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Growth of narrow-neck, epitaxial as well as non-epitaxial and nearly spherical Ge islands on air-exposed Si(111)-(7\times7) surfaces has been investigated by in-situ scanning tunnelling microscopy (STM) and ex-situ high resolution cross-sectional transmission electron microscopy (HRXTEM). A thin oxide is formed on Si(111)-(7\times7) surfaces via air exposure. Ge islands are grown on this oxide. STM measurements reveal the growth of very small (~2 nm diameter) Ge islands with a high number density of about 1.8\times10^12 cm-2. The island size has been found to depend on the amount of deposited Ge as well as the substrate temperature during Ge deposition. HRXTEM micrographs reveal that the islands are nearly spherical in shape, making narrow-neck contact with the substrate surface. At 520{\deg}C growth temperature both epitaxial and non-epitaxial islands grow. However, at 550{\deg}C, Ge islands predominantly grow epitaxially by a narrow-contact with Si via voids in the oxide layer. Growth of vertically elongated Ge islands is also observed in HRXTEM measurements with a very small diameter-to-height aspect ratio (~0.5-1), a hitherto unreported feature of epitaxial Ge growth on Si surfaces. In addition, stacking fault and faceting are observed in islands as small as 5 nm diameter. Ge islands, not even in contact with the Si substrate, appear to be in epitaxial alignment with the Si substrate. The island size distribution is essentially monomodal. As the contact area of Ge islands with Si through the voids in the oxide layer can be controlled via growth temperature, the results indicate that tunability of the potential barrier at the interface and consequently the tunability of electronic levels and optical properties can be achieved by the control of growth temperature., Comment: 27 Pages and 10 Figures

Clean Si(111)-(7{x7) surfaces, followed by air-exposure, have been investigated by reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Fourier transforms (FTs) of STM images show the presence of short range (7x7) order on the air-oxidized surface. Comparison with FTs of STM images from a clean Si(111)-(7x7) surface shows that only the 1/7th order spots are present on the air-oxidized surface. The oxide layer is ~ 2-3 nm thick, as revealed by cross-sectional transmission electron microscopy (XTEM). Growth of Ag islands on these air-oxidized Si(111)-(7x7) surfaces has been investigated by in-situ RHEED and STM and ex-situ XTEM and scanning electron microscopy. Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while preferred orientation evolves when Ag is deposited at higher substrate temperatures. For deposition at 550{\deg}C face centered cubic Ag nanoislands grow with a predominant epitaxial orientation [1 -1 0]Ag || [1 -1 0]Si, (111)Ag || (111)Si along with its twin [-1 1 0]Ag || [1 -1 0]Si, (111)Ag || (111)Si, as observed for epitaxial growth of Ag on Si(111) surfaces. The twins are thus rotated by a 180{\deg} rotation of the Ag unit cell about the Si [111] axis. It is intriguing that Ag nanoislands follow an epitaxial relationship with the Si(111) substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently the short range order on the oxide surface influences the crystallographic orientation of the Ag nanoislands., Comment: 10 figures

Ion-irradiation-induced modifications of a periodic Pt/C multilayer system containing Fe impurity have been analyzed by transmission electron microscopy (TEM). The multilayer stack with 16 Pt/C layer pairs (period 4.23 nm) was fabricated on a glass substrate. A 2 MeV Au$^{2+}$ ion beam was rastered on the sample to obtain uniformly irradiated strips with fluences from 1$\times10^{14}$ to 1$\times10^{15}$ $ions/cm^2$. Ion-irradiation has been found to cause preferential migration of Fe towards Pt layers [Nucl. Instr. Methods Phys. Res. B212 (2003) 530]. Cross-sectional transmission electron microscopy (XTEM) shows considerable atomic redistribution for irradiation at the highest ion fluence (1$\times10^{15}$ $ions/cm^2$). Individual entities in this structure is like a cluster. Periodic multilayers have periodicity only in the direction normal to the multilayer surface. However, Fourier transform of the XTEM images of the sample irradiated at the highest-fluence shows new off-normal Fourier components of superlattice periodicities arising due to ion irradiation. With a proper understanding of this phenomenon it may be possible to fabricate three dimensional periodic structures of nanoclusters., Comment: 12 pages, 6 figures

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the morphology of ultrathin Ge films eposited on pristine Si(100) and ion-irradiated Si(100) self-affine fractal surfaces. The pristine and the ion-irradiated Si(100) surface have roughness exponents of alpha=0.19+/-0.05 and alpha=0.82+/-0.04 respectively. These measurements were carried out on two halves of the same sample where only one half was ion-irradiated. Following deposition of a thin film of Ge (~6 A) the roughness exponents change to 0.11+/-0.04 and 0.99+/-0.06, respectively. Upon Ge deposition, while the roughness increases by more than an order of magnitude on the pristine surface, a smoothing is observed for the ion-irradiated surface. For the ion-irradiated surface the correlation length xi increases from 32 nm to 137 nm upon Ge deposition. Ge grows on Si surfaces in the Stranski-Krastanov or layer-plus-island mode where islands grow on a wetting layer of about three atomic layers. On the pristine surface the islands are predominantly of square or rectangular shape, while on the ion-irradiated surface the islands are nearly diamond shaped. Changes of adsorption behaviour of deposited atoms depending on the roughness exponent (or the fractal dimension) of the substrate surface are discussed., Comment: 5 pages, 2 figures and 1 table

Growth and strain behavior of thin Ag films on Si substrate have been investigated by scanning tunneling microscopy, cross-sectional transmission electron microscopy and high resolution x-ray diffraction studies. Ag islands formed on Si at room temperature growth show strongly preferred heights and flat top. At low coverage, islands containing two atomic layers of Ag are overwhelmingly formed. At higher coverages island height distribution shows strong peaks at relative heights corresponding to an even number of Ag atomic layers. This appears to be a quantum size effect. Hexagonal disc-like islands with flat top are formed upon annealing. The annealed film shows two closely-spaced Ag(111) diffraction peaks - one weak and broad and the other narrow and more intense. The intense peak corresponds to a shorter Ag(111) planar spacing compared to the bulk value. This can be explained in terms of changes in the Ag lattice during the heating-cooling cycle due to thermal expansion coefficient mismatch between Ag and Si., Comment: 9 pages, 11 figures