Electron affinity and work function of differently oriented and doped diamond surfaces determined by photoelectron spectroscopy

Abstract

We investigate band bending, electron affinity and work function of differently terminated, doped and oriented diamond surfaces by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The diamond surfaces were polished by a hydrogen plasma treatment and present a mean roughness below 10 Å. The hydrogen-terminated diamond surfaces have negative electron affinity (NEA), whereas the hydrogen-free surfaces present positive electron affinity (PEA). The NEA peak is only observed for the boron-doped diamond (100)-(2×1):H surface, whereas it is not visible for the nitrogen-doped diamond (100)-(2×1):H surface due to strong upward band bending. For the boron-doped diamond (111)-(1×1):H surface, the NEA peak is also absent due to the conservation of the parallel wavevector component ( k ‖) in photoemission. Electron emission from energy levels below the conduction band minimum (CBM) up to the vacuum level E vac allowed the electron affinity to be measured quantitatively for PEA as well as for NEA. The emission from populated surface states forms a shoulder or a peak at lower kinetic energies, depending on the NEA behavior and additionally shows a dispersion behavior. The low boron-doped diamond (100)-(2×1):H surface presents a high-intensity NEA peak with a FWHM of 250 meV. Its cut-off is situated at a kinetic energy of 4.9 eV, whereas the upper limit of the vacuum level is situated at 3.9 eV, resulting in a NEA of at least −1.0 eV and a maximum work function of 3.9 eV. The high-boron-doped diamond (100) surface behaves similarly, showing that the NEA peak is present due to the downward band bending independent of the boron concentration. The nitrogen-doped (100)-(2×1):H surface shows a low NEA of −0.2 eV but no NEA peak due to the strong upward band bending. The (111)-(1×1):H surface does not show a NEA peak due to the k ‖ conservation in photoemission; E vac is situated at 4.2 eV or below, resulting in a NEA of at least −0.9 eV and a maximum work function of 4.2 eV. The high-intensity NEA peak of boron-doped diamond seems to be due to the downward band bending together with the reduced work function because of hydrogen termination. Upon hydrogen desorption at higher annealing temperatures, the work function increases, and NEA disappears. For the nitrogen-doped diamond (100) surface, the work function behaves similarly, but the observation of a NEA peak is absent because of the surface barrier formed by the high upward band bending.