Electron spectroscopy of condensed multilayers: Line shape changes due to beam damage and excitation mode

Surface Science 88 (1979) 121-137 © North-Holland Publishing Company

ELECTRON SPECTROSCOPY OF CONDENSF.D M U L T I L A Y E R S : LINE SHAPE CHANGES DUE TO BEAM DAMAGE AND EXCITATION MODE * P.H. HOLLOWAY **, T.E. MADEY ***, C.T. CAMPBELL t , R.R. RYE and J.E. HOUSTON Sandia Laboratories, Albuquerque, New Mexico 87115, USA Received 8 March 1979; manascript received in final form 18 June 1979

Auger line-shape analysis, photoelectron spectroscopy, and the:final desorption spectroscopy have been used to study the effects of electron bombardment on condensed multilayers of (CH3)20 , CHaOH , and H20. The data show that electron doses as low as 5 x 10"~ C/cm 2 (a 0.5 mm diameter, 1 ~A beam for 1 sec) can cause detectable damage. New chemical species are created in the condensed layex by this electron beam interaction, and the data surest that water and hydrocarbons are the most abundant. Auger spectra excited by X-rays and by electrons were shown to be different both before and afler electron damage. This difference probably results from shake-up or shake-off processes which are sensitive to the specific mode of core-level excitation.

1. Introduction A variety of means have been employed in studies of Electron Stimulated Desorption (ESD) of ~dsorbed monolayers and multilayers to assess whether or not an electron beam has produced irreversible changes in the surface, regions [ l ]. These include direct detection of ions, ground state neutrals or metastables released in ESD, as well as the detection of changes in surface work function, LEED patterns, or AES signal intensity. During the course of a recent study [2,3] of chemical effects in Auger electron spectroscopy (AES), damaging effects of electron beam interactions with condensed solids were evktent in raany ways. In the present work, ~ve discuss some of the electron beam effects on condensed multilayers of the homologous series of H20, CHsOH and (CH3)120. These molecules all have struc* This work suppc:,rted by the US Department of Energy. ** Present address: Department of ~aterials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA. *** Visiting summer scientist at Sandia Laboratories, 1,977; permanent address: National Bureau of Slandards, Washington, DC 20234, USA. t Visiting summer graduate student, Sandia Laboratories, 1977" present address: Department of Chemistry, University of Texas, Texas 87812, USA; NSF trainee. 121

122

P.II. HoUoway et al. /Electron spectroscopy of condensed multilayers R

F

tures of the form ~ O where R and R are either H or CH3; all are characterized R by lone pair, nonbounding orbitals located primarily on the O atom. The observed beam effects include delection of beam damage by irreversible changes in Auger, photoelectron, and thermal desorption spectra (indicative of beam-created chemical species). The relative merits of electron excited and X-ray excited Auger electron spectroscopy (EAES and XAES, respectively) in the characterization of the local chemical environment of atoms in multilayers will be discussed, with particular emphasis on the limitations of EAES. In addition to beam damage effects, we have ,observed differences between EAES and XAES. These differences appear to originate from shake-up or shake-off processes whos': transition probabilities differ for EAES and XAES.

