V. V. Zuev

Properties of tungsten oxide thin films formed by ion-plasma and laser deposition methods for MOSiC-based hydrogen sensors

Thin-film structures based on gas-sensitive tungsten oxide and catalytic platinum are fabricated by room-temperature deposition on a silicon carbide wafer using pulsed laser and ion-plasma methods. Oxide layer annealing in air to 600°C caused the formation of microstructured and nanostructured crystalline states depending on the deposition conditions. Structural differences affect the electrical parameters and the stability of characteristics. The maximum response to hydrogen is detected in the structure fabricated by depositing a low-energy laser-induced flow of tungsten atoms in oxygen. The voltage shift of the currentvoltage curves for 2% H2 in air at 350°C was 4.6 V at a current of ∼10 μA. The grown structures’ metastability caused a significant decrease in the shift after long-term cyclic testing. The most stable shifts of ∼2 V at positive bias on the Pt contact were detected for oxide films deposited by ion-plasma sputtering.

Influence of the energy parameters of the deposited laser-induced flow of platinum atoms on characteristics of a Pt/n-6H-SiC thin-film structure

The features of platinum film formation on n-6H-SiC substrates were experimentally studied for different methods of pulsed laser deposition, i.e., by both the conventional method in vacuum and in an inert gas atmosphere. In the latter case, a disk screen was placed on the plume expansion axis between the laser target and substrate to protect the latter from micrometer and submicrometer particles. A numerical model of such a process was developed, which makes it possible to predict the deposited film distribution over the substrate surface, energy and angular parameters of the atomic flow as laser plume properties, inert gas pressure, and screen position are varied. Simulation results were used to explain electrical properties of Pt/n-6H-SiC thin-film structures fabricated by different methods.

Effect of hydrogen on the electrical characteristics of structural elements of the Pt/WOx/6H-SiC

The formation conditions of the Pt/WOx/SiC thin-film system on a silicon carbide (6H-SiC) single crystal are optimized. The prepared system possesses stable characteristics and makes it possible to effectively record hydrogen at low concentrations in air at a temperature of ∼350°C, as well as to hold hydrogen in the WOx lattice at room temperature for a long time. The voltage shift of reverse portions of the current–voltage characteristics at a hydrogen concentration of ∼0.2% reach 6.5 V at a current of 0.4 µA because of large series resistance, which is defined by space-charge regions in WOx and SiC. Structural-phase investigations of the oxide layer are performed under various effect modes of the hydrogen-containing medium on the Pt/WOx/SiC system. A correlation in the variations of its electrical properties (ability to accumulate charge and vary the resistivity) and structural state of the oxide layer is revealed. An explanation for the variation in the current transport through the Pt/WOx/SiC and its contact regions (barrier layers) under the effect of hydrogen is proposed.

Functional micro- and nanostructured layers based on tungsten oxide for high-temperature hydrogen detectors on the “Pt-metal oxide-SiC” platform

The features of the structural formation and chemical composition of thin films of tungsten oxide grown under various conditions of pulsed laser deposition on a substrate of single crystal silicon carbide followed by annealing have been investigated. To obtain doped films of tungsten oxide deposited by the laser plume expanding from a tungsten target, an additional deposition of Pt, Ti, and Ta atoms was carried out. In some cases a thin film of a catalytically active metal (platinum) was deposited on the oxide layer formation. The structural state of the films was investigated by X-ray diffraction, electron and atomic force scanning microscopy, and Raman spectroscopy. The gas-sensing properties of the Pt-oxide-SiC structures were investigated by measuring the current-voltage characteristics at 300°C in air and air mixtures with hydrogen (2 vol %). It has been established that the deposited metal oxide films are significantly different in morphology and structure on micro- and nanoscales. This had a significant impact on the magnitude of the response to hydrogen and the mechanisms determining the gas-sensing properties: current passage in oxide and the magnitude of the potential barriers at the interfaces of the structure.

Ion implantation of platinum from pulsed laser plasma for fabrication of a hydrogen detector based on an n-6H-SiC crystal

A fairly simple method of surface doping of a SiC substrate with a catalytic metal (in particular, platinum) from a pulsed laser plasma is suggested. Doping is attained due to implantation of high-energy ions and ion mixing of the plasma-deposited film with the surface layer of the SiC substrate. The developed mathematical model makes it possible to conduct predictive calculations of the energy spectrum of implanted platinum ions on the basis of experimental data on the main physical characteristics of the pulsed laser plasma and technical parameters of the high-voltage system. The study of ion-implanted layers in the n-6H-SiC crystals have revealed features of structural and phase variations in the surface layers of the crystal at various doses of “hot” (600°C) ion implantation. On the basis of an analysis of experimental data, it is suggested that there are different solid-state reactions of platinum with silicon carbide in relation to the dose of implantation; these reactions cause a loss or acquisition of catalytic properties of nanosystems in the formation of platinum silicides or metal-like clusters, respectively. Optimization of the dose of ion implantation of platinum makes it possible to fabricate an on-chip n-6H-SiC structure, which varies the electrical parameters in a hydrogen-containing environment at elevated temperatures.

