Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

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In situ growth of nanoparticles through control of non-stoichiometry

Cation segregation on perovskite oxide surfaces affects vastly the oxygen reduction activity and stability of solid oxide fuel cell (SOFC) cathodes. A unified theory that explains the physical origins of this phenomenon is therefore needed for designing cathode materials with optimal surface chemistry. We quantitatively assessed the elastic and electrostatic interactions of the dopant with the surrounding lattice as the key driving forces for segregation on model perovskite compounds, LnMnO3 (host cation Ln = La, Sm). Our approach combines surface chemical analysis with X-ray photoelectron and Auger electron spectroscopy on model dense thin films and computational analysis with density functional theory (DFT) calculations and analytical models. Elastic energy differences were systematically induced in the system by varying the radius of the selected dopants (Ca, Sr, Ba) with respect to the host cations (La, Sm) while retaining the same charge state. Electrostatic energy differences were introduced by varying the distribution of charged oxygen and cation vacancies in our models. Varying the oxygen chemical potential in our experiments induced changes in both the elastic energy and electrostatic interactions. Our results quantitatively demonstrate that the mechanism of dopant segregation on perovskite oxides includes both the elastic and electrostatic energy contributions. A smaller size mismatch between the host and dopant cations and a chemically expanded lattice were found to reduce the segregation level of the dopant and to enable more stable cathode surfaces. Ca-doped LaMnO3 was found to have the most stable surface composition with the least cation segregation among the compositions surveyed. The diffusion kinetics of the larger dopants, Ba and Sr, was found to be slower and can kinetically trap the segregation at reduced temperatures despite the larger elastic energy driving force. Lastly, scanning probe image contrast showed that the surface chemical heterogeneities made of dopant oxides upon segregation were electronically insulating. The consistency between the results obtained from experiments, DFT calculations, and analytical theory in this work provides a predictive capability to tailor the cathode surface compositions for high-performance SOFCs.

Cation Size Mismatch and Charge Interactions Drive Dopant Segregation at the Surfaces of Manganite Perovskites

In the present brief review modern views on the reasons of time instability of gas sensors parameters, as well as approaches, which could be used for the improvement of this important sensor's parameter, are summarized. In particular, the influence of factors such as structure transformation, phase transformation, poisoning, degradation of contacts and heaters, bulk diffusion, errors in design, change of humidity, fluctuations of temperature in the surrounding atmosphere, and interference effect was analyzed. It was shown that while designing devices such as solid-state gas sensors, there are no secondary issues or tasks—all are important. Sensors work in extreme temperatures in the presence of active gases, and therefore every element of the sensor could be responsible for its long-term stability. The conclusions, regarding the efficiency of approaches such as optimization of technological processes and optimization of exploitation processes used for improvement of stability of conductometric metal oxide gas sensors, were made as well.

Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement (short survey)

While SOFC perovskite oxide cathodes have been the subject of numerous studies, the critical factors governing their kinetic behavior have remained poorly understood. This has been due to a number of factors including the morphological complexity of the electrode and the electrode- electrolyte interface as well as the evolution of the surface chemistry with varying operating conditions. In this work, the surface chemical composition of dense thin film SrTi1−xFexO3-δelectrodes, with considerably simplified and well-defined electrode geometry, was investigated by means of XPS, focusing on surface cation segregation. An appreciable degree of Sr-excess was found at the surface of STF specimens over the wide composition range studied. The detailed nature of the Sr-excess is discussed by means of depth and take-off angle dependent XPS spectra, in combination with chemical and thermal treatments. Furthermore, the degree of surface segregation was successfully controlled by etching the films, and/or preparing intentionally Sr deficient films. Electrochemical Impedance Spectroscopy studies, under circumstances where surface chemistry was controlled, were used to examine and characterize the blocking effect of Sr segregation on the surface oxygen exchange rate.

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Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes

H2-induced changes of electrical conductivity in polycrystalline, undoped β-Ga2O3 thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law σ□ ∼ p
1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga2O3 film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10−4 ML for pH2=0.05 bar and T=600° C. A possible explanation of the σ□ ∼ p
1/3 power law is provided.