2. Experimental method The details of the ultrahigh vacuum system and experimental procedures employed in these studies have been described previously [2]. Briefly, H20, CH3OH, and (CH3)20 were deposited onto the surface of a liquid nitrogen-cooled ('~1 IO K) Ni(lO0) crystal mounted on an X Y Z rotary manipulator. A collimating gas doser was used to insure uniformity of coverage over the surface of the substrate, and to minimize the increase in background pressure in the vacuum chamber. For all of the multilayer doses, complete obliterelion of :;ubstrate Ni XPS and AES insured that the AES spectra resulted from multilayers sufficiently thick (estimated to be >100 A,) as to be unaffected by suOstrate emissio1~, and substrate-molecule interactions. The electron energy, analyzer was a Physical Electro, tics Industri.'s double-pass cylindrical mirror analyzer (CMA) operated in tl~e retarding mode. InLerchangeable X-ray sources (600 W A1 and 400 W Mg) were used for X-ray photoelectron spectroscopy (XPS) and XAES. For EAES studies, an electron gun was operated at an elects'on energy of 1.5 keV, the energy for which the C(KVV) and O ( K W ) cross sections are near their maximum values [4]. Beam currents were deliberately maintained az low values, ~<5 × 10 -8 A (~3 × 10 -s A/cm2). Even under these conditions, evidence for beam-induced damage was s ,~n in times of the order of minutes. To increase measurement times and reduce beam damage, the electroi~ beam could also be :astered over a 16 mm ~ area of the sample, resulting in a current density of "-3 × 1C-7 A/era 2. Unless otherwise specified, all the beam damage data reported here was ~.aken while rastering. The rastered area was :sufficiently laq,e that all of the XPS data were from a damaged area. For all of the studies discussed below, data were accumulated u.,;ing multiple scan, pulse counting methods with accumulation times ranging fi'om 1C s ~Io 90 rain. Data were stored in a multichannel analyzer, then recorded onto magnetic tape for subsequent computer reduction and manipulation. The experimental AES spectra labeled as N(E) and cL,V/dJ~;are "integral" and "derivative" spectra, respectively, and

P.H. Holloway et al. /Electron spectroscopy of condensed multilavers

123

are uncorrected for instrument transmission, background, etc. The data were all accumulated in the N(E) mode and dN/dt:-" spectra wert, determined digitally using a three-point finite differences method. Thermal desorption spectra were taken by heating the sample linearly with time ( " 2 K/min always starting at 110 K) and fol. lowing gas phase concentrations of various species with a UTI quadrupole mass analyzer. The quadrupole was usually continuously scanned from 10 to 50 m/e during desorption while simultaneously recording the temperature with a W-Re thermocouple spot welded to the sample.

3. Results 3.1. Electron beam-induced damage in condensed multilayers

In the following figures, electron doses are expressed as the product / • t where / is current density and t is bombardment time;/, t has units of C/¢m 2. Note that / . t = l0 -3 C/cm 2 corresponds to a 0.5 mm diameter, 1/aA beam bombarding a surface for only 2 s! Thus, as will be seen in the following discussion, the typical LEED/Auger electron beams used for surface studies can create extensive damage in surface layers in times considerably ~orter than typical measurement times. This critical fact is frequently overlooked in AES studies of adsorbed layers. The XAES O(KVV) spectra from solid (CH3)20 are shown in fig. I. The solid curves result from a fresh multilayer while the dotted curves result from a total electron dose of oaly 1.2 × lO -3 C,'cm 2. At this dose the changes that occur in the Auger spectra are subtle in the N(E) form (curves A and B). The derivative data (curves C and D) show only a slight change in the negative peak at ~510 eV for the damaged film. The close similarity in the O(KVV) spectra indicates that little change has occurred in the local electronic structure about oxygen [2,3]. In contrast, fig. 2 contains the XAES C(KVV) spectra for the same (CHa)20 multilayer. In this case it is clear even in the integral spectra (curves A and B) that a rastered dose of 1.2 × l0 -3 C/cm 2 is capable of producing large changes in the carbon Auger line shape. In the derivative spectra (curves C and D) the electron damage causes a large broadening in the negative peak at -"268 eV and iacreases the positive derivative at "232 eV. The more pronounced change with damage in the C(KW) spectra relative to that found for the O(KVV)spectra is suggestive of a larger change in the local electronic environment about carbon as compared to oxygen. The relative changes in the cxygen and carbon Auger peaks are consistent with the effects of electron damage on the photoelectron peaks (fig. 3). The C ls peah is both broadened (3.0 eV before and 4.4 eV FWHM after) and shifted -'-I eV lower in binding energy after 1.2 × 10 -3 (.;/cm 2. At the same time, the O ls photoelectron peak only broadened from 3.0 to 3.6 eV FWHM upon exposure to 1.2 × 10 -~ C/ cm:, and did not shift in energy. The multilayer was visibly darkened by electron bombardment.

