In situ growth of Ge-rich poly-SiGe:H thin films on glass by RF magnetron sputtering for photovoltaic applications

Hydrogenated polycrystalline Si_xGe_1−x films, with a varying silicon fraction x ≤ 0.246, were in situ deposited in an argon and hydrogen mixture at 500 °C using radio frequency sputtering with an aim to develop a material for the bottom cell of a low cost monolithic tandem solar cell. Silicon and germanium atomic compositions of the films were determined by X-ray photoelectron spectroscopy (XPS). Structural evolution revealed by Raman and X-ray diffraction (XRD) indicated that the crystallinity of the films was improved with decreasing silicon fraction, accompanied with an increase of surface roughness verified by atomic force microscopy (AFM). Optical band gaps of these films derived from Tauc plots, which were calculated from reflectance/transmittance measurements, decreased with decreasing silicon fraction. Resistivity of the films, determined by four-point-probe technique, significantly decreased as well. High quality with low thermal budget obtained in this work suggests the films could be used in thin film solar cells on glass.