Monoclinic Films Research Articles

In situ carbothermal reduction of oxygen vacancies in monoclinic WO3-x film for dual-band electrochromic windows

Dual-band electrochromic windows have emerged as a promising approach to reduce building energy consumption. However, developing facile and effective method for preparing single-component dual-band electrochromic material remains a challenge. Herein, an in situ carbothermal reduction strategy was utilized to fabricate monoclinic oxygen-deficient tungsten oxide (WO3-x) for high-performance dual-band electrochromism. Polyvinylpyrrolidone (PVP–K90) underwent transformation into amorphous carbon at high temperature, serving as the reducing agent embedded in the tungsten oxide film, and in situ inducing the formation of oxygen vacancies during annealing treatment. Lastly, the amorphous carbon was gasified without weakening the transmittance of WO3-x film. The content of oxygen vacancies in WO3-x could be modulated by controlling the added amount of PVP-K90. The intrinsic generation of oxygen vacancies not only obtained a strong local surface plasmon resonance (LSPR) absorption in near-infrared (NIR) range, but also improved lithium ion (Li+) diffusion in WO3-x film. The WO3-x electrochromic film exhibited excellent dual-band electrochromic performance in optical modulation, switching time, coloring efficiency, bistability, and cycle stability. Furthermore, the WO3-x film was applied to electrochromic window to verify its available regulation in solar illumination and heat, indicating that the designed modification strategy has potential for developing a single-component dual-band electrochromic window with outstanding performance.

A Highly Transparent β-Ga2O3 Thin Film-Based Photodetector for Solar-Blind Imaging

Ultra-wide bandgap Ga2O3-based optoelectronic devices have attracted considerable attention owing to their special significance in military and commercial applications. Using RF magnetron sputtering and post-annealing, monoclinic Ga2O3 films of various thicknesses were created on a c-plane sapphire substrate (0001). The structural and optical properties of β-Ga2O3 films were then investigated. The results show that all β-Ga2O3 films have a single preferred orientation (2(_)01) and an average transmittance of more than 96% in the visible wavelength range (380–780 nm). Among them, the sample with a 90-minute sputtering time has the best crystal quality. This sample was subsequently used to construct a metal-semiconductor-metal (MSM), solar-blind, ultraviolet photodetector. The resulting photodetector not only exhibits excellent stability and sunblind characteristics but also has an ultra-high responsivity (46.3 A/W) and superb detectivity (1.83 × 1013 Jones). Finally, the application potential of the device in solar-blind ultraviolet imaging was verified.

Al Incorporation up to 99% in Metalorganic Chemical Vapor Deposition‐Grown Monoclinic (AlxGa1–x)2O3 Films Using Trimethylgallium

Growths of monoclinic (AlxGa1−x)2O3 thin films up to 99% Al contents are demonstrated via metalorganic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa) as the Ga precursor. The utilization of TMGa, rather than triethylgallium, enables a significant improvement of the growth rates (>2.5 μm h−1) of β‐(AlxGa1−x)2O3 thin films on (010), (100), and (01) β‐Ga2O3 substrates. By systematically tuning the precursor molar flow rates, growth of coherently strained phase pure β‐(AlxGa1−x)2O3 films is demonstrated by comprehensive material characterizations via high‐resolution X‐ray diffraction (XRD) and atomic‐resolution scanning transmission electron microscopy (STEM) imaging. Monoclinic (AlxGa1−x)2O3 films with Al contents up to 99, 29, and 16% are achieved on (100), (010), and (01) β‐Ga2O3 substrates, respectively. Beyond 29% of Al incorporation, the (010) (AlxGa1−x)2O3 films exhibit β‐ to γ‐phase segregation. β‐(AlxGa1−x)2O3 films grown on (01) β‐Ga2O3 show local segregation of Al along (100) plane. Record‐high Al incorporations up to 99% in monoclinic (AlxGa1−x)2O3 grown on (100) Ga2O3 are confirmed from XRD, STEM, electron nanodiffraction, and X‐ray photoelectron spectroscopy measurements. These results indicate great promises of MOCVD development of β‐(AlxGa1−x)2O3 films and heterostructures with high Al content and growth rates using TMGa for next‐generation high‐power and high‐frequency electronic devices.

