The Benefits of a High Vacuum for Electrical Scanning Probe Microscopy

A huge scientific interest in two-dimensional (2D) layered materials has developed as a result of the discovery of graphene as the prototype of a new class of materials in 2004.1 Ever since, a large variety of 2D materials have been created and explored.2-6 Among these materials, the family of transition metal dichalcogenides (TMDs) has drawn specific attention from the semiconductor industry, which is due to an inherent band gap, a small dielectric constant, high mobilities, and ultrathin materials. These are hailed as promising candidates for scaling logic technology beyond the 5 nm node.

On the other hand, integrating such materials in a 300 mm compatible manufacturing environment still comes up against multiple obstacles. Because the applauded properties have mainly been seen very locally, in flakes or singular grains, controlling the growth, transfer and processing of excellent quality TMD layers remains a pivotal challenge.

Scanning probe microscopy, as an inherently high-resolution 2D technique, is a very strong way to enable the investigation of the morphological and electrical components of TMDs. This technical note demonstrates the benefits of high vacuum for electrical measurements, which utilize the capability of the Park NX-Hivacatomic force microscopy system (Park Systems) with the use of the material MoS2 as an example.

Investigation: Material and Methods

MoS2

A succession of MoS2 samples with a variety of layer thickness were grown by metal-organic chemical vapor deposition (MO-CVD) on sapphire substrates. Every measurement is performed on the as-grown, un-transferred MoS2/sapphire. The room temperature mobility for devices that originate from the same material is discovered to be up to µm ~ 30 cm2/Vs, with greater average mobilities for thicker samples.12

(a-c) AFM topography images of the samples studied. (d) Schematic of C-AFM setup used to measure multilayer MoS2 on a sapphire. (e) Cartoon demonstrating how the cantilever twists as it scans over a high friction region. (f) Cross section of topography which corresponds to the black line in (b) demonstrating 0.6nm step at the MoS2 island edge and 0.2 nm step at the sapphire terrace. Every image is plotted in Gwyddion. Scale bar is 500 nm.

Figure 1: (a-c) AFM topography images of the samples studied. (d) Schematic of C-AFM setup used to measure multilayer MoS2 on a sapphire. (e) Cartoon demonstrating how the cantilever twists as it scans over a high friction region. (f) Cross section of topography which corresponds to the black line in (b) demonstrating 0.6 nm step at the MoS2 island edge and 0.2 nm step at the sapphire terrace. Every image is plotted in Gwyddion. Scale bar is 500 nm.

Atomic force microscopy (AFM) images of every sample measured are illustrated in Figure 1. In all, three samples were measured with layer thicknesses of 1-2 layers, 3-4 layers, and lastly with pyramid structures, referenced here as multi-layered MoS2.

The 1-2 layer sample is made up of an entirely closed monolayer MoS2 film with the addition of monolayer islands that forms on top. The monolayer islands make up the start of the growth of the second layer and can be viewed in the topography image as the light-colored regions.

Similarly, the 3-4 layer sample constitutes an entirely closed tri-layer MoS2 film with added monolayer islands. An example of the 3-4 layer sample structure can be seen in figure 1(d). In this instance, each green layer is representative of one layer of MoS2. Additionally to MoS2 islands, we can also view diagonal lines crossing each sample.

These terraces originate from the sapphire substrate, which are able to be viewed through the 2D film. The sapphire terraces can be very clearly distinguished from MoS2 layers by the step height, 0.2 nm for c-plane sapphires. 0.6 nm for a monolayer MoS2 step, as shown from the cross-section in figure 1 (f).13,14 The multiple-layered sample is different from the other two in that the MoS2 surface is marked by 3D pyramid-like structures. These pyramids rest on an entirely closed tri-layer, and their formation is caused by a variance in the growth mechanism from layer-by-layer to 3D with growing layer thickness. Details of the increase can be located in ref.12.

