Heteroepitaxial Thin Films Research Articles

Stress-induced domain structure formation in nanodimensional barium strontium titanate films

Thin heteroepitaxial films of BaxSr1−x TiO3 (BST) with x = 0.80 possessing high structural perfection have been grown by RF magnetron sputtering on MgO(001) substrates. The microstructure and crystallographic characteristics of the BST films of various thicknesses have been studied using electron microscopy and X-ray diffraction. It is established that there is a threshold thickness at which the BST crystal lattice parameter in the direction perpendicular to the substrate exhibits a sharp change.

Structure and Dielectric Properties of Heteroepitaxial PMNT Thin Films

Thin films of (1−x)PbMg 1/3 Nb 2/3 O 3 -xPbTiO 3 (PMNT) with x = 0 to 0.35 were epitaxially grown on (100)SrTiO 3 (ST) and (100) MgO substrates by magnetron sputtering. Typical film thickness was 300 to 2000 nm. La-doped conductive (100) ST and Pt coated MgO, (100) Pt/MgO, were used for their dielectric measurements. XRD and cross-sectional TEM studies suggested that the hetero-epitaxial PMNT thin films comprised single crystal-like structure with (001) orientation. The dielectric response showed frequency dispersion similar to bulk relaxor-like behavior except extremely broad temperature anomaly. The maximum relative dielectric constant, ϵ* m , for PMNT thin films at x = 0.23 was 900 to 2000. The ϵ* m ncreased with the increase of PbTiO3. For PMNT thin films at bulk morphotropic phase boundary condition, x = 0.35, the ϵ* m was 5000 which was still lower than bulk values. The low dielectric constant is probably due to the presence of strained hetero-epitaxial layer having temperature independent dielectric properties. The strained PMNT thin films of 1800 nm thick showed d 33 = 120 pC/N at ac applied voltage of 50 V.

Growth and structure of heteroepitaxial thin films of homologous compounds RAO 3( MO) m by reactive solid-phase epitaxy: Applicability to a variety of materials and epitaxial template layers

A reactive solid-phase epitaxy (R-SPE) method combines deposition of a thick amorphous or polycrystalline layer with a desired chemical composition and post-deposition solid-phase epitaxial growth. The solid-phase epitaxial growth is invoked by thermal annealing with an assistance of a sacrificial layer working as an epitaxial template. Thereby it enables us to grow high-quality epitaxial films of complex oxides whose epitaxial films are not grown by conventional high-temperature growth techniques. It was reported that 2-nm-thick ZnO layers worked as template for growing InGaO 3(ZnO) m ( m = integer) epitaxial films. The present study extended the R-SPE technique to growth of various complex oxides with chemical compositions of RAO 3( MO) m and to use of various epitaxial template layers. We found that mono oxide epitaxial layers such as In 2O 3 and Ga 2O 3 work as template layers as well. Alternatively, a ZnO epitaxial layer is also applicable to ZnO-free compounds. The films obtained were grown heteroepitaxially on YSZ(111) and single-crystalline when the fabrication conditions are optimized.

Growth, structure and carrier transport properties of Ga 2O 3 epitaxial film examined for transparent field-effect transistor

Growth conditions of heteroepitaxial thin films of tin-doped Ga 2O 3 were surveyed from the viewpoint of visible application to field-effect transistors (FETs). Films were deposited by pulsed laser deposition, and post-annealing was examined to improve film structures. Atomically flat surfaces were obtained for films grown on yttria-stabilized zirconia (111) plane and post-annealed at 1400 °C, but they were insulating. Conductive heteroepitaxial films applicable to FETs were obtained on α-Al 2O 3 (0001) at specific deposition conditions, i.e. substrate temperatures from 500 to 550 °C and oxygen pressures from 5 × 10 − 4 to 1 × 10 − 3 Pa. It was found that the resulting epitaxial films have a crystal structure different from that of β-Ga 2O 3. The crystal lattice for the films is determined to be orthorhombic with a large possibility of a higher-symmetry hexagonal or rhombohedral system. The films exhibited high transparency in the near infrared–deep ultraviolet region and had bandgap of ∼ 4.9 eV. The operation of top-gate MISFETs using the Ga 2O 3 film for the n channel was demonstrated.

