J.G. Partridge

Microstructural investigation supporting an abrupt stress induced transformation in amorphous carbon films

The intrinsic stress of carbon thin films deposited by filtered cathodic arc was investigated as a function of ion energy and Ar background gas pressure. The microstructure of the films was analyzed using transmission electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy. The stress at given substrate bias was reduced by the presence of an Ar background gas and by the presence of a Cu underlayer deposited onto the substrate prior to deposition. Auger electron spectroscopy depth profiles showed no evidence of Ar incorporation into the films. A sharp transition from a sp 2 to sp 3 rich phase was found to occur at a stress of 6.51.5 GPa, independent of the deposition conditions. The structural transition at this value of stress is consistent with available data taken from the literature and also with the expected value of biaxial stress at the phase boundary between graphite and diamond at room temperature. The microstructure of films with stress in the transition region near 6.5 GPa was consistent with a mixture of sp 2 and sp 3 rich structures.

The role of pulse length in target poisoning during reactive HiPIMS: application to amorphous HfO2

In conventional reactive magnetron sputtering, target poisoning frequently leads to an instability that requires the reactive gas flow rate to be actively regulated to maintain a constant composition of the deposited layers. Here we demonstrate that the pulse length in high power impulse magnetron sputtering (HiPIMS) is important for determining the surface conditions on the target that lead to poisoning. By increasing the pulse length, a smooth transition can be achieved from a poisoned target condition (short pulses) to a quasi-metallic target condition (long pulses). Appropriate selection of pulse length eliminates the need for active regulation, enabling stable reactive magnetron sputter deposition of stoichiometric amorphous hafnium oxide (HfO 2 ) from a Hf target. A model is presented for the reactive HiPIMS process in which the target operates in a partially poisoned mode with a distribution of oxide on its surface that depends on the pulse length.

Memristor and selector devices fabricated from HfO2−xNx

Monoclinic HfO2−xNx has been incorporated into two-terminal devices exhibiting either memristor or selector operation depending on the controlled inclusion/suppression of mobile oxygen vacancies. In HfO2 memristors containing oxygen vacancies, gradual conductance modulation, short-term plasticity, and long-term potentiation were observed using appropriate voltage-spike stimulation, suggesting suitability for artificial neural networks. Passivation of oxygen vacancies, confirmed by X-ray absorption spectroscopy, was achieved in HfO2−xNx films by the addition of nitrogen during growth. Selector devices formed on these films exhibited threshold switching and current controlled negative differential resistance consistent with thermally driven insulator to metal transitions.

Synthesis of highly tetrahedral amorphous carbon by mixed-mode HiPIMS sputtering

Tetrahedral amorphous carbon films with an sp 3 content of 80% have been produced by high power impulse magnetron sputtering (HiPIMS) operating in a mixed sputtering/arc mode. In this mode, short-lived cathode spots form in the magnetic racetrack and produce large numbers of carbon ions. The spots move rapidly, inhibiting the formation of macroparticles. An argon pressure below 2.5 mTorr was critical for obtaining films with high sp 3 content, high stress, large Tauc gap and symmetrical Raman spectra, and all four quantities were strongly correlated.

Influence of nitrogen-related defects on optical and electrical behaviour in HfO2−xNx deposited by high-power impulse magnetron sputtering

HfO2−xNx films have been deposited by high-power impulse magnetron sputtering in an Ar-O2-N2 atmosphere with a series of nitrogen partial pressures. X-ray absorption spectroscopy revealed the optimum deposition conditions required to passivate O vacancies in the HfO2−xNx films by nitrogen. Low-mobility interstitial species prevent crystallisation of nitrogen-incorporated films. These effects combine to remove leakage paths resulting in superior breakdown strengths compared to films deposited without nitrogen. The bandgap was maintained at ∼5.9 eV in the films in which nitrogen passivated the oxygen vacancies. This is essential to provide sufficient band offsets for HfO2−xNx films to be used an effective gate dielectric.

The near edge structure of hexagonal boron nitride.

