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Wide-bandgap oxide semiconductors are essential for the development of high-speed and energy-efficient transparent electronics. However, while many high-mobility n-type oxide semiconductors are known, wide-bandgap p-type oxides have carrier mobilities that are one to two orders of magnitude lower due to strong carrier localization near their valence band edge. Here, we report the growth of bilayer beta tellurium dioxide (β-TeO2), which has recently been proposed theoretically as a high-mobility p-type semiconductor, through the surface oxidation of a eutectic mixture of tellurium and selenium. The isolated β-TeO2 nanosheets are transparent and have a direct bandgap of 3.7 eV. Field-effect transistors based on the nanosheets exhibit p-type switching with an on/off ratio exceeding 106 and a field-effect hole mobility of up to 232 cm2 V−1 s−1 at room temperature. A low effective mass of 0.51 was observed for holes, and the carrier mobility reached 6,000 cm2 V−1 s−1 on cooling to −50 °C. Bilayer beta tellurium dioxide nanosheets with p-type characteristics can be formed through the surface oxidation of a mixture of tellurium and selenium, and used to create transistors with performance that matches their n-type oxide counterparts.

Graphitic (sp2-rich) carbon Schottky contacts to ( $\overline {2}01$ ) $\beta $ -Ga2O3 were fabricated using sputtering and energetic ion deposition with and without the use of an oxygen plasma. As-deposited contacts exhibited room temperature effective barrier heights between 0.80 and 1.20 eV, depending on the deposition technique. After moderate heat treatment (595 K in N2) the work function of the oxygenated graphitic contacts increased by 0.5–0.8 eV. This resulted in a permanent increase in barrier height (up to 1.6 eV) and a corresponding 3–4 orders of magnitude increase in current rectification (at ±3 V). The barrier heights of the heat-treated oxygenated graphitic Schottky contacts agree well with the Mott–Schottky model, suggesting an absence of Fermi level pinning at the C/ $\beta $ -Ga2O3 interface.

Amorphous carbon-based memristors with transparent conductive electrodes can be programmed both optically and electronically. We demonstrate neuromorphic logic and memory functions using light-gated amorphous carbon (a-C) memristors with indium-free transparent electrodes. The devices consist of insulating tetrahedral a-C layers with Pt top contacts supported on conductive zinc tin oxide electrodes with intentionally-oxygenated a-C interfaces. Resistive switching occurs by electron trapping/de-trapping, allowing low power, self-rectifying operation. Broadband light pulses evoke paired-pulse facilitation, emulating photonic coincidence detection. Coincidence detection was also achieved using heterogeneous optoelectronic inputs demonstrating dendritic computation at the single memory cell level.

Energetically deposited graphitic carbon (C) is known to form high-endurance rectifying contacts to a variety of semiconductors. Graphitic contacts to n-type 6H-SiC have demonstrated current rectification ratios (at ± 1.5 V) up to 1:106. In this article, the current voltage temperature (I–V–T) characteristics of these devices are examined to reveal more detail on the junction/barrier properties that are critical to performance. Analysis of the I–V–T characteristics and disparity between barrier heights extracted from the I–V–T data and C–V data show inhomogeneity in the contacts and this has been quantified. Accounting for the inhomogeneity, the homogeneous Richardson constant of the n-type 6H-SiC can be extracted from the I–V–T data, and this value agrees with the reported theoretical value.

Amorphous carbon films have many applications that require control over their sp3 fraction to customise the electrical, optical and mechanical properties. Examples of these applications include coatings for machine parts, biomedical and microelectromechanical devices. In this work, we demonstrate the use of a magnetic field with a high-power impulse magnetron sputtering (HiPIMS) source as a simple, new approach to give control over the sp3 fraction. We provide evidence that this strategy enhances the deposition rate by focusing the flux, giving films with high tetrahedral bonding at the centre of the deposition field and lower sp3 fractions further from the centre. Resistive switching appears in films with intermediate sp3 fractions. The production of thin amorphous carbon films with selected properties without the need for electrical bias opens up applications where insulating substrates are required. For example, deposition of sp3 rich films on polymers for wear resistant coatings as well as fabrication of resistive switching devices for neuromorphic technologies that require tuning of the sp3 fraction on insulating substrates are now possible.

