Local band bending and grain-to-grain interaction induced strain nonuniformity in polycrystalline CdTe films

The local physical properties of polycrystalline semiconducting films drive their performances in a wide variety of optoelectronic devices but are still not completely elucidated. These properties are investigated and correlated on the same region of polycrystalline CdTe films by combining electron backscattered diffraction, mu-Laue x-ray experiments using synchrotron radiation, electron beam-induced current, and cathodoluminescence. The local band bending is revealed at random grain boundaries: its characteristics vary from one grain to another, depending on the nature of grain boundaries and the doping level in the nearby grains, in agreement with the theoretical approach of the double Schottky potential barriers. In contrast, no local band bending occurs at Sigma 3 growth twins since these extended defects have no dangling bonds in their center. Additionally, the density of unpaired dislocations and the components of the strain and stress tensors are found to be highly nonuniform from one grain to another and within the grains themselves. This reveals that grain-to-grain interactions (i.e., collective effects) occur during the Volmer-Weber-type growth. These findings emphasize the critical importance of grain boundary design engineering. They also highlight how polycrystalline semiconducting films work locally and show the complexity of the local physical processes governing their macroscopic performances in optoelectronic devices.