Development of dual bias modulation electrostatic force microscopy for variable frequency measurements of capacitance

Abstract

We propose dual bias modulation electrostatic force microscopy (DEFM) for variable frequency measurements of surface depletion capacitance on a semiconductor. In DEFM, dual alternating current bias voltages at angular frequencies of ω1 and ω2 are applied to generate an electrostatic force, and we detect the high order term at an angular frequency of ω2 − 2ω1 in the electrostatic force from which a derivative of surface depletion capacitance by voltage (∂C/∂V) can be evaluated. Even with a fixed value of ω2 − 2ω1 at a specific resonant frequency of the cantilever, to ensure sufficient sensitivity of the electrostatic force, a pair of ω1 and ω2 can be varied; this enables variable frequency measurements of ∂C/∂V by DEFM. The validity of the quantitation and spatial resolution of DEFM were assessed through the analysis of metal-oxide-silicon and Zn(O, S)/Cu(In,Ga)(Se,S)2 samples.

Quantitative capacitance measurements in frequency modulation electrostatic force microscopy

“…This equation will allow us to convert the frequency shift value acquired by FM-EFM into the capacitance if Q tip (V, z 0 ) is given. We previously proposed the dual bias modulation EFM (DEFM), 20,21) in which the dual AC voltages w V t sin 1 1 and w V t sin 2 2 were applied between the tip and sample together with DC voltage V DC , and, in DEFM, Q tip (V, z) can be approximated as…”

“…taken on the n-and p-type Si substrates, respectively. Here, the amplitudes of AC voltages V 1 and V 2 were equally set at 1 V. The modulation frequency ω 1 /2π was set at 300 kHz or 1 MHz, the latter of which was used as the high-frequency condition, being typical in the conventional C-V measurements, 20,[35][36][37] while the former one was chosen to be close to the resonant frequency ω 0 of the cantilever. Another modulation frequency ω 2 /2π was set at ω 1 /2π +200 Hz, meaning that ω d /2π was 200 Hz.…”

“…Hence, broadband and variable frequency measurements are difficult in SMIM, although some slow behaviors of electric charges, such as response of isolated charge traps in a semiconductor and charge migration in an organic material, are very interesting subjects to investigate locally. Using electrostatic force microscopy (EFM), 17) such broadband and variable frequency measurements of the capacitance-related signals have been realized by multiple bias modulation methods [18][19][20][21] or amplitude modulated voltage application techniques, 22,23) and an influence of surface states has been investigated. 20,24) Especially when the electrostaticforce modulated by the external bias between the tip and sample is detected by the frequency shift of the cantilever resonance, 25,26) which is a similar way to frequency modulation (FM)-AFM 27) and is referred to as FM-EFM here, the detected force is mainly dominated by the very local force acting on the tip itself by reducing a contribution from background force acting on the body of cantilever, 28) from which the tip-sample capacitance can be sensitively evaluated.…”

“…Using electrostatic force microscopy (EFM), 17) such broadband and variable frequency measurements of the capacitance-related signals have been realized by multiple bias modulation methods [18][19][20][21] or amplitude modulated voltage application techniques, 22,23) and an influence of surface states has been investigated. 20,24) Especially when the electrostaticforce modulated by the external bias between the tip and sample is detected by the frequency shift of the cantilever resonance, 25,26) which is a similar way to frequency modulation (FM)-AFM 27) and is referred to as FM-EFM here, the detected force is mainly dominated by the very local force acting on the tip itself by reducing a contribution from background force acting on the body of cantilever, 28) from which the tip-sample capacitance can be sensitively evaluated. When, however, we want to quantitatively evaluate a capacitance on a semiconductor sample by FM-EFM, the following point should be carefully taken into account:…”

Visualization of Charge Migration in Conductive Polymers via Time-Resolved Electrostatic Force Microscopy

“…Electrostatic force microscopy (EFM), which is based on atomic force microscopy, can be used to probe isolated local charges , and single electrons . This technique has previously been used to examine nanoscale charge behavior in natural systems and artificial devices, including inorganic semiconductors, catalysts, batteries, organic field-effect transistors, and organic photovoltaics. − These literature reports have demonstrated the power and utility of EFM for time-resolved charge tracking.…”