2O3 Structures Research Articles

Improved β-Ga2O3 Schottky Barrier Diodes via thermal oxidation of titanium insertion layer

In realm of β-Ga2O3 Schottky Barrier Diodes (SBDs), insertion layer appeared as an effective strategy for enhancing breakdown voltage. Presently, insertion layer techniques typically employed atomic layer deposition (ALD) to grow insulators with low electron affinity (e.g., Al2O3, HfO2). Although these insertion layers enhanced the reverse break-down characteristics, substantially degraded forward con-duction was also induced. In this study, a TiO2 thin layer by thermally oxidized Titanium metal was inserted to elevate the breakdown voltage and power figure of merit (PFOM) of β-Ga2O3 SBDs, while maintaining the on-resistance nearly unchanged. In comparison with the conventional met-al/semiconductor-SBD (MS-SBD), the thermally oxidized metal/insulator/semiconductor-SBD (MIS-SBD) exhibited 2.75 times elevation of breakdown voltage with only 11.6% reduction of forward current at +5V. The PFOM of the thermally oxidized MIS-SBD was enhanced by 7.6-fold compared to conventional MS-SBD, and by 15.7-fold com-pared to the counterpart ALD MIS-SBD. Analysis of the TiO2/β-Ga2O3 interface characteristics through X-ray photo-electron spectroscopy (XPS) and capacitance-voltage (C–V) measurements revealed that the thermally oxidized TiO2-SBD possesses smaller band offset and larger interface trap density (Dit) compared to the ALD counterpart, which contribute to the higher PFOM of the thermally oxidized TiO2-SBD. This novel TiO2/β-Ga2O3 structure exhibited superior electrical properties, providing potential solution for high performed SBD.

Perspectives of miniaturization of β-Ga2O3 devices with graphene electrodes

Micrometer-scale circular graphene/β-Ga2O3 structures are fabricated by transferring graphene layers to (2‾01) and (010) bulk gallium oxide substrates. Focused ion beam etching is used to downscale the area of the graphene/β-Ga2O3 junctions, and a nanoprobe-based approach is employed for their electrical characterization in the chamber of the scanning electron microscope. Using this approach, the impact of defects in graphene on the electrical characteristics of the junctions is suppressed. A significant difference between the structures fabricated on (2‾01) and (010) gallium oxide surfaces is further demonstrated. By numerical fitting of the measured data, several electric charge transport mechanisms are distinguished depending on the applied bias and orientation of the gallium oxide surface.

Optimized intermediate layer formation and electroless plating methods to obtain supported dense Pd surfaces.

Dense metallic membranes, especially Pd and Pd alloys, have been intensely investigated to provide an alternative and economical way to obtain H2 with ultrahigh purity. To overcome the high cost of Pd, composite membrane structures that comprise a thin layer of Pd are utilized. However, it is a challenge to obtain a thin, dense, and uniform Pd layer on the support materials. This study investigates the parametric analysis of γ-Al2O3 interlayer formation and the electroless Pd plating (Pd ELP) procedures on α-Al2O3 supports with the aim to achieve a thin, uniform Pd surface without annealing. Adjustments in PEG/PVA concentration, dipping time, and heat treatment enabled creating a thin γ-Al2O3 interlayer on α-Al2O3, minimizing pore size and density. Hydrazine concentration, heat treatment, and bath temperature were adjusted to optimize Pd ELP to achieve maximum yield from the plating bath and a dense, uniform surface without annealing. Pd/γ-Al2O3/α-Al2O3 structures were analyzed using scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis to observe the impact of varied parameters on surface structures. Optimized sample was compared to an annealed Pd/α-Al2O3 prepared in accordance with literature methods and a Pd/graphite/α-Al2O3 sample to validate the use of optimized ELP procedure and the γ-Al2O3 interlayer. Results show that a dense and uniform 13 μm Pd coating was achieved on a γ-Al2O3-coated α-Al2O3 support without annealing, using three fresh ELP baths. This was done using sequential hydrazine addition with a decreased concentration (1 M) into the ELP baths at 30 °C, and applying heat treatment at 120 °C between each fresh ELP bath.

Ultrahigh Photoresponsivity of W/Graphene/β-Ga2O3 Schottky Barrier Deep Ultraviolet Photodiodes.

