Photoemission studies on in situ prepared hydrogenated amorphous silicon films

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Abstract
Photoemission studies involving the valence and core levels of doped and undoped amorphous hydrogenated silicon (a-Si: H) prepared by the glow-discharge method are reported. The valence-band spectra show states identified as hydrogen ls/silicon 3p—3s bonding orbitals which are sensitive to the bonding configuration of hydrogen. Singly and multiply bonded hydrogen have been distinguished unambiguously. A recession of the valence-band edge (E v) of up to 1 eV is observed for the highest hydrogen concentrations (∼50%). The amounts of boron and phosphorus incorporated into a-Si: H as dopants have been estimated through the corresponding core-level intensities. The incorporation efficiencies are 80% (P) and 70% (B) respectively. However, only 10% of the incorporated atoms act as dopants. The position of the Fermi level relative to E v as a function of dopant concentration has also been determined. By combining these positions with activation energies obtained from conductivity measurements, the location of the transport regions for p- and n-type conductivity was deduced. Electron transport takes place at the bottom of the conduction band, while a region of high density of states ∼0·3 eV above E v is responsible for the p-type conduction. This latter region of gap states also shows up in photoemission spectra.