Gamma-Ray Response of Semi-Insulating CdMnTe Crystals
Semi-insulating Cd0.9Mn0.1Te:In crystals are grown by vertical Bridgman method. The segregation coefficient of Mn in CdTe is nearly 1 so that all the CdMnTe samples obtained from one ingot have nearly a same Mn composition. Also sulfur-based passivation effectively prevent the formation of Te-oxide but large amount of Mn exist as a MnO on the CdMnTe surface. The resistivity of CdMnTe samples are low 1010 Omegacm and well resolved 241Am gamma peaks are seen for all detectors. The difference in spectral response can be attributed to the effect of excess Te and conductivity change due to over-compensation induced by indium segregation in CdMnTe. The mobility-lifetime products evaluated from the dependence of peak location on the bias voltage are 1 times 10-3 cm2/V. The higher mobility-lifetime products in our CdMnTe crystals than other previous reports are assumed due to minimization of impurity contents in MnTe by several zone refining process.
New Approaches for Making Large-Volume and Uniform CdZnTe and CdMnTe Detectors
Although CdZnTe (CZT) and CdMnTe (CMT) materials are leading contenders for room-temperature semiconductor detectors, nonetheless, both materials have limitations hindering their full usage in producing economical, uniform, large-volume devices due to their grain/twin boundaries, material purity, secondary-phase Te defects and material segregation. We tried to prevent the generation of twin and subgrain boundaries to achieve large-volume CZT crystals by means of local temperature control between the CZT melt and quartz crucible. Also, we have expanded the understanding of the electrical and structural properties of coherent/incoherent twin boundaries. The high residual impurities in the starting source materials, especially in manganese, were identified as obstacles against obtaining high-performance CMT detectors. We found that purifying manganese telluride (MnTe) via a floating Te melt-zone very effectively removes impurities, leading to better detectors. CMT detectors fabricated with purified material give a 2.1% energy resolution for 662 keV with a $^{137}$Cs gamma source without any electron-loss corrections. Secondary-phase Te defects deteriorate detector performance due to incomplete charge collection caused by charge trapping. In situ growth interface studies reveal the thermo-migration of Te inclusions to CZT melts and the dependence of Te-inclusion size on the cooling rate. The effective segregation coefficient of Zn in the CdTe host is nearly 1.3, so about 5%–6% of Zn deviation was reported in Bridgman-grown CZT (Zn $\,=$10% ingots. Such uncontrolled Zn variations cause a significant variation of the band-gap throughout the ingot and, consequently, affect the nonuniformity of the detectors' responses. Practically, this means that manufacturers cannot cut the ingot parallel to the crystal growth direction. We also demonstrated that the segregation of Zn can be controlled by creating particular thermal environments after growth.
Purification methods of Cd, Te and CdTe and periodicity of segregation coefficients of admixtures
The purification methods and their advantages and disadvantages for refining of Cd, Te and CdTe are summarized. Equilibrium segregation coefficients of admixtures in Cd, Te and CdTe were calculated from phase diagrams and compared with literature values. Their periodical dependence on the atomic number of admixtures is discussed. Theoretical and technological aspects of the production of high-purity materials are described via rectification, zone melting, and normal freezing with removal of non-purified material.