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.     

Raman study of stress effect on Ge nanocrystals embedded in Al2O3

Ge nanocrystals (NCs) embedded in Al₂O₃ were grown by RF-sputtering. X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy techniques were used to characterize the stresses on the NCs. While small NCs (< 10 nm) have been observed to be spherical and fully relaxed in the matrix, the larger ones (> 17 nm) demonstrated a compressive stress effect. This could be linked to the crystal structure of the adjacent Al₂O₃ matrix.     

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.     

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.     

Characterization of stepwise flash-evaporated CuInSe2 films

CuInSe₂ (CIS) films were deposited by stepwise flash evaporation from polycrystalline powder source onto glass substrates held at various temperatures ranging from 100 to 560 K. The phase purity and microstructure were analyzed by transmission electron microscopy. The investigations show that films grown at 300 K and below were amorphous, whereas those grown at 370 K and above were polycrystalline in nature. The grain size in polycrystalline films were found to improve with increase in substrate temperature and during post-deposition annealing. The films had near stoichiometric composition as revealed by Rutherford backscattering spectrometry. Analysis of the optical transmittance spectra of CIS films deposited at 520 K yielded a value of ∼0.97 eV for the fundamental band gap.     

Native oxide consumption during the atomic layer deposition of TiO2 films on GaAs (100) surfaces

The consumption of the surface native oxides is studied during the atomic layer deposition of TiO₂ films on GaAs (100) surfaces. Films are deposited at 200 °C from tetrakis dimethyl amido titanium and H₂O. Transmission electron microscopy data show that the starting surface consists of ~2.6 nm of native oxide and X-ray photoelectron spectroscopy indicates a gradual reduction in the thickness of the oxide layer as the thickness of the TiO₂ film increases. Approximately 0.1-0.2 nm of arsenic and gallium suboxide is detected at the interface after 250 process cycles. For depositions on etched GaAs surfaces no interfacial oxidation is observed.     

Microstructures of YBa2Cu3O7/La0.7Ca0.3MnO3 and La0.7Ca0.3MnO3/YBa2Cu3O7 bi-layers grown on (001)LaAlO3

Epitaxial YBa₂Cu₃O₇/La_0.7Ca_0.3MnO₃ (YBCO/LCMO) bi-layers and La_0.7Ca_0.3MnO₃/YBa₂Cu₃O₇ (LCMO/YBCO) bi-layers were grown on (001)LaAlO₃ by pulsed laser deposition, and their microstructures were compared by transmission electron microscopy investigation. In the YBCO(100 nm)/LCMO(150 nm) bi-layers, the LCMO layer consists of columnar grains of ~ 17 nm in diameter and contains mixed orientation domains of [100]_c, [010]_c and [001]_c. The YBCO layer is totally c-axis oriented and the YBCO lattices are tilted − 2.5° to + 2.5° as they grew on the rough surfaces of LCMO columnar grains. For the LCMO(140 nm)/YBCO(140 nm) bi-layers, the LCMO/YBCO interface is sharp and flat. The initial 12-nm thickness of the YBCO layer is composed of c-axis oriented domains, and the upper part of YBCO layer is [100] oriented. The LCMO layer was predominantly [001]_c oriented while [100]_c-oriented domains were occasionally observed.     

Structural, dielectric, impedance and optical properties of CaBi2B2O7 glasses and glass-nanocrystal composites

Optically clear glasses were fabricated by quenching the melt of CaCO₃–Bi₂O₃–B₂O₃ (in equimolecular ratio). The amorphous and glassy characteristics of the as-quenched samples were confirmed via the X-ray powder diffraction (XRD) and differential scanning calorimetric (DSC) studies. These glasses were found to have high thermal stability parameter (S). The optical transmission studies carried out in the 200–2500 nm wavelength range confirmed both the as-quenched and heat-treated samples to be transparent between 400 nm and 2500 nm. The glass-plates that were heat-treated just above the glass transition temperature (723 K) for 6 h retained ≈60% transparency despite having nano-crystallites (≈50–100 nm) of CaBi₂B₂O₇ (CBBO) as confirmed by both the XRD and transmission electron microscopy (TEM) studies. The dielectric properties and impedance characteristics of the as-quenched and heat-treated (723 K/6 h) samples were studied as a function of frequency at different temperatures. Cole–Cole equation was employed to rationalize the impedance data.     

