Tellurium is one of the major constituents of II-VI compound semiconductors. The high purity (HP) tellurium is required for preparation of the compound semiconductors. The tellurium based semiconductor devices are used in infrared (IR) detectors, solar cells, thermo-electric coolers, etc. These devices are mainly used in strategic electronics such as defence, space, etc. Purification was done by electro-refining, vacuum distillation and zone refining. Majority of impurities are removed by electro-refining and vacuum distillation and analyzed by atomic absorption spectrometry (AAS). The impurities at ppb/ ppt level are removed by zone refining and analyzed by Mass spectrometry such as GDMS.
3N purity tellurium used as raw material for preparation of 7N tellurium. The purification are divided into two main process stages viz.,
The raw tellurium (3N pure) was kept in a graphite crucible and distilled at about 525°C under dynamic vacuum. The material deposited over the water cooled SS fingers was then subjected to ICP–OES analysis. The overall purity of trace impurity elements was more than 99×995% except selenium which is around 115 ppm. The gaseous impurities like oxygen and nitrogen in vacuum distilled tellurium was analysed on LECO make (model TC-236) nitrogen/oxygen determinator and observed both as < 1 ppm.
The chunks of vacuum distilled tellurium (5N pure except selenium) of nearly 1000 g weight was taken in a thoroughly cleaned and etched quartz (GE214 grade) boat container of dimensions 34 × 38 × 560 mm (ID × OD × L) and melted at a temperature, Tm, nearly 13 to 15% more than the melting point of tellurium in a uniform temperature gradient furnace under a constant flow of hydrogen gas (IOLAR I grade). After completely melting, the H2 gas flow continued till the tellurium bar temperature was cooled down to room temperature (RT) to avoid any possible oxidation. After taking out the tellurium bar, a small portion of the tellurium material at one end was cut and analysed to check for any added impurities during melting. It was observed from the analysis that the selenium impurity content was 115 ppm and the sum of the amounts of the remaining elemental impurities content indicated the purity of tellurium bar remaining nearly as 99.999 at.%.
A pre-stage fifteen-pass cycle zone refining experiment with circulation of coolant at (+ 5 to -80C) on the homogenized sample will be carried out in order to sensitize the system parameters and establish the suitable experimental conditions and narrow zone(s). In the second stage refining, to further narrow down the liquid region, a twenty-five–pass cycle zone refining experiment with circulation of coolant at a temperature of +5 to -15°C is carried out. In third stage, a twenty-five pass zone-leveling experiment on the sample is conducted with coolant temperature at +5 to –20°C.