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

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Abstract
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 (≥5×1015 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.