PbS Quantum Dots-Decorated BiVO4 Photoanodes for Highly Efficient Photoelectrochemical Hydrogen Production.

While metal oxides such as TiO2, Fe2O3, WO3, and BiVO4 have been previously studied for their potential as photoanodes in photoelectrochemical (PEC) hydrogen production, their relatively wide band-gap limits their photocurrent, making them unsuitable for the efficient utilization of incident visible light. To overcome this limitation, we propose a new approach for highly efficient PEC hydrogen production based on a novel photoanode composed of BiVO4/PbS quantum dots (QDs). Crystallized monoclinic BiVO4 films were prepared via a typical electrodeposition process, followed by the deposition of PbS QDs using a successive ionic layer adsorption and reaction (SILAR) method to form a p-n heterojunction. This is the first time that narrow band-gap QDs were applied to sensitize a BiVO4 photoelectrode. The PbS QDs were uniformly coated on the surface of nanoporous BiVO4, and their optical band-gap was reduced by increasing the number of SILAR cycles. However, this did not affect the crystal structure and optical properties of the BiVO4. By decorating the surface of BiVO4 with PbS QDs, the photocurrent was increased from 2.92 to 4.88 mA/cm2 (at 1.23 VRHE) for PEC hydrogen production, resulting from the enhanced light-harvesting capability arising from the narrow band-gap of the PbS QDs. Moreover, the introduction of a ZnS overlayer on the BiVO4/PbS QDs further improved the photocurrent to 5.19 mA/cm2, attributed to the reduction in interfacial charge recombination.

Band alignment and photocatalytic activity of monoclinic BiVO4 (0 1 0) and (1 0 0) films with SrTiO3

The crystal plane with the highest electron and hole mobility in a material is very important to physical and chemical properties. Herein, the BiVO4 (BVO) thin films exposed (010) or (100) planes which pointed out as the facets with the highest electron and hole mobility, and formed the interface with SrTiO3 (STO) were fabricated by magnetron sputtering. The band edge offsets calculation results of oriented BVO and STO show that the conduction band and valence band positions of the BVO (010) plane are 0.23 eV higher than those of (100). The average generation rates of H2O2 in BVO(100) are 5.3 × 10−2 and 3.8 × 10−1 µmol h−1 under 440 nm monochromatic light and full arc Xe lamp irradiation, which are 1.8 and 1.6 times higher than those in BVO(010), respectively.

Exciton Luminescence and Optical Properties of Nanocrystalline Cubic Y2O3 Films Prepared by Reactive Magnetron Sputtering.

This paper presents a comprehensive study of the energy structure, optical characteristics, and spectral-kinetic parameters of elementary excitations in a high-purity nanocrystalline cubic Y2O3 film synthesized by reactive magnetron sputtering. The optical transparency gaps for direct and indirect interband transitions were determined and discussed. The dispersion of the refractive index was established based on the analysis of interference effects. It was found that the refractive index of the Y2O3 film synthesized in this study is higher in order of magnitude than that of the films obtained with the help of other technologies. The intrinsic emission of Y2O3 film has been discussed and associated with the triplet–singlet radiative relaxation of self-trapped and bound excitons. We also studied the temperature behavior of the exciton luminescence of Y2O3 for the first time and determined thermal activation barriers. The optical energy and kinetic parameters of cubic Y2O3 films were analyzed in comparison with those of the monoclinic films of yttrium oxide. The main difference between the optical properties of cubic and monoclinic Y2O3 films was established, which allowed for a supposition of their application prospects.

Controlling the crystalline orientation and textual morphologies of the VO2 film and the effect on insulator–metal transition properties

Monoclinic VO2 films were prepared over quartz substrates through thermal oxidation of sputtered vanadium films under continuous O2 flow in a vacuum. It was found that an additional vacuum pre-heating before the thermal oxidation could have a great effect on film crystalline orientation and textual morphologies. The film exhibits a preferable growth along [011] direction and consists of uniform small inter-connected nanoparticles, while the untreated film shows (200) orientation and is composed of large irregular nanoparticles that connect via disorder boundaries. The (011) orientated film shows slightly higher full solar and IR light modulations. The electric resistance results also show that the metal–insulator transition (MIT) of the (011) orientated film presents a larger amplitude, higher sharpness, and narrower hysteresis as compared to the (200) orientated film. The difference in textual structures of the orientated films is the main physical reason that affects the MIT of m-VO2 films.

