Investigation of Effects of Processing and Impurities on the Properties of CdTe Using Microscopic Two-Dimensional Photoluminescence Imaging Technique

We fabricated glass/SnO₂/CdS/CdTe samples with 100-nm-thick CdS by chemical-bath deposition and 8-mm-thick CdTe by close-spaced sublimation. A CdCl₂ heat treatment was done in the presence of CdCl₂ vapor at 400°C for 5 min. Our photoluminescence (PL) mapping is a direct-imaging technique in which the entire two-dimensional sample surface is imaged onto an electron-multiplying-CCD camera. PL at 850 nm corresponds to 1.458 eV, i.e., a Te-rich CdS_xTe_1-x alloy. For the asdeposited sample, the PL intensity is higher at the grain boundaries, indicating a higher concentration of the alloy in the grain-boundary region. The PL intensity image for 900 nm (1.38 eV) is similar to that of the 850-nm image. This indicates that Te-rich CdS_xTe_1-x is segregated preferentially in the grain-boundary region. PL images of the same area at 740 and 800 nm show widespread bright spots with a uniform background that is brighter for the 800-nm image. Results for the CdCl₂-treated samples for the 850- and 900-nm PL are similar to the as-deposited sample; however, the PL intensity is almost 5 times higher when the brighter regions are compared. This indicates a similar accumulation of Te-rich alloy for the CdCl ₂-treated sample as well, but with reduced nonradiative recombination and/or higher degree of alloying. The PL at 740 nm is similar to the as-deposited sample but with higher intensity (2.5 times) again, indicating lower nonradiative recombination.