Light-Induced Tellurium Enrichment on CdZnTe Crystal Surfaces Detected by Raman Spectroscopy

Abstract

Synthetic CdZnTe or "CZT" crystals can be grown under controlled conditions to produce high quality crystals to be used as room temperature radiation detectors. Even the best crystal growth methods result in defects, such as tellurium secondary phases, that affect the crystal's performance. In this study, CZT crystals were analyzed by micro Raman spectroscopy. The growth of Te rich areas on the surface was induced by low powered lasers. The growth was observed versus time with low power Raman scattering and was observed immediately under higher power conditions. The detector response was also measured after induced Te enrichment.

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

“…Topographic images show that these areas of increased conductivity correspond to a pile-up of material that occurs on the edges of the laser damaged areas-consistent with a combination of both of the aforementioned hypotheses that (1) Cd material is being evaporated from the surface and (2) free Te atoms are migrating to these heated areas. These data, in combination with previous Raman results, 16 suggest that the increased conductivity is a result of a localized increase in Te in the areas exposed to the laser. We also note that the difference in conductivity between the Te rich laser damaged areas and the surrounding bulk is an order of magnitude smaller than that of similar C-AFM studies of surfaceterminated Te rich SPs.…”

“…Using the Raman laser, a series of highly damaged fiducial marks (~20 mW) were created as well as a series of lower laser power exposure areas (~160 mW to 1.7 mW) for subsequent AFM evaluation (see Figure 2). Raman spectra were collected simultaneously with the surface laser exposure with a 100X objective and a cooled charge-coupled device (CCD) camera detector.The Raman spectra of the untreated, freshly polished surface are consistent with those reported previously, 16 and were collected with lower laser power (less than 100 µW) in order to reduce the damage to the crystal.The surface of CZT3-7-8 was also exposed to a Raman laser (632.8 nm, HeNe) in four different regions for ~30 seconds (s) each. Previous studies of this crystal showed that while it has a high concentration of SPs within the crystal, the detector performance was not adversely influenced.…”

“…This behavior has been compared to that of melted Te metal. 16 This suggests that the laser, being focused very tightly on the surface, even at low power, causes a localized surface annealing effect and the enrichment of Te relative to the surrounding bulk CZT material. Thermomigration has been suggested as a probable mechanism to explain the increase in Te concentration near the heated area.…”

“…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.…”

“…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.…”