The vaporisation of Hg_1-xCd_xTe crystals-a case of gross incongruency

The vaporisation of Hg_0.8Cd_0.2Te crystals under Knudsen effusion conditions has been investigated by the technique of modulated-beam mass spectrometry. Hg is the only dissociation vapour species detected at temperatures below 200 degrees C. Between 200 degrees C and 450 degrees C both Hg and Te₃ was present but Cd was detected only above 450 degrees C. This grossly incongruent vaporisation leads to considerable concentration gradients within the charge crystals which have been observed directly using SEM, EDEX and X-ray diffraction techniques. Crystals heated to 420 degrees C for several hours under Knudsen effusion conditions lose HgTe via the vapour species Hg and Te₂ and develop a surface layer of CdTe which, though porous, remains epitaxial. The development of concentration gradients within the charge at lower temperature leads to a diffusion rate limitation of the mercury vapour pressure developed within the Knudsen oven.