Effective p + -Doping of a-Si:H and a-SiC:H Layers by Plasma Assisted Boron Diffusion
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EFFECTIVE p+-DOPING OF a-Si:H AND a-SiC:H LAYERS BY PLASMA ASSISTED BORON DIFFUSION H.-D. MOHRING, G.H. BAUER, G. BILGER, AND C.E. NEBEL Institut fuer Physikalische Elektronik, Universitaet Stuttgart Pfaffenwaldring 47, D-7000 Stuttgart 80, Fed. Rep. Germany ABSTRACT Doping of a-Si:H and a-SiC:H has been performed by plasma enhanced boron migration from thin (5-30 nm) Sin B0 8 layers on top of the substrate into the growing i-film. Transport coef cients~for thjý plasm1 7 ass~sted diffusion (PH) epaluated from SIMS profiles (D = 10-
. .. 10
cm /s for
cm /s for ajiC:ý) by far exceed those for thermal diffusion a-Si:H, 10 cm /s). Boron profiles are strongly governed by after film deposition (10deposition parameters like substrate temperature, types and energies of radicals growing the film. Boron migration can be modelled assuming particle extraction from the network to the surface by bombardment of ions and atoms and high surface mobility of BH x radicals as a result from interaction with the plasma. Temperature dependent dark conductivity measurements show doping efficiencies comparable to B H6 gas phase doping, but leaving plasma parameters optimized for i-film Leposition unchanged. Thin a-SiC:H layers are SiB p -dopeddetectable without affecting effectively after PAD.transmission, as no residual layer is optically
INTRODUCTION Boron incorporation into the amorphous silicon network by adding B H to the silane gas in glow discharges is accompanied by a perceptible moaification of the deposition process; subsequently structural properties of Bdoped films in comparison to intrinsic a-Si:H like growth rate, hydrogen content, optical band gap, refractive index are affected, and granular structure is induced /1-6/. Furthermore there are indications that plasma deposition creates interface states at the p+/i-interface which reduce solar cell efficiency /7/. We have performed p+-layer doping in a-Si:H and a-SiC:H by plasma enhanced boron migration from thin B containing layers on top of the substrate into the growing intrinsic film. In this way p-doped layers can be prepared without changing plasma parameters optimized for i-film deposition. EXPERIMENTAL The samples for analyses of B-diffusion and doping effects were grown onto 7059 glass substrates, one half coated with thin (5 - 30 nm) rfsputtered Si 0 B8 layers. Intrinsic films have been deposited by different preparation mthooas: - dc glow discharge in SiH4 and SiH 4 /CH4 mixtures
Mat. Res. Soc. Symp. Proc. Vol. 70. 1986 Materials Research Society
332
- modified CVD method with predissociation of SiH 4 in a plasma region which is locally separated from the deposition region in order to avoid interaction of charged particles from the plasma with the growing film and to reduce kinetic energy of hot radicals by molecular impacts in the gas flow to the substrate - rf-sputtering in Ar/H 2 mixtures. SIMS depth profiling procedure (depth resolution 2 nm) has recently been described in detail /8/. Temperature dependent electrical conductivity measurements have been car
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