Single-crystal Epitaxial Films Research Articles

High-performance solar-blind photodetector based on Si-doped α-Ga2O3 thin films grown by mist chemical vapor deposition

α-Ga2O3 has gained increasing attention in the field of photodetectors because of its wide bandgap and exceptional chemical and physical properties. Si-doped α-Ga2O3 single-crystal epitaxial films were successfully deposited on sapphire substrates using an affordable and non-vacuum-based mist chemical vapor deposition (Mist-CVD) technique. An interdigital metal-semiconductor-metal (MSM) photodetector with excellent performance was prepared by using the thin film. At 254 nm illumination and a bias voltage of 20 V, the photodetector has a high light-to-dark current ratio of 3.24×106, a responsivity of 3.23×102 A/W, a detectivity of 8.80×1015 Jones, and an external quantum efficiency of 1.58×105%. The results show that Si-doped α-Ga2O3 thin films have great potential for application in high-performance photodetectors.

Structural characterization of Zn2SnO4 epitaxial single crystal films prepared on c-cut α-Al2O3 films by magnetron sputtering

Amorphous zinc stannate (Zn2SnO4) films were deposited on α-Al2O3 (0006) substrate by magnetron sputtering. And single crystal Zn2SnO4 films were obtained after thermal-annealing. The structural and optical properties of Zn2SnO4 films at different annealing temperatures were systematically studied. The structural analyses revealed that the sample annealed at 900 °C exhibited the best crystallization quality with an inverse spinel structure. The heteroepitaxy mechanisms were further elucidated by XRD and HETEM. The epitaxial relationship between thin film and substrate can be concluded as Zn2SnO4 (111)//α-Al2O3 (0006) with Zn2SnO4 [0 11‾]//α-Al2O3 <1‾ 100>. The obtained Zn2SnO4 films showed excellent transparency with an average transmittance of 88.8% in the visible region. The optical band gaps of Zn2SnO4 films annealed at temperatures ranging from 600 to 1100 °C was found to be within the range of 3.7–4.2 eV.

Preparation of Single-Crystal Zinc Oxide Films on a Metal Sublayer

To increase the efficiency of devices based on piezoelectric transducers made of zinc oxide, it is proposed to use single-crystal layers of platinum as the bottom electrode. Epitaxial single-crystal films of zinc oxide on metal have been obtained. It is shown that this combination of layers makes it possible to minimizethe mismatch in the lattice parameters of the film and substrate, which significantly improves the quality of the obtained piezoelectric film.

Gaseous Catalyst Assisted Growth of Graphene on Silicon Carbide for Quantum Hall Resistance Standard Device

AbstractDirect growth of graphene on silicon carbide (SiC) is a very promising method for preparing high‐quality graphene. However, high quality single crystal epitaxial graphene films on SiC always form at a temperature higher than 1800 °C. Here, the synthesis of graphene on the silicon surface (0001) of SiC at ≈1300 °C by gaseous catalyst‐assisted chemical vapor deposition (CVD) method is reported. As the step height of terraces on SiC surface can influence the performance of graphene Hall devices, low‐temperature growth of graphene benefits for keeping the steps of the SiC surface at a small height, which can be achieved by a pre‐treatment before growth. A graphene quantum Hall resistance standard (G‐QHRS) device fabricated on the SiC surface with a small step height of ≈0.5 nm exhibits an accuracy of 1.15 × 10−8 and a reproducibility of 3.6 × 10−9 within 7 days in the measurement of quantum resistance at B = 6 T and T = 4.5 K. The gaseous catalyst‐assisted CVD on SiC is an effective scalable growth approach that produces high‐quality graphene for the resistance metrology, and it represents a promising step toward a low magnetic field QHRS setting the basis of low‐cost and transportable QHRS in a near future.

Effect of nitrogen vacancies on the growth, dislocation structure, and decomposition of single crystal epitaxial (Ti1-xAlx)Ny thin films

