Effect of an inter-electrode distance in VHF-PECVD and gas flow ratios on a-SiGeC:H films and solar cells by using monomethyl germane as a germanium source

Hydrogenated amorphous silicon–germanium–carbide (a-SiGeC:H) has been fabricated by monomethyl germane (MMG, GeH3CH3) under various deposition conditions, inter-electrode distance (dele) in VHF-PECVD and gas flow ratios. With decreasing dele, it is observed from optical emission spectroscopy (OES) that the generation of atomic hydrogen in plasma gradually increases. It is also found that the enhanced atomic hydrogen tends to take both Ge and C away from growing surfaces, leading to the decrease in Ge and C contents. The total content of hydrogen bonds increases as a result of the increase in both SiH and GeH bonds as the dele decreases. Consequently, the a-SiGeC:H solar cells fabricated at narrower dele exhibit the improved performance. Even though the optical band gap (Eopt) of the a-SiGeC:H increases with decreasing dele, the quantum efficiency (QE) spectra reveal even an increasing trend of long wavelength regions because of the significant improvement in i-layers. It is also confirmed that a lot lower MMG/SiH4 was needed for the films having certain Eopt, when fabricated near amorphous-to-crystalline transition, and the solar cell fabricated near the transition region shows the better performance.