Article pubs.acs.org/JPCC
Competitive Growth and Etching of Epitaxial Graphene Lianchang Zhang,*,†,§ Ming Ni,†,§ Donghua Liu,‡ Dongxia Shi,‡ and Guangyu Zhang‡ †
Department of Physics and Key Laboratory of Yunnan Provincial Higher Education Institutions for Organic Optoelectronic Materials and Devices, Kunming University, Kunming, 650214, China ‡ Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Science, Beijing 100190, China ABSTRACT: In this paper, we studied the competition of growth and etching during graphene epitaxial growth in the remote plasma enhanced chemical vapor deposition (rPECVD) system. Epitaxial growth of graphene on HOPG substrates with a simultaneous etching process was systematically explored at various temperatures. It was found that etching of graphene by hydrogen radicals generated in the rPECVD system was a critical factor during graphene’s growth for controlling the nucleation densities, lateral growth rates, and layer thickness. At temperatures lower than 490 °C, the etching effect is dominant, and there is no graphene nucleation. And at temperatures higher than 490 °C, the etching effect decreases gradually with rising temperature and the growth effect stands out. The optimized epitaxial growth was at 520 °C, and at that temperature a monolayer graphene single crystal was achieved with near perfect lattice structure on HOPG substrates.
G
raphene, a single layer of sp2-bonded carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice, has received considerable attention owing to its novel and exceptional properties1,2 and has been considered to be a promising candidate material for future electronics and a successor of silicon in the post-Moor’s-law era.2 Over the past few years, several approaches have been developed, such as micromechanical exfoliation,3 liquid phase exfoliation,4−6 graphene oxide reduction,7−10 catalytic growth on multicrystalline metal films,11−14 epitaxial growth on single crystal transition metals,15,16 high temperature decomposition of SiC,17−19 etc. In the cases of epitaxial growth of graphene on single crystal metals and SiC substrate, the growth temperatures are about 1000 and 1400 °C, respectively, meanwhile wrinkles are inevitable in these two growth techniques. Recently, we developed a new epitaxial growth technique for graphene by a homemade remote plasma enhanced chemical vapor deposition (r-PECVD) system at temperature as low as 520−540 °C and without catalyst.20 Under optimized conditions, continuous, uniform, and quasi-monolayer graphene films are successfully grown on a freshly cleaved HOPG surface and on atomically smooth SiC surface following a layerby-layer growth model. The configuration of the r-PECVD setup is shown in Figure 1. However, the growth mechanism is still unclear and needs to be further explored. In this paper, we systematically studied the competition between the growth effect and the simultaneous etching effect by CxHy radicals20 and atomic H21 generated in methane plasma, respectively, and discovered that the etching effect of atomic H in methane plasma plays a critical part in controlling the nucleation, lateral growth rates, and layer thickness. © 2012 American Chemical Society
Figure 1. Configuration of the r-PECVD system.
The competition between growth and etching depends critically on the growth temperatures, as shown in Figure 2. At a low temperature of 470 °C, etched hexagonal pits occur, caused by atomic H in methane plasma, similar to hydrogen plasma etching,21 and methane plasma can also etch the grain boundary into a wider gap, as shown in Figure 2a. At this temperature, no homoepitaxial graphene was deposited on the surface. At a raised temperature of 490 °C, in addition to the etched pits, small epitaxial graphene islands are grown on the surface (Figure 2b). At a relatively high temperature of 520 °C, the etching effect becomes weaker and negligible while the growth becomes faster and the growth mode is layer by layer (Figure 2c). At a higher temperature of 550 °C, the growth rate is further raised, and the second and third graphene nucleation takes place (Figure 2d), suggesting that high temperatures favor a 3-D growth instead of layer-by-layer growth. Received: October 13, 2012 Revised: November 20, 2012 Published: December 5, 2012 26929
dx.doi.org/10.1021/jp310134g | J. Phys. Chem. C 2012, 116, 26929−26931
The Journal of Physical Chemistry C
Article
Figure 3. Homoepitaxial growth at graphene edges. (a) Growth at the edges generated by anisotropic etched hexagonal pits. Dotted line hexagons indicate anisotropically etched pits on the HOPG surface. Arrows indicate growth at the edges of the etched hexagonal pits. (b) Growth at the cleaved top layer HOPG edges. Scale bars, 250 nm.
of graphene fabricated from reduced graphene oxide (rGO). The orientations of the etched pits and the grown graphene islands are in the same direction (Figure 3a), implying that the edges of the grown grahene islands is zigzag type as those of the etched pits.21 Besides, the epitaxial growth can also happen at the freshly cleaved edges, as shown by the arrows in Figure 3b. These effects of homoepitaxial growth by methane plasma and anisotropic etching by hydrogen plasma can be expressed by the following reversible reactions:
Figure 2. Competition between etching of the HOPG surface and homoepitaxial growth of graphene with pure methane precursor at different temperatures. (a) At 470 °C, the HOPG surface is etched and no graphene is grown. (b) At 490 °C, etching of the HOPG surface and growth of homoepitaxial graphene coexist. (c) Graphene grows homoepitaxially at 520 °C with layer-by-layer growth mode; weak etching restrains the nucleation on top. (d) Graphene grows homoepitaxially at 550 °C with 3-D island growth mode, with no etching effect. Scale bars, 250 nm.
