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

We investigate band bending, electron affinity and work function of differe ntly terminated, doped and oriented diamond surfaces by X-ray and ultraviol et photoelectron spectroscopy (XPS and UPS). The diamond surfaces were poli shed by a hydrogen plasma treatment and present a mean roughness below 10 A ngstrom. The hydrogen-terminated diamond surfaces have negative electron af finity (NEA), whereas the hydrogen-free surfaces present positive electron affinity (PEA). The NEA peak is only observed for the boron-doped diamond ( 100)-(2 x 1):H surface, whereas it is not visible for the nitrogen-doped di amond (100)-(2 x 1):H surface due to strong upward band bending. For the bo ron-doped diamond (111)-(1 x 1):H surface, the NEA peak is also absent due to the conservation of the parallel wavevector component (k(parallel to)) i n photoemission. Electron emission from energy levels below the conduction band minimum (CBM) up to the vacuum level E-vac allowed the electron affini ty to be measured quantitatively for PEA as well as for NEA. The emission f rom populated surface states forms a shoulder or a peak at lower kinetic en ergies, depending on the NEA. behavior and additionally shows a dispersion behavior. The low boron-doped diamond (100)-(2 x I):H surface presents a hi gh-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 si tuated 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 sim ilarly, showing that the NEA peak is present due to the downward band bendi ng independent of the boron concentration. The nitrogen-doped (100)-(2 x 1) :H surface shows a low NEA of -0.2 eV but no NEA peak due to the strong upw ard band bending. The (111)-(1 x 1):H surface does not show a NEA peak due to the k(1) conservation in photoemission; E-vac is situated at 4.2 eV or b elow, 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 becau se of hydrogen termination. Upon hydrogen desorption at higher annealing te mperatures, the work function increases, and NEA disappears. For the nitrog en-doped diamond (100) surface, the work function behaves similarly, but th e observation of a NEA peak is absent because of the surface barrier formed by the high upward band bending. (C) 1998 Elsevier Science B.V. All rights reserved.