Impurity segregation in horizontal Bridgman grown cadmium zinc telluride

Segregation of impurities that cause low infrared (IR) transmission in hori zontal Bridgman (HB) grown cadmium zinc telluride (CdZnTe) has been investi gated. This segregation was characterized using IR transmission, glow disch arge mass spectrometry (GDMS), and IR microscopy measurements. In the studi ed HB CdZnTe ingots, impurity segregation causes the formation of a small v olume in the last-to-freeze portion of the ingot that has high impurity con centration and low IR transmission. Outside this region the concentration o f impurities is low and the material shows high IR transmission The region is visibly observable on CdZnTe slices and appears as a dark area with a sh arp boundary. Free carrier absorption within the region causes a decrease i n IR transmission with an increase in wavelength and correlates with the co ncentration of lithium and sodium impurities. Impurity segregation in HB in gots is described and explains the location of the high impurity region. Th e location of the visible boundary correlates with the first measurable cha nge in IR transmission as compared to the high IR transmission of the surro unding material and supports the hypothesis that the darkening of the regio n is due to a reduction of the reflectivity due to free carrier absorption. With st properly controlled cool-down recipe, the impurities segregated in the last-to-freeze section of the ingots can remain localized, thereby imp roving the purity of the remaining bulk of the material.