A-oriented Films Research Articles

Terahertz photoconductive antennas based on silicon-doped GaAs (111)A

In this study we investigate a relatively new material for terahertz (THz) photoconductive antennas (PCAs): GaAs epitaxial films grown on (111)A-oriented semi-insulated GaAs substrates and doped with Si. GaAs:Si (111)A films with the required high resistance without postgrowth annealing have been grown by molecular-beam epitaxy. The Si doping of (111)A-oriented GaAs was expected to lead to the formation of acceptors that facilitate the activation of As[Formula: see text] traps. The relaxation times of nonequilibrium charge carriers in the films were measured in pump-probe experiment, and, finally, the electron mobility was estimated from the carrier lifetime and current–voltage characteristics measured under pulsed pumping by Ti:sapphire femtosecond laser. Dipole and bow-tie PCAs were fabricated, and the THz emission spectra and the total THz emission power were measured with varying applied voltage and optical excitation power. The properties of GaAs:Si (111)A films and the PCAs based on them are compared with LTG-GaAs (100) and (111)A-oriented films.

Crystallography and Microstructure of 7M Martensite in Ni-Mn-Ga Thin Films Epitaxially Grown on (1 1 2¯ 0)-Oriented Al2O3 Substrate.

Epitaxial Ni-Mn-Ga thin films have been extensively investigated, due to their potential applications in magnetic micro-electro-mechanical systems. It has been proposed that the martensitic phase in the <1 1 0>A-oriented film is much more stable than that in the <1 0 0>A-oriented film. Nevertheless, the magnetic properties, microstructural features, and crystal structures of martensite in such films have not been fully revealed. In this work, the <1 1 0>A-oriented Ni51.0Mn27.5Ga21.5 films with different thicknesses were prepared by epitaxially growing on Al2O3(1 1 0) substrate by magnetron sputtering. The characterization by X-ray diffraction technique and transmission electron microscopy revealed that all the Ni51.0Mn27.5Ga21.5 films are of 7M martensite at the ambient temperature, with their Type-I and Type-II twinning interfaces nearly parallel to the substrate surface.

Characteristics of ferroelectric field effect transistors composed of a ferroelectric Bi3TaTiO9 gate stack and a single-layer MoS2 channel

Polycrystalline Bi3TaTiO9 (BTTO) thin films of layered perovskite ferroelectric materials were deposited on a Pt/Ta/glass substrate via a pulsed laser deposition (PLD) method using two different PLD deposition rates of 0.03 and 0.5 nm/pulse under the same substrate temperature conditions. The BTTO thin film grown by the low PLD deposition rate exhibited a highly a-oriented crystal structure, which was confirmed by X-ray diffraction experiments. The crystallinity of the a-oriented film caused an improvement in the ferroelectric polarization and piezoelectric coefficients; at the same time, the leakage current characteristics were slightly deteriorated. Ferroelectric field effect transistors (FeFETs), made of monolayer MoS2 channels and BTTO thin films as a gate dielectric layer, exhibited clearly different memory windows as a nonvolatile memory. In addition, the reliability of the FeFETs was confirmed by fatigue tests up to 1012 switching cycles and retention tests up to 106 s.

Nanoscale investigation of ferroelectric and piezoelectric properties in (Pb,Ca)TiO3 thin films grown on LaNiO3/LaAlO3(1 0 0) and Pt/Si(1 1 1) using piezoresponse force microscopy

