High-resistivity CdTe Crystals Research Articles

Influence of Laser Radiation on Optical Properties of High Resistivity CdTe Crystals and Cd1 – xZnxTe Solid Solutions in the Field of the Fundamental Optical Transition E0

Influence of Laser Radiation on Optical Properties of High Resistivity CdTe Crystals and Cd1 – xZnxTe Solid Solutions in the Field of the Fundamental Optical Transition E0

Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors

Spectrometer-grade CdTe single crystals with resistivities higher than 109 Ω cm have been grown by the modified Bridgman method using zone-refined precursor materials (Cd and Te) under a Cd overpressure. The grown CdTe crystals had good charge-transport properties (μτe = 2 × 10−3 cm2 V−1, μτh = 8 × 10−5 cm2 V−1) and significantly reduced Te precipitates compared with crystals grown without Cd overpressure. The crystal growth conditions for the Bridgman system were optimized by computer modeling and simulation, using modified MASTRAPP program, and applied to crystal diameters of 14 mm (0.55′′), 38 mm (1.5′′), and 76 mm (3′′). Details of the CdTe crystal growth operation, structural, electrical, and optical characterization measurements, detector fabrication, and testing using 241Am (60 keV) and 137Cs (662 keV) sources are presented.

Growth and Deep Level Characterisation of Undoped High Resistivity CdTe Crystals

High resistivity CdTe crystals were grown by Bridgman and by physical vapour transport techniques without intentional doping. Novel synthesis and pre-growth treatment procedures were developed for controlling background impurities concentration and stoichiometry of the source material. A detailed characterisation of the deep levels responsible for the high resistivity behaviour is reported.

Crystal growth of undoped semi-insulating CdTe

In the frame of a research project aimed at obtaining high-resistivity CdTe crystals, the authors report on the preparation of (nominally) undoped semi-insulating (SI) CdTe crystals. The method is based on the use of polycrystalline CdTe charges synthesized from 7 N pure Cd and Te elements, in such a way as to ensure an atomic stoichiometric ratio (Cd/Te) equal to 1 within the weighing error. The charges are further stoichiometrically adjusted by subliming off residual excess components through effusion in semi-open ampoules vented to vacuum. By making use of these charges, undoped SI CdTe ingots, made up of large single-crystal grains, have been grown from both vapour, by closed-ampoule physical vapour transport and melt, by liquid encapsulation Bridgman technique. The yields in SI material turned out to be 100% for vapour-grown samples and about 50% for melt-grown ones. Preliminary results of this research work are presented here and briefly discussed.

Improved spectrometric performance of CdTe radiation detectors in a p-i-n design

CdTe radiation detectors were fabricated using a p-i-n design and a significant improvement in the spectral properties was obtained during room temperature operation. An iodine doped n-CdTe layer was grown on the Te faces of the (111) oriented high resistivity CdTe crystals at the low substrate temperature of 150 °C. An aluminum electrode was evaporated on the n-CdTe side for the n-type contact, while a gold electrode on the opposite side acted as the p-type contact. Very low leakage currents, typically 60 pA/mm2, were attained at room temperature (25 °C) for an applied reverse bias of 250 V. Detectors exhibited excellent spectral responses with an energy resolution of 1.42, 1.7, and 4.2 keV FWHM at 59.5, 122, and 662 keV γ peaks, respectively.

Characterization of high-resistivity CdTe crystals for γ-ray detectors

High-resistivity CdTe crystals were grown from Te-rich solution doped with Cl by the temperature gradient solution growth (TGSG) method. The room-temperature resistivity of the undoped crystals was about 10 3 Ω cm, and rose abruptly to > 10 9 Ω cm for a doping concentration of Cl ranging from 500 to 5000 ppm. The mobility—lifetime ( μτ) product for both electrons and holes decreased with increasing Cl concentration. The best μτ values of 7.22 × 10 −4 cm 2 V −1 for electrons and 6.24 × 10 −5 cm 2 V −1 for holes were obtained at a Cl concentration of 500 ppm. The quality of the crystals was characterized by means of Fourier-transform infrared spectroscopy, photoluminescence analysis and γ-ray detection test. γ-ray detectors with a size of 3 × 3 × 1 mm 3, fabricated from high resistivity TGSG crystals, showed energy resolutions (FWHM) of 6.9 keV for 122 keV γ-rays from 57Co and 22.4 keV for 662 keV γ-rays from 137Cs.

Accurate evaluations of thermally stimulated current curves and defect parameters for CdTe crystals

More accurate expressions for the evaluations of the trap depth E and the capture cross section S related to the thermally stimulated current process are developed. The different T−1, T−2, T−3, and T−4 dependencies of S on the temperature T have been considered for the slow retrapping case. In the fast retrapping case, the energy E and the product τNT have been expressed independently, where τ is the lifetime of the free electron and NT the total trap density. The calculations depend on the temperatures Tm and T1 that correspond to the peak and the half-height of the current, respectively. A new experimental method providing the determination of Tm and T1 with high precision has been developed that allows extensive experimental examination of thermally stimulated current curves in high-resistivity CdTe crystals. Application to defect parameters of fourteen levels in CdTe has been carried out in the slow and fast retrapping limits. In the slow retrapping case, the results differ from those obtained by the widely used Grossweiner formula. For the fast retrapping limit the values of E corresponding to different sets of Tm and T1 are calculated independently of τNT. This again has not been the case published by the different authors.

Space charge limited currents in high resistivity CdTe crystals

DC-measurements of SCLC of high resistivity Cl-compensated p-type crystals of CdTe have been performed in the temperature range of 230–300 K. The evaluation of voltage-current characteristics by means of a simple model with discrete trap levels led to activation energies of approx. 0·65 and 0·4 eV above the valence band and trap concentrations of the order 1011 to 1012 cm−3.