Understanding graphene formation mechanisms on different silicon carbide polytypes – a way to optimize graphene quality
In short: We use a high temperature sublimation technique to grow a single layer of graphene on silicon carbide (SiC) substrates. The structural and electronic properties of the resulting graphene are dependent on the detailed characteristics of the surface on which it grows, which is different for different SiC polytypes. We have grown, for the first time, large area graphene with superior uniformity on the unconventional cubic polytype of SiC (3C). A comparison is made with results on two hexagonal polytypes. The present work contributes to understanding and quality control of graphene growth on SiC.
We are applying high temperature sublimation technique to grow a single layer of graphene on silicon carbide (SiC) substrates. This allows formation of graphene which is spatially highly uniform in thickness, carrier concentration and carrier mobility. This is vital for performance of devices processed on the graphene sheets.
Structural and electronic properties of graphene produced on SiC by sublimation are substrate mediated. However due to the crystal structure of SiC a step formation on the crystal surface is taking place during sublimation, a phenomenon known as step bunching. The step bunching is being dependent on the exact stacking sequence of the atomic layers in the SiC crystal. Each stacking sequence corresponds to one so-called polytype of SiC.
We have studied thickness uniformity of graphene grown at the same conditions on two hexagonal polytypes and one cubic polytype of SiC (silicon face nominally on-axis), denoted 4H-SiC(0 0 0 1), 6H-SiC(0 0 0 1), and 3C-SiC (1 1 1). The graphene layer was probed by means of atomic force microscopy topology measurements and low-energy electron microscopy images. Since the cubic type SiC is not a common substrate, the electronic energy-momentum relation in this graphene layer was evidenced by angle-resolved photoemission spectroscopy.
Graphene formation at high temperature (2000 °C) and argon atmosphere is influenced by the step bunching process and surface decomposition energy differences created by the SiC stacking sequence on different SiC polytypes. We have demonstrated that a single layer of graphene can be grown on all available SiC polytypes. We have grown, for the first time, large area, over 50 × 50 square micrometers, graphene on the cubic 3C-SiC (1 1 1). The sublimation rate of cubic SiC is the same over the whole defect-free substrate surface due to the similar decomposition energy on all terraces, this is providing a uniform source of carbon on the surface which results in a superior uniformity of the grown graphene layer. It is worth noting that three layers of carbon in cubic 3C-SiC are sufficient to feed the formation of one layer of graphene. Therefore, no extra carbon is released which preserves the structure from second graphene layer inclusions.
The hexagonal 6H-SiC polytype shows a quality of graphene close to that on the cubic 3C-SiC polytype. The results for the other hexagonal polytype 4H-SiC show that graphene formation process has narrower window of growth parameters. As graphitization of 4H-SiC starts at a higher temperature in comparison with other SiC polytypes, an increased growth temperature should be used to have more uniform graphene thickness and larger coverage by one layer graphene on 4H-SiC. Although the cubic 3C-SiC substrates are not commercially available, the present work may contribute to understanding and quality control of graphene growth on SiC.
Details can be found in Carbon 57, 477 (2013)
- FP7 EU project Concept Graphene.
- Swedish Research Council (VR).
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Last updated: 01/23/15