Novel Substrates for Graphene based Electronics
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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Name of University: University of ManchesterCandidate full name: Rashid JalilDegree Title: Doctor of Philosophy in the Faculty of Engineering and Physical sciencesThesis Title: Novel Substrates for Graphene based ElectronicsDate: 21-JULY-2012
The application of the layered material hexagonal boron nitride for graphene based electronics is introduced and studied in this thesis. The by now well-known two dimensional material graphene is strongly considered a promising material for the future nano-world. An attractive feature of graphene is its very high electronic quality that is generally expressed by the mobility of its charge carriers. In this thesis, graphene devices were initially fabricated on oxidised silicon substrates. Transport measurements revealed no improvement in graphene’s charge carrier mobility of ~ 5000-15,000 cm2V-1s-1, which is far less than graphene’s predicted intrinsic mobility of ~200,000 cm2V-1s-1. This drew attention to use other, better, smoother and inert substrates for graphene to help improving the charge carrier mobility. Hexagonal boron nitride (hBN) has shown very remarkable first results in this regards. In the work presented here, boron nitride layers were isolated and identified successfully. Large boron nitride flakes were prepared on oxidized silicon by micromechanical cleavage, a conventional and routinely used technique for the fabrication of graphene and other two-dimensional materials and afterwards characterised by a number of techniques. Using optical microscopy, a low optical contrast of the thin and ultrathin boron nitride layers is observed. To allow for an unambiguous identification of the number of atomic layers, the use of optical filters and different thicknesses of oxide for the underlying oxidised silicon substrate are suggested to improve the optical contrast of these boron nitride layers. The thickness of boron nitride layers has been further studied by atomic force microscopy. Additionally, the use of Raman spectroscopy for counting the number of boron nitride is described. Raman studies show that the number of boron nitride layers is proportional to the corresponding Raman intensity. A characteristic Raman peak of boron nitride is observed at ~ 1366 cm-1. The peak position and intensity for layers of various thicknesses with respect to bulk boron nitride is analysed carefully. An upward shift in frequency is seen in monolayer boron nitride whereas all other layers show a down shift. These thin boron nitride layers can be employed in making graphene boron nitride heterostruture. A precise flake transfer technique has been developed and is discussed in detail that has allowed for transferring graphene and other layered materials on any desired location with an accuracy of few microns. Two different flake transfer methods wet and dry were used for the fabrication of graphene heterostrutures. The graphene layers were successfully transferred on boron nitride substrate and TEM grids. Raman and AFM analysis of transferred flakes were also carried out to confirm the presence of graphene. A comparison between wet and dry transfer methods and their possible applications in fabrication of artificial heterostrutures are discussed. The last part of this thesis deals with the study of micrometer ballistic transport in high mobility graphene samples at room temperature. The devices studied for this purpose were fabricated by transferring graphene on a hBN substrate and encapsulating it by another hBN crystal. Bend resistance measurements were employed to study the ballistic effects in our four terminal graphene boron nitride devices. A negative bend resistance is observed, showing that the devices exhibit room temperature ballistic transport over 1μm distance. A room temperature mobility of >100,000 cm2V-1s-1 at n ≈1011 cm-2 is determined by analysing the field effect mobility. The role of diffusive scattering at the boundary of the samples in limiting the longitudinal conductivity of 1μm devices is described for higher n ≈1012 cm-2. At low temperature, the mean free path reaches l ≈ 3μm which translates into ultrahigh mobility for graphene. Our findings show a significant improvement in the mobility of graphene charge carriers when employing boron nitride substrates. Encapsulating the graphene devices with hBN is an even better addition, as it provides a barrier against environmental effects and can also serve as top dielectric for graphene boron nitride heterostrutures.