We report on the growth and characterization of epitaxial YBa2Cu3O7−δ (YBCO) complex oxide thin films and related heterostructures exclusively by Pulsed Laser Deposition (PLD) and using first harmonic Nd:Y3Al5O12 (Nd:YAG) pulsed laser source (λ = 1064 nm). High-quality epitaxial YBCO thin film heterostructures display superconducting properties with transition temperature ∼ 80 K. Compared with the excimer lasers, when using Nd:YAG lasers, the optimal growth conditions are achieved at a large target-to-substrate distance d. These results clearly demonstrate the potential use of the first harmonic Nd:YAG laser source as an alternative to the excimer lasers for the PLD thin film community. Its compactness as well as the absence of any safety issues related to poisonous gas represent a major breakthrough in the deposition of complex multi-element compounds in form of thin films.
Here, we present an integrated ultra-high-vacuum (UHV) apparatus for the growth of complex materials and heterostructures. The specific growth technique is the Pulsed Laser Deposition (PLD) by means of a dual-laser source based on an excimer KrF ultraviolet and solid-state Nd:YAG infra-red lasers. By taking advantage of the two laser sources—both lasers can be independently used within the deposition chambers—a large number of different materials—ranging from oxides to metals, to selenides, and others—can be successfully grown in the form of thin films and heterostructures. All of the samples can be in situ transferred between the deposition chambers and the analysis chambers by using vessels and holders’ manipulators. The apparatus also offers the possibility to transfer samples to remote instrumentation under UHV conditions by means of commercially available UHV-suitcases. The dual-PLD operates for in-house research as well as user facility in combination with the Advanced Photo-electric Effect beamline at the Elettra synchrotron radiation facility in Trieste and allows synchrotron-based photo-emission as well as x-ray absorption experiments on pristine films and heterostructures.
V2O3 presents a complex interrelationship between the metal–insulator transition and the structural rhombohedral-monoclinic one in temperature, as a function of sample thickness. Whilst in bulk V2O3 the two transitions coincide on the temperature scale, at 15 nm thickness a fully independent Mott-like transition occurs at lower temperature, with no corresponding structural changes perhaps related to epitaxial strain. It is therefore of relevance to investigate the thin and ultrathin film growth to pinpoint the chemical, electronic and structural phase phenomenology and the role of the interface with the substrate. Here we present results on the thickness dependent properties of V2O3 from 1 nm up to 40 nm thick as grown on c-plane Al2O3 substrates by exploiting variable sampling depth probes. The surface morphology of stoichiometric ultra-thin V2O3 layers evolves from islands-like to continuous flat film with thickness, with implications on the overall properties.
Phase transitions are key in determining and controlling the quantum properties of correlated materials. Here, by using the combination of material synthesis and photoelectron spectroscopy, we demonstrate a genuine Mott transition undressed of any symmetry breaking side effects in the thin films of V2O3. In particular and in contrast with the bulk V2O3, we unveil the purely electronic dynamics approaching the metal–insulator transition, disentangled from the structural transformation that is prevented by the residual substrate-induced strain. On approaching the transition, the spectral signal evolves slowly over a wide temperature range, the Fermi wave-vector does not change, and the critical temperature is lower than the one reported for the bulk. Our findings are fundamental in demonstrating the universal benchmarks of a genuine nonsymmetry breaking Mott transition, extendable to a large array of correlated quantum systems, and hold promise of exploiting the metal–insulator transition by implementing V2O3 thin films in devices.
The structural, electronic, and magnetic properties of Sr-hole-doped epitaxial La1–xSrxMnO3 (0.15 ≤ x ≤ 0.45) thin films deposited using the molecular beam epitaxy technique on 4° vicinal STO (001) substrates are probed by the combination of X-ray diffraction and various synchrotron-based spectroscopy techniques. The structural characterizations evidence a significant shift in the LSMO (002) peak to the higher diffraction angles owing to the increase in Sr doping concentrations in thin films. The nature of the LSMO Mn mixed-valence state was estimated from X-ray photoemission spectroscopy together with the relative changes in the Mn L2,3 edges observed in X-ray absorption spectroscopy (XAS), both strongly affected by doping. CTM4XAS simulations at the XAS Mn L2,3 edges reveal the combination of epitaxial strain, and different MnO6 crystal field splitting give rise to a peak at ∼641 eV. The observed changes in the occupancy of the eg and the t2g orbitals as well as their binding energy positions toward the Fermi level with hole doping are discussed. The room-temperature magnetic properties were probed at the end by circular dichroism.
