P. R. Markworth

Selective-area atomic layer epitaxy growth of ZnO features on soft lithography-patterned substrates

Templated ZnO thin-film growth from the vapor phase is achieved on docosyltrichloro- silane-patterned Si substrates using atomic layer epitaxy (ALE) combined with soft lithography. Patterned hydrophobic self-assembled monolayers (SAMs) are first transferred to single-crystal Si surfaces by hot microcontact printing. Using diethylzinc and water as ALE precursors, crystalline ZnO layers are then grown selectively on the SAM-free surface regions where native hydroxy groups nucleate growth from the vapor phase. High-resolution ZnO patterns with 1.0–40 μm feature sizes are readily achieved, demonstrating that soft lithography combined with ALE is a simple and promising methodology for selective area in situ vapor phase fabrication of patterned oxide thin films.

Coherent island formation of Cu2O films grown by chemical vapor deposition on MgO(110)

Cuprous oxide (Cu 2 O) films have been grown on single-crystal MgO(110) substrates by a chemical vapor deposition process in the temperature range 690–790 °C. X-ray diffraction measurements show that phase-pure, highly oriented Cu 2 O films form at these temperatures. The Cu 2 O films are observed to grow by an island-formation mechanism on this substrate. Films grown at 690 °C uniformly coat the substrate except for micropores between grains. However, at a growth temperature of 790 °C, an isolated, three-dimensional island morphology develops. Using a transmission electron microscopy and atomic force microscope, both dome- and hut-shaped islands are observed and are shown to be coherent and epitaxial. The isolated, coherent islands form under high mobility growth conditions where geometric strain relaxation occurs before misfit dislocation can be introduced. This rare observation for oxides is attributed to the relatively weak bonding of Cu 2 O, which also has a relatively low melting temperature.

Buffers for high temperature superconductor coatings. Low temperature growth of CeO2 films by metal–organic chemical vapor deposition and their implementation as buffers

Abstract Smooth, epitaxial cerium dioxide thin films have been grown in-situ in the 450–650°C temperature range on (001) yttria-stabilized zirconia (YSZ) substrates by metal–organic chemical vapor deposition (MOCVD) using a new fluorine-free liquid Ce precursor. As assessed by X-ray diffraction, transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM), the epitaxial films exhibit a columnar microstructure with atomically abrupt film-substrate interfaces and with only minor bending of the crystal plane parallel to the substrate surface near the interface and at the column boundaries. With fixed precursor temperature and gas flow rate, the CeO 2 growth rate decreases from ∼10 A/min at 450°C to ∼6.5 A/min at 540°C. The root-mean-square roughness of the films also decreases from 15.5 A at 450°C to 4.3 A at 540°C. High-quality, epitaxial YBa 2 C 3 O 7− x films have been successfully deposited on these MOCVD-derived CeO 2 films grown at temperatures as low as 540°C. They exhibit T c =86.5 K and J c =1.08×10 6 A/cm 2 at 77.4 K.

In‐plane orientation control of (001)YBa2Cu3O7−δ grown on (001)MgO by pulsed organometallic beam epitaxy

Thin films of (001) YBCO are grown on epitaxially polished (001) MgO by pulsed organometallic beam epitaxy. The in‐plane orientation of the film is controlled by the thickness of a BaO layer, grown in situ, prior to the YBCO growth. For thin BaO layers (<≊7×1014 Ba/cm2) the films grown [110]YBCO∥[100]MgO. For thick BaO layers (≳≊11×1014 Ba/cm2) the films grow [100]YBCO∥[100]MgO. A mechanism that relates the change in YBCO in‐plane orientation to a change in the structure of the initial BaO layers with BaO thickness is described.

Epitaxial stabilization of orthorhombic cuprous oxide films on MgO(110)

Continuous epitaxial films of cuprous oxide (Cu 2 O) have been formed by the thermal oxidation of 1.5-μm-thick Cu metal films deposited on MgO(110) substrates. These films melted at 1118 °C in air, in agreement with equilibrium phase diagrams. Upon cooling from the liquid, a highly crystalline, epitaxial, 2.5-μm-thick Cu 2 O film was formed. X-ray diffraction spectroscopy revealed that the Cu 2 O film crystal structure was orthorhombically distorted from the bulk cubic crystal structure. High-resolution transmission electron microscopy showed that the film is coherent, and energy dispersive x-ray spectroscopy showed that interdiffusion is limited to the interface. These results suggest that a new epitaxially stabilized phase of Cu 2 O has been formed.