Single phase polycrystalline and strong textured Pb0.76Ca0.24TiO3 (PCT24) thin films have been grown on platinum coated silicon and LaAlO3(100) (LAO) substrates using LaNiO3 (LNO) as a buffer layer by chemical solution deposition. X-ray diffraction measurements showed that the PCT24 thin films crystallize in a highly a-oriented single phase on LNO/LAO(100). The effects of the LNO buffer layer, nature of the substrate, and film orientation on the ferroelectric and piezoelectric properties were investigated in the nanoscale range using piezoresponse force microscopy (PFM). Local piezoelectric hysteresis loops for PCT24/LNO/LAO(100) oriented films and PCT24/Pt/Si polycrystalline films were measured on selected grains. From these piezoloops, PCT24/LNO/LAO(100) oriented films show a higher response (maximum relative d33 value) than PCT24/Pt/Si polycrystalline films. Furthermore, the comparison of simultaneously acquired surface morphology and piezoresponse images of the PCT24/Pt/Si films revealed the presence of inactive grain regions. In contrast, highly a-oriented films showed a higher presence of active grains. Our observations suggest that the improvement of ferro/piezoelectric properties is greatly associated with the use of LNO buffer layers and the growth of highly a-oriented films.

Continuous change of supersaturation and evolution of oriented structure in dipping LPE process of YBa2Cu3O7−δ

Liquid phase epitaxy (LPE) of YBa2Cu3O7−δ (YBCO) films was performed by vertical dipping along both the [001] and the [110] directions of (110) NdGaO3 (NGO) substrates. Remarkably, an evolution of oriented structure from c-axis to a-axis, corresponding to the supersaturation (σ) change from high to low level, was explicitly observed on a single NGO substrate. Distinctively, creeping along the [001] direction and forming a low-σ-related a-oriented film with a crack-free macrostructure, the liquid presented a strong anisotropic wettability with the NGO substrate. Most importantly, this work provides a unique method to achieve high-quality a-axis YBCO LPE films, which are potentially appropriate for Josephson junction devices.

On defects in highly a-oriented MFI membranes

Previously, uniformly a-oriented MFI films were synthesized via the secondary growth of an a-oriented seed layer, which was obtained by chemically depositing ∼2μm thin-along-the-a-axis particles on a silica-coated α-Al2O3 porous disc (Choi et al., Angew. Chem. Int. Ed. 45 (2006) 1154–1158). Despite the uniformity of the a-orientation throughout the film, we found that the MFI membranes are not applicable for p-/o-xylene separation since they have significant crack and grain boundary defects. Accordingly, a-oriented MFI films were fabricated via alternative methods, motivated by the expected high separation performance that could be comparable to p-/o-xylene separations (100–400 separation factor) of a b-oriented MFI membrane. First, smaller 500nm thin-along-the-a-axis MFI particles (small seeds) were used instead of 2μm thin-along-the-a-axis particles (large seeds) for seed deposition. Second, a seed layer was formed by a sonication-assisted physical deposition of the large seeds. Finally, an a-oriented seed layer comprising of the large seeds was epitaxially-grown to be the a-oriented MFI film by conducting two consecutive hydrothermal growth processes. The separation performance of the resultant MFI membranes was investigated via n-/i-butane and p-/o-xylene permeation tests. Maximum separation factors (SF) of ∼4 and ∼17 were achieved for p-/o-xylene and n-/i-butane isomers, respectively. These values are below the expected performance, indicating the presence of non-zeolitic pathways in a-oriented films.

Mechanism of Growth of Cr2O3 Thin Films on (11̄02), (112̄0), and (0001) Surfaces of Sapphire Substrates by Direct Current–Radio Frequency Magnetron Sputtering

With the aim of directly controlling the magnetization in a ferromagnetic (FM) metal/Cr2O3 multilayer by means of an electric field without field cooling, the mechanism of growth of magnetoelectric (ME) Cr2O3 thin films on r-, a-, and c-cut sapphire substrates is investigated. In all the films, epitaxial growth is observed with a unit step on the film surface. The growth difference, which appears during the initial growth stage (at about 1 nm film thickness), is observed in the oriented films on each substrate surface. This growth difference arises from the different surface energies and the presence of twin grains due to the dislocation of Cr atoms. The twin-free r-oriented film contains coalesced grains with deep trenches at the grain boundaries, which release film stress and maintain flat grain surfaces because of the lowest surface energy. The a-oriented film shows anisotropic grains with the unit step and no trenches because the film starts to grow in a three-dimensional mode. The c-oriented film contains twin grains with the lowest full width at half-maximum of the rocking curve for the Bragg diffraction and a surface roughness of 0.17 nm. The twin grains in the c-oriented film contain a mixture of Cr atoms with up and down spins, which would simultaneously induce both an increase and a decrease in the exchange bias magnetic field (HEB) under an applied electric field through the ME effect. Therefore, the magnetization depends on the ratio of the twin grains. The Cr spins of the twin-free r-oriented film are expected to show FM ordering, indicating that HEB can be increased or decreased by an applied electric field. This r-oriented film is thus a promising surface for direct magnetization control using an electric field.

