Niall Tumilty

University College London, Londinium, England, United Kingdom

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Publications (8)19.75 Total impact

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    ABSTRACT: Nanocrystalline diamond (NCD) has been grown on oxide coated silicon wafers by microwave plasma assisted chemical vapour deposition using a novel seeding technique followed by optimised growth conditions, and leads to a highly-dense form of this material with grain sizes around 100nm for films approximately 1.5 microns thick. The electrical properties of these films have been investigated using Impedance Spectroscopy, which enables the contributions from sources characterised by differing capacitances, such as grain boundaries and grain interiors, to be isolated. After an initial acid clean the electrical properties of the film are not stable, and both grain boundaries and grains themselves contribute to the frequency dependant impedance values recorded. However, following mild oxidation grain boundary conduction is completely removed and the films become highly resistive (>1013 ohm/sq). This is most unusual, as conduction through NCD material is more normally dominated by grain boundary effects. Interestingly, the AC properties of these films are also excellent with a dielectric loss value (tan δ) as low as 0.002 for frequencies up to 10MHz. The dielectric properties of these NCD films are therefore as good as high quality free-standing (large grain) polycrystalline diamond films, and not too dissimilar to single crystal diamond, and are therefore ideally suited to future ‘silicon-on-diamond’ applications.
    MRS Online Proceeding Library 01/2011; 1039. DOI:10.1557/PROC-1039-P13-02
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    ABSTRACT: A method for attaching nanodiamonds (NDs) to H-terminated diamond devices displaying surface conductivity, configured as an ion-sensitive field-effect transistor and resistor sensor, is demonstrated. From Hall effect measurements, there was minimal sign of degradation of the p-type surface conductivity after ND coating ( ∼ 1013 carriers/cm2, ∼ 27 cm2/V s). In response to pH changes, the device showed an improved response to the as-hydrogenated sensor, from 19 mV/pH to a maximum of 37 mV/pH. Configured in resistor mode, exposure to 2,4-dinitrotoluene vapor gave rise to sensitive detection, while the uncoated H-terminated device exhibited reaction instability. The mechanisms behind these observations are discussed.
    Applied Physics Letters 11/2010; 97(20):203503-203503-3. DOI:10.1063/1.3518060 · 3.52 Impact Factor
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    ABSTRACT: Vertically aligned multiwalled carbon nanotubes (MWCNTs) have been synthesised on Ni coated single crystal diamond substrates using a glow-discharge technique. A mixture of gases including CH4, H2 and N2 has been used for growth. The effect of the CH4/H2 gas mixture and growth temperature on the structure and yield of the MWCNTs has been studied. Atomic force microscopy is used to characterise the annealed Ni film prior to growth. Scanning electron microscope studies have also been carried out to observe the yield, height and diameter of MWCNTs produced under various experimental conditions. Raman spectroscopy has been performed to provide quantitive information on the crystallinity of our as-grown MWCNTs. It has been shown that highly adherent, vertically aligned MWCNTs can be grown on type Ib diamond (100) substrates with an interface free from metal catalyst.
    Carbon 09/2010; 48(11):3027-3032. DOI:10.1016/j.carbon.2010.04.023 · 6.16 Impact Factor
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    ABSTRACT: Nanocrystalline diamond (NCD) has been grown using a nanodiamond seeding technique, leading to a dense form of this material, with grain sizes around 100 nm. The electrical properties of both intrinsic and lightly boron-doped NCD have been investigated using impedance spectroscopy and Hall effect measurements. For intrinsic material, both grain boundaries and grains themselves initially contribute to the frequency dependant impedance values recorded. However, boundary conduction can be removed and the films become highly resistive. Interestingly, the ac properties of these films are also excellent with a dielectric loss value ∼0.004 for frequencies up to 10 MHz. The dielectric properties of these NCD films are therefore as good as high quality large grain polycrystalline diamond films. In the case of boron-doped material, p -type material with good carrier mobility values (10–50 cm <sup>2</sup>/ V   s ) can be produced at carrier concentrations around 10<sup>17</sup> cm <sup>-3</sup> .
