Structural and dielectric properties of (001) and (111)-oriented BaZr0.2Ti0.8O3 epitaxial thin films

We have grown and characterized BaZr_0.2Ti_0.8O₃ (BZT) epitaxial thin films deposited on (001) and (111)-oriented SrRuO₃-buffered SrTiO₃ substrates by pulsed laser deposition. Structural and morphological characterizations were performed using X-ray diffractometry and atomic force microscopy, respectively. A cube-on-cube epitaxial relationship was ascertained from the θ-2θ and φ diffractograms in both (001) and (111)-oriented films. The (001)-oriented films showed a smooth granular morphology, whereas the faceted pyramid-like crystallites of the (111)-oriented films led to a rough surface. The dielectric response of BZT at room temperature was measured along the growth direction. The films were found to be ferroelectric, although a well-saturated hysteresis loop was obtained only for the (001)-oriented films. High leakage currents were observed for the (111) orientation, likely associated to charge transport along the boundaries of its crystallites. The remanent polarization, coercive field, dielectric constant, and relative change of dielectric permittivity (tunability) of (111)-oriented BZT were higher than those of (001)-oriented BZT.     

Structural investigations of homoepitaxial Si films grown at low temperature by pulsed magnetron sputtering on Si(111) substrates

Using pulsed magnetron sputtering at low substrate temperature (T_s = 580 °C) the homoepitaxial growth on Si(111) was studied. The films were comprehensively characterized by cross-section transmission electron microscopy and various diffraction methods. Up to a film thickness of 1240 nm no breakdown of the epitaxial growth was observed. The surface microstructure, characterized by electron backscatter diffraction, exhibits exclusively crystalline structure with (111) orientation. Careful analysis of selected area electron diffraction patterns and high-resolution X-ray diffraction data clearly proves the existence of twinning/stacking faults in the {111} planes. Besides these defects – which are typical for low-temperature epitaxy – no additional significant defects related to the energetic particle bombardment by the sputter deposition method are observed.     

High-quality epitaxial Bi(111) films on Si(111) by isochronal annealing

Bi(111) films grown on Si(111) at room temperature show a significantly higher roughness compared to Bi films grown on Si(100) utilizing a kinetic pathway based on a low-temperature process. Isochronal annealing steps of 3 min duration each with temperatures up to 200 °C cause a relaxation of the Bi films' lattice parameter toward the Bi bulk value and yield an atomically flat Bi surface. Driving force for the relaxation and surface reordering is the magic mismatch of 11 Bi atoms to 13 Si atoms that emerges at annealing temperatures above 150 °C and reduces the remaining strain to less than 0.2%.     

Morphological modification of β-FeSi2 on Si(111) by high temperature growth and post-thermal annealing

β-FeSi₂ crystals have been grown on Si(111) substrates, and morphological modification of the β-FeSi₂/Si(111) by high temperature growth and post-thermal annealing was investigated. The morphological feature of the β-FeSi₂ crystals significantly depends on the growth conditions, especially, substrate temperature during growth. The β-FeSi₂ continuous layers with relatively smooth surfaces were grown at the low substrate temperatures of 650-700 °C with exposure of the grown layers to Sb flux during the growth. On the other hand, nano-scaled islands have been grown at the higher substrate temperature of 850 °C. The structural property, interfacial morphology and growth evolution of the β-FeSi₂ islands were examined, and compared with those for the layers grown at a lower substrate temperature. In addition, the morphological evolution of the β-FeSi₂/Si layers by post-thermal annealing was examined, and it was found that the interfacial smoothness between the β-FeSi₂ layers and the Si(111) substrates was improved by the post-thermal annealing on condition that a thin SiO_x amorphous overlayer should be formed on the β-FeSi₂ layer during the post-thermal annealing. The mechanisms of the morphological modification at the β-FeSi₂/Si(111) interface by the post-thermal annealing will also be discussed.     

Texture of CoSi2 films on Si(111), (110) and (001) substrates

Synchrotron radiation was used to study the texture of polycrystalline CoSi₂ films that were formed by a solid-state reaction between a 30 nm Co film and Si(111), (110) and (001) substrates. All films were strongly textured, and several texture components were identified. We discuss the simultaneous occurrence of axiotaxy (i.e. alignment of lattice planes across the interface) and several different types of epitaxy in each of the films. Comparison of the different texture components observed on the three substrate orientations suggests a strong preference for the alignment of CoSi₂{110} planes in the film with Si{110} planes in the substrate, and twinning around Si[111] directions.     