Magnetic van der Waals (vdW) materials, including ferromagnets (FM) and antiferromagnets (AFM), have given access to the investigation of magnetism in two-dimensional (2D) limit and attracted broad interests recently. However, most of them are semiconducting or insulating and the vdW itinerant magnets, especially vdW itinerant AFM, are very rare. Here, we studied the anomalous Hall effect of a vdW itinerant magnet Fe$_5$GeTe$_2$ (F5GT) with various thicknesses down to 6.8 nm (two unit cells). Despite the robust ferromagnetic ground state in thin-layer F5GT, however, we show that the electron doping implemented by a protonic gate can eventually induce a magnetic phase transition from FM to AFM. Realization of an antiferromagnetic phase in F5GT highlights its promising applications in high-temperature antiferromagnetic vdW devices and heterostructures., 27 pages

The weak interlayer coupling in van der Waals (vdW) magnets has confined their application to two dimensional (2D) spintronic devices. Here, we demonstrate that the interlayer coupling in a vdW magnet Fe_{3}GeTe_{2} (FGT) can be largely modulated by a protonic gate. With the increase of the protons intercalated among vdW layers, interlayer magnetic coupling increases. Because of the existence of antiferromagnetic layers in FGT nanoflakes, the increasing interlayer magnetic coupling induces exchange bias in protonated FGT nanoflakes. Most strikingly, a rarely seen zero-field cooled (ZFC) exchange bias with very large values (maximally up to 1.2 kOe) has been observed when higher positive voltages (V_{g}≥4.36 V) are applied to the protonic gate, which clearly demonstrates that a strong interlayer coupling is realized by proton intercalation. Such strong interlayer coupling will enable a wider range of applications for vdW magnets.

We describe the structural and electrical characteristics of a resistive switching device fabricated from amorphous carbon. The device consisted of two low resistance sp2-rich contacts separated by a resistive layer with mixed sp2 and sp3 hybridisation states deposited using energetic deposition. Systematic bipolar switching between high- and low-resistance states (HRS/LRS) was observed. The ratio between the HRS and LRS exceeded 100:1 and the device provided stable operation up to at least 10 k cycles. Unlike conventional metal-oxide memristors, this device does not rely on the formation and breakage of conductive metal filaments. A mechanism based on the formation of conductive pathways caused by transitions between sp2 and sp3 hybridisation states is proposed. The current-voltage characteristics were modelled assuming a conducting pathway formed either at the interface regions where barriers could be present or within the interior of the resistive layer where Poole-Frenkel conduction occurred. The switching was attributed to reversible electric field induced alteration of the hybridisation states of carbon atoms. The ability to fabricate these devices entirely from carbon suggests the possibility of a large scale manufacture of durable, biocompatible memristors.

Lateral memristors consisting of planar Ag electrodes (with sub-micrometer separation) supported on thin films of amorphous zinc-tin-oxide have been characterized. After an initial filament-forming process, each device exhibited volatile, resistive switching. In the low resistance state, the transport mechanism and conductance depended on prior activity and on the imposed current limit, mimicking biologic synaptic plasticity. Microscopic observations performed on each device revealed nanoscale filaments between the electrodes. These filaments were subject to Rayleigh instability and exhibited relaxation times determined by their effective radii. The relaxation times and on:off resistance ratios suggest suitability for threshold switching selector devices.

Two dimensional (2D) van der Waals (vdW) materials have demonstrated fascinating optical, electrical and thickness-dependent characteristics. These have been explored by numerous authors but reports on magnetic properties and spintronic applications of 2D vdW materials are scarce by comparison. By performing anomalous Hall effect transport measurements, we have characterised the thickness dependent magnetic properties of single crystalline vdW Fe3GeTe2. The nanoflakes of this vdW metallic material exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, we confirmed the existence of magnetic coupling between vdW atomic layers and obtained an estimated coupling length of ~ 5 vdW layers in Fe3GeTe2. Furthermore, the hard magnetic behaviour of Fe3GeTe2 can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into vdW magnetic heterostructures, paving the way for spintronic research and applications based on these devices., Accepted by Nature Communications

Multi-state amorphous carbon-based memory devices have been developed that exhibit both bipolar and unipolar resistive switching behaviour. These modes of operation were implemented independently to access multiple resistance states, enabling higher memory density than conventional binary non-volatile memory technologies. The switching characteristics have been further utilised to study synaptic computational functions that could be implemented in artificial neural networks. Notably, paired-pulse inhibition (PPI) is observed at bio-realistic timescales (

Junctions between energetically deposited graphitic carbon and n-type 6H-SiC have been fabricated. Their current-voltage characteristics have been measured and compared with simulations using Sentaurus TCAD finite element software. Agreement between the experimental and simulated current-voltage characteristics was achieved using parameters derived from electrical measurements and electron microscopy. Using the best-fit models, the effects of interfacial layers and contact work function variations were elucidated to provide guidance for improved device performance.

Cubic boron nitride (cBN) is a synthetic wide band gap material that has attracted attention due to its high thermal conductivity, optical transparency and optical emission. In this work, defects in cBN have been investigated using experimental and theoretical X-ray absorption near edge structure (XANES). Vacancy and O substitutional defects were considered, with O substituted at the N site (ON) to be the most energetically favorable. All defects produce unique signatures in either the B or N K-edges and can thus be identified using XANES. The calculations coupled with electron-irradiation / annealing experiments strongly suggest that ON is the dominant defect in irradiated cBN and remains after annealing. This defect is a likely source of optical emission in cBN.