The effect of varying nitrogen vacancies on the growth, microstructure, spinodal decomposition and hardness values of predominantly single crystal cubic phase c-(Ti1-xAlx)Ny films was investigated. Epitaxial c-(Ti1−xAlx)Ny films with y = 0.67, 0.79, and 0.92 were grown on MgO(001) and MgO(111) substrates by magnetron sputter deposition. High N vacancy c-(Ti1−xAlx)N0.67 films deposited on MgO(111) contained coherently oriented w-(0001) structures while segregated conical structures were observed on the films grown on MgO(001). High resolution STEM images revealed that the N-deficient growth conditions induced segregation with small compositional fluctuations that increase with the number of N vacancies. Similarly, strain map analysis of the epitaxial c-(Ti1−xAlx)Ny (001) and (111) films show fluctuations in strain concentration that scales with the number of N vacancies and increases during annealing. The spinodal decomposition coarsening rate of the epitaxial c-(Ti1−xAlx)Ny films was observed to increase with decreasing N vacancies. Nanoindentation showed decreasing trends in hardness of the as-deposited films as the N vacancies increase. Isothermal post-anneal at 1100 °C in vacuum for 120 min revealed a continuation in the increase in hardness for the film with the largest number of N vacancies (y = 0.67) while the hardness decreased for the films with y = 0.79 and 0.92. These results suggest that nitrogen-deficient depositions of c-(Ti1-xAlx)Ny films help to promote a self-organized phase segregation, while higher N vacancies generally increase the coherency strain which delays the coarsening process and can influence the hardness at high temperatures.

Applying Configurational Complexity to the 2D Ruddlesden–Popper Crystal Structure

The layered Ruddlesden-Popper crystal structure can host a broad range of functionally important behaviors. Here we establish extraordinary configurational disorder in a layered Ruddlesden-Popper (RP) structure using entropy stabilization assisted synthesis. A protype A2CuO4 RP cuprate oxide with five cations on the A-site sublattice is designed and fabricated into epitaxial single crystal films using pulsed laser deposition. When grown on a near lattice matched substrate, the (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 film features a T'-type RP structure with uniform A-site cation mixing and square-planar CuO4 units. These observations are made with a range of combined characterizations using X-ray diffraction, atomic-resolution scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray absorption spectroscopy measurements. It is further found that heteroepitaxial strain plays an important role in crystal phase formation during synthesis. Compressive strain over ∼1.5% results in the formation of a non-RP cubic phase consistent with a CuX2O4 spinel structure. The ability to manipulate configurational complexity and move between 2D layered RP and 3D cubic crystal structures in cuprate and related materials promises to enable flexible design strategies for a range of functionalities, such as magnetoresistance, unconventional superconductivity, ferroelectricity, catalysis, and ion transport.

Plasmonic Photovoltaics Enabled By Single-Crystal Metal Films and Nanostructures

In contrast to conventional photovoltaic devices which rely on bulk semiconductor material absorption and separation of electron-hole pairs, surface plasmon-based solar energy harvesting employs rectifying metal/dielectric interfaces to capture light and separate charges. Here, we describe the requirements for efficient hot electron extraction in plasmonic photovoltaic devices and demonstrate a new scalable and environmentally friendly electroless deposition method for single crystal epitaxial noble metals films and nanostructures. The method produces ultra-smooth, low loss, single-crystal noble metal films ideal for subtractive patterning of nanostructures through ion beam milling, and high definition, sub-wavelength single crystal nanostructures through lithographic patterning methods. We describe the nucleation and growth of these metal films and nanostructures in the absence and presence of anionic shape-control agents and examine the role of specific anions in determining the resulting film and nanostructure morphologies via scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). These effects have been exploited to yield large area patterned, and shape-controlled nanoarrays of single crystal metal nanostructures for plasmonic and metamaterial applications. These approaches offer new and cost effective routes to achieve crystalline, shape-controlled surface nanostructure to enable efficient hot electron extraction for energy harvesting applications.

Intrinsically p-type cuprous iodide semiconductor for hybrid light-emitting diodes

Cuprous halides, characterized by a direct wide band-gap and a good lattice matching with Si, is an intrinsic p-type I-VII compound semiconductor. It shows remarkable optoelectronic properties, including a large exciton binding energy at room temperature and a very small piezoelectric coefficient. The major obstacle to its application is the difficulty in growing a single-crystal epitaxial film of cuprous halides. We first demonstrate the single crystal epitaxy of high quality cuprous iodide (CuI) film grown on Si and sapphire substrates by molecular beam epitaxy. Enhanced photoluminescence on the order of magnitude larger than that of GaN and continuous-wave optically pumped lasing were found in MBE grown CuI film. The intrinsic p-type characteristics of CuI were confirmed using an n-AlGaN/p-CuI junction that emits blue light. The discovery will provide an alternative way towards highly efficient optoelectronic devices compatible with both Si and III-nitride technologies.

Magnon profile on SrRuO3 films studied by inelastic neutron scattering.

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic SrRuO3 single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the SrRuO3 films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadratic relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.

Epitaxial UN and α-U2N3 thin films

Single crystal epitaxial thin films of UN and α-U2N3 have been grown for the first time by reactive DC magnetron sputtering. These films provide ideal samples for fundamental research into the potential accident tolerant fuel, UN, and U2N3, its intermediate oxidation product. Films were characterised using x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS), with XRD analysis showing both thin films to be [001] oriented and composed of a single domain. The specular lattice parameters of the UN and U2N3 films were found to be 4.895 Å and 10.72 Å, respectively, with the UN film having a miscut of 2.6°. XPS showed significant differences in the N-1 s peak between the two films, with area analysis showing both films to be stoichiometric.

