Correlated structural reordering and dopant redistribution in annealed polycrystalline CdTe

Postdeposition heat treatments are found to strongly influence the structural and optical properties of chlorine doped polycrystalline CdTe films. From scanning electron microscopy images and diffraction measurements, annealing is shown to induce a major structural reordering. The strong texturation along the << 100 >> orientation indicates that the overall crystalline quality has been considerably improved. Upon annealing, a bimodal distribution of grain sizes is also observed, which points out an abnormal grain growth mechanism. Additionally, deformation twins, which are detrimental for the transport properties, are replaced by thicker annealing twins. Through time-of-flight secondary ion mass spectroscopy and 4 K cathodoluminescence measurements, an overall decrease in the chlorine concentration and a spatial redistribution of chlorine atoms are evidenced and clearly correlated with the observed structural reordering: beta acceptor complexes, which are commonly predominant in as-grown chlorine doped polycrystalline CdTe, are found to dissociate into chlorine donors and A centers upon annealing. This is attributed to the thermally activated diffusion of chlorine, which leads to its re-evaporation from the films. Furthermore, the low chlorine concentration in the annealed films allows one to infer a value of 135 +/- 3 meV for the ionization energy of the chlorine A centers, as deduced from the donor-acceptor pair position in the cathodoluminescence spectra.