I

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.-.

........ D

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1

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[Lt[CTRON ENERGY-eV

1

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........... UNP ~ ~AAGE [~

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550

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CfKVV) tCH 3) 20

I

265

,//F

.

ELECTRON [NER(;Y eV

225

UNDAMAGED

............ ]. 2 x 10 -3 coullcm 2

•

Fig. 2. X-ray c×cited C(KVV) Auger spectra. The spectra have the same origin as those in fig. 1.

i"ig. 1. X-ray excited O(KVV) Auger spectrum from undamaged (A and C) electron damaged (1.2 × 10 -3 C/cm 2 (B and D) multilayer of (('!-13)20. Speclra A and B are N(L") data while C and D are dN/dl'," data digitally derived fron. & and B, respectively.

~30

i':

........... 1 2 ;~ I0 1 ct)ul/cm 2

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P.H. Hollo~vay et al. / Electron spectroscopy of condensed:multllayers

125

.t

,

Ols

(CH3)20

Cls

1CH3)20

10eV

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-

-

-

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Fig. 3. C ls (A and B) and O Is (C and D) photoelectron peaks from undamaged (A and C) and damaged (1.2 x 10-3 C/cm2 - B and D) layers of (CHa)20.

The results for CHaOH are similar to those for (CHa)20. The XAES spectra for a CHaOH layer before and after electron bombardment ~ o w n in fig. 4 demonstrate that an exposure of !.3 X 10 -a C/cm 2 caused extensive changes in the CHaOH layer. Again the change is more pronounced in the C(KVV) spectra than for O(KVV). The C and O 1s photoelectron peaks changed with electron exposure in a manner aimilar to those for (CH3)20. The C ls peaLkbroadened from 2.9 to 3.9 eV and shifted "1 eV lower in energy. No shift was detected for O Is, but the peak broadened from 2.9 to 3.6 eV after 1.3 × 10 -3 C/cm 2. Subtle change:~ hi background slope and peak width were detected for doses as low as 2.5 X 10 -4 C/cm 2, These data suggest that new chemical species, containing C and O, are being created on the surface. The data also indicate that the change in electronic environment is greater for C than for O [2,3]. To assist in determiniLg what species might be forming, both damaged and undamaged layers were analyzed ufing Thermal Desorption Spectroscopy (TDS) [5,6]. The behavior of selected mass spectTa'peaks from CH3C H versus temperature is shown in fig. 5. For a fresh (undamaged)multi-

126

P.H. Hoilowa), et aL /Electron spectroscopy of condensed multilayers

UNDAMAGED

O|KVV) CH]OH

A B

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470

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1

265

305

ELEC'IRONENERGY-eV i~. 4, X-r~; ex,,ited ,~3 iA and B'~ and C iC and D} Auger spectra from undamaged (A and C) ,~n,t d,~mjgcd t l.3 x 11)-" ( ' / c m 2 (B and l)' multilaycrs of CH3OH.

layer, a ('I~3Ot| desorption peak at --150 K and a smallt mass 18 peak at " 1 6 5 K are observed. t h e mass 18 peak is assigned to H20 since this dcsorption tempeiature agrees ,,,,ell with the desorption temperatures observed for multilayers of H 2 0 on Ru(O0]) [7]. :\t~er a dose of 1.3 × l0 -3 C/cm 2, there is still a detectable Ctt3OH

P.H. Holloway et aL / Electron spectroscopy o f condensed mulalayers I

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127

.....

