Papers by Author: Irina P. Nikitina

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: Trenched implanted vertical JFETs (TI-VJFETs) with self-aligned gate and source contacts were fabricated on commercial 4H-SiC epitaxial wafers. Gate regions were formed by aluminium implantation through the same silicon oxide mask which was used for etching mesa-structures. Self-aligned nickel silicide source and gate contacts were formed using a silicon oxide spacer formed on mesa-structure sidewalls by anisotropic thermal oxidation of silicon carbide followed by anisotropic reactive ion etching of oxide. Fabricated normally-on 4H-SiC TI-VJFETs demonstrated low gate leakage currents and blocking voltages exceeding 200 V.

Abstract: 3.3 kV rated 4H-SiC diodes with nickel monosilicide Schottky contacts and 2-zone JTE regions were fabricated on commercial epitaxial wafers having a 34 m thick blocking layer with donor concentration of 2.2×1015 cm-3. The diodes were fabricated with and without additional field stop rings to investigate the impact of practically realizable stopper rings on the diode blocking characteristics. The field stop ring was formed by reactive ion etching of heavily doped epitaxial capping layer. The diodes with field stop rings demonstrated significantly higher yield and reduction of reverse leakage current. The diodes demonstrated blocking voltages in excess of 4.0 kV and very low change of leakage current at ambient temperatures up to 200 °C.

Abstract: Free standing AlN wafers were grown on pre-patterned and in situ patterned 4H-SiC substrates by a physical vapor transport method. It is based on the coalescence of AlN microrods, which evolve from the apex of SiC pyramids grown on the SiC substrate during a temperature ramp up for in situ patterned substrate and SiC pyramids formed by reactive ion etching (RIE). This process yields stress-free (according XRD and Raman results) AlN single crystals with a thickness up to 400 µm and low dislocation density.

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: 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.