Invited talk abstract

Diamond despite being a bona fide insulator with a gap of 5.4 eV exhibits a special kind of surface conductivity (SC) of the order of 10^-5 W^-1. The conductivity is confined to a surface layer of a few tens of nm and is carried by an areal hole concentration of about 10^13 cm-2. The SC can be controlled by a gate electrode and has therefore been utilized in the form of a unique kind of field effect transistor. Despite ten years of research the mechanism responsible for the hole accumulation layer remained unsolved. The only reliable fact was that surface hydrogenation is required to induce SC. Consequently, a number of publications have suggested that an as yet unspecified hydrogen related subsurface defect acts as acceptor that provides the necessary hole concentration.

We have combined conductivity measurements in ultra high vacuum (UHV) and under atmospheric conditions with spectroscopic methods such as photoelectron emission spectroscopy and infrared (ir) spectroscopy in the attenuated total reflection (ATR) mode to elucidate the connection between surface conditions and SC. We find that surface hydrogenation is a necessary but not a sufficient condition for SC. In addition, species from the atmosphere are required that act as acceptors. Furthermore, the acceptor level has to coincide with the valence band maximum (VBM) in order to induce the surface band bending that is connected with the subsurface hole concentration referred to above, i. e., the VBM has to coincide with the Fermi level. This in turn requires that the adsorbate has an electron affinity of at least 4.2 eV which is higher than that of the most electronegative adsorbates imaginable, namely halogen atoms. However, charge exchange between diamond and a thin water layer as it forms automatically on all surfaces exposed to air is possible by the neutralisation of oxonium ions according to the reaction 2H3O+ + 2e- n H2 + 2H2O. The electrochemical potential of this reaction turns out to be of the correct magnitude (4.2 eV below the vacuum level for a pH of 6 that is maintained by dissolved CO2 from the atmosphere) to allow electron transfer from the diamond to the water layer (electrolyte). The holes remaining in the VB induce an upward band bending and form the subsurface accumulation layer. The electron transfer comes to a halt when the band bending has reached a degree such that the Fermi level of diamond and the electrochemical potential are aligned (the electrochemical potential is tied to the vacuum level and thus to the VBM of diamond). At this point the hole concentration has the value of 10^13 cm-2 required for the SC. The neutralizing negative charges reside in the form of anions, presumably carbonate (HCO3-) in the electrolyte and they can even reside on the surface after the sample is brought into UHV and other species have desorbed from the surface. By this the surface conductivity is maintained in a metastable configuration and can be gradually removed by thermal annealing. The hydrogenation of diamond is necessary in this scenario because it results in the lowering of the vacuum level (negative electron affinity of diamond) such that the ionisation energy (vacuum level-VBM) reaches a value (4.2 eV) comparable with the electrochemical potential of a slightly acidic water layer. It is this last point that makes hydrogenated diamond unique. No other semiconductor has such low ionisation energy.