

Weyher

Zappettini³

et al. 2005

Cryst. Res. Technol.

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The effect of DS(L) (Diluited Sirtl with or without Light) solution on CadmiumTelluride crystals has been studied in comparison with the actions due to Inoue and Nakagawa etching solutions. The use of chemical etching to reveal extended defects is a fast and useful technique for characterizing the crystals with the aim of improving the growth technology and better devices performance. In fact it is well known that extensive defects in CdTe crystals have a relevant role on the material properties and finally on the devices performance. DSL solution previously used on Si, GaAs and InP, here has been used for the first time on CdTe crystals. The etching solutions have been used to characterize crystals with two different characteristics: the first group shows high electrical resistivity and close to stoichiometry composition, the second one shows low resistivity behaviour and large Te deviation. We report here the preliminary results of the characterization of these crystals with the DSL etching.

“…2 Experiments CdTe crystals were grown both by the physical vapour transport (PVT) method and by the liquid encapsulated Bridgman method [10]. The stoichiometry of the feed charge was carefully con-trolled according to the procedure described in [11] in order to obtain crystals with different values of conductivity (both n-and p-type).…”

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confidence: 99%

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

Zappettini

Zha

et al. 2004

phys. stat. sol. (c)

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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.