AVS2001 Session TF-WeA: Nucleation and Growth

Wednesday, October 31, 2001 2:00 PM in Room 123

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		TF-WeA-1 Spectroscopic Ellipsometry Studies on Film Smoothness of Sputtered Thin Films¹ C. Liu, J. Erdmann, A. Macrander (Argonne National Laboratory) For x-ray mirrors, it is very important to have the surface roughness controlled at less than a few Å rms. Sophisticated polishing techniques have been developed to obtain substrate roughness less than 3 Å. Quite often, x-ray mirrors are coated with various metals for optimum reflectivity at selected energies. One needs to make sure that the coating process will not introduce excess roughness. Spectroscopic ellipsometry has been broadly used for film thickness and morphology measurements. Here we report a new method to study the smoothness for films that are usually too thick for an ellipsometer to measure. We found that a thin C film usually follows the substrate morphology and will not introduce additional roughness. A thin C film was grown on the film to be studied; ellipsometry measurements were then carried out. The ellipsometry data were fit with both a flat-film and rough-film model to obtain the film smoothness information. Results of C/Au, C/W, and C/Cu films on Si and glass substrates will be discussed. ¹This work is supported by the U. S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-ENG-38.
3:20 PM		TF-WeA-5 Quantum Size Effects in 2D Pb Islands on Si(111) C.S. Chang, W.B. Su (Academia Sinica, Taiwan, ROC); S.H. Chang (National Tsing Hua University, Taiwan, ROC); W.B. Jian (Academia Sinica, Taiwan, ROC); L.J. Chen (National Tsing Hua University, Taiwan, ROC); T.T. Tsong (Academia Sinica, Taiwan, ROC) When the thickness of a metallic film approaches a few atomic layers, the de Broglie wavelength of its Fermi electrons becomes comparable to film thickness and quantum size effects (QSE) begin to appear. These effects, though originated from the electron confinement, can have a profound effect on various nano scale physical properties. For instance, Schulte¹ earlier calculated the film thickness dependence of electron densities, potentials and work functions for free standing metal films and found oscillations in all these quantities. In addition, when the effect of discrete lattice is taken into account, the QSE can invoke the structural variations also. All these findings exemplify the unusual behavior of thin metal films varying with their thickness. However, clear experimental evidence directly relating quantized electronic states to interlayer relaxations of individual islands is still lacking. The first observation of the QSE for individual Pb islands grown on the Si(111) surface has been made by Altfeder et al.,² and we have extended their work toward thinner films. Also in this study, individual 2D lead (Pb) islands of varying heights grown on the Si(111)7x7 surface at low temperature are investigated concurrently with real-space and local-probe scanning tunneling microscopy and spectroscopy. Quantum size effects, manifested as quantized electronic states and oscillatory relaxations in interlayer spacings are found perfectly correlated to each other. ¹F. K. Schulte, Surf. Sci. 55, 427 (1976). ²I.B. Altfeder, K.A. Matveev, and D.M. Chen, Phys. Rev. Lett. 78, 2815 (1997).
3:40 PM		TF-WeA-6 In situ and Ex Situ Surface Dangling Bond Measurements on a-Si:H by Means of Surface Cavity Ring Down Absorption A.H.M. Smets, J.H. van Helden, M.C.M. van de Sanden (Eindhoven University of Technology, The Netherlands) The study of surface dangling bonds during growth of hydrogenated amorphous silicon (a-Si:H) is of great importance because of their possible role as dominant growth sites. In this contribution we will introduce a new technique, surface cavity ring down absorption (SCRDA), which enables us to measure directly small absorptions (10^-5 - 10^-2) by surface dangling bonds. The sensitive SCRDA technique is based up on the rate of absorption of a light pulse confined between two highly reflective mirrors in an optical cavity configuration. Ex situ single wavelength measurements (λ = 1064 nm, τpulse = 10 ns, Epulse in cavity = ± 1 µJ) on a-Si:H films (10-3000 nm) on Corning glass show that the SCRDA technique is able to detect the surface dangling bonds. The obtained surface defect density of the a-Si:H oxidized surface is around 10¹² cm^-2 and is in agreement with electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS) results. Furthermore, it is demonstrated that the surface roughness, as determined from AFM measurements, correlates with the surface defect density. In situ SCRDA measurements are performed using a mirror - prism (suprasil) - mirror cavity configuration in which the light pulse incidents perpendicular on the two 70° tilted prism surfaces and has a total internal reflection on the backside surface. The initial a-Si:H growth phase (approx. 20 nm) has been monitored. The surface defect density increases up to the not yet saturated value of about 10¹³ cm^-2 in agreement with reported results using in situ ESR measurements. This value is higher than expected from reported growth models (10⁹ - 10¹² cm^-2) in which the defect density is determined by the creation and passivation of surface defects by weakly absorbed radicals. The implications for the growth mechanism of a-Si:H will be discussed.
4:00 PM	Invited	TF-WeA-7 Nanostructured Responsive Surfaces J. Liu (Lucent Bell Laboratories) This presentation discusses the design and synthesis of nanostructured materials and films with multifunctional responsive surfaces. The electronic, optical, and dielectric properties of various nanostructured films have been widely investigated. More recently, there has been a growing interest in these materials for health and biomedical applications. Two key areas of research have been pursued: (1) the formation of the nanostructures themselves on a multilength scale, and (2) the incorporation of the recognition and responsive properties. On a molecular level, techniques such as molecular imprinting have been explored. On the nano- and sub micrometer length scale, self-assembly and super-molecular templating have attracted wide attention. On the micro- and macrolength scale, micro machining and lithography have been extensively used. In addition, novel synthesis methods have been developed to incorporate active sites and functionalities so that the nanostructured hosts can selectively recognize, or even respond to the change of the environment. For molecular recognition, selective ligands, or size-and-shape selective cage structures, are introduced. For responsive properties, environmental sensitive polymers, or other switchable organic molecules, are investigated. The challenge is to integrate the nanostructured materials and the functionalities on different length scales so that the whole material will have the proper chemical and physical properties, and will detect and respond to the change in the environment. Many examples from our own research, as well as from other groups, will be given to illustrate the potential of the integrated approaches.
4:40 PM		TF-WeA-9 Connecting the Evolution and Coalescence of 3-dimensional Grain Structures to Reactor-scale Phenomena M.O. Bloomfield, D.F. Richards, O. Klaas, J. Lu, A.M. Maniatty, M.S. Shepard, T.S. Cale (Rensselaer Polytechnic Institute) We have created a finite-element based, multiple level-set code to model the evolution and coalescence of grains and atomic scale islands during thin film growth. Our software tool can simulate the evolution of N grains or atomic-scale proto-grains. Grain boundaries are represented implicitly by a set of N+1 scalar fields, phi_i(r,t) expressed on an unstructured mesh, subject to the condition that phi_i(r,t) = 0 for all r on the boundary of grain i at time t.¹ By extracting the zero contour, we can recover the grain boundaries at any time. Because each grain is associated with its own scalar field, properties such as lattice orientation can be easily retained on a grain-by-grain basis. The evolution of each grain is computed separately using the usual level set equation. We use an explicit positive coefficient scheme for this evolution. Level sets representing different regions are then reconciled to bring the them into agreement. To address distortions in the scalar fields, we implement a "redistancing" algorithm that corrects these distortions. This step stabilizes the evolution, allowing for simulations that include the coalescense of proto-grains and islands into complex grain structures. Demonstrations of this code are presented, including applications within a multiscale framework. Reactor scale simulations of reactant transport are performed using an FEM code. Reactant data are passed to this scale from the grain scale in the form of boundary conditions. This allows us to establish concentration fields of reactant both on the scale of 0.1 m, and using local refinement, on the 0.1 mm scale. The reactor-scale simulation passes reactant data back down to a grain-scale level set simulation. This allows us to show the interaction of phenomena such as reactor-scale reactant depletion on the resulting grain structure. ¹Osher, S. and Sethian, J.A., J. Comput. Phys. 79, 12 (1988).
5:00 PM		TF-WeA-10 Sputter Deposition of Metallic Sponges A.F. Jankowski (Lawrence Livermore National Laboratory) The growth of thin-film metallic sponges is of interest in several electrochemical applications, for example, conductive porous electrodes for gas transport and processing.¹ Determination of the experimental parameters needed to yield metallic sponges should be tractable for physical vapor deposition processes. The general guidelines on how to stabilize the basic coating morphologies are found in the classic zone model for film growth.² It's known that morphologies in structure can range from porous columnar to dense polycrystalline as the process conditions are manipulated for either sputter deposition or evaporation. However, there's no reference to the three-dimensional structure of a sponge - that is, a polycrystal with continuous open porosity but without the definitive columnar features characteristic of vapor deposits. Herein, conditions for deposition are resolved that produce the sponge morphology augmenting the classic zone model of structure. For sputter deposition using planar magnetrons, the general conditions are an increased working gas pressure and an intermediate substrate temperature. Example results of the sponge morphology in a 1 to 2 µm thick coating are presented for metals including aluminum, gold, nickel, and silver. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. ¹ A. Jankowski, et al., J. Vac. Sci. Technol. A, 13 (1995) 658; 18 (2000) 2003. ² J. Thornton, J. Vac. Sci. Technol., 14 (1974) 666.