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

Abstract

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 ...

Composition dependence of structural and optical properties of Cd1−Zn Te thick films obtained by the close-spaced sublimation

“…We assume that the appearance of the strong Te-related modes is caused by local heating of the sample surface, as this was shown in [75,77,78]. This assumption was confirmed by RS measurement using 633 nm excitation wavelength.…”

“…In this case, the penetration depth of the green wavelength radiation for CdTe and ZnTe is only 200 and 400 nm, respectively [75]. Thus, the excitation light is absorbed in a thin near-surface layer that could lead to local heating of the material followed by Te enrichment on the surface, even at low green laser excitation power [75,77,78]. In order to determine the position of Te-related modes, the tellurium crystal was measured as a reference, using three available laser excitations wavelengths.…”

Synchrotron radiation x-ray absorption fine-structure and Raman studies on CdZnTe ternary alloys

“…As a result, this observation might be suggest that the ZnTe bonding was broken due to the highly focused laser which causes a localized surface heating effect and the free Te atoms migrate to the heated areas. Lucile C. Teague et al [42] had been used AFM and Raman to study the localized surface damage in the areas exposed to the Raman laser beam on Cd 1-x Zn x Te crystal surface with x=0.10. They found that (1) localized surface heating causes Cd material to be evaporated from the surface, which in some cases causes voids in the surface, and (2) free Te atoms are migrating to these heated areas.…”

Photo-induced currents in CdZnTe crystals as a function of illumination wavelength

“…[7] More information on these crystals has been published elsewhere. [5,8,12,13] The surfaces of each crystal were mechanically polished using standard techniques, with a final 0.05 µm alumina polish. Gold (Au) contacts were sputtered onto the as-polished crystal and the bulk currents were measured at room temperature as described below.…”