First-principles study of substrate-mediated interactions on a compressed Ag(111) surface

We used density-functional theory (DFT) to resolve interactions between Ag atoms on a compressed Ag(111) surface from the first to the 53rd neighbor distance, a separation range of over $2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. We find that these interactions are primarily pairwise and electronic in origin. A spatially resolved plot of the pair interactions reveals that they form concentric rings that alternate between attraction and repulsion. Calculations of the band structure of the strained Ag(111) surface reveal that there is a surface state at the $\overline{\ensuremath{\Gamma}}$ point just below the Fermi level, similar to what is observed for unstrained Ag(111). The band structure of the strained Ag(111) surface is consistent with the magnitude and spatial dependence of the oscillatory interactions that we find in the DFT calculations. A comparison between a theory developed to describe adsorbate pair interactions mediated by surface-state electrons and our DFT results indicates that the theory describes the DFT results in an average way at distances beyond the third neighbor---although the DFT results contain angular variations in the interactions not predicted by the theory. At short adsorbate separations, we find that interactions at the first-neighbor and second-neighbor distances are strongly attractive and indicative of direct chemical bonds between the adatoms. The third-neighbor interaction is repulsive and is possibly mediated by bulk electrons. The results of these studies may be useful in the design of metal thin heteroepitaxial films.

Pulsed Laser Deposition에 의해 SrRuO3/SrTiO3기판위에 여러 가지 증착온도에서 증착된 Pb(Zr0.2Ti0.8)O3박막의 특성

[ ] heteroepitaxial thin films were deposited at various temperatures on single crystal substrates by pulsed laser deposition and characterized for the microstructural and ferroelectric properties. The substartes etched by buffered oxide etch solution for 20s followed by the thermal annealing at for 1h showed the terrace ledges with a 0.4nm height. The bottom electrodes with a thickness of 52nm grown on single crystal also exhibit a terrace ledge similar to that of . The PZT thin films were grown with an epitaxial relationship and showed typical P-E hysteresis loops shown at the epitaxial films. The 56nm thick-PZT films deposited at exhibit a remanent polarization of and a coercive field of 160kV/cm.

β -phase-domain-free α-MnAs thin films on GaAs(001) by postgrowth annealing

Postgrowth annealing effects on a heteroepitaxial MnAs thin film grown on a GaAs(001) substrate have been investigated. The β-MnAs phase domains of an as-grown sample, observed as dark stripes in the surface topography at room temperature, disappear completely after postgrowth annealing. In support of this finding, the paramagnetic contribution to the magnetic hysteresis loop arising from the β-MnAs phase domains is also not observed at 300K. We attribute the origin of these effects to relaxation of the elastic strain in the MnAs thin film.

Phase-field modeling of stress-induced surface instabilities in heteroepitaxial thin films

A phase-field model for investigating the surface morphological evolution of a film is developed, taking into account the surface energies of film and substrate, the interfacial energy between the film and substrate, and the elastic energy associated with the lattice mismatch between the film and substrate. Using the lattice mismatch and the surface energies for the Ge∕Si heteroepitaxial system, the morphology of islands and the formation of a wetting layer are investigated using two-dimensional simulations. The results show that the wetting angle increases continuously with the increase in the lattice mismatch, and the surface angle of the island on wetting layer varies with the island size. It is demonstrated that the anisotropy of elastic interactions alone is not sufficient to cause surface angle discontinuity or faceting that is observed in experiments.

