David B. Beach

Decomposition mechanisms of SiHx species on Si(100)‐(2×1) for x=2, 3, and 4

Silane adsorption at a surface temperature of 150 K and the surface decomposition of SiH3 and SiH2 have been investigated on the Si(100)‐(2×1) surface using static secondary ion mass spectrometry (SSIMS) and temperature programmed desorption (TPD). Silane dissociatively chemisorbs at 150 K to form SiH3 and H. At saturation, the combined coverage of these two is approximately 0.4 groups/1st layer Si atom (0.2 SiH4 adsorbed/1st layer Si atom). Using SiH4, the surface coverage of SiH3 species is varied, and the coverage‐dependant kinetics of SiH3 decomposition are examined using temperature programmed SSIMS. Changes in SiH4 exposure and source of SiH3 (di‐ vs monosilane) cause changes in surface SiH3 stability. The stability changes are interpreted as due to blocking of empty sites (dangling bonds, db) required for SiH3 decomposition to SiH2 and H. It is shown here that the decomposition temperature of SiH3 can vary from 200 to 600 K, depending on the dangling bond coverage (θdb). Subsequently, evidence for ...

Decomposition of silane on Si(111)‐(7×7) and Si(100)‐(2×1) surfaces below 500 °C

Using static secondary ion mass spectrometry (SIMS) to observe the silicon hydride species formed by silane adsorption on atomically clean single crystal silicon surfaces, two distinct adsorption mechanisms are identified. Dissociation to SiH3 plus H occurs on the Si(100)‐(2×1) surface, which contains pairs of dangling bonds located on Si dimers (with Si–Si distance ≊2.4 A). In contrast, SiH2 formation in the adsorption step is indicated on the Si(111)‐(7×7) surface, where adjacent dangling bonds are separated by more than 7 A. Lower limits on the silane reactive sticking coefficient (SR) are evaluated using hydrogen coverage (ΘH) measurements after calibrated SiH4 exposures, and this limit is ≊10−5 for 25 °C gas and 100–500 °C surface temperatures. Within experimental error, SR is the same for both mechanisms on the two clean surfaces (ΘH near zero). Dependence of SR on ΘH is reported at 400 °C for both surfaces, and differences appear as ΘH exceeds 0.1 H/Si. Silane adsorption is weakly activated on Si(1...

Epitaxial growth of La2Zr2O7 thin films on rolled Ni-substrates by sol-gel process for high Tc superconducting tapes

Abstract A solution process was used to grow epitaxial La 2 Zr 2 O 7 (LZO) buffer layers on roll-textured Ni (100) substrates to produce YBa 2 Cu 3 O 7− δ (YBCO)-coated conductors. The LZO precursor solution was prepared by an all alkoxide sol–gel route using mixed metal methoxyethoxides in 2-methoxyethanol. The partially hydrolyzed solution was either spin-coated or dip-coated onto the textured Ni substrates. The amorphous thin film was then heat treated at 1150°C under (96%)Ar/(4%)H 2 atmosphere for 1 h. X-ray diffraction (XRD) of the buffer layer indicated a strong c -axis orientation on the Ni (100) substrate. The LZO (222) pole figure revealed a single cube-on-cube texture. SEM images of the LZO buffer layer showed a dense microstructure without cracks. The YBCO deposited on the sol–gel LZO-buffered Ni substrates with sputtered YSZ and CeO 2 top layers had a critical current density of 480,000 A/cm 2 at 77 K and self-field.

Fabrication of long lengths of YBCO coated conductors using a continuous reel-to-reel dip-coating unit

A low-cost, non-vacuum, solution precursor route has been developed to produce epitaxial Gd/sub 2/O/sub 3/ and Eu/sub 2/O/sub 3/ buffer layers and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) superconductors on biaxially textured metal substrates. On sol-gel Eu/sub 2/O/sub 3/ seed layers with sputtered YSZ and CeO/sub 2/ top layers, a YBCO film with a J/sub c/ of over 1 MA/cm/sup 2/ at 77 K was obtained. On all solution buffer layers (CeO/sub 2//Eu/sub 2/O/sub 3//Ni), YBCO film with a J/sub c/ of 200,000 A/cm/sup 2/ at 77 K was grown using pulsed laser deposition (PLD). Meter lengths of epitaxial and crack-free Gd/sub 2/O/sub 3/ buffer layers were fabricated on cube textured Ni-W (3 at.%) substrates for the first time. High quality YBCO films were deposited on Rolling-Assisted Biaxially Textured Substrates (RABiTS) using a trifluoroacetate (TFA) precursor approach. The precursors were either spin-coated or dip-coated and decomposed in a newly developed fast 3-hour burn-out step followed by post-annealing. In a stationary burn-out route, we have produced 40 cm long crack-free YBCO TFA precursors on RABiTS. On short segments, YBCO films with a J/sub c/ of over 500,000 A/cm/sup 2/ at 77 K were grown on all PLD buffered-Ni substrates (CeO/sub 2//YSZ/CeO/sub 2//Ni).

