Papers by Author: Alton B. Horsfall

Abstract: We have investigated the annealing of fixed oxide charge and interfacial traps in MISiC strucures by means of the photo capacitance voltage technique at temperatures up to 500°C. Elevated temperature measurements show reduced hysteresis and reduced fixed oxide charge at the interface. The photo capacitance technique shows a real-time measurement at elevated temperatures, in which electrons are populated by photo energy, in a 4H-SiC MIS structure. We also confirm the reduction of fixed oxide charge at the interface by means of high temperature post deposition annealing, which occurs during the high temperature measurements.

Abstract: This paper demonstrates the rst high temperature silicon carbide based energy harvesting module suitable for use in hostile environments. The system comprises a of SiC pin photovoltaic cell, HfO2 based capacitive storage bank and Schottky blocking diode. The system demonstrates the ability to harvest energy from a UV rich environment and store this energy on a HfO2 metal - insulator - metal (MIM) capacitor bank. The system unies work thathas focussed on developing high temperature energy harvesting technologies, a key technology in facilitating the deployment of resilient wireless sensor nodes into hostile environments. The system demonstrates the capability to store an initial voltage of 2.3V decaying to 0.5V in 300ms with a Schottky based system. Replacing the Schottky diode with a switched system, a much lower decay rate to 1.5V in over 8s was observed. This shows that an effective harvester could be made with a switched power controller.

Abstract: This work presents the amplitude modulation radio transmission system for communications in hostile environments. The commissioning of a high temperature oscillator and AM mixer system for the purpose of Amplitude Shift Keyed modulation is presented. While previous work has demonstrated oscillators in the Ultra High Frequency (UHF) band, these have been targeted at applications such as radar and mobile telephones. In this paper we have concentrated on the shortwave bands to maximize the range between a sensor unit and the receiver within wireless networks. The work demonstrates that simple communication systems are already possible for hostile environments and allow for simple sensor data to be wirelessly transmitted to safer working areas.

Abstract: 4H-SiC diodes with 0.60 mm2 nickel silicide Schottky contacts were fabricated on commercial epitaxial layers. At room temperature, the diodes have specific on-resistances (RON-SP) down to 10.5 mΩcm2 and blocking voltages (VBL) up to 4.6 kV, which is equal to 93 % of the calculated parallel plane breakdown voltage for used epitaxial structure. The corresponding figure-of-merit, defined as (VBL)2/RON-SP, is equal to 2015 MW/cm2 and is among the highest FOM values reported to date. The diodes demonstrated stable operation at forward current of 1 A and VBL value in excess of 3.3 kV at ambient temperatures up to 200 °C.

Abstract: The nature of the interaction between the substrate and the graphene is critical in terms of impact upon the graphene electron dispersion relation, and in terms of charge transfer. We present here the results of density functional simulations of 4H-SiC–graphene heterostructures using large, periodic simulation supercells. We show that covalent bonding between the substrate and graphene leads both to changes in the electronic structure, and extensive charge transfer, but that the larger simulation system yields qualitatively different electronic structure to that from the more usual p3 × p3R30◦ cell.

Abstract: Few Layers Graphene (FLG) films were grown on the carbon-terminated surface of 4H-SiC from nickel silicide supersaturated with carbon. The process was realised by annealing of thin Ni films deposited on silicon carbide followed by wet processing to remove the nickel silicide. To identify and characterize the fabricated FLG films, micro-Raman scattering spectroscopy, AFM and optical microscopy have been used. The films grown on samples with initially deposited nickel thinner than 20 nm show clear graphene footprints in micro-Raman scattering spectra, namely a single component, Lorentzian shape 2D band with FWHM remarkably lower than that of the 2D peak of graphite.

Abstract: The development of silicon carbide technologies has allowed for the development of sensors and electronics to measure the changes in a variety of hostile environments. A problem has been identified with reliable and efficient ways to power such sensors in these hostile environments. It is likely to be impractical to run power cables to these sensors and battery power has a finite lifetime. Recent research has demonstrated many energy scavenging techniques but to date none have been developed with a view of operation in hostile environments. To investigate the power density achievable from a SiC based energy scavenging device a SiC pin diode was exposed to both broad spectrum light form a tungsten halogen bulb and a 255 nm UV source. IV and CV measurements were used to determine the structural properties and photovoltaic response of the device, dark saturation current, induced photo current and the fill factor. We present the characteristics and maximum power density of these devices at temperatures between 300 K and 600 K. We demonstrate that the maximum power density achievable decreases with temperature. This is mostly due to the reduction in the built in potential from the pn junction, and the reduction of the generated photocurrent.

Abstract: Buried gate static induction transistors (BGSITs) were fabricated on commercial 4H-SiC wafer with 20 m thick n-type epilayer having a net donor density of 0.71015 cm-3. Buried gate regions were formed by the selective implantation of high energy (up to 2 MeV) aluminium performed at 600 °C. Nitrogen was implanted at temperature of 400 °C to form a heavily doped blanket source region. Post-implantation annealing was carried out at the atmospheric pressure in argon using a graphite capping layer. Fabricated normally-on devices with source contact diameter of 0.2 mm were tested at temperatures up to 500 °C and current densities up to 270 A/cm2. The specific on-resistance of a completely open 4H-SiC BGSIT was 34 mcm2 and showed a thermally activated behaviour at temperatures up to 500 °C.

Abstract: Nickel silicide Schottky contacts were formed on 4H-SiC by consecutive deposition of a titanium adhesion layer, 4 nm thick, and nickel, 100 nm thick, followed by annealing at temperatures from 600 to 750 °C. It was found that contacts with barrier heights of 1.45 eV, consisting mainly of NiSi phase, formed in the 600-660 °C temperature range, while annealing at around 750 °C led to the formation of Ni2Si phase with barrier heights of 1.1 eV. Annealing at intermediate temperatures resulted in the nucleation of Ni2Si grains embedded in the NiSi film which were directly observed by micro-Raman mapping. It was concluded that the thermodynamically unfavourable NiSi phase appeared in the 600-660 °C temperature range due to the fact that the solid state chemical reaction between Ni and SiC at these temperatures is controlled by nickel diffusion through the titanium barrier.

Abstract: Trenched and implanted vertical JFETs (TI-VJFETs) with blocking voltages of 700 V were fabricated on commercial 4H-SiC epitaxial wafers. Vertical p+-n junctions were formed by aluminium implantation in sidewalls of strip-like mesa structures. Normally-on 4H-SiC TI-VJFETs had specific on-state resistance (RO-S ) of 8 mW×cm2 measured at room temperature. These devices operated reversibly at a current density of 100 A/cm2 whilst placed on a hot stage at temperature of 500 °C and without any protective atmosphere. The change of RO-S with temperature rising from 20 to 500 °C followed a power law (~ T 2.4) which is close to the temperature dependence of electron mobility in 4H-SiC.