Inline Microwave-Excited Plasma Deposition of Thin Amorphous Silicon Layers

Intrinsic hydrogenated amorphous silicon (a-Si:H(i)) passivation layers with a thickness < 7 nm were deposited by an industrial scale inline deposition technique with a very high dynamic deposition rate of around 35 nm*m/min. An implied open circuit voltage near an injection level of one sun of 714 mV on flat mono-crystalline silicon wafers was achieved. The a-Si:H(i) layers were characterized by spectral ellipsometry, infra-red spectroscopy and the quasi-steady-state-photo-conductance method. The highest passivation quality for the inline microwave excited plasma enhanced chemical vapor deposition (MW-PECVD) of a-Si:H(i) layers was achieved at deposition temperatures of 325 350 °C. Due to the high deposition temperature no additional annealing is necessary to improve the passivation quality. The layer roughness and the band gap of the deposited a-Si:H(i) layers decrease, as well as the Si-H2 concentration with an increased deposition temperature. A hydrogen dilution of the deposition plasma can be advantageous for the passivation quality of a-Si:H(i) layers at lower temperatures, but detrimental for higher temperatures. Furthermore, a hydrogen dilution was obtained as a method to change the band gap and the surface roughness of the deposited a-Si:H(i) layers. The growth dynamic of the inline deposited a-Si:H(i) layers was studied and a formation of silicon di-hydrid was found to build the initial layer of growing a-Si:H(i) films, as already observed for static deposited a-Si:H(i) in the literature earlier. The inline deposition of these thin a-Si:H(i) layers is the first step to produce silicon heterojunction solar cells by a complete inline process.