Conducting Scanning Probe Microscopy

Here we implement two conducting scanning probe microscopy (SPM) techniques to distinguish the electronic properties of MoS2: Conductive Atomic Force Microscopy (C-AFM) and Scanning Tunneling Microscopy (STM). In C-AFM, the cantilever touches the material’s surface and we record topography and current at the same time.

The electrical current measurement is obtained by an external current amplifier connected to a conducting AFM probe, after a bias is applied to the sample chuck. Electrical contact to the material is achieved by the application of silver paint to the top and the material’s sides.

Commercially available Pt-Ir coated probes are used, for instance PPP-CONTSCPt or PPP-NCSTPt, with nominal spring constants in the range of 0.2 to 7 N/m. C-AFM is a contact-based AFM technique, allowing for the lateral force to be recorded together with the other C-AFM channels. Lateral force microscopy (LFM) measures the lateral deflection of the laser on the PSD caused by torsion or twisting, of the cantilever as it scans across the surface, as illustrated in figure 1(e).

The differential between the forward and reverse LFM images is in proportion to the friction of the materialist differing from C-AFM in that a conductive wire, cut Pt-Ir in this example, is used in the measuring of the tunneling current between the probe and the sample when the probe is a few angstroms above the surface. Performing STM is possible by either maintaining the height consistently and recording the current (referred to as constant height mode) or by the use of the feedback to maintain the current level consistently and to record the height (constant current mode). The height image consists of both topographic and electronic information in constant current mode.

C-AFM in Air vs High Vacuum

To be able to show the significance of the water layer on the surface of 2D materials, C-AFM was performed on the same MoS2 sample in air and high vacuum (HV), figure 2 (a-b) and (c-d), respectively. While the topography images for the scan in air and HV are fairly comparable, the C-AFM images are considerably different. Most conspicuously, the measured current grows by three orders of magnitude in HV. The average current level in air is 1.4 nA at 5 V bias, while in HV it is 1.1 µA.

The growth in current level is caused by the taking away of the thin water layer that is continually present on the surface of the sample in air. This water layer is a particular problem for MoS2 as it p-dopes the material, essentially switching it off electrically. From electrical transport of similar CVD-grown MoS2 devices, the on-state current is harshly deteriorated and mobility is lowered by 40% after being exposed to DI water for two hours.15

C-AFM from the same 3-4 MoS2 sample illustrating the higher current level and sensitivity under high vacuum. (a) Topography and (b) current images in air at 5 V bias. (c) Topography and (d) current images taken straight after pumping to high vacuum at 0.5V bias. The data taken in air and high vacuum was obtained with the use of the exact same parameters: same probe with spring constant k of 7 N/m, set point of 10 nN, and 1 Hz scan rate. Scale bar is 500 nm.

Figure 2: C-AFM from the same 3-4 MoS2 sample illustrating the higher current level and sensitivity under high vacuum. (a) Topography and (b) current images in air at 5 V bias. (c) Topography and (d) current images taken straight after pumping to high vacuum at 0.5V bias. The data taken in air and high vacuum was obtained with the use of the exact same parameters: same probe with spring constant k of 7 N/m, set point of 10 nN, and 1 Hz scan rate. Scale bar is 500 nm.

Aside from simply an increase in current, the C-AFM image in HV demonstrates greater detail. From the image in the air, it seems that current is relatively homogeneous. Other than the current level, not a lot of information can be extracted for the C-AFM in air on this sample. In contrast, it is easy to perceive the grain boundaries in the MoS2 layer from the current map taken in HV.

Although the C-AFM probe is in direct contact with the material, the low force applied has over the repeated scans resulted in no MoS2 material being removed. Figure 3 illustrates the topography image from the same sample after 5 scans in HV at ~30 nN force with a probe with nominal spring constant at ~7 N/m.

Topography image of a 3-4 layer MoS2 (a) at first and (b) after a sequence of 5  scans at 0.1 V set point using a PPP-NCSTPt probe with spring constant of ~7 N/m. Scale bar is 50 nm.

