Ozone-assisted Molecular Beam Epitaxy Research Articles

Real-space characterization of tetragonal CuO epitaxial films

It is widely believed that the undoped Mott insulator is crucial to understand the origin of high-TC superconductivity and the complex phase diagram in cuprates. Tetragonal CuO, the simplest structure among cuprates, is regarded as an archetypal model to explore the ground state physics of Mott insulator. Here, we report on the epitaxial growth and surface characterization of tetragonal CuO thin film by combining ozone-assisted molecular beam epitaxy and multi-analytical techniques. In situ scanning tunneling microscopy measurement reveals two sets of Cu2+ square lattices with a reversed contrast of their apparent heights between neighboring domains. A spectroscopy study demonstrates an upper bound of the charge transfer gap as 3.68 eV, and quantitatively agrees with the previous photoemission and x-ray photoelectron results. Our work will deepen the understanding of high-energy electronic structures in parent cuprates.

Molecular beam epitaxy of PdO on MgO (001)

PdO, widely used in catalysis in powder form for decades, has been predicted recently to be a Dirac semimetal. Synthesis of high-quality single crystals of this material is thus of great interest. Here, by using ozone-assisted molecular beam epitaxy, PdO thin films were grown on a MgO (001) substrate. X-ray diffraction and transmission electron microscopy indicate the film is $a/b$ axis oriented, with the $c$ axis lying in plane. Fully oxygenated PdO films have a low density of holelike carriers, and are insulating at the lowest temperatures. Our density functional theory calculations using the Heyd-Scuseria-Ernzerhof exchange-correlation functional suggest a \ensuremath{\sim}1.0 eV band gap at the $M$ point, where the gap can be reduced by tensile strain along the $c$ axis and the bands begin to invert when the tensile strain is as high as 18%. Although tensile strain of this magnitude is experimentally not viable using epitaxy, electrons can be doped by oxygen reduction. Our results emphasize the need for careful consideration of electron correlation effects and stoichiometry in ab initio modeling of topological semimetals involving transition metal oxides.

Bandgap Tailoring of Monoclinic Single‐Phase β‐(AlxGa1−x)2O3 (0 ≤ x ≤ 0.65) Thin Film by Annealing β‐Ga2O3/Al2O3 Heterojunction at High Temperatures

Herein, bandgap tuning of monoclinic (−201)‐oriented β‐(AlxGa1−x)2O3 thin films is achieved through a β‐Ga2O3/Al2O3 heterojunction by a feasible annealing process with an O2 atmosphere. During the annealing process, Al atoms of the Al2O3 substrate outdiffuse easily into the β‐Ga2O3 thin film deposited by ozone‐assisted molecular beam epitaxy (OMBE). The Al compositions in the β‐(AlxGa1−x)2O3 samples are tuned through adjusting the annealing temperature from 800 to 1300 °C and experimentally determined from the result of X‐ray photoelectron spectroscopy (XPS) measurements combined with Vegard's law. Successive Al‐composition‐gradient β‐(AlxGa1−x)2O3 thin films with controlled bandgap are constructed. On these bases, β‐Ga2O3 thin films are deposited on β‐(AlxGa1−x)2O3 (0 ≤ x ≤ 0.65) substrates through OMBE, yielding β‐Ga2O3/β‐(AlxGa1−x)2O3 (0 ≤ x ≤ 0.65) heterojunction structures, and the band offsets of this heterojunction are determined by XPS accordingly. This methodology to achieve high‐quality β‐(AlxGa1−x)2O3 thin films with adjustable Al composition and tunable band offsets of the β‐Ga2O3/β‐(AlxGa1−x)2O3 interface will provide guidance for potential strategies to develop and fabricate β‐(AlxGa1−x)2O3‐based deep‐UV photodetectors and power devices.

Delta-doped β-(AlxGa1−x)2O3/Ga2O3 heterostructure field-effect transistors by ozone molecular beam epitaxy

Heterojunction field-effect transistors based on the β-(AlxGa1−x)2O3/Ga2O3 heterostructure grown by ozone-assisted molecular beam epitaxy were demonstrated for the first time. Al composition ratios in the 14%–23% range were validated using x-ray diffraction on the three samples grown for this study. Electrochemical capacitance-voltage (ECV) measurements showed the presence of a charge sheet in the delta-doped (AlxGa1−x)2O3 barrier layer. Secondary ion mass spectroscopy and ECV measurements also revealed an unintentional Si peak at the (AlxGa1−x)2O3/Ga2O3 interface. Direct current (IDS-VGS) and transconductance (Gm-VGS) measurements demonstrated depletion-mode transistor operation as well as the presence of a parallel conduction channel. A one-dimensional Poisson model suggested that dopant redistribution in the delta-doped region could cause a secondary channel to form in the barrier in addition to the primary channel near the (AlxGa1−x)2O3/Ga2O3 interface under certain conditions met in these samples. Fabricated devices on sample A did not exhibit breakdown up to the measurement limit of 1100 V, with stability after ten cycles. A maximum output drain current density of 22 mA/mm was measured on sample B. Room temperature Hall measurements yielded a sheet carrier density of 1.12 × 1013 cm−2 with corresponding Hall mobility of 95 cm2/V s in sample C.

