Preferential etching of Si–Si bond in the microcrystalline silicon germanium

Hydrogenated microcrystalline silicon germanium (μc-Si1−xGex:H) films were investigated as a bottom cell absorber in multi-junction solar cells. μc-Si1−xGex:H films were prepared using very high frequency (VHF, 60 MHz) plasma enhanced chemical vapor deposition (PECVD) systems working pressure of about 1.5 Torr. The precursor flow rates were carefully controlled to determine the phase transition point and to improve the crystallinity of μc-Si1−xGex:H. A relatively high plasma power was necessary to have the high hydrogen (H2) dilution. Raman spectroscopy study showed transition steps from amorphous to microstructure morphology as hydrogen dilution increasing. Crystallite Si–Ge and Ge–Ge bonds were occurred at relatively higher H2 dilution compare to crystallite Si–Si bond. The rapidly increased Ge content as increasing the H2 dilution is believed mainly due to the different decomposition rate of silane (SiH4) and germane (GeH4). The other reason of high Ge content even at the low GeH4 precursor flow rate is probably due to the preferential etching of silicon atom by H2. The preferential etching of Si–H possibly occurred in very highly concentrated H2 plasma due to the preferential attachment of Si–H. The compositions of μc-Si1−xGex:H films measured using RBS were Si0.83Ge0.17, Si0.67Ge0.33 and Si0.59Ge0.41 at H2/SiH4 flow rate of 60, 80 and 100, respectively. μc-Si1−xGex:H films showed the dark (σd) and photo conductivity (σp) of about 10−7 and 10−5 S/cm, respectively and photo response (σp/σd) was about 102. This study will present the comprehensive evaluation of crystallization behavior of μc-Si1−xGex:H films.