Figure 3: Topography image of a 3-4 layer MoS2 (a) at first and (b) after a sequence of 5  scans at 0.1 V set point using a PPP-NCSTPt probe with spring constant of ~7 N/m. Scale bar is 50 nm.

C-AFM + LFM for Grain Boundary Analysis

While taking images with a low spring constant probe, such as PPP-CONTSCPt with nominal spring constant of 0.2 N/m, frictional data can be acquired at the same time with C-AFM, which allows for correlation between topographic, electrical, and material properties.

Figure 3 illustrates the height, friction, and current images from a 1-2 layer MoS2 sample. The 1st layer and 2nd layer regions are labeled as 1Ly and 2Ly, respectively, in Figure 3(a). The friction at grain boundaries is greater than the pristine regions so that they appear as dark lines in the friction. Through the comparison of the current and friction, we are able to observe that the dark lines in the friction image are comparable to the dark lines in the current. Nevertheless, the current image demonstrates more features caused by the effect of the substrate on the local conductivity of the 2D film.

(a) Topography, (b) Friction, and (c) current obtained at the same time on an 1-2 layer as-grown MoS2/sapphire sample. The layer thickness of each region is shown in (a). Scale bar is 200 nm.

Figure 4: (a) Topography, (b) Friction, and (c) current obtained at the same time on an 1-2 layer as-grown MoS2/sapphire sample. The layer thickness of each region is shown in (a). Scale bar is 200 nm.

Scanning Tunneling Microscopy on MoS2

Using the Park NX-Hivac, it was possible to also obtain excellent quality STM images omitting the necessity of complex UHV systems and special sample preparation/handling. Figure 4 illustrates a 500 nm scan of the multi-layer MoS2 sample imaged in constant current mode, with Iset=0.5 nA and Vbias=1 V. As STM gives a convolution of topography with electronic structure, both islands and boundary of the grain in the image of the height can be observed.

STM image of multilayer MoS2/sapphire.Cut Pt-Ir wire in constant current mode.

Figure 5: STM image of multilayer MoS2/sapphire.Cut Pt-Ir wire in constant current mode. Iset=0.5nA, Vbias=1V. Scale bar is 200 nm.

Conclusion

Through this case, molybdenum disulfide (MoS2), one of the most fascinating 2D materials of transition metal dichalcogenides (TMDs) family, was investigated on morphological and electrical features using Park NX-Hivac AFM (Park Systems). The differentials between single and multi-layer were recorded on AFM topography images. Moreover, the details of 3D pyramid-like structures caused by layer-by-layer growth mechanism were found at the multi-layer image.

With the use of conducting SPM (C-AFM and STM), electrical components of MoS2 were looked at both in ambient and high vacuum (HV) condition. Despite the existence of the oxide layer, clear, homogeneous and higher current signal was measured while in HV condition. Finally, topographical, electrical, and mechanical details were gained for grain boundary analysis with the use of C-AFM and LFM put together. This type of undertaking allowed the discovery of more specific and detailed structures on grain boundary.

2D layered material is in wide usage for a variety of research field both industry and academy. The characterization and investigation of electrical and mechanical properties of 2D material are one of the most vital matters in materials research field. The atomic force microscope, a flexible imaging and measurement instrument, allows the evaluation of 2D materials in multidirectional viewpoints with the use of a variety of imaging modes. This study emphasizes the advancing strategy of material analysis. Furthermore, these results underscore the high significance of multidirectional and multichannel analysis on 2D materials, with the inclusion of the transition metal dichalcogenides that are of great interest to the semiconductor industry.

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Acknowledgments

Produced from materials originally authored by Jonathan Ludwig, Marco Mascaro, Umberto Celano, Wilfried Vandervorst, and Kristof Paredis from IMEC, Leuven, Belgium, and Department of Physics and Astronomy, University of Leuven, Leuven, Belgium.

This information has been sourced, reviewed and adapted from materials provided by Park Systems.

For more information on this source, please visit Park Systems.

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