The effect of Cs on photon and negative ion emission was discussed for the situations where the sputtered atom interacts either very weakly or very strongly with the target surface. The experimental data seem to favor the strong interaction case. 5 references

Yu, Ming L.

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MASTER BNL-24397A Comment on the Effect of Cs on Photon and*Secondary Ion Emission During SputteringMing L. YuBrookhaven National LaboratoryUpton, New York 11973NOTICETfiis report was prepared as an accounl of worksponsored by the United Stales Govemmenl. Neither theUnited Slates nor the United Siaies Department off'nergy, nor any of iheir employees, nor any of theircontractors, subcontractors, w Ifieji employees, makesdny warranty, express or implied, or assumes any legalIjahihiy or responsibility fot tfie accuracy, complelencaor tHefulness of any informaiion, appar3ius, product orMS disclosed, or represents tiiai its use wouldinfringe privately owned nghls.AbstractThe effect of Cs on photon and negative ion emission wasdiscussed for the situations where the sputtered atom inter-acts either very weakly or very strongly with the target sur-face. The experimental data seem to favor the strong inter-action rase.*Research supported by the Division of Basic Energy Sciences,Department of Energy, under Contract No. EY-76-C-O2-OO16.Accordingly, the U.S. Government retains a nonexclusive,royalty-free license to publish or reproduce the publishedform of this contribution, or allow others to do so, for U.S.Government purposes.-2-In the past few years there were two experiments reported indetail on the effect of adsorbed Cs on the photon and negative2secondary ion ' yields during ion bombardment of solid surfaces. Inboth cases the adsorbed Cs was used to lower the work function of thetarget surface. Hopefully, from the work-function dependences ofthese" inelastic processes, we can improve our understanding of theelectronic transitions involved.In a sophisticated experiment by Thomas et al, it wasdemonstrated that Cs adsorption did not enhance photon emission fromclean Cu and Al surfaces. It was proposed that the Cs dipole layercaused a relative displacement of the energy levels of the sputteredatoms with respect to the Fermi level. With the right amount of Cs,the excitation level should be shifted well below the Fermi level.This blocked the resonance tunneling channel and the excited atomcould only decay by photon emission. Since experimentally, no enhance-ment in optical emission was observed, the result casted doubt on thevalidity of the resonant electron tunneling model in photon emission.2In another recent experiment, the effect of Cs on negative ionformation was investigated. It was found that Cs adsorption enhancedthe negative ion yields by few orders of magnitude. It was suggestedthat resonant electron tunneling was the mechanism for the formationof the negative ions. In this tunneling model, the energy level associ-ated with the negative ion was assumed not to change much with re-spect to the Fermi level of the target. The Cs overlayer only loweredthe tunnel barrier so that the probability to fill the negative ionenergy level was enhanced. So there is an apparent disagreementbetween the two interpretations. This paper is an attempt to clarifythe situation.The effect of Cs is a surface phenomenon. The interactionbetween a sputtered atom and the surface can be represented by anenergy diagram. It is convenient to use the vacuum level at infinityas the energy reference for the various energy diagrams. As shown inFig. 1, the Fermi level of the metal substrate lies below the vacuumlevel at infinity by an amount equal to the work function c of thepure metal. If Cs is deposited onto the right hand side of the sampleto lower the work function by £<}., the "local" vacuum level on theright will be lowered by this same amount. As a consequence, it takesless energy (<} -£9) to remove an electron from the inside of the metalsample. With this choice of energy reference, the Fermi level of themetal is fixed in the diagram with the vacuum level at infinity, whilethe "local" vacuum level is allowed to move. An interesting observa-tion is since the local vacuum level on the right side of the metal islower than that at infinity, there is an electric field induced betweenthe cesiated metal surface and infinity. This field is so weak thatit can be neglected under ordinary circumstances. However, if thevacuum level at infinity is replaced by the vacuum level of a close byreference metal, the induced electric field provides the way to measurethe work function difference, for example, by Kelvin prcte.When the sputtered particle is sufficiently far away from thetarget surface (region 1) so that the chemical interaction between thetwo is very small, the situation assumed in ref. 1 applies. This isillustrated in Fig. 2. Consider an atom outside the target metalsurface. The vacuum level of the pure metal has to match the vacuumlevel of the isolated atom. So when the work function and hence the"local" vacuum level of the metal is shifted by an amount A$ with respect-4-to the Fermi level, the energy difference between the atomic leveland the Fermi level is decreased by the same amount Lg. The tunnelbarrier height, however, remains the same. The conclusions drawn inref. 