We report the integration of high-quality epitaxial La2/3Sr1/3MnO3 (LSMO) thin films onto SrTiO3 buffered Silicon-on-Sapphire (SOS) substrates by combining state-of-the-art thin film growth techniques such as molecular beam epitaxy and pulsed laser deposition. Detailed structural, magnetic and electrical characterizations of the LSMO/STO/SOS heterostructures show that the LSMO film properties are competitive with those directly grown on oxide substrates. X-ray magnetic circular dichroism measurements on Mn L2,3 edges show strong dichroic signal at room temperature, and angular-dependent in-plane magnetic properties by magneto-optical Kerr magnetometry reveal isotropic magnetic anisotropy. Suspended micro-bridges were thus finally fabricated by silicon micromachining, thus demonstrating the potential use of integrating LSMO magnetic layer on industrially compatible SOS substrates for the development of applicative MEMS devices.
The occurrence of oxygen-driven metal–insulator-transition (MIT) in SrNbO3 (SNO) thin films epitaxially grown on (110)-oriented DyScO3 has been reported. SNO films are fabricated by the pulsed laser deposition technique at different partial O2 pressure to vary the oxygen content and their structural, optical, and transport properties are probed. SNO unit cell has been found to shrink vertically as the oxygen content increases but keeping the epitaxial matching with the substrate. The results of Fourier-transform infra-red spectroscopy show that highly oxygenated SNO samples (i.e., grown at high oxygen pressure) show distinct optical conductivity behavior with respect to oxygen deficient films, hence demonstrating the insulating character of the formers with respect to those fabricated with lower pressure conditions. Tailoring the optical absorption and conductivity of strontium niobate epitaxial films across the MIT will favor novel applications of this material.
V2O3 has long been studied as a prototypical strongly correlated material. The difficulty in obtaining clean, well ordered surfaces, however, hindered the use of surface sensitive techniques to study its electronic structure. Here we show by means of X-ray diffraction and electrical transport that thin films prepared by pulsed laser deposition can reproduce the functionality of bulk V2O3. The same films, when transferred in-situ, show an excellent surface quality as indicated by scanning tunnelling microscopy and low energy electron diffraction, representing a viable approach to study the metal-insulator transition in V2O3 by means of angle-resolved photoemission spectroscopy. Combined, these two aspects pave the way for the use of V2O3 thin films in device-oriented heterostructures.
The understanding of the origin of a two-dimensional electron gas (2DEG) at the surface of anatase TiO2 remains a challenging issue. In particular, in TiO2 ultra-thin films, it is extremely difficult to distinguish intrinsic effects, due to the physics of the TiO2, from extrinsic effects, such as those arising from structural defects, dislocations, and the presence of competing phases at the film/substrate interface. It is, therefore, mandatory to unambiguously ascertain the structure of the TiO2/substrate interface. In this work, by combining high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), density functional theory calculations, and multislice image simulations, we have investigated the nature of strainless anatase TiO2 thin films grown on LaAlO3 substrate. In particular, the presence of oxygen vacancies in anatase TiO2 has been proved to stabilize the formation of an extra alloy layer, Ti2AlO4, by means of interface rearrangement. Our results, therefore, elucidate why the growth of anatase TiO2 directly on LaAlO3 substrate has required the deposition of a TiOx extra-layer to have a 2DEG established, thus confirming the absence of a critical thickness for the TiO2 to stabilize a 2DEG at its surface. These findings provide fundamental insights on the underlying formation mechanism of the 2DEG in TiO2/LAO hetero-interfaces to engineer the 2DEG formation in anatase TiO2 for tailored applications.