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Fig. 5. Thermal desorption spectra from fresh and damaged (1.3 × X0.3~C/cm 2) multilayers of CH3OH, before electroa bombardment, the desorption is primarily CH3OH (role --- 31) with some H20 (role = 18). After bombardment, the CH3OHpeak is smaller,the H20 peak is larger and shifted slightly higher in temperature, and an m/e - 28 desorption peak is observed.

desorption peak at "~150 K, but the H20 peak has increased in araplitude, broadened, and shifted to higher temperatures (~170 K). Ir, addition there is a detectable peak for mass 28 (presumably CO) ~tt " 1 7 0 K for tile damaged film. There were also small iacreases in the concentration of masses 26, 27, 39, 41,and 43 with heating which indicates the possible exis~tence of CaHx and C3Hy products (or fragments from higher molecular weight hydrocarbons)from damaged CH3OH. While it is believed that these, changes represent electron beam damage on the surface, the possibility exists that some of the products may be formed during the heating process by reactions involving radical fragments. The effect of electron damage upon the TDS spectra of (CH3)20 was similar. (CH3)20 was desorbed from th,, surface at-0120 K. Some H=O desorption occurred from undamaged f'tlms at ~160 K. After 6 × 10 -4 C/cm 2, (CH~)20 still desorbed, but the H20 peak had increased in size mad ,,~ifted higher in temperatare to "-185 K. Again small peaks near 26 and 43 role were detected indicating the presence of hydrocarbon products. Electron bombardment damage was also detected for water multilayers, kiut at much larger electron doses. Fig. 6 shows the E/d~S O ( K W ) spectra for this case

128

P.H. Holloway et aL / Electron spectroscopy of condensed multilayers O(KVV)

5

~

].l

H20

i 1.4

l. 5 x 10-3

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..... 430

I ,170

1 ....

i 510

550

~ 1 U I I R O N EN[R(;Y e V

l.ig. 6. F:cctron excited O Aut:cr spectra after varying amounts of electron bombardment on tt 20 multilaycrs.

taken with a current density of 3 × 10 -s A/cm: (nonrastered beams). The corresponding XAES spectrum has been discussed previously [2]. The different signal-tonoise ratios for the various spectra are due to different measurement times. For electron bombardment doses much in excess of 1.4 × 10 -2 C/cm 2, distinct changes are seen in the O(KVV) spectra shown in fig 5. The relative intensities of the vario~s ~tr~lctural features are altered, and ~there is a dramatic growth in intensity of the "shouk!er'" at 508 eV wit~ increasing bombardment time. In the case of solid H20 ~oth the number and variely of potential stable reaction products is limited. Attestin,,L~ to n~s molecular stability, erosion (., ~¢ ice fihns by MeV ions results in stoichiometric loss of H,O ~7]. However, the electron-damaged layer is probably hydrogen deficier~t since H" is the (lominant ion liberated by electron bombardment of ice I9]. Thee nature of the oxygen-rich product trapped in the condensed layer after electron bombardment is not known.

P.H. Holioway et aL /Electron spectroscopy of condensed multilaye,s

129

3.2. Comparisons between XAES and EAES spectra for condensed CH30H and (CH3):O During the studies of condensed multilayers, differences between the EAES and XAES spectra were noticed in several instances. Most prominent were the differ. ences observed in the C(KVV) species of methanol and methyl ether. Fig. 7 shows a comparison between EAES and XAES from a (CH3)20 multilayer. Fig. 7A is a plot of the XAES C(KVV) spectrum [2], while fig. 7B is the EAES C ( K W ) spectrum obtained following a 3.6 X 10 -s C/cm 2 electron bombardment dose using a low intensity, rastered, !500 eV beam (Je = 4 X 10 -'T A/cm2). Electron bombardment has broadened the peak at " 2 5 2 eV, but there is also a lack of intensity at 233 eV in the EAES as compared to the XAES spectrum. Further electron bombardment (7C) caused a further broadening of the dominant feature at --252 eV, but an XAES spectrum taken immediately following the electron bombardment (7D) reveals that the peak at 233 eV is still present. Thus, the reduction of intensity at 233 eV in

CiKVV) (CH3120

I)

°

D - - 6 x 10 -4 co

2

~-

A m U'N~AMAGED XAES

185

" 2152

..._ I

?65

_

.__

305

ELECTRON ENERGY-eV

Fig. 7. Comparison of electron and X-ray excited C Auger spectra both before and after damage. Note the decreased emission at -~233 eV ir, the electron excited case.