Reactive sticking coefficient of silane on the Si(111)-(7×7) Surface

Abstract The reactive sticking coefficient, S R , of SiH 4 on the Si (111)-(7×7) surface has been studied as a function of hydrogen coverage (Θ H ) in the temperature range from 100 to 500 °C. Evidence is seen for two adsorption regimes which are proposed to correspond to minority and majority surface sites. On the minority sites (Θ H =0 to 0.08), S R is approximately 10 −5 and essentially no dependence of S R on surface temperature, T S , is found. Reactive sticking becomes a complicated function of T S and Θ H , with S R decreasing, on the majority sites (Θ H ⪢008).

Composition and structure of boron nitride films deposited by chemical vapor deposition from borazine

The composition and structure of boron nitride films prepared by thermal and plasma enhanced chemical vapor deposition (CVD) using borazine as a precursor have been studied. Thermal CVD at temperatures between 475 and 550 °C using either a hot‐wall or cold‐wall reactor results in amorphous boron‐rich films of approximate composition BN0.67. Plasma enhanced CVD consistently gives films of 1:1 boron to nitrogen stoichiometry, but the hydrogen content of films deposited below 300 °C is so high that the films react with atmospheric moisture. Optimum conditions for the growth of stoichiometric BN with relatively low hydrogen content were found to be low plasma power, hydrogen–borazine gas mixtures, and a substrate temperature of 550 °C. Films deposited under these conditions are mixtures of poorly crystalline hexagonal and cubic boron nitride.

Absolute rate constants for silylene reactions with diatomic molecules

Abstract Absolute rate constants have been determined for the reaction of SiH2 with the diatomics HCl, Cl2, NO and O2. Upper limits are reported for rate constants for the reaction of SiH2 with N2 and CO. Comparisons are made between the reactivity of silylene, singlet methylene and halogenated silylenes.

Growth of Si on Si(100) via H/Cl exchange and the effect of interfacial boron

Using alternating exposures of Si2H6 and Si2Cl6, very thin Si layers have been grown on the Si(100) surface at temperatures (T) as low as 475 °C. Although this growth method is not truly self‐limiting, some of the desired features for Si atomic layer epitaxy (ALE) are retained, as discussed here. The growth rate of new Si on Si(100) using this method is limited by the thermal desorption of H2 and HCl. Doping the surface with boron atoms can lower the growth temperature, due to a weakening of the Si—H and Si—Cl bonds on the surface as observed in the temperature programmed desorption results from H2, HCl, and SiCl2 desorption from the clean and the boron‐doped Si(100) surfaces.

Epitaxial growth of BaZrO3 films on single crystal oxide substrates using sol-gel alkoxide precursors

Abstract Epitaxial BaZrO 3 (barium zirconium oxide) films were grown on single crystal substrates. A BaZrO 3 precursor solution was prepared by sol-gel synthesis using an all-alkoxide route. The barium precursors were prepared by reacting barium metal with 2-methoxyethanol, and zirconium precursors were prepared by exchanging ligands between zirconium n-propoxide and 2-methoxyethanol. The resulting BaZrO 3 precursor solution was partially hydrolyzed and spin-coated on sapphire (100), SrTiO 3 (strontium titanium oxide) (100), and LaAlO 3 (lanthanum aluminum oxide) (100) substrates. The films were post-annealed in oxygen at 800 °C for 2 min in a rapid thermal annealer. The coating and the annealing procedures were repeated three times to obtain the desired thickness, 300 nm. X-ray diffraction studies showed the presence of a single (100) cube texture for BaZrO 3 films on SrTiO 3 and LaAlO 3 substrates. The BaZrO 3 films grown on sapphire had a random texture. The BaZrO 3 films grown on SrTiO 3 substrates had a sharp texture compared to that on LaAlO 3 substrates. This may be due to the relatively smaller lattice mismatch between SrTiO 3 and BaZrO 3 .

SOL-GEL SYNTHESIS OF LAALO3; EPITAXIAL GROWTH OF LAALO3 THIN FILMS ON SRTIO3 (100)

A LaAlO{sub 3} precursor solution was prepared via an all alkoxide sol-gel route. The solution of lanthanum methoxyethoxide and aluminum methoxyethoxide in 2-methoxyethanol was prepared via ligand exchange starting from lanthanum isopropoxide and aluminum sec-butoxide and was used to make both LaAlO{sub 3} powders and films. Complete hydrolysis of the solution formed a gel that yielded well-crystallized LaAlO{sub 3} powders when fired in air at 800{degree}C. A partially hydrolyzed solution was spun-cast on SrTiO{sub 3} (100) single crystal substrates. Epitaxial films of LaAlO{sub 3} were subsequently formed during pyrolysis in O{sub 2} at 800{degree}C in a rapid thermal annealing furnace for a total of 8 min. The films were strongly c-axis oriented, verified by x-ray rocking curve results from the (003) plane with full-width at half-maximum (FWHM)=0.87{degree}, and had good in-plane texture shown by a {phi} scan of the (202) plane with FWHM=1.07{degree}. {copyright} {ital 1997 Materials Research Society.}