Ultra Low Specific Contact Resistivity in Metal-Graphene Junctions via Atomic Orbital Engineering

A systematic investigation of graphene edge contacts is provided. Intentionally patterning monolayer graphene at the contact region creates well-defined edge contacts that lead to a 67% enhancement in current injection from a gold contact. Specific contact resistivity is reduced from 1372 {\Omega}m for a device with surface contacts to 456 {\Omega}m when contacts are patterned with holes. Electrostatic doping of the graphene further reduces contact resistivity from 519 {\Omega}m to 45 {\Omega}m, a substantial decrease of 91%. The experimental results are supported and understood via a multi-scale numerical model, based on density-functional-theory calculations and transport simulations. The data is analyzed with regards to the edge perimeter and hole-to-graphene ratio, which provides insights into optimized contact geometries. The current work thus indicates a reliable and reproducible approach for fabricating low resistance contacts in graphene devices. We provide a simple guideline for contact design that can be exploited to guide graphene and 2D material contact engineering.Comment: 26 page

Anisotropic Vapor HF etching of silicon dioxide for Si microstructure release

Damages are created in a sacrificial layer of silicon dioxide by ion implantation to enhance the etch rate of silicon-dioxide in liquid and vapor phase hydrofluoric acid. The etch rate ratio between implanted and unimplanted silicon dioxide is more than 150 in vapor hydrofluoric acid (VHF). This feature is of interest to greatly reduce the underetch of microelectromechanical systems anchors. Based on the experimentally extracted etch rate of unimplanted and implanted silicon dioxide, the patterning of the sacrificial layer can be predicted by simulation

Contact Resistance Study of Various Metal Electrodes with CVD Graphene

In this study, the contact resistance of various metals to chemical vapour deposited (CVD) monolayer graphene is investigated. Transfer length method (TLM) structures with varying widths and separation between contacts have been fabricated and electrically characterized in ambient air and vacuum condition. Electrical contacts are made with five metals: gold, nickel, nickel/gold, palladium and platinum/gold. The lowest value of 92 {\Omega}{\mu}m is observed for the contact resistance between graphene and gold, extracted from back-gated devices at an applied back-gate bias of -40 V. Measurements carried out under vacuum show larger contact resistance values when compared with measurements carried out in ambient conditions. Post processing annealing at 450{\deg}C for 1 hour in argon-95% / hydrogen-5% atmosphere results in lowering the contact resistance value which is attributed to the enhancement of the adhesion between metal and graphene. The results presented in this work provide an overview for potential contact engineering for high performance graphene-based electronic devices

Fabrication and characterization of silicon nanowires for electromechanical and gas sensing application

When dimensions of material approach nanoscale, it often reveals startling properties which pave way to innovative functionality. These unique properties when compared to bulk material make them interesting candidates for new technologies. In a race to sustain Moore’s Law, silicon nanowires which possess remarkable properties diverse from bulk-silicon have gained notable attention. With advancement in technology engineers have mastered the art of fabrication of nanowires, but there exists a big gap in understanding various phenomena at this scale. The aim of this thesis is to bridge the gap and give an insight into some interesting properties and application of silicon nanowires. Top-down method with electron beam lithography is used to fabricate silicon nanowires. The nanowires are released with various methods to determine their mechanical properties such as maximum release length without stiction, residual stress and stress gradient. Followed by this, a novel method to determine fracture strain based on “internal stress relaxation” method is presented. Fracture strain of 5% is measured in silicon nanowires when compared to 0.3% for bulk-silicon. Very large piezoresistance is observed in nanowires although the detailed mechanism behind this is not yet well understood. Finally, the use of functionalized silicon nanowires for gas detection is demonstrated with very large sensitivity and detection window reported for the first time.(FSA 3) -- UCL, 201

Piezoresistance of nano-scale silicon up to 2GPa in tension

The piezo-resistance of 100 nm-thick, [110] oriented, p-type, mono-crystalline Si beams has been investigated under large uniaxial tension up to 2 GPa using an original on-chip tensile testing set-up. The piezo-resistance coefficient (π) was found to increase by a factor of 6 compared with ∼1.5 for Si bulk, when decreasing the dopant concentration from Na ∼ 1 × 1019 cm−3 down to Na ∼ 5 × 1017 cm−3. Reduction of resistance by a factor of 5.8, higher than theoretical maximum of 4.5, is reported for Na ∼ 5 × 1017 cm−3 under a stress of 1.7 GPa, without any sign of saturation