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.
Mechanically exfoliated 2D hexagonal boron nitride (h‐BN) is currently the preferred dielectric material to interact with graphene and 2D transition metal dichalcogenides in nanoelectronic devices, ...as they form a clean van der Waals interface. However, h‐BN has a low dielectric constant (≈3.9), which in ultrascaled devices results in high leakage current and premature dielectric breakdown. Furthermore, the synthesis of h‐BN using scalable methods, such as chemical vapor deposition, requires very high temperatures (>900 °C) , and the resulting h‐BN stacks contain abundant few‐atoms‐wide amorphous regions that decrease its homogeneity and dielectric strength. Here it is shown that ultrathin calcium fluoride (CaF2) ionic crystals could be an excellent solution to mitigate these problems. By applying >3000 ramped voltage stresses and several current maps at different locations of the samples via conductive atomic force microscopy, it is statistically demonstrated that ultrathin CaF2 shows much better dielectric performance (i.e., homogeneity, leakage current, and dielectric strength) than SiO2, TiO2, and h‐BN. The main reason behind this behavior is that the cubic crystalline structure of CaF2 is continuous and free of defects over large regions, which prevents the formation of electrically weak spots.
Calcium fluoride (CaF2) is an attractive high‐k dielectric for 2D electronics because its surface is terminated with fluorine atoms, which leads to a defect‐free van der Waals interface with 2D materials. Ultrathin CaF2 ionic crystals (grown by molecular beam epitaxy at 250 ºC) are shown to exhibit outstanding dielectric performance (i.e., homogeneity, leakage current, and dielectric strength), which is much better than that observed in SiO2, TiO2, and h‐BN.
Conductive atomic force microscopy (CAFM) is a powerful technique to investigate electrical and mechanical properties of materials and devices at the nanoscale. However, its main challenge is the ...reliability of the probe tips and their interaction with the samples. The most common probe tips used in CAFM studies are made of Si coated with a thin (∼20 nm) film of Pt or Pt-rich alloys (such as Pt/Ir), but this can degrade fast due to high current densities (>102A/cm2) and mechanical frictions. Si tips coated with doped diamond and solid doped diamond tips are more durable, but they are significantly more expensive and their high stiffness often damages the surface of most samples. One growing alternative is to use solid Pt tips, which have an intermediate price and are expected to be more durable than metal-coated silicon tips. However, a thorough characterization of the performance of solid Pt probes for CAFM research has never been reported. In this article, we characterize the performance of solid Pt probes for nanoelectronics research by performing various types of experiments and compare them to Pt/Ir-coated Si probes. Our results indicate that solid Pt probes exhibit a lateral resolution that is very similar to that of Pt/Ir-coated Si probes but with the big advantage of a much longer lifetime. Moreover, the probe-to-probe deviation of the electrical data collected is small. The use of solid Pt probes can help researchers to enhance the reliability of their CAFM experiments.
Conductive atomic force microscopy (CAFM) has become the preferred tool of many companies and academics to analyze the electronic properties of materials and devices at the nanoscale. This technique ...scans the surface of a sample using an ultrasharp conductive nanoprobe so that the contact area between them is very small (<100 nm2) and it can measure the properties of the sample with a very high lateral resolution. However, measuring relatively low currents (∼1 nA) in such small areas produces high current densities (∼1000 A/cm2), which almost always results in fast nanoprobe degradation. That is not only expensive but also endangers the reliability of the data collected because detecting which data sets are affected by tip degradation can be complex. Here, we show an inexpensive long-sought solution for this problem by using a current limitation system. We test its performance by measuring the tunneling current across a reference ultrathin dielectric when applying ramped voltage stresses at hundreds of randomly selected locations of its surface, and we conclude that the use of a current limitation system increases the lifetime of the tips by a factor of ∼50. Our work contributes to significantly enhance the reliability of one of the most important characterization techniques in the field of nanoelectronics.
The influence of the probe tip type on the electrical oxide thickness result was researched for four differently coated conductive tip types using SiO2 (oxide) films with optical thickness of ...1.7-8.3nm. For this purpose, conductive atomic force microscopy (C-AFM) was used to measure more than 7200 current-voltage (IV) curves. The electrical oxide thickness was determined on a statistical basis from the IV-curves using a recently published tunnelling model for C-AFM application. The model includes parameters associated with the probe tip types used. The evolution of the tip parameters is described in detail. For the theoretical tip parameters, measured and calculated IV-curves showed excellent agreement and the electrical oxide thickness versus the optical oxide thickness showed congruent behaviour, independent of the tip type. However, differences in the electrical oxide thickness were observed for the different tip types. The theoretical parameters were modified experimentally in order to reduce these differences. Theoretical and experimental tip parameters were compared and their effect on the differences in the electrical oxide thickness is discussed for the different tip types. Overall, it is shown that the proposed model provides a comprehensive framework for determining the electrical oxide thickness using C-AFM, for a wide range of oxide thicknesses and for differently coated conductive tips.
Mechanically exfoliated 2D hexagonal boron nitride (h-BN) is currently the preferred dielectric material to interact with graphene and 2D transition metal dichalcogenides in nanoelectronic devices, ...as they form a clean van der Waals interface. However, h-BN has a low dielectric constant (≈3.9), which in ultrascaled devices results in high leakage current and premature dielectric breakdown. Furthermore, the synthesis of h-BN using scalable methods, such as chemical vapor deposition, requires very high temperatures (>900 °C) , and the resulting h-BN stacks contain abundant few-atoms-wide amorphous regions that decrease its homogeneity and dielectric strength. Here it is shown that ultrathin calcium fluoride (CaF
) ionic crystals could be an excellent solution to mitigate these problems. By applying >3000 ramped voltage stresses and several current maps at different locations of the samples via conductive atomic force microscopy, it is statistically demonstrated that ultrathin CaF
shows much better dielectric performance (i.e., homogeneity, leakage current, and dielectric strength) than SiO
, TiO
, and h-BN. The main reason behind this behavior is that the cubic crystalline structure of CaF
is continuous and free of defects over large regions, which prevents the formation of electrically weak spots.
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