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 (mu c-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 m Torr. ECR-deposited a-Si:H films show a high Tauc bandgap (similar to 1.9eV), low dark conductivity (similar to 10(-11) Omega(-1) cm(-1)), relatively high Urbach energy (greater than or equal to 55 meV) and high defect density (greater than or equal to 5x10(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 mu(c)-Si:H. Raman spectra of the mu c-Si:H suggest small grain size (similar to 4 nm) and crystalline fraction (similar to 60%). A growth model is proposed that includes silane excitation both by the ECR electrons and by the excited hydrogen species.