Atomic force microscopy (AFM) enables the characterization of various sample properties through local force detection. Electrostatic forces acting between the tip and sample can also be measured using AFM, an approach known as electrostatic force microscopy (EFM). In conventional EFM, the DC component or a specific frequency component of the electrostatic force is detected to investigate the surface potential and electrical tip–sample capacitance. However, the frequency range accessible to EFM is severely limited by the narrow bandwidth of conventional force-detection methods, such as amplitude-modulation (AM) and frequency-modulation (FM) techniques. Furthermore, in FM-EFM, the cantilever frequency shift induced by the electrostatic force is proportional to the force gradient, making it difficult to quantitatively determine the tip–sample capacitance from the measured frequency shift.

To overcome these limitations, we have developed dual-bias modulation EFM (DEFM), which enables capacitance measurements over a wide frequency range as well as quantitative evaluation of the capacitance. In DEFM, two AC voltages are applied between the tip and sample, and the electrostatic-force component at their difference frequency is detected using a lock-in amplifier. The two AC frequencies can be varied while keeping their difference frequency constant. This scheme enables measurement of the frequency-dependent capacitance associated with deep-level states in semiconductors. In addition, we have analytically derived a formula for quantitatively determining the tip–sample capacitance, including the semiconductor capacitance, from the measured cantilever frequency shift.

In this talk, we demonstrate the capabilities and practical applicability of DEFM through measurements of Cu(In,Ga)(Se,S)₂ photovoltaic materials and metal–oxide–semiconductor structures.