The properties of half-metallic manganite thin films depend on the composition and structure in the atomic scale, and consequently, their potential functional behavior can only be based on fine structure characterization. By combining advanced transmission electron microscopy, electron energy loss spectroscopy, density functional theory calculations, and multislice image simulations, we obtained evidence of a 7 nm-thick interface layer in La0.7Sr0.3MnO3 (LSMO) thin films, compatible with the formation of well-known dead layers in manganites, with an elongated out-of-plane lattice parameter and structural and electronic properties well distinguished from the bulk of the film. We observed, for the first time, a structural shift of Mn ions coupled with oxygen vacancies and a reduced Mn valence state within such layer. Understanding the correlation between oxygen vacancies, the Mn oxidation state, and Mn-ion displacements is a prerequisite to engineer the magnetotransport properties of LSMO thin films.
Quantum materials are central for the development of novel functional systems that are often based on interface specific phenomena. Fabricating controlled interfaces between quantum materials requires adopting a flexible growth technique capable to synthesize different materials within a single-run deposition process with high control of structure, stoichiometry, and termination. Among the various available thin film growth technologies, pulsed laser deposition (PLD) allows controlling the growth of diverse materials at the level of single atomic layers. In PLD the atomic species are supplied through an ablation process of a stoichiometric target either in form of polycrystalline powders or of a single crystal. No carrier gases are needed in the deposition process. The ablation process is compatible with a wide range of background pressure. We present results of thin-film growth by PLD obtained by using an Nd:YAG infrared pulsed laser source operating at its first harmonics. With respect to the traditional PLD systems—based on excimer KrF UV-lasers—optimal conditions for the growth of thin films and heterostructures are reached at large target-to-substrate distance. Merits and limitations of this approach for growing oxide and non-oxide thin films are discussed. The merits of an Nd:YAG laser to grow very high-quality thin films suggest the possibility of implementing compact in-situ setups e.g. integrated with analytical instrumentation under ultra-high vacuum conditions.
In this work, we investigate the effects of the V2O3 structural phase transition on the magnetic properties of an amorphous magnetic thin film of CoFeB in contact with it. V2O3 thin films are deposited epitaxially on sapphire substrates, reaching bulklike properties after few nm of growth. By means of temperature dependent Kerr effect characterizations, we prove that crossing the V2O3 structural phase transition induces reproducible and reversible changes to CoFeB magnetic properties, especially to its coercive field. By decreasing the oxide layer thickness, its effects on the magnetic layer decreases, while reducing the magnetic layer thickness maximizes it, with a maximum of 330% coercive field variation found between the two V2O3 structural phases. By simply tuning the temperature, this systematic study shows that the engineering of V2O3 structural transition induces large interfacial strain and thus strong magnetic property variations to an amorphous thin film, opening wide possibilities in implementing strain-driven control of the magnetic behavior without strict requirements on epitaxial coherence at the interface.
Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.
Perovskite-based heterostructures have recently gained remarkable interest, thanks to atomic-scale precision engineering. These systems are very susceptible to small variations of control parameters, such as two-dimensionality, strain, lattice polarizability, and doping. Focusing on the rare-earth nickelate diagram, LaNiO3 (LNO) catches the eye, being the only nickelate that does not undergo a metal-to-insulator transition (MIT). Therefore, the ground state of LNO has been studied in several theoretical and experimental papers. Here, we show by means of infrared spectroscopy that an MIT can be driven by dimensionality control in ultrathin LNO films when the number of unit cells drops to 2. Such a dimensionality tuning can eventually be tailored when a physically implemented monolayer in the ultrathin films is replaced by a digital single layer embedded in the Ruddlesden–Popper Lan+1NinO3n+1 series. We provide spectroscopic evidence that the dimensionality-induced MIT in Ruddlesden–Popper nickelates strongly resembles that of ultrathin LNO films. Our results can pave the way to the employment of Ruddlesden–Popper Lan+1NinO3n+1 to tune the electronic properties of LNO through dimensional transition without the need of physically changing the number of unit cells in thin films.