130

P.H. Holloway et aL / Electron spectroscopy o f condensed multilayers

O(KVV)

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L

"

t

8 ........... 1 . 2 x lo -3 cou,l~mZ

_ A ~l. .~"

l.

/" ~!

3x 10"4coul/cm2 ljf~ I

.

GED H20 C............ UNDAMAGED (CH3~20 l 1

430

410

510

5%'

ELr CTRON [NER(;Y oV

Fig. 8. Electron and X-ray excited O Auger spectra from (CH3)20 and t120 multilayers after correcting for background and inelastic scattering processes. (A) electron exci~cd (C|t3)20, ~ ? ~ 10-4 C/c~,-a2 (B) electron excited (CH3):O, 1.2 × l0 -3 C/cm2" (C) X-ray excited from an undamaged (CH3)20 multilayer; (D) X-ray excited from an undamaged H20 multilayer.

figs. 7B and 7C is clearly not due to an irreversible electron-induced change in the chemistry of the (CH3)20 layer. Rather, it ~ppears that this intensity difference reflects a basic difference in the Auger structure as excited by electrons and X-rays, respectively. To check for a possible dependence of ,Auger intensity on electron e:~citation energy, the primary beam energy was varied between 500 and 3000 eV in 500 eV steps. These EAES spectra replicated those in fig. 7B. Essentially the same differences observed for (CH3)20 were found for the C(KVV) spectra of condensed CHaOH using EAES and XAES (not shown here). The O(KVV) spectra were equivalent when comparing EAES with XAES. This is shown in fig. 8 where spectra A and C should be compared. These peaks have been corrected for inelastic scattering as will be discussed shortly. Generally, the signalto-noise ratio for EAE$ of O(KVV) is much worse than for C(KV~I).

P.H. Holloway et al. /Electron spectroscopy of condensed muitilayers

131

4. Discussion

4.1. Beam damage Procedures for the determination of cross sections for ESD processes in adsorbed monolayers are well established [ 1] but the sensitivity of AES to several molecular layers (electron escape depth ~ ~ 10 A for 500 eV electrons) and the complex chemistry in the bombarded region make this a less straightforward process for multilayers. On the basis of the data in figs. 1 to 4, it appears that for 1500 eV, doses of "1 X 10 -3 C/cm 2 will cause damage easily detectable by AES. By analogy with ESD in monolayers: we assume that the concentration of undamaged species in the surface region at tin:e t, N(t), obeys the first order relation

N(t)/No = exp(--D't/e),

(1)

