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

The survival of holes as thru-holes in thru-hole epitaxy: How can holes in 2D layers not sealed off during multiple stacking of those layers?

Thru-hole epitaxy was recently reported to be able to grow readily detachable domains crystallographically aligned with the underlying substrate over 2D mask material transferred onto a substrate. While the experimental demonstration of thru-hole epitaxy of GaN over multiple stacks of $h$-BN was evident, the detailed mechanism of how small holes in each stack of $h$-BN survived as thru-holes during multiple stacking of $h$-BN was not intuitively clear. Here, we show through Monte Carlo simulations that in what conditions holes in each stack of 2D mask layers can survive as thru-holes during multiple stacking. If more holes are laterally connected, the survival rate of thru-holes increases resulting in the epitaxial connectedness. This computational work verifies and supports that thru-hole epitaxy is attributed to the epitaxial connectedness established by thru-holes survived even through multiple stacks.Comment: 14 pages, 6 figure

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

Formation and photoluminescent properties of embedded ZnO quantum dots in ZnO/ZnMgO multiple-quantum-well-structured nanorods

ZnO/Zn0.8Mg0.2O multiple-quantum-well (MQW) nanorods with a different number of periods and well widths were grown by catalyst-free metal-organic vapor phase epitaxy. Their optical and structural characteristics were investigated using photoluminescence, transmission electron microscopy, and field emission scanning electron microscopy. Unlike ZnO/ZnMgO MQW thin films, it was observed that embedded quantum dots played a more important role in the optical characteristics of ZnO/ZnMgO MQW nanorods than quantum confined Stark effect due to polarization field.The work at POSTECH was supported by the National Creative Research Initiative Project of the Korean Ministry of Science and Technology

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.)

Zn-triggerd critical behavior of the formation of highly coherent domains in a Mg1-xZnxO thin film on Al2O3

A series of Mg1-xZnxO (x=0, 0.05, 0.10, and 0.15) thin films were grown by metal-organic chemical vapor deposition on a (0001) sapphire substrate, and the structural characteristics of Mg1-xZnxO thin films were investigated by synchrotron x-ray diffraction. The increasing amount of Zn was found to gradually enhance the structural coherence of Mg1-xZnxO films. For a sample with 15 at. % of Zn, in particular, the formation of highly coherent domains in Mg1-xZnxO was observed to be triggered, with an accompanying phase separation of ZnO. An integrated intensity analysis predicted that the critical concentration x(c) of Zn at which the phase separation occurred was 0.086+/-0.015, and that the highly coherent domains in Mg1-xZnxO accounted for 12+/-1%

Strain relaxation mechanism of semiconductor thin films

For the atoms at clean surface of crystal, the atomic bonding environment is very different from that of deep inside bulk crystal. This fact forces those atoms to rearrange themselves in such a way that reconstruction occurs with the total energy being minimized. Reconstruction also takes place when foreign atoms are deposited on a clean surface. This strain experienced by individual atoms will propagate into a bulk crystal down to a few layers, sometimes even to several layers. Using x-ray diffraction, the positions as well as thermal vibrational amplitudes of these strained atoms can be determined. In the first part of this thesis, we present the surface reconstruction and a strain analysis of Sb/Si(ll1) using synchrotron radiation x-ray diffraction. Another case when we can observe macroscopic strain is heteroepitaxial film growth. Even though there is lattice mismatch between substrate and film, pseudomorphic film growth is usually possible at the initial stage of growth, so the film is grown under strain. However, it becomes energetically favorable to introduce misfit dislocations above a certain thickness of film growth, which is called a critical thickness. In this case, strain is considered in the continuum elastic sense, defined by the lattice constant of the material. From measurement of the positions of multiple Bragg reflections, accurate values of lattice constant can be obtained using x-ray diffraction as a function of film thickness and a critical thickness can be determined. The second part of the thesis is a study of III-nitride thin films grown on sapphire (0001) substrate with molecular beam epitaxy. Irradiation experiments were also performed to investigate whether strain relaxation takes place in SiGe films and GaN films to look for indications of metastability.U of I Onlydissertation/thesi

Heteroepitaxial Growth of MgO Thin Films on Al2O3(0001) by Metalorganic Chemical Vapor Deposition

MgO thin films were epitaxially grown on Al2O3(0001) substrates using metalorganic chemical vapor deposition. For film growth, bis-cyclopentadienyl-Mg and oxygen were employed as reactants. X-ray diffraction (XRD) theta-2theta scan and theta-rocking curve data showed a preferred film orientation of [111] direction along Al2O3[0001]. The XRD rocking curve of films grown at 600degreesC showed a full width at half maximum as narrow as 0.45degrees, indicating good crystallinity. The pole figures, of MgO thin films, which show clearly six poles with 60degrees of rotational symmetry, imply that the MgO films were epitaxially grown with, either 60degrees twins or inversion domains. Based on the pole figure and azimuthal,scan measurements, the epitaxial relationship of MgO(111)//Al2O3(0001) and MgO[110]//Al2O3[10 (1) over bar0] was determined. More importantly, the vertical alignment and surface morphology of the thin films were significantly improved by doping Zn during MgO film growth.This work was supported by the Electrical Engineering & Science Institute (EESRI) (contract no. 01-056) funded by Korean Electric Power Corporation