Single-phase Epitaxial Films Research Articles

Epitaxial Film Growth of LiBH4 via Molecular Unit Evaporation

Complex hydrides have attracted considerable attention in fields including fast ion conduction and hydrogen storage. To understand the physical properties and to expand the fields of application of complex hydrides, physically well-defined epitaxial films that can facilitate the investigation of intrinsic properties and interfacial effects need to be fabricated. However, epitaxial films of complex hydrides remain difficult to produce. This study reports the growth of single-phase epitaxial films of the complex hydride LiBH4 and their high Li-ion conductivity of 1 × 10–2 S cm–1 at 423 K. To achieve this, we used a low-power infrared laser to induce the evaporation of LiBH4 molecular units; with this technique, we were able to deposit [BH4]− complex anions while preserving their molecular integrity, producing epitaxial films with high crystallinity, flat surface, and high Li-ion conductivity. These achievements will establish a solid basis for epitaxial growth of complex hydrides and pave the way for advanced studies of complex hydrides including surface and interfacial phenomena.

Tuning of magnetic properties for epitaxial Y2NiMnO6 thin film: Substrate is crucial

The effects of epitaxial strain induced by lattice mismatch and substrate type on the structure and magnetic properties of Y2NiMnO6 thin films have been systematically investigated. Y2NiMnO6 thin films grown on (001)-oriented LaAlO3 (LAO), (La,Sr)(Al,Ta)O3 (LSAT), and SrTiO3 (STO) substrates with varying film thickness are obtained by a simple polymer assisted deposition method. X-ray diffraction and Raman scattering observations indicate that the single-phase epitaxial films are successfully obtained. By magnetic measurements, it is found that all the films show an obvious ferromagnetic transition with a lower transition temperatures (Tc) than that of the Y2NiMnO6 bulk. With the biaxial tensile strain decreasing or the film thickness increasing, the Tc of the film increases. Besides, due to the different strain states of the films and the different surface migrations of the substrates, the structure and magnetic properties show a strong dependence on the type of substrate. It is suggested that the biaxial tensile strain and substrate type have crucial effects on the structure, magnetic properties and the related Tc of the thin film, which can be utilized to engineer the magnetic properties of the films and the related ferroelectricity.

Structural characteristics, magnetic properties of Re2NiMnO6 (Re = La, Pr, Nd, Sm, Y) thin films on (001) LaAlO3 by simple polymer assisted deposition

Re2NiMnO6 (Re=La, Pr, Nd, Sm, Y) epitaxial thin films grown on (001) LaAlO3 substrates were obtained via a simple polymer assisted deposition method. The XRD θ/2θ scans, ω scans, and φ scans indicate that the films are single-phase epitaxial films. It is found that there is a ferromagnetic (FM) transition with the temperature decreasing from room temperature for all the films. The Raman mode positions shift toward the low frequency with the substitution of La by a smaller rare earth element. The FM transition temperature Tc decreases as the A-site ionic radius decreasing. Moreover, all the FM transition temperatures (Tc) of the Re2NiMnO6 films are lower than those corresponding values of the bulks, indicating the ferromagnetic interaction weakening. It is suggested that the ferromagnetic interaction is affected by the strain existing in the film through affecting the B(B′)O6 octahedra and the cation disorder related to the ionic radius of Re, which can be used as an effective way to improve the magnetic properties of the film and obtain the higher Tc functional materials.

Adsorption-controlled growth of BiVO4 by molecular-beam epitaxy

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.

Epitaxial, cation-ordered, ferroelectric PbSc0.5Ta0.5O3 thin films prepared by pulsed laser deposition

We report on growth and ferroelectric properties of cation-ordered, epitaxial PbSc0.5Ta0.5O3 (100) films. Single phase epitaxial films were grown on vicinal SrTiO3 (100) substrates with a layer of SrRuO3 as bottom electrode by pulsed laser deposition. Their crystal orientation, topography, and microstructure were analyzed by x-ray diffraction, atomic force microscopy, and transmission electron microscopy (TEM), respectively. The in-plane and out-of-plane epitaxial relationships were studied by ϕ and θ-2θ scans, respectively. Superstructure reflections obtained in TEM diffraction patterns of the films confirm cation ordering. Polarization-field and switching current-voltage hysteresis curves demonstrate ferroelectric behavior of the films at room temperature.

