In-line deposition of silicon-based films by hot-wire chemical vapor deposition

Silicon-based films such as hydrogenated amorphous silicon (a-Si:H), nanocrystalline silicon (nc-Si:H), and hydrogenated amorphous silicon nitride (a-SiNx:H) can be deposited by hot-wire chemical vapor deposition (HW-CVD). The HW-CVD technology differs from conventional plasma-enhanced (PE)-CVD in a number of technological aspects, such as soft activation, high growth rates, and low system costs. To evaluate the HW-CVD technology for thin film deposition in solar industry an in-line hot-wire CVD system was used to deposit a-Si:H films for passivation of crystalline silicon solar cells as well as for the fabrication of thin film silicon solar cells. The HW-CVD system consists of seven vacuum chambers including three hot-wire systems with maximum deposition areas of 500 mm by 600 mm for each hot-wire activation source. The deposition processes were investigated by applying design of experiment to identify the effects and interactions of the process parameters on the deposition characteristics and film properties. The process parameters investigated were silane flow, deposition pressure, substrate temperature, film thickness, as well as temperature, diameter and number of wires, respectively. Growth rates up to 2.5 nm/s were achieved for a-Si:H films. Intrinsic a-Si:H films for passivation of different crystalline solar cell types yielded carrier lifetimes of more than 1000 μs for film thickness values below 20 nm. For n-doped a-Si:H films prepared with PH3 as dopant gas, electrical resistivity is in the range of 102 Ω cm. P-doped a-Si:H films prepared with B2H6 as dopant gas show electrical resistivity of about 105 Ω cm. Crystalline silicon heterojunction solar cells with intrinsic thin layer (HIT cells) exhibit energy conversion efficiencies of more than 17% when fabricated with intrinsic HW-CVD amorphous silicon films as passivation layers.