where / is the current density (A/cm:), Q is the cross section (cm 2) for dest~ction of the species in the region detected with AES (~-'10 A), e is the electronic charge, and No is the initial concentration. Assuming further that detectable changes will occur in the AES spectra for 0.1 ~< Q/He ~< 1, then 10 -t6 -< Q ~< 10 -~s cm 2. The cross section per molecule for the destruction of the condensed species [(CH3)20 or CH3OH] is Q × n, where n is the (approximate) number of molecular layers in the AES region of sensitivity. For the present case, n is " 3 and thus 3 × 10 -t6 ~< Q < 3 × 10 -1 s cm 2. This range of values is higher than typical dissociative ionization cross sections measured for simple gaseous molecular species for electron energies >1000 eV [8]. However, the total damage cross section by a 1500 eV beam in a solid will be significantly greater than the primary ionization cross sections. Most of the damage will be due to multiple inelastic scattering of the primary beam as well as the low energy secondaries generated during scattering in the condensed layer (see ref. [9] for discussions of beam-induced damage). Gerlach and DuCharme [4] have measured the K shell ionization cross sections for electron bombardment of surface C and O atoms, and found values of 3 × 10 -~9 and 7 X 10 -:° cm 2, respectively, at ~-1500 eV electron energy. Thus, the cross sections for damage of the surface layer due to desorption, decomposition, etc. are several orders of magnitude greater than the cross s,,ctions for the initial step in the Auger process, viz. K shell ionization. As discussed above, this large difference is due to damage by multiple inelastic collisions of the primary beana and by ~oennd,ary electrons. It is not surprisi_n_g~ therefore, that si~ificant molecular damage can occur during the course of typical Auger meast.:ements unless extreme care is taken to minimize electron beam exposure. Coad and Rivi6re [10] have studied electron beam damage in layers adsorbed on a number of different and oxide samples. Using both EAES and XPS, they observed damage i,a contamination on Ni at doses of "-'2.4 X I0 -~- C/cm 2. In fact, they recommend that EAES data be taken at doses of le~ than-'2 X 10 -3 C/cm 2 when possible. These values are quite consistent with the present results. In addition, sen-

132

P.H. Hoiloway et al. ] Electron spectroscopy o f condensed multilayers

sitivities to electron beam damage of the order of 10 "6 C/cm 2 at 10 keV has been found for the high molecular weight polymeric species used as negative resists in electron beam lithography [ 11 ]. This extreme sensitivity arises from the large molecular size, the penetration depth of the primary beam, the generation of low energy secondaries, and high cross section for "splitting off" ligands from the giant molecules. Previous ESD studies of H:O multilayers [6] have indicated that H + is the dominant ionic desorptie.~ product, and bombardment of ice with low energy electrons at certain temperatures has produced clusters of the form H÷(HaO)n [12]. Clearly an electrori beam induces dissociation of molecules in the solid(as well as possible neutrat~ desorption) apparently leaving the film rich in oxygen. The chemically unsaturated species produced in the f'dmr probably undergo further reaction with adjacent n,.'utral species analogous to the methyl radical formation and subsequent H abstrac :ion reactions observed for UV irradiation of methyl halides in alcohol "glasses" [' 3;!. In the case of electron bombardment of CHsOH and (CH3):O the present T~3 data clearly show that new chemical species (H20, C2Hx, C3Hy) are desorbed foUowing electron bombardment. We have previ:~usly investigated the use of Auger peak structure to characterize the chemical species present on the surface [2,3], therefore it is interesting to examine the shape of Auger peaks which result from electron damage in light of the TDS evidence for election beam cl+emical modification. To compare peak shapes, the spectra were corrected for the se~.ondary electron background according to the prescription of Madden and Schreiner [14] and then deconvoluted following the Madden-Houston [151 procedure. An approximate loss function for the deconvolution was obtained by measuring the spectrum of electrons backscattered from the multilayer when the ir.cident beam had a kinetic energy equal to that of the Auger electrons (e.g., "+250 eV for C) [2,3]. Th," results of such corrections are shown in fig. 8 for O(KVV) from (CH3)20. In this case the spectra excited from a multilayer by electrons (b~th low and high damage) are compared with an X-ray excited multilayer spectrum. Note that the EAES spectrum from a highly damaged !.2 × 10 -3 C/cm 2 layer (fig. 8A) is only slightly broader than the low damage (8B)or X-ray excited (8C) spectra. Also included in fig. 8 is the corrected X-ray spectrum from an tt20 multilayer (8D). With the exception of a slight broadening, only small differences are observed between figs. 8A and 8B in the damaged O(KVV) spectra even though H20 is known (from TDS) to be present. With the presence of H20, one might expect a high energy ( ' 5 0 0 eV) shoulde to develop upon creation of H20. However, this unique shoulder for the solid H20 spectrum has been attributed to the fact that H20 is bound in the 3-dimensional, tetrahedral structure of ice [2,31. In the present case, the H20 resulting from electron beam damage is trapped in a |lm,ttltA ~,~usolid t~+N:+p2~, " " ' " . . . . .anu . . . . . . .tins . . strong binding feature at 500 eV should not be (gbscrved. consistent with fig. 8. Siafila~° data for carbon arc shown m "ig 9. Fig. 9A shows the electron excited gas phase spectrum from C2H 6. Because of its vapor pressure, C2H 6 will probably ao~ be present in the condensed layer, b~tt it is the prototype for all higher mole-