As discussed in the previous paper,21 the etching rate of atomic H increases with temperature raised and reaches the peak value at about 450 °C and then decreases with temperature further raised. In the case of homoepitaxial growth, at 470 °C, because the etching effect is comparatively strong, it predominates over the growth effect of CxHy radicals; therefore, no graphene nucleation takes place and only etched marks occur in defects regions or grain boundaries on the HOPG surface (Figure 2a). As the temperature is raised, the etching effects become weaker gradually, and the growth effect gradually emerges (Figure 2b) and dominates the total effects (Figure 2c). More importantly, variation of etching rates at different temperature influences the nucleation and growth model. At 520 °C, the etching effect is weak enough to allow the epitaxal growth of graphene and eliminate etching of the HOPG surface, but strong enough to inhibit graphene nucleation on the top layer; therefore, this temperature favors a layer-by-layer growth of graphene. While at the temperature of 550 °C, the etching effect is so weak that the second and third layer graphene nucleation can easily take place and a 3D growth model is favored. Besides, the etching effect can also eliminate the formation of amorphous carbon, which is much less robust than graphene and can be easily etched away by atomic H. In addition to the homoepitaxial growth on the surface, the growth can also occur at graphene edges. After hydrogen plasma etching, hexagonal pits are formed in the top layer/ layers of the HOPG surface (Figure 3a). The edges of these pits are atomically smooth.21 Subsequent growth using methane precursor showed that the growth also occurs at the edges of etched hexagonal pits, as shown by the arrows in Figure 3b. It indicates that this growth can be used to repair the hole defects
When pure methane precursor is used, methane is dissociated into CxHy radicals and atomic H. Because there are comparable amounts of the two species, their total effect of either growth or etching varies with temperature, as discussed above. When pure hydrogen is used, because there are dominant amounts of atomic H and negligible amounts of CxHy radicals, the total effect is along the backward reaction, resulting in etching of the HOPG surface. As we discussed above, the variation of etching rate with growth temperature is the most crucial factor that affects the nucleation, growth rate and growth model. Other factors including the RF power and pressure and flow rate of methane precursor were also studied, which mainly influence the growth rate of graphene. At 520 °C following a layer-by-layer 2D growth with 100 W RF power, 0.20 Torr pressure, and 30 sccm flow rate of methane precursor, we successfully synthesized a quasi-monolayer graphene by elongating the growth duration to 2 h, as shown in Figure 4. The first layer homoepitaxial graphene almost reaches one monolayer and the second layer begins to nucleate, and the different growth stages are also clearly indicated. As previously discussed,20 the growth followed a typical nucleation−enlargement−coalescence process. At the early nucleation stage, the deposited carbon atoms form tiny islands of irregular shapes as shown by arrow A in Figure 4. As the nuclei grow bigger, the islands tend to form a regular hexagonal shape, as shown by arrow B. This corresponds well with the intrinsic 6-fold symmetry of the inplane carbon atoms in graphene. Moreover, because the growth is homoepitaxial, the deposited graphene crystal lattice possesses the same orientation as the underlying HOPG surface layer; therefore, all the hexagonal shape epitaxial graphene islands are in the same orientation. Arrow C, D and 26930
dx.doi.org/10.1021/jp310134g | J. Phys. Chem. C 2012, 116, 26929−26931
The Journal of Physical Chemistry C
Article
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Figure 4. STM characterization of homoepitaxial graphene with growth duration of 2 h. Arrows A and B denote a graphene nucleation and a larger graphene islands. Arrows C, D, and E denote neighbor graphene islands that are about to coalesce, are coalescing, and have coalesced.
E shows that adjacent islands at different coalesce stages, are about to coalesce, are coalescing, and are completely coalesced. In conclusion, we have studied the competition between the growth effects of CxHy radicals and the etching effect of atomic H in methane plasma for the homoepitaxial growth of graphene in r-PECVD system. It was discovered that the intensity of the etching effect of atomic H is temperature dependent and is a crucial factor that controls the nucleation, growth rate, and growth model and eliminates the formation of undesired amorphous carbon. Finally, under the optimized conditions for monolayer graphene growth, a quasi-monolayer epitaxial graphene was successfully synthesized.
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AUTHOR INFORMATION
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[email protected]. Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Talents Introduction Project of Kunming University (Grant No. YJL12009). REFERENCES
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