The growth of (001) YBa2Cu3O(7−δ) thin films on (001) MgO by pulsed organo—metallic beam epitaxy with controlled in-plane orientation

Abstract Thin films of (001) YBCO are grown on epitaxially polished (001) MgO by pulsed organo–metallic beam epitaxy. The in-plane orientation of the film is controlled by the thickness of a BaO layer, grown in situ, prior to the YBCO growth. For thin BaO layers ( ≈1.1×1015 Ba cm−2) the films grow [100]YBCO|[100]MgO. For YBCO films grown with thin BaO layers (>≈0.7 times 1015 Ba cm−2), ex situ, low energy Ar+ ion sputtering of the MgO surface prior to film growth will also induce a change of in-plane orientation from [110]YBCO|[100]MgO on epitaxial polished MgO to [100]YBCO|[100]MgO on Ar+ sputtered MgO. A mechanism that relates the change in YBCO in-plane orientation to a change in the structure of the initial BaO layers with BaO thickness is described.

Heteroepitaxial oxide structures grown by pulsed organometallic beam epitaxy (POMBE)

We describe the design, construction, and use of pulsed organometallic beam epitaxy (POMBE), a plasma-enhanced CVD technique to grow oxide heterostructures. Solid-state precursors are sampled in the gas line via quartz crystal monitors andinjectedinto the O 2 microwave plasma with pulse time durations of a few seconds. The precursors are injectedthrough pneumatic valves in a heatedvalve box. The valves andmicrowave power are under computer control. The microwave plasma is rampedbetween a forwardpower of 600 and1500 W to improve film epitaxy. We use POMBE to grow epitaxial BaYZrO3/MgO, Y–ZrO2/LAO, andYBa 2Cu3O7/Y–ZrO2/LAO structures. The processing parameters leading to the heteroepitaxy are described. The best epitaxy results in X-ray FWHM of 0.121, 0.381, and 0.871 for BaYZrO3, Y–ZrO2, andYBa 2Cu3O7, respectively. We show the advantages of the POMBE technique over that of plasma-enhancedCVD. SelectedTEM results of the heteroepitaxial oxid e structures are shown, andthe role that temperature plays in the oxide epitaxy. The epitaxy of BaYZrO3 is the first described in the literature, and that of YSZ is among the best reported. r 2002 Elsevier Science B.V. All rights reserved.

The mechanism of sputter-induced epitaxy modification in YBCO (001) films grown on MgO (001) substrates

The sputter-induced epitaxy change of in-plane orientation occurring in YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} (001) thin films grown on MgO (001) substrates by pulsed organo-metallic beam epitaxy (POMBE) is investigated by a series of film growth and characterization experiments, including RBS and TEM. The factors influencing the orientation change are systematically studied. The experimental results suggest that the substrate surface morphology change caused by the ion sputtering and the Ar ion implantation in the substrate surface layer are not the major factors that affect the orientation change. Instead, the implantation of W ions, which come from the hot filament of the ion gun, and the initial Ba deposition layer in the YBCO film growth play the most important roles in controlling the epitaxy orientation change. Microstructure studies show that a Ba{sub x}Mg{sub 1{minus}x}O buffer layer is formed on top of the sputtered substrate surface due to Ba diffusion into the W implanted layer. It is believed that the formation of this buffer layer relieves the large lattice mismatch and changes the YBCO film from the 45{degree} oriented growth to the 0{degree} oriented growth. {copyright} {ital 1998 Materials Research Society.}

The effect of the reactant gas on YBa2Cu3Ox film formation

Abstract The reactant gas used as the deposition ambient has a large effect on the composition and properties of cuprate superconductor films. In this study YBa2Cu3Ox thin films are deposited by pulsed organometallic beam epitaxy (POMBE). The barium precursor used contains fluorine and it is therefore possible to incorporate unwanted fluorine into the deposited film. For our deposition process both the fluorine content and oxygen content of the film are highly dependent on the reactant gas used as the deposition ambient. Many of the reaction conditions that remove fluorine also deplete the film of oxygen. In this study a set of reaction conditions that produce YBa2Cu3Ox films which are both fluorine-free and fully oxygenated are presented.