The integration of graphene with semiconductor materials has been studied for developing advanced electronic and optoelectronic devices. Here, we propose ultrahigh photoresponsivity of β-Ga2O3 photodiodes with a graphene monolayer inserted in a W Schottky contact. After inserting the graphene monolayer, we found a reduction in the leakage current and ideality factor. The Schottky barrier height was also shown to be about 0.53 eV, which is close to an ideal value. This was attributed to a decrease in the interfacial state density and the strong suppression of metal Fermi-level pinning. Based on a W/graphene/β-Ga2O3 structure, the responsivity and external quantum efficiency reached 14.49 A/W and 7044%, respectively. These values were over 100 times greater than those of the W contact alone. The rise and delay times of the W/graphene/β-Ga2O3 Schottky barrier photodiodes significantly decreased to 139 and 200 ms, respectively, compared to those obtained without a graphene interlayer (2000 and 3000 ms). In addition, the W/graphene/β-Ga2O3 Schottky barrier photodiode was highly stable, even at 150 °C.

Ultraviolet Photodetector Based on a Beta-Gallium Oxide/Nickel Oxide/Beta-Gallium Oxide Heterojunction Structure.

In this paper, an n-p-n structure based on a β-Ga2O3/NiO/β-Ga2O3 junction was fabricated. The device based on the β-Ga2O3/NiO/β-Ga2O3 structure, as an ultraviolet (UV) photodetector, was compared with a p-n diode based on a NiO/β-Ga2O3 structure, where it showed rectification and 10 times greater responsivity and amplified the photocurrent. The reverse current increased in proportion to the 1.5 power of UV light intensity. The photocurrent amplification was related to the accumulation of holes in the NiO layer given by the heterobarrier for holes from the NiO layer to the β-Ga2O3 layer. Moreover, the device could respond to an optical pulse of less than a few microseconds.

CdSe@CdS quantum dot-sensitized Au/α-Fe2O3 structure for photoelectrochemical detection of circulating tumor cells.

Circulating tumor cells (CTCs) are the important biomarker for cancer diagnosis and individualized treatment. However, due to the extreme rarity of CTCs (only 1-10 CTCs are found in every milliliter of peripheral blood) high sensitivity and selectivity are urgently needed for CTC detection. Here, a sandwich PEC cytosensor for the ultrasensitive detection of CTCs was developed using thephotoactive material Au NP/-Fe2O3 and core-shell CdSe@CdS QD sensitizer. In the proposed protocol, theCdSe@CdS QD/Au NP/α-Fe2O3-sensitized structure with cascade band-edge levels could evidently promote the photoelectric conversion efficiency due to suitable light absorption and efficient electron-hole pair recombination inhibition. Additionally, a dendritic aptamer-DNA concatemer was constructed for highly efficient capture of MCF-7 cells carrying CdSe@CdS QDs, a sensitive material. The linear range of this proposed signal-on PEC sensing method was 300 cell mL-1 to 6 × 105 cell mL-1 with a detection limit of 3 cell mL-1, and it demonstrated an ultrasensitive response to CTCs. Furthermore, this PEC sensor enabled accurate detection of CTCs in serum samples. Hence, a promising strategy for CTC detection in clinical diagnosis was developed based on CdSe@CdS QD-sensitized Au NP/α-Fe2O3-based PEC cytosensor with dendritic aptamer-DNA concatemer.

Correlating the Structural Evolution of ZnO/Al2O3 to Spinel Zinc Aluminate with its Catalytic Performance in Propane Dehydrogenation

Zn-based Al2O3-suported materials have been proposed as inexpensive and environmentally friendly catalysts for the direct dehydrogenation of propane (PDH), however, our understanding of these catalysts’ structure and deactivation routes is still limited. Here, we correlate the catalytic activity for PDH of a series of Zn-based Al2O3 catalysts with their structure and structural evolution. To this end, three model catalysts are investigated. (i) ZnO/Al2O3 prepared by atomic layer deposition (ALD) of ZnO onto γ-Al2O3 followed by calcination at 700 °C, which yields a core-shell spinel zinc aluminate/γ-Al2O3 structure. (ii) Zinc aluminate spinel nanoparticles (ZnxAlyO4 NPs) prepared via a hydrothermal method. (iii) A reference core-shell ZnO/SiO2 catalyst prepared by ALD of ZnO on SiO2. The catalysts are characterized in detail by synchrotron X-ray powder diffraction (XRD), Zn K-edge X-ray absorption spectroscopy (XAS), and 27Al solid state nuclear magnetic resonance (ssNMR). These experiments allowed us to identify tetrahedral Zn sites in close proximity to Al sites of a zinc aluminate spinel phase (ZnIV–O–AlIV/VI linkages) as notably more active and selective in PDH relative to the supported ZnO wurtzite phase (ZnIV–O– ZnIV linkages) in ZnO/SiO2. The best performing catalyst, 50ZnO/Al2O3 gives 77% selectivity to propene (gaseous products based) at 9 mmol C3H6 gcat−1 h−1 space time yield (STY) after 3 min of reaction at 600 °C. On the other hand, the core-shell ZnO/Al2O3 catalyst shows an irreversible loss of activity over repeated PDH and air-regeneration cycles, explained by Zn depletion on the surface due to its diffusion into subsurface layers or the bulk. ZnxAlyO4 NPs gave a comparable initial selectivity and catalytic activity as 50ZnO/Al2O3. With time on stream, ZnxAlyO4 NPs deactivate due to the formation of coke at the catalyst surface, yet the extend of coke deposition is lower than for the ZnO/Al2O3 catalysts, and the activity of ZnxAlyO4 NPs can be regenerated almost fully using calcination in air.

