Regensburg 2016 – scientific programme
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O: Fachverband Oberflächenphysik
O 51: Organic-Inorganic Systems II: Energy Level Alignment
O 51.2: Talk
Wednesday, March 9, 2016, 11:00–11:15, S054
Work Function Increase of GaN and the Influence of Surface Gap States — •Thorsten Schultz1, Raphael Schlesinger1, Jens Niederhausen1,2, and Norbert Koch1,2 — 1Institut für Physik & IRIS Adlershof, Humboldt-Universität zu Berlin, 12489 Berlin, Germany — 2Helmholtz-Zentrum Berlin für Materialien und Energie, 12489 Berlin, Germany
The two molecular organic acceptors HATCN and F6-TCNNQ were vacuum-deposited on non-intentionally doped GaN (0001), which had a residual donor concentration of 1016 cm-3. By means of ultraviolet photoelectron spectroscopy, a huge work function increase (up to 1.5 eV for HATCN and 1.7 eV for F6-TCNNQ) was observed for monolayer coverage, which stems mostly from an interface dipole between substrate and molecules. The contribution of band bending within GaN (ca. 0.35 eV, as determined from X-ray photoelectron spectroscopy) was found to be significantly smaller than expected from theoretical calculations that predict the contribution of band bending to be dominating for low doping concentrations. A qualitative model is introduced, which assigns the low band bending contribution to the presence of substrate surface gap states. These states donate electrons to the acceptor molecules, so less charge is transferred from the GaN donors. This results in less band bending change than expected from calculations, which did not take surface states into account. Our investigations show that the energy level tuning scheme via acceptor interlayers, already successfully employed for ZnO, holds great promise for GaN as well.