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Technical Paper

Applied Science and Convergence Technology 2024; 33(2): 50-52

Published online March 30, 2024

https://doi.org/10.5757/ASCT.2024.33.2.50

Copyright © The Korean Vacuum Society.

Scanning Gate Microscopic Investigation of Graphene Nanoribbon Underneath Dielectric Layer

Seo Gyun Jang , Oh Hun Gwon , BeomKyu Shin , Daehyun Ryu , and Young-Jun Yu*

Department of Physics, Chungnam National University, Daejeon 34134, Republic of Korea

Correspondence to:E-mail: yjyu@cnu.ac.kr

Received: March 11, 2024; Accepted: March 26, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.

In this report, we introduce a scanning gate microscope (SGM) to characterize nanoscale conductive channels under dielectric materials. We comprehensively review the electrical characterization of a graphene nanoribbon (GNR) using the SGM. We present a method to measure partial electrical gating on GNR via an SGM probe. Furthermore, by employing the assumption of GNR width difference for the partial position of the GNR, we attempt to elucidate the conductance distribution on the GNR by partial gating.

Keywords: Graphene, Nanoribbon, Scanning gate microscope

The scaling down of semiconductor devices and an increase in their integration have been emphasized to improve their performance in various applications. Accordingly, the characterization of nanoscale electric structures is crucial for nanoscale fabrication processes. Although conventional optical microscopes have a spatial resolution limitation ascribed to the diffraction limit, scanning electron microscopes (SEM) have been extensively employed for measuring nanoscale surface topography [14]. However, because conductive surfaces are typically examined using SEM, the surfaces of nonconductive specimens require metal coatings before analysis [57]. Various nanoscale surfaces on specimens require nondestructive characterization, and the atomic force microscope (AFM) is a remarkable characterization tool for measuring the topography of these surfaces without damaging them [810]. Furthermore, this scanning probe microscope can be extended to analyze nanoscale topography exhibiting various electrical, thermal, and magnetic properties, which are generally measured using an electric force microscope (EFM), scanning thermal microscope, and magnetic force microscope, respectively [1115]. In particular, EFM can be used to measure the work function of a specimen surface with a scanning Kelvin probe microscope and the conductance variation on the specimen surface under tip gating with a scanning gate microscope (SGM). Recently, because the conductance variation with potential on the nanoscale area of graphene has been measured by tuning the Fermi level [16,17], the SGM characterization of nanoscale graphene nanoribbons (GNR), which are fabricated from two-dimensional graphene, is important for understanding the electrical properties of onedimensional carbon networks [18,19].

In this study, we introduce SGM measurement for GNR. The SGM is employed to measure the conductance of nanoscale metal and semiconductor channels by applying a drain-source bias voltage (VDS). Because the SGM locally applies an electric gate field from the conductive tip to the nanoscale surface region, the nanoscale conductance properties can be determined by applying VDS. To characterize the local conductance properties of the GNR under VDS by the SGM, the GNR is prepared via plasma etching, using hydrogen silsesquioxane (HSQ), which is an insulator, as a mask on single-layer graphene (SLG). The electrical properties of the GNR under the HSQ dielectric layer, determined by SGM measurements, are used to explain the variation in carrier density depending on the width difference of the GNR. These results clarify the width-dependent doping differences in nanoscale GNR structures and suggest that GNR structures can be predicted from electrical property measurements.

The conductance of semiconductors or metal channels encapsulated by a dielectric layer can be measured through SGM characterization. As illustrated in Fig. 1, when the semiconductor channel is measured using two-terminal characterization, the conductance is tuned by electric gating via the dielectric layer. For the SGM, partial gating can be applied using a conductive AFM tip. The following steps were used to obtain the SGM images: Initially, the topography was measured in non-contact mode with a cantilever dithering frequency of ~100 kHz. Subsequently, by employing a constant-height mode with a distance of ~30 nm between the tip and the sample surface, a partial gate voltage (Vtip) was applied via a conductive tip (Au-coated SGM tip) in addition to measuring the drain-source current (IDS) in the GNR channel. An SGM image was generated by plotting the IDS values of the GNR channel for each scanning area of the SGM tip. The charge distribution in the channel was characterized based on the conductance image data. In particular, nanoscale channels have been reported to exhibit SGM signal variation owing to charge disorders or dimensional fluctuations of the channel, which change the electric field density with the SGM tip, resulting in current flow variation in the channel [20].

Figure 1. Schematic of SGM characterization leading to partial tip gating (Vtip) on conducting (semiconductor or metal) channels under a dielectric layer, with the evaluation of conductance by two-terminal (VDS−IDS) measurement.