OPTICAL CHARACTERISTICS OF Cr2O3/Al2O3 COMPOSITE STRUCTURE

The absorption, luminescence excitation, and photoluminescence (PL) spectra were studied for the nanocomposite chromium oxide (Cr2O3)/aluminum oxide (Al2O3) structure in the crystalline state produced by annealing pre-impregnated anodic alumina chromium anhydride at high temperatures. A PL band with maximum intensity at 615 nm, which is not characteristic of bulk materials, was found in a Cr2O3/γ-Al2O3, structure annealed at Тa = 900°С. This band was attributed to hexavalent chromium (Cr6+) ions formed as a result of trivalent chromium (Cr3+)-Cr6+ redox processes on the surface of Cr2O3 crystallites at annealing temperatures of 400-900°С. The emergence of an additional excitation band of Cr6+ ions at 520 nm was explained, with good agreement with the experimental results. It is shown that the photoluminescent properties of the nanocomposite Cr2O3/γ-Al2O3 structure are different for different ambient temperatures: at liquid nitrogen temperature (Т = 77.4 K), two broad bands were observed at 500 and 670 nm, while there was no PL at room temperature. For the nanocomposite Cr2O3/α-Al2O3 structure annealed at 1300°C, the photoluminescent properties were caused by radiative transitions in the structure of the energy levels of the Al2O3 defects of the anion-vacancy type and by impurity centers. In addition, the R-line half-width of Cr3+ for the nanocomposite Cr2O3/ Al2O3 structure was ~ 250 cm-1, which was larger than ~ 100 cm-1 for the original anodic alumina. This may be due to the greater defectiveness of the crystal lattices. Also, for the Cr2O3/Al2O3 structures, a significant increase in absorption in the ultraviolet range was found compared to the original Al2O3.

A flexible and transparent β-Ga2O3 solar-blind ultraviolet photodetector on mica

In the present work, we report a flexible transparent β-Ga2O3 solar-blind ultraviolet (UV) photodetector (PD) fabricated on a mica substrate. A laminated a-Ga2O3/Ga/a-Ga2O3 structure is thermally annealed at 1050 °C, forming a β-Ga2O3 film incorporating Ga nanospheres. A PD based on this nanocomposite film has a spectrum response peak at 250 nm, an extremely low dark current of 0.6 pA at a 10 V bias, a very high Ilight/Idark ratio of 3 × 106, and a fast recovery speed of less than 50 ms. Robust flexibility is demonstrated by bending tests and 10 000 cycles of a fatigue test with a radius as small as 8 mm. Compared to a room-temperature-fabricated flexible amorphous Ga2O3 (a-Ga2O3) PD, the flexible β-Ga2O3 PD on mica exhibits improved solar-blind UV photoresponse characteristics. The insertion of a gallium interlayer and treatment by high-temperature post annealing are proposed to contribute to a better stoichiometry and lattice order of the β-Ga2O3 thin film, as evidenced by the pronounced Raman peaks related to the GaI(OI)2 and GaIO4 vibration modes in β-phase Ga2O3. Our research is believed to provide a simple and practical route to achieving flexible transparent β-Ga2O3 solar-blind UV PDs, as well as other devices such as flexible transparent phototransistors and power rectifiers.