Van der Waals (vdW) ferromagnetic materials are rapidly establishing themselves as effective building blocks for next generation spintronic devices. When layered with non-magnetic vdW materials, such as graphene and/or topological insulators, vdW heterostructures can be assembled (with no requirement for lattice matching) to provide otherwise unattainable device structures and functionalities. We report a hitherto rarely seen antisymmetric magnetoresistance (MR) effect in van der Waals heterostructured Fe3GeTe2/graphite/Fe3GeTe2 devices. Unlike conventional giant magnetoresistance (GMR) which is characterized by two resistance states, the MR in these vdW heterostructures features distinct high, intermediate and low resistance states. This unique characteristic is suggestive of underlying physical mechanisms that differ from those observed before. After theoretical calculations, the three resistance behavior was attributed to a spin momentum locking induced spin polarized current at the graphite/FGT interface. Our work reveals that ferromagnetic heterostructures assembled from vdW materials can exhibit substantially different properties to those exhibited by similar heterostructures grown in vacuum. Hence, it highlights the potential for new physics and new spintronic applications to be discovered using vdW heterostructures., 35 pages (Accepted by Science Advances)

Lateral memristors configured with inert Pt contacts and mixed phase tin oxide layers have exhibited immediate, forming-free, low-power bidirectional resistance switching. Activity dependent conductance and relaxation in the low resistance state resembled short term potentiation in biological synapses. After scanning probe microscopy, x-ray photoelectron spectroscopy and electrical measurements, the device characteristics were attributed to Joule heating induced decomposition of the minority SnO phase and formation of a SnO2 conducting filament with higher effective n-type doping. Finally, the devices recognized input voltage pulse sequences and spectral data by returning unique conductance states, suggesting suitability for bio-inspired pattern recognition systems.

Amorphous carbon films with an intermediate content of sp3 atoms are finding applications as resistive switches in devices for bio-sensing and for neuromorphic pattern recognition. To understand resistive switching and photoconductivity in amorphous semiconductors dominated by hopping conduction, we present a theory that unifies the optical and electronic properties. The theory considers all of the states to be localized to various extents instead of being extended electronic states. The electronic density of states (eDOS) is modeled with Gaussian functions, symmetric in energy around the Fermi energy. A “hopping mobility” between localized states that is explicitly both energy and temperature dependent is introduced. We describe an example application to amorphous carbon films prepared by using high power impulse magnetron sputtering that have a range of sp3 hybridization fractions of the carbon atoms. The electronic bandgaps of the films are observed to correlate with their optical bandgaps. The eDOS is benchmarked against optical property measurements made by ellipsometry. The theory explains the temperature dependence of the resistivity and predicts that the films should show a temperature dependent hopping photoconductivity. Measurements confirm the presence of the photoconductivity and reveal its spectral dependence. A link is made between persistent hopping photoconductivity and resistive switching.

Electrical contacts to n-type 6H–SiC deposited from carbon fluxes with differing energies (0.12–1.0 keV) have exhibited deposition energy dependent microstructural and electrical properties. Lower deposition energies resulted in resistive amorphous carbon contacts in which transport occurred by tunneling through an interfacial potential barrier consisting of a mixed oxide/carbon layer. With higher average energy and moderately elevated temperature, the interfacial native oxide was removed by implantation/sputtering and Schottky contacts were formed with ideality factors of 1.3 and barrier heights of 0.97 eV. These contacts consisted of graphitic sheets aligned perpendicular to the growth direction. Energetic deposition offers control over microstructural/device properties, compatibility with photolithography and excellent adhesion to the underlying 6H–SiC.

Finite element modelling has been used to optimise the current/ voltage (I/V) characteristics of metal/ n-SiC and metal/ n-Si diodes incorporating a thin interfacial layer. The electrical properties of the diodes have been examined in relation to the polytype of SiC (3H, 4H or 6C), the doping level, NA, (1015 - 1018cm3) of the substrate, the defect state density, Dit and the work function of the Schottky metal, Φm. The modelling by Technology Computer-Aided Design (TCAD) has shown that the presence of an interfacial insulating layer with a thickness of 1.0 nm has reduced the reverse leakage current of the diode by a factor of ∼102 in Si and 1013 (from 10-19 A to 10-6 A) for SiC with only a minor reduction (∼ 0.8 times) in the forward current in SiC. The properties of the diodes have been modelled at room temperature without thermal annealing.

