Influence of Dynamic Temperature Adjustments During Growth on the Material Properties of CZT Radiation Devices

The advantages of Cadmium Zinc Telluride (CZT) based detectors are their wide bandgap, high atomic number, and room temperature operability. CZT-based detectors provide higher energy resolution compared with scintillator detectors (which behave as indirect convertors and require a second stage, such as photomultiplier tubes, to convert low energy scintillation photons into electronic charge, decreasing the signal to noise ratio), while commercially available semiconductors such as Ge require cryogenic working temperatures (and equipment) due to the high leakage currents associated with a low energy band gap (0.67eV at 300K respectively). There are several challenges to producing detector grade material based on CZT, which includes material synthesis, growth of electrically compensated crystals, as well as the extraction of high resistivity material suitable for device applications. Principle among these is the choice of the growth method use for material synthesis and crystal growth. One of the challenges towards producing large volumes of compensated material using the Vertical Gradient Freeze (VGF) method is the axial and radial variation in material homogeneity. For CZT, the segregation coefficient of Zn has been reported near 1.35 (1). A discussion of how zinc segregation affects mechanical strain in CZT is presented in (2), and in general describes how changes in the concentration of Zinc leads to misfit strain in the crystal. In fact, strain is one of the more important factors in the crystal growth of CZT because of the low critically resolved sheer stress (CRSS) value for this material. Sources of strain may be attributed to several factors including variation of Zn composition along the growth axis, melt-crucible interactions, as well as the applied temperature gradients at the SLI. The volume coefficients of thermal expansion for CdTe and ZnTe have been reported to be βCdTe = 1.5× 10-5 K-1 and βZnTe = 2.9× 10-5 K-1 (3) (4). As a result, the stress induced by a thermal gradient of 10 ºC/cm is near 2.5 MPa, which is on the same order of magnitude as the CRSS for CZT (1). More importantly, using global temperature gradients on the order of 10 ºC/cm leads to a difference in temperature between the tip and tail of up to 100ºC. Taking into account the melting point of CZT is near 1100ºC and the softening point of quartz is near 1160º, these types of temperature gradients should not be applied for extended durations such as those required by crystal growth (200+ hours). In this work, methods to reduce the thermo-mechanical stress imparted into the crystal have been implemented. Specifically, crystals have been grown under dynamic temperature gradients to minimize the temperature gradient across the ingot, while maintaining relatively higher temperature gradients at the Solid Liquid Interface (SLI). How this adjustment affects bulk resistivity and photoconductivity has been investigated. In this study, we investigate test devices harvested from the ingot along the axial direction, and studied in terms of their optical and electrical properties to investigate (1) how these temperature adjustments may lead to variations in material properties and (2) how this variation may affect test device performance.