Nanoclustering of vacancies in thin metal films revealed by x-ray diffuse scattering

doi:10.1063/1.2779097 http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=APPLAB000091000009093131000001&idtype=cvips&prog=normal&doi=10.1063/1.2779097The authors report the incorporation of unexpectedly large vacancy clusters into homoepitaxial Ag(001) films. These results, which are for a simple noble metal system, have important implications for understanding the atomic-scale kinetics of surfaces where current models have mostly ignored the role of vacancies. For films grown at 150 K, an average vacancy cluster exhibits a local dilatation volume of 750 Å3, which leads to a 1% compressive strain of the film. Vacancy clusters are observed even for films grown near room temperature. These in situ diffuse x-ray scattering experiments measure the local deformation around the cluster and, therefore, provide conclusive evidence of vacancy clusters.Financial support is gratefully acknowledged from the University of Missouri Research Board, the National Science Foundation under Grant No. DMR0706278, the Petroleum Research Fund under Grant No. 41792-AC10 P.F.M. and C.K. , the Canim Scientific Group E.H.C. and R.F. , and the Seoul Research and Business Development Program under Grant No. 10583 C.K. . The Advanced Photon Source is supported by the DOE Office of Basic Energy Sciences under Contract No. W-31-109-Eng-38. The CAT beam line is supported through Ames Laboratory, operated for the U.S. DOE by Iowa State University under Contract No. W-7405-Eng-82

Temperature and orientation dependence of kinetic roughening during homoepitaxy: A quantitative x-ray-scattering study of Ag

URL:http://link.aps.org/doi/10.1103/PhysRevB.54.17938 DOI:10.1103/PhysRevB.54.17938Kinetic roughening during homoepitaxial growth was studied for Ag(111) and Ag(001). For Ag(111), from 150 to 500 K, the rms roughness exhibits a power law, σ∝tβ over nearly three decades in thickness. β≈1/2 at low temperatures, and there is an abrupt transition to smaller values above 300 K. In contrast, Ag(001) exhibits layer-by-layer growth with a significantly smaller β. These results are the first to establish the evolution of surface roughness quantitatively for a broad thickness and temperature range, as well as for the case where growth kinetics are dominated by a step-ledge diffusion barrier.Support is acknowledged from the University of Missouri Research Board, the NSF under Contract Nos. DMR-9202528 and DMR-9623827, and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DE-FG02-90ER45427. The SUNY X3 beamline is supported by the DOE under Contract No. DE-FG02-86ER45231, and the NSLS is supported by the DOE, Div. of Materials Sciences and Div. of Chemical Sciences. One of us ~W.C.E.! acknowledges support from the GAANN program of the U.S. Department of Education. We thank Ian Robinson for the Ag~111! crystal

Temperature dependence of surface roughening during homoepitaxial growth on Cu(001)

URL:http://link.aps.org/doi/10.1103/PhysRevB.64.125427 DOI:10.1103/PhysRevB.64.125427X-ray scattering has been used to study the roughening of the Cu(001) surface during homoepitaxial growth, as a function of temperature. Between 370 and 160 K, the mean-square roughness σ2, obtained from specular reflectivity data, was found to increase as a power law σ2=Θ2β for coverages Θ, ranging from 3 to 96 ML. The roughening exponent β was observed to depend on the temperature of the substrate: it monotonically increases with decreasing temperature from β≈1/3 at T=370K to β≈1/2, at T=200K. At 110 K a smoother growth re-enters in the presence of a large vacancy concentration in the deposited film.Support is acknowledged from the National Science Foundation under contracts ~P.W.S.! DMR-9202528 and~P.F.M. and C.E.B.! DMR-9623827 and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DH-FG02-90ER45427. The SUNY X3 beamline was supported by the DOE, under Contract No. DE-FG02-86ER45231, and the NSLS was supported by the DOE, Division of Material Sciences, and Division of Chemical Sciences. We thank Ian Robinson for the Cu crystal and for valuable discussions

Temperature-dependent vacancy formation during the growth of Cu on Cu(001)

URL:http://link.aps.org/doi/10.1103/PhysRevB.66.195413 DOI:10.1103/PhysRevB.66.195413X-ray diffraction measurements show that a large number of vacancies are incorporated in thin Cu films grown on Cu(001) at low temperatures. At any given deposition temperature between 110 and 160 K, the vacancy concentration cv, obtained from reflectivity data, does not change with the coverage Θ, for 2.5ML<~Θ<~20ML. However, cv is temperature dependent: for 15-ML-thick films, grown at different temperatures, it monotonically decreases with increasing T from cv≈2% at 110 K to zero at T=160K. A different “cv vs T” dependence is observed for films grown at 110 K and then annealed at progressively higher temperatures. Here cv≈2% persists over a broad temperature interval (between 110 and 200 K) and cv exhibits a slower decrease upon heating, reaching zero at 300 K.Support is acknowledged from the National Science Foundation under Contract Nos. ~P.W.S.! DMR-9202528 and ~P.F.M., C.E.B.! DMR-9623827 and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DEFG02-90ER45427. The SUNY X3 beamline is supported by the DOE, under Contract No. DE-FG02-86ER45231 and the NSLS is supported by the DOE, Division of Material Sciences and Division of Chemical Sciences

Wetting-layer transformation for Pb nanocrystals grown on Si(111)

doi:10.1063/1.1812593We present the results of in situ x-ray scattering experiments that investigate the growth of Pb nanocrystalline islands on Si(111). It is conclusively shown that the Pb nanocrystals do not reside on top of a Pb wetting layer. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer occupying the region between the islands. As the Pb nanocrystals coalesce at higher coverage we observe increasing disorder that is consistent with misfit strain relaxation. These results have important implications for predicting stable Pb island heights

