Papers by Keyword: Transfer Length Method

Abstract: In this work, fabrication and characterisation of graphene photodiodes and transfer length method structures is presented. Graphene growth is carried out using a thermal chemical vapor deposition process on copper foils and subsequently transferred onto silicon-dioxide/silicon substrate. Comparison of electrical and optical characteristics of the photodiodes, which are fabricated on both n-type and p-type silicon, is shown. The photodiodes fabricated on n-type silicon show good rectifying behaviour when compared with photodiodes fabricated on p-type silicon. Spectral response of graphene photodiodes is measured to be less than 0.2 mAW^-1 which is attributed to the light absorbance of 2.3% for single layer graphene. Transfer length method device structures are also fabricated and contact resistance is calculated and plotted as a function of spacing between the contacts. The calculated contact resistance (R_cW) is 0.87 kΩ.µm. The latter structures are also characterised under various ambient conditions, before and after annealing. The value of contact resistance reduces from 0.87 kΩ.µm to 0.75 kΩ.µm after annealing. This reduction is attributed to the improvement in bonding between graphene and metal. Measurements under vacuum show an increase in contact resistance which is attributed to the removal of adsorbed water molecules on the surface on graphene. The sheet resistivity of graphene is calculated to be between 1.17 kΩ/□ and 3.67 kΩ/□.

Abstract: Epitaxial graphene on silicon carbide (SiC) can easily be grown by thermal decomposition. A well-defined epitaxial interface between graphene and substrate is formed, especially when the silicon face of hexagonal polytypes is employed. It is found that as-grown monolayer graphene with interfacial buffer layer provides perfectly ohmic contacts to n-type SiC – even to low-doped epitaxial layers without contact implantation. Contact resistances to highly doped samples are competitive with conventional annealed nickel (Ni) contacts; a direct comparison of Ni and graphene contacts on 4H-SiC resulted in an one order of magnitude reduction of the contact resistance in the case of graphene contacts. On highly doped 6H-SiC, a specific contact resistance as low as ρ_C = 5.9·10^-6 Ωcm² was found. This further improvement compared to 4H-SiC is assigned to better matching of work functions at the Schottky-like interface.

Abstract: We report on the influence of titanium thickness on the structural and electrical properties of annealed Ti/Ni ohmic contacts on highly doped n-type 3C-SiC. Electrical analysis by means of circular transfer length method demonstrate that an interlayer of titanium with thickness in the range of 25-150 nm has no significant influence on specific contact resistance. However, from a structural point of view, the formation of nickel silicides as well as Ti₃SiC₂ is severely affected by the titanium thickness. Moreover, the Kirkendall effect due to the reaction between Ni and SiC is influenced by the titanium thickness. In fact, Scanning Electron Microscopy analysis demonstrates that the adjunction of titanium affects the distribution of Kirkendall voids in the contact. Current maps determined by conductive Atomic Force Microscopy reveal significant variation of uniformity according to the titanium thickness.