High-quality Single-crystal Films Research Articles

Structural and compositional characterization of single crystal uranium dioxide thin films deposited on different substrates

Uranium dioxide thin films were deposited on single crystal TiO2, Al2O3, YSZ, ZnO and NdGaO3 substrates to optimize conditions for the growth of high quality single crystal films. X-ray diffraction results show that all the films have one growth direction and well defined peaks in the specular scans with the expected symmetry for each growth orientation. The UO2/Al2O3, TiO2, and ZnO films have high concentration of misfit dislocations that increase with the lattice mismatch. The UO2 film on YSZ is found to be in registry with the substrate. The film has narrow mosaic component that is imposed upon a broader component arises from the diffuse scattering due to defects in the film. Meanwhile, UO2/NdGaO3 film shows a splitting of the X-ray diffraction peaks which is attributed to the in-plane asymmetry of the orthorhombic substrate.

Growth control of oxygen stoichiometry in homoepitaxial SrTiO3 films by pulsed laser epitaxy in high vacuum.

In many transition metal oxides, oxygen stoichiometry is one of the most critical parameters that plays a key role in determining the structural, physical, optical, and electrochemical properties of the material. However, controlling the growth to obtain high quality single crystal films having the right oxygen stoichiometry, especially in a high vacuum environment, has been viewed as a challenge. In this work, we show that, through proper control of the plume kinetic energy, stoichiometric crystalline films can be synthesized without generating oxygen defects even in high vacuum. We use a model homoepitaxial system of SrTiO3 (STO) thin films on single crystal STO substrates. Physical property measurements indicate that oxygen vacancy generation in high vacuum is strongly influenced by the energetics of the laser plume, and it can be controlled by proper laser beam delivery. Therefore, our finding not only provides essential insight into oxygen stoichiometry control in high vacuum for understanding the fundamental properties of STO-based thin films and heterostructures, but expands the utility of pulsed laser epitaxy of other materials as well.

Epitaxial growth of ultra-thin NbN films on AlxGa1−xN buffer-layers

The suitability of AlxGa1−xN epilayers to deposit onto ultra-thin NbN films has been demonstrated for the first time. High quality single-crystal films with 5 nm thickness confirmed by high resolution transmission electron microscopy (HRTEM) have been deposited in a reproducible manner by means of reactive DC magnetron sputtering at elevated temperatures and exhibit critical temperatures (Tc) as high as 13.2 K and residual resistivity ratio (RRR) ∼1 on hexagonal GaN epilayers. On increasing the Al content x in the AlxGa1−xN epilayer above 20%, a gradual deterioration of Tc to 10 K was observed. Deposition of NbN on bare silicon substrates served as a reference and comparison. Excellent spatial homogeneity of the fabricated films was confirmed by R(T) measurements of patterned micro-bridges across the entire film area. The superconducting properties of these films were further characterized by critical magnetic field and critical current measurements. It is expected that the employment of GaN material as a buffer-layer for the deposition of ultra-thin NbN films will prospectively benefit terahertz electronics, particularly hot electron bolometer (HEB) mixers.

Epitaxial growth of cubic Mg0.45Zn0.55O thin films on SrTiO3 (001) substrate with MgO buffer layer

Cubic Mg0.45Zn0.55O thin films with MgO buffer layer were epitaxially grown on SrTiO3 substrate by radio frequency oxygen plasma assisted molecular beam epitaxy. X-ray diffraction measurements indicate high quality single crystal films of (200) orientation. The growth follows cubic-on-cubic alignment with epitaxial relationship of Mg0.45Zn0.55O (001)[100]//MgO (001)[100]//SrTiO3 (001)[100]. Atomic force microscopy measurements reveal a smooth surface with the rms roughness of 0.36nm in an area of 5×5μm2. The band gap of the film is calculated to be 4.8eV and in the solar blind range. Since the MgO buffer layer can suppress the sub-band-gap photoresponse from the SrTiO3 substrate and does not absorb moisture like the bare MgO substrate, the Mg0.45Zn0.55O thin films with MgO buffer layer grown on SrTiO3 substrate have promising applications in solar blind ultraviolet photodetectors.

Electron Microscopy Study of Single Crystal BaZr0.9Y0.1O3-x Films Prepared by Chemical Solution Deposition

Single crystal thin films of 10% yttrium doped barium zirconate (BZY10) have been prepared using chemical solution deposition on magnesium oxide substrates and annealed at 800 or 1000ºC. A focused ion beam lift out technique was used to prepare thin (< 100 nm) specimens for transmission electron microscopy (TEM) analysis. A variety of TEM based techniques were used for characterization, including high resolution imaging, X-ray energy dispersive spectroscopy and energy filtered TEM. High resolution TEM imaging indicated a high quality single crystal film with an epitaxial cube on cube type interface between BZY10 and MgO. For the sample annealed 1000ºC, annular dark field imaging in scanning transmission electron microscopy (STEM) mode showed a layered pattern of lower intensity in the single crystal film. Energy filtered TEM thickness map together with scanning electron microscopy (SEM) indicate the pattern consists of partially repeating voids.

The FMR behavior of an ultrathin single Fe layer on a GaAs substrate

Single-layer Fe films 800 Å thick, with and without a 10-Å-thick Ag buffer layer, were prepared at various growtht emperature,T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> , on GaAs(001) substrates by molecular-beam epitaxy. For T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> around 200 °C, high-quality single crystal films were obtained without the Ag buffer layer. For T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ⩽100 °C, a defect structure was observed in the iron layer. However, the addition of an ultrathin Ag buffer layer between GaAs and Fe has the desirable effects of eliminating the defect structure and significantly enhancing the FMR absorption.