A-axis YBa2Cu3O7−x/PrBa2Cu3O7−x/YBa2Cu3O7−x trilayers with subnanometer rms roughness

We demonstrate a-axis YBa2Cu3O7−x/PrBa2Cu3O7−x/YBa2Cu3O7−x trilayers grown on (100) LaAlO3 substrates with improved interface smoothness. The trilayers are synthesized by ozone-assisted molecular-beam epitaxy. The thickness of the PrBa2Cu3O7−x layer is held constant at 8 nm, and the thickness of the YBa2Cu3O7−x layers is varied from 24 nm to 100 nm. X-ray diffraction measurements show all trilayers to have >97% a-axis content. The rms roughness of the thinnest trilayer is <0.7 nm, and this roughness increases with the thickness of the YBa2Cu3O7−x layers. The thickness of the YBa2Cu3O7−x layers also affects the transport properties: while all samples exhibit an onset of the superconducting transition at and above 85 K, the thinner samples show wider transition widths, ΔTc. High-resolution scanning transmission electron microscopy reveals coherent and chemically sharp interfaces and that growth begins with a cubic (Y,Ba)CuO3−x perovskite phase that transforms into a-axis oriented YBa2Cu3O7−x as the substrate temperature is ramped up.

Electronic inhomogeneity and band structure in superstructural CuO2 planes of infinite-layer Sr0.94La0.06CuO2+y films

Scanning tunneling microscopy and spectroscopy are utilized to study the atomic-scale structure and electronic properties of infinite-layer Sr0.94La0.06CuO2+y films prepared on SrRuO3-buffered SrTiO3(001) substrate by ozone-assisted molecular beam epitaxy. Incommensurate structural supermodulation with a period of 24.5{\AA} is identified on the CuO2-terminated surface, leading to characteristic stripes running along the 45o direction with respect to the Cu-O-Cu bonds. Spatially resolved tunneling spectra reveal substantial inhomogeneity on a nanometer length scale and emergence of in-gap states at sufficient doping. Despite the Fermi level shifting up to 0.7 eV, the charge-transfer energy gap of the CuO2 planes remains fundamentally unchanged at different doping levels. The occurrence of the CuO2 superstructure is constrained in the surface region and its formation is found to link with oxygen intake that serves as doping agent of holes in the epitaxial films.

Molecular beam epitaxy growth and surface structure of Sr1−xNdxCuO2 cuprate films

We report on the epitaxial growth and surface structure of infinite-layer cuprate Sr1-xNdxCuO2 films on SrTiO3(001) substrates by combining ozone-assisted molecular beam epitaxy and in situ scanning tunneling microscopy. Careful substrate temperature and flux control has been used to achieve single-phase, stoichiometric, and c-axis oriented films. The surface of the films is usually characterized by a mixed CuO2 surface and gridlike superstructure. The superstructure exhibits a periodicity of 3.47 nm that corresponds to a coincidence lattice between the overlayer peroxide SrO2 and underlying CuO2 plane, and gives rise to a conductance spectrum that is distinct from the Mott-Hubbard band structure of CuO2. At a higher Nd composition x > 0.1, a (2 x 2) surface characteristic of the hole-doped CuO2 emerges, which we ascribe to the intake of apical oxygens in the intervening Sr planes.

Growth of PdCoO2 by ozone-assisted molecular-beam epitaxy

We report the in situ, direct epitaxial synthesis of (0001)-oriented PdCoO2 thin films on c-plane sapphire using ozone-assisted molecular-beam epitaxy. The resulting films have smoothness, structural perfection, and electrical characteristics that rival the best in situ grown PdCoO2 thin films in the literature. Metallic conductivity is observed in PdCoO2 films as thin as ∼2.0 nm. The PdCoO2 films contain 180° in-plane rotation twins. Scanning transmission electron microscopy reveals that the growth of PdCoO2 on the (0001) surface of Al2O3 begins with the CoO2 layer.