Control of carrier concentration and surface flattening of CuGaO 2 epitaxial films for a p-channel transparent transistor

A p-type transparent oxide semiconductor, CuGaO 2, was grown epitaxially on (111) single-crystalline yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD). Single-phase epitaxial films were obtained in narrow conditions: i.e. 725 ≤ T s ≤ 780 °C at PO 2 = 6 Pa, and 3 Pa ≤ PO 2 ≤ 6 Pa at T s = 750 °C. The electrical conductivity of the as-deposited films was controlled from 3.3 × 10 − 5 S∙cm − 1 to 1.7 × 10 − 2 S∙cm − 1 by increasing the oxygen partial pressure from 3 to 6.5 Pa. The hole concentration and Hall mobility of the most conductive film were 5 × 10 17 cm − 3 and 0.2 cm 2∙V − 1 ∙s − 1 , respectively. Estimated from the conductivity, the hole concentration was controlled from ∼ 10 14 cm − 3 to ∼ 10 17 cm − 3 by the oxygen partial pressure. Post-annealing at 1215 °C smoothed the films surface to 0.56 nm in the root-mean-squares roughness and increased the Hall mobility to 0.8 cm 2 V − 1 ∙s − 1 , which is the largest value among CuGaO 2 films reported to date.

The road to magnesium diboride thin films, Josephson junctions and SQUIDs

The remarkably high critical temperature at which magnesium diboride (MgB2) undergoes transition to the superconducting state, Tc ≈ 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB2 films. An overview of the deposition techniques for MgB2 thin film growth will be given, with a special focus on the in situ two-step process.Although, meanwhile, many problems to obtain suitable films have been solved, such as oxygen impurities and magnesium volatility, the question of how single-phase epitaxial films can be grown still remains. The possibility of growing single-crystalline epitaxial films will be discussed from the deposition conditions' point of view as well as substrate choice. Necessary conditions are discussed and possible routes are reviewed.The applicability of MgB2 in superconducting electronic devices depends on the possibility of making well-controlled, i.e., reproducible and stable, Josephson junctions. The first attempts to make MgB2–MgO–MgB2 ramp-type junctions and SQUIDs from MgB2 nanobridges are discussed.

Stability and Structural Characterization of Epitaxial NdNiO3 Films Grown by Pulsed Laser Deposition

ABSTRACTThin films of NdNiO3 were grown using pulsed laser deposition on single crystal substrates of [100]-oriented LaAlO3 and SrTiO3. X-ray diffraction and reflectivity, scanning electron microscopy, and atomic force microscopy were used to characterize the chemical, morphological and structural traits of the thin films. Single-phase epitaxial films are grown on LaAlO3 and SrTiO3 at 625°C in an oxygen pressure of 200 mTorr. At higher temperatures, the films partially decompose to Nd2NiO4 and NiO. The films are epitaxial with the (101) planes (orthorhombic Pnma notation) parallel to the substrate surface. Four in-plane orientational variants exist that correspond to the four 90° degenerate orientations of the film's [010] with respect to the in-plane substrate directions. Films are observed to be strained in accordance with the structural mismatch to the underlying substrate, and this leads, in the thinnest films on LaAlO3, to an apparent monoclinic distortion to the unit cell.

A structural investigation of high-quality epitaxial thin films

The (PZT 53/47) and (PZT 20/80) films deposited on substrates by RF magnetron sputtering are single-phase epitaxial films. In order to prevent large thermal stresses being induced in the films during the preparation process, the PZT films were cooled slowly through the Curie temperature. Scanning electron microscopy (SEM) showed that the PZT 53/47 film had a pyramid structure on its surface, whereas the PZT 20/80 film had a smooth surface without any observable features. Transmission electron microscopy (TEM) observations showed similar features: the PZT 53/47 film had a coarse columnar structure, whereas the PZT 20/80 film had a fine crystalline structure; this is due to the better lattice matching in the latter.

Thin-film growth of the charge-density-wave oxide Rb0.30MoO3

We report on the thin-film fabrication of a charge-density wave (CDW) compound. Single-phase epitaxial films of the model CDW oxide Rb0.30MoO3 have been grown by pulsed-laser deposition. Detailed analyses show that the Rb0.30MoO3 films have μm-size grains with the CDW chains oriented parallel to the substrate. On SrTiO3 (510), the CDW chains align into a single direction within the film plane. The electrical resistance of the films demonstrates a CDW state below about 182 K. Structures patterned in films will permit unprecedented studies of phase-coherent CDW transport, as well as the exploration of devices based on CDWs.