Tuning the work function of GaN with organic molecular acceptors

We demonstrate the capability of two molecular organic acceptors [1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile and 2,2\ensuremath{'}-(perfluoronaphthalene-2,6-diylidene)dimalononitrile] to tune the work function (\ensuremath{\Phi}) of intrinsically doped GaN(0001) and for comparison of intrinsically doped ZnO(0001). With ultraviolet photoelectron spectroscopy we determine the accessible \ensuremath{\Phi} range as 4.2--6.0 eV for GaN and 3.9--6.3 eV for ZnO. The contribution to the \ensuremath{\Phi} change (\ensuremath{\Delta}\ensuremath{\Phi}) of acceptor-induced surface band bending within GaN was significantly smaller than in ZnO (0.35 versus 1 eV), which we attribute to surface gap states. We introduce a model that takes these surface states into account and thus allows quantifying the impact of the semiconductor bulk doping level and surface state density on \ensuremath{\Delta}\ensuremath{\Phi}. The lower the doping level is, the lower the surface state density needs to be to reduce the band bending contribution to \ensuremath{\Delta}\ensuremath{\Phi}. This limits the maximum value of \ensuremath{\Phi} tuning for GaN with molecular acceptors, whereas, on the other hand, it renders the method robust against surface structural disorder.