R. J. Wilson

Optical self‐regulation during laser‐induced oxidation of copper

The oxidation of copper induced by a cw Ar+ laser beam is investigated using time‐resolved reflectance measurements at λ=632.8 and 514.5 nm, together with supporting studies of film properties by scanning Auger and electron microscopies. The optical measurements and film composition analyses indicate that the oxide layer is mainly Cu2O. Since this material is partially transparent to both the probe and oxidizing laser beams, interference of each beam within the film results. This gives rise to strong reflectance variations which can be used to gain information about the oxidation reaction via theoretical simulations of the process. A model which explicitly treats the intimate relationship between temperature, oxide growth, and optical absorption is proposed. Free of adjustable parameters, the theory uses only optical, thermal, and furnace oxidation data from the literature, and gives calculated curves in good agreement with experiment. The results of this study have important consequences for investigatio...

Nucleation and growth of ultrathin Fe and Au films on Cu(100) studied by scanning tunneling microscopy

Cu(100) surfaces modified by depositing Au and Fe in the monolayer (ML) thickness regime are studied with the scanning tunneling microscope (STM). STM results confirm that Au deposition of 0.5 ML at 300 K creates a CuAu c(2×2) alloy monolayer, with nearly regular disruptions of structure that relieve misfit strain. The alloy forms by Au replacement of Cu atoms in the surface. Submonolayer Fe deposition at 300 K yields monolayer islands and additional features interpreted as Fe patches in the surface Cu layer. This suggests that Fe atoms replace Cu atoms in the surface. Deposition of ∼4 ML Fe yields a more homogeneous surface with height variations that suggest structural disorder in the Fe layer. Fe deposition at ∼110 K and warming to 300 K yields many monolayer and bilayer islands, with no evidence of Fe incorporation into the surface. Unusual tip behavior is also discussed which appears to yield STM sensitivity to the chemical difference between Fe and Cu areas.

Growth and morphology of partial and multilayer Fe thin films on Cu(100) and the effect of adsorbed gases studied by scanning tunneling microscopy

Fe epitaxy on Cu(100) is investigated for Fe coverages θ≤3.0 ML. The layer filling statistics are quantitatively related to an evolving growth process, which includes intermixing at the substrate overlayer interface. The resulting inhomogeneous substrate surface and first layer affect the processes by which arriving Fe atoms add to the growth front. Our results explain the previously reported covering of initial Fe not as bilayer growth, but instead as the result of island growth on top of Fe incorporated in the top substrate layer. First layer composition and structure influence the nucleation and growth of the second layer. Island coalescence and formation of a first layer percolation network change the connected first layer area thereby changing the nucleation and growth behavior of the second layer. After both first and second layer growth are completed, images show additional growth is much more layer‐by‐layer in nature. Oxygen exposure after Fe deposition changes the layer filling by both promotion ...

Surface structure and metal epitaxy: STM studies of ultrathin metal films on Au(111) and Cu(100)

Abstract The scanning tunneling microscope (STM) is an important tool for studying the growth of ultrathin metal structures. The behavior of atoms arriving at the surface determines nanometer-scale structure that is readily measured with the STM. These structural features are important in determining properties. The variety of structural possibilities is illustrated with the difference between substrate-controlled island nucleation of Ni on Au(111) and diffussion-controlled aggregation of Ag on Au(111). The STM also provides a fairly complete picture of the intermixing that occurs in the early stages of room-temperature growth of Fe on Cu(100).

Epitaxial growth of ultrathin Pt films on basal-plane sapphire: the emergence of a continuous atomically flat film

Abstract Epitaxial Pt films of various thicknesses were grown on basal-plane sapphire using ion beam sputtering. Scanning tunneling microscope images are used to characterize the film morphologies and their dependence on deposition temperature, T D . Epitaxial characteristics are determined, in a qualitative sense, by low-energy electron diffraction (LEED). The film morphologies are very sensitive to T D — exceeding the narrow ideal temperature range of 580–590°C means the difference between a relatively flat Pt film and a surface with tall Pt mounds separated by bare sapphire patches. Within this temperature range, Pt islands are observed at 10 A, these islands begin to coalesce at 15 A, and continuous films are observed at 30 A and 50 A. The continuous films are nearly ideal atomically flat surfaces, but both show several exposed Pt layers and the 30 A film has many pinholes. Also, images of 5 A Pt films deposited at room temperature are used to characterize the substrate morphology.