P.H. Holloway et el. / Electron spectroscopy o[ condensed multilay~,

133

C(KVVI

u'l Z a6 e,-

x

10.4 coultcm2

z

~ A S E 1~-

| .....

2?0 [I f£IRI'IN fNft~f;Y eV

2~o . . . .

C?H6

310

Fig. 9. Comparison of the true C Auger line shape from damaged (6 x 10"~ C/cm=) (CH.a)zO layers to that from gas phase CzH6. (A) electron excited gas phase C2H6" (B) electron damaged (CH3)20 multilayer; (c) spectrum generated by adding a fresh (CH3)20 spectrum to a C2H6 spectrum (see text).

cular weight linear alkanes [16]. Note that the breadth of the C2H6 peak is similar to that of the damaged (CH3)20 multilayer (fig. 9B). In fact, fig. 9C is the result of summing the spectra from gas phase C:H6 (attentuated by 0.55)and fresh (CH3)20 multilayers (XAES), then renormalizing. ~ e agreemem between spectra B and C is quite good considering the fact that C2H6 is used for illustrative purposes. In actual fact, the TDS data suggest that more than one new species is present and each has its own characteristic Auger spectra [16,17]. Thus, exact agreement is not expected, but in fig. 9 is consistent with the creation of new hydrocarbon products by electron bombardment: The comparison between corrected Auger peaks from damaged and undamaged CH3OH multilayers gave similar results. Electron damage broadened the ~ K W r) spectrum, and the breadth and shape of the C(KVV) spectrum was consistent with the formation of hydrocarbon species. It is reasonable therefore to presume that

134

P.H. Holloway et al. / Electron spectroscopy o f condensed multilayers

Auger peak shape analysis :may be used as a tool to study the chemical state of unknown species, once a l~rger body of information is gathered on known materials. 4.2. Excitation effects on line shapes

A major questior, arising from this work concerns the origin of the differences between the EALS and XAES C(KVV) spectra from condensed multilayers of CH3OH and (CH3)20. Data in fig. 7 and fig. 10 show that the major difference revolves the int,;nsity of emission of electrons with ~ 2 0 eV less energy than the main C peak. T a ensure that this difference was not due to electron transport phenomena, the ~pectra in fig. 7 were corrected as described earlier. The results, shown in fig. I c demonstrate that the intensity difference at "233 eV is real and

CIKVV) (CH.)J20

C ~

×AES

6 x l0 -4 couffcm 2 Z

o•

D............ EAES

6 x l0 -4 coultcm 2

c~

Z

A ~

XAES UNDAMAGED

B ........... EAES

3. 6 x I0 -5 coullcm 2

!

fl

ELECTr~ON ENERGY-eV

~ i~.,. ]0. l.ilcctron and X-ray excited C Auger spectra from (CH3)20 multilayers after correcting g~ ]~ackg~u~,d and i~,,:lastic scattecing pro~:csses. {A) X-ray exched, undamaged; (B) electron c~,c~cd, 3.~ :~ ltl ~ ('~on) 2 ~('~ X-ray excited, 6 x 10 -4 C/cm 2" (D) clec!ro!~, c~:cited, 6 × 10 -4 ( ~c~ 2