1 was based on this model.On the other hand, when the atom is close to the surface, thereis a strong chemical interaction between the atom and the target(region 2). The chemical bond determines the position of the valenceelectron level with respect to the Fermi level. Changes in the workfunction only indirectly affect the energy levels. This has beendemonstrated by photoemission in many cases. For example, the hydrogeninduced photoelectron peaks of the H-W(IOO) chemisorption system didnot shift in energy with respect to the Fermi level when the hydrogencoverage was changed. On the other hand the work function increasedlinearly with coverage. Saturation coverage was found to give a A<j> of0.85 eV. Smith et al^ also looked at the Cs levels during Cs deposi-tion on W(100). The Cs 5p-/;;, level only shifted by 1.0 eV betweenvery small Cs coverage and the Cs coverage for minimum work function withA<f> = -2.6 eV. So to the first order approximation, the work function£ only determines the surface potential barrier (Fig. 3). Thus as anatom moves away from the surface, there is a gradual transition fromregion 2 where the positions of atomic levels are determined by thechemical bonding and not by g, to region 1 w.iere the levels followexactly the changes in g.In ref. 1 the excited particles were assumed to be in region 1.However, there is some difficulty in trying to explain the negativeion results if we assume the same physical picture. In ref. 2, threenegative ion species M , H and 0 , all originating from a cesiated-5-Mo(lOO) surface were investigated. The electron affinities of theseions are 1.13, 0.77 and 1.46 eV, respectively, but the minimum workfunction of a cesiated Mo surface is 1.54 eV. So in region 1, thenegative ion levels are all above the Fermi level and the electronswould not be able to tunnel out from the Uo surface to forn negativeions. In the experiment, Cs enhancement of negative ion yields wasobserved even when the work function was still close to 4 eV. Onepossible explanation is the electronic transitions actually occurredvery close to the surface so that the image potential e"/4r couldlower the negative ion level below the Fermi level of Mo. At thisdistance, the region 1 situation would not be applicable anymore. Inref. 2, the region 2 approximation was assumed. It was posulf.ted thatthere was a finite probability to convert the escaping atoms into nega-tive ions through resonant electron tunneling. The tunneling probability2p is related to the tunnel barrier height V Q and barrier width a byr 1/2 1/2 _p a exp -2/^|) (Vo-E) a | (1)where E is the energy of the electron. It was further assumed thatthe Cs overlayer changes the barrier height V by A<j> so thatVQ-E -V1+L* , (2)where V, is the initial barrier height when there was no Cs.This gives a functional relationship between p and A4>. The modelfitted the negative ion enhancement data very well. Estimates of V2and a from the data have very reasonable physical values.-6-It is of interest to see the effect of Cs on photon emissionin region 2. The image potential would raise the excited level of theneutral atom by e M r approximately, placing it above the Ferni levelof the metal target. In fact, the excitation levels of the atoms of thetarget all lie above the Fermi level. If the atoms are excited duringsputtering, deexcitation can occur through resonant electron tunnelingback to the metal. The Cs overlayer lowers the potential barrier,making the deexcitation process even more probable. So a lower workfunction would not enhance the photon yield. This model thus arrivesat a conclusion which is different from that in ref. 1. It is, however,consistent with the experimental observation. So if the region 2 inter-action is taken into account for photon emission, the experimentalresult in ref. 1 could not conclusively rule out the electron tunnelingmechanism.The actual state of an escaping particle is obviously far morecomplicated than the approximations taken in region 1 or region 2. Itshould be a spatially dependent intermediate state between the two ex-tremes. An investigation on the gradual evolution of the particle-surface interaction along the trajectory of the moving particle wouldbe very useful.The author wishes to acknowledge Dr. R. Smith of BrookhavenNational Laboratory and Dr. G. E. Thomas of Philips Research Laboratory,for their helpful discussions.FIGURE CAPTIONSFig. 1 Energy diagram of a metal target with Cs on the righthand side to lower the local work function. Thevacuum level at infinity and the Fermi level of themetal are fixed in this diagram while the local vacuumlevel changes with the work function.Fig. 2 Energy diagram of an atom in region 1. The atomiclevel shifts with work function change A$ while thepotential barrier A for tunneling remains unchanged.Fig. 3 Energy diagram of an adsorbed atom in region 2. Theatomic level is fixed by the chemical bond. The potentialbarrier for tunneling changes with A(j>.References1. G. E. Thomas and E. E. deKluizenaar, Nucl. Inst. andMethods 132!, 449 (1976).2. M. L. Yu, Phys. Rev. Lett. _40, 574 (1978).3. For simplicity, the surface dipole layer of the pure metalsurface is neglected.4. B. Feuerbacher and B. Fitton, Phys. Rev. B8_, 4890 (1973).5- N. V. Smith, P. K. Larsen, and S. Chiang, Phys. Rev. B,16, 2699 (1977).1TVACUUMLEVELATINFINITYFig- 1L- -"•""to• - iAMETAL ATOMFig. 3

Comment on the effect of Cs on photon and secondary ion emission during sputtering

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Comment on the effect of Cs on photon and secondary ion emission during sputtering

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