Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO₃ thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al₂O₃ and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al₂O₃, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO₃ films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.     

Measurement of epitaxial misorientations and related effects in thin films of YBa2Cu3O7-δ grown on nominally (001)MgO substrates by pulsed laser deposition

Heteroepitaxial YBa₂Cu₃O_7-δ thin films have been deposited onto nominal (001)MgO substrates by pulsed laser deposition. For some of these films, Rutherford backscattering spectrometry analysis revealed anomalous variations in the backscattered yield at the substrate level during [001] YBa₂Cu₃O_7-δ axial channelling. A novel angular-dependent ion channelling technique has been developed to investigate these effects. It has been shown that the anomalies arise from an epitaxial misorientation between the lattices of the substrate and deposit. Strong correlations were found between the magnitudes of the misorientations, the densities of spiral features on the deposit surface and the critical current densities exhibited by the deposits. It is proposed that the misorientation arises due to vicinal offcut of the substrate surface, which leads to a lower density of threading dislocations and hence less flux pinning.     

Highly (100)-oriented CeO2 films prepared on amorphous substrates by laser chemical vapor deposition

CeO₂ films were prepared on amorphous silica substrates by laser chemical vapor deposition using cerium dipivaloylmethanate precursor and a semiconductor InGaAlAs (808 nm in wavelength) laser system. The laser spot size was about 20 mm, which was sufficient to cover the whole substrate. Highly (100)-oriented CeO₂ films were obtained at extraordinary high deposition rates ranging from 60 to 132 μm/h. Films exhibited a columnar feather-like structure with a large number of nano-sized voids, and a surface morphology consisting of either nearly flat or pyramidal top-ending columns depending on the laser power. Nearly flat top-ending columns could be fairly (100)-oriented at the top and (111)-oriented laterally.     

Growth and structural investigations of epitaxial hexagonal YMnO3 thin films deposited on wurtzite GaN(001) substrates

Epitaxial hexagonal YMnO₃ (h-YMnO₃) films having sharp (00l) X-ray diffraction peaks were grown above 700 °C in 5 mTorr O₂ via pulsed laser deposition both on as-received wurtzite GaN/AlN/6H-SiC(001) (w-GaN) substrates as well as on w-GaN surfaces that were etched in 50% HF solution. High-resolution transmission electron microscopy revealed an interfacial layer between film and the unetched substrate; this layer was absent in those samples wherein an etched substrate was used. However, the substrate treatment did not affect the epitaxial arrangement between the h-YMnO₃ film and w-GaN substrate. The epitaxial relationships of the h-YMnO₃ films with the w-GaN(001) substrate was determined via X-ray diffraction to be (001)_YMnO3 ‖ (001)_GaN : [11¯0]_YMnO3 ‖ [110]_GaN; in other words, the basal planes of the film and the substrate are aligned parallel to one another, as are the most densely packed directions in planes of the film and the substrate. Interestingly, this arrangement has a larger lattice mismatch than if the principal axes of the unit cells were aligned.     

Structure of epitaxial Mn-stabilized ZrO2 layers on yttria-stabilized zirconia single crystals prepared by sputtering

Epitaxial Mn-stabilized zirconia layers on yttria-stabilized ZrO₂ (YSZ) single crystals were prepared by sputtering a Mn_0.32Zr_0.68O₂ target. Substrates with (001), (110) and (111) orientation were used. Transmission electron microscopy cross sections showed a homogeneous thickness of the layers. Layers prepared at a substrate temperature of 600 °C grew epitaxially. X-ray diffraction showed that these layers are not exactly cubic. We found a slightly larger out-of-plane lattice parameter compared to the in-plane lattice parameter. This means the out-of-plane lattice misfit is lower than the in-plane lattice misfit, contrary to behaviour expected for epitaxially grown layers. This effect was largest for layers on YSZ(111) and small for layers on YSZ(001).     

Structural properties of epitaxial SrCuO2 thin films on SrTiO3 (001) substrates

Thin films of SrCuO₂ with tetragonal structure have been epitaxially grown on SrTiO₃ (001) substrates by high-oxygen pressure sputtering technique. The interface structure between SrCuO₂ and SrTiO₃ and configuration of defects in SrCuO₂ thin films have been characterized by means of high-resolution transmission electron microscopy. Two types of film-substrate interface structure coexist and are determined as bulk-SrO-TiO₂-Sr(O) -CuO₂-Sr-bulk and bulk-SrO-TiO₂-SrO-Sr(O) -CuO₂-Sr-bulk. The planar faults with double SrO atomic layers in {100} planes in SrCuO₂ thin films are observed, which mainly arise from the coalescence of these two types of film-substrate interface structure. Meanwhile, planar faults in {110} planes are observed in thin films and structural models are proposed.     

