2, University of Toledo, Toledo, Ohio, United States
Three sets of solar cell quality CdTe thin film samples were prepared: two sets were grown by rf magnetron sputtering whereas the other was grown by closed space sublimation. Each set was subsequently divided into 3 subsets: one subset was left as-grown, one was treated with CdCl2, and one was treated with CdCl2 and further etched with Br2/MeOH. Grazing incidence X-ray diffraction and photothermal deflection spectroscopic measurements were performed on all samples in order to study the effects of the treatments on the structural and optical properties of the resultant polycrystalline thin films. For sputtered CdTe, grazing incidence X-ray diffraction shows that the as-grown preferred surface orientation is the <111> direction. After CdCl2 treatment, however, the intensity of the <111> peak is greatly reduced, and other peaks appear, representing a random orientation. Further treatment with Br2/MeOH leaves the sample unchanged from this polycrystalline form. The samples grown by closed space sublimation do not have a preferred <111> orientation and are structurally insensitive to post-growth treatments. Photothermal deflection spectroscopy was performed on all samples to determine the shape of the optical absorption spectrum of the samples. For the sputtered films, the optical absorption edge becomes sharper when the films are treated with CdCl2, with an additional small sharpness increase when the sample is further etched in a Br2/MeOH solution. For the sputtered films, the Urbach parameter, E0, which is inversely proportional to the sharpness, decreasing after CdCl2 treatment and showed little to no change after etching. A direct comparison of the optical absorption curves shows that the absorption from the gap states increases after CdCl2 treatment and further increases after the Br2/MeOH etching.