Amorphous and Microcrystalline Silicon Deposited by Low-Power Electron-Cyclotron Resonance Plasma-Enhanced Chemical-Vapor Deposition

The structural and optoelectronic properties of intrinsic amorphous silicon (a-Si:H) and microcrystalline silicon ( µc-Si:H) deposited using electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) with a microwave power of 150 W were studied as a function of the ECR source-to-substrate distance, d ss , process pressure, hydrogen dilution and substrate temperature. Hydrogen was used as the excitation gas and silane was injected into the main chamber. Deposition rates show a maximum as a function of the deposition pressure. For d ss =6 cm this maximum occurs between 5 and 10 mTorr. ECR-deposited a-Si:H films show a high Tauc bandgap (∼1.9 eV), low dark conductivity (∼10 -11 Ω -1 cm -1 ), relatively high Urbach energy (≥55 meV) and high defect density (≥5×10 15 cm -3 ) compared with a-Si:H grown by RF glow discharge. Hydrogen evolution and infrared spectroscopy reveal the presence of voids and/or columnar structure. The transition from amorphous to microcrystalline silicon occurs under conditions of high hydrogen dilution, low deposition pressure, and low d ss . The higher the hydrogen dilution, the lower the substrate temperature needed to achieve µc-Si:H. Raman spectra of the µc-Si:H suggest small grain size ( ∼4 nm) and crystalline fraction (∼60%). A growth model is proposed that includes silane excitation both by the ECR electrons and by the excited hydrogen species.