M. G. Jubber

Diamond-based glucose sensors

Abstract A series of diamond-based glucose sensors, based on the interaction of glucose with the enzyme glucose oxidase (GOD), has been produced. For each sensor, the sensitivity to glucose was assessed and, with some devices, the range of glucose concentrations over which the sensor showed a linear response was determined. The sensing electrode formed one electrode of an electrochemical cell, and a tungsten counter electrode formed the other. All the sensors were diamond-based, with the nature of the working electrode being the distinguishing feature. The first device was a diamond platinum-GOD sensor. However, this device was prone to interference from other electroactive chemicals in the blood such as vitamin C and acetaminophen. To minimise the metal content of the sensors, two further sensors were produced using heavily boron doped diamond as the conducting electrode in place of the platinum. In the first case, the GOD was immobilised on to the surface of the diamond by electrochemical deposition, and in the second, the GOD was “wired” directly to the electrode by covalent bonding to the electrode surface.

Modelling of self-limiting laser ablation of rough surfaces: application to the polishing of diamond films

Abstract We present a theoretical model for the interaction of excimer laser radiation with rough polycrystalline CVD diamond films, showing the influence of angle of incidence, irradiation intensity and number of laser pulses. A new regime of laser treatment — self-limiting laser ablation — was found, which allows faceted films to be smoothed without wasteful ablation of the bulk. Multipulse XeCl laser (308 nm) irradiation of our diamond films, grown on silicon wafers, confirms that such polishing is accomplished.

Epitaxy of diamond on silicon

Abstract Localized heteroepitaxial growth of diamond on (100) silicon was investigated using microwave plasma deposition. The final growth was performed in hydrogen-carbon-monoxide-methane gas mixtures conducive to (100) texturing of the crystallites. The deposition was preceded by either a single or two-stage in situ bias treatment to enhance the nucleation density. An initial carburization stage was not found to be essential although the inclusion of this step reduced the bias period required for epitaxy. The bias current-time characteristics exhibit features which could be interpreted as changes in the chemical composition of the nucleation layer. Epitaxial films were produced with grain sizes of 15–20 μm.

Design of a UHV reactor for microwave plasma deposition of diamond films

Abstract A UHV compatible deposition system has been constructed for the growth of diamond films by 2.45 GHz microwave plasma chemical vapour deposition (CVD). The system comprises a loadlock and a spherical deposition chamber where the heated 100 mm diameter substrate is exposed to a reactive plasma environment. The design provides ports for in-situ monitoring by ellipsometry, optical emission spectrometry, mass spectrometry, and laser reflectometry and allows for the later addition of analysis chambers such as XPS. Computer control is provided for all major components and operations, including pumps, valves, gas flows, pressure and temperature adjustment. The system has four pumping groups, two for the main growth chamber providing base vacuum and for pumping the process gases, one for evacuating the loadlock and the fourth for the mass spectrometer. Microwaves enter the chamber via an antenna-based microwave applicator with a water-cooled quartz window. A key feature of this design is the ability to have a free standing ball plasma which touches neither the chamber walls nor the substrate.

Laser writing of high-purity gold lines

Gold tracks of better than 98% purity have been deposited onto oxidized silicon wafers from gaseous methyl(triethylphosphine) gold(I), AuMe(Et3 P), by using 514 nm radiation from a focused cw argon ion laser. Room‐temperature resistivities of 4.2 μΩ cm, comparable with bulk gold, were attained at a writing speed of 35 μm s−1 . The track profiles suggest that deposition is more rapid on the gold surface than on the SiO2 substrate.

IR attenuated total reflectance studies of d.c. biased growth of diamond films

Abstract The IR spectra of polycrystalline diamond films grown under negative d.c. biased conditions were recorded using both conventional transmission and total internal reflectance spectrometry. The films were grown by microwave enhanced plasma deposition on singlecrystal silicon using methane-hydrogen mixtures at powers of up to 0.8 kW. IR spectra in the carbon-hydrogen stretching region were recorded over the wavenumber range 4800–400 cm−1 using Fourier transform techniques. The profiles of the IR bands of the biased and non-biased samples showed the presence of differing carbon-hydrogen groupings. In the non-biased samples only hydrogen bound to sp3 hybridized carbon was detected. In contrast, the biased films showed the presence of sp2 hybridized C-H groupings for films deposited with a 10% methane in hydrogen mixture. The spectroscopic evidence could not distinguish between the sp2 C7 H derived from either graphitic or =CH containing phases. When growth at normal conditions, 0.5% methane in hydrogen, was performed after biased enhanced nucleation, the overall hydrogen content of the film remained similar although the distribution of chemically bound hydrogen amongst the sp3 hybridized CH groupings altered significantly.