Phase transformation of sputtered hafnium oxide by post annealing treatment and its effect on the amorphous Si-In-Zn-O thin film transistor

The effect of post annealing treatment on the phase transformation and the electrical properties of the sputtered hafnium oxide (HfO2) is investigated. As-deposited HfO2 films were annealed at 200 and 400 ∘C in ambient conditions. Film annealed at 200 ∘C is amorphous, but further annealing at 400 ∘C crystallizes it into the monoclinic structure. Amorphous films exhibit a higher bandgap (≥ 5.8 eV) than the monoclinic film (≈ 5.65 eV). The electrical characteristics of the Metal-Insulator-Metal devices reveal a nonlinear capacitance with respect to the voltage. The nonlinearity is predominant in the as-deposited film due to the oxygen vacancy related defects formed during the deposition. However, excess Hf-O bond formation occurs with annealing at higher temperatures, reducing the capacitance-voltage curve’s nonlinearity. The dielectric constant of the as-deposited, 200, and 400 ∘C annealed films are estimated as 22.35, 22.64, and 20.57, respectively. The increment in the dielectric constant at 200 ∘C is due to its amorphous phase and reduced defects. Amorphous Si-In-Zn-O thin film transistors fabricated using the as-deposited and annealed hafnium oxide as gate insulator show almost similar threshold voltage of ≈ −1 V. The transistor with hafnium oxide annealed at 200 ∘C shows the highest on current (1.04 × 10−6 A) followed by as-deposited (6.82 × 10−7 A) and 400 ∘C annealed device (5.86 × 10−7 A), respectively. Moreover, the interface state density increases monotonically from 3.98 × 1012 to 1.11 × 1013 cm−2 for as-deposited and 400 ∘C, respectively. The increment in the interface state density is due to the grain boundary formation due to the crystallization of HfO2.

Effect of epitaxial growth rate on morphological, structural and optical properties of β-Ga2O3 films prepared by MOCVD

This study investigated the effects of growth rate on the properties of monoclinic β-Ga2O3 films grown on SrTiO3 (100) substrates by Metal-organic Chemical Vapor Deposition. The β-Ga2O3 films deposited at the low rate exhibited monocrystalline structure, for which the structural analyses identified the epitaxial relationship of β-Ga2O3 (100) // SrTiO3 (100) with β-Ga2O3 〈001〉 // SrTiO3 〈011〉. In the case of the same film thickness, as the growth rate increased from 2.1 to 10.1 nm•min−1, the surface root-mean-square roughness of the β-Ga2O3 film increased from 6.96 to 8.41 nm, and the crystalline quality decreased obviously. The prepared β-Ga2O3 films exhibited a band gap of about 4.9 eV, and the refractive index in 450–800 nm wavelength range was 2.25–2.02. Moreover, the transmittance of all samples in the visible region was around 75%.

Observation of the crystalline orientation dependence of the semiconductor–metal transition for thermal oxidation induced VO2 films over amorphous quartz glasses

Polycrystalline VO2 films were obtained through a vacuum annealing of sputtered V-rich films over quartz substrates and were characterized with x-ray diffraction, field-emission scanning electron microscopy, and x-ray photoelectron spectroscopy, respectively. The semiconductor–metal transition (SMT) was studied with the temperature-variable electric resistances. It was seen that the VO2 film crystalline orientation changes with the O2 partial pressure during the vacuum annealing. We observed a relation between the thermal hysteresis of the SMT and the crystalline orientation of the monoclinic VO2 films. The (011) oriented monoclinic film presents a narrower thermal hysteresis and a larger transition amplitude as compared to the (200) orientated films. In addition, a transition shoulder appears in the thermal hysteresis of the SMT for the (200) oriented VO2 films during the cooling process and becomes absent for the (011) orientated VO2 films.