Tunable perpendicular exchange bias in oxide heterostructures

The exchange bias effect is an essential component of magnetic memory and spintronic devices. Whereas recent research has shown that anisotropies perpendicular to the device plane provide superior stability against thermal noise, it has proven remarkably difficult to realize perpendicular exchange bias in thin-film structures. Here we demonstrate a strong perpendicular exchange bias effect in heterostructures of the quasi-two-dimensional canted antiferromagnet La$_2$CuO$_4$ and ferromagnetic (La,Sr)MnO$_3$ synthesized by ozone-assisted molecular beam epitaxy. The magnitude of this effect can be controlled via the doping level of the cuprate layers. Canted antiferromagnetism of layered oxides is thus a new and potentially powerful source of uniaxial anisotropy in magnetic devices.

Interfacial stability of ultrathin films of magnetite Fe3O4 (111) on Al2O3(001) grown by ozone-assisted molecular-beam epitaxy

The thin films of iron oxides including magnetite (Fe3O4) and hematite (α-Fe2O3) have many important applications. Both forms of oxide can occur naturally during film growth by iron deposition under various oxidation environments; an important issue is to understand and control the process resulting in a single-phase film. We have performed the in-situ -time studies using X-ray diffraction of such film growth on sapphire (001) under pure ozone by monitoring the (00L) rod. Stable magnetite growth can be maintained at growth temperatures below 600 °C up to a certain critical film thickness, beyond which the growth becomes hematite. The results demonstrate the importance of interfacial interaction in stabilizing the magnetite phase.

Clean superconductivity in electron doped Pr2−xCexCuO4+δthin films hetero-epitaxially grown on SrTiO3by reactive molecular beam epitaxy

Abstract

Charge transfer and interfacial magnetism in (LaNiO3)n/(LaMnO3)2superlattices

(LaNiO3)n/(LaMnO3)2 superlattices were grown using ozone-assisted molecular beam epitaxy, where LaNiO3 is a paramagnetic metal and LaMnO3 is an antiferromagnetic insulator. The superlattices exhibit excellent crystallinity and interfacial roughness of less than 1 unit cell. X-ray spectroscopy and dichroism measurements indicate that electrons are transferred from the LaMnO3 to the LaNiO3, inducing magnetism in LaNiO3. Magnetotransport measurements reveal a transition from metallic to insulating behavior as the LaNiO3 layer thickness is reduced from 5 unit cells to 2 unit cells and suggest a modulated magnetic structure within LaNiO3.

Electronically driven superconductor-insulator transition in electrostatically doped La2CuO4+δthin films

Using an electronic double layer transistor we have systematically studied the superconductor-to-insulator transition in La${}_{2}$CuO${}_{4+\ensuremath{\delta}}$ thin films grown by ozone-assisted molecular-beam epitaxy. We have confirmed the high crystalline quality of the cuprate films and have demonstrated the suitability of the electronic double layer technique to continuously vary the charge density in a system that is otherwise characterized by the presence of miscibility gaps. The transport and magnetotransport results highlight the role of electron-electron interactions in the mechanism of the transition due to the proximity of the Mott-insulating state.

Ultrathin BaTiO3 templates for multiferroic nanostructures

The structural, electronic and dielectric properties of high-quality ultrathin BaTiO3 films were investigated. The films, which were grown by ozone-assisted molecular beam epitaxy on Nb-doped SrTiO3(001) substrates and have thicknesses as low as 8 unit cells (u.c.) (3.2 nm), are unreconstructed and atomically smooth with large crystalline terraces. A strain-driven transition to three-dimensional (3D) island formation is observed for films of 13 u.c. thickness (5.2 nm). The high structural quality of the surfaces, together with dielectric properties similar to bulk BaTiO3 and dominantly TiO2 surface termination, makes these films suitable templates for the synthesis of high-quality metal-oxide multiferroic heterostructures for the fundamental study and exploitation of magneto-electric effects, such as a recently proposed interface effect in Fe/BaTiO3 heterostructures based on Fe–Ti interface bonds.

Composition Shift as a Function of Thickness in Fe3-δO4(001) Epitaxial Films

We report on a thickness-dependent composition shift and a valency change in B-site Fe in Fe3-δO4(001) epitaxial films prepared by ozone-assisted molecular beam epitaxy. Conversion electron Mössbauer spectroscopy (CEMS) measurements for locally inserted 57Fe-enriched probe layers showed that a pure γ-Fe2O3 film can be obtained only if the film is sufficiently thin, and that the composition shift occurs when the thickness increases. The resistivity of Fe3-δO4 films having different thicknesses exhibited a significant decrease with an increase in the thickness, consistent with the CEMS experiment. These results indicate that γ-Fe2O3 films do not preferably grow homoepitaxially and that careful control of the growth is necessary to apply γ-Fe2O3 films to spintronics devices.