Twinning of YBa2Cu3O7 thin films on different substrates

The twinning of YBa₂Cu₃O₇ (YBCO) thin films with c-axis orientation on (001) MgO, (001) SrTiO₃, (012) LaAlO₃, (110) NdGaO₃ and (001) NdGaO₃ substrates, prepared by laser ablation, has been examined using a combination of and θ/2θ scans at a four-circle diffractometer. On all substrates, except for (001) NdGaO₃, the tetragonal to orthorhombal phase transition results in four different orientations of YBCO twins relating to the substrate. On (001) NdGaO₃ only two different twin orientations, accompanied by a slight lattice monoclinization, has been observed.     

Structural models for epitaxy of hexagonal phases of boron nitride on Si(001)

Interface models for matching either w-BN or h-BN layers to an Si(001) substrate, are considered. Aligning the [2 − 1 − 10] direction of h-BN (or equivalently the [−1 − 120] direction in w-BN) along [1 − 10] the Si(001), chemical saturation of interface bonds can be achieved. For h-BN, orientations of the basal plane parallel and perpendicular to the surface are both considered. The total energy of atomic clusters constructed to represent these models, are calculated using the AM1 quantum chemical Hamiltonian, and atomic geometries optimised allowing full symmetric relaxation of interface atoms. Only the perpendicular h-BN basal plane model is found to be energetically favourable compared to earlier calculations for cubic boron nitride.     

Growth mechanism of epitaxial CoSi2 on Si and reactive deposition epitaxy of double heteroepitaxial Si/CoSi2/Si

The growth mechanism of epitaxial CoSi₂ was studied using Co/Ti/Si multilayer solid phase reaction. Results showed that phase formation was controlled by diffusion of Co through the growing CoSi_x, although at the early stage of CoSi₂ growth the diffusion of Co could be controlled by a Ti layer. A reactive deposition technique was also evaluated by using a conventional magnetron sputtering system. Results showed that an epitaxial CoSi₂ layer was formed by controlling the Co sputtering rate not to exceed the Co diffusion rate through CoSi_x. However, the surface of CoSi₂ became rough when the deposition rate was much slower than the Co diffusion rate through CoSi_x. The roughness was caused by the formation of CoSi₂ (111) facets at the interface between CoSi₂ and the Si substrate. Si/CoSi₂/Si double heteroepitaxial structures were fabricated when Si and Co were sequentially sputter-deposited on a Si (100) substrate.     

Preparation and electrical properties of Ca5Si3 and Sr5Si3 powders

Single phase Ca₅Si₃ and Sr₅Si₃ powders were prepared, and their electric and thermoelectric properties were investigated. The Ca₅Si₃ and Sr₅Si₃ powders are synthesized by exposure of the Si powders to Ca and Sr fluxes, respectively. It is found that both silicides show a p-type conduction and semiconductor-like behavior. The electronic band structures of the silicides are calculated using the first-principles total-energy calculation program in pseudopotential schemes with plane-wave basis functions. The calculated result predicts the possibility of a semiconductor-like property with a sharp pseudogap at the Fermi level for Ca₅Si₃, as experimentally obtained. The silicide would be expected to be a new semiconductor-like conducting material.     

Characterization of asymmetric rhombohedral twin in epitaxial α-Cr2O3 thin films by X-ray and electron diffraction

Epitaxial chromium oxide (α-Cr₂O₃) films grown by atomic layer deposition at 375 °C from CrO₂Cl₂ and CH₃OH on (1 1¯ 0 2) oriented α-Al₂O₃ have been studied by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and X-ray reflection (XRR). The thickness of the films ranged from 10 to 310 nm, and the average growth rate was 0.1 nm per deposition cycle. According to the XRD analysis, the orientation relationship in thinner films was (1 1¯ 0 2)[1 1 0]Cr₂O₃ || (1 1¯ 0 2)[1 1 0]Al₂O₃. Confirmed by the RHEED and XRD analyses, (1¯ 1 0 2) became the preferred growth plane at the thicknesses above 40 nm. This change has been interpreted as the appearance of an asymmetric rhombohedral twin with the orientation relationship between the layers (1¯ 1 0 2)[1 1 0]top || (1 1¯ 0 2)[1 1 0]bottom and (1¯ 1 0 2)[1 1¯ 1]top || (1 1¯ 0 2)[1¯ 1 1]bottom. The match of the anion and cation sublattices of both layers was characterized in terms of the structural model of the twin interface.     