The mechanical behaviour of carbon films prepared with a variety of densities and microstructures was investigated using nanoindentation. Deposition energies between 25 and 600 eV and temperatures in the range 25–600 °C were used. Films prepared at low temperatures and moderate energies were amorphous with a high density. Finite element methods were used to model the stress fields, reproduce the indentation behaviour and evaluate elastic properties. Young's moduli up to 670 GPa and a low Poisson's ratio of ∼0.17 were found, comparable to polycrystalline cubic boron nitride, one of the hardest materials known. Films with the same density did not always show the same behaviour, emphasising the role of microstructure in determining mechanical response. Extended graphite-like regions within the films grown at high energy and high temperature, observed in transmission electron microscopy caused plastic deformation and failure to recover after a complete indentation cycle. At low deposition energies, the graphite-like regions were smaller in size causing plastic deformation but with complete recovery after indentation.

Stoichiometric amorphous HfO2 films have been deposited by reactive High Power Impulse Magnetron Sputtering (HiPIMS) from a Hf target in a 1:1 Ar:O2 atmosphere at pressures 2–4.5 mTorr. An optimum pressure was found for depositing smooth, high refractive index and amorphous films. Stress and refractive index reached a maximum as deposition pressure was increased to 3.5 mTorr. At 3.5 mTorr, HfO2 films were deposited with a refractive index of 2.15 at 500 nm, low leakage currents, moderate fixed charge density and a high dielectric constant of ∼18.3. The intensification of energetic ion bombardment upon the film with increase in HiPIMS pressure plays a dominant role in film properties. Increase in pressure above the optimum relieved the stress in the films and degraded the optical and electrical properties. HiPIMS pressure enables to gain indirect control of ion flux and energy in the plasma and can be used to modify the properties of depositing films.

We report on the hardness response of vacuum heat treated (VHT) M35 HSS as a function of the thickness of oxide present during VHT. An almost linear relationship existed between the hardening response post-VHT and the oxide thickness pre-VHT. A similar relationship existed between the thickness of oxide pre-VHT and the carbon content post-VHT. CO evolution during VHT was observed from ∼800 °C. In combination, these measurements show that CO evolution and carbon loss are increased in M35 HSS covered with surface oxide whilst hardening response is reduced. A full hardening response and unchanged carbon content were measured in M35 HSS VHT with surface oxide removed.

Transparent conducting oxides are fundamental for the fabrication of optoelectronic devices including touchscreen displays, solar cells, and light emitting diodes. However, they mostly rely on rare elements and expensive vacuum-based deposition methods that negatively affect the overall cost of optoelectronics. Here, a detailed investigation on the synthesis of antimony-doped tin oxide films using an ultrasonic spray coating system is presented. High-quality, crystalline SnO2 films are deposited via decomposition of metal precursors sprayed directly onto hot (>400 °C) substrates. Doping is easily achieved by adding the dopant salt to the spray solution, and the presence of the dopant atoms heavily influences the optical, electrical, and structural properties of SnO2. These coatings are characterized using a comprehensive suite of techniques including X-ray diffraction, electron microscopy, X-ray and ultraviolet photoelectron spectroscopies, Hall effect measurements, optical spectroscopy in the visible and near infrared, and atomic force microscopy, in order to elucidate the relationship between the synthetic conditions and functional properties. Through a careful optimization process, Sb-doped SnO2 coatings showing transmittance values in the visible spectrum between 80% and 90%, and sheet resistances of 10–20 Ω/sq−1 are achieved. Such values are suitable for immediate applications of these Sb-doped SnO2 films as high-performance transparent conductors.

Interconnected SnOx nanoparticle (NP) networks are electroformed within a semi‐insulating SnOx thin‐film and between lateral electrodes. During this top‐down process, Joule heating, disproportionation, and de‐wetting of the SnOx thin‐film precede the formation of the NP networks. The same lateral electrodes used for electroforming are used to probe the network and reveal its complex electrical characteristics. Higher‐order harmonic generation is observed and the internal short‐term memory effects of the NP networks enable temporal inputs to be mapped into reservoir states for subsequent linear readout without training. Reservoir computing functionality is demonstrated with no requirement for high‐vacuum or cryo‐cooled environments.

We describe a duplex nitriding and TiAlN coating process performed with no break in vacuum in an industrial scale deposition chamber. Plasma nitriding at 480 °C resulted in a fracture tough diffusion zone with no evidence of either a compound layer or grain boundary precipitation. Untreated, plasma-nitrided, coating-only and duplex-coated High Speed Steel (HSS) M2 coupons were microstructurally and mechanically characterised whilst similarly treated M2 ¼-inch jobber drills were subjected to accelerated drill testing (using D2 tool steel). In these tests, the duplex treated drills exhibited significantly improved wear performance and tool lives when compared with the other drills. After sectioning and electron microscopy, this extended tool life was attributed to increased toughness of the nitrided cutting edges coupled with improved adhesion at the substrate–TiAlN interface.

