Role of growth temperature and the presence of dopants in layer-by-layer plasma deposition of thin microcrystalline silicon (μc-Si:H) doped layers

Abstract

Microcrystalline silicon (μc-Si:H) p- and n-type layers have been developed by Layer-by-Layer (LbL) deposition at high temperatures. The LbL deposition consists of alternating boron or phosphorus doped amorphous silicon depositions and hydrogen plasma treatments by Very High Frequency Chemical Vapor Deposition (VHF PECVD). The layers are developed to be resistant to the temperature and hydrogen flux of a micro- of polycrystalline intrinsic layer grown at a high deposition rate in a p-i-n or an n-i-p solar cell device. It is concluded that the LbL method is suitable to produce device quality μc-Si:H p- and n-type doped layers in a temperature range from 250 to 400 °C. This is not possible with standard continuous PECVD employing high hydrogen dilution of silane, where the addition of dopants reduces the crystallinity. An optimum effective thickness per deposition cycle (total thickness divided by the number of cycles) of 1.5 nm/cycle is needed for the crystallization. This optimal effective sub layer thickness is independent of dopants and deposition temperature. However, a minimum thickness of the first layer is needed for a sustaining growth in the LbL process. The doped layers grown by LbL are smoother than reference samples grown by continuous wave (cw). The doping efficiencies in our LbL deposited layers are structurally higher than those in cw deposition (for p layers a doping efficiency of 39% in case of LbL, compared to 1% for cw). The properties of the best high-temperature doped layers are as follows: for LbL p-type μc-Si:H (Ts=350 °C, 29 nm), activation energy=0.11 eV and dark conductivity=0.1 Ω−1 cm−1; for LbL n-type μc-Si:H (Ts=400 °C, 31 nm), activation energy=0.056 eV and dark conductivity=2.7 Ω−1 cm−1. Test solar cells have been deposited using Hot-Wire CVD (HWCVD) and VHF PECVD deposited μc-Si:H i-layers on top of the high-temperature LbL μc-Si:H n-type doped layer in an n-i-p configuration on a stainless steel substrate without a back reflector. A high open circuit voltage of 0.56 V and a fill factor of 0.7 show the high doping efficiency and crystallinity of the n-type doped layer and the resistance to the impinging atomic hydrogen during the HWCVD deposition. The mechanism behind the LbL μc-Si:H growth phenomenon is a controversial subject. We studied the LbL growth and nucleation mechanism as well as the incorporation of dopant atoms in the μc-Si:H layers. Etching, abstraction, and hydrogen diffusion are analyzed and it is concluded that our observations support the nucleation model that is based on hydrogen diffusion, while chemical transport and epitaxial growth are excluded to be the mechanism behind the crystallization.