    Journal of Applied Physics 03/2010; 107(3-107):033716 - 033716-8. DOI:10.1063/1.3291118 · 2.19 Impact Factor
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    ABSTRACT: Impedance spectroscopy has been used to investigate the conductivity displayed by diamond doped with boron in an intrinsic-delta-layer-intrinsic multilayer system with differing delta-layer thicknesses. Carrier transport within 5 nm delta-layer structures is complex, being dominated by conduction in the interfacial regions between the delta-layer and the intrinsic regions, as well as conduction within the delta-layer itself. In the case of 3.2 nm thick delta-layers the situation appears improved with uncapped samples supporting only two conduction paths, one of which may be associated with transport outside of the delta-layer, the other low transport within the delta-layer complex diamond structures. Introduction of the capping layer creates a third conduction path associated with unwanted boron in the capping layer-delta-layer interface.
    Journal of Applied Physics 11/2009; 106(10). DOI:10.1063/1.3261759 · 2.19 Impact Factor
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    ABSTRACT: Impedance spectroscopy has been used to investigate conductivity within boron-doped diamond in an intrinsic/delta-doped/intrinsic (i-δ-i) multilayer structure. For a 5 nm thick delta layer, three conduction pathways are observed, which can be assigned to transport within the delta layer and to two differing conduction paths in the i-layers adjoining the delta layer. For transport in the i-layers, thermal trapping/detrapping processes can be observed, and only at the highest temperature investigated (673 K) can transport due to a single conduction process be seen. Impedance spectroscopy is an ideal nondestructive tool for investigating the electrical characteristics of complex diamond structures.
    Applied Physics Letters 02/2009; 94(5). DOI:10.1063/1.3075860 · 3.52 Impact Factor
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    ABSTRACT: The use of diamond as a semiconductor for the realization of transistor structures, which can operate at high temperatures (>700 K), is of increasing interest. In terms of bipolar devices, the growth of n-type phosphorus doped diamond is more efficient on the (111) growth plane; p-type boron-doped diamond growth has been most usually grown in the (100) direction and, hence, this study into the electronic properties, at high temperatures, of boron-doped diamond (111) homoepitaxial layers. It is shown that highly doped layers (hole carrier concentrations as high as 2×1020 cm−3) can be produced without promoting the onset of (unwanted) hopping conduction. The persistence of valance-band conduction in these films enables relatively high mobility values to be measured ( ∼ 20 cm2/V s) and, intriguingly, these values are not significantly reduced at high temperatures. The layers also display very low compensation levels, a fact that may explain the high mobility values since compensation is required for hopping conduction. The results are discussed in terms of the potential of these types of layers for use with high temperature compatible diamond transistors.
    Journal of Applied Physics 03/2008; 103(5):054503-054503-7. DOI:10.1063/1.2837114 · 2.19 Impact Factor
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    ABSTRACT: A diamond based insulated gate bipolar transistor is incorporated into a two-dimensional device simulator (MEDICI) to examine the current gain (β) and potential distribution across the device. Initially, work has focused on an important component of IGBT structure, the PNP bipolar transistor, which has been simulated and is reported upon in this paper. Empirical parameters for emitter and collector regions were used. Various carrier concentrations for base region were used to optimize the simulation. It was found that decreasing the thickness of base region leads to an increase in current gain. A buffer layer is needed to prevent the punch-through at low carrier concentration in the base region. Various approaches of increasing the current gain are also discussed in this paper.
    MRS Online Proceeding Library 12/2005; 956. DOI:10.1557/PROC-0956-J09-17

Publication Stats

27 Citations
19.75 Total Impact Points

Institutions

  • 2008–2010
    • University College London
      • • Department of Electronic and Electrical Engineering
      • • London Centre for Nanotechnology
      Londinium, England, United Kingdom