William J. DeSisto

GROWTH MODEL FOR GAN WITH COMPARISON TO STRUCTURAL, OPTICAL, AND ELECTRICAL PROPERTIES

A kinetic model is presented to explain the metal organic vapor phase epitaxy (MOVPE) growth of GaN. The model is based upon measured desorption rates and assumptions on the precursor dissociation and sticking probabilities. The model shows how the growth temperature and V/III ratio are linked for the growth of high quality GaN films. From a comparison of growth conditions cited in the literature to the quality of GaN produced, optimal film growth appears to occur when the V/III ratio is chosen to be slightly larger than the N to Ga desorption ratio. The relationship between the growth temperature, V/III ratio, and GaN quality are explained in terms of how the growth parameters influence the incorporation of Ga and N atoms into the growing film. The Ga and N diffusion lengths are estimated to be 2–20 nm and <1 nm at 1050 °C, respectively, for practical MOVPE growth rates. Growth conditions for smooth (0001) surface morphology are described in terms of the growth model, as well as possible origins for defe...

Ba(1−x)SrxTiO3 thin films by off-axis cosputtering BaTiO3 and SrTiO3

Thin films of Ba(1−x)SrxTiO3 (BST) have been deposited by off-axis cosputtering targets of SrTiO3 (STO) and BaTiO3 (BTO). The films were grown on (100) MgO and LaAlO3 substrates at 550 °C at an argon/oxygen gas pressure of 20 Pa. Reaction of the sputtered species in the plasma resulted in the growth of BST films. The film composition was set by the relative power levels of the STO and BTO targets. X-ray diffractometry indicated the as-deposited films were predominantly (h00) oriented. The calculated lattice parameter was 3.955±0.01 A, slightly larger than bulk Ba0.5Sr0.5TiO3. Postannealing the films in flowing O2 at 780 °C resulted in greater peak heights and no change in orientation. Capacitance–voltage characteristics measured at 10 GHz yielded Q’s of 65 with nearly 14% tuning for BST on LAO and Q>1000 with a 2% tuning for films on MgO.

Deposition of (100) oriented MgO thin films on sapphire by a spray pyrolysis method

Thin films of magnesia have been deposited by a novel spray pyrolysis method. An aqueous magnesium acetylacetonate solution was ultrasonically nebulized, transported in flowing oxygen, and thermally decomposed on silicon (100), sapphire, and fused silica at temperatures between 400 and 550 °C. The films were from 0.1 to 0.5 μm thick, optically transparent, and smooth. The MgO films were poorly crystalline as deposited. MgO films on sapphire crystallized with strong (100) orientation after annealing at 700 and 930 °C in flowing oxygen.

Metalorganic chemical vapor deposition of low microwave surface resistance YBa2Cu3O7 on (100) LaAlO3 and (100) SrTiO3

Thin films of YBa2Cu3O7−δ on (100) LaAlO3 and (100) SrTiO3, up to 5000 A thick, were grown by metalorganic chemical vapor deposition. The as‐deposited films were c‐axis oriented, and had transition temperatures between 87 and 89 K, transition widths less than or equal to 1 K, and critical current densities of 1–3×106 A/cm2 at 77 K. Microwave surface resistance measurements at 36 GHz on (100) LaAlO3 showed significant improvements over copper metal at 77 K. Films grown on (100) SrTiO3 exhibited slightly better properties than those grown on (100) LaAlO3. Films which were slightly deficient in barium and rich in yttrium consistently demonstrated superior properties.

Preparation and characterization of MgO thin films deposited by spray pyrolysis of Mg(2,4-pentanedionate)2

Abstract Thin films of magnesia (0.1 – 0.5 μm) were deposited by a spray pyrolysis method. Aqueous and alcoholic solutions of magnesium (2,4-pentanedionate) 2 were ultrasonically nebulized and thermally decomposed onto a variety of substrates at temperatures between 400 and 540°C. Strong (100) orientation of MgO was observed on alumina soated sapphire as deposited from alcoholic solutions at 470°C. In addition, strong (100) orientation was observed on samples prepared from thermally decomposing aqueous solutions on sapphire followed by oxygen anneals at 700 and 93°C.

