Electrostatic force microscopy studies of surface defects on GaAs/Ge films

We apply electrostatic force microscopy (EFM) to study defects in GaAs films grown on Ge. On a GaAs film with surface antiphase boundaries (APBs), we reproducibly measure the surface contact potential (SCP) at the APBs to be (30±5) mV higher than that of the domains, due to the surface Fermi level at APBs being pinned closer to the valence band maximum. On a thick film which contains buried APBs and wedge-shaped depressions on the surface, we find that the SCP of the wedge-shaped depressions is (25±5) mV lower than that of the GaAs surface. Hence, these wedge-shaped depressions have defect electronic states different from those of APBs. The capacitance gradient $\mathrm{(\partial C/\partial z)}$ contrasts on the two samples are also shown to arise from different origins. Factors that can affect the measured SCP and $\mathrm{\partial C/\partial z}$ values are discussed. We demonstrate a new application of EFM to distinguish different types of defects by measuring variations in relative SCP (thus the work function or position of Fermi level) and/or $\mathrm{\partial C/\partial z}$ on sample surfaces. The spatial resolutions of SCP and $\mathrm{\partial C/\partial z}$ are 30 nm, limited by the tip size.