Aluminum tunes the electronic properties of graphene on silicon carbide
In short: Graphene has unique and superior electronic properties. It can be grown on silicon carbide by a graphitization process, enabling development of single layer graphene based electronic devices for high temperature and high voltage operation. Aluminum is a commonly used contact material for electronics, but its stability on graphene has not been investigated. We show that a distinct change in the electronic properties of graphene occurs for temperatures above 350 °C, when aluminum penetrates into the graphene-silicon carbide interface. Our results evidence the importance of temperature of the graphene device, when selecting aluminum as contact material.
Graphene is one monolayer of carbon atoms bonded in a honeycomb network. The linear energy-momentum relation is unique in graphene, see figure (b), and results in effectively massless charge carriers with superior electronic properties. A continuous and homogeneous graphene layer covering a large surface area can be achieved by thermal graphitization of silicon-terminated silicon carbide, see figures (a)-(b). This provides opportunity for the development of single layer graphene based electronic devices, compatible with operation at high voltage and in high temperature environments.
The choice of metal as contact material can have a crucial influence on the performance of graphene-based electronic devices, since functionalization with metals is an efficient way to tune the electronic structure of graphene. Aluminum is one of the common metals used in silicon carbide based electronics. Moreover, the combination of aluminum and silicon carbide has been reported to provide good rectification characteristics with low reverse leakage current. Nonetheless, whether aluminum provides a good choice as contact material for graphene based electronic devices has not been investigated. Some compounds formed between aluminum and silicon carbide have desirable ceramic properties. Theoretical studies predict aluminum-doped graphene to be significantly more reactive, which can be beneficial in sensor applications. Nevertheless, if those properties are not stable when varying the temperature a few hundred degrees, the device may become unreliable. Therefore a detailed study of the effects of aluminum layers deposited on graphene grown on silicon carbide and after subsequent annealing at temperatures up to 1200°C was carried out.
Graphene was formed on the (0001) crystal surface of silicon carbide (polytype 6H) by heating it at a temperature of 1300 °C for a few minutes at ultra-high vacuum. Aluminum was then thermally deposited on the graphene sample at room temperature. The effects induced on the graphene sample were studied after deposition and after subsequent heating at different temperatures. The morphology, chemical composition, electronic structure and atomic structure were investigated using low energy electron microscopy (LEEM), X-ray photoelectron microscopy (XPEEM), photoelectron spectroscopy (PES), angle resolved photoelectron spectroscopy (ARPES), and selected area low energy electron diffraction (micro-LEED) at beamlines I311 and I4 at the MAX Laboratory in Lund, Sweden.
Our results show that deposited aluminum does not induce any noticeable change in the energy-momentum relation and no chemical reactions before annealing. However, distinct changes occur when the temperature is raised above 350°C. Aluminum then penetrates into the graphene-silicon carbide interface and the aluminum remaining on the sample surface form micrometer sized islands, as illustrated in figure (c). Moreover, the energy-momentum relation becomes strongly modified at this temperature, adopting the characteristic of two graphene layers, see figure (d). A chemical reaction with silicon is also detected in the recorded spectra and additional reactions are observed to occur when the sample temperature is raised further to 700-1200°C. This also induces distinct changes in the surface morphology and electronic properties, as seen in figures (e) and (f).
These results evidence that it is important to consider the desirable operation temperature range of the graphene device when selecting aluminum as contact material, since the electronic properties are shown to vary significantly within a temperature range of a few hundred degrees.
Details of the research are described in Materials Research Express 1 015606 (2014)
Chao Xia, PhD student
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- Swedish Research Council (VR).
- Linköping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM).
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Last updated: 03/11/14