Switchable single c-domain formation in a heteroepitaxial PbTiO3 thin film on a (001) Nb–SrTiO3 substrate fabricated by means of hydrothermal epitaxy

We used hydrothermal epitaxy to fabricate a heteroepitaxial PbTiO3 (PTO) thin film on a Nb-doped (001) cubic SrTiO3 (NSTO) substrate while avoiding the phase transition. By means of transmission electron microscopy and piezoresponse force microscopy analyses, it was confirmed that no a domain formed in the heteroepitaxial PTO film; the film had a single +c-domain structure at an as-synthesized state. From the measurement of the polarization-voltage hysteresis curve, large remanent polarization (2Pr:144μC∕cm2) resulting from the single +c-domain structure and an imprint induced by an asymmetric electrode configuration were observed. It is suggested that the single +c-domain structure was caused by the interface of the PTO film and solution rather than the interface of the PTO film and NSTO electrode under the hydrothermal conditions. This hydrothermally synthesized PTO film is anticipated to be very suitable for high-performance engineering applications.

Understanding organic–inorganic heteroepitaxial growth of molecules on crystalline substrates: Experiment and theory

The physical structure of organic--inorganic heteroepitaxial thin films is usually governed by a fine balance between weak molecule--molecule interactions and a small lateral variation of the molecule--substrate interaction potential. In order to investigate the energetics of such a layer system, one has to consider large molecular domains [Mannsfeld and Fritz, Phys. Rev. B 69, 75416 (2004)]. So far, layer potential calculations for large domains of organic thin films on crystalline substrates were difficult to perform concerning the computational effort which stems from the huge number of atoms which have to be included. Here, we present a technique which enables the calculation of the molecule--substrate interactions for large molecular domains by utilizing potential energy grid files. As an application example, we discuss the growth of titanylphthalocyanine (TiOPc) on Au(111). TiOPc is found to grow homogeneously in extended domains on top of the alternating domains of the Au(111) surface reconstruction, locally exhibiting a point-on-line coincident epitaxy with the substrate lattice. Here, we demonstrate that the optimal structure and orientation of the TiOPc lattice in such extended overlayer domains can be predicted by potential optimization calculations using the technique described herein.

Hetero-Epitaxial Growth of YBCO Thin Films on the A-Cut Plane Sapphire Substrates

We examined the hetero-epitaxial growth of YBCO thin films on the a-cut plane sapphire substrates for the passive microwave applications, because the a-cut plane sapphire substrates have smaller in-plane anisotropy of the dielectric constant compared with that of r-cut plane sapphire substrates. The CeO/sub 2/ buffer layers and YBCO thin films were prepared by an inductive-coupled plasma sputtering method. We found that perfect in-plane alignment of the CeO/sub 2/ buffer layer could be obtained on the a-cut plane sapphire substrates. Also we could obtain the hetero-epitaxial YBCO thin films on CeO/sub 2//a-cut plane sapphire substrates. Tc of the YBCO thin films was approximately 89 K, and the surface resistance (Rs) of the YBCO thin films was approximately 1 m/spl Omega/ at 50 K and at 22 GHz, equivalently to values for YBCO thin films fabricated on the r-cut plane sapphire substrates.

Characterization of stress induced in SOS and Si/γ-Al 2O 3/Si heteroepitaxial thin films by Raman spectroscopy

Raman spectroscopy technique has been performed to investigate the stress induced in as-grown silicon-on-sapphire (SOS), solid-phase-epitaxy (SPE) re-grown SOS, and Si/γ-Al 2O 3/Si double-heteroepitaxial thin films. It was demonstrated that the residual stress in SOS film, arising from mismatch and difference of thermal expansion coefficient between silicon and sapphire, was reduced efficiently by SPE process, and that the stress in Si/γ-Al 2O 3/Si thin film is much smaller than that of as-grown SOS and SPE upgraded SOS films. The stress decrease for double heteroepitaxial film Si/γ-Al 2O 3/Si mainly arises from the smaller lattice mismatching of 2.4% between silicon top layer and the γ-Al 2O 3/Si epitaxial composite substrate, comparing with the large lattice mismatch of 13% for SOS films. It indicated that γ-Al 2O 3/Si as a silicon-based epitaxial substrate benefits for reducing the residual stress for further growth of silicon layer, compared with on bulk sapphire substrate.

