THERMAL ANNEALING IMPROVES THE PERFORMANCE OF CADMIUM ZINC TELLURIDE ( CZT ) SEMICONDUCTOR MATERIAL USED FOR NUCLEAR RADIATION DETECTORS

For the past 20 years, extensive research was conducted to investigate the suitability of cadmium zinc telluride (CZT) crystals as a room-temperature nuclear detector material. Large-volume CZT crystals with extended thicknesses and large effective-areas are needed for nuclear-radiation detection in homeland-security applications. However, due to the defects present in the as-grown ingot, we rarely obtain perfect single crystals; most include structural defects, such as voids, pipes, impurities from source materials, tellurium inclusions and precipitates, vacancies, and vacancy-impurity complexes produced during crystal growth. Other extended defects include grain boundaries, micro twins, and walls of dislocations (subgrain boundaries). Identifying and controlling these defects in the bulk CZT material remain important issues that lessen the yield of material suitable for detectors. In this study, we employed a novel post-growth thermal annealing technique to remove one of the performance-limiting defects, i.e., tellurium inclusions, in CZT room-temperature material. This method employed state-of-the-art instruments at Brookhaven National Laboratory (BNL) encompassing 3D-infrared transmission spectroscopy and X-ray diffraction topography to probe and collect data before and after the post-growth annealing process. We believe that the experimental results offer a good understanding of the critical parameters and the dynamic properties of post-growth annealing processes. INTRODUCTION It was earlier demonstrated that both point defects and tellurium inclusions in CZT trap the charge carriers generated by nuclear radiation [1]. The amount of charge trapped by point defects is proportional to the carriers’ drift time, and can be corrected electronically [2]. For tellurium inclusions, the charge loss depends on their randomness of their locations with respect to the electron cloud. As a result, the inclusions introduce fluctuations in the charge signals, which cannot be electronically corrected. Sub-grain boundaries are a common defect in CZT crystals, among others. However, they cannot be revealed with infrared microscopy unless they are decorated by tellurium inclusions. Consequently, we used White-Beam X-ray Diffraction Topography (WBXDT) measurements, a technique that directly reflects the small lattice distortion they cause, and sensitively monitors sub-grain boundaries [3]. There always is a thermodynamic driving force for the formation of secondary phase particles during the growth of CZT. Larger particles are believed to arise from liquid inclusions at the growth interface, while smaller particles are thought to precipitate and grow in the solid upon cooling. There still is a significant lack of understanding of their dominant formation mechanism and changes during cool-down. While different growth conditions may be exploited to reduce their size and/or concentration, these particles likely always will be present. There are two common practical methods of treating such crystals: 1) Assuring the migration of tellurium ASME 2012 Early Career Technical Journal Vol. 11 47 inclusions via a temperature gradient and, 2) annealing in Cd vapor to reduce tellurium inclusions, followed by annealing in tellurium vapor to recover their high resistivity. For the first method, (i.e., causing the movement of tellurium inclusions using a temperature gradient), the approach causes the secondary phase particles to migrate away from part of the sample and accumulate elsewhere, leaving high-quality regions in the crystal that contain far fewer particles than the rest of it. This can be achieved by heating a part of the sample to certain temperature and so establishing a temperature gradient across it. Under such conditions, the now-liquid particle dissolves on the hot side and re-solidifies on the cool side, with a net effect of a migration toward the hotter region. This process is termed “temperature gradient zone melting,” or TGZM. Tiller [4] presented the first theoretical analysis of this phenomenon, and there are many general reviews [5–8]. TGZM reportedly caused the movement of tellurium inclusions under thermal gradients during the growth of CdTeand CZTcrystals [9,10]. Lee et al. [11] employed thermal gradients in a post-growth strategy to reduce the number of tellurium particles in CZT crystals, reporting migration velocities as fast as 50 μm/h. More recently, this idea was revisited by Meier et al. [12] also for CZT crystals. Where they showed laser-induced migration rates of 40 to 50 u μm/h The second method (i.e., annealing in Cd vapor to reduce tellurium inclusions followed by an annealing in tellurium vapor to recover the material’s high resistivity), comprises two steps: (a) Annealing in Cd vapor to reduce the number and size of tellurium