SiC surface orientation and Si loss rate effects on epitaxial graphene
We have explored the properties of SiC-based epitaxial graphene grown in a cold wall UHV chamber. The effects of the SiC surface orientation and silicon loss rate were investigated by comparing the characteristics of each formed graphene. Graphene was grown by thermal decomposition on both the silicon (0001) and carbon (000-1) faces of on-axis semi-insulating 6H-SiC with a "face-down" and "face-up" orientations. The thermal gradient, in relation to the silicon flux from the surface, was towards the surface and away from the surface, respectively, in the two configurations. Raman results indicate the disorder characteristics represented by ID/IG down to < 0.02 in Si-face samples and < 0.05 in C-faces over the 1 cm2 wafer surface grown at 1,450°C. AFM examination shows a better morphology in face-down surfaces. This study suggests that the optimum configuration slows the thermal decomposition and allows the graphene to form near the equilibrium. The Si-face-down orientation (in opposition to the temperature gradient) results in a better combination of low disorder ratio, ID/IG, and smooth surface morphology. Mobility of Si-face-down orientation has been measured as high as approximately 1,500 cm2/Vs at room temperature. Additionally, the field effect transistors have been fabricated on both Si-face-down and C-face-down showing an ambipolar behavior with more favorable electron conduction.
KeywordsGraphene Film Epitaxial Graphene High Quality Graphene Field Effect Mobility Grown Graphene
Graphene is a sheet of graphite consisting of sp2-bonded carbon atoms . The unique material properties of graphene such as extremely high-carrier mobility, semi-metallic characteristics, and two-dimensional [2-D] very thin sheet of carbon have attracted a great interest and will lead to the development of nanoelectronics [2, 3]. The graphene was first obtained by cleaving the graphite, but this drawing/exfoliation method is only useful for the demonstration of scientific or engineering concept rather than a large volume manufacturing [1, 4]. The result of exfoliation is not predictable, and the available size is too small (< 100 um) for practical application. In order to obtain a large area graphene, the chemical vapor deposition [CVD] growth on catalytic metals or thermal decomposition of SiC has been extensively studied [5, 6, 7, 8]. Large area good quality graphene was produced using the CVD method, but the grown graphene has to be transferred to an insulating substrate since the graphene cannot be used on the metals in most applications . This transfer method needs costly processes and likely causes damages to the grown graphene. Thermal decomposition of SiC produces the so-called epitaxial graphene and has shown high crystal quality [10, 11]. Since this epitaxial graphene can be directly formed on an insulating large area substrate compatible with the already established semiconductor processing technique, it is a promising route for commercialization of graphene devices. The epitaxial graphene is grown as a result of Si evaporation at high temperature and can be grown under the ultra high vacuum [UHV] or atmospheric Argon [Ar] environment. The growth under Ar requires higher-annealing temperatures (1,500°C to approximately 2,000°C) for Si to be evaporated overcoming the Ar pressure near the substrate surface . This method reduces the Si evaporation rate and enhances the surface diffusion resulting in the formation of higher quality graphene. However, this technique needs extremely high temperature and may not be compatible with some samples such as SiC epi on Si substrate. The epitaxial graphene can be formed at relatively lower temperature (1,150°C to approximately 1,450°C) in the case of UHV, but the film quality is usually lower than that grown under the Ar at higher temperature (1,500°C to approximately 2,000°C) due to the fast and uncontrollable Si loss. Here, we explore the UHV growth of epitaxial graphene using a face-down configuration. In order to investigate the effects of silicon evaporation rate on the film quality, we designed a growth configuration that would allow the minimization of silicon loss and at the same time provide a way to examine the effects of a much higher rate of Si evaporation. Both the Si-face and C-face of SiC were chemically and mechanically polished [CMP], and the growth was done on both surfaces with the thermal gradient directed towards one surface and away from the other. This arrangement provides a condition for low and high silicon loss at the same time.
Results and discussions
By comparing the face-down (low Si evaporation rate) and face-up (high Si evaporation rate) mounting method, the optimum growth condition producing a higher quality graphene in a cold wall UHV chamber was found. The use of graphite enclosure and the face-down scheme provide a condition that can reduce Si loss rate and allow higher temperature growth. The best quality film was obtained with the Si-face-down configuration with the ID/IG ratio below 0.02 with smooth and uniform surface morphology. Electrical properties were characterized by test structure-fabricated and Hall-effect measurements. In transistor characteristics, it shows an ambipolar behavior, and the electron conduction is favored over the hole conduction, which is determined by the polarity of initial majority carriers in graphene layer during the formation of graphene. The Hall mobility at RT is as high as 1,500 cm2/Vs, and the value of field effect mobility is close to that of silicon devices. This approach to form a graphene film could be used to grow high quality epitaxial graphene in case a relatively lower-annealing temperature is required.
This work was partially supported by Air Force MURI/SBIR and monitored by Dr. Harold Weinstock, NSF through Cornell Center for Materials Research, Center for Nanoscale Systems and NYSTAR.
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