Pulsed Organometallic Beam Epitaxy Research Articles

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 and injected into the O 2 microwave plasma with pulse time durations of a few seconds. The precursors are injected through pneumatic valves in a heated valve box. The valves and microwave power are under computer control. The microwave plasma is ramped between a forward power of 600 and 1500 W to improve film epitaxy. We use POMBE to grow epitaxial BaYZrO 3/MgO, Y–ZrO 2/LAO, and YBa 2Cu 3O 7/Y–ZrO 2/LAO structures. The processing parameters leading to the heteroepitaxy are described. The best epitaxy results in X-ray FWHM of 0.12°, 0.38°, and 0.87° for BaYZrO 3, Y–ZrO 2, and YBa 2Cu 3O 7, respectively. We show the advantages of the POMBE technique over that of plasma-enhanced CVD. Selected TEM results of the heteroepitaxial oxide structures are shown, and the role that temperature plays in the oxide epitaxy. The epitaxy of BaYZrO 3 is the first described in the literature, and that of YSZ is among the best reported.

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 YBa2Cu3O7-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 BaxMg1-xO 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° oriented growth to the 0° oriented growth.

ChemInform Abstract: Stability of Bilayer Films of YBa2Cu3O7 and Y—ZrO2 Grown on LaAlO3 by Pulsed Organometallic Beam Epitaxy (POMBE).

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The effect of the reactant gas on YBa 2Cu 3O x film formation

The reactant gas used as the deposition ambient has a large effect on the composition and properties of cuprate superconductor films. In this study YBa 2Cu 3O x 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 YBa 2Cu 3O x films which are both fluorine-free and fully oxygenated are presented.

The growth of (001) YBa 2Cu 3O (7−δ) thin films on (001) MgO by pulsed organo—metallic beam epitaxy with controlled in-plane orientation

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 (<≈0.7×10 15 Ba cm −2) the films grow [110]YBCO|[100]MgO. For thick BaO layers (>≈1.1×10 15 Ba cm −2) the films grow [100]YBCO|[100]MgO. For YBCO films grown with thin BaO layers (>≈0.7 times 10 15 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.

Bi-epitaxial grain boundaries in YBa2Cu3O7−x thin films prepared by pulsed laser deposition and pulsed organometallic beam epitaxy: Direct comparison of transport properties and grain boundary structure

A set of 45° [001] bi-epitaxial YB2Cu3O7−x thin film grain boundaries was studied to compare the effects of the microstructure on transport properties. The grain boundaries were made using two different deposition techniques: pulsed laser deposition (PLD) and pulsed organometallic beam epitaxy (POMBE). The transport properties were highly dependent on the specific growth conditions used, resulting in both fully resistive and superconducting grain boundaries. Subsequent microstructural analysis of the measured boundaries showed that both types (superconducting and resistive) meandered on the length scale of hundreds of nanometers. The major structural difference between the boundaries was at the atomic scale where the resistive boundary had a 1 nm wide disordered region. The direct correlation of microstructure to transport properties demonstrates the importance of the atomic scale structure in the resulting transport behavior.

The formation, transport properties and microstructure of 45° [001] grain boundaries induced by epitaxy modification in YBa 2Cu 3O 7− x thin films

Tilt grain-boundary junctions with a 45° [001] misorientation were formed in YBa 2Cu 3O 7− x (YBCO) thin films grown by pulsed organometallic beam epitaxy on (100) MgO substrates. The junctions were introduced at predetermined locations due to a modification of the orientation relation between the thin film and substrate following a low-energy argon ion irradiation of specific areas of the substrate surface prior to film deposition. Rutherford backscatter spectrometry and certain surface features observed by atomic force microscopy indicate that implantation of ions is necessary to cause the modified epitaxy. The low-temperature transport characteristics of individual isolated grain boundaries were determined by electromagnetic characterizations of the junction behavior. The same grain boundaries were examined by transmission electron microscopy and high-resolution electron microscopy and it was found that the boundaries are for the most part free of precipitates and well structured at the atomic scale. Regardless of the average grain boundary inclination, asymmetric (110)(100) facets dominate the microstructure of the junctions. Possible mechanisms for epitaxy modification and the transport properties in relation to the observed microstructure are discussed.

Microstructure of artificial 45° [001] tilt grain boundaries in YBCO films grown on (001) MgO

It is well known that high-angle grain boundaries in YBa2Cu3O7−x (YBCO) show weak-link effects and behave as Josephson junctions. This kind of grain boundary junction (GBJ) has potential applications in magnetic field measurement and electronic devices. This work studies the microstructure of artificially made GBJs in YBCO films on (001) MgO and the mechanism of the boundary formation, with the goal to improve the GBJ quality and obtain a better understanding of the junctions’ transport properties.Ion-sputter-induced epitaxy is used to form YBCO films with isolated 45° [001] tilt grain boundaries. Prior to YBCO film growth the (001) MgO substrate is selectively sputtered by a low energy Ar ion beam. A portion of the substrate remains non-sputtered by protecting the surface with a patterned photoresist mask. After removing the mask, a YBCO film is grown on the substrate using pulsed organometallic molecular beam epitaxy (POMBE). Under suitable conditions single crystal YBCO c-axis films can be reproducibly obtained in both the sputtered and non-sputtered regions. The orientation between the films and the substrate has been examined by both x-ray diffraction and electron diffraction. The in-plane orientation relation is [110]YBCO//[100]MgO on the non-sputtered epitaxially polished MgO and [100]YBCO//[100]MgO on the sputtered MgO. Thus, 45° tilt boundaries are formed in the film at the boundary between the sputtered and non-sputtered substrate regions.