The electronic properties of hole- and electron-doped manganites were probed by a combination of x-ray absorption and photoemission spectroscopies. Hole-doped La0.7Ba0.3MnO3 and electron-doped La0.7Ce0.3MnO3 thin films were epitaxially grown on SrTiO3 substrates by means of pulsed laser deposition. Ex-situ x-ray diffraction demonstrated the substrate/film epitaxy relation and in-situ low energy electron diffraction provided evidence of high structural order of film surfaces. By combining synchrotron x-ray absorption and x-ray photoemission spectroscopy, evidence of Mn ions into a 2+ state as a result of the Ce substitution in the electron-doped manganites was provided. Angular resolved photo-emission spectroscopy (ARPES) results showed a predominance of z2-orbitals at the surface of both hole- and, unexpectedly, electron-doped manganites thus questioning the validity of the commonly accepted scenario describing the electron filling in manganites’ 3d orbitals in oxide manganites. The precise determination of the electronic and orbital properties of the terminating layers of oxide manganites paves the way for engineering multi-layered heterostructures thus leading to novel opportunities in the field of quantum electronics.
Oxygen vacancies are known to play a crucial role in tuning the physical properties and technological applications of titanium dioxide TiO2. Over the last decades, defects in substoichiometric TiO2 have been commonly associated with the formation of TinO2n–x Magnéli phases, which are extended planar defects originating from crystallographic shear planes. By combining advanced transmission electron microscopy techniques, electron energy-loss spectroscopy and atomistic simulations, we reach new understanding of the oxygen vacancy induced structural modulations in anatase, ruling out the earlier shear-plane model. Structural modulations are instead shown to be due to the formation of oxygen vacancy superstructures that extend periodically inside the films, preserving the crystalline order of anatase. Elucidating the structure of oxygen defects in anatase is a crucial step for improving the functionalities of such material system and to engineer devices with targeted properties.
The electronic properties of strontium ruthenate SrRuO3perovskite oxide thin filmsare modified by epitaxial strain, as determined by growing on different substrates by pulsedlaser deposition. Temperature dependence of the transport properties indicates that tensilestrain deformation of the SrRuO3unit cell reduces the metallicity of the material as well as itsmetal-insulator-transition (MIT) temperature. On the contrary, the shrinkage of the Ru–O–Rubuckling angle due to compressive strain is counterweighted by the increased overlap of theconduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting intoan increased MIT temperature, i.e., a more conducting material. In particular, in the more metallicsamples, the core level X-ray photoemission spectroscopy lineshapes show the occurrence of anextra-peak at the lower binding energies of the main Ru-3d peak that is attributed to screening,as observed in volume sensitive photoemission of the unstrained material.
The electronic properties of anatase titanium dioxide (TiO2) thin films epitaxially grown on LaAlO3 substrates are investigated by synchrotron-x-ray spectroscopy [x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES)] and infrared spectroscopy. The Ti3+ fraction in TiO2−x is varied either by changing the oxygen pressure during deposition or by postgrowth annealing in ultrahigh vacuum (UHV). Structural investigation of the TiO2 thin films provides evidence of highly uniform crystallographic order in both as-grown and in situ UHV-annealed samples. The increased amount of Ti3+ as a consequence of UHV annealing is calibrated by in situ XPS and XAS analysis. The as-grown TiO2 samples, with a low Ti3+ concentration, show distinct electronic properties with respect to the annealed films, namely, absorption in the midinfrared (MIR) region correlated with polaron formation, and another peak in the visible range at 1.6 eV correlated with the presence of localized defect states (DSs). With the increasing level of Ti3+ induced by the postannealing process, the MIR peak disappears, while the DS peak is redshifted to the near-infrared region at about 1.0 eV. These results indicate the possibility of tailoring the optical absorption of anatase TiO2 films from the visible to the near-infrared region.
Spintronics exploits the magnetoresistance effects to store or sense the magnetic information. Since the magnetoresistance strictly depends on the magnetic anisotropy of a system, it is fundamental to set a defined anisotropy to the system. Here, we investigate half-metallic La0.67Sr0.33MnO3 thin films by means of vectorial Magneto-Optical Kerr Magnetometry and found that they exhibit pure biaxial magnetic anisotropy at room temperature if grown onto a MgO (001) substrate with a thin SrTiO3 buffer. In this way, we can avoid unwanted uniaxial magnetic anisotropy contributions that may be detrimental for specific applications. The detailed study of the angular evolution of the magnetization reversal pathways and critical fields (coercivity and switching) discloses the origin of the magnetic anisotropy, which is magnetocrystalline in nature and shows fourfold symmetry at any temperature.