Liquid metal intercalation of epitaxial graphene: Large-area gallenene layer fabrication through gallium self-propagation at ambient conditions

S. Wundrack, D. Momeni, W. Dempwolf, N. Schmidt, K. Pierz, L. Michaliszyn, H. Spende, A. Schmidt, H. W. Schumacher, R. Stosch, and A. Bakin

Phys. Rev. Materials 5, 024006 – Published 19 February 2021

Abstract

We demonstrate the fabrication of an ultrathin gallium film, also known as gallenene, beneath epitaxial graphene on 6H-SiC under ambient conditions triggered by liquid gallium intercalation. Gallenene has been fabricated using liquid metal intercalation, achieving lateral intercalation and diffusion of Ga atoms at room temperature on square centimeter areas limited only by the graphene samples’ size. The stepwise self-propagation of the gallenene film below the epitaxial graphene surface on the macroscopic scale was observed by optical microscopy shortly after the initial processing without further physical or chemical treatment. Directional Ga diffusion of gallenene occurs on SiC terraces since the terrace steps form an energetic barrier (Ehrlich-Schwoebel barrier), retarding the gallenene propagation. The subsequent conversion of the epitaxial graphene into quasi-free-standing bilayer graphene and the graphene-gallenene heterostack interactions have been analyzed by x-ray photoelectron spectroscopy and Raman measurements. The results reveal an alternative approach for the controlled fabrication of wafer-scale gallenene as well as for two-dimensional heterostructures and stacks based on the interaction between liquid metal and epitaxial graphene.

Received 28 June 2020
Revised 2 December 2020
Accepted 4 January 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.024006

Physics Subject Headings (PhySH)

Graphene

Epitaxy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

S. Wundrack ^1,*, D. Momeni ¹, W. Dempwolf^3,4, N. Schmidt², K. Pierz¹, L. Michaliszyn ¹, H. Spende ^2,3, A. Schmidt^2,3, H. W. Schumacher¹, R. Stosch ¹, and A. Bakin ^2,3,†

¹Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany
²Institut für Halbleitertechnik, Technische Universität Braunschweig, Hans-Sommer Straße 66, D-38106 Braunschweig, Germany
³Laboratory of Emerging Nanometrology (LENA) der Technischen Universität Braunschweig, Langer Kamp 6 a/b, 38106 Braunschweig, Germany
⁴Institut für Technische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany

^*stefan.wundrack@ptb.de
^†a.bakin@tu-braunschweig.de

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Issue

Vol. 5, Iss. 2 — February 2021

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Images

Figure 1
(a) Schematic representation of gallenene self-propagation caused by the liquid metal intercalation technique (LiMIT). I. Initiation of the process by liquid Ga deposition and wiping the metal across the substrate. The inset shows schematically the sample structure consisting of a monolayer graphene sheet on top of a graphene buffer layer that is covalently bonded to the underlying SiC substrate. II. Self-propagation of gallenene shortly after processing. III. Self-propagation of gallenene after 2 h. IV. Gallenene completely intercalating the epitaxial graphene sample after $> 24 h$ . (b) I. Time-dependent self-propagation of gallenene. The red dashed line is used as a start marker. II. Time-dependent self-propagation of gallenene after 20 min. The inset indicates the terrace orientation of SiC steps measured with AFM on a pure epitaxial graphene surface. III. Time-dependent self-propagation of gallenene after 30 min. Vectors a and b indicate the self-propagation direction of Ga derived from AFM measurements. (c) Optical micrograph of the Ga intercalated graphene.
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Figure 2
(a) Optical micrograph of the area considered for the LA-ICP-MS measurements. Scale bar corresponds to a width of 100 $μ m$ . (b) False color mapping across the transition of nonintercalated epitaxial graphene to Ga-intercalated graphene using LA-ICP-MS. Scale bar corresponds to a width of 20 $μ m$ . (c) Mass spectrum of pure epitaxial graphene (blue) and Ga-intercalated graphene (red), showing signal intensities of gallium isotopes $^{69} Ga$ and $^{71} Ga$ . Both spectra result from areas from LA-ICP-MS mapping (marked with a blue and red circle). (d) XPS spectra of the C $1 s$ and Si $2 p$ core-level before and after Ga intercalation of epitaxial graphene. Ga $3 d$ core-level demonstrating the presence of elemental Ga as well as Ga-oxide contents.
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Figure 3
(a) and (b) AFM topography and phase image of gallenene on epitaxial graphene. The inset shows a magnified representation of the measured topography (red dashed box), specifically the transition from epitaxial graphene substrate to gallenene covered areas. The line profile extracted from the topography (blue, red, and green line) represents the thickness of gallenene (red) measured at the intercalated/nonintercalated border, the step height of the terraces (green), and the thickness of Ga film across a gallenene area (blue). (c) and (d) Scanning electron microscopy image (at 2 keV) of epitaxial graphene and a Ga intercalated graphene area measured with an in-beam mode SEM image. White stripes indicate terrace edges of SiC.
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Figure 4
(a) Raman spectra of pure epitaxial graphene (black line) and intercalated area consisting of SiC/Ga/QFBLG (red line). The Raman signal related to gallenene appears below $250 c m^{- 1}$ (dashed purple line). SiC phonon bands are marked with an asterisk. (b) and (c) Subtracted Raman spectrum of epitaxial graphene (blue line) and intercalated QFBLG (red line). (d) Time-dependent evolution of the phonon bands of graphene during the gallenene self-propagation. The intercalation of epitaxial graphene to QFBLG appears after 6 s.
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Figure 5
(a) Optical micrograph of the gallene-graphene boundary indicating the area selected for Raman mapping (red square). (b) Measured Raman intensity signal of gallenene. Dark areas represent areas of pure epitaxial graphene substrate. Green areas correspond to gallenene (c) Lateral 2D peak width distribution along the graphene-gallenene boundary. Green areas indicate regions of monolayer epitaxial graphene, whereas pink areas indicate the intercalation of epitaxial graphene caused by gallenene. The scale bar corresponds to a width of 5 μm. (d) $G$ peak width distribution indicating electronic doping within graphene on gallenene-covered areas (blue areas). (e) Strain-doping trajectory of epitaxial graphene (yellow dots) and intercalated QFBLG (green dots). (f) Histogram of the 2D peak width of epitaxial graphene and intercalated QFBLG extracted from Raman mapping. (g) Histogram of the $G$ peak position width of epitaxial graphene and intercalated QFBLG extracted from Raman mapping.
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