Gas phase ultraviolet spectroscopy of high‐temperature superconductor precursors for chemical vapor deposition processing

UV spectroscopy has been used as an in situ diagnostic to measure the gas phase concentration of 2,2,6,6‐tetramethyl‐3,5‐heptanedionate (thd) complexes of Cu and Y in metalorganic chemical vapor deposition (MOCVD) bubbler effluent. These Cu and Y precursors for MOCVD synthesis of YBa2Cu3O7−δ (YBCO) show marked instability in gas phase concentration as a function of time during the initial bubbler purge. The precursors show ideal behavior as predicted by the Clausius–Clapeyron relation. The UV diagonstic technique has the potential of controlling precursor concentration for MOCVD fabrication of high‐temperature superconducting thin films.

Athermal annealing of phosphorus-ion-implanted silicon

A 1 cm2 area in phosphorus-implanted silicon samples is annealed by irradiation of a much smaller 0.002 cm2 area with a single laser pulse. Resistivity of the annealed region is uniform and similar to that measured after thermal annealing. Electrically activated donors did not diffuse into the sample and only slightly towards the sample surface. The process is 100% reproducible. We present evidence that the annealing is not caused by heat.

Metalorganic chemical vapor deposition of double‐sided YBa2Cu3O7 thin films

Thin films of YBa2Cu3O7−δ were deposited on both sides of (100) LaAlO3 substrates by metalorganic chemical vapor deposition. Using a gold‐coated wafer carrier, improvements in both the transition temperature and critical current density of the film exposed to the carrier (first side) were demonstrated. Tc’s of 86–88 K and Jc’s≥106 A/cm2 at 77 K were achieved. The microwave surface resistance of both sides of double‐sided samples measured at 77 K was approximately 8 mΩ at 36 GHz, scaling to less than 700 μΩ at 10 GHz.

Critical current characteristics of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on [110] SrTiO/sub 3/

The material and electrical characteristics of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) thin films deposited by inverted cylindrical magnetron sputtering on [110] SrTiO/sub 3/ (STO) were investigated. X-ray diffractometry shows the grain orientations to be predominantly the YBCO [110] and [103] with no evidence of c-axis grains, Electron micrographs show the film surface to consist of coupled elongated grains parallel to the [110] STO edge. The films were patterned into small 2.5 mm squares parallel to the substrate edges for electrical characterization. Transport currents parallel and perpendicular to the [110] substrate edge showed a 945:1 anisotropy in film resistance and a factor of two in critical current density for temperatures below 60% of the transition temperature (T/sub c/). The temperature dependence of the critical current near T/sub c/ was quadratic-like and strongly dependent on the value of T/sub c/ used in the analysis. For the two orientations, there was nearly a 6 K difference in T/sub c/ as determined by the point at which the critical current became zero. The response of the critical current to small magnetic fields was greater for transport current along the c-axis direction and was observable over a temperature interval nearly four times greater than for current along the basal plain. These YBCO thin films have good response to small magnetic fields and are suitable for vortex flow device development.

Microwave properties of double-sided YBa/sub 2/Cu/sub 3/O/sub 7- delta / thin films deposited by metalorganic chemical vapor deposition

High-quality thin films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / have been deposited by metalorganic chemical vapor deposition on both sides of 15-mm*15-mm <100> LaAlO/sub 3/ substrates without requiring the use of heat-sinking compounds. Typical transport data exhibited T/sub c/s >87 K, transition widths <1 K, and J/sub c/s >10/sup 6/ A/cm/sup 2/ (measured inductively at 77 K, 0 T). Microwave surface resistance measurements (36.4 GHz) at temperatures between 20 K and 100 K, made using a copper-cavity end-wall-replacement technique, showed a sharp drop through the transition temperature and yielded values at 77 K that were better than a 20* improvement over copper metal when scaled to 10 GHz. Double-sided coatings have been obtained by sequentially depositing each side of the substrate. Contamination of the backside during growth of the first-side film and of the first-side film during growth of the second-side film has been avoided by coating the susceptor with gold.<<ETX>>