Plasma-enhanced chemical vapor deposition of intrinsic microcrystalline silicon from chlorine-containing source gas

Abstract

Microcrystalline silicon (μc-Si:H) of truly intrinsic character can be deposited by plasma-enhanced chemical vapor deposition when dichlorosilane (SiH2Cl2) is added to the SiH4–H2 source gas. A dark conductivity of 5×10−8 S/cm, activation energy of 0.62 eV, and photoconductivity of 1×10−5 S/cm are obtained. The optical band gap for this material is approximately 1.1 eV. No special gas purification or microdoping is required. SiH2Cl2 added in small amounts has the additional effects of enhancing the crystallinity, and of reducing the oxygen incorporation by over a factor of 2. Sub-band gap absorption spectroscopy indicates a low defect density. Very high frequency deposition yields material with lower defect density and higher photoconductivity than material deposited using dc plasma excitation. Transition from amorphous to microcrystalline growth occurs during the first 100–150 nm of film growth. The oxygen content increases as the crystallinity increases. A first p-i-n solar cell with a 1.8 μm thick μc-Si:H(:Cl) i layer exhibits Voc=0.35 V, Isc=4.14 mA/cm2, and FF=55%, demonstrating device-quality material.