Epitaxial growth of mono- and (twisted) multilayer graphene on SiC(0001)

To take full advantage of twisted bilayers of graphene or other two-dimensional materials, it is essential to precisely control the twist angle between the stacked layers, as this parameter determines the properties of the heterostructure. In this context, a growth routine using borazine as a surfactant molecule on SiC(0001) surfaces has been reported, leading to the formation of high-quality epitaxial graphene layers that are unconventionally oriented, i.e., aligned with the substrate lattice <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>(</a:mo><a:mi mathvariant="normal">G</a:mi><a:mtext>−</a:mtext><a:mi>R</a:mi><a:msup><a:mn>0</a:mn><a:mo>∘</a:mo></a:msup><a:mo>)</a:mo></a:mrow></a:math> [Bocquet ]. Since the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:mi mathvariant="normal">G</c:mi><c:mtext>−</c:mtext><c:mi>R</c:mi><c:msup><c:mn>0</c:mn><c:mo>∘</c:mo></c:msup></c:mrow></c:math> layer sits on a buffer layer, also known as zeroth-layer graphene (ZLG), which is rotated <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msup><e:mn>30</e:mn><e:mo>∘</e:mo></e:msup></e:math> with respect to the SiC substrate and still covalently bonded to it, decoupling the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mi>ZLG</f:mi><f:mtext>−</f:mtext><f:mi>R</f:mi><f:msup><f:mn>30</f:mn><f:mo>∘</f:mo></f:msup></f:mrow></f:math> from the substrate can lead to high-quality twisted bilayer graphene (tBLG). Here, we report the decoupling of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:mi>ZLG</g:mi><g:mtext>−</g:mtext><g:mi>R</g:mi><g:msup><g:mn>30</g:mn><g:mo>∘</g:mo></g:msup></g:mrow></g:math> by increasing the temperature during annealing in a borazine atmosphere. While this converts <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mi>ZLG</h:mi><h:mtext>−</h:mtext><h:mi>R</h:mi><h:msup><h:mn>30</h:mn><h:mo>∘</h:mo></h:msup></h:mrow></h:math> to <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi mathvariant="normal">G</i:mi><i:mtext>−</i:mtext><i:mi>R</i:mi><i:msup><i:mn>30</i:mn><i:mo>∘</i:mo></i:msup></i:mrow></i:math> and thus produces tBLG, the growth process at elevated temperature is no longer self-limiting, so that the surface is covered by a patchwork of graphene multilayers of different thicknesses. We find a 20% coverage of tBLG on ZLG, while on the rest of the surface tBLG sits on one or more additional graphene layers. In order to achieve complete coverage with tBLG only, alternative ways of decoupling the ZLG, e.g., by intercalation with suitable atoms, may be advantageous.