Annealing effects on the structural properties of IrO2 thin films

We have demonstrated the structural and morphological changes of iridium oxide (IrO₂) films by the thermal annealing process. We have characterized the samples by using the X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The Ir-related XRD peaks predominantly appeared after the thermal annealing at 750-1000 °C. SEM images revealed that the films became quite uneven in thickness by annealing at 750 °C, whereas island-like structures were found to agglomerate on substrate surfaces by annealing at 1000 °C. From EDX and XRD analysis, we suggested that the agglomerated structures mainly consisted of Ir phase.     

Solid phase growth and properties of Mg2Si films on Si(111)

Solid phase technology of growing of epitaxial and textured Mg₂Si films on Si(111) is developed. It includes fabrication of template layer, deposition of multiplayer sandwich structure Mg/Si/ and final annealing. Research results are presented on morphology, structure and optical properties of thin Mg₂Si films grown on Si(111) by the method of solid-phase annealing on a template layer.     

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Optimization of ZnSe-SiO2 nanostructures deposited by radio-frequency magnetron sputtering: Correlations between plasma species and thin film composition, structural and microstructural properties

ZnSe nanoparticle doped SiO₂ films have been grown on various substrates at different deposition temperatures, radio-frequency power, Argon pressures and substrate to target distances, by means of reactive magnetron sputtering. A detailed study of the correlations between plasma species and thin film composition, structure and morphology is investigated using X-ray reflectivity and diffraction, Raman and optical emission spectroscopies and Rutherford backscattering technique. It is evidenced that the most sensitive species in the plasma is the Selenium and that the optimal deposition parameters correspond to random stress-free films with a high content of quasi-stoechiometric ZnSe cubic nanocrystallites. A few amount of ZnSe in the hexagonal structure is also evidenced in these films. Using proper deposition parameters, the SiO₂/ZnSe proportion in the films and the mean ZnSe particles size around 3 nm are easily monitored.     

Effects of p(O2) and p(CO2) on epitaxial growth of BaTiO3 thin films on MgO(100) substrates by using metal organic acid salts

BaTiO₃ thin films were prepared by using metal organic acid salts on MgO(100) substrates, which have large lattice-misfit with BaTiO₃. Amorphous films prefired at 470°C were crystallized to BaTiO₃ phase by heat treatment at higher temperature. Crystallinity and in-plane alignment of the prepared films were found to depend on the heat-treatment conditions. BaTiO₃ films with high crystallinity but poor (100)-orientation were obtained in air at higher than 1200°C. Whereas, (100)-oriented epitaxial BaTiO₃ film was fabricated by annealing at 900°C under low oxygen partial pressure (p(O₂)). Low carbon dioxide partial pressure (p(CO₂)) is also found to be essential for preparation of epitaxial BaTiO₃ films on MgO substrates by using metal organic acid salts.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates using pulsed-laser deposition technique. The crystalline structure and the surface morphology of the CCTO thin films were greatly affected by the substrate temperature and oxygen pressure. Thin films with a (2 2 0) preferential orientation were obtained at the substrate temperature above 700 °C and oxygen pressure above 13.3 Pa. The 480-nm thin films deposited under 720 °C and 26.6 Pa have a fairly high dielectric constant of near 2000 at 10 kHz and room temperature. The values of the dielectric constant and loss and their temperature-dependence under different frequency are comparable with those obtained in the epitaxial CCTO films grown on oxide substrates.     

Correlation between structural and opto-electronic properties of a-Si1 − xCx:H films deposited by plasma enhanced chemical vapour deposition

Hydrogenated amorphous silicon carbon alloy films of different carbon content were prepared by Plasma Enhanced Chemical Vapor Deposition using silane and methane with helium dilution and were characterized to study their opto-electronic, structural and defective properties. A linear correlation between micro structural disorder and overall disorder has been demonstrated. Further, it has been shown that the increase in the intrinsic disorder leads to an increase in the defect density while the increase in voids results in the decrease in the mass density for the studied films.     

Epitaxy of atomically flat CaF2 films on Si(1 1 1) substrates

The growth of CaF₂ films with a thickness of approximately 3–4 nm on well-oriented Si(1 1 1) substrates by molecular beam epitaxy at temperatures between 410 and 560 °C were investigated by ex vacuo atomic force microscopy. Layer-by-layer growth producing atomically flat CaF₂ surfaces has been observed in a very narrow growth temperature window between approximately 430 and 470 °C. Perfect triangular shaped islands of one CaF₂ layer height are found on the surface with all corners aligned with the Si directions, indicating a pure B-stacking of the CaF₂ film. Surprisingly, also the substrate steps have been overgrown without visible defects. Below 410 °C, two different island orientations revealed a mixture of A- and B-stacking areas in the films. Above 520 °C non-wetting of the CaF interface layer leads to epitaxial films with a rough surface morphology.     

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.