Photoelectron spectroscopic investigations of very thin a-Si:H layers

We report on a detailed study on gap-state distribution in thin amorphous silicon layers (a-Si:H) with film thicknesses between 5 nm and 20 nm on c-Si wafers performed by UV excited photoelectron spectroscopy (UV-PES). We measured how the work function, the gap state density, the position of the Fermi-level and the Urbach-energy depend on the layer thickness and the doping level of the ultra thin a-Si:H(n) layers. It was found, that for phosphorous doping the position of the Fermi level saturates at E-F-E-V=1.47 eV. This is achieved at a gas phase concentration of 10000 ppm PH3 in the SiH4/H-2 mixture which was used for the PECVD deposition process. The variation of the doping level from 0 to 20000 ppm PH3 addition results in an increase of the Urbach energy from 65 meV to 101 meV and in an increase of the gap state density at midgap (E-V-E-i=0.86eV) from 3.10(18) to 2.10(19) cm(-3) eV(-1).