X- and gamma-ray detectors have broad applications ranging from medical imaging to security, non-proliferation, high-energy physics and astrophysics. Detectors with high energy resolution, e.g. less than 1.5% resolution at 662 keV at room temperature, are critically important in most uses. The efficacy of adding selenium to the cadmium zinc telluride (CdZnTe) matrix for radiation detector applications has been studied. In this paper, the growth of a new quaternary compound Cd_0.9Zn_0.1Te_0.98Se_0.02 by the Traveling Heater Method (THM) is reported. The crystals possess a very high compositional homogeneity with less extended defects, such as secondary phases and sub-grain boundary networks. Virtual Frisch-grid detectors fabricated from as-grown ingots revealed ~0.87–1.5% energy resolution for 662-keV gamma rays. The superior material quality with a very low density of defects and very high compositional homogeneity heightens the likelihood that Cd_0.9Zn_0.1Te_0.98Se_0.02 will be the next generation room-temperature detector material.

Because of its ideal band gap, high density and high electron mobility-lifetime product, cadmium zinc telluride (CdZnTe or CZT) is currently the best room-temperature compound-semiconductor X- and gamma-ray detector material. However, because of its innate poor thermo-physical properties and above unity segregation coefficient for Zn, the wide spread deployment of this material in large-volume CZT detectors is still limited by the high production cost. The underlying reason for the low yield of high-quality material is that CZT suffers from three major detrimental defects: compositional inhomogeneity, high concentrations of dislocation walls/sub-grain boundary networks and high concentrations of Te inclusions/precipitates. To mitigate all these disadvantages, we report for the first time the effects of the addition of selenium to the CZT matrix. The addition of Se was found to be very effective in arresting the formation of sub-grain boundaries and its networks, significantly reducing Zn segregation, improving compositional homogeneity and resulting in much lower concentrations of Te inclusions/precipitates. Growth of the new quaternary crystal Cd_1−xZn_xTe_1−ySe_y (CZTS) by the Traveling Heater Method (THM) is reported in this paper. We have demonstrated the production of much higher yield according to its compositional homogeneity, with substantially lower sub-grain boundaries and their network, and a lower concentration of Te inclusions/precipitates.

Both material quality and detector performance have been steadily improving over the past few years for the leading room temperature radiation detector material cadmium-zinc-telluride (CdZnTe). However, although tremendous progress being made, CdZnTe still suffers from high concentrations of performance-limiting defects, such as Te inclusions, networks of sub-grain boundaries and compositional inhomogeneity due to the higher segregation coefficient of Zn. Adding as low as 2% (atomic) Se into CdZnTe matrix was found to successfully mitigate many performance-limiting defects and provide improved compositional homogeneity. Here we report record-high performance of Virtual Frisch Grid (VFG) detector fabricated from as-grown Cd_0.9Zn_0.1Te_0.98Se_0.02 ingot grown by the Traveling Heater Method (THM). Benefiting from superior material quality, we achieved superb energy resolution of 0.77% at 662 keV (as-measured without charge-loss correction algorithms) registered at room temperature. The absence of residual thermal stress in the detector was revealed from white beam X-ray topographic images, which was also confirmed by Infra-Red (IR) transmission imaging under cross polarizers. Furthermore, neither sub-grain boundaries nor their networks were observed from the X-ray topographic image. However, large concentrations of extrinsic impurities were revealed in as-grown materials, suggesting a high likelihood for further reduction in the energy resolution after improved purification of the starting material.

X- and gamma-ray detectors are increasingly becoming essential tools for science and technology in various fields. These detectors offer broad applications such as homeland security, nonproliferation, nuclear security, medical imaging, astrophysics, and high energy physics. All these applications demand high-resolution detectors operable at room temperature and available at a reasonable cost. CdZnTe (CZT) is the material of choice for this purpose; however, the material still suffers from intrinsic defects such as highly decorated subgrain boundary networks and a high concentration of secondary phases. These defects not only hinder the charge transport but also create a spatial inhomogeneity in the charge transport properties, subsequently causing substantial degradation in detector response particularly for relatively thick (>1 cm) detectors. Some of the material deficiencies suffered by CZT have been addressed by adding selenium into the CZT matrix. Selenium was found to be very effective in producing material that is principally free from a subgrain boundary network with the occasional appearance of subgrain boundaries with reduced secondary phases decorating these boundaries. The resulting quaternary compound CdZnTeSe (CZTS) showed excellent material quality mitigating some major deficiencies suffered by CZT. Virtual Frisch grid detectors were fabricated from the as-grown CZTS ingots, and they demonstrated high resolution spectroscopic grade. The excellent CZTS material contained very low defects and was found to potentially increase the yield of high-quality detectors as compared to CZT.

Detection of X- and gamma-rays is essential to a wide range of applications from medical imaging to high energy physics, astronomy, and homeland security. Cadmium zinc telluride (CZT) is the most widely used material for room-temperature detector applications and has been fulfilling the requirements for growing detection demands over the last three decades. However, CZT still suffers from the presence of a high density of performance-limiting defects, such as sub-grain boundary networks and Te inclusions. Cadmium zinc telluride selenide (CZTS) is an emerging material with compelling properties that mitigate some of the long-standing issues seen in CZT. This new quaternary is free from sub-grain boundary networks and possesses very few Te inclusions. In addition, the material offers a high degree of compositional homogeneity. The advancement of CZTS has accelerated through investigations of the material properties and virtual Frisch-grid (VFG) detector performance. The excellent material quality with highly reduced performance-limiting defects elevates the importance of CZTS as a potential replacement to CZT at a substantially lower cost.

The electron- and hole-transport properties in cadmium zinc telluride selenide (CZTS) crystals are studied using a laser-induced transient-current technique with pulsed and dc bias. The internal electric field profile and velocity of surface recombination are determined by Monte Carlo simulations of electron and hole transient currents combined with a numerical solution of the drift-diffusion equation coupled with Poisson’s equation. Electron and hole drift mobilities of ${\mu }_e$ = 830 ${\mathrm{cm}}^2$/Vs and ${\mu }_h$ = 40 ${\mathrm{cm}}^2$/Vs, respectively, are determined. We also develop a simple technique for evaluating surface recombination directly from measured current waveforms without the need for numerical simulation. The good quality of the prepared detector at pulsed bias, with electron- and hole-mobility-lifetime products of $(\mu \tau {)}_e$ = 1.9 × ${10}^{-3}$ ${\mathrm{cm}}^2$/V and $(\mu \tau {)}_h$ = 1.4 × ${10}^{-4}$ ${\mathrm{cm}}^2$/V, respectively, are observed. The formation of a positive space charge, originating from hole injection combined with a recombination level, is found. We observe a significant position dependence of the lifetime of electrons and holes in dc bias due to hole injection. The experiment is successfully fitted by a simple model dominated by a single deep recombination level with an energy of $E_t=E_C-0.73\mathrm{eV}$; concentration of 7.3 × ${10}^{11}$ ${\mathrm{cm}}^{-3}$; and electron- and hole-capture cross sections of 3.5 × ${10}^{-14}{\mathrm{cm}}^2$ and 6.5 × ${10}^{-14}{\mathrm{cm}}^2$, respectively.