The growth of (100) orientated diamond films

Abstract Quasi-equilibrium calculations were performed on the C H and C H O systems to calculate the thermodynamic stability of diamond and graphite phases under CVD conditions. Exclusive diamond growth is dependent, in part, on the increased thermodynamic stability of H-terminated crystallographic surfaces. This feature was included in the methodology by estimating the surface enthalpies by group additivity methods. The lower boundary in the Bachmann ternary diagram which defines the demarcation line between the growth and non-growth regions was calculated. The gradient of the line is a strong function of the degree of stabilization of the H-terminated diamond surfaces. Using mixtures of CH 4 CO H 2 of compositions close to the CO line of the phase diagram, (100) faceted films were grown by microwave assisted CVD at rates of 1.5–2.0 μm h −1 .

THE SELECTIVE AREA DEPOSITION OF DIAMOND FILMS

Diamond films were selectively nucleated and grown on single crystal (100) silicon by microwave plasma assisted chemical vapor deposition with submicron spatial resolution. A thermal silicon dioxide layer on the wafers was patterned by standard photolithography. Nucleation was performed by applying a dc bias of −250 to −350 V in a hydrogen-methane plasma. Lifting off the oxide layer by HF etching prior to growth delineated the nucleation pattern which was replicated by the diamond film after growth. The growth of polycrystalline diamond was performed in a hydrogen-carbon monoxide-methane mixture selected to facilitate (100) texturing. Individual faceted crystallites were grown on a square matrix of sites, with a pitch of 3 μm, by controlling the nucleation densities within the windows exposing the prenucleated silicon. However, the orientation of the crystallites was randomly aligned with respect to the (100) silicon lattice within the micron scale windows employed in this study.

Atom beam treatment of diamond films

Abstract Diamond films produced by microwave plasma chemical vapour deposition were exposed, at room temperature, to a flux of thermal H, O or N atoms (plus excited N 2 ∗ ) produced from a microwave-powered beam source. Optical changes were observed in samples treated with N atom doses in the range 3.24 × 10 17 –1.67 × 10 18 atoms cm −2 , whereas samples treated with H atoms (1.64 × 10 22 atoms cm −2 ) and O atoms (2.81 × 10 17 atoms cm −2 ) showed no obvious alteration. Scanning electron micrographs of the N-treated samples showed etching of the faceted crystallites. This resulted in reduced surface roughness as evidenced by stylus profilometry. X-Ray photoelectron spectroscopy and laser ionization mass analysis both detected the presence of nitrogen in the surface layers in these samples. Fourier transform IR analysis showed this to be covalently bound as both NH and CN. Cathodoluminescence studies did not show the characteristic emission lines associated with nitrogen vacancy or nitrogen interstitial centres. Electrical conductivity measurements by four-point probe techniques showed an increase in the resistivity of as-prepared films after O and H atom treatment. For a dose of 1.64 × 10 22 H atoms cm −2 , the electrical resistivity was dramatically increased to greater than 2 × 10 5 Ω cm. In contrast, etching of the films by an H 2 microwave plasma for 30 min immediately after deposition at 835 °C produced films with a low resistivity of around 50 Ω cm. The resistivity was increased to 5.8 × 10 3 Ω cm after an additional 3 h, but did not attain the values produced by room temperature thermal H atom treatment.

An FTIR study of the heteroepitaxy of diamond on silicon

Abstract A Fourier transform IR (FTIR) study is reported of the TO band in SiC, observed at 800 cm −1 at each of the three stages leading to the formation of epitaxial diamond films on (100) single-crystal silicon. Due to the large oscillator strength of the TO band this technique can detect, in transmission, layers as thin as a few nanometres. The radial profile of the SiC band intensity across the wafer, formed during the initial carburization step, is radically altered after the bias step. Within the annular region of epitaxial growth, the SiC thickness exhibits a point of inflection. Following the growth of the epitaxial diamond film, the interfacial SiC thickness is reduced further. An average SiC thickness of a few nanometres is observed at the interface, although the thickness is greater in regions in which the azimuthal alignment of the (100) diamond crystallites is lost.