Loick Verger

Gamma- and X-ray detectors manufactured from Cd1−xZnx Te grown by a high pressure bridgman method

Results of recent efforts in the growth of Cd1−xZnxTe crystals by a high pressure Bridgman (HPB) method and their use in gamma- and X-ray detector applications are presented. Evidence for crystals of relatively low defect content include etch pit densities of ≤ 104cm−2, double crystal rocking curve linewidths of 10–15″ and sharp, bright emission lines with excitonic features in low temperature photoluminescence measurements. Resistivities in excess of 1011 ohm cm are achieved without impurity doping. The resulting low leakage currents lead to good energy resolution, <6% at 59.5 keV for example. The dependence of leakage current on temperature from 233 K to 373 K implies a Fermi level at mid-gap for x=0.2. The results of flash X-ray experiments indicate that the high current sensitivity, low leakage current and good temporal response of Cd1−xZnxTe detectors make them attractive candidates for applications involving short pulses at high dose rates.

Applications of CdTe detectors in x-ray imaging and metrology

Operating as a photoconductor, the sensitivity and the impulse response of semi-insulating materials greatly depend on the excitation duration compared to electron and hole lifetimes. The characteristic of ohmic contact for these compounds is briefly discussed. Before developing picosecond measurements with integrated autocorrelation system, this paper explains high energy industrial tomographic application with large CdTe detectors (25 X 15 X 0.9 mm3) where spatial resolution, contrast, and wide dynamic are the main criteria. The excitation is typically microsecond(s) range. X-ray flash radiography with 10 ns burst is in an intermediate time domain where excitation is similar to electron life-time in cadmium telluride. In a laser fusion experiment the excitation is in the range of 50 ps and we develop for such high band devices photoconductive structures able to study very short x-ray emission. Thin polycrystalline MOCVD CdTe films with picosecond response are an alternative material suitable to perform optical correlation measurements of single shot pulses with a very large bandwidth (approximately 50 GHz).

Power switching with CdTe:Cl

Abstract CdTe material with its high molecular weight (240) is today widely used for radiation detection. The band-gap energy of CdTe is 1.45 eV. When doped with chlorine, which compensates the acceptor level introduced by cadmium vacancies, CdTe is intrinsic and gives a very high resistivity (more than 10 8 Ω cm). The contacts were made by electroless metal deposition and further annealing; they were characterized with d.c. and pulsed voltages. For the first time, this paper presents power switching experiments with CdTe material. We investigated different types of crystals and contact geometries with gap sizes varying from less than 1 mm to a few millimetres. The switches were activated by a YAG laser with 10 ns FWHM pulses (1.06 μm) or with 160 kV X-rays with 30 ns FWHM pulses. The time constant of recovery was found to be more than 10 ns. In some cases, for high voltages, corresponding to fields higher than a few kilovolts per centimetre, large recovery times of more than 100 ns were measured. This apparently longer carrier lifetime, combined with the high resistivity, make CdTe an alternative material to silicon and GaAs for some switching applications.

Power switching with a new material: CdTe:Cl

CdTe material, with its high molecular weight (240), is widely used for radiation detection. The bandgap energy of CdTe is 1.45 eV. When doped with chlorine, which compensates the acceptor level introduced by cadmium vacancies, CdTe is intrinsic and presents a very high resistivity (> 108 (Omega) cm). The contacts were made by electroless metal deposition and further annealing; they were characterized with DC and pulsed voltages. For the first time, this paper presents power switching experiments with CdTe material. We investigated different types of crystals and contact geometries with gap size varying from less than one mm to a few millimeters. The switches were activated by a YAG laser 10 ns FWHM pulses (1.06 micrometers ) or with 160 kV X-rays 30 ns FWHM pulses. The time constant of recovery was found to be more than 10 ns. In some cases, for high voltages corresponding to fields higher than a few kV/cm, large recovery times of more than 100 ns were measured. This apparently long carrier lifetime, combined with the high resistivity, make CdTe an alternative material to Si and GaAs for some switching applications.