Single-crystal high entropy perovskite oxide epitaxial films

The first examples of single crystal epitaxial thin films of a high entropy perovskite oxide are synthesized. Pulsed laser deposition is used to grow the configurationally disordered ABO3 perovskite, Ba(Zr0.2Sn0.2Ti0.2Hf0.2Nb0.2)O3, epitaxially on SrTiO3 and MgO substrates. X-ray diffraction and scanning transmission electron microscopy demonstrate that the films are single phase with excellent crystallinity and atomically abrupt interfaces to the underlying substrates. Atomically-resolved electron energy loss spectroscopy mapping shows a uniform and random distribution of all B-site cations. The ability to stabilize perovskites with this level of configurational disorder offers new possibilities for designing materials from a much broader combinatorial cation pallet while providing a fresh avenue for fundamental studies in strongly correlated quantum materials where local disorder can play a critical role in determining macroscopic properties.

The synthesis and microstructural, optical, magnetic characterizations of m 0 0-oriented epitaxial Bi2Fe4O9 thin film by pulsed laser deposition

Mullite Bi2Fe4O9 single crystal epitaxial thin film has been successfully synthesized on SrTiO3 (100) substrate by pulsed laser deposition. The single crystal thin film is grown along (m00) orientation (m=2, 4) which is identified by X-ray diffraction. Morphology analysis shows orderly assigned grains with some small defects due to the volatilization of Bi2O3 tested by scanning electron microscope and atomic force microscopy. The chemical composition of film is confirmed by energy dispersive X-ray spectroscopy, nearly in accord with the chemical formula. M-H hysteresis curve confirms the ferromagnetism in single crystal thin film which can be attributed to canted non-collinear anti-ferromagnetic spin order caused by structural distortion. The band gap of thin film is calculated as 1.5eV through transmission spectrum which is very suitable for solar cell. This research not only lay a solid foundation on further researching Bi2Fe4O9 single crystal epitaxial film, but also open an avenue to design various multifunctional devices.

Controlled Phase and Tunable Magnetism in Ordered Iron Oxide Nanotube Arrays Prepared by Atomic Layer Deposition.

Highly-ordered and conformal iron oxide nanotube arrays on an atomic scale are successfully prepared by atomic layer deposition (ALD) with controlled oxidization states and tunable magnetic properties between superparamagnetism and ferrimagnetism. Non-magnetic α-Fe2O3 and superparamagnetic Fe3O4 with a blocking temperature of 120 K are in-situ obtained by finely controlling the oxidation reaction. Both of them exhibit a very small grain size of only several nanometers due to the nature of atom-by-atom growth of the ALD technique. Post-annealing α-Fe2O3 in a reducing atmosphere leads to the formation of the spinel Fe3O4 phase which displays a distinct ferrimagnetic anisotropy and the Verwey metal-insulator transition that usually takes place only in single crystal magnetite or thick epitaxial films at low temperatures. The ALD deposition of iron oxide with well-controlled phase and tunable magnetism demonstrated in this work provides a promising opportunity for the fabrication of 3D nano-devices to be used in catalysis, spintronics, microelectronics, data storages and bio-applications.

Epitaxial single-crystal thin films of MnxTi1 − xO2 − δ grown on (rutile)TiO2 substrates with pulsed laser deposition: Experiment and theory

Epitaxial rutile-structured single-crystal MnxTi1−xO2−δ films were synthesized on rutile- (110) and -(001) substrates using pulsed laser deposition. The films were characterized by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and aberration-corrected transmission electron microscopy (ACTEM). Under the present conditions, 400°C and PO2=20mTorr, single crystal epitaxial thin films were grown for x=0.13, where x is the nominal average mole fraction of Mn. In fact, arbitrarily thick films could be grown with near invariant Mn/Ti concentration profiles from the substrate/film interface to the film surface. In contrast, at x=0.25, Mn became enriched towards the surface and a secondary nano-scale phase formed which appeared to maintain the basic rutile structure but with enhanced z-contrast in the tunnels, or interstitial sites. Ab initio thermodynamic calculations provided quantitative estimates for the destabilizing effect of expanding the β-MnO2 lattice parameters to those of TiO2-rutile, the stabilizing effect of diluting Mn with increasing Ti concentration, and competing reaction pathways for surface oxide formation.

