Band diagrams of intrinsic and p-type diamond with hydrogenated surfaces

We investigated the correlation between surface conductivity and surface band profiles of hydrogenated single crystal diamond (type IIa and IIb) by a combination of total photoelectron yield spectroscopy and I-V measurements. Electron and exciton contributions in the photoyield spectra are clearly separated and their response to thermal annealing and subsequent air exposure of the samples is found to be strictly correlated with the occurrence of surface conductivity. From this correlation we prove a long-ranging upward band bending in samples that show surface conductivity irrespective of bulk doping. This upward band bending drops to zero or even turns to downward bending when samples are annealed to temperatures above 200°C in vacuum and loose their surface conductivity. Both changes are fully reversible by air exposure. The results are analyzed in terms of the two currently discussed models for the surface-near hole layer that gives rise to the p-type surface conductivity, namely the electrochemical surface transfer model and the hydrogen induced sub-surface acceptor model. Whereas the former offers a straightforward interpretation, the latter requires a number of conditions to be met in order to be consistent with our results. One is that donor-like surface defects such as dangling bonds with densities exceeding the typical carrier sheet density of the conductive layer have to be assumed. Furthermore, a critical upper limit exists for the product of the carrier sheet density and the extension of the sub-surface acceptor layer into the bulk.