Zn1−xMgxO thin films have been energetically deposited from a filtered catholic vacuum arc at moderate temperatures and microstructurally characterised. Partial oxidation (‘poisoning’) of the Zn0.8Mg0.2 cathode caused layering and phase separation in the films. However, periods of non-reactive ablation steps incorporated into the deposition process minimised the effects of cathode poisoning and enabled dense, phase-pure, wurtzite Zn1−xMgxO to be grown at room temperature and 200 °C. Elevated substrate temperature resulted in enlarged grains and increased surface roughness. Increased substrate bias caused reduced crystalline order. X-ray absorption spectra from the homogeneous Zn1−xMgxO films, revealing local atomic bonding, were similar to spectra from single crystal ZnO but with features indicative of defects related to oxygen deficiency.

Carbon films deposited by filtered cathodic vacuum arc have been used to form high quality Schottky diodes on p-Si. Energetic deposition with an applied substrate bias of -1 kV and with a substrate temperature of 100 °C has produced carbon diodes with rectification ratios of ∼ 3 × 106, saturation currents of ∼0.02 nA and ideality factors close to unity (n = 1.05). Simulations were used to estimate the effective work function and the thickness of an interfacial mixed (C/SiO2) layer from the current/voltage characteristics of the diodes.

Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor with applications including gate insulation layers in graphene transistors, far-ultraviolet light emitting devices and as hydrogen storage media. Due to its complex microstructure, defects in hBN are challenging to identify. Here, we combine x-ray absorption near edge structure (XANES) spectroscopy with ab initio theoretical modeling to identify energetically favorable defects. Following annealing of hBN samples in vacuum and oxygen, the B and N $K$ edges exhibited angular-dependent peak modifications consistent with in-plane defects. Theoretical calculations showed that the energetically favorable defects all produce signature features in XANES. Comparing these calculations with experiments, the principle defects were attributed to substitutional oxygen at the nitrogen site, substitutional carbon at the boron site, and hydrogen passivated boron vacancies. Hydrogen passivation of defects was found to significantly affect the formation energies, electronic states, and XANES. In the B $K$ edge, multiple peaks above the major $1s$ to ${\ensuremath{\pi}}^{*}$ peak occur as a result of these defects and the hydrogen passivated boron vacancy produces the frequently observed doublet in the $1s$ to ${\ensuremath{\sigma}}^{*}$ transition. While the N $K$ edge is less sensitive to defects, features attributable to substitutional C at the B site were observed. This defect was also calculated to have mid-gap states in its band structure that may be responsible for the 4.1-eV ultraviolet emission frequently observed from this material.

Carbon films and underlying copper template-layers have been deposited energetically in the same filtered cathodic vacuum arc system at 750 °C. The high quality copper template-layers were supported on either silicon or silica and were subsequently sacrificially etched. On silicon, copper silicide formed during the copper deposition process, inhibiting ordered growth in the carbon film. On silica, large areas of multi-layer graphene (up to ∼10 layers) oriented parallel to the substrate were synthesised and these remained intact after the sacrificial etching process. The ability to produce both copper and multilayer graphene layers in one system enables simplified fabrication of this material.

Carbon allotropes are of great interest recently due to theirs novel applications. This paper examines the electrical behavior of amorphous carbon film, deposited on n-Si substrate by filtered cathodic vacuum arc (FCVA) deposition methodology, after treating it at high temperature. XPS data is also used to further understand the material. FCVA amorphous carbon film deposited at 1000V and post deposition heat-treated at 8000C is suggested to achieve a better Ohmic contact to n-Si substrates with total resistance 1200Ω compared to other settings with total resistance up to 2000Ω.

A filtered cathodic vacuum arc system has been used to reactively deposit nano-composite high speed steel (HSS)-TiN films from a cold-sprayed HSS-Ti cathode. The microstructure of the films depended on both the energy of the depositing flux (controlled by substrate bias) and the substrate temperature. With a low substrate bias of − 25 V at room temperature, a fine nanocrystalline microstructure was produced. An increase in preferred orientation of the FCC TiN phase was observed in films deposited at RT with elevated substrate biases. Elevated temperature in the absence of bias promoted orientation of the BCC Fe phase. Moderate substrate bias (~− 200 V) and high substrate temperature (> 250 °C) improved the hardness, elastic modulus and elastic recovery of the HSS-TiN thin films. Potential applications for these films, with tunable mechanical properties, include matching layers between HSS tools and TiN protective coatings.