Growth of double-barrier β-(AlGa)2O3/Ga2O3 structure and heavily Sn-doped Ga2O3 layers using molecular-beam epitaxy

I report on the growth of a double-barrier β-(Al0.15Ga0.85)2O3/Ga2O3/(Al0.15Ga0.85)2O3 heterostructure and heavily Sn-doped β-Ga2O3 layers toward the application of resonant-tunneling diodes. The Ga2O3 and (AlGa)2O3 layers were grown on a β-Ga2O3 (010) substrate by plasma-assisted molecular-beam epitaxy. The heavily Sn-doped β-Ga2O3 layer had a layer resistivity of 2 × 10−3 Ω cm and a specific contact resistivity of 9 × 10−6 Ω cm−2. The diode with the double-barrier (AlGa)2O3/Ga2O3/(AlGa)2O3 structure sandwiched between the heavily Sn-doped Ga2O3 layers exhibited negative differential resistance with a peak-to-valley current ratio of 2 at room temperature.

First-principle investigation of monoclinic (AlxInyGa1−x−y)2O3 quaternary alloys

First-principle density functional theory calculations were performed to explore electronic and structural properties of β-(AlxInyGa1−x−y)2O3 quaternary alloys with both Al-content (x) and In-content (y) ranging from 0% to 18.75%. The β-(AlxInyGa1−x−y)2O3 quaternary alloys exhibit indirect band gap property with the bandgap energy varying from 4.432 to 5.171 eV. Electron effective masses are also presented for β-(AlxInyGa1−x−y)2O3 quaternary alloys, showing a general reduction with In-content increases but a general increment with Al-content increases in the material. Further analysis indicates the possibility of achieving lattice-matched or near-lattice-matched β-(AlxInyGa1−x−y)2O3/β-Ga2O3 structures system, which is critical for high performance field effect transistor and deep ultraviolet photodetector applications. Our work shows that the β-(AlInGa)2O3 alloys with proper tuning of Al- and In-content have strong potential to be used as part of the epitaxial layers for β-Ga2O3-based material system.

Separation of arsenic from arsenic—antimony-bearing dust through selective oxidation—sulfidation roasting with CuS

Abstract The feasibility of a new method for separating arsenic from arsenic—antimony-bearing dusts using CuS was put forward, in which Sb was transformed into Sb2O4 and Sb2S3 that stayed in the roasted calcine while As was volatilized in the form of As4O6. The factors such as roasting temperature and CuS addition amount were studied using XRD, EPMA and SEM—EDS. CuS has an active effect on the separation of arsenic due to the destruction of (Sb,As)2O3 structures in the original dust and the simultaneous release of As in the form of As4O6. At a roasting temperature of 400 °C and CuS addition amount of 130%, the volatilization rates of arsenic and antimony reach 97.80 wt.% and 8.29 wt.%, respectively. Further, the high As volatile matter can be used to prepare ferric arsenate after it is oxidized, with this treatment rendering the vapor harmlessness.

Magnetic properties of Co film in Pt/Co/Cr2O3/Pt structure

Magnetic properties of Co film in Pt/Co/α-Cr2O3/Pt/α-Al2O3 structure were investigated. Co layer thickness tCo dependence of perpendicular magnetic anisotropy energy density K reveals that the bulk magnetic anisotropy plays an important role in the system in addition to the interfacial anisotropy. Damping constant α monotonically increases with the decrease of tCo but not proportionally to 1/tCo. Both K and α increase with the increase of Pt layer thickness tPt from 3 nm to 5 nm and keeps almost constant in the tPt range between 5 nm to 20 nm. These results are of importance to understand the magnetization switching behavior driven by the magneto-electric (ME) effect as well as to design the spintronics device using the ME effect.

An investigation on synthesis and characterization of superhard Cr-Zr-O coatings

Coatings with superior hardness, high temperature thermal stability, and oxidation resistance are required to protect cutting tools in machining applications. Hence, the synthesis of such coatings was investigated in a ternary Cr–Zr–O system by reactive radio-frequency magnetron sputtering technique. For this purpose, the Cr–Zr–O coatings with chemical compositions up to 9 at. % Zr were deposited on Si (100) substrates. The coatings were then characterized by energy dispersive spectroscopy, X-ray diffraction, Raman spectroscopy, and Nanoindentation and a correlation between their chemical composition, crystal structure, phase composition, and mechanical properties was established. The structural stability of the coatings was also evaluated after annealing treatments. The results indicated that adding zirconium to Cr2O3 coatings shifted the onset of crystallization for the coatings to higher temperatures. The hardness measurements showed that Cr-Zr-O coatings have the potential to reach a hardness value over 40 GPa. Based on the XRD and Raman spectroscopy analysis, it was assumed that grain size refinement and solid solution hardening were the most responsible mechanisms for the enhanced hardness. Annealing treatments at elevated temperatures showed that the thermal stability of the superhard Cr-Zr-O coatings was higher than pure chromium oxide coatings. Moreover, the metastable (Cr, Zr)2O3 structure in the superhard coatings decomposed to Cr2O3 and ZrO2 phases at 1000 °C, indicating that Zr has <2 at. % solubility in Cr2O3 in an equilibrium condition.

