Interpreting Kelvin probe force microscopy on semiconductors by Fourier analysis

Abstract

Kelvin probe force microscopy (KPFM) has become a popular surface scanning tool for functional materials and devices, and it has been widely interpreted by the contact potential difference (CPD) theory as the precedent Kelvin probe method. In this article, we developed a Fourier analysis framework for KPFM on the basis that the probe in KPFM is excited by a sinusoidal ac voltage, which is different from the classical Kelvin method. As a result, it was found that the KPFM signal will deviate from the CPD value if the sample charge quantity is not an odd function of the external bias, i.e., the CPD interpretation is invalid on those samples such as doped semiconductors. In order to further estimate the signal deviation from the CPD in the KPFM measurement on semiconductors, the tip–sample system was simulated as a one-dimensional metal–insulator–semiconductor capacitor using Fermi–Dirac statistics. The simulation results showed that the KPFM signals on doped semiconductors behave like those on an intrinsic one when the ac voltage is large, and therefore, the KPFM signal contrast on a pn junction could be flattened even if the sample surface has a clear CPD contrast without any Fermi level pinning due to surface states. Finally, possible ways for tuning KPFM operation parameters to suppress the signal deviation effect were also discussed.