The intrinsic stress, film density and nitrogen content of carbon nitride (CN x ) films deposited from a filtered cathodic vacuum arc were determined as a function of substrate bias, substrate temperature and nitrogen process pressure. Contour plots of the measurements show the deposition conditions required to produce the main structural forms of CN x including N-doped tetrahedral amorphous carbon (ta-C:N) and a variety of nitrogen containing graphitic carbons. The film with maximum nitrogen content (~ 30%) was deposited at room temperature with 1.0 mTorr N 2 pressure and using an intermediate bias of − 400 V. Higher nitrogen pressure, higher bias and/or higher temperature promoted layering with substitutional nitrogen bonded into graphite-like sheets. As the deposition temperature exceeded 500 °C, the nitrogen content diminished regardless of nitrogen pressure, showing the meta-stability of the carbon–nitrogen bonding in the films. Hardness and ductility measurements revealed a diverse range of mechanical properties in the films, varying from hard ta-C:N (~ 50 GPa) to softer and highly ductile CN x which contained tangled graphite-like sheets. Through-film current–voltage characteristics showed that the conductance of the carbon nitride films increased with nitrogen content and substrate bias, consistent with the transition to more graphite-like films.

We report temperature dependent electrical characteristics of two-terminal Ag/a-COx/ta-C/Pt memristors. In these asymmetric devices, defects at the Ag/a-COx interface are passivated by oxygen. This alleviates Fermi level pinning and hence increases the height of the Schottky barrier formed at the interface. Electric-field-induced detrapping of electrons from sp2-related defects in the ta-C causes the observed resistive switching. This occurs entirely in the insulating regime, i.e., with conductance ≪ 2e2/h, enabling ultralow power resistive switching (∼6 nW). Nonlinear temperature dependent ON/OFF ratios and short-term memory characteristics (governed by thermal detrapping kinetics) suggest suitability for temporal neuromorphic computing and sensing applications.

We report an electrically conductive carbon film with controllable hydrophilic properties that offers a covalent binding surface containing radicals for biomolecule attachment without using chemical linkers. Films were deposited from an acetylene-containing plasma using plasma immersion ion implantation during growth and subsequently annealed under vacuum. Electrical conductivity, spin density, contact angle, surface energy, surface composition, and covalent binding capability were studied as a function of annealing temperature, revealing three distinct regions. In the first region, surface energy is dominated by polar groups. In the second region, the polar groups are expelled, creating unpaired electrons that dominate the polar component of the surface energy. In the third region, the electrical conductivity rises and the polar component of surface energy falls as the unpaired electrons recombine, leading to an optimum combination of surface energy, spin density, and electrical conductivity for biological applications. It is proposed that persistent radicals are responsible for both high wettability and covalent binding properties. Covalently attached enzyme molecules on the C film can resist stringent washing with detergents. The C films offer the functions of conducting polymers, but with the added features of controllable wettability and a covalent binding capability.We report an electrically conductive carbon film with controllable hydrophilic properties that offers a covalent binding surface containing radicals for biomolecule attachment without using chemical linkers. Films were deposited from an acetylene-containing plasma using plasma immersion ion implantation during growth and subsequently annealed under vacuum. Electrical conductivity, spin density, contact angle, surface energy, surface composition, and covalent binding capability were studied as a function of annealing temperature, revealing three distinct regions. In the first region, surface energy is dominated by polar groups. In the second region, the polar groups are expelled, creating unpaired electrons that dominate the polar component of the surface energy. In the third region, the electrical conductivity rises and the polar component of surface energy falls as the unpaired electrons recombine, leading to an optimum combination of surface energy, spin density, and electrical conductivity for biologic...

The electrical resistance of oriented graphitic carbon films supporting catalytic Pt pads decreases upon exposure to hydrogen gas with a magnitude dependent on the crystallographic order in the film. This resistance change is attributed to defects responding to atomic hydrogen spilled over from the catalytic Pt.

In this paper we report on the structural, electrical and optical characteristics of unintentionally doped ZnO films grown on a-plane sapphire substrates using the Filtered Cathodic Vacuum Arc (FCVA) technique. The resulting films showed considerable promise for device applications with properties including high transparency, moderate intrinsic carrier concentrations (10(17)-10(19) cm(-3)), electron mobilities up to 30 cm(2)/Vs, low surface roughness (typically

Oxidized iridium (IrOx) anodes fabricated on n-type ZnO single crystal wafers using reactive pulsed laser deposition are known to produce high quality Schottky barriers with ideality factors approaching the image-force-controlled limit for laterally homogeneous interfaces. These high performance IrOx/ZnO Schottky contacts were cross-sectioned and analyzed using transmission electron microscopy, revealing an amorphous interfacial layer of 2–3 nm thickness. Electron energy loss spectroscopy, used to study the composition of the interface region, showed evidence of significant zinc diffusion across the interface into the IrOx film, which leads to the creation of Zn vacancies (acceptors), in the ZnO sub-interface region. There is also evidence for oxygen passivation near the interface resulting from the use of an active oxygen ambient during the IrOx deposition. Both these factors may explain the outstanding electrical performance of these Schottky devices.

