Recipe to minimize Te precipitation in CdTe and (Cd,Zn)Te crystals

IntroductionHigh quality CdZnTe (or CZT) crystals have the potential for use in room temperature gamma-ray and X-ray spectrometers. 1, 2 Over the last decade, the methods for growing high quality CZT have improved the quality of the produced crystals however there are material features that can influence the performance of these materials as radiation detectors. The presence of structural heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity), and secondary phases (SPs) [3][4][5][6][7][8][9][10][11] can have an impact on the detector performance. There is considerable need for reliable and reproducible characterization methods for the measurement of crystal quality. With improvements in material characterization and synthesis, these crystals may become suitable for widespread use in gamma radiation detection.Characterization techniques currently utilized to test for quality and/or to predict performance of the crystal as a gamma-ray detector include infrared (IR) transmission imaging, synchrotron X-ray topography, photoluminescence spectroscopy, transmission electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy. 1,6,7,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] In some cases, damage caused by characterization methods can have deleterious effects on the crystal performance. The availability of non-destructive analysis techniques is essential to validate a crystal's quality and its ability to be used for either qualitative or quantitative gamma-ray or Xray detection. The work presented herein discusses the damage that occurs during characterization of the CZT surface by a laser during Raman spectroscopy, even at minimal laser powers.Previous Raman studies have shown that the localized annealing from tightly focused, low powered lasers results in areas of higher Te concentration on the CZT surface. 16 This type of laser damage on the surface resulted in decreased detector performance which was most likely due to increased leakage current caused by areas of higher Te concentration. In this study, AFM was used to characterize the extent of damage to the CZT crystal surface following exposure to a Raman laser. AFM data reveal localized surface damage and increased conductivity in the areas exposed to the Raman laser beam. Materials and MethodsTwo different CZT crystals (CZT3-7-4 and 3-7-8) were used in this study and were obtained from Yinnel Tech (South Bend, IN). They were grown by the modified vertical Bridgman (MVB) method 27 and are composed of ~ 10% Zn content (Cd 1-x Zn x Te, with x=0.1).Both faces of the crystal were finely polished with a series of alumina grit sizes; the finest of which was 0.05 µm.CZT3-7-4 was identified as a moderate performer for gamma-ray spectrometry using an 241 Am source (Figure 1). Both the Cd-terminating face and the Te-terminating face of this crystal were exposed to an Argon ion laser (514.5 nm) in a Raman microscope system. Using the Raman laser, a series of highly damaged fiducial marks (~20 ...

Section: Resultsmentioning

confidence: 99%

AFM Characterization of Raman Laser-Induced Damage on CdZnTe Crystal Surfaces

Teague

Hawkins

Duff

et al. 2009

“…Consequently, the Te inclusions will move towards the high-temperature end. Vydyanath et al 4 first proposed such a migratory mechanism during their annealing work to improve the quality of CdTe and Cd 0.96 Zn 0.04 Te as a substrate for HgCdTe epilayer. We note that, compared with large-size Te inclusions, small ones are relatively difficult to migrate even under the same temperature gradient.…”

Section: Methodsmentioning

confidence: 99%

Post-growth Annealing of Cadmium Zinc Telluride Crystals for Room-Temperature Radiation Detectors

Yang

Bolotnikov

Fochuk

et al. 2012

We investigated the effects of post-growth annealing on cadmium zinc telluride crystals intended for use as room-temperature radiation detectors. Annealing under Cd vapor effectively eliminated Te inclusions. The material's resistivity was lowered, and loss of Zn component was observed. Annealing under Cd + Zn vapors similarly removed Te inclusions. Furthermore, after exposure to the two vapors, we noted a change in the crystal surface morphology, i.e., formation of patterns of parallel lines. In contrast to annealing under Cd vapor alone, high resistivity was maintained after suitably controlling the Cd and Zn pressures.

“…Defects such as inclusions and precipitates reduce the IR transmittance of the substrates 1,2 and strongly affect the final quality of the epitaxial films. [3][4][5] In the case of detector-grade material, large structural defects deteriorate the charge collection in the detector. 6 Inclusions are formed by capturing of the melt droplets on the growth interface during crystallization in contrast to precipitates, which are created during cooling of the crystal due to the retrograde solubility of the excess component in the solid.…”

Section: Introductionmentioning

confidence: 99%

Reduction of Inclusions in (CdZn)Te and CdTe:In Single Crystals by Post-Growth Annealing

Belas

Bugár

Grill

et al. 2008

CdZn)Te with the composition of 3% Zn and In-doped CdTe single crystals were annealed at various annealing temperatures and under various Cd or Te pressures with the aim of eliminating Te or Cd inclusions. Te inclusions were reduced by Cd-saturated annealing at temperatures above 660°C. Only small (<1 lm) residual dark spots, located at the original position of as-grown inclusions, were observed after annealing. The size of Cd inclusions was reduced by Te-rich annealing at temperatures higher than 700°C. A specific cooling regime was used to eliminate new small Te precipitates ($1 lm) concurrently formed on dislocations during Te-rich annealing. Poor infrared transmittance of samples with Cd inclusions was detected after Te-rich annealing; therefore, Cd-saturated re-annealing of annealed samples was used for increasing infrared transmittance to a value above 60%. Alternative models explaining the formation of star-shaped corona-surrounding inclusions are discussed.