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-(perfluoronaphthalene-2,6-diylidene)dimalononitrile] to tune the work function () of intrinsically doped GaN(0001) and for comparison of intrinsically doped ZnO(0001). With ultraviolet photoelectron spectroscopy we determine the accessible range as 4.2–6.0 eV for GaN and 3.9–6.3 eV for ZnO. The contribution to the change () 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 . The lower the doping level is, the lower the surface state density needs to be to reduce the band bending contribution to . This limits the maximum value of tuning for GaN with molecular acceptors, whereas, on the other hand, it renders the method robust against
surface structural disorder.