Aluminium oxide based coatings were reactively deposited using a filtered cathodic vacuum arc and their microstructure and optical properties were determined using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, energy filtered transmission electron microscopy and spectroscopic ellipsometry. Coatings with compositions ranging from near stoichiometric Al 2 O 3 to aluminium rich aluminium oxide were produced using different oxygen process pressures. The aluminium rich films consisted of Al metal clusters in an aluminium oxide matrix and exhibited optical and electronic properties which depended on the Al metal content. The refractive index at a wavelength of 500 nm varied systematically from 1.75 to 1.9 as the Al content increased to approximately 50 at.%. Films with high Al content were also found to be more electrically conductive, consistent with the presence of metallic Al. A coating was also successfully prepared in which the refractive index was varied with thickness by varying the oxygen pressure during deposition. The optical absorption of this coating showed near zero reflectance at 450 nm which indicated that these gradient index coatings could be successfully used as solar selective coatings.

A Pt/oriented graphitic carbon heterojunction has been fabricated and tested as a conductometric hydrogen gas sensor. The carbon layer, deposited between planar Au substrate contacts using a filtered cathodic vacuum arc (with an applied substrate bias of -500 V), has a low density of 1.7 g/cm3 and with 67% of the atoms bonded in sp2 or graphitic configurations. Electron diffraction analysis showed evidence that the film has a microstructure consisting largely of vertically oriented graphitic sheets. These sheets have good through-film electrical conductivity and contributed to a low device resistance of ~ 60 Ω. A 3-nm Pt layer was deposited subsequently on the carbon layer. A change in the device resistance of >; 2% was exhibited upon exposure to 1% hydrogen gas (in synthetic, zero humidity air) at room temperature. The time for the sensor resistance to decrease by 2% under these conditions was 60 s and the baseline (zero hydrogen exposure) resistance remained constant to within 0.03% during and after the exposure tests.

Aluminium nitride (AlN) thin films have been reactively deposited using a filtered cathodic vacuum arc system. A pulsed substrate bias was applied in order to increase the average energy of the depositing species. The stress and microstructure of the films were determined as a function of the deposition rate and pulse bias amplitude/frequency. The stress generated in films grown with high voltage pulsed bias depended on the deposition rate and a transition from tensile stress to compressive stress occurred as the deposition rate increased. This trend was accompanied by progressive changes in the microstructure. In order of increasing deposition rate, the films exhibited: a porous structure with tensile stress; a dense AlN film with compressive stress; and a dense AlN film showing evidence of a thermally induced reduction in stress.

Graphitic contacts to semiconductors have been shown to provide highly rectifying current-voltage characteristics coupled with high thermal/chemical stability. Energetically deposited graphitic contacts to p-type Si have exhibited rectification ratios (at ± 1.0 V) up to 106:1 and ideality factors approaching unity. Here, we report temperature dependent current-voltage (I-V-T) measurements performed on such devices. The measurements and subsequent analysis show that during energetic carbon deposition, deleterious oxide/contaminants are removed from the Si substrate surface. The Richardson constant of the p-type Si extracted from the I-V-T measurements agrees with the theoretical value, indicating that the surface contaminants are removed without significant damage to the underlying Si. Therefore, by energetic deposition of C on Si, C-Si junctions can be formed with low lateral inhomogeneity and low interface defect density. These attributes of the junctions enable the observed near-ideal Schottky diode characteristics.

Energetic deposition of high speed steel (HSS) films has been performed using a filtered cathodic vacuum arc system fitted with a AISI M2 HSS cathode. The mechanical properties and microstructure of the films were investigated as a function of the deposition energy. The stoichiometry and microstructure of the deposited films were different to that of the cathode and a dependence on the deposition energy was found. Films deposited at low deposition energies (< 100 eV) exhibited reduced concentrations of Mo and W and a smaller average crystallite size than the M2 HSS cathode. These films were found to be significantly harder than the M2 HSS cathode, possibly due to their smaller average crystallite size. A HSS film consisting of an amorphous phase with isolated crystals of bcc Fe was produced using high deposition energy and the deposition rate was found to decrease, likely due to sputtering.

Active screen plasma nitriding (ASPN) was conceived in order to reduce negative effects observed in direct current plasma nitriding arising from the application of bias to the components. The mechanism of nitrogen mass transfer in ASPN is still not fully understood. Here, we compare the microstructure, composition and hardness response of AISI P20 and H13 steels after nitriding. A set of samples was nitrided with sample bias applied directly and another set was nitrided at floating potential under an active screen. Similar nitrogen content and hardness profiles were obtained for the samples treated using a bias and under an active screen separated from the samples by 12 mm. When the sample-screen separation was increased from 12 to 70 mm the hardness response improved. The principle processes occurring during ASPN are proposed based on the experimental results. In ASPN, a flux of energetic nitrogen species is generated by the active screen which, provided that the samples are within the range of the energetic species, bombards the surface of the samples being treated. This flux is critical in establishing a nitrogen potential and a satisfactory response in the components.