Electronic Property of Thin Single-Crystal Films of α-Al2O3on Ru(0001)

High-quality single-crystal films of α-Al 2 O 3 with various thicknesses below 35 A have been fabricated on the Ru(0001) surface. The phase transtion from γ- to α-Al 2 O 3 has been used for fabrication. The thickness dependence of the band gap for the oxide layer has been measured by the interband transition using electron energy loss spectroscopy. The band gap has been found to decrease with decreasing oxide thickness. The same tendency of this extraordinary result has been observed in scanning tunneling spectroscopy. These results can be interpreted by the many-body effect.

Dielectric Investigation of BaTiO3 Thin-Film Capacitor

The dielectric properties of BaTiO3 thin film were investigated on a highly c-axis-oriented epitaxial thin crystal. It was confirmed by X-ray diffraction that the film thickness was 120 Å and that it was high-quality single-crystal film. The spread of crystal orientation of the BaTiO3 thin film was the same as that of SrTiO3 substrate. However, the dielectric properties of this thin film were different from those of the BaTiO3 bulk crystals and were explained by the space-charge effect. The space-charge polarization of this film strongly depends on temperature and this behavior was observed after the incubation time had passed. The experimental results are discussed in terms of the interaction between the polarities of domain and space charges.

Growth of b-axis rare earths on sapphire by molecular beam epitaxy

A process is described whereby hcp rare-earth metals can be grown heteroepitaxially as high-quality single-crystal films with the b axis normal to the growth plane. The growth employs molecular beam epitaxy, starting from available sapphire substrates. The results of characterization by several techniques are described.

Crystal Ion-Slicing of Magnetic and Ferroelectric Oxide Films

AbstractThe epitaxial separation of single-crystal magnetic and ferroelectric oxide films is presented. Ion implantation is used to create a buried damage layer beneath the surface. The high etch-selectivity of this sacrificial layer makes it possible to detach high quality single-crystal films from bulk samples. Magnetic and electrical properties of the films are discussed.

Epitaxial growth of β-SiC thin films using bis-trimethylsilylmethane on Si(100) with a polycrystalline buffer layer

Epitaxial β-SiC thin films were successfully grown on carburized Si(100) substrates using bis-trimethylsilylmethane (BTMSM) as an organo-silicon precursor at deposition temperatures as low as 1100 °C. TEM analysis revealed that the carburized buffer layer was not a single crystal but well-aligned polycrystals. Very smooth and high quality single crystal films were obtained at 4.8 × 10 4 Pa. The defect density of the β-SiC thin films deposited to 5 μm thickness using BTMSM was as low as that of other commercial films deposited using gas sources. It appears that the carburized buffer layer, which was partially polycrystalline, reduces the out-diffusion of the free Si species from the substrate.

Hydrogen surface coverage: Raising the silicon epitaxial growth temperature

The chemisorption of molecular hydrogen onto the Si (100) surface is shown to disrupt the epitaxial growth of silicon and silicon/germanium alloys grown by molecular beam epitaxy. It is only after the substrate temperature is raised above the hydrogen desorption temperature, or the deposition rate is lowered, that high quality single-crystal films can be grown. The results also suggest the surface segregation of hydrogen during growth.

MBE growth of single crystal α-Fe films on ZnSe (001) and (110)

We report the growth by molecular beam epitaxy oh high quality single crystal films of ferromagnetic α-Fe on ZnSe (001) and (110) epilayers grown on chemically etched and annealed GaAs wafers. The films were characterized by reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), X-ray diffraction (θ-2θ), ferromagnetic resonance (FMR) and vibrating sample magnetometry. RHEED demonstrates single crystal Fe film growth in registry with the ZnSe surface, and indicates that the surface order of the Fe (110) films is superior to that of the Fe (001) films. The FMR linewidth and measured coercive fields are significantly smaller than previously reported for similar Fe films grown directly on the GaAs substrates. Room temperature FMR spectra (35 GHz) of thin Fe (001) films (∼140 Å) exhibit symmetric Lorentzian lineshapes with a linewidth of 45 Oe, which represents the narrowest 35 GHz FMR linewidth ever measured in single crystal iron, and approaches the theoretical lower limit set by Landau-Lifschitz broadening.

Growth of single crystal bcc α-Fe on ZnSe via molecular beam epitaxy

Molecular beam epitaxy methods have been used to grow high quality single crystal films of bcc α-Fe on fcc (zinc blende) ZnSe (001) epilayers on GaAs (001). These films were characterized by reflection high-energy electron diffraction, Auger electron spectroscopy, ferromagnetic resonance (FMR), and vibrating sample magnetometry. The FMR linewidth and measured coercive field are significantly smaller than previously reported for single crystal Fe films.

Properties and ion implantation of AlxGa1−xN epitaxial single crystal films prepared by low pressure metalorganic chemical vapor deposition

High quality single crystal films of AlxGa1−xN have been grown on sapphire substrates using low pressure metalorganic chemical vapor deposition at 915 °C over the entire range of x. Electrical and optical properties of the layers were measured. Electrical compensation by Be+ and N+ implants in the layers was studied. Schottky barriers were fabricated and their current voltage characteristics were studied. We also report some initial results of a study of GaN growth kinetics.