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 (&amp;lt;≊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.

Fabrication and characteristics of weak links between â- and ĉ-axis normal grains of Y1Ba2Cu3O7−x

We have used pulsed organometallic beam epitaxy (POMBE) to simultaneously deposit â- and ĉ-axis oriented Y1Ba2Cu3O7−x (YBCO) thin films at arbitrary locations on LaAlO3(100) substrates. Using photolithography and ion milling, several types of â-ĉ weak links have been fabricated at the boundary between the two films. The current-voltage (I-V) characteristics show a flux flow type behavior. The resistive transitions are broad and the critical current density is low, indicating weak coupling across these boundaries. With magnetic field applied parallel to the grain boundary plane, nonhysteretic I-V curves are obtained and the critical current goes to zero at an applied magnetic field of ∼7500 G.

The mechanism of sputter-induced orientation change in YBCO films on MgO (001)

AbstractThe mechanisms of the sputter-induced orientation change in YBa2Cu3O7-x(YBCO) films grown on MgO (001) substrates by pulsed organometallic beam epitaxy (POMBE) are investigated by x-ray diffraction, Rutherford backscatter spectroscopy (RBS), cross-section TEM (XTEM) and microanalysis. It is found that the W atom implantation concurring with the ion sputtering plays an important role in effecting the orientation change. This implantation changes the surface structure of the substrate and induces an intermediate layer in the initial growth of the YBCO film, which in turn acts as a template that induces the orientation change. It seems that the surface morphology change caused by ion sputtering has only a minor effect on the orientation change.

YBa2Cu3O7−δ on MgO films grown by pulsed organometallic beam epitaxy and a grain boundary junction application

MgO films and YBa2Cu3O7−δ/MgO multilayer films were developed with the pulsed organometallic beam epitaxy (POMBE) growth technique, and grain boundary junctions were fabricated from the films to demonstrate the utility of the multilayers. High-quality MgO films were grown on LaAlO3 substrates by POMBE using a Mg(dpm)2 precursor. MgO crystallinity, as assessed by x-ray diffraction rocking curves, improved with the use of CuOx or YBa2Cu3O7−δ buffer layers. YBa2Cu3O7−δ films grown on the MgO layer by POMBE exhibited a Tc0 of 83 K and a Jc (12 K) exceeding 106 A/cm2 for applied magnetic fields up to 3 × 104 G. Grain boundary junctions were formed by growing YBa2Cu3O7−δ on MgO films that had been pretreated with a simple sputtering technique. This sputtering induces a controlled, 45°grain boundary in subsequently deposited YBa2Cu3O7−δ films. The resulting boundary showed weak-link current-voltage behavior and an IcRn product of 52 μV at 10 K, demonstrating that sputter-induced grain boundary junctions are compatible with multilayer technology.

Integrated multilayer sputter-induced 45° YBa2Cu3O7−x grain boundary junctions

A versatile multilayer technique has been developed to form 45° YBa2Cu3O7−x [001] tilt grain boundary junctions on LaAlO3 substrates. An epitaxial MgO layer is initially deposited on a (100) LaAlO3substrate using pulsed organometallic beam epitaxy (POMBE). After a pregrowth sputter treatment, an YBa2Cu3O7−x thin film is then grown using POMBE. The resultant film is c-axis oriented with a cube-on-cube orientation over the unsputtered portion of the MgO, and rotated by 45° about the [001] axis on the sputtered region of the substrate. The resulting grain boundary junction shows weak-link behavior. The advantage of this technique is the ability to place the grain boundary anywhere on the substrate in any configuration, and the potential to use any substrate upon which MgO can be epitaxially grown.

Sputter-induced grain boundary junctions in YBa2Cu3O7−x thin films on MgO

A low voltage argon ion sputter technique was used to form grain boundary junctions in YBa2Cu3O7−x thin films on MgO. The YBa2Cu3O7−x thin film grown on a pre-sputtered region of MgO was rotated 45° about the [001] axis relative to the YBa2Cu3O7−x thin film grown on an adjacent unsputtered region of the substrate. YBa2Cu3O7−x thin films were grown using pulsed organometallic beam epitaxy (POMBE). The current-voltage and resistance-temperature characteristics of individual grain boundary junctions demonstrated weak-link-type behavior. Sputter-induced 45° grain boundary junctions are advantageous in device applications because they are planar and simple to form in many configurations.