It has been proposed that in plasma nitriding, sputtering of material from biased components within the chamber assists in the nitrogen mass transfer process. Here, we investigate the effects of this sputter deposition process on the nitriding response of biased and unbiased AISI P20 steel samples mounted in a large-scale plasma nitriding system operated at 520 °C. Films with nanostructured morphologies resulting from Volmer–Weber film growth were observed on Si substrates placed adjacent to AISI P20 steel substrates after nitriding experiments. Auger depth profiling revealed that the films on the Si substrates had a stoichiometric ratio of 4:1 Fe:N. This suggested that the particles consisted largely of Fe4N and it was concluded that they were formed from atoms and small clusters sputtered from biased components in the chamber. Despite the deposition of these films, no significant improvement in surface hardness was observed in the steel samples unless bias was applied to them. Furthermore, the maximum hardness achieved in biased P20 samples after the nitriding process occurred in the samples positioned adjacent to the Si samples supporting the thinnest deposited films. These findings do not support the proposition that in plasma nitriding, nitrogen mass transfer occurs predominantly by sputter deposition of iron nitride.

Nitrogen-doped multiwall carbon nanotubes (NCNTs), having an average diameter of approximately 20 nm, were synthesized at 650 oC by chemical vapor deposition using a pyridine precursor. Pt-Ni alloyed nanoparticles with approximate diameter 3 nm and with different Pt to Ni molar ratios were deposited on the NCNTs by a microwave-polyol method. Electron microscopy revealed that the nanoparticles were deposited homogeneously on the outer surface of the NCNTs and were immobilized at active nitrogen sites. A dielectrophoresis technique was used to selectively align the Pt-Ni-coated NCNTs between metallic electrodes to form conductometric hydrogen gas sensors. Gas sensing measurements performed with different concentrations of hydrogen revealed that the sensor based upon Pt/NCNTs exhibited the fastest response and recovery and best sensitivity. The sensing mechanism in the Pt/NCNT sensors can be explained by a combination of responses from the nitrogen-induced defects and the supported Pt nanoparticles, with the latter providing significantly faster response and recovery.

This paper describes the formation of self-organized nanopores in thin films of titanium prepared using a Filtered Cathodic Vacuum Arc (FCVA) deposition system. The post-deposition anodization was performed using 0.5% (wt) NH 4 F in ethylene glycol and an aqueous based solution containing 0.5% (wt) NH 4 F and 1 M (NH 4 ) 2 SO 4 electrolytes. Homogenously distributed nanopores with dimensions in the range of 10 to 20 nm were obtained. Nanoporous TiO 2 thin films were obtained after annealing the anodized samples at 600 °C for 4 h. Scanning electron microscopy (SEM) and Raman spectroscopy were used to characterize these nanoporous films. Raman measurements revealed that the rutile TiO 2 polymorph dominates these structures along with imperfect titanium oxidation resulting in the formation of defect structures, particularly when aqueous electrolyte was used for the anodization.

Nanoporous titanium dioxide (TiO 2 ) based conductometric sensors have been fabricated and their sensitivity to hydrogen (H 2 ) gas has been investigated. A filtered cathodic vacuum arc (FCVA) system was used to deposit ultra-smooth Ti thin films on a transducer having patterned inter-digital gold electrodes (IDTs). Nanoporous TiO 2 films were obtained by anodization of the titanium (Ti) thin films using a neutral 0.5% (wt) NH 4 F in ethylene glycol solution at 5 V for 1 h. After anodization, the films were annealed at 600 °C for 8 h to convert the remaining Ti into TiO 2 . The scanning electron microscopy (SEM) images revealed that the average diameters of the nanopores are in the range of 20 to 25 nm. The sensor was exposed to different concentrations of H 2 in synthetic air at operating temperatures between 100 °C and 300 °C. The sensor responded with a highest sensitivity of 1.24 to 1% of H 2 gas at 225 °C.

The structure of non-crystalline carbon films produced using physical vapour deposition is studied as a function of ion impact energy and substrate temperature. The average ion energy was varied from 10 eV to 820 eV using magnetron sputtering and cathodic arc deposition systems while the substrate temperature was varied from room temperature to 640 °C. The intrinsic stress, film density and through-film electrical resistance were mapped in the ion energy–temperature plane. These contour plots show the deposition conditions which give rise to stressed tetrahedral amorphous carbon (ta-C), stress relieved ta-C and lower density films with a variety of structural forms. Electron microscopy revealed that the microstructure of the low-density forms ranges from highly disordered to various types of oriented graphitic material. The microstructure is determined by the local structural rearrangements that occur due to ion impacts on the picosecond timescale (thermal spikes) and slower relaxation processes which depend on the substrate temperature.