Phase stability of heteroepitaxial polydomain BaTiO3 thin films

The phase stability of ferroelectric, epitaxial, polydomain BaTiO3 thin films was examined using temperature-dependent x-ray diffraction (XRD) and in-plane electronic polarization measurements. The epitaxial BaTiO3 thin films were grown on MgO(100) substrates by a metal-organic chemical vapor deposition process. As-deposited and annealed BaTiO3 thin films with different domain structures were examined. Temperature-dependent plane-normal XRD analysis reveals well-defined phase transitions at 140 and 169 °C in the c- and a-oriented films, respectively. The measured Curie temperatures are consistent with those predicted by Landau-Ginsburg-Devonshire theory as applied to polydomain BaTiO3 thin films. Temperature-dependent in-plane electronic polarization measurements confirm that the 140 °C Curie temperature observed in the c-oriented film is a well-defined second-order paraelectric-ferroelectric transition.

Defects of YBCO films grown by liquid-phase epitaxy on NdGaO 3

Defects observed on a- and c-oriented YBa 2Cu 3O 7− x (YBCO) films grown by liquid-phase epitaxy (LPE) on (1 1 0) and (0 0 1) NdGaO 3 (NGO) are discussed. The orientation-dependent cracking and twinning of the films is shown and related to the misfit-induced strain. The film thicknesses and crack distances of LPE-grown YBCO layers were measured by Nomarski microscopy and by scanning electron microscopy. For a-oriented films on (1 1 0) NGO, with an estimated uniaxial effective film strain of 1.8% ( c f ||d s ), a good fit with theory is found. For c-oriented layers on (1 1 0) NGO, an effective strain of 0.08% ( a f /b f ||d s ) is found. This value is lower than that predicted from the theory and this might be explained by the strain relaxation due to twinning.

Structural characteristics of [formula omitted]-axis-oriented PrBa 2Cu 3O 7 thin films grown by pulsed laser deposition

PBCO films have been successfully fabricated at different temperatures. X-ray diffraction results show that the orientation of PBCO films varies with decreasing deposition temperature. The optimal deposition condition for a-axis oriented PBCO films is 15 Pa oxygen pressure and 700°C. Raman scattering measurements indicate that the 514 cm −1 of A g phonon mode can be used as a criterion for determining whether the film is a-axis-oriented or not, e.g., the intensity of 514 cm −1 mode is the strongest for c-axis-oriented film but almost disappears for a-oriented film. High-resolution transmission electron microscopy confirms that our a-axis-oriented film has a good structural quality.

Study of doping of InxGa1−xAs films with germanium

InxGa1−xAs: Ge films (x ≦ 0.12) grown form the solution in a broad temperature range are investigated. It is found that the donor density in the solid phase increases with increasing growth temperature and fraction of In in the solution while the acceptor density is practically independent of these parameters and the growth direction. It is observed that Nd(111)B > Nd(111)A and (Nd/Na)(111)B > (Nd/Na)(111)A for all the liquid phase compositions investigated. Therefore p(111)B < p(111)A, and this inequality becomes stronger with increasing x. The (111)B films have a less homogeneous profile of the carrier distribution along the thickness than the (111)A-oriented films. An inhomogeneity of this profile becomes greater with increasing fraction of In in the solution. Under some conditions, a p–n junction, whose distance from a substrate rises with increasing x, is formed in films having the (111)B orientation. [Russian Text Ignored].