Growth and Electrical Properties of Chemical Vapour Deposited Low Dimensional sp2 Carbons.

This thesis describes the growth of sp2 carbon materials - namely graphene and carbon nanotube (CNT) materials using a chemical vapour deposition (CVD) process. A novel CVD process tool based on a photothermal process (PT-CVD) that differs from standard thermal CVD has been developed. This thesis reports the investigations into the properties of the deposited carbon nanomaterials and applications that exploit their electronic properties. The first investigation is into the growth of vertically aligned MWCNT forests. Growth of CNTs at 370°C by a one-step PT-CVD method was demonstrated. The growth rate can reach ~1.3 μm/min, which is faster than most other reported thermal CVD methods. The use of bimetallic catalyst (Fe/Ti) and the use of rapid thermal process are the keys to this process. AFM topography studies showed that the fast top-down heating mode of the PT-CVD leads to the formation of a Fe/Ti uniform solid solution, which is believed to improve the CNT growth. These CNTs are composed of a few layer crystalline graphene sheets with a 5-6 nm diameter. Raman scattering provides supporting evidence that the as-grown CNTs are of high quality, better than some CNTs grown at higher temperatures by traditional CVD methods. CVD growth of graphene was investigated using Cu foils as substrate, with the field-effect in the graphene subsequently demonstrated by transferring it to a back-gate bottom contact transistor arrangement using poly-4-vinyl-phenol gate dielectric as an alternative to oxide based insulators. This graphene transistor showed a simple, inexpensive fabrication method that is completely compatible to large scale fabrication of organic devices, to demonstrate a field effect hole mobility of 37 cm2/Vs. Despite the mobility being lower than that found in exfoliated graphene, it demonstrates the potential of a graphene based all carbon transistor for large area electronics. The fabrication and electrical performance of a 3 terminal graphene device is further reported. This device displayed characteristics similar to a p-type graphene FET. While past investigations of distortion and saturation in transfer characteristics of graphene FET indicated that metal-graphene interaction may be the controlling mechanism, this device operation is based on the design of transferring graphene onto a Diamond-like-carbon DLC/p-Si heterostructure with Si as the back contact and with the DLC acting as the dielectric support in contact to graphene. Thus, this provided a mechanism for the DLC/p-Si heterojunction to moderate the I-V characteristics of this device, resulting in a p-type only conduction process in graphene that is also saturable. Following the work on using conventional thermal CVD (T-CVD) for graphene growth, we demonstrated the possibility of using the PT-CVD to develop a graphene growth process. It is found that the non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down, thus allows the reduction of the overall growth time for graphene. The choice of performing growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates. To characterize the film’s electrical and optical properties, pristine PT-CVD grown graphene was used as the transparent electrode material in an organic photovoltaic devices (OPV) and is found to be comparable to that reported using pristine graphene prepared by conventional CVD

Integration of planar Gunn diodes and HEMTs for high-power MMIC oscillators

This work has as main objective the integration of planar Gunn diodes and high electron mobility transistors (HEMTs) on the same chip for the realisation of high-power oscillators in the millimeter-wave regime. By integrating the two devices, we can reinforce the high frequency oscillations generated by the diode using a transistor-based amplifier. The integration of the planar Gunn diode and the pseudomorphic HEMT was initially attempted on a combined gallium arsenide (GaAs) wafer. In this approach, the active layers of the two devices were separated by a thick buffer layer. A second technique was examined afterwards where both devices were fabricated on the same wafer that included AlGaAs/InGaAs/GaAs heterostructures optimised for the fabrication of pHEMTs. The second approach demonstrated the successful implementation of both devices on the same substrate. Planar Gunn diodes with 1.3 μm anode-to-cathode separation (Lac) presented oscillations up to 87.6 GHz with a maximum power equal to -40 dBm. A new technique was developed for the fabrication of 70 nm long T-gates, improving the gain and the high frequency performance of the transistor. The pHEMT presented cut-off frequency (fT) equal to 90 GHz and 200 GHz maximum frequency of oscillation (fmax). The same side-by-side approach was applied afterwards for the implementation of both devices on an indium phosphide (InP) HEMT wafer for the first time. Planar Gunn diodes with Lac equal to 1 μm generated oscillations up to 204 GHz with -7.1 dBm maximum power. The developed 70 nm T-gate technology was applied for the fabrication of HEMTs with fT equal to 220 GHz and fmax equal to 330 GHz. In the end of this work, the two devices were combined in the same monolithic microwave integrated circuit (MMIC), where the diode was connected to the transistor based amplifier. The amplifier demonstrated a very promising performance with 10 dB of stable gain at 43 GHz. However, imperfections of the material caused large variations at the current density of the devices. As a consequence, no signals were detected at the output of the complete MMIC oscillators

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.Peer ReviewedPostprint (published version

On a Pd/InAlAs metamorphic high electron mobility transistor (MHEMT)-based hydrogen sensor

[[abstract]]The hydrogen sensing properties of a Pd/InAlAs metamorphic high electron mobility transistor (MHEMT) are investigated. Experimentally, a threshold voltage shift (Delta V(th)) of 260 mV is observed upon exposing to a 1% H(2)/air gas. The drain current sensing response (S(R)) shows the strong dependence on the gate bias voltage V(GS). A maximum S(R) of 107% is found at the applied voltage of V(GS) = -0.5 V. In addition. the temperature behavior of S(R) is predominantly determined by the hydrogen sticking coefficient and the exothermic reaction of hydrogen adsorption. It is also found that rectification ratio R can be changed with different hydrogen concentrations Furthermore, the response rate analyses reveal that the initial response rate is increased with the hydrogen concentration and temperature. The activation energy E(a) of 2.88 kJ mol(-1) suggests that the studied Pd/InAlAs MHEMT hydrogen sensor has a low barrier for the adsorption of hydrogen. (C) 2009 Elsevier B.V. All rights reserved.[[note]]SC