The traditional compensation model to explain the high resistivity properties of CdTe is based on the presence of a deep acceptor level of the cadmium vacancy in the middle of the band gap. A new compensation model based on a deep intrinsic donor level is presented. The compensation model is used together with an appropriate segregation model to calculate axial distributions of resistivity which are compared with spatially resolved resistivity measurements. The Te-antisite defect is discussed as a possible origin cause of this intrinsic defect, which is also supported by theoretical calculations.

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 × 109 Ω · cm.

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.

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.

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.

The measurement of piezobirefringence in CdTe is a useful tool for the detection of long-range and local stress fields. By means of infrared polariscopy, we obtained both the orientation of principal stresses, and the size mapping of their differences. Wall contact of the crystal, self-seeding and formation of facets at the phase boundary are origins of highly strained regions. Local stress fields can be assigned to microscopic defects such as dislocations and inclusions. We will contribute to the classification of inclusions concerning their size and shape versus local stress generation.

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.

Several CdTe crystals doped with vanadium (V) or titanium (Ti) were grown by a vertical Bridgman technique and the Sublimation Travelling Heater Method (STHM). Spatially resolved resistivity measurements were performed using the Time Dependent Charge Measurement (TDCM) method. It is shown that V doping leads to a more homogeneous resistivity distribution in contrast to Ti. Photoinduced Current Transient Spectroscopy (PICTS) has been used to characterize the deep levels. Up to three additional levels are introduced through the doping with transition metals.

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.

Spatially resolved resistivity measurements of CdTe crystals doped with Titanium (Ti) and Vanadium (V) were performed. From the temperature dependence of the resistivity the spatial variation of the thermal activation energy was deduced. Variations in axial as well as radial direction were observed and qualitatively explained by a combined segregation and compensation model. It is based on the deep donor levels of Ti and V at 0.95 eV below the conduction band.

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 109 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 cm2/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.

As a nondestructive, contactless characterization method time dependent charge measurements (TDCM) are used for the investigation of high resistivity CdTe doped with vanadium or titanium. TDCM is presented as a multi-purpose technique which allows for the examination of the resistivity, the thermal activation energy of the charge carriers, the photosensitivity and the surface voltage (SPV). Strong axial variations of the physical properties are observed as a consequence of the segregation of the dopants.

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.

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.

There are several experimental methods which give information about the thermal relaxation times of the deep levels in a semiconductor. Analyzing the temperature dependence of the relaxation times, the activation energy and the cross section of the corresponding deep levels can be determined. An essential problem of such methods is the identification of the relaxation times in the measured signal. In the context of time-dependent measurements such as photoinduced current transient spectroscopy and deep level transient spectroscopy, Tikhonov regularization was recently proposed as a high resolution method for this purpose. In this contribution it is proposed to apply Tikhonov regularization in order to identify the thermal relaxation times in admittance spectroscopy data. The method is tested and discussed using simulated data. Finally, admittance spectroscopy data of a GaAs diode are analyzed. The results demonstrate that the resolution of an ordinary admittance spectroscopy setup can considerably be improved by the application of Tikhonov regularization.

AbstractA detailed analysis of photoinduced current transients of differntly grown CdTe:Cl samples was performed in the 100–130 K range in order to investigate the influence of the different growth techniques (sublimation, Bridgman method and travelling heater method (THM)) on the compensation defects. The transients were evaluated using a regularisation method (fast Tikhonov regularisation) as implimented in the program FTIKREG. The advantages of the regularisation method in comparison with the customary two‐gate technique are demonstrated by the analysis of simulated data. It can be shown that the different growth techniques have only one level in common. Furthermore, the superposition of different traps can lead to wrong results using the conventional two‐gate technigque. The temperature dependence of the relaxation times is evaluated and the corresponding trap parameters are determined.

CdTe is one of the most encouraging semiconductor materials in the field of room temperature γ- and X-ray spectroscopy. To improve the detector properties, the ternary systems (Cd,Zn)Te and Cd(Te,Se), and CdTe were grown by vertical Bridgman technique. To achieve low noise detectors, the resistivity of all materials was increased by chlorine doping. The crystals were characterized by electrical (Hall measurements, photoinduced current transient spectroscopy) methods. The numbers of deep levels influencing the resistivity were reduced by introducing Se into the CdTe system. A common deep level close to the middle of the bandgap has been identified, responsible for the compensation effect in all three systems. In addition high resistivity and n-type conductivity were achieved in CdTeSe materials for the first time. Detectors were tested by irradiation with α particles and low γ-rays. Best charge collection efficiency (CCE) of 91% was recorded for CdTe0.9Se0.1 for both radiation types and smallest FWHM of about 10% at 60 keV.

Using spark-source mass spectrometry, atomic absorption spectrometry and various optical techniques, a study is made of the concentrations of impurities (total and point-defect concentrations), charge carriers and precipitates as well as of their axial distributions in Bridgman crystals grown with and without an extra Cd source. It is found that the concentrations of substitutional impurities are controlled by Cd and Te vacancies which form during the growth process. When the crystals are cooled after growth, part of the Cd vacancies are occupied by impurity atoms, while part of them cluster to form Te precipitates. The vacancy concentrations are modified, and the latter reaction is suppressed, in the case of growth under Cd overpressure. Intentional doping with halogens leads to formation of persistent Te-site donor/Cd-vacancy complexes.

Impedance or admittance spectroscopy has been shown to be a very convenient tool for the investigation of deep levels in semiconductor junctions. At constant temperature a frequency sweep is performed. After that the impedance signal is analysed by a regularization method based on Tikhonov regularization in order to obtain the thermal relaxation times of the deep levels present in the junction. The high resolution of the regularization method in comparison to conventional techniques is demonstrated using simulated data. The temperature dependence of the thermal relaxation times provides information about the properties of the deep levels such as activation energy or capture cross section. Two donor levels with activation energies dE1 =0.58 eV and dE2 =0.68 eV are observed in our detector diodes. It can be shown that the concentration of level 2 is increased after irradiation.