Influence of oxygen partial pressure on properties of monoclinic Ga2O3 deposited on sapphire substrates

Thin monoclinic Ga2O3 films were deposited on c-plane sapphire substrates by low pressure chemical vapor deposition. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen gas (O2) as precursors. The effect of oxygen volume percentage on the growth rate of thin films was observed at two growth temperatures. Within the investigated growth window, a maximum growth rate of ∼2.9 μm/h was obtained for an oxygen volume percentage of 4.8% with a growth temperature at 800 °C. The film growth rate decreased as growth temperature increased when other growth parameters were kept the same. X-ray diffraction indicates that all films have the β-Ga2O3 structure with (−201) orientation, and those deposited with higher oxygen partial pressure are thicker and have improved crystalline quality. Polarized micro-Raman scattering is consistent with small grains of (−201) β-Ga2O3 having random in-plane orientations. The large variation of the relative intensities of overlapping emission bands contributing to the broad luminescence emission extending between 1.5 and 4.5 eV (∼825 and 275 nm) suggest that deposition conditions strongly affect different defect concentrations. Films deposited at 800 °C with a higher oxygen partial pressure yielded higher resistance, which may result from the incorporation of gallium vacancies, identified as a compensating point defect affecting the electrical conductivity of bulk monoclinic Ga2O3.

Atomic Properties of Monoclinic Ag2Se Thin Film Grown on SrTiO3 Substrate by Molecular Beam Epitaxy.

Silver chalcogenides have attracted a great deal of interest due to their promise for exhibiting novel topological properties. Using scanning tunneling microscopy/spectroscopy (STM/S), we have characterized the atomic structure and electronic properties of a monoclinic Ag2Se thin film, similar to β-Ag2Te, grown on a SrTiO3 (STO)(001) substrate by molecular beam epitaxy (MBE). Three different types of Ag2Se atomic terminations are observed on the surface: (i) homogeneous hexagonal-like, (ii) rough mixed, and (iii) flat zigzag-striped structures. Structural analysis indicates that the different atomic terminations stem from different growth directions, which can be attributed to the lattice mismatch between the substrate and the Ag2Se film. STS analysis of these atomic terminations uncovers different features near the Fermi level, indicating constituent- and direction-dependent electronic properties. This Letter presents a practical method to grow monoclinic thin film Ag2Se and provides insight into its physical properties.

Investigating the ranges of (meta)stable phase formation in (InxGa1−x)2O3 : Impact of the cation coordination

We investigate the phase diagram of the heterostructural solid solution (InxGa1-x)2O3 both computationally, by combining cluster expansion and density functional theory, and experimentally, by means of TEM measurements of pulsed laser deposited (PLD) heteroepitaxial thin films. The shapes of the Gibbs free energy curves for the monoclinic, hexagonal and cubic bixbyite alloy as a function of composition can be explained in terms of the preferred cation coordination environments of indium and gallium. We show by atomically resolved STEM that the strong preference of indium for six-fold coordination results in ordered monoclinic and hexagonal lattices. This ordering impacts the configurational entropy in the solid solution and thereby the (InxGa1-x)2O3 phase diagram. The resulting phase diagram is characterized by very limited solubilities of gallium and indium in the monoclinic, hexagonal and cubic ground state phases respectively but exhibits wide metastable ranges at realistic growth temperatures. On the indium rich side of the phase diagram a wide miscibility gap is found, which results in phase separated layers. The experimentally observed indium solubilities in the PLD samples are in the range of x=0.45 and x=0.55 for monoclinic and hexagonal single-phase films, while for phase separated films we find x=0.5 for the monoclinic phase, x=0.65-0.7 for the hexagonal phase and x>0.9 for the cubic phase. These values are consistent with the computed metastable ranges for each phase.

