Surface Photovoltage Effects in Photoemission from Diamond Surfaces
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)-(2x1) diamond surfaces
performed at temperatures ranging from 170 to 425 K. Photoelectron energy distributions demonstrate that, in contrast to the reconstructed (111 )-(2x 1) surface, the in-situ "rehydrogenated" diamond surface exhibits an apparenttemperature dependent Fermi level pinning position. These results can be understood within Hecht's model for photovoltaic charging during photoemission [6-8]. Furthermore, we find that photovoltaic charging in diamond can be significant even at room temperature. Therefore, the effects of photovoltaic charging must be addressed in photoelectron spectroscopy (PES) studies of wide band gap semiconductors such as diamond, cBN, AIN, etc.. Over the past several decades photoelectron emission has been used to study the electronic structure of semiconductor interfaces. Results from such photoemission studies were generally thought to represent the equilibrium band structure at the interface. Recently however, it has been shown that this is not always a well justified assumption, and that the photon flux used during the photoemission experiments can cause significant deviation from equilibrium even at room temperature due to photovoltaic charging [7, 9, 10]. Hecht [6-8] has calculated the effect of the surface photovoltage (AV) resulting from separation of electron-hole pairs in the band bending region for GaAs, and found that, at low temperatures, photovoltaic charging can be significant. After Hecht, in Fig. 1a we show the surface (no illumination) of a p-type semiconductor with an arbitrary density of surface states which exhibits downward band bending, EF-EVBM=Vb (EF is the energy of the Fermi level, and EVBM is the energy of the valence band maximum, VBM, at the surface). Under illumination with band gap radiation (Fig. lb), the photovoltage (AV) due to separation of the electron-hole pairs in the band bending region opposes the voltage associated 759 Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
Illuminated surface
No photons
Conduction Band
-e
:Band:Ga~p::) FF
F +e
Vb
Valence Band
V
(a)
(b)
FIG. 1 Energy band diagrams of a p-type semiconductor surface with an arbitrary, partially filled, density of states localized at the semiconductor-vacuum interface, (a) in the dark, and (b) with photon illumination. with the band bending. This, in the case of an electrically isolated surface, results in flattening of the bands (Fig. lb) and an apparent EF-EVBM=Vd (=Vb-AV) which is determined by the temperature dependent restoring current in the semiconductor. Hecht derived this restoring current in terms of thermionic emission over the surface barrier plus field emission through the barrier and found that it depends exponentially on temperature [6, 7]. EXPERIMENT Our photoemission experiments were performed on both the in-situ "re-hydrogenated" and reconstructed (I 11) surfaces of a 4 carat type Ilb (p-type, boron doped) natural single crystal diamond (sample D5). Before each study the diamond surface was mechanically polished on a ten-inch cast
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