Measurement of interface state density on pn-paterned Si surface using high- and low-frequency Kelvin probe force spectroscopy

抄録

With the recent miniaturization of semiconductor devices, understanding the physical and electrical properties of semiconductor devices, such as the dopant concentration, dopant distribution and defect level distribution, at the nanoscale has become important. Among the physical properties of semiconductors, information on semiconductor interface states is particularly important. For example, in semiconductor devices such as field-effect transistors, the presence of semiconductor interface states is known to significantly affect device operation characteristics. Therefore, direct observation of semiconductor surfaces with nanoscale spatial resolution will become even more important for understanding and controlling the effects of these properties on devices and for evaluating semiconductor device operation.

Recently, we proposed high—low frequency Kelvin probe force microscopy (high—low frequency KPFM) as a technique to solve the above problem [1]. High—low frequency KPFM is a method for measuring the magnitude and direction of band bending due to interface states by applying low-frequency and high-frequency AC bias voltages between the tip and the sample with respect to the cutoff frequency of carrier transport between the bulk and interface states and measuring the difference in CPD by KPFM. In high-low frequency KPFM, frequency modulation (FM) KPFM (FM-KPFM) combined with FM-AFM is used to detect the tip-sample interaction force. FM-KPFM has several advantages, namely, high sensitivity to the electrostatic force gradient, high detection sensitivity using a cantilever with a weak spring constant at the first resonance, ease of implementation in adding FM-AFM, and no need to enhance the bandwidth of the cantilever deflection sensor. FM-KPFM is used to apply an AC bias voltage at frequencies lower than the cutoff frequency of carrier transport, and heterodyne FM-KPFM, based on the heterodyne effect (frequency conversion effect) between mechanical oscillation of the cantilever and electrostatic force oscillation, is used to apply an AC bias voltage at frequencies higher than the cutoff frequency of carrier transport. To date, high—low frequency KPFM has successfully visualized the surface band bending of pn-patterned silicon substrates [1]. However, in high—low frequency KPFM, the CPD is compensated by a DC bias voltage, so a certain DC voltage, determined by the CPD, is applied to the semiconductor sample. Therefore, the surface potential of the semiconductor is fixed at a certain energy, and only the surface state near the Fermi level of the surface is reflected in CPD measurements, making measurement of the energy distribution of the interface states within the band gap difficult. Thus, a method for measuring the energy distribution of the interface states must be developed.

In this study, we propose high-low KPFS using high- and low-frequency AC bias voltages to measure the interface state density inside semiconductors [2]. We derive an analytical expression for the electrostatic force between the tip and the sample that takes into account the charge transfer between the bulk and interface states in the semiconductor. We show that the electrostatic force between the tip and the semiconductor sample strongly depends on the capacitance of the charge depletion region on the surface and that the analysis of the electrostatic force at low- and high-frequency AC bias voltages can provide information on the interface state density in the semiconductor band gap (Fig.1). We also demonstrate using a pn-patterned silicon substrate that the interface state density can be measured.

References

[1] R. Izumi, Y. J. Li, Y. Naitoh, Y. Sugawara, Microscopy, 2022, 71, 98.

[2] R. Izumi, M. Miyazaki, Y. J. Li, Y. Sugawara, Beilstein Journal of Nanotechnology, 2023, 14, 175.