Cadmium zinc telluride (CZT) is a II-VI compound semiconductor widely used for X-ray and gamma-ray detection due to its high atomic number, excellent stopping power, and room-temperature operability. The material’s performance is heavily influenced by crystal growth techniques, defect control, and electrode fabrication. Two primary methods for growing CZT crystals are the Bridgman method and the traveling heater method (THM), each with distinct advantages and challenges in producing high-quality detector-grade material.
The Bridgman method is a vertical growth technique where a stoichiometric melt of cadmium, zinc, and tellurium is slowly cooled in a sealed quartz ampoule. The ampoule is translated through a temperature gradient, allowing the melt to solidify progressively from one end to the other. The growth parameters, such as temperature gradient (typically 5–15 K/cm), translation rate (0.5–3 mm/h), and melt composition, are critical in determining crystal quality. A slight excess of tellurium (0.1–0.5%) is often used to compensate for cadmium vacancies, which act as trapping centers and degrade charge transport properties. However, this excess can lead to the formation of tellurium inclusions, a major defect in Bridgman-grown CZT. These inclusions, ranging from 5 to 50 µm in size, act as recombination centers and reduce charge collection efficiency. Post-growth annealing in cadmium vapor at 600–800°C can reduce these defects by promoting tellurium diffusion and filling cadmium vacancies.
The traveling heater method is a solution-based growth technique that offers better control over stoichiometry and defect formation. In THM, a molten zone of tellurium-rich solvent moves through a solid feed rod of CZT, dissolving and recrystallizing the material. The lower growth temperature (900–1100°C compared to 1100–1300°C in Bridgman) reduces thermal stress and minimizes tellurium inclusion formation. THM-grown crystals typically exhibit fewer and smaller tellurium inclusions (1–10 µm) and lower dislocation densities. The method also allows for better zinc distribution uniformity, which is crucial for maintaining consistent detector performance. However, THM has slower growth rates (1–10 mm/day) and requires precise control of the solvent composition and temperature profile to avoid secondary phases like cadmium telluride (CdTe) or zinc telluride (ZnTe) precipitates.
Defect control in CZT is critical for achieving high-resolution detection. Tellurium inclusions, the most common defect, arise from constitutional supercooling and tellurium-rich conditions during growth. These defects can be minimized by optimizing the growth rate, temperature gradient, and melt composition. Grain boundaries, another major issue, form due to uncontrolled nucleation and can act as charge trapping sites. Single-crystal growth is preferred, but even in single-crystalline material, subgrain boundaries and dislocations may persist. Techniques like seed selection, controlled nucleation, and post-growth annealing help reduce these defects. Deep-level traps, often caused by impurities like iron or copper, can be mitigated through high-purity starting materials (6N or better) and gettering during growth.
Electrode fabrication is another key factor in CZT detector performance. Planar electrodes are the simplest configuration, with gold or platinum contacts deposited by sputtering or electroless plating. However, these can introduce electric field inhomogeneities due to surface states and Schottky barriers. Ohmic contacts are preferred, achieved by using indium or aluminum electrodes with appropriate surface treatments like bromine-methanol etching. Frisch-grid and pixelated electrode designs improve charge collection efficiency by shaping the electric field to minimize trapping effects. For high-energy applications, segmented electrodes with guard rings reduce leakage current and edge effects. The electrode material and deposition process must avoid introducing additional defects or contaminating the CZT surface.
The choice between Bridgman and THM depends on the application requirements. Bridgman is more scalable and cost-effective for large-volume production but may require extensive post-growth processing to achieve detector-grade quality. THM offers superior material homogeneity and lower defect densities but is slower and more complex. Advances in both methods, such as accelerated crucible rotation in Bridgman or solvent optimization in THM, continue to improve CZT crystal quality.
Recent research has focused on alternative growth techniques like the high-pressure Bridgman method, which suppresses tellurium inclusions by increasing the cadmium partial pressure during growth. Another approach is the use of dopants like indium or aluminum to compensate for deep-level traps and improve charge transport. However, these methods are still under development and not yet widely adopted for industrial production.
In summary, CZT crystal growth for radiation detection requires careful balancing of growth parameters, defect control, and electrode design. The Bridgman method offers practicality for large-scale production, while THM provides higher material quality at the expense of slower growth rates. Defect mitigation strategies, such as post-growth annealing and high-purity starting materials, are essential for achieving high-performance detectors. Electrode fabrication must be tailored to the specific detector geometry and application needs. Continued advancements in growth techniques and material processing will further enhance CZT’s capabilities for X-ray and gamma-ray detection.