Hexagonal boron nitride (hBN) is a promising material for a range of applications including deep-ultraviolet light emission. Despite extensive experimental studies, some fundamental aspects of hBN remain unknown, such as the type of stacking faults likely to be present and their influence on electronic properties. In this paper, different stacking configurations of hBN are investigated using CASTEP, a pseudopotential density functional theory code. AB-b stacking faults, in which B atoms are positioned directly on top of one another while N atoms are located above the center of BN hexagons, are shown to be likely in conventional AB stacked hBN. Bandstructure calculations predict a single direct bandgap structure that may be responsible for the discrepancies in bandgap type observed experimentally. Calculations of the near edge structure showed that different stackings of hBN are distinguishable using measurements of core-loss edges in X-ray absorption and electron energy loss spectroscopy. AB stacking was found to best reproduce features in the experimental B and N K-edges. The calculations also show that splitting of the 1s to π* peak in the B K-edge, recently observed experimentally, may be accounted for by the presence of AB-b stacking faults.

Engineering titanium and aluminum oxide composites using atomic layer deposition

Mixed metal oxides provide a convenient means to produce coatings with tailored physical properties. We investigate the possibility of synthesizing novel coatings of mixed titanium and aluminum oxide using atomic layer deposition (ALD). Results show that ALD films were prepared with compositions ranging between Al2O3 and TiO2 having refractive indices between 1.6 and 2.4 (at λ = 550 nm) at low temperature. The microstructure and bonding environment within the films was investigated using electron microscopy and x-ray absorption spectroscopy. The films were amorphous, and the Ti and Al atoms were mixed at the atomic scale. The electrical breakdown characteristics of the films were measured and showed that films with intermediate compositions had poor leakage current properties, believed to be caused by the presence of distorted bonding configurations. This study shows that ALD can be used to deposit high quality thin films with tailored optical properties, particularly suitable for applications in which co...

Effect of Schottky gate type and channel defects on the stability of transparent ZnO MESFETs

Transparent metal–semiconductor field-effect transistors (MESFETs) were fabricated on ZnO thin films deposited using a filtered cathodic vacuum arc. MESFETs with silver oxide (AgOx) and iridium oxide (IrOx) Schottky gates with mobilities up to 70 cm2 V−1 s−1 were subjected to extensive bias and illumination stress tests. Device instability appeared to be strongly dependent on gate metal type, bias conditions and ZnO film morphology. Positive bias stress of AgOx gated devices resulted in irreversible damage that is thought to be due to Ag electromigration across the gate interface. IrOx gated devices showed superior stability with only a small recoverable decay in channel current that was associated with the charging of grain boundary defects.

Structural and dielectric properties of energetically deposited hafnium oxide films

Amorphous hafnium oxide films, energetically deposited at room temperature from a filtered cathodic vacuum arc (FCVA) onto Si substrates, exhibit low current leakage (11 μA cm−2 in an electric field of 100 kV cm−1), a dielectric constant (k) of 17 and a refractive index exceeding 2.1 over the visible spectrum. Cross-sectional transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy loss spectroscopy revealed an amorphous microstructure and higher film density when compared with HfO2 deposited by reactive direct-current magnetron sputtering. The superior properties and higher density of the FCVA HfO2 are attributed to the elevated energy of the depositing flux.

Energetic deposition of carbon in a cathodic vacuum arc with a biased mesh

Carbon films were deposited in a filtered cathodic vacuum arc with a bias potential applied to a conducting mesh mounted in the plasma stream between the source and the substrate. We determined the stress and microstructural properties of the resulting carbon films and compared the results with those obtained using direct substrate bias with no mesh. Since the relationship between deposition energy and the stress, sp2 fraction and density of carbon are well known, measuring these film properties enabled us to investigate the effect of the mesh on the energy and composition of the depositing flux. When a mesh was used, the film stress showed a monotonic decrease for negative mesh bias voltages greater than 400V, even though the floating potential of the substrate did not vary. We explain this result by the neutralization of some ions when they are near to or passing through the negatively biased mesh. The microstructure of the films showed a change from amorphous to glassy carbonlike with increasing bias. ...