SnO2 thin-films prepared by a spray-gel pyrolysis: Influence of sol properties on film morphologies

Nanostructured tin oxide films were prepared by depositing different sols using the so-called spray-gel pyrolysis process. SnO₂ suspensions (sols) were obtained from tin (IV) tert-amyloxide (Sn(t-OAm)₄) or tin (IV) chloride pentahydrate (SnCl₄·5H₂O) precursors, and stabilized with ammonia or tetraethylammonium hydroxide (TEA-OH). Xerogels from the different sols were obtained by solvent evaporation under controlled humidity.The Relative Gelling Volumes (RGV) of these sols strongly depended on the type of precursor. Xerogels obtained from inorganic salts gelled faster, while, as determined by thermal gravimetric analysis, occluding a significant amount of volatile compounds. Infrared spectroscopic analysis was performed on raw and annealed xerogels (300, 500 °C, 1 h). Annealing removed water and ammonium or alkyl ammonium chloride, increasing the number of Sn-O-Sn bonds.SnO₂ films were prepared by spraying the sols for 60 min onto glass and alumina substrates at 130 °C. The films obtained from all the sols were amorphous or displayed a very small grain size, and crystallized after annealing at 400 °C or 500 °C in air for 2 h. X-ray diffraction analysis showed the presence of the cassiterite structure and line broadening indicated a polycrystalline material with a grain size in the nanometer range. Results obtained from Scanning Electron Microscopy analysis demonstrated a strong dependence of the film morphology on the RGV of the sols. Films obtained from Sn(t-OAm)₄ showed a highly textured morphology based on fiber-shape bridges, whereas the films obtained from SnCl₄·5H₂O had a smoother surface formed by “O-ring” shaped domains.Lastly, the performance of these films as gas sensor devices was tested. The conductance (sensor) response for ethanol as a target analyte was of the same order of magnitude for the three kinds of films. However, the response of the highly textured films was more stable with shorter response times.     

Study of silicon-on-insulator substrates incorporated with buried MoSi2 layer

Silicon-on-Insulator (SOI) substrates incorporated with buried MoSi₂ were fabricated using room temperature plasma bonding technology and smart cut technology. The molybdenum disilicide phase formation and morphology were studied by means of four-point probe measurements, X-ray diffraction analysis, atomic force microscopy and transmission electron microscopy examination. It is found that the transition of high-resistance phase Mo₃Si to low-resistance phase h-MoSi₂ occurs at approximately 750 °C. The t-MoSi₂ phase emerges at approximately 900 °C. SOI substrate incorporated with buried silicide layer of complete t-MoSi₂ phase can be achieved by 900 °C annealing for 20 min.     

Dislocations in Bi0.4Ca0.6MnO3 epitaxial film grown on (110) SrTiO3 substrate

Bi_0.4Ca_0.6MnO₃ (BCMO) film with a thickness of 110 nm was epitaxially grown on a (110) SrTiO₃ (STO) substrate using pulsed laser ablation technique. The microstructure of the epitaxial films was investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) in details. Two different kinds of dislocations, one being perpendicular to the BCMO/STO interface, the other being parallel to the interface, have been commonly observed. The formation mechanism for these dislocations has been discussed. All the dislocations are thought to relieve the local strain in the epitaxial film.     

Properties of multilayer composite thin films based on morphotropic phase boundary Pb(Mg1/3Nb2/3)O3-PbTiO3

It has been reported that ferroelectric and piezoelectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMNT) thin films, with compositions close to the morphotropic phase boundary (MPB), show lower values than those reported for bulk ceramics with the same composition, which has been attributed to a reduction of the remnant polarization caused by the small size of the grains in the films. An alternative has been proposed to take full advantage of the excellent piezoelectric properties of polycrystalline PMNT in thin film form: a multilayer configuration that uses ferroelectric layers with large remnant polarization, in this case PbTiO₃, to generate an internal electric bias within the PMNT layers and, thus, anchor an induced polarization on them, resulting in a consequent large piezoelectric behavior. The detailed study of the properties of these multilayer composite films reveals the complex correlations that arise in these heterostructures, which are key for the design of optimized piezoelectric films based on MPB PMNT.     

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.     

Epitaxial growth of topological insulator Bi2Se3 film on Si(111) with atomically sharp interface

Atomically sharp epitaxial growth of Bi₂Se₃ films is achieved on Si(111) substrate with molecular beam epitaxy. Two-step growth process is found to be a key to achieve interfacial-layer-free epitaxial Bi₂Se₃ films on Si substrates. With a single-step high temperature growth, second phase clusters are formed at an early stage. On the other hand, with low temperature growth, the film tends to be disordered even in the absence of a second phase. With a low temperature initial growth followed by a high temperature growth, second-phase-free atomically sharp interface is obtained between Bi₂Se₃ and Si substrate, as verified by reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and X-ray diffraction. The lattice constant of Bi₂Se₃ is observed to relax to its bulk value during the first quintuple layer according to RHEED analysis, implying the absence of strain from the substrate. TEM shows a fully epitaxial structure of Bi₂Se₃ film down to the first quintuple layer without any second phase or an amorphous layer.     

