Abstract
Conductive atomic force microscopy (CAFM) is one of the most powerful techniques in studying the electrical properties of various materials at the nanoscale. However, understanding current fluctuations within one study (due to degradation of the probe tips) and from one study to another (due to the use of probe tips with different characteristics), are still two major problems that may drive CAFM researchers to extract wrong conclusions. In this manuscript, these two issues are statistically analyzed by collecting experimental CAFM data and processing them using two different computational models. Our study indicates that: (i) before their complete degradation, CAFM tips show a stable state with degraded conductance, which is difficult to detect and it requires CAFM tip conductivity characterization before and after the CAFM experiments; and (ii) CAFM tips with low spring constants may unavoidably lead to the presence of a ~1.2 nm thick water film at the tip/sample junction, even if the maximum contact force allowed by the setup is applied. These two phenomena can easily drive CAFM users to overestimate the properties of the samples under test (e.g., oxide thickness). Our study can help researchers to better understand the current shifts that were observed during their CAFM experiments, as well as which probe tip to use and how it degrades. Ultimately, this work may contribute to enhancing the reliability of CAFM investigations.
Highlights
Since its invention in 1993 by Murrel et al [1], conductive atomic force microscopy (CAFM) has experienced continuous developments, and nowadays it has become one of the most powerful tools in studying the electrical properties of materials and devices at the nanoscale [2,3]
One of the main advantages of CAFM is that it allows collecting topographic and current information about the samples simultaneously and independently. This is possible because the topographic information is collected using an optical system, and the electrical information is collected using a preamplifier that is connected to the CAFM tip [2]
The samples that were used in this study consisted of 2 nm TiO2 grown by plasma enhanced atomic layer deposition system (PEALD, Savannah, Cambridge Nanotech, Cambridge, UK) on highly doped n-type Si (n++ Si) wafers with a resistivity of 0.008–0.02 Ω·cm−1
Summary
Since its invention in 1993 by Murrel et al [1], conductive atomic force microscopy (CAFM) has experienced continuous developments, and nowadays it has become one of the most powerful tools in studying the electrical properties of materials and devices at the nanoscale [2,3]. This problem might be minimized (up to a certain degree) by using stable CAFM tips (e.g., solid metallic tips [5]) These tips are much more expensive than standard metal-coated Si tips, but they may produce a reduction of the lateral resolution of the technique due to their larger RTIP. Manufacturers of solid highly-doped diamond tips claimed sub-nanometer lateral resolution [6], but these tips are so stiff that they can damage almost every sample Another possibility is to use metal-coated Si tips protected with a thin layer of graphene (which does not increase RTIP ), but this solution is still in an experimental stage [7,8,9]. It is very important to detect when these phenomena take place, otherwise the interpretation of the current signals that are detected by the CAFM would be erroneous
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