Temperature-dependent growth of hexagonal and monoclinic gallium sulfide films by pulsed-laser deposition

We demonstrate the selective, pulsed-laser deposition of hexagonal GaS and monoclinic Ga2S3 films on sapphire substrates from a single Ga2S3 target in high-vacuum conditions. Growth at substrate temperatures below 550 °C causes GaS film formation, which indicates non-stoichiometric transfer from the target to the film. Surprisingly, stoichiometric transfer occurs at substrate temperatures above 650 °C with monoclinic Ga2S3 as the preferred, higher S-content phase. Through a series of growth and annealing experiments, we show that GaS nucleation under S-deficient conditions leads to the preferred growth of this layered, hexagonal phase below 550 °C. Furthermore, GaS films annealed above 650 °C under high vacuum are transformed to Ga2S3, reflecting the greater stability of the monoclinic phase. By first growing Ga2S3 at a higher temperature and subsequently growing GaS at a lower temperature, we can fabricate GaS/Ga2S3 heterostructures in a single growth process.

Photoelectrochemical water oxidation on PbCrO4 thin film photoanode fabricated via Pechini method: Various solution-processes for PbCrO4 film synthesis

Abstract PbCrO4 thin film photoanode was fabricated by utilizing various solution-processes (precipitation, solution-combustion, sol-gel, and the Pechini methods) and its photocatalytic activity for photoelectrochemical water oxidation was characterized. During film formation, secondary phases such as Pb2CrO5, Pb5CrO8, and Cr2O3 were easily formed owing to the incomplete oxidation of Cr(III) in the precursor. The Pechini method inherently suppressed the formation of the secondary phase, whereas for the other methods optimization of the composition of the precursor for the solution-processes or the electrochemical removal of Cr2O3 were required in order to form a pure monoclinic PbCrO4 film. The photoactivity of the PbCrO4 films fabricated with the various solution-processes was characterized in conjunction with the film properties such as the morphology, crystal structure, and light absorbance. The low film coverage of PbCrO4 fabricated via the precipitation and solution-combustion methods resulted in low light absorbance, and consequently, low photoresponse whereas PbCrO4 prepared by the sol-gel and Pechini methods was photoresponsive. For water oxidation, only PbCrO4 from the Pechini method exhibited a reasonable photoresponse (0.57 mA/cm2 at 1.23 V vs RHE under 100 mW/cm2), whereas the photoacitivity of PbCrO4 from sol-gel method was deteriorated due to surface recombination of the photogenerated carriers. The photoactivity PbCrO4 from the Pechini method could be improved further to 0.91 mA/cm2 by decorating with a Co-Pi cocatalyst to enhance the kinetics of the water oxidation reaction. Co-Pi decoration also improved the faradaic efficiency for the water oxidation, which was roughly estimated from scanning electrochemical microscopy. However, the notable photoactivity of PbCrO4 deteriorated rapidly due to the photodegradation.

Microstructure, hardness and optical properties of Er2O3 films deposited on diamond-coated and Si(100) substrates by radio frequency magnetron sputtering

Er2O3 anti-reflection films were deposited on the surface of the chemical vapor deposited diamond films and Si(100) substrates by radio frequency magnetron sputtering. The microstructure of the Er2O3 thin films and the interfacial layer were investigated by means of scanning electron microscopy, X-ray diffraction, atomic force microscopy and transmission electron microscopy. The refractive index, hardness and transmittance of the films were measured by ellipsometry, nanoindentation measurements, and Fourier transformed infrared spectroscopy. Results revealed that the bias induces the formation of a monoclinic phase and the nucleation of the Er2O3 thin films is mainly observed along the (222) plane. A high-resolution TEM image revealed a 2.5-nm-thick amorphous layer at the interface between the Si substrate and Er2O3 film. Notably, monoclinic Er2O3 films can considerably improve the transmittance of the diamond film in the long-wavelength infrared range of 8–12 μm via the reduction of the refractive index. However, the introduction of the monoclinic phase led to the decrease of the hardness and elastic modulus of the films; hence, controlling a suitable amount of the monoclinic phase may play an extremely important role in the mechanical and optical properties of the Er2O3 films.

