Surface charge accumulation of InN films grown by molecular-beam epitaxy

A series of thin InN films down to 10 nm in thickness were prepared by molecular-beam epitaxy on either AlN or GaN buffers under optimized growth conditions. By extrapolating the fitted curve of sheet carrier density versus film thickness to zero film thickness, a strong excess sheet charge was derived, which must come from either the surface or the interface between InN and its buffer layer. Since metal contacts, including Ti, Al, Ni, and a Hg probe, can always form an ohmic contact on InN without any annealing, it is determined that at least part of the excess charge is surface charge, which was also confirmed by capacitance–voltage measurements.