Papers by Author: Nicolas G. Wright

Abstract: The electrical characteristics of oxygen functionalized epitaxial graphene and Ti/Au metal contact interfaces were systematically investigated as a function of temperature. As the temperature was increased from 300 K to 673 K, the contact resistance and the sheet resistance decreased by 75% and 33%, respectively. The resistance of oxygen functionalized graphene vs temperature exhibited Arrhenius type behavior with activation energy of 38 meV. The results showed no hysteresis effects in resistance measurements over the temperatures studied here, suggesting the contact interfaces remain stable at high temperatures.

Abstract: In this work SiC-based MIS capacitors have been fabricated with different contact/high-k dielectric combinations and the temperature dependence of the characteristics have been examined in an N₂ ambient at temperatures between 323K and 673K. The structures utilise either a Pt or Pd catalytic gate contact and a TiO₂ or HfO₂ high-k dielectric, all of which are grown on a thin SiO₂ layer, thermally grown on the Si face of a 4H SiC epitaxial layer. The MIS capacitors have been studied in an N2 ambient between 323K and 673K and observations show that V_FB reduces with increasing temperature. The majority of this variation is caused a reduction in the D_it influencing the structures electrical characteristics, due to a shift in the semiconductors bulk potential, which is due to the lower V_TH of SiC-based MOSFETs at high temperatures.

Abstract: We investigated the chemical sensing mechanism of epitaxial graphene grown on 6H-SiC (0001) to different polar solvents and their behavior at higher temperatures. We show that at 300 K the sensitivity of the graphene sensor increases exponentially with the dipole moment of a solvent and decreases significantly as the temperature increased to 425 K. Using electrical measurements, we also show that graphene can effectively discriminate between polar protic and polar aprotic solvents with the shift in device electrical resistance at 300 K.

Abstract: Patterned Few Layers Graphene (FLG) films were grown by local solid phase epitaxy from nickel silicide supersaturated with carbon. The process was realised by annealing of thin Ni films deposited on the carbon-terminated surface of 6H-SiC semi-insulating wafer followed by wet processing to remove the resulting nickel silicide. Raman spectroscopy was used to investigate both the formation and subsequent removal of nickel silicide during processing. Characterisation of the resulting FLG films was carried out by Raman spectroscopy and Atomic Force Microscopy (AFM). The thickness of the final FLG film estimated from the Raman spectra varied from 1 to 3 monolayers for initial Ni layers varying from 3 to 20 nm thick. AFM observations revealed process-induced surface roughening in FLG films, however, electrical conductivity measurements by Transmission Line Model (TLM) structures confirmed that roughness does not compromise the film sheet resistance.

Abstract: Recent progress in the field of silicon carbide sensor technology, such as wireless communications and sensors, has demonstrated the need for a resilient energy supply as an alternative to conventional batteries. Previous work has shown that silicon carbide is an effective energy harvester of UV light in high temperature and hostile environments. Until now however, there has been little work undertaken to assess the long-term effects of elevated temperature on such devices. Although it is understood that silicon carbide is unaffected by long-term temperature exposure below 400 °C, there has been little research into the overall device response and how changes in contact metallisation affect the photovoltaic behaviour.

Abstract: In the aerospace industry where the weight and power density are important design parameters, high frequency operation results in smaller passive components. Furthermore, to achieve a large voltage conversion ratio, which is a goal for payload systems, the use of transformers increases the size and power losses of the system. To fulfill the space and weight requirements, a transformer-less SiC-based DC-DC multilevel converter providing high voltage conversion ratios without an extremely high duty cycle has been realized. The experimental high switching frequency and low current results for a conventional, 3-level and 4-level converter utilizing Si and SiC based COTS diodes are presented. SiC-based multilevel converters show a higher efficiency due to the low reverse recovery and fast switching of the diodes, which results in a higher voltage conversion ratio. This translates to a lower duty cycle to obtain the required output voltage, whilst eliminating the need for complex filtering even under light load conditions.

Abstract: The chemical termination of diamond strongly impacts its electron affinity and thermal stability. We have performed density functional calculations examining up to a monolayer of selected transition metals (Ti, V, Ni and Cu) on the 2×1 reconstructed (001) surface. We find that addition of the carbide forming species, Ti and V, results in significantly higher binding adsorption energies at all surface coverages relative to those of the non-carbide-forming species. For monolayer coverage by Cu or Ni, and sub-monolayer coverage by Ti and V, we observe a negative electron affinity. We propose that based upon the electron affinities and binding energies, metal coated 2×1 reconstructed (001) diamond surfaces are promising candidates for electron emitters.

Abstract: We show that it is possible to obtain information relating to deep level interface traps, or so called ‘slow states’, by using the photo-CV characterisation method. Sub-bandgap illumination has been chosen in order to avoid band-to-band excitation for the creation of minority carriers. This enables information to be extracted from trapping states at the SiO2/SiC interface that are energetically deep within the band gap. Empirical observations of deep level trapping states with life times in the order of tens of hours are reported and the interface trap density as a function of energy has been extracted using the Terman method. Characterisation of these interface states will aid the development of new fabrication processes, with the aim of reducing the interface trap density to the same level as that of the SiO2/Si interface and facilitating the production of higher quality SiC based devices.

Abstract: This paper reports on direct frequency modulation of a RF Colpitts oscillator, realised from silicon carbide devices and proprietary components, capable of transmitting sensor data whilst operating at 300°C. Utilizing a reversed biased Schottky diode as a varactor in an LC oscillator, it is possible to modulate the frequency of an RF carrier by applying external voltage signals. These experiments have shown that a 10V bias will increase the frequency by as much as 10%, however signals as low as 10mV are easily detectable with standard silicon receivers.

Abstract: The recent development of silicon carbide complimentary metal-oxide-semiconductor (CMOS) is a key enabling step in the realisation of low power circuitry for high temperature applications, such as aerospace and well logging. This paper describes investigations into the properties of the gate dielectric as part of the development of the technology to realize monolithic fabrication of both n and p channel devices. A comparison of the oxide quality of the silicon carbide CMOS transistors is performed to examine the feasibility of this technology for high temperature circuitry.