Layer-by-layer epitaxial growth of monoclinic SrIrO3 thin films on (111)-oriented SrTiO3 through interface engineering

When transition metal oxides is grown along (111) direction, a variety of intriguing phenomena is expected to arise. One of important system, SrIrO3, crystallizes into monoclinic hexagonal-based 6M structure when grown on (111)-oriented perovskite SrTiO3, and is expected to host novel topological states. However, the growth of high quality SrIrO3 on (111)-oriented substrate is still a challenge. We report on high-quality layer-by-layer epitaxial growth of monoclinic SrIrO3 on SrTiO3(111) substrate by inserting the CaTiO3 buffer layer. The scanning transmission electron microscopy image shows perfect epitaxial growth of monoclinic SrIrO3. Reflection high-energy electron diffraction and low-energy electron diffraction patterns exhibit smooth film surface with reconstruction. Inserting CaTiO3 buffer layer proves to be an effective way of interface engineering in realizing layer-by-layer growth by reducing the lattice mismatch. The study paves way for the future research on novel quantum states in monoclinic SrIrO3 films and heterostructures.

Wafer-size VO2 film prepared by water-vapor oxidant

The growth of wafer-scale and uniform monoclinic VO2 film with excellent phase transition property was a challenge if considering the multivalent nature of vanadium atom and the various phase structures of VO2 compound. Direct oxidation of metallic vanadium film by annealing in oxygen gas or in air was suggested to be an easy way for VO2 film preparation, while the oxidation parameters including the gas pressure, gas flux and annealing temperature were extremely sensitive due to the critical preparation window. Here we proposed a facile water-vapor assisted thermal oxidation to produce wafer-scale VO2 films with good uniformity. Results indicated that by using the water vapor as the oxidizing agent, the temperature window for VO2 growth was greatly broadened. In addition, the obtained wafer-size VO2 film showed very uniform surface and sharp resistance change, which was attributed to the improved crystallinity and the high oxygen stoichiometry. The chemical reaction route was calculated based on Gibbs free energy, which confirmed the preferred growth of VO2 film by wet oxidation. Our results not only demonstrated that the water vapor could be used as a modest oxidizing agent, but also showed the unique advantage for large size VO2 film preparation, which was useful for practical devices applications in the future.

Magneto-dielectric effect in relaxor superparaelectric Tb2CoMnO6 film

We report magneto-dielectric properties of partially B-site ordered monoclinic Tb2CoMnO6 double perovskite thin film epitaxially grown by metalorganic aerosol deposition technique. Transmission electron microscopy and electron energy loss spectroscopy mapping shows the presence and distribution of both Co2+ and Co3+ ions in the film, evidencing a partial B-site disorder, which was further confirmed by the observation of reduced saturation magnetization at low temperatures. The ferromagnetic Curie temperature, TC=110 K, is slightly higher as compared to the bulk value (100 K) probably due to an in plane tensile strain. Remarkably, a short range ordering of spins at T*~190 K>>TC was established and assigned to the B-site disorder in the film. Two different dielectric relaxation peaks have been observed; they merge at the same temperature T* of short range spin correlations. Moreover, an unexpected high temperature dipolar relaxor-glass-like transition at T~T* was observed, at which a coupling to short range magnetic correlations results in a 4% magneto-dielectric coupling.

Accelerated Hydrogen Diffusion and Surface Exchange by Domain Boundaries in Epitaxial VO2 Thin Films.

Electronic phase modulation based on hydrogen insertion/extraction is kinetically limited by the bulk hydrogen diffusion or surface exchange reaction, so slow hydrogen kinetics has been a fundamental challenge to be solved for realizing faster solid-state electrochemical switching devices. Here we accelerate electronic phase modulation that occurs by hydrogen insertion in VO2 through vertically aligned 2D defects induced by symmetry mismatch between epitaxial films and substrates. By using domain-matching epitaxial growth of monoclinic VO2 films with lattice rotation and twinning on hexagonal Al2O3 substrates, the domain boundaries naturally align vertically; they provide a "highway" for hydrogen diffusion and surface exchange in VO2 films and overcome the limited rates of bulk diffusion and surface reaction. From the quantitative analysis of the deuterium (2H) isotope tracer exchange, it is confirmed that the tracer diffusion coefficient (D*) and surface exchange coefficient (k*) were increased by several orders of magnitude in VO2 films that had domain boundaries. These results yield fundamental insights into the mechanism by which mobile ions are inserted along extended defects and provide a strategy to overcome a limitation to switching speed in electrochemical devices that exploit ion insertion.