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An elementary model of neutral and ion sputtering yields
A19 Surface Science 102 (1981) 578-587 © North-Holland Publishing Company A N E L E M E N T A R Y M O D E L O F N E U T R A L A N D ION S P U T T E R I N G Y I E L D S S.A. S C H W A R Z * a n d C .R. H E L M S
Stanford Electronics Laboratories, Stanford University, Stanford, California 94305, USA Received 15 April 1980; accepted for publication 26 August 1980
We have recently developed a simple statistical model of sputtering which indicates that the magnitudes of elemental sputtering yields are consistent with a neutralization model of atom emission. In this model, displaced ion cores at the target surface have a finite probability of neutralization; unneutralized ions must surmount an additional coulombic barrier and therefore have a much reduced probability of escape. The neutral yield will be proportional to the probability of neutralization. The positive ion yield will be proportional to one .minus this probability. These suppositions are supported by experimental data in the literature. We also suggest that oxidation enhancement of positive and negative ion yields may result from a reduction of the coulombic force.
Surface Science 102 (1981) 588-609 © North-Holland Publishing Company S U R F A C E D I F F U S I O N , ATOMIC J U M P R A T E S AND T H E R M O D Y N A M I C S * David A. R E E D and Gert E H R L I C H
Coordinated Science Laboratory ** and Department of Metallurgy, University of Illinois at Utbana-Champaign, Urbana, Illinois 61801, USA Received 10 June 1980
The connection is established between the surface diffusivity D(O) and F(0), the rate of atom jumps in an adsorbed gas at a coverage 0. For a Langmuir layer, the two quantities are related at all coverages by D(O) = F ( 0 ) h 2, where h is the jump distance. For interacting adatoms, however, the diffusivity also depends upon the chemical potential u of the adatoms through D(O) = F(0) h 2 [a(~/kT)/a In O]T, Techniques are presented for evaluating the thermodynamic factor [a(u/kT)/a In 0IT in the diffusivity from adsorption isotherms as well as from fluctuation studies. The extent to which this factor affects the interpretation of diffusion measurements is tested in several ways: diffusion of a nearest neighbor lattice gas is formulated in the quasi-chemical approximation, the behavior of adatoms with interactions of longer range is examined by Monte Carlo simulations, and the available experimental measurements are analyzed. Both the jump rate and the thermodynamic factor are found to be affected by interactions between adatoms; the formation of superlatttices results in anomalies in both. These studies suggest that for layers of interacting adatoms, knowledge of the thermodynamics of the adsorbed layer is essential in any attempt to derive quantitative information about atomic jump rates.
Stanford Electronics Laboratories, Stanford University, Stanford, California 94305, USA Received 15 April 1980; accepted for publication 26 August 1980
We have recently developed a simple statistical model of sputtering which indicates that the magnitudes of elemental sputtering yields are consistent with a neutralization model of atom emission. In this model, displaced ion cores at the target surface have a finite probability of neutralization; unneutralized ions must surmount an additional coulombic barrier and therefore have a much reduced probability of escape. The neutral yield will be proportional to the probability of neutralization. The positive ion yield will be proportional to one .minus this probability. These suppositions are supported by experimental data in the literature. We also suggest that oxidation enhancement of positive and negative ion yields may result from a reduction of the coulombic force.
Surface Science 102 (1981) 588-609 © North-Holland Publishing Company S U R F A C E D I F F U S I O N , ATOMIC J U M P R A T E S AND T H E R M O D Y N A M I C S * David A. R E E D and Gert E H R L I C H
Coordinated Science Laboratory ** and Department of Metallurgy, University of Illinois at Utbana-Champaign, Urbana, Illinois 61801, USA Received 10 June 1980
The connection is established between the surface diffusivity D(O) and F(0), the rate of atom jumps in an adsorbed gas at a coverage 0. For a Langmuir layer, the two quantities are related at all coverages by D(O) = F ( 0 ) h 2, where h is the jump distance. For interacting adatoms, however, the diffusivity also depends upon the chemical potential u of the adatoms through D(O) = F(0) h 2 [a(~/kT)/a In O]T, Techniques are presented for evaluating the thermodynamic factor [a(u/kT)/a In 0IT in the diffusivity from adsorption isotherms as well as from fluctuation studies. The extent to which this factor affects the interpretation of diffusion measurements is tested in several ways: diffusion of a nearest neighbor lattice gas is formulated in the quasi-chemical approximation, the behavior of adatoms with interactions of longer range is examined by Monte Carlo simulations, and the available experimental measurements are analyzed. Both the jump rate and the thermodynamic factor are found to be affected by interactions between adatoms; the formation of superlatttices results in anomalies in both. These studies suggest that for layers of interacting adatoms, knowledge of the thermodynamics of the adsorbed layer is essential in any attempt to derive quantitative information about atomic jump rates.