1806 エピタキシャル薄膜のひずみ誘起島形態形成Phase-fieldシミュレーション(J06-1 マイクロ・ナノ構造と変形解析,J06 計算マイクロ・ナノメカニクス)

The morphological evolution of a strained heteroepitaxial thin film of Si_<1-x>Ge_x/Si during deposition has been investigated by two-dimensional phase-field simulations. The developed phase-field model can create the facet morphologies which are modeled using the generalized gradient correction coefficient for a crystal with a high anisotropy of surface energy, and can simulate the sequence of shape transitions which is well known in Si_<1-x>Ge_x/Si(001) system: 2D wetting layer, faceted 3D pyramids, and multifaceted domes. The effects of Ge composition, mobility for phase field and deposition rate on the island formation process and the island evolutions have been studied here. Some typical and important phenomena observed, in previous experimental studies, during Si_<1-x>Ge_x/Si(001) heteroepitaxy have been able to simulate by the developed phase-field model.

Growth of heteroepitaxial ZnO thin film and ZnO∕(Mg,Zn)O nanomultilayer by off-axis rf magnetron sputtering

Heteroepitaxial ZnO thin film was deposited on a sapphire (001) substrate using off-axis radio-frequency magnetron sputtering. The crystallinity of the ZnO thin film was affected by deposition pressure, substrate temperature, and postannealing temperature. The low-temperature photoluminescence of heteroepitaxial ZnO thin film grown under optimum conditions of 650°C, 120W, and 10mTorr showed strong UV emission at 3.36eV with a full width at half maximum (FWHM) of 16.0meV. After annealing in an O2 ambient at 950°C, the high-resolution x-ray diffraction rocking curve FWHM of the ZnO thin film markedly decreased from 0.38° to 0.19° although UV emission decreased. These results indicated that a heteroepitaxial ZnO thin film with strong UV emission can be grown by off-axis rf sputtering and that O2 annealing helps enhance the crystallinity of ZnO thin film which can be used as a buffer layer in ZnO∕(Mg,Zn)O multiple quantum well structures. To demonstrate an application of the ZnO thin film, a ZnO∕(Mg,Zn)O nanomultilayer was grown on a ZnO-buffered sapphire substrate and its optical properties were measured.

Chirally oriented heteroepitaxial thin films grown by pulsed laser deposition: Pt(621) on SrTiO3(621)

Pulsed laser deposition has been used to grow Pt(621) thin films having a chiral orientation on SrTiO3(621) substrates. Films were deposited over a range of conditions, then characterized for their crystallinity and epitaxy using x-ray diffraction and for their surface morphologies using atomic force microscopy (AFM). Films deposited at 600°C grew epitaxially, but with an island growth mode. Pt deposited at 250°C displayed smooth surfaces but did not grow epitaxially. A three-step process wherein films were seeded at 600°C, deposited at 250°C, and postannealed was shown to result in excellent epitaxial growth and flat surface morphologies. Films grown using the three-step process exhibited excellent epitaxy, having the orientation relationship (621)Pt‖(621)SrTiO3:[01¯2]Pt‖[01¯2]SrTiO3. When postannealed at 800°C, they exhibited flat surfaces (rms≈20Å) having long terraces separated by steps running along the expected [01¯2] direction of the SrTiO3(621) substrate. Films postannealed at 600°C were flatter (rms≈10Å) with no evident step directionality. The SrTiO3(621) substrates were observed to be extremely flat (rms≈2Å) and to have morphologically featureless surfaces, as expected for a (621) surface. The good crystalline quality of the SrTiO3 substrate surface is considered to drive the growth of the chirally oriented heteroepitaxial Pt films. Similarities and differences between Pt growth on chiral surfaces and on low-index achiral surfaces are discussed.

Heteroepitaxial growth of wide gap p-type semiconductors: LnCuOCh (Lr=La, Pr and Nd; Ch=S or Se) by reactive solid-phase epitaxy

Heteroepitaxial thin films of Cu-based oxychalcogenides, LnCuOCh (Ln = La, Pr and Nd; Ch = mixture of S and Se), were fabricated and their resulting film structures were characterized. A reactive solid-phase epitaxy method successfully yielded heteroepitaxial films on MgO(001) substrates. A high-resolution electron microscopic examination of a LaCuOS film revealed a sharp film-substrate interface. Four-axes high-resolution X-ray diffraction measurements revealed that crystalline lattices in the films are fully relaxed and that the crystallographic orientation is (001)[110] LnCuOCh || (001)[110] MgO. Furthermore, systematic variations in the lattice constant by lanthanide or chalcogen ion substitutions were observed.