Here, we introduced a partial conductance variation in the GNR via SGM characterization. The GNR was fabricated from SLG on a SiO2 substrate (thickness = ~280 nm) [Fig. 2(a)]. This SLG on SiO2 was prepared via mechanical exfoliation and then brought into contact with Cr/Au (0.5/40 nm thickness) electrodes via normal e-beam lithography process. The nanoscale etch mask on the SLG was fabricated with a negative-tone e-beam resist (i.e., HSQ), and the unprotected SLG area was etched by O2 plasma treatment. The HSQ mask on GNR was prepared with a width of ~100 nm and length of 1 μm [Figs. 2 and 3(a)] [2124]. Figure 2(b) illustrates an AFM image of the GNR. Consequently, we observed SGM images that exhibited three different current variation areas in the GNR, marked as A, B, and C in the SGM images presented in Fig. 3(b). The topography and SGM images were measured using a commercial AFM (XE-100, Park Systems Corp.) in air at room temperature. Upon applying Vtip via the SGM tip, IDS of GNR was measured at VDS = 0.5 V, leading to an initial current level of 8.7 μA. Notably, this initial IDS is influenced by dangling bonds on the SiO2 substrate, and the current level can be improved using the ultraclean surface of a hexagonal boron nitride substrate [25,26].

Figure 2. (a) Schematic of GNR fabrication with HSQ mask by O2 plasma treatment. (b) Topography image of GNR with Au electrodes (The width and length of HSQ on GNR are measured to be approximately 100 nm and 1 μm, respectively).

Figure 3. (a) Schematic of SGM process for measuring GNR under HSQ mask on SiO2. (b) SGM images of GNR with different Vtip (−3, −1, 0, 1, and 3 V) exhibiting substantial variations in current, with positions marked by A, B, and C. The dashed lines correspond to the GNR boundary, as confirmed from the topography [Fig. 2(b)]. (c) Current profiles extracted along the GNR length direction of SGM images with different Vtip (−3, −1, 0, 1, and 3 V) in (b).

To study the conductance variation of the GNR with Vtip, we investigated SGM images of GNR for different values of Vtip (−3, −1, 0, 1, and 3 V) [Fig. 3(b)]. The decrease and increase in the current levels for positive (3 and 1 V) and negative (−3 and −1 V) Vtip were observed as compared with the current level for Vtip = 0 V, respectively. The origin of these areas with considerable variations in the current (i.e., areas A and C) could be attributed to a partially undesired width fluctuation in the GNR, leading to a partial conductance variation on the GNR, as reported previously [1618]. Herein, we focus on the width fluctuation in the GNR; additional experimental measurements are necessary to confirm the origin of the areas with substantial current variations, which will be investigated more comprehensively in the future. Owing to the indistinguishable topography of the HSQ, we can consider that the permeation of O2 plasma under HSQ randomly etches the carbon bonds at the edge of graphene, leading to a width fluctuation in the GNR under the HSQ mask [19,20]. This partially induced current difference in the GNR can be manifested by employing the electric field density difference for different widths at positions A, B, and C of the GNR [Fig. 3(c)]. Since the electric field density between the SGM tip and area A (i.e., narrowest width region) is higher than that for areas B and C, the Fermi level tuning range in area A by Vtip is larger than that in areas B and C. Furthermore, the variation in IDS induced by Vtip values of −3 to 3 V for position B is smaller than that for positions A and C, indicating that the width of B is the widest in the GNR. The larger Fermi level tuning under the same Vtip in positions A and C than in position B of the GNR underneath the HSQ structure can be attributed to a narrower width region on the GNR, leading to a high electric field density between the tip and the sample, as reported previously [20].

Consequently, we infer that the width fluctuations in the GNR with changes in the IDS are induced through partial gating in the GNR. This is useful for predicting the structure through nondestructive inspection in an environment where the structure of the sample cannot be identified by topographic measurements using AFM.

Herein, we introduced the SGM and the corresponding measurement process. The conductance on the GNR underneath the HSQ was investigated. In particular, the current level of the GNR varied with the application of a partial gate via the SGM tip. Based on previous results [2022], we propose that partial width fluctuations in the GNR vary the current in the GNR under the HSQ due to Vtip gating. This finding can be applied to analyze the electrical properties of nanoelectronic structures. The proposed method of evaluating conductance using the SGM is expected to contribute markedly to the development and performance optimization of nanostructure processes.

This work was supported by the Research Fund of the Chungnam National University.