Design of high-coercivity epitaxial magnetic garnet films for thermomagnetic recording and nanotechnology

Tuning the magnetic hysteresis and optical absorption in single crystal epitaxial magnetic garnet films is investigated. It is demonstrated that high values of coercivity and optical absorption are caused by lattice mismatch stress and/or oxygen vacancies which compensate calcium anions in Ca-containing garnet films. Several methods for enhancing coercivity in Bi-containing films are studied: by doping bivalent impurity anions, by mismatch stress and by treatment in negative corona discharge. Optimization and comparative analysis of these methods are carried out discussing their pros and cons.

Structural and optical properties of β-Ga2O3 films deposited on MgAl2O4 (1 0 0) substrates by metal–organic chemical vapor deposition

β-Ga2O3 films have been prepared on MgAl2O4 (100) substrates at different temperatures (550–700°C) by metal–organic chemical vapor deposition (MOCVD). Microstructure analysis revealed that the sample deposited at 650°C was the single crystal epitaxial film with the best crystallinities. The epitaxial relationship was β-Ga2O3 (100) || MgAl2O4 (100) with β-Ga2O3 [001] || MgAl2O4 〈011〉. A schematic diagram was proposed to explain the domain structure in the film layer. The average transmittance for the films in the visible range was over 90%. An ultraviolet (UV)–green photoluminescence (PL) from about 350–600nm was observed at room temperature (RT). The corresponding PL mechanisms were investigated.

Structural and transport properties of epitaxial PrNiO3 thin films grown by molecular beam epitaxy

Epitaxial single crystal films of praseodymium nickelate (PrNiO3) have been grown by molecular beam epitaxy on LaAlO3(001) substrates. In-situ electron diffraction and ex-situ X-ray diffraction techniques confirm an epitaxial relationship to the underlying substrate. Crystalline quality depends strongly on the substrate temperature and activated oxygen flux during growth with the highest quality films formed at 600°C under high oxygen plasma fluxes. The metal–insulator transition for PrNiO3 films was suppressed as the crystalline quality of the layers was improved.

A versatile metal-halide vapor chemistry for the epitaxial growth of metallic, insulating and semiconducting films

A metal-chloride-based chemistry that bypasses many traditional pitfalls present in the epitaxy of refractory-metal-containing films is described. Methods of dealing with the corrosive chloride-based chemistry in a Molecular Beam Epitaxy (MBE) chamber are detailed. The present state of refractory metal oxide thin film heteroepitaxy is presented along with an explanation of the deposition chemistry involved. To demonstrate the chemical diversity of this technology, oxidation-limited conditions in a vacuum environment are used to grow single crystal epitaxial films of; lithium niobate (LiNbO 3), a wide bandgap ferroelectric; lithium niobite (LiNbO 2), a semiconducting lithium-metal-oxide; and epitaxial metals niobium and iron. Additionally, the chemistry can easily be applied to other chemical vapor deposition and atomic layer epitaxial growth methods. The novel LiNbO 2 thin films are confirmed to have a 2.0 eV bandgap, and several are found to have unusually high p-type conductivities of up to 2500 S/cm.

An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts

A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical∕thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques and in situ reflectivity measurements and (ii) the adsorption∕reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005-1.5 × 10(15) molecules cm(-2) s(-1) and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190-3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJ cm(-2)) corresponds to the detection limit for adsorption of less than 1.5 × 10(12) CO molecules cm(-2) or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10(15) surface Pt atoms cm(-2)). The absolute accuracy in energy is within ∼7%-9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K.

Fabrication and characterization of the multiferroic birelaxor lead–iron–tungstate/lead–zirconate–titanate

We compare chemical solution deposition (CSD) and pulsed laser deposition (PLD), specimens of the room temperature, single-phase, multiferroic magnetoelectric, [PbFe0.67W0.33O3]x[PbZr0.53Ti0.47O3]1−x (PZTFWx∼0.40&amp;lt;x&amp;lt;0.20) with high polarization, low loss (∼2%–5%), and high resistivity (typically 108 Ω cm). Single phase polycrystalline multiferroics PZTFWx thin films were fabricated on platinized silicon substrate by CSD and as epitaxial single-crystal films on MgO (100) substrate by PLD. PLD grown samples showed high dielectric constants (1200–3000), high polarization (30–60 μC/cm2), and weak saturation magnetization (2.11–5.47 emu/cm3) compare to the dielectric constants (800–1400), polarization (11–22 μC/cm2), and magnetization (0.48–4.53 emu/cm3) of CSD grown films, where as a broad dielectric temperature peak, high-frequency dispersion, low dielectric loss, and low leakage current were observed in these materials synthesized by both methods, suggesting the family as candidates for room temperature multiferroic devices. The ferroelectric switching in these materials can be suppressed or quenched with applied magnetic field.