Electrical properties of thin-film structures formed by pulsed laser deposition of Au, Ag, Cu, Pd, Pt, W, Zr metals on n-6H-SiC crystal

Diode structures with ideality factors of 1.28–2.14 and potential barriers from 0.58 to 0.62 eV on the semiconductor side were formed by pulsed laser deposition of Au, Ag, Cu, Pd, Pt, W, and Zr metal films on n-6H-SiC crystal without epitaxial layer preparation. A high density of surface acceptor and donor states was formed at the metal-semiconductor interface during deposition of the laser-induced atomic flux, which violated the correlation between the potential barrier height and metal work function. The barrier heights determined from characteristic currents and capacitance measurements were in quite good agreement. For the used low-resistance semiconductor and contact elements, the sizes of majority carrier (electron) depletion regions were determined as 26–60 nm.

Comparative studies of monoclinic and orthorhombic WO3 films used for hydrogen sensor fabrication on SiC crystal

Amorphous WOx films were prepared on the SiC crystal by using two different methods, namely, reactive pulsed laser deposition (RPLD) and reactive deposition by ion sputtering (RDIS). After deposition, the WOx films were annealed in an air. The RISD film possessed a m-WO3 structure and consisted of closely packed microcrystals. Localized swelling of the films and micro-hills growth did not destroy dense crystal packing. RPLD film had layered β-WO3 structure with relatively smooth surface. Smoothness of the films were destroyed by localized swelling and the micro-openings formation was observed. Comparative study of m-WO3/SiC, Pt/m-WO3/SiC, and P-WO3/SiC samples shows that structural characteristics of the WO3 films strongly influence on the voltage/current response as well as on the rate of current growth during H2 detection at elevated temperatures.

Execution of energy efficient detection of hydrogen using Pt/WO x /SiC semiconductor structure

It has been shown that, at elevated temperatures (∼350°C), the most distinct response to H2 from the thin film structure Pt/WOx/SiC is achieved at registration of change in voltage for the reverse branch of a current-voltage characteristic. Comparative studies of electric current conduction through the structure and over its surface (with deposited Pt film) have led to the conclusion that a change in properties of the Pt/WOx and WOx/SiC interfaces under action of H2 mostly determines efficiency of response of the structure in the case of “transverse” measuring geometry. In the case of a 2% concentration of H2 in air the voltage shift for the reverse branch at a current of ∼10 μA reached 5 V against 2 V on the forward branch and “planar” geometry of measurements.

A high-temperature hydrogen detector with Pt/Pt+/n-6H-SiC structure

A combined method of implantation of Pt+ and subsequent deposition of a Pt film from pulsed-laser plasma was used to form gas-sensitive structures on n-6H-SiC monocrystals. High-temperature implantation ensures formation of a layer that improves adhesion of the film to the substrate and varies the current flow parameters due to the effect of atomic hydrogen, which appears during the reaction between H2 and catalytically active Pt. Breakage of the Pt film at elevated detector-operating temperatures (∼500°C) has no pronounced negative effect on the sensor properties of the Pt/Pt+/SiC structure. Similar effects in the traditionally used Pt/SiC diode structure lead to severe degradation of its characteristics.

Effect of hydrogen on the electrical properties of a Ag/WO x /W thin-film structure exhibiting resistive switching behavior

Thin-film structures based on gas-sensitive tungsten oxide Ag/WOx/W exhibiting a bipolar resistive switching effect were prepared. The current-voltage characteristics of the structures were studied in air with laboratory humidity and after hydrogen feeding into the chamber to a concentration of 2% in the air. The chemical state of the surface layers of the resulting structures was analyzed by X-ray photoelectron spectroscopy. The morphology and structural condition were studied by atomic force and scanning electron microscopy and micro-Raman spectroscopy. A severe reaction to hydrogen was found in the Ag/WOx/W structure subjected to additional heat treatment in the air at 200°C. The annealing of this structure gave rise to the formation of silver nanoparticles with an oxide coating on the surface of tungsten oxide. The formation of nano-particles resulted in a change in contact properties and offered hydrogen efficient access to the contact areas, as is evidenced by an increase in the flow of current in the low- and high-resistive states of this structure. A comparison of the reaction to hydrogen exhibited by WOx/W and Ag/WOx/W structures suggested that hydrogen had a significant effect on the mechanism of formation of conductive silver threads in the oxide layer and the chemical state of the interface owing to a change in the kinetics of the electrochemical oxidation-reduction processes in the contact areas of Ag/WOx. The results show the possibility of designing hydrogen sensors operating on new physical principles.