Local redistribution of dopants and defects induced by annealing in polycrystalline compound semiconductors

The annealing of chlorine-doped polycrystalline CdTe films leads to the occurrence of an abnormal grain structure with the formation of both primary and secondary grains, the latter being larger and growing at the expense of the former. The spatial distribution of dopants and defects has been investigated within both types of grains by time-of-flight secondary-ion-mass spectroscopy imaging and spatially resolved cathodoluminescence. It is found that chlorine atoms similarly segregate in the vicinity of grain boundaries in both primary and secondary grains, whereas chlorine donors are homogeneously distributed away from grain boundaries. It is shown that, contrary to primary grains, secondary grains exhibit specific concentration processes around grain boundaries, such as cadmium vacancy accumulation and dislocation piling up. The existence of long-range stresses around grain boundaries only within secondary grains is also evidenced and attributed to piezoelectric effects. Grain boundaries thus act as getters for dopants and defects by draining them from the interior of secondary grains. These physical mechanisms emphasize efficient purifying effects associated with the beneficial formation and growth of secondary grains within an abnormal grain structure as induced by annealing. The depicted processes can be applied to a wide variety of polycrystalline compound semiconductors and can find direct applications in the field of solar cells based on these materials.