We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

Cadmium zinc telluride and its use as a nuclear radiation detector material

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Transition metal-doped zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and /spl gamma/-ray detection. The high atomic number of the materials (Z/sub Cd/=48, Z/sub Te/=52) gives a high quantum efficiency in comparison with Si. The large bandgap energy (Eg/spl sim/1.5 eV) allows us to operate the detector at room temperature. However, a considerable amount of charge loss in these detectors produces a reduced energy resolution. This problem arises due to the low mobility and short lifetime of holes. Recently, significant improvements have been achieved to improve the spectral properties based on the advances in the production of crystals and in the design of electrodes. In this paper we summarize 1) advantages and disadvantages of CdTe and CdZnTe semiconductor detectors and 2) the technique for improving energy resolution and photopeak efficiencies. Applications of these imaging detectors in future hard X-ray and /spl gamma/-ray astronomy missions are briefly discussed.

Recent progress in CdTe and CdZnTe detectors

We discuss the potential benefits of using compound semiconductors for the detection of X- and γ-ray radiation. While Si and Ge have become detection standards for energy dispersive spectroscopy in the laboratory, their use for an increasing range of applications is becoming marginalized by one or more of their physical limitations; namely the need for ancillary cooling systems or bulky cryogenics, their modest stopping powers and radiation intolerance. Compound semiconductors encompass such a wide range of physical properties that it is technically feasible to engineer a material to any application. Wide band-gap compounds offer the ability to operate in a wide range of thermal and radiation environments, whilst still maintaining sub-keV spectral resolution at hard X-ray wavelengths. Narrow band-gap materials, on the other hand, offer the potential of exceeding the spectral resolution of both Si and Ge, by as much as a factor of 3. Assuming that the total system noise can be reduced to a level commensurate with Fano noise, spectroscopic detectors could work in the XUV, effectively bridging the gap between the ultraviolet and soft X-ray wavebands. Thus, in principle, compound semiconductor detectors can provide continuous spectroscopic coverage from the far infrared through to γ-ray wavelengths. However, while they are routinely used at infrared and optical wavelengths, in other bands, their development has been plagued by material and fabrication problems. This is particularly true at hard X- and γ-ray wavelengths, where only a few compounds (e.g., GaAs, CdZnTe and HgI 2 ) have evolved sufficiently to produce working detection systems. In this paper, we examine the current status of research in compound semiconductors and by a careful examination of material properties and future requirements, recommend a number of compounds for further development. In the longer term, when material problems are sufficiently under control, we believe the future lies in the development of heterostructures and inserted interface layers to overcome contacting problems and quantum heterostructures and superlattices to facilitate low-noise readout.

Compound Semiconductor Radiation Detectors

Transition metal (TM) doped II-VI chalcogenide laser materials offer a unique blend of physical, spectroscopic, and technological parameters that make them the gain media of choice for cost effective broadly tunable lasing in the Mid-IR. The II-VI semiconductor hosts provide a low phonon cut-off, broad IR transparency, and high thermal conductivity. When doped with transition metal ions, these materials feature ultrabroadband gain, low saturation intensities, and large pump absorption coefficients. This combined with the low-cost mass production technology of crystal fabrication by postgrowth thermal diffusion, as well as broad availability of convenient pump sources, make these materials ideal candidates for broadly tunable mid-IR lasing in CW, gain-switched, free running, and mode-locked regimes of operation. This review summarizes experimental results on optically pumped lasers based on Cr and Fe doped II-VI wide band semiconductors providing access to the 1.9-6 μm spectral range with a high (exceeding 60%) efficiency, multi-Watt-level (18 W in gain switch and 30 W in pure CW) output powers, tunability in excess of 1000 nm, short-pulse (<;50 fs) multi-watt oscillation, multi-Joule long-pulse output energy, and narrow spectral linewidth (<;100 kHz).

Progress in Mid-IR Lasers Based on Cr and Fe-Doped II–VI Chalcogenides

We report room-temperature operation of an all-solid-state broadly tunable continuous-wave Cr(2+):ZnSe laser. Output power of 250 mW, an absorbed power slope efficiency of 63%, and continuous tunability from 2138 to 2760 nm are demonstrated.

Continuous-wave broadly tunable Cr2+:ZnSe laser.

Crystal growth, fabrication and evaluation of cadmium manganese telluride gamma ray detectors

Preparation conditions of chromium doped ZnSe and their infrared luminescence properties

Lead iodide purification was performed by repeated sublimation at 390 °C. Studies of purity and stoichiometry were done by Inductively Coupled Plasma and Differential Scanning Calorimetric methods. These results were compared with those obtained from the same starting material through zone melting purification and with previous reports. Palladium contacts were attached to Teflon substrates with areas of 2 cm2 and lead iodide films 50 to 500 μm in thickness were deposited onto it by sublimation of the purified material at near 390 °C. Deposition conditions such as source state (powder, bulk), source-substrate distance and deposition time were optimized. Front palladium thermal deposition contacts and acrylic encapsulation were done on the radiation penetrating surface and electrical properties such as apparent resistivity and current density were measured. The X-ray film detection capabilities were checked by light pulses, X and gamma irradiation. Finally, the film properties were correlated with the starting material, the deposition process and previous data.

Lead iodide film deposition and characterization

ZnSe:Cr2+ is an attractive candidate as a room-temperature tunable solid-state laser with output in the 2–3 µm range. Passive absorption losses in this emission range currently limit laser performance. In this study, we use absorption and photoluminescence spectroscopies at 5 and 296K to address the origin of these optical losses. A series of diffusion-doped ZnSe:Cr single-crystal samples with Cr2+ concentrations in the range from 2×1017 cm−3 to 9×1019 cm−3 were obtained using CrSe powder as the dopant source. We find that trace amounts of Fe2+ produce absorption in the 2–3 µm range. Also, we have obtained data on a 680 nm absorption band observed in ZnSe:Cr which has been assigned to an internal transition of Cr2+. In our series of samples, the relative intensities of the 680 nm absorption band do not track the relative intensities of the 1.8 µm band (known to be due to Cr2+), although excitation near 680 nm does produce weak Cr2+ luminescence. Our absorption data do not support the current assignment of the 680 nm absorption as being an internal transition of the Cr2+ ion.

X. Ma

Papers

Continuous-wave broadly tunable Cr2+:ZnSe laser.

Crystal growth, fabrication and evaluation of cadmium manganese telluride gamma ray detectors

Preparation conditions of chromium doped ZnSe and their infrared luminescence properties

Lead iodide film deposition and characterization

Absorption and photoluminescence spectroscopy of diffusion-doped ZnSe:Cr 2p