Origin of contaminants in photochemically deposited chromium films

A systematic investigation of the composition of Cr films deposited by cw 257 nm photolysis of Cr(CO)6 has been performed in order to identify sources of C and O contamination. In the absence of oxygen, the films have the approximate stoichiometry CrCO, independent of light intensity (40–2900 W/cm2). Five carbonyl groups are removed by a combination of surface photochemical and spontaneous processes; the sixth cannot be desorbed in the absence of heating. Deposits grown by purely gas phase photolysis have the approximate composition Cr(CO)2. Oxygen containing species present in the cell during deposition and/or exposure to air rapidly and efficiently convert these films to Cr2O3.

Temperature dependence of the epitaxial growth of Pt on basal-plane sapphire

Abstract We have investigated the temperature-dependent growth characteristics of epitaxial Pt(111) on Al 2 O 3 (0001) using in-situ scanning tunneling microscopy. For temperatures near the onset of epitaxy (600°C), the Pt films grown by ion-beam sputtering, are flat and well-ordered. With increasing substrate deposition temperature, the surfaces grow rougher and at 700°C display island-on-island growth with up to 12 monosteps visible. When subjected to a post-deposition anneal of 950°C, the Pt becomes very smooth and the initial growth temperature becomes less important. Finally, we show that the Pt provides an excellent seed layer on which to grow and investigate metals such as Co and Cu.

A structural model and mechanism for Fe epitaxy on Cu(100)

A structural and mechanistic model for initial room temperature Fe epitaxy on Cu(100) is presented, based on scanning tunneling microscopy data. Changes in Fe atom attachment kinetics with coverage θ yield several growth regimes: Fe incorporation into the surface by atomic exchange with Cu (θ < 0.2), growth of first-layer Fe islands (0.2 < θ < 0.7), and simultaneous layer-1 and layer-2 growth (0.7 < θ < 2). These results reconcile qualitative disparities in previous interpretations of experimental results.

IN SITU SCANNING TUNNELING MICROSCOPY OBSERVATION OF SURFACE EVOLUTION IN MAGNETICALLY COUPLED CO/CU MULTILAYERS

A series of bilayers of the form Cu(100)/[Co(21 A)/Cu(21 A)]n were grown by both ion beam and DC magnetron sputtering techniques. Scanning tunneling microscopy images of the developing layers demonstrate a marked difference in the way in which roughness evolves through the films. The higher energy ion beam sputtered systems show a nonconformal roughness that is characterized by comparatively large lateral length scales. The less energetic magnetron‐formed systems exhibit an island‐upon‐island growth that is conformal from layer to layer. Kerr effect measurements show that the former is ferromagnetically coupled and the latter is antiferromagnetically coupled. An explanation is presented that attributes the differences in roughness to the potential barriers at step edges. Adatom mobility and incident energy are shown to be the determining factors for this kind of conformal growth.

Surface roughness in Cu(100)/[Co/Cu]n systems grown by ion‐beam sputtering

The development of surface roughness in Co/Cu systems was investigated through the use of in situ scanning tunneling microscopy. Multilayers and single layers of cobalt and copper were sequentially grown on a Cu(100) substrate. We show that the growth mode of cobalt on copper is quite different from that of copper on cobalt. We characterize these differences by looking at the lateral variations and obtaining a mean measure of island length. The cobalt tends to nucleate in small (<5 nm) islands. An overlayer of copper broadens this length scale while maintaining approximately the same peak‐to‐peak roughness of 5 monolayers. These growth mechanisms are shown to affect the way in which roughness propagates through multilayers. The impact of deposition temperature is also examined and seen to alter the degree of roughness in these samples. We conclude by discussing the implications for the magnetoresistance of these structures.