W. Joerger

Growth of twin-free CdTe single crystals in a semi-closed vapour phase system

Abstract V and Ga doped CdTe single crystals with one inch diameter were grown without wall contact in a semi-closed vapour phase system (modified Markov method). By vapour transport modelling, we demonstrate that volatile impurities and excess species are enabled to condense in a heat sink connected to the growth chamber. Vapour composition and component fluxes are controlled by the temperature profile, in particular by the sink temperature. The grown crystals exhibit pronounced {{1 1 1}}, {{1 1 0}} and {{1 0 0}} facetting. The influence of the deep heat sink on interface stability is discussed in terms of growth morphology and formation of inclusions. Piezobirefringence measurements indicate nearly stress-free growth. The resistivities of the grown crystals are up to 3 × 10 9 Ω · cm .

Radiation detector properties of CdTe0.9Se0.1:Cl crystals grown under microgravity in a rotating magnetic field

Abstract The influence of growth conditions on radiation detector performance was studied by examining Cd(Te,Se):Cl crystals. They were grown by the travelling heater method under microgravity conditions during the PHOTON 8 mission. An additional forced convection was produced by a rotating magnetic field (400 Hz) of 2 mT. The influence on crystal quality was analyzed by several characterization methods such as time dependent charge measurement, photo-induced current transient spectroscopy and admittance spectroscopy. The charge collection efficiency of detectors was increased from 10% without magnetic field up to 66% by the influence of the rotating magnetic field. The improved detector performance is explained by the increased resistivity and a reduced number of deep levels.

Studies of the compensation mechanism in CdTe grown from the vapour phase

Abstract Semi-insulating titanium and vanadium doped cadmium telluride crystals were grown from the vapour phase. We show results of the electrical characterisation obtained by Hall measurements, photoinduced current transient spectroscopy (PICTS), admittance spectroscopy and time-of-flight measurements. The crystals with a resistivity of about 10 9 Ω · cm showed a deep level, which was identified for CdTe:V at −0.77 eV and a similar value at −0.66 eV for CdTe:Ti, both with reference to the conduction band. The μτ products are almost the same for both materials in the range of 10 −5 cm 2 /V. A compensation model, which was developed for semi-insulating GaAs, is adapted to semi-insulating vanadium- and titanium-doped cadmium telluride. We show that the data from the compensation model correspond to the measured properties.

Characterization of cadmium telluride crystals grown by different techniques from the vapour phase

Abstract Semi-insulting CdTe bulk crystals were grown from the vapour phase in both closed and semi-open arrangements. The results of the growth experiments are discussed in terms of various electrical and optical characterization methods. Van der Pauw measurements and time dependent charge measurements (TDCM) were used to determine the resistivity. Deep level defects were investigated by means of photoinduced current transient spectroscopy (PICTS). For one of the most important fields of application, detector spectra of the vapour phase material are measured and discussed.

Investigation of CdTe:Cl grown from the vapour phase under microgravity conditions with time dependent charge measurements and photoinduced current transient spectroscopy

Abstract CdTe: Cl crystals were grown from the vapour phase by the sublimation travelling heater method (STHM) in space on board of the unmanned EURECA I mission. They were characterized by time dependent charge measurement (TDCM) and photoinduced current transient spectroscopy (PICTS). Axial cuts of the space grown crystals investigated by TDCM reveal an axial symmetric resistivity distribution along the growth direction, which can be explained by segregation effects of chlorine.

Characterization of CdTe:Cl crystals grown under microgravity conditions by time dependent charge measurements (TDCM)

Abstract CdTe:Cl crystals were grown from the liquid and from the vapour phase under microgravity (μg) conditions on board the unmanned EURECA I mission. The resistivity distribution of the grown crystals was measured by time dependent charge measurement (TDCM). Photo induced current transient spectroscopy (PICTS) was used to investigate the deep level properties. The axial resistivity distributions of the crystals grown in space differ significantly from 1 g reference crystals. In the case of vapour growth, these differences can be explained by an additional laminar flow under 1 g conditions. Supercooling has to be considered in growth from a Te zone under μg.

Closed tube vapour growth of CdTe:V and CdTe:Ti and its characterization

The vapour growth of CdTe:V and CdTe:Ti by the sublimation travelling heater method (STHM) is reported for the first time. Characterization of the crystals has been performed by time dependent charge measurement (TDCM) and photoinduced current transient spectroscopy (PICTS). Axial cuts of CdTe:V crystals showed a very homogeneous and high resistivity distribution in the range of 1010 ω · cm. The PICTS spectra depict three levels which can be attributed to transition metal doping. The expected deep donor level can be identified clearly to be at Ev + 0.95 and 0.94 eV in CdTe:V and CdTe:Ti, respectively.

Noncontact characterization of CdTe doped with V or Ti

Time dependent charge measurements (TDCM) are used for noncontact characterization of CdTe crystals doped with vanadium or titanium. Several extensions of the basic technique are presented, which allow for the investigation of the thermal activation energy of the charge carriers, the photosensitivity and the surface photovoltage (SPV). Results of noncontact DLTS measurements show that the formation of defects can change suddenly during the crystal growth process.

Vapor crystal growth of CdTe under terrestrial and microgravity conditions

CdTe:Cl crystals were successfully grown in a closed vapor transport system under terrestrial and microgravity conditions within the framework of the EURECA-1 mission. The axial as well as radial resistivity distribution differs strongly in 1 g and (mu) g crystals. This is assumed to be due to different vapor transport of major components and dopants and different dopant segregation. For CdTe crystal growth in closed systems, the partial pressures depend sensitively on the composition of the feed material and its pretreatment. This affects the reproducibility of vapor transport: Smallest deviations from the congruently subliming composition yield significant changes in the pCd/pTe(2) ratio and transport-limiting diffusion barriers. Therefore, vapor pressure control by means of a semi-closed growth system is proposed for CdTe vapor transport experiments in microgravity. In ground-based experiments, CdTe crystals were grown in the semi-closed system applying Cl, Ga and V as dopants. The conditions for reproducible transport are discussed. Results of electrical material characterization are presented.

Resistivity and deep-level investigations of detector-grade CdTe: a comparison of different growth techniques

The electrical properties of semi-insulating CdTe crystals were characterized considering the application of the material as a room temperature radiation detector. Requirement for the detector application is a high resistivity material which is achieved by the compensation of shallow levels by deep donors in the middle of the band gap. These deep donors are obtained by doping the CdTe crystals with titanium or vanadium or by the influence of chlorine and intrinsic defects like the antisite defect. The energies of the deep levels are within the range of 0.31 eV and 0.95 eV determined by photoinduced current spectroscopy and admittance measurements. To calculate the compensation effects we applied a model originally developed for semi-insulating GaAs. To form detectors with a high local resolution, homogeneous material is needed. Homogeneity was tested by time dependent charge measurements. While CdTe:Cl showed variations of about 20% other high resistivity materials could vary up to one order of magnitude across one wafer. The variation of resistivity in CdTe crystals could be attributed to the crystal growth by a combined segregation-compensation model.