Deposition of CeO2 films including areas with the different orientation and sharp border between them

Epitaxial films from one material, with sharp borders between contacting regions having different film orientation are grown on one surface of the substrate for the first time. The main reason for the deposition of thin ceria layers with mixed (001) and (111) orientations on a (1 02) sapphire substrate is determined. We suggest that this is related to the availability of surface defects which, in thin near-surface layers, deviate from stoichiometric composition. This in turn is connected with the loss of oxygen.

A technique for influencing CeO₂ film orientation is demonstrated. This involves specific preliminary processing of the substrate, and the selection of oxygen partial pressure during the deposition process.

High quality thin (30–50 nm) “protective” (001) CeO₂ epitaxial layers are prepared on (1 02) Al₂O₃. Structures comprising two epitaxial protective CeO₂ layers, orientations (001) and (111), are made on the base of (0001) and (1 02) sapphire substrates. The interface between the epitaxial layers is <1 000 nm.

Preliminary results using this method are described, and the possibility of creating a “bi-epitaxial” transition in thin YBa₂Cu₃O_7−x layers is explored.     

Epitaxial growth of CeO2/yttria-stabilized ZrO2 double layer films on biaxially textured Ni tape via electrostatic spray assisted vapour deposition

Epitaxial growth of CeO₂ and yttria-stabilized ZrO₂ (YSZ) double layer films has been successfully carried out on biaxially textured nickel substrates at a temperature between 400 and 600 °C using electrostatic spray assisted vapour deposition method. The structure of the double layer was characterized by X-ray diffraction and scanning electron microscopy. The results show that highly oriented CeO₂/YSZ double buffer films were formed epitaxially onto biaxially textured Ni substrates. The orientation relationships between YSZ layer and Ni substrate are 001_YSZ//001_Ni and 110_YSZ//100_Ni, while the orientation relationships between CeO₂ and YSZ are 001_CeO2//001_YSZ and 100_CeO2//100_YSZ.     

Biaxial CdTe/CaF2 films growth on amorphous surface

A continuous and highly biaxially textured CdTe film was grown by metal organic chemical vapor deposition on an amorphous substrate using biaxial CaF₂ nanorods as a buffer layer. The interface between the CdTe film and CaF₂ nanorods and the morphology of the CdTe film were studied by transmission electron microscopy (TEM) and scanning electron microscopy. Both the TEM and X-ray pole figure analysis clearly reveal that the crystalline orientation of the continuous CdTe film followed the {111}<121> biaxial texture of the CaF₂ nanorods. A high density of twin faults was observed in the CdTe film. Furthermore, the near surface texture of the CdTe thin film was investigated by reflection high-energy electron diffraction (RHEED) and RHEED surface pole figure analysis. Twinning was also observed from the RHEED surface pole figure analysis.     

Orientation control of epitaxial Ge thin films growth on SrTiO3 (100) by ultrahigh vacuum sputtering

We report the orientation control of crystalline Ge (111) and Ge (001) growth on SrTiO₃ (100) substrate by adjusting the temperature of substrate. It is found that the substrate temperature plays an important role for the formation of crystalline Ge with different surface orientations and interfacial chemical configuration during the sputtering process. At 500 °C, Ge (111) with good crystalline quality is formed, while Ge (001) is preferably grown on SrTiO₃ substrate at 650 °C. Our results show the possibility of manipulating the surface orientations during Ge growth on SrTiO₃ by controlling the substrate temperatures.     

Growth structure investigation of MgF2 and NdF3 films grown by molecular beam deposition on CaF2(111) substrates

The growth structure of MgF₂ and NdF₃ films grown on polished CaF₂(111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF₂ and NdF₃ films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K.     

Pulsed Laser Deposition of epitaxial SrTiO3 films: Growth,structure and functional properties

Langmuir ion probe and in situ RHEED were applied to study the effects of low oxygen pressure on SrTiO₃ (STO) film growth by Pulsed Laser Deposition (PLD). Contributions of different oxygen fluxes were analysed and parameters of STO epitaxial film growth were evaluated using physical model of adiabatic expansion of the ablation products and its interaction with ambient gas. Film surface undergoes reconstruction at growing temperatures > 600 °C indicating complete or partial relaxation of top layer without changing growth mechanism of smooth multilayered film. All films have a tetrahedral lattice distortion in the direction of growth that varying with deposition temperature and oxygen pressure. STO lattice distortion is the relevant factor in determining both agility and dielectric loss for tuneable microwave devices. Annealing in oxygen at 1100 °C improves significantly functional properties of STO films, but only the layers deposited under the pressure lower than 10^− 3 Pa possesses low dielectric losses in combination with high agility.     

