T. Feltgen

Compensation of CdTe by Doping With Gallium

Semi-insulating CdTe single crystals doped with Ga were grown from the vapour phase by the modified Markov technique MMT. The study of the resistivity map in the cross-sections cut along the growth direction has been performed. The compensation phenomenon is analysed in the framework of the three levels Fermi-statistic model. It is shown that a semi-insulation behaviour throughout the ingot is due to the compensation of shallow impurities by the deep level. From the low-temperature photoluminescence spectra it was concluded that shallow donors (Ga Cd ) are partly compensated by (Ga Cd -V Cd ) - and (Ga Cd -Cd Te ) complexes and by residual acceptors (Na Cd , Cu Cd ). The microscopic structure of (Ga Cd -Cd Te ) complex is proposed based on the value of its local phonon mode and the growth conditions. A native defect like Teed which has a deep level near the middle-band-gap is suppose to give a stable compensation and a tolerance for variation in shallow impurity concentrations.

P–T–X phase equilibrium studies in Zn–Te for crystal growth by the Markov method

Abstract Synchrotron topography characterization of vapor grown ZnTe crystals showed that the available data in this system are not sufficient to control the growth parameters. First P – T – X phase equilibrium studies confirm that the stoichiometric plane X =50 at.% Te is completely outside of the single-phase volume of ZnTe. The Te boundary exhibits retrograde solubility. The maximum Te non-stoichiometry of 0.0186 at.% was found at T =1344 K.

P–T–X Phase Equilibrium in the Zn–Te System

P-T-X data are the thermodynamic basis for modeling the crystal growth of materials with controlled stoichiometry. In this paper results of the first direct experimental measurement of the total vapor pressure are reported for three-phase equilibria SLV (solid-liquid-vapor) and VLS (vapor-liquid-solid) in Zn-Te, and the P-T projection of the P-T-X phase diagram is constructed. Geometrical analysis of the phase equilibrium showed that three congruent processes are observed in the Zn-Te system: congruent melting (S = L), sublimation (S = V), and vaporization (L = V). All three congruent curves are tangent to SLV. Consequently, all three congruent points are on the Te-side of the maximum melting point of ZnTe. The vapor pressure scanning method was applied to determine the maximum non-stoichiometry as a function of the temperature. The solidus volume of ZnTe was shown to be on the Te-side of the stoichiometric plane.

Defect engineering in CdTe, based on the total energies of elementary defects

This paper presents data on experimentally and theoretically deduced total energies and energy level positions in the band-gap for eight elementary defects in CdTe. Based on these data, the total energy dependence on the Fermi-level position in the band-gap is graphically analysed. Two types of defect reactions, which are responsible for cadmium vacancy (VCd) creation and transformation, are discussed. It is shown that the most probable candidate for the native deep-level donor is a tellurium antisite.

Identification of Cl and Na Impurities in Inclusions of a Vapor-grown CdTe Doped with Zn and Cl

Morphology and analysis of composition of inclusions were done by secondary electron microscopy and spatially resolved energy-dispersive analysis of x-ray on semiintrinsic CdTe:Cl and CdTe:Zn:Cl crystals grown from the vapor phase by the modified Markov technique and on undoped CdTe crystals grown from the melt by the Bridgman method. In CdTe:Cl and CdTe:Zn:Cl crystals nonstoichiometric inclusions of about 10-20 μm were found, which contain high concentrations of Cl and Na impurities. The Cl is concentrated in small precipitates of 1-2 μm inside these inclusions. After short-time low-temperature annealing (600 °C), the inclusions mostly disappeared.

Compensation in semi-intrinsic CdTe-based materials

In this report a brief review of the semi-intrinsic conductivity phenomenon in doped CdTe:Cl, CdTe:Ga and Cd1- xZnxTe materials used for room temperature X and (gamma) -ray detectors is discussed. The upper limit of the resistivity is analyzed in a framework of a general three- levels Fermi-statistic model. The role of the residual impurities and impurity-defect interaction as well as segregation of impurity in Te inclusions are discussed. Dependence of the elementary native defects energy formation on the Fermi-level position in CdTe is shown and some reactions between them are taken into consideration for the Fermi-level stabilization near the middle of the band-gap. On the bases of the Fermi-level stabilization phenomenon it is shown that a self-compensation and a maximum doping level in CdTe:Cl, CdTe:Ga and Cd1-xZnxTe depend on the absolute energy of the C (V)-band position. Experimental results of EDAX compositional measurements, photoluminescence are used for illustration of these problems.

Characterization of CdTe:Zn:V crystals grown under microgravity conditions

CdTe:Zn:V crystals grown by the seeded Bridgman method in microgravity conditions during the STS95-Spacelab-AGHF-1 mission and in the ground laboratory (l-g) were analyzed and compared. The results obtained clearly show that the structural quality of the space crystal is better. Density of inclusions, concentration of dislocations, and presence of stresses are lower in the microgravity-grown (μ-g) crystal. The l-g crystal contains twins and grains from the beginning of the growth process, that is, from the near-seed region. In general, the concentration of inclusions and amount of segregated impurities on the l-g crystal are larger than in the μ-g crystal. X-ray rocking curves and low-temperature photoluminescence spectra demonstrate the relatively high quality of both crystals on a microscale at the beginning of the growth and show that the l-g conditions were worse at the end. The results of this investigation demonstrate a positive role of contactless growth and μ-g conditions in the melt in suppressing the creation of inclusions and dislocations.

Self-cleaning effect and compensation mechanisms in Cl-doped high-resistivity cadmium telluride

In this report a new up to dated view on the compensation mechanism in CdTe bulk crystals doped with Cl in concentration up to 1019 cm-3 is given. This concentration of Cl doping gives a high resistivity material. The chlorine atoms can act as shallow donors being in Te sites or can form complex with cadmium vacancies, so called A-centers. Spatially resolved EDAX mapping of CdTe doped revealed nonstoichiometrical areas distributed over the surface. In these areas there are a high concentration of impurities, where the Cl is located in a very small inclusions, while Na is distributed all over the volume of the big inclusions. After a short-time annealing (4 hours) at low temperature (500 degree(s)C) in Cd atmosphere the areas with deviation from stoichiometry mostly disappeared as well as the inclusions which were present inside. This paper includes a review of the different aspects, which can influence a precise compensation mechanism in a non-doped and Cl doped CdTe.

Detached growth of CdTe:Zn:V (STS-95): preliminary results

The wall free growth of CdTe by the Bridgman technique under microgravity was performed. This phenomenon called Detached or Dewetting growth was studied in the flight mission STS- 95. At this mission two CdTe crystals were grown co-doped with vanadium and zinc. Identical samples were grown with different ampoule designs. The effect of Detached or Dewetting growth was observed by analysis of crystal surface and roughness by optical and mechanical methods. All results of the crystals grown under microgravity are compared to the earth grown reference samples. Surface differences can be found between the (mu) g and the 1g samples. The surface roughness measurements demonstrate that the detaching was partially successful.