inclusions; and, (b) annealing in tellurium vapor to recover the high resistivity of the crystal. There are two possible mechanisms operating during annealing. One is the diffusion mechanism wherein Cd atoms diffuse into the CZT crystals and react with excess tellurium to form CdTe. The other mechanism is that of migration mechanism where liquidphase tellurium droplets move to the crystal’s surface in a thermal-gradient field and react with Cd vapor. It is unclear which mechanism is the dominant one. In the second annealing method, the Cd vapor in the ampoule is saturated, and the vapor pressure of Cd annealing source is always higher than the Cd partial pressure of a crystal at the same temperature. Recently, it has been shown that small tellurium inclusions are eliminated from CZT crystals after annealing at 650 °C for 24 hours, whereas the larger inclusions persisted [13]. Also, inclusions lying on grain boundaries were removed faster during annealing under this condition than in the other one. High resistivity is required for nuclear radiation detectors as it helps reduce leakage currents and thus reduces the noise of the detector. Resistivity measurements are therefore a good metric for the determination on the performance of the crystals before and after annealing. Unfortunately, annealing in Cd overpressure results in a decrease of resistivity due to the addition of Cd interstitials. It has been proposed that the most effective method of retrieving the high resistivity of CZT crystals is through annealing with Te overpressure following annealing with Cd overpressure. The Te overpressure is proposed to reduce the Cd interstitials introduced during the first Cd overpressure annealing step. Recently, several groups have shown that the recovery of resistivity due to Te overpressure annealing is a possible and effective method [14]. MCA . Multi-Channel Analyzer CZT . Cadmium Zinc Telluride TGZM . Temperature Gradient Zone Melting Cd . Cadmium Te . Tellurium Zn . Zinc Ge . Germanium BNL . Brookhaven National Laboratory NSLS . National Synchrotron Light Source keV . kiloelectron volt THM . Traveling Heater Method IR . Infrared I-V . Current Voltage EXPERIMENTAL SETUP & PROCESS Before preparing the CZT crystals, we grew large ingots (10-15-cm long and roughly 3 cm in radius) via the Bridgeman or Travelling growth method. These ingots then were cut to a usable sample size, i.e. 5×5×5 mm. Samples were mechanically polished to diminish cutting damage first using an 800-grit paper, then a series of Al2O3 polishing powders, the grits declining in size from 5to 0.1-microns. Any remaining surface damage was removed through chemical etching with a Bromine-Methanol (B-M) solution. After final polishing of the crystal, we employed an infrared transmission microscopy to capture images of areas where Te inclusions are highly concentrated. After mapping areas where large concentrations of small Te inclusions (< 5 m diameter) or large Te inclusions (> 5 m), we recorded the coordinates of their locations. Data collected from the images of the samples were labeled as “before annealing”. After IR microscopy images were taken from the polished crystal, individual samples of crystallized CZT then were sealed in a quartz tube under vacuum (10 torr) along with a small piece of Cd, Te, or Zn. These single elements create the overpressure during the annealing process. The prepared ampoule quartz tube then was inserted into a high temperature computer-controlled furnace, capable of generating temperatures up to 1000 oC. The furnace attains these temperatures by using resistive-wire heating elements, wrapped around the outside of the ceramic work tube, so making it an integral part of the tube furnace’s heating element. Outside the ceramic work tube and the heating element, a thermocouple is located in a protected position, allowing the diameter of the ceramic work-tube to be used at its full potential and preventing mechanical damage to the thermocouple. The computer-controlled furnace can hold a set temperature with a tolerance of +/1.0 oC. This furnace design also supports the easy insertion and placement of an ampoule ASME 2012 Early Career Technical Journal Vol. 11 48 holder, with an attached thermocouple, into the ceramic work tube. During the annealing process, this thermocouple allows us to monitor temperatures present in the ampoule. Once the furnace is closed and sealed, the computer automatically controls the chosen time and temperature profile. In this study, the annealing conditions for the CZT samples were set to 510 C for 60 hours with a temperature gradient of 30 C/cm. After the CZT sample was annealed, the surface of the crystal became rough. At this point, the sample then is polished again and prepared for another IR microscopy study. Images of the sample are taken for “after annealing”, and data on the I-V measurements then are taken and compared with the results obtained before th