Measurement Condition for Atomic Resolution imaging of Elementary Charge Using Electrostatic Force Microscope

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The electrostatic force microscope (EFM) offers new opportunity to measure a variety of electrostatic properties on the surface on a sub-micron scale, which result from nonuniform charge distribution and variations in surface work function. For example, potentiometry, imaging of contact-electrified charge and its dissipation process on insulating surfaces and so on have been demonstrated. Unfortunately, so far, the lateral resolution of the EFM has been insufficient. This is due to the following two main reasons : (i) It is rather difficult to measure weak distance dependence of the electrostatic force with a good signal-to-noise (S/N) ratio. (ii) It is much difficult to separate the electrostatic force from the van der Waals force.In this project, we demonstrated a novel method to detect the van der Waals and the electrostatic force interactions simultaneously, which was based on the Frequency modulation (FM) detection method in UHV. For the first time, the surface structure and the surface charge at atomic-scale point defects on the GaAs (110) surface were clearly resolved with true atomic resolution. The contrast of the electrostatic force image at a point defect showed monotonous change in the positive sample bias region, while it complicatedly changed in the negative sample bias region. From this bias voltage dependence, we could verify that the sign of the atomically resolved surface charge at the point defect was positive. Furthermore, we investigated the measurement condition of the atomic resolution imaging of the charge using the EFM.