Microwave surface resistance of YBa 2Cu 3O 7-δ thin films deposited by pulsed organometallic beam epitaxy

The microwave surface resistance of superconducting YBa 2Cu 3O 7-δ thin films deposited by pulsed organometallic beam epitaxy (POMBE) has been characterized using the parallel plate transmission line resonator method. POMBE is an advanced organometric chemical vapor deposition technique where precursor vapors are precisely metered onto the substrate under computer control. In this study, the POMBE reactor was used to deposit epitaxial films of varying thickness onto LaAlO 3 substrates. The deposition procedure and surface-resistance results for films of varying thicknesses are described. The reduction of surface resistance achieved supports the use of the POMBE technique as a possible method for preparing device-quality high- T c films and multi-layer structures.

Surface morphology studies of Y-Ba-Cu-oxide thin films prepared by pulsed organometallic beam epitaxy

Pulsed organometallic beam epitaxy is a new technique for the deposition of complex metal oxide films. With this technique each metal precursor species is sequentially pulsed onto the substrate. The flux of the precursor is monitored and computer feedback control is used to adjust the size of each precursor pulse, allowing precise control of the metal stoichiometry. Smooth films of YBa 2Cu 3O 7−δ, as examined by SEM, can be made with this technique in a temperature range that includes 640–680 °C using the β-diketonate precursors, Y(dipivalolymethanate) 3, Ba(hexafluoroacetylacetonate) 2-tetraglyme, and Cu(acetylacetonate) 2. The morphology of the films is dependent on the ratio of the three precursors delivered to the substrate. The ratio of the three precursors must be held to better than +-10% in order to produce smooth films. The ratio between the three precursors needed to produce a smooth film changes with the deposition temperature. The relative amount of yttrium precursor delivered to the substrate must be decreased as the temperature is increased in order to maintain a smooth film. The metal composition of the smooth films is Y 1Ba 2Cu 3 (±4%).

Direct Correlation of Transport Properties and Microstructure In Y1Ba2Cu3O7-x Thin Film Grain Boundaries

AbstractIndividual 45° [001] tilt grain boundaries in Y1Ba2Cu3O7-x thin films grown on biepitaxial substrates were studied. The thin films were grown using both pulsed organometallic beam epitaxy (POMBE) and laser ablation. Transport characteristics of the individual grain boundaries were measured including resistance - temperature (R-T) and current - voltage (I-V) dependencies with and without an applied magnetic field. In order to elucidate possible structural origins of the differences in transport behavior, the same grain boundaries which were electrically characterized were subsequently thinned for electron-microscopy analysis. Transmission-electron-microscopy and high-resolution-electron-microscopy were used to structurally characterize the grain boundaries. The macroscopic and microscopic structures of two boundaries, a nominally resistive and a superconducting grain boundary, are compared.

Superlattices of YBa2Cu3O7−δ/PrBa2Cu3O7−δ grown by the pulsed organometallic beam epitaxy method

We report on the growth of YBa2Cu3O7−δ/PrBa2Cu3O7−δ superlattices by pulsed organometallic beam epitaxy technique. X-ray diffraction and analytical high resolution transmission electron microscopy measurements show high-quality superlattice crystalline structures are being grown. Electrical measurements of these layered oxides have also shown very promising results. This work demonstrates the capabilities of pulsed organometallic chemical vapor deposition as an alternative method for growing high temperature superconducting oxides in a controlled manner.

Microstructures of YBCO Thin Films Prepared by Pulsed Organometallic Beam Epitaxy

ABSTRACTThe microstructure of Y1Ba2Cu3Oy films and Y1Ba2Cu3Oy / Pr1Ba2Cu3Oy multilayers prepared by a pulsed organometallic beam epitaxy (POMBE) technique have been characterized by transmission electron microscopy (TEM). The microstructure of the films is observed to vary as a function of substrate temperature. At low temperatures, films are polycrystalline and exhibit some impurity phases. At higher substrate temperatures, films grow epitaxially with smooth surfaces and few impurities. Controlled growth at sufficiently high substrate temperatures allows uniform growth of alternating Y1Ba2Cu3Oy and Pr1Ba2Cu3Oy layers.

Pulsed organometallic beam epitaxy of complex oxide films

We report the results of a pulsed organometallic beam epitaxy (POMBE) process for growing complex oxide films at low background gas pressure (10−4–10−2 Torr) and low substrate temperature (600–680 °C) using organometallic precursors in an oxygen plasma environment. Our results show that POMBE can extend the capability of organometallic chemical vapor deposition to growing complex oxide films with high precision both in composition and structure without the need for post-deposition oxidation and heat treatments. The growth of phase-pure, highly oriented Y-Ba-Cu-O superconducting oxide films {[Tc (R=0)=90.5 K] and Jc (77 K, 50 K gauss)=1.1×105 A/cm2} is given as an example. Similar to the pulsed laser deposition process, the POMBE method has the potential for in situ processing of multilayer structures (e.g., junctions).