Tuning between the metallic antiferromagnetic and ferromagnetic phases ofLa1−xSrxMnO3nearx=0.5by digital synthesis

We investigated cation-ordered La{sub 1-x}Sr{sub x}MnO{sub 3} about the half-doping level x {approx} 0.5 in superlattices of alternating, single-unit-cell layers of LaMnO{sub 3} and SrMnO{sub 3}. The effect of La/Sr cation order was addressed by comparing the structural, magnetic and transport properties of these superlattices with random-alloy films of equivalent composition. The samples were synthesized by ozone-assisted molecular-beam epitaxy onto SrTiO{sub 3} substrates. The superlattices could be tuned between ferromagnetic and antiferromagnetic metallic states by inserting extra single-unit-cell layers of LaMnO{sub 3} and SrMnO{sub 3}, respectively. For x<0.5, a ferromagnetic, metallic phase was observed. For x=0.50 and 0.55, A-type antiferromagnetic order was confirmed by neutron diffraction, with a Neel temperature of 300 K, significantly higher than bulk values. The enhanced Neel temperature was attributed to lattice strain rather than cation order.

Onset of metallic behavior in strained(LaNiO3)n/(SrMnO3)2superlattices

(LaNiO{sub 3}){sub n}/(SrMnO{sub 3}){sub 2} superlattices were grown using ozone-assisted molecular beam epitaxy. In situ reflection high-energy electron diffraction and x-ray scattering has been used to characterize the structural properties of the superlattices, which are strained to the SrTiO{sub 3} substrates. The superlattices exhibit excellent crystallinity and interfacial roughness of less than 1 unit cell. A metal-insulator transition is observed as n is decreased from 4 to 1. Analysis of the transport data suggests an evolution from gapped insulator (n=1) to hopping conductor (n=2) to metal (n=4) with increasing LaNiO{sub 3} concentration.

Thickness dependence of the exchange bias in epitaxial manganite bilayers

Exchange bias has been studied in a series of ${\text{La}}_{2/3}{\text{Ca}}_{1/3}{\text{MnO}}_{3}/{\text{La}}_{1/3}{\text{Ca}}_{2/3}{\text{MnO}}_{3}$ bilayers grown on (001) ${\text{SrTiO}}_{3}$ substrates by ozone-assisted molecular-beam epitaxy. The high crystalline quality of the samples and interfaces has been verified using high-resolution x-ray diffractometry and $Z$-contrast scanning transmission electron microscopy with electron-energy-loss spectroscopy. The dependence of exchange bias on the thickness of the antiferromagnetic layer has been investigated. A critical value for the onset of the hysteresis loop shift has been determined. An antiferromagnetic anisotropy constant has been obtained by fitting the results to the generalized Meiklejohn-Bean model.

Iron Vacancy Ordered γ-Fe2O3(001) Epitaxial Films: The Crystal Structure and Electrical Resistivity

We report the structural characterization and transport properties of highly-ordered epitaxial γ-Fe 2 O 3 thin films grown by a pure ozone-assisted molecular beam epitaxy method. X ray diffraction (XRD) measurements at a synchrotron facility indicated that the composition of films was close to that of γ-Fe 2 O 3 with Fe vacancy only occurring at octahedral sites of a spinel. We revealed that the crystal symmetry was reduced from a cubic, giving rise to extra superlattice reflections along the c * axis and this showed good agreement with a tetragonal structure with \(c\lesssim 3a\). We succeeded in determining the position of vacancy sites as well as the unit cell by comparing the XRD results with calculated reflection patterns for plausible vacancy ordered models. The temperature dependent resistivity of the vacancy ordered γ-Fe 2 O 3 was several orders higher than that of Fe 3 O 4 .

Electronic and crystal structure of fully strainedLaNiO3films

First-principles band structure calculations have been used to investigate epitaxially strained ${\mathrm{LaNiO}}_{3}$ films. Experimentally, tensile biaxial strain has been realized in pseudomorphic ${\mathrm{LaNiO}}_{3}$ films grown on ${\mathrm{SrTiO}}_{3}$ (001) substrates using ozone-assisted molecular beam epitaxy. Measured and calculated out-of-plane lattice parameters are in excellent agreement. This demonstrates the viability of the computational method as well as the high quality of the films.