Spin-polarized neutron reflectivity: A probe of vortices in thin-film superconductors

URL:http://link.aps.org/doi/10.1103/PhysRevB.59.14692 DOI:10.1103/PhysRevB.59.14692It is demonstrated that the specular reflectivity of spin-polarized neutrons can be used to study vortices in a thin-film superconductor. Experiments were performed on a 6000 Å thick c-axis film of YBa2Cu3O7-x with the magnetic field applied parallel to the surface. A magnetic hysteresis loop was observed for the spin-polarized reflection and, from these data, the average density of vortices was extracted. A model is presented which relates the specular reflectivity to the one-dimensional spatial distribution of vortices in the direction perpendicular to the surface. Unlike other techniques, neutron reflectivity observes vortices in a geometry where they are parallel to the interface.Support ~P.F.M., S.W.H.! from the Midwest Superconductivity Consortium ~MISCON! under the U.S. DOE Grant No. DE-FG02-90ER45427, the NSF DMR Grant No. 96-23827, and ~L.H.G., E.P.! from the NSF DMR Grant No. 94-21957, and ONR Grant No. N-00014-95-1-0831 is gratefully acknowledged. We thank E. Fullerton for useful discussions and D.H. Lowndes for help in understanding the surface roughness of oxide superconductors

Thermal expansion of the Ag(111) surface measured by x-ray scattering

URL:http://link.aps.org/doi/10.1103/PhysRevB.63.113404 DOI:10.1103/PhysRevB.63.113404We have investigated the structure of the Ag(111) surface, for temperatures between 300 and 1100 K (90% of the bulk melting point), using synchrotron x-ray diffraction. Our data show no evidence of the anomalously large surface thermal expansion previously reported by medium-energy ion-scattering [Phys. Rev. Lett. 72, 3574 (1994)]. At all temperatures we find that the interlayer separations at the surface differ from their bulk counterparts by less than 1%, indicating that the surface expands similarly to the underlying bulk crystal. This behavior is in good agreement with results from molecular dynamics simulations.Support is acknowledged from the National Science Foundation under Contract Nos. ~PWS! DMR-9202528 and ~PFM, CEB, WCE! DMR-9623827 and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DE-FG02-90ER45427. The SUNY X3 beam line is supported by the DOE, under Contract No. DE-FG02-86ER45231, and the NSLS was supported by the DOE, Division of Material Sciences and Division of Chemical Sciences

Vacancy formation in homoepitaxially grown Ag films and its effect on surface morphology

URL:http://link.aps.org/doi/10.1103/PhysRevB.66.075418 DOI:10.1103/PhysRevB.66.075418Synchrotron x-ray diffraction was used to investigate the low-temperature homoepitaxial growth on Ag(001) and Ag(111) surfaces. For both orientations, the Ag films deposited at T=100K were observed to exhibit a 1% surface-normal compressive strain, indicating that an appreciable vacancy concentration (∼2%) is incorporated in the growing film. Concomitantly with the incorporation of vacancies, the growth on Ag(111) leads to the formation of pyramidlike structures with a non-Gaussian distribution of heights, whereas a similar effect was not observed for Ag(001).Support is acknowledged from the National Science Foundation under Contract ~P.W.S.! No. DMR-9202528 and ~P.F.M., C.E.B., W.C.E.! No. DMR-9623827 and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DE-FG02-90ER45427. The SUNY X3 beam line is supported by the DOE, under Contract No. DE-FG02-86ER45231, and the NSLS is supported by the DOE, Division of Material Sciences and Division of Chemical Sciences

Optical and structural properties of ZnO films deposited on GaAs by pulsed laser deposition

doi:10.1063/1.373643ZnO films were synthesized on GaAs substrates at different growth conditions by pulse laser deposition. High-purity (99.999%) oxygen was used as the ambient gas. The pressure of the ambient oxygen gas for ZnO film growth was varied from 20 to 50 mTorr, and the growth temperature from 300 to 450 °C. ZnO films showed very strong bound exciton peaks located between 3.37 and 3.35 eV. The full width at half maximum of the bound exciton peak is less than 5 meV. These results indicate ZnO films on GaAs substrates can be used for optical devices such as light-emitting diodes. The other significant properties of textured ZnO films on GaAs substrates are described.This work was supported in part by University of Missouri Research Board under Grant No. RB95-061, the U.S. Army Research Office ~Research Triangle Park, N.C.! under Grant No. DAAH04-94-G-0305, and the Office of Naval Research-Electronics Division, Contract No. N00014-99-1-0288

Influence of Quantum Size Effects on Island Coarsening

DOI:10.1103/PhysRevLett.96.10610 http://prl.aps.org/pdf/PRL/v96/i10/e106105Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si 111 - 7 7 . It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., 1=F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.The Advanced Photon Source is supported by the DOE Office of Basic Energy Sciences, Contract No. W-31-109-Eng-38. The -CAT beam line is supported through Ames Laboratory, operated for the U.S. DOE by Iowa State University under Contract No. W-7405-Eng-82. Research funding was supported, in part, by Ames Laboratory (M. C. T.), Canim Scientific Group (E. H. C.), the Missouri University Research Board, the National Science Foundation DMR-0405742, and the Petroleum Research Fund No. 41792AC10 (P. F. M., C. A. J., C. K.), the Natural Sciences and Engineering Research Council (NSERC) of Canada (C. A. J.), the Center for Nanostructured Materials Technology under 21st Century Frontier R&D Programs of the Ministry of Science and Technology (No. 05K1501-02520), Korea (C. K.)