Surface photovoltage, band-bending and surface states on a-Si : H

Abstract

We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between −168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at −168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (∼min), saturation at photon fluxes of about 10 11/cm 2 · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at −168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1−2) × 10 11/ cm 2, a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge.