Photoemission Characterization of Thin Film Nucleation on Inert Substrates
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PHOTOEMISSION CHARACTERIZATION OF THIN FILM NUCLEATION ON INERT SUBSTRATES G. HAUGSTAD, A. RAISANEN, C. CAPRILE, X. YU AND A. FRANCIOSI Department of Chemical Engineering and Materials Science University of Minnesota, Minneapolis, MN 55455 ABSTRACT Synchrotron radiation photoemission was used to characterize Sm and Mn thin film nucleation and growth on solid Xe substrates, in the 3x10 1 4 - 2x10 1 6 atoms/cm2 coverage range. Film growth is well approximated by a model in which the nucleation site density remains constant and hemi-ellipsoidal particles increase in size until coalescence is achieved. Site density and average cluster size are determined from the coverage-dependence of metal and Xe photoemission intensities. Size estimates are confirmed by experimental fingerprints of coalescence. INTRODUCTION Many thin film nucleation experiments employ chemically inert substrates of theoretical and/or technological interest [1-5]. Recently rare gas solid substrates have been used in photoemission studies of metal particles (2-4], and in the first stages of unreactive metal-semiconductor interface synthesis [5]. Thin films grown in situ on solid Xe exhibit three-dimensional island growth, and the inportant parameters are therefore particle concentration, size and shape [4). Film characterization must be non-destructive and is best performed in situ, to avoid changes in film morphology. In this paper we apply synchrotron radiation photoemission to estimate Sm and Mn cluster size, shape and nucleation site density on Xe, and to measure the size-dependent valence electronic structure. We employed a simple experimental procedure to minimize work-function induced shifts of cluster and Xe photoemission features. In the particle size range explored (35-90 A in diameter), we find that growth can be well approximated by a model which assumes a constant nucleation site density and hemi-ellipsoidal particles with varible z-aspect ratio. Site density and particle aspect ratio are nearly identical for Sm and Mn clusters on Xe. EXPERIMENT A bellow-mounted, closed-cycle refrigerator was used as sample manipulator in the photoelectron spectrometer, with a polished polycrystalline Cu cold finger serving as the sample substrate. A radiation shield at 60-70 K was used to minimize the thermal load. An operating base pressure < 6x10-II Torr was routinely achieved. Metal deposition was accomplished through direct sublimation from a W coil evaporator. A water-cooled quartz thickness monitor was employed to measure metal coverage. Sm or Mn films 100 A thick were deposited at room temperature on the polished region of the cold finger. The area was then cooled to 12-20 K for Xe condensation. By using identical metals for both the "underlayer" (below the Xe) and clusters, we avoided having Mat. Res. Soc. Symp. Proc. Vol. 159. ©1990 Materials Research Society
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the Xe film in contact with two metals of different work functions during the cluster studies. The presence of different work functions would induce Xe lineshapes shifted relative to
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