New LaCu0.5Mn0.5O3 thin films deposited by the sol-gel process on different substrates

New LaCu_0.5Mn_0.5O₃ thin films deposited by the sol-gel process on ceramic Al₂O₃ and ZrO₂, glass ceramic and 101 single-crystal quartz, using the sol-gel process. It was found in all cases that the films are polycrystalline and single phase without preferred orientation. The morphology of the films depends strongly on the nature of the substrate. The films deposited on Al₂O₃ exhibit morphological characteristics making them suitable as possible sensors and catalysts.     

Synthesis and structural characterization of mesoporous V2O5 thin films for electrochromic applications

Mesoporous vanadium pentoxide (V₂O₅) films have been synthesized by hydrolysis of vanadium tri-isopropoxide (VO(OC₃H₇)₃) in the presence of polyethylene glycol (PEG) as a structure-directing agent. The structure, the stoichiometry and the morphology of the films have been studied as a function of the thermal annealing by X-ray diffraction (XRD), micro-Raman spectroscopy, optical microscopy, scanning electron microscopy and atomic force microscopy. XRD patterns and Raman spectra show the presence of two previously unreported crystalline phases. The PEG:V₂O₅ molar ratio affects the temperature of phase formation, the amount and even the order in which the phases appear. The morphological characterization underlines the role of the surfactant to promote porous networks, formed by micrometric clusters of controlled shapes and patterns embedded in a homogeneous host matrix.     

Effect of deposition temperature on the properties of CeO2 films grown by atomic layer deposition

Cerium oxide dielectric thin films have been grown on n-type silicon by atomic layer deposition using a monomeric homoleptic Ce^IV alkoxide precursor with water vapour. Herein we report the dielectric properties of CeO₂ films deposited from tetrakis(1-methoxy-2-methyl-2-propanolate)cerium. The resulting films exhibit permittivities in the range 25-42 at 1 MHz with a strong dependency on the deposition temperature. The microstructural origin of this behaviour has been investigated. The as-deposited films were found to be crystalline and they exhibited the cubic fluorite structure for deposition temperatures in the range 150 °C to 350 °C. Variations in the crystallite sizes are governed by the deposition temperature and have been estimated using a Debye-Scherrer analysis of the X-ray diffraction patterns. The changing crystallite size correlates with changes seen in the triply-degenerate F_2g first-order Raman line half-width at 465 cm^− 1. It is concluded that the frequency dependency of the film dielectric properties is strongly influenced by the crystallite size which in turn is governed by the growth temperature.     

Growth and structural characterisation of vanadium oxide ultrathin films on TiO2 (110)

The research activity of our group in the last few years has mainly been devoted to the study of ultrathin vanadium oxide films deposited on a (110)-oriented TiO₂ single crystal, in order to prepare systems which may be largely thought of as simplified models for the investigation of the structure/properties relationships in real world catalysts, sensing and optical devices. The main objective of our work consists of setting up reproducible synthesis routes for the deposition of vanadium oxide ultrathin films on TiO₂ (110), through strict control of the reaction parameters. The films obtained are then characterised from a chemical, electronic and structural point of view, and their properties are compared to those of their bulk-related phases. Results are presented concerning growth procedures and structural and electronic properties of vanadium oxide ultrathin films on titania, with a stoichiometry ranging from VO₂, down to approximately VO. In particular, it will be shown that the oxidation product of metallic vanadium in an oxygen or water atmosphere (in the 10⁻⁶ mbar range) retains the rutile lattice structure typical of stoichiometric VO₂, despite the increasing degree of oxygen defectiveness and the electronic properties, very similar to those pertaining to bulk V₂O₃. The peculiar behaviour of vanadium oxide on titania demonstrates how important the epitaxial influence of the substrate is on the nature of the overlayer. These results could represent a good starting point to understand why vanadium oxides on TiO₂ show an enhanced catalytic activity and selectivity in many industrially relevant reactions.     

Influence of secondary phases during annealing on re-crystallization of CuInSe2 electrodeposited films

Electrodeposited CuInSe₂ thin films are of potential importance, as light absorber material, in the next generation of photovoltaic cells as long as we can optimize their annealing process to obtain dense and highly crystalline films. The intent of this study was to gain a basic understanding of the key experimental parameters governing the structural-textural-composition evolution of thin films as function of the annealing temperature via X-ray diffraction, scanning/transmission electron microscopy and thermal analysis measurements. The crystallization of the electrodeposited CuInSe₂ films, with the presence of Se and orthorhombic Cu_2 − xSe (o-Cu_2 − xSe) phases, occurs over two distinct temperature ranges, between 220 °C and 250 °C and beyond 520 °C. Such domains of temperature are consistent with the melting of elemental Se and the binary CuSe phase, respectively. The CuSe phase forming during annealing results from the reaction between the two secondary species o-Cu_2 − xSe and Se (o-Cu_2 − xSe + Se → 2 CuSe) but can be decomposed into the cubic β-Cu_2 − xSe phase by slowing down the heating rate. Formation of liquid CuSe beyond 520°C seems to govern both the grain size of the films and the porosity of the substrate-CuInSe₂ film interface. A simple model explaining the competitive interplay between the film crystallinity and the interface porosity is proposed, aiming at an improved protocol based on temperature range, which will enable to enhance the film crystalline nature while limiting the interface porosity.     

