Spatial and time-resolved single-dopant band bending fluctuations in semiconductors measured with electrostatic force microscopy*

In frequency-modulated atomic force microscopy (fm-AFM) the measured frequency shift is quadratic in applied bias for metallic samples and probes. However, for semiconducting samples, band bending effects must be considered, resulting in non-parabolic bias curves. We have developed a theoretical framework to quantitatively describe a metal-insulator semiconductor (MIS) device formed out of a metallic AFM tip, vacuum gap, and semiconducting sample [1]. This framework allows us to measure dopant concentration, bandgap and band bending timescales of different types of defects on semiconductors with nm scale resolution.

Using fm-AFM we experimentally demonstrate two-state fluctuations on a MoSe2 sample [1]. We show that potential noise in this device is intrinsically bias-dependent due to the bias-dependent surface potential. Importantly, this source of potential fluctuations does not require that the frequency or magnitude of individual dopant fluctuations are themselves bias-dependent.

At a semiconductor surface, polarization is related to the time dependent charging of a quantum capacitor. In such a system, dielectric dispersion occurs because there is an intrinsic timescale to establish a surface potential, due to charge screening and finite carrier mobility. In MoSe2, we measured spatial inhomogeneities in this band bending (or charge reorganization) timescales, with an average of 30ns. The spatial variability of interfacial screening at a Si(001) surface was measured with nm resolution and ns precision for the dielectric dispersion as a function of dopant concentration [2]. We find that the dispersion time scales are dopant density dependent and highly sensitive to spatially localized states.

Work performed in collaboration with Megan Cowie, Rikke Plougmann, Zeno Schumacher, Josephine Spiegelberg, Adam Prus-Czarnecki (all McGill University); Taylor Stock and Neil Curson (both University College London).

[1] Single-dopant band bending fluctuations in MoSe2 measured with electrostatic force microscopy

M. Cowie, R. Plougmann, Z. Schumacher, and P. Grutter

Phys. Rev. Materials 6, 104002 (2022)

[2] M. Cowie et al., manuscript in preparation