Thickness dependent microstructural changes in La0.5Ca0.5MnO3 thin films deposited on (111) SrTiO3

The epitaxial strain can modify the physical properties of complex oxide thin films considerably. The strain effect is expected to be less pronounced for relatively thick films and the physical properties should resemble to the bulk material. However, it has been recently observed that the electronic and magnetic properties of La_0.5Ca_0.5MnO₃ thin films deposited on (111) SrTiO₃ substrates thicker than a threshold value differ considerably from the bulk material. This observation is a hint for some interesting microstructural features in these films. In the present study, the microstructure of La_0.5Ca_0.5MnO₃ thin films on (111) SrTiO₃ substrates is investigated by X-ray diffraction and high resolution transmission electron microscopy.     

Electrical characterization of high-pressure reactive sputtered ScOx films on silicon

Al/ScO_x/SiN_x/n-Si and Al/ScO_x/SiO_x/n-Si metal-insulator-semiconductor capacitors have been electrically characterized. Scandium oxide was grown by high-pressure sputtering on different substrates to study the dielectric/insulator interface quality. The substrates were silicon nitride and native silicon oxide. The use of a silicon nitride interfacial layer between the silicon substrate and the scandium oxide layer improves interface quality, as interfacial state density and defect density inside the insulator are decreased.     

Growth and structure of epitaxial CeO2 films on yttria-stabilized ZrO2

Thirty to a hundred-nm thick epitaxial CeO₂ layers are grown on YSZ (100), (110) and (111) surfaces of yttria-stabilized ZrO₂ (YSZ) by electron beam evaporation of Ce in oxygen at reduced pressure. Their growth, structure and thermal stability are studied with several bulk and surface sensitive techniques including Rutherford backscattering spectrometry, cross-sectional high resolution electron microscopy, low energy electron diffraction and low energy reflection electron microscopy. Excellent epitaxy is obtained on all YSZ surfaces at a growth temperature of 750 K. The surfaces of films grown on (111)-oriented substrates are flat, whereas those on the other substrates are faceted into small (111) planes. The grain sizes in the films are in the 10 nm range and smaller.     

Investigations on TlBa2Ca2Cu3Ox superconducting thin films formation on LaAlO3 substrate

Investigations on pure superconducting phase TlBa₂Ca₂Cu₃O_x (Tl-1223) thin films formation, of about 100-125 nm in thickness, on (001) LaAlO₃ single crystal substrate, were made using radio-frequency sputtering deposition of Ba₂Ca₂Cu₃O_x precursor films and ex-situ thallination in sealed quartz tube. The precursor films were thallinated under different conditions of partial oxygen pressure, temperature, time and y thallium source content using unreacted pellets of composition Tl_yBa₂Ca₂Cu₃O_x. In all cases, strongly c-oriented multiphase films were obtained. A correlation between the Tl-1223 phase purity and the precursor film conditions of thallination is established. Temperature and time of thallination as well as the thallium source content and the partial pressure of oxygen play a key role in the quality of the obtained film. The films' onset temperature of the superconducting transition ranges between 90 and 103 K. It is shown that the best samples can be obtained from a dense precursor film and relatively medium thallination time.     

Anatase TiO2 films crystallized on SnO2:F substrates in an aqueous solution

Anatase TiO₂ films were fabricated on SnO₂:F substrates in an aqueous solution. The films were constructed of assemblies of nano-TiO₂. Surface of the films showed large roughness due to nano/micro-asperity of the assemblies. The thickness was increased to 260 nm, 360 nm, 600 nm and 760 nm with the deposition time of 2 h, 5 h, 25 h and 48 h, respectively. The films showed strong intensity of 004 X-ray diffraction possibly because the needles, i.e., nano-TiO₂, elongated along the c-axis. Low-temperature synthesis of crystalline anatase TiO₂ films has a great deal of potential in the development of electronic devices, optical devices, dye-sensitized solar cells, photocatalytic devices and biomolecule sensors.     

Structural property of β-FeSi2 layers deposited on FeSi from a molten salt

The microstructural properties of the β-FeSi₂/FeSi structure prepared from a molten salt have been characterized using transmission electron microscopy (TEM). The β-FeSi₂ films were grown on FeSi substrates at the heat treatment temperature of 900 °C from 1 min to 24 h using the molten salt technique. It is found that the films consisted of a thin surface layer and a thick underlying layer with columnar-shaped domains. The crystallographic directions of the domains are mostly randomly oriented. The β-FeSi₂ domains in the film, however, have specific crystallographic orientation relationships with the adjoining domains and the FeSi substrate. A high density of the stacking faults on the β-FeSi₂ (100) planes was also observed through the films. Moreover, the growth evolution of the β-FeSi₂ domains, the defect characteristics and the formation mechanism of the defects are discussed.