Formation of amorphous anodic oxide films of controlled composition on aluminium alloys

Binary, non-equilibrium Al-29at%Nb, Al-44at%Ta, Al-19at%Ti, Al-25at%Ti and Al-32at%Zr alloys were prepared by magnetron sputtering and subsequently anodized at high Faradaic efficiency to grow barrier-type anodic films. Examination in the transmission electron microscope revealed amorphous anodic films of relatively uniform compositions across the film thicknesses, except for a layer of relatively pure alumina, of about 5% of the film thickness, present at the film/electrolyte interface of the Al-Ta alloy. The film compositions, from Rutherford backscattering spectroscopy, indicate that the alloy constituents are oxidized in their alloy proportions to form films comprising intimately mixed units of the various oxides, namely alumina, niobia, titania, tantala and zirconia. The films grow by co-operative transport of metal and oxygen ions under the electric field with formation of film material by both migration of metal ions outwards and of oxygen, and possibly hydroxyl, ions inwards. The average migration rates of Al³⁺, Nb⁵⁺, Ti⁴⁺ and Zr⁴⁺ ions are similar, to within 10%, but Ta⁵⁺ ions migrate more slowly than Al³⁺ ions. The results of the study show that a wide range of compositions of amorphous oxide films can be readily formed by anodic oxidation of appropriate alloys, including compositions containing units of normally crystalline anodic oxides, namely TiO₂ and ZrO₂.     

Enhancement of photoluminescence in Er-doped Ag-SiO2 nanocomposite thin films: A post annealing study

Silver-silica (with Ag 2.8 at.% and 8.7 at.%) nanocomposite (NC) thin films doped with Er⁺³ (0.1-0.9 at.%) were synthesized by atom beam co-sputtering using 1.5 keV Ar atoms. Optical absorption and photoluminescence (PL) studies of pristine and annealed films were performed, together with Rutherford backscattering and secondary ion mass spectroscopy studies for elemental characterization of the NC films. Optical absorption results of pristine and annealed NC film (with Ag ∼8.7 at.%) confirmed the formation of Ag nanoparticles evidenced by the appearance of characteristic surface plasmon resonance absorption features. Photoluminescence (PL) studies, carried out using Ar Laser pumping at 0.488 μm, the wavelength that the Er ions can absorb resonantly, indicated the presence of PL emission around 1.54 μm in the case of all the as-synthesized samples. The observed PL peak corresponds to the atomic transitions of Er as reported in literature. A relative enhancement in the intensity of PL peak has been observed after annealing the NC films. In the case of NC film with 0.9 at.% Er and 8.7 at.% Ag, the enhancement in PL intensity is almost twice, with respect to the as-deposited sample, for a heat treatment of about 1 h at 600 °C in a nitrogen atmosphere. However for a NC film with Er 0.1 at.% + Ag 2.8 at.%, the PL intensity is enhanced by approximately 3.7 times after annealing at 400 °C for 1 h in nitrogen atmosphere. Since the samples with surface plasmon resonance (SPR) did not show the PL enhancement, the role of SPR in the enhancement of the PL is ruled out. The enhanced PL emission from Er and Ag codoped silica indicates that the Er photo-stimulation is mediated by the energy transfer from Ag nanostructures or ions to Er. Lifetimes of PL peaks for the pristine and annealed samples were also studied. The observed lifetime ∼10 ms is a good indication of excellent PL efficiency.     

Microstructural changes induced by the pyrolysis step on epitaxial La2Zr2O7 thin films grown by metalorganic decomposition

La₂Zr₂O₇ (LZO) thin films are used as buffer layers in second generation high Tc superconductor tapes. The microstructure of LZO films grown by metalorganic decomposition is characterized by the presence of nanovoids throughout the whole thickness of the films. We introduced an out-gassing plateau under vacuum during the pyrolysis process to decrease the size of voids. The temperature of this plateau was determined by Fourier transformed infrared spectroscopy, electron back scattering diffraction and X-Ray diffraction characterizations. The dwelling time was also varied. Transmission Electron Microscopy (TEM) studies revealed that a high heating ramp in combination with a less than an hour pyrolysis plateau decreased pore size. The deposition rate during dip-coating was also decreased to enhance out-gassing at the plateau. Successive LZO layers were deposited and energy filtered TEM images at C K-edge were performed to identify the role of carbon in the nucleation mechanisms.