Room-temperature magneto-optical activity of InMnAs thin films

Room-temperature magneto-optical activity of In1−xMnxAs heteroepitaxial thin films grown on InAs (001) and GaAs (001) substrates has been studied using the longitudinal magneto-optical Kerr effect. For single-phase layers grown on InAs and GaAs with x ranging from 0.03 to 0.09, square hysteresis loops that indicate ferromagnetic order were observed. The magnitude of the saturation Kerr rotation of all the InMnAs samples ranged from 0.29to1.1mdeg, and scaled with the manganese concentration. The shape of the hysteresis curves for samples of InMnAs grown GaAs was invariant with respect to rotation about the sample normal, indicating in-plane magnetic isotropy. In contrast, InMnAs layers on InAs exhibited strong uniaxial in-plane anisotropy with the easy axis of magnetization along the [110] and [1¯1¯0] directions. The magnetic anisotropy is attributed to the effect of epitaxial strain.

Nanocrystalline Alkaline Earth Titanates and Their Conductivity Characteristics Under Changing Oxygen Ambients

This work presents the first systematic study of conductivity characteristics of alkaline earth titanates in the form of polycrystalline and heteroepitaxial thin films as well as nanocrystalline ceramics as a function of temperature (between 600_∘C and 1000_∘C) and continuously adjustable oxygen partial pressures ranging from 10− 20 bar to 1 bar. Compared to the well-known log,σ-log, pO2 profiles of single crystals, the conductivity behavior of CSD-prepared, polycrystalline SrTiO3 thin films with a feature size of about 50 nm differs radically. The most prominent characteristics are a sharp drop under reducing conditions followed by a broad plateau region. Tailored investigations on heteroepitaxial as well as polycrystalline thin films grown by PLD and especially by studies on nanocrystalline BaTiO3 ceramics with a mean grain size of ≤100 nm allowed an unambiguous assignment of the described effects to the nanocrystalline morphology of the samples.

Efficient phase-field simulation of quantum dot formation in a strained heteroepitaxial film

A Cahn–Hilliard evolution equation possessing a source term is employed to study the morphological evolution of a strained heteroepitaxial thin film, during continuous mass deposition, on a substrate with an embedded coherent island. The elastic properties and the surface energy are anisotropic, with the surface energy anisotropy being strong enough to result in missing orientations and facets. A sophisticated finite-difference/multigrid method and an implicit time integration scheme are combined to make an efficient numerical method, one which enables numerically tractable computation in both two and three dimensions. Herein we present preliminary two-dimensional results demonstrating the utility of our finite difference/multigrid algorithms. The strain localization effects produced by a buried, coherent inclusion are shown to produce laterally organized quantum dots during the morphological evolution of the film.

Fabrication of heteroepitaxial thin films of layered oxychalcogenides LnCuOCh (Ln = La–Nd; Ch = S–Te) by reactive solid-phase epitaxy

Processes and preparation conditions for growing epitaxial thin films of Cu-based, layered oxychalcogenides LnCuOCh (Ln = La, Ce, Pr or Nd; Ch = S1-xSex or Se1-yTey) are reported. Epitaxial thin films on MgO (001) substrates were prepared by a reactive solid-phase epitaxy method. Four-axes high-resolution x-ray diffraction measurements revealed that the crystallographic orientation is (001)[110] LnCuOCh || (001)[110] MgO and the internal stress of the crystalline lattices in the films are relaxed during thermal-annealing process of the reactive solid-phase epitaxy. Furthermore, except for CeCuOS, systematic variations in the lattice constant by chalcogen or lanthanide ion substitutions were observed. These results demonstrated that the reactive solid-phase epitaxy is an efficient technique for fabricating LnCuOCh epitaxial films.