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Thin solid films with thicknesses of some nm play a crucial role in heterostructures and quantum well superlattices. The Fermi level position, gap state densities and band offsets are important quantities which dominate their electronic properties. A method for measuring these quantities is the photoemission spectroscopy with excitation by near ultra-violet light. The near UV-excitation of photoelectrons in thin solid layers with photon energies below 10eV avoids the excitation of plasmons and entails a drastically enhanced photo-ionisation probability of the occupied states. This results in photoelectron escape depth up to 10nm and optical excitation probabilities enhanced by several orders of magnitude compared to excitation with soft X-rays. This allows the direct observation of the density distribution Nocc(E) of occupied states down to 1015states/cm3. The constant final state yield spectroscopy mode (CFSYS) was used to measure Nocc(E) for a-Si:H with nanometer (2-20nm) thin n- and p-doped a-Si:H layers of varying doping level. The Fermi level shifts from 0.4 eV below the conduction band via 0.2 eV above the intrinsic level to 0.45 eV above the valence band edge for PH3-doped, intentionally undoped and B2H6-doped a-Si:H layers, respectively. For i-, n- and p-type a-Si:H layers prepared at different temperatures on c-Si substrates we found that the correlation between Urbach energies and dangling bond (db) densities, as measured at µm thick layers is qualitatively fulfilled for thin layers (<20nm), too. However, the dangling bond density is enhanced by 1-2 orders of magnitude. A minimum of db-density of states is reached at deposition temperatures of 230 °C. For extremely thin a-Si:H layers on c-Si a contribution of c-Si valence band states to the photoemission signal was obtained. Using the CFSY method it is possible to determine the band offset between a-Si:H and c-Si exactly. This was cross-checked for all combination of p- and n-type c-Si substrate and all three a-Si:H layer doping types. As expected, we obtained in all cases same values of ΔEv=0.45±0.1eV and ΔEc=0.15eV±0.2eV independent of the doping sequence. |