Billy J. Murdoch

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.

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.

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.

Co-deposition of band-gap tuned Zn1-xMgxO using high impulse power- and dc-magnetron sputtering

High impulse power- and direct current- magnetron sputtering have been used to reactively co-deposit Zn1-xMgxO onto a 100 mm diameter a-plane sapphire wafer at 200 C. The Zn1-xMgxO film exhibited low surface roughness, high transparency, high electrical resistivity and a Mg fraction (x) depending on substrate location. The optical bandgap of the film varied monotonically with x up to the miscibility limit of ∼0.32, beyond which a mixed cubic/wurtzite structure formed. Annealing at 550 C in forming gas (95% N2, 5% H2), caused reduced compressive stress and dramatically reduced electrical resistivity. The latter was attributed to shallow doping by hydrogen bound to oxygen vacancies and these changes occurred in the wurtzite Zn1-xMgxO without detectable phase transformation. A filtered UV detector, with active and filter layers fabricated from the co-deposited film, exhibited sensitivity to UV in a 330-355 nm pass-band and approximately three orders of magnitude UV-to-visible rejection.

Relationship between microstructure and electronic properties of energetically deposited zinc tin oxide

Thin films of amorphous n-type zinc tin oxide have been energetically deposited from a filtered cathodic vacuum arc at moderate temperatures. The characteristics of these films span a range suitable for semiconductor devices and transparent conducting oxide interconnects with carrier concentration and mobility dependent on local bonding. X-ray photoelectron spectroscopy (XPS) and electron diffraction have revealed that acceptor-like Sn(II) bonding in the films decreased with increasing growth temperature, resulting in higher n-type carrier concentrations. XPS and in situ Ar plasma treatment showed that downward surface band bending resulted from OH attachment. Persistent photoconductivity was attributed to the photoionization of oxygen vacancies.

Ultraviolet detection from energetically deposited titania films

Thin films of unintentionally doped n-type titania have been energetically deposited from a filtered cathodic vacuum arc. All films were dense, smooth, and transparent with crystallinity depending on the deposition/annealing temperature. At a growth temperature of 600 °C, the preferred phase could be changed from rutile to anatase by increasing the oxygen process pressure thereby reducing dynamic annealing. Pt/TiOx/Pt ultraviolet detectors exhibiting rectifying current-voltage characteristics and ultraviolet-visible rejection ratios exceeding 104:1 were formed on selected films.

Non-volatile and volatile memory behaviour in oxygenated amorphous carbon electrochemical metallisation devices

The resistive switching behaviour of oxygenated amorphous carbon electrochemical metallisation devices is investigated. The effect of temperature on the microstructure and composition of the oxygenated carbon matrix is also investigated by annealing in situ in a transmission electron microscope. The devices exhibit controllable bipolar non-volatile and bi-directional volatile resistive switching behaviour that is dependent on the resistance state of the device and the polarity of the RESET voltage. The characteristics presented suggest suitability for incorporation into neuromorphic computing and memory storage technologies as memory cells, selector devices, or synaptic emulators.The resistive switching behaviour of oxygenated amorphous carbon electrochemical metallisation devices is investigated. The effect of temperature on the microstructure and composition of the oxygenated carbon matrix is also investigated by annealing in situ in a transmission electron microscope. The devices exhibit controllable bipolar non-volatile and bi-directional volatile resistive switching behaviour that is dependent on the resistance state of the device and the polarity of the RESET voltage. The characteristics presented suggest suitability for incorporation into neuromorphic computing and memory storage technologies as memory cells, selector devices, or synaptic emulators.

Synaptic plasticity and oscillation at zinc tin oxide/silver oxide interfaces

Short-term plasticity, long-term potentiation, and pulse interval dependent plasticity learning/memory functions have been observed in junctions between amorphous zinc-tin-oxide and silver-oxide. The same junctions exhibited current-controlled negative differential resistance and when connected in an appropriate circuit, they behaved as relaxation oscillators. These oscillators produced voltage pulses suitable for device programming. Transmission electron microscopy, energy dispersive X-ray spectroscopy, and electrical measurements suggest that the characteristics of these junctions arise from Ag+/O− electromigration across a highly resistive interface layer. With memory/learning functions and programming spikes provided in a single device structure, arrays of similar devices could be used to form transistor-free neuromorphic circuits.

Voltage Controlled Hot Carrier Injection Enables Ohmic Contacts Using Au Island Metal Films on Ge

We introduce a new approach to creating low-resistance metal-semiconductor ohmic contacts, illustrated using high conductivity Au island metal films (IMFs) on Ge, with hot carrier injection initiated at low applied voltage. The same metallization process simultaneously allows ohmic contact to n-Ge and p-Ge, because hot carriers circumvent the Schottky barrier formed at metal/n-Ge interfaces. A 2.5× improvement in contact resistivity is reported over previous techniques to achieve ohmic contact to both n- and p- semiconductor. Ohmic contacts at 4.2 K confirm nonequilibrium current transport. Self-assembled Au IMFs are strongly orientated to Ge by annealing near the Au/Ge eutectic temperature. Au IMF nanostructures form, provided the Au layer is below a critical thickness. We anticipate that optimized IMF contacts may have applicability to many material systems. Optimizing this new paradigm for metal-semiconductor contacts offers the prospect of improved nanoelectronic systems and the study of voltage controlled hot holes and electrons.