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

The influence of corrosive gases (H2, CO, NO, Cl2, SO2) on the composition of sputtered polycrystalline StTiO3, CeO2 and Ga2O3 thin films was studied. Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) were applied for studying the depth profiles of atomic composition of the thin films, treated for several hundred hours at 800 °C in diluted (20 ppm, 5%) corrosive gases. SrTiO3 was strongly attacked by Cl2 and SO2. During the Cl2 treatment the layer thickness was reduced and the structure altered near the grain boundaries. SO2 treatment caused a new surface phase formation (probably SrO). the Ga2O3 layer had a fast reaction with Cl2 giving rise to volatile Ga products but the other corrosive gases had no deteriorating effect on the layer. CeO2 proved to be the most resistant to all the gases mentioned above.

Auger and SIMS study of segregation and corrosion behaviour of some semiconducting oxide gas-sensor materials

The electrical conductivity of sputtered 2 μm thick films of the oxide semiconductor α-Ga2O3 was studied in function of the partial pressure of CO or H2 above the solid and also in the presence of CO+O2, H2+O2 and CO+H2 mixtures. The concentrations of the examined gases varied in the range 0.01–1 vol.% in nitrogen carrier gas, while the experiments were carried out at 823, 873 and 923 K. Under the influence of H2 and CO the conductivity of the samples increased with the partial pressures of the gases according to the relations: lg σ∼ (1/n)lgp co and lg σ∼(1/m)lgp H 2, where n and m are greater than unity. As a second step the effect of coadsorption of CO+O2 and H2+O2 on the conductivity was investigated. The β-Ga2O3 sensors, connected electrically by Pt electrodes, manifested a moderate catalytic activity. Due to this property the conductivity of the solid changed steeply in the vicinity of the stoichiometric composition of the gas mixtures. In the third step the coadsorption of CO+H2 was studied. In function of the CO partial pressure the conductivity of the solid passed through a minimum. The shape of the experimental curves was attributed to the formation of certain organic intermediate surface compounds.

Effect of coadsorption of reducing gases on the conductivity of β-Ga2O3 thin films in the presence of O2

The operation of high temperature oxide semiconductor gas sensors is strongly influenced by their surface properties and the processes taking place in the topmost atomic layers such as adsorption, coadsorption, gas—gas interactions and the catalytic processes on them. From our experimental and theoretical results a resistance limit was calculated; owing to the measured resistance it is possible to estimate when the surface phenomena dominate. According to our results the effect of water on the resistance is the result of at least two processes. The adsorption of molecular water is fast and results in a donor effect while the parallel formation of surface OH groups is relatively slow and results in an acceptor effect. When the temperature is increased the OH formation becomes the dominant process and at a definite temperature its influence passes a maximum. Our results prove that on β-Ga2O3 and n-SrTiO3 the adsorption and coadsorption processes are strongly influenced by the presence of water. For both substances the effects are very similar. We suggest that similar mechanisms take place on every oxide semiconductor.

Coadsorption and cross sensitivity on high temperature semiconducting metal oxides: water effect on the coadsorption process☆

The surface-segregation properties of perovskite titanate-based solid-state sensor materials are investigated by Auger and secondary ion mass spectroscopy (SIMS) methods. The experimental findings are interpreted in terms of a thermodynamic model, which considers the so-called ‘generalized surface free enthalpies’ and the effect of the ambient gas atmosphere as the major driving forces of segregation. As well as the qualitative comparison of the experimental results and the model predictions, a semi-empirical calculation is also performed to estimate the actual values of these main driving forces. It is found that, although both effects have considerable impact on the surface composition, the influence of the ‘generalized surface free enthalpies’ is predominant.

J. Giber

Papers

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

Auger and SIMS study of segregation and corrosion behaviour of some semiconducting oxide gas-sensor materials

Effect of coadsorption of reducing gases on the conductivity of β-Ga2O3 thin films in the presence of O2

Coadsorption and cross sensitivity on high temperature semiconducting metal oxides: water effect on the coadsorption process☆

Segregation driving forces in perovskite titanates