  1. Wang, Z.-J. et al, ACS Nano 9, 1506 (2015).
    Pubmed CrossRef
  2. Cazaux, J., J. Microsc. 217, 16 (2005).
    Pubmed CrossRef
  3. Tsiper, S., Dicker, O., Kaizerman, I., Zohar, Z., Segev, M., and Eldar, Y. C., Nano Lett. 17, 5437 (2017).
    Pubmed CrossRef
  4. Marnautov, N. A., Matveev, M. V., Gulin, A. A., Kálai, T., Bognár, B., Rebrikova, A. T., and Chumakova, N. A., J. Phys. Chem. C 128, 2543 (2024).
    CrossRef
  5. Golding, C. G., Lamboo, L. L., Beniac, D. R., and Booth, T. F., Sci. Rep. 6, 26516 (2016).
    Pubmed KoreaMed CrossRef
  6. Asahi, Y., Miura, J., Tsuda, T., Kuwabata, S., Tsunashima, K., Noiri, Y., Sakata, T., Ebisu, S., and Asahi, M. H., AMB Express 5, 6 (2015).
    Pubmed KoreaMed CrossRef
  7. Li, J., He, Y., Han, Y., Liu, K., Wang, J., Li, Q., Fan, S., and Jiang, K., Nano Lett. 12, 4095 (2012).
    Pubmed CrossRef
  8. Alsteens, D., Dague, E., Rouxhet, P. G., Baulard, A. R., and Dufrêne, Y. F., Langmuir 23, 11977 (2007).
    Pubmed CrossRef
  9. Bhushan, B., Kwak, K. J., and Palacio, M., J. Phys. Condens. Matter 20, 365207 (2008).
    CrossRef
  10. Kodera, N., Yamamoto, D., Ishikawa, R., and Ando, T., Nature 468, 72 (2010).
    Pubmed CrossRef
  11. Kuntze, S. B., Ban, S., Sargent, D., Dixon-Warren, E., White, S., Hinzer, J., and Hinzer, K., Crit. Rev. Solid State Mater. Sci. 30, 71 (2005).
    CrossRef
  12. Yu, Y.-J., Zhao, Y., Ryu, S., Brus, L. E., Kim, K. S., and Kim, P., Nano Lett. 9, 3430 (2009).
    Pubmed CrossRef
  13. Zhou, X., Dayeh, S. A., Wang, D., and Yu, E. T., Appl. Phys. Lett. 90, 233118 (2007).
    CrossRef
  14. Wilson, N. R. and Cobden, D. H., Nano Lett. 8, 2161 (2008).
    Pubmed CrossRef
  15. Kazakova, O., Puttock, R., Barton, C., Corte-León, H., Jaafar, M., Neu, V., and Asenjo, A., J. Appl. Phys. 125, 060901 (2019).
    CrossRef
  16. Deshpande, A. and LeRoy, B. J., Physica E 44, 743 (2012).
    CrossRef
  17. Mackenzie, D. M. A., Pamchal, V., Corte-León, H., Petersen, D. H., and Kazakova, O., 2D Mater. 6, 025023 (2019).
    CrossRef
  18. Pascher, N., Bischoff, D., Ihn, T., and Ensslin, K., Appl. Phys. Lett. 101, 063101 (2012).
    CrossRef
  19. Soudi, A., Aivazian, G., Shi, S.-F., Xu, X. D., and Gu, Y., Appl. Phys. Lett. 100, 033115 (2012).
    CrossRef
  20. Yu, Y.-J. et al, Nanoscale 11, 4735 (2019).
    Pubmed CrossRef
  21. Yu, Y.-J., Han, M. Y., Berciaud, S., Georgescu, A. B., Heinz, T. F., Bru, L. E., Kim, K. S., and Kim, P., Appl. Phys. Lett. 99, 183105 (2011).
    CrossRef
  22. Yu, Y.-J., J. Korean Phys. Soc. 76, 727 (2020).
    CrossRef
  23. Han, M. Y., Özyilmaz, B., Zhang, Y., and Kim, P., Phys. Rev. Lett. 98, 206805 (2007).
    Pubmed CrossRef
  24. Han, M. Y., Brant, J. C., and Kim, P., Phys. Rev. Lett. 104, 056801 (2010).
    Pubmed CrossRef
  25. Martini, L. et al, ACS Appl. Mater. Interfaces 15, 37794 (2023).
    Pubmed KoreaMed CrossRef
  26. Karak, S., Paul, S., Negi, D., Poojitha, B., Srivastav, S. K., Das, A., and Saha, S., ACS Appl. Nano Mater. 4, 1951 (2021).
    CrossRef

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