Acceptor doping of β-Ga2O3 by Mg and N ion implantations

Deep acceptor doping of β-Ga2O3 with Mg and N was demonstrated by implantation of the impurity ions into n-type bulk substrates. Systematic physical and electrical characterizations were performed to demonstrate recovery of the implantation-damaged crystals and electrical activation of the dopant atoms by thermal annealing at 1000–1200 °C in an N2 atmosphere. N was found to exhibit much lower thermal diffusivity than Mg, thus enabling the use of higher annealing temperatures to maximize N activation efficiency without significantly altering the impurity profile. Consequently, an n-Ga2O3/Ga2O3:N/n-Ga2O3 structure was capable of sustaining a much larger voltage across its end terminals than its Mg-doped counterpart. The development of an ion implantation technology for acceptor doping of β-Ga2O3 creates unique opportunities for designing and engineering a variety of high-voltage β-Ga2O3 devices.

Microstructure and phase evolution of gradually-structured arc evaporated Al0.25Cr0.75-based oxide coatings

The structural evolution from the (inter)metallic to the oxide-stoichiometric regime within oxygen-gradient Al0.25Cr0.75-based coatings was investigated by transmission electron microscopy (TEM). Grain refinement of the (inter)metallic Cr(Al) structure — induced by the addition of oxygen to the growth process — is less significant than for the higher Al-containing Al0.50Cr0.50-based coatings. Our detailed TEM studies clearly show that between 110 and 120 sccm O2 per active Al0.25Cr0.75-source (corresponding to 0.7–0.9 Pa), the coating structure changes from partially oxidised — where (inter)metallic Cr(Al) particles are encapsulated by amorphous-like Al-rich oxides — to a columnar, corundum-type sesquioxide structure. Contrary to the higher Al-containing Al0.50Cr0.50-based coatings, this transformation takes place without the formation of metastable cubic (Al1-xCrx)2O3 structures.

Z-Scheme in a Co3(PO4)2/α-Fe2O3 photocatalysis system for overall water splitting under visible light

A Z-scheme Co3(PO4)2/α-Fe2O3 structure was designed as an efficient photocatalyst for overall water splitting under visible light.

Nickel-aluminum diffusion: A study of evolution of microstructure and phase

Microstructural and phase evolution of an aluminum deposit on nickel, after heat treatment at 883K, is studied by means of various microscopy techniques, i.e. energy dispersive X-ray spectroscopy, backscattered electron imaging, electron backscatter diffraction, ion channeling contrast imaging and scanning transmission electron microscopy. AlNi3 crystallites are observed on the aluminum grain boundaries after only 3min of heat treatment indicating that nickel and nickel rich phases are the initially diffusing and forming species. Heat treatment for 120min or longer results in the formation of Al3Ni2 and a porous Al3Ni2/γ-Al2O3 structure at the surface. The Al3Ni2 layer is composed of two different grain morphologies, indicating the position of a Kirkendall plane, and hence, there is a high diffusion rate of aluminum in this phase.

Synthesis of GaN network by nitridation of hexagonal ε-Ga2O3 film

GaN network structure was synthesized on a hexagonal e-Ga2O3 film by performing thermal nitridation under ammonia atmosphere in III-nitride MOCVD system. High-resolution X-ray diffraction revealed that the GaN had a (0002) preferred orientation, and no other polymorphs of GaN were detected. X-ray photoelectron spectroscopy results confirmed the formation of Ga–N bonds as well as other element status induced from nitridation process. According to field-emission scanning electron microscopy and atomic force microscopy images, the surface of GaN layer shows network morphology, which was caused by etching from hydrogen gas along GaN grain boundary. Our results indicated the potential applications of nitrided GaN/e-Ga2O3 structure in fabricating novel electronic and optoelectronic devices.

Structural stability and electronic properties in Al2O3–Cr2O3 mixed crystal

Structural stability and electronic properties of a corundum (Al1−xCrx)2O3 structure over the whole Cr composition are investigated by means of the full-potential linearized augmented plane-wave method within the local-spin-density approximation (LSDA)+U. Calculated results predict that ordered and disordered states are energetically less favorable with respect to a phase separation, and that interatomic distances do not change over the whole Cr composition, where the Cr–O distance is always larger than the Al–O distance. An antibonding Cr 3d–O 2p hybridization leads to the phase-separated-type phase stability and the structural property.