Si(111)

A181 T h o m a s H. U P T O N *, W. H e n r y W E I N B E R G a n d W d h a m A. G O D D A R D III

Dtvtsion of Chemtstry and Chemical Engmeermg **, California Institute of Technology, Pasadena, Cahforma 91125, USA Recewed 18 September 1979. accepted for publication 4 March 1980 Seml-emptncal L o n d o n - E y r m g - P o l a n y i - S a t o (LEPS) calculaUons are reported, comparing the energetics of H 2 dissociation at linear, twofold, and fourfold sites on NI(001). Parameters for the LEPS method were obtained from the results of ftrst principles calculations of both atomic and molecular hydrogen adsorption on model Ni(001) surfaces. Several pathways are found to require no actwatlon energy for dissociation, of which the most favorable is dlssocmtion across a twofold site with subsequent atomic adsorption at fourfold sites. The expermaentally observed 131 (high coverage) state is found to be consistent with a geometry m which two hydrogen atoms are adsorbed at a single fourfold site Surface Science 95 (1980) 4 0 3 - 4 1 0 © North-Holland Publishing Company D I R E C T I O N A L M E M O R Y E F F E C T S IN I N E L A S T I C P H O T O E L E C T R O N SCATTERING D.P. W O O D R U F F a n d D. N O R M A N *

Physics Department, Umverszty ol Warwwk, Coventry CV4 7AL, UK and H.H. F A R R E L L , M.M. T R A U M a n d N.V. S M I T H

Bell Laboratorws, Murray Hdl, New Jersey 0 79 74, USA Recewed 2 October 1979; accepted for publication 14 January 1980 We have recently shown that photoemission from core levels of adsorbates on surfaces shows strong angular effects attributable to diffraction processes. In order to gain structural reformation from this anisotropy it is important to separate out variations in the lnelastically scattered background. In a study of the Nl (100) surface both clean and with adsorbed Na we find substantial anisotropies in the background which appear to be directly related to the strong anisotropies in the Ni 3d photoemission. The data suggest that inelastic loss processes contributing to the photoemission "tail" involve predominantly small changes in the direction of the electron wave vector k and so allow much of the noqoss directionality to be retained. The implications for the adsorbate measurements are not severe, however, as our experiments also show the Ni 3d emission angular dependence to be insensitwe to the presence of an overlayer.

Surface Science 95 (1980) 4 1 1 - 4 3 0 © North-Holland Publishing Company DIRECT OBSERVATION OF THE NUCLEATION AND GROWTH MODES

OF Ag]Si(11 l) J.A. V E N A B L E S , J. D E R R I E N * a n d A.P. J A N S S E N **

School of Mathematwal and Physwal Sciences, Umverstty of Sussex. Brtghton BN1 9QH, Sussex, England Received 13 November 1979, accepted for publication 15 February 1980 Direct observation in an ultrahigh vacuum scanning electron microscope (SEM) has revealed the growth modes of Ag on $1(111), previously deduced indirectly from analysis of Auger elec-

A182 iron spectra, and the form and number of crystalhtes as a function of deposition tune and temperature At high temperatures, Ts > 200°C, the sdver grows defmitwely in a Stranskl Krastanov (layer plus island) growth mode, with a very strong dependence of the island density on the deposition temperature, varying from ~106 to ~1010 cm -2 between Ts = 500 and 200°C The crystals have more or less regular hexagonal forms with (111) and (111) faces predominating with a height to width ratio which decreases with deposition time in the range 0 1 - 0 6 At temperatures below 200°C, the island density Is too high to be observed directly by SEM, and silver appears to grow m a F r a n k - v a n der Merwe (layer by layer) mode However, this uniform deposit is unstable on heating above 200°C, and annealed deposits recreate rather similar islands An analysis of published Auger a m p l i t u d e - t i m e curves is made to estimate the initial island density produced by depositions below Ts = 200°C, and it is shown that these densities constitute a very reasonable extrapolation of the higher temperature SEM values A model of Stranski-Krastanov growth is gwen in outline and applied to Ag/Sffl 11) It is argued that the island density is determined largely by island mstabihty at high temperatures, even though condensatlon is complete At low temperatures it is argued that Stranskl-Krastanov growth becomes essentially equivalent to F r a n k - v a n der Merwe growth for high enough island density, for kinetic rather than thermodynamic reasons, without any change in the basic StransklKrastanov mechanism

Surface Science 95 (1980) 4 3 1 - 4 4 6 © North-Holland Publishing Company PARTIAL CHARGE INJECTION FROM SURFACE AND ADSORBATE STATES INTO SEMICONDUCTOR SPACE CHARGE LAYERS W. L O R E N Z a n d C. E N G L E R

Sektton Chemte, Karl-Marx-Umversttat, Leipzig, GDR Received 7 December 1979; accepted for publication 28 January 1980 Chemisorption processes on semiconductor interfaces are generally coupled with partial charge rejection into a space charge layer. A computational approach of this phenomenon Is closely related to the problem of discriminating surface and space charges on semiconductors. We report on the present state of model computations of partial charge injection, based on a local density of states treatment of semiconductor surfaces and on a superpositron approximation of local chemical states and macroscopic band bending. Relation to macroscopic dynamic theory of interface reactrons is indicated. As examples, ideal and (1 × 1) reconstructed GaAs(110) surfaces and chemlsorption of a L1 atom are discussed in some detail.

Surface Science 95 (1980) 4 4 7 - 4 6 4 © North-Holland Pubhsinng Company Q U A N T I T A T I V E P R E D I C T I O N S OF TIlE H E A T OF A D S O R P T I O N OF METALS ON METALLIC SUBSTRATES A . R M I E D E M A a n d J.W.F. D O R L E I J N

Phtlips Research Laboratories and Phihps Lighting Division. Eindhoven, The Netherlands Received 4 December 1979; accepted for publication 19 February 1980 The enthalpy of adsorption of metal atoms on metalhc substrates can be calculated by means of a semi-empirical model in which metal atoms are treated as if they were macroscopic pieces of metal In this model the enthalpy of adsorption of a metal A on a substrate B can be