CdS_xTe_1-x: bulk vapour growth, twin formation and the electrical activity of twin boundaries

CdS_xTe_1-x has been used as a model substance to investigate crystal growth from the vapour, twinning phenomena and the electrical properties of grain boundaries. A self-selecting vapour growth method produced solid solutions of CdS_xTe_1-x (x≈0.067) with compositional uniformity of <2{%} of x (i.e. <0.0013) as determined by x-ray diffraction and photoluminescence spectroscopy. The CdS_xTe_1-x crystals were highly twinned. Statistical tests showed that the twin boundaries are spatially correlated. A model for twin formation based on nucleation and grain size development is proposed. Band bending at grain and twin boundaries in CdTe and in CdS_xTe_1-x was measured using the scanning electron microscope remote electron beam induced current technique. Boundaries in CdTe have downward band bending, but those in CdS_xTe_1-x have upward band bending. Decoration of grain boundaries with Te inclusions reduced the band bending effect in CdTe. It is postulated that the grain boundary passivation in CdTe/CdS solar cells is partly caused by enrichment of the grain boundaries in CdTe with CdS_xTe_1-x, and partly by the Te enrichment that occurs during processing.