P.H. Hoiloway er al. / Electron spectrost'opy ~

~

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r

s

135

not a result of electron transport phenomena in the multila~r. This eahancement at 233 eV was observed with both Mgl~ and ~ radiation which elin'dnates the possibility that "impurity" X-rays were causing artifacts; in addition, the~ spectra show no enhanced emission " 2 0 eV below the main feature; eliminating the possibility of spuriousanaiyzer response. . . . . . . . . . . . -...................................... Battle and Brundle [18] have compared the electron ~ ex electron peaks from a rmonolayer of CO and adsorbed o n M o , U d detected differences dependent upon the excitation ~mode ~ difgerencesare ~ ~ changes in the intensity of emission and Barrie and Brundledid notattempt to explain their origin Moddeman et a l [ 19] have reported enhanced emission for electron V e ~ r X r a y emission from Na attd Oa. In their case, electron excitation caused onthe ~ high kinetic energy side of the main Auger features; these~ !o~the occupation of Rydberg states (i.e., trapping of electrons in unoccupied molecular orbitals). Such phenomena cannot explain the ~233 eV peak enhancement in the present case, however, since it is observed for X-ray excitation (and notforelectron excitation) and is on the low kinetic energy side. Jenni~n's calculations [3] of the Auger spectra for CH3OH demonstrate that a featum:at~233~eVi~in~ spectrum is a normal transition, based on the known m o l e c ~ orbital s t r u c k . But, the enhancement of the intensity of this ~233 eV feature may be due to secondary processes, such as shake-up or shake.off, whose transition probability depends on the mode of excitation. In fact, Hussain and Newns [20] have ~own ~hat shake-up peaks from the O 2s level can cause structure ~24 eV below the main XPS peak. Similar structure can occur below an Auger peak, e.g, from the C2s lev¢! which has a binding energy ~ 16 eV. Evidence to date has indicated that the probaability of shake-up and shake-off processes is essentiaUy the same independent of whether excitation ~ ccurs by photo-ionization or electron impact, i.e. the "Sudden Approximation Theory" is obeyed [21 ], The observation of e difference in the present case represents a breakdown in this theory. Similar bre~downs have previously been observed, but only near the threshold energy for the core level i0ni~tion [21]. In this instance, both AI and Mg radiation are well above the critical energy of the C l s level, and a breakdown of this theorem is not expected. Thus, while differences in the probability of shake.up and shake-off best explain the differences in Auger peaks excited by X-rays and electrons, we have no conclusive proof that such differences are the origin of our observation.

5. Summary Auger line-shape analysis has been used to study the effects cf electron i~npact on thick layers of adsorbed methanol (CH3OH), methyl ether [(CH3)20] and water (H20). The data shc,w that doses as low as ~--5× 10-4 C/era ~" can cause detectable changes in the Auge: spectra. This is equivalent to the dose accumulated

136

P.H. !iollo~), et at. /Electron spectmscopy of condens~ muitilayers

in 1 s using a I vA beam, 0.5 mm in diameter; therefore electron damage is a serious obstacle to the use of EAES in studying fragile molecules. Core-level photoelectron changes, Auger spectral changes, and thermal desorption spectroscopy (TDS) all indicate that new chemical species are formed by electron damage. TDS data showed that H20 ~s produced in CH3OH and (CH3)20 layers along with unknown types of C2Hx and C3Hy products. This was shown to be consistent witb changes in the Auger features. An enhanced emission (not an electron damage effect) was obse,rved in the C ( K W ) spectra (at "-233 eV) when the Auger process was initiated with an X-ray rather than an electron, The origin of this enhanced emission was discussed, and the most likely explanation is a shake-up or a shake-off process which is sensitive to the specific mode of core-level excitation. This would represent a b .....~.,down in the "'Suddetl~ Approximation T h e o r y " at energies well above the criti~ ~1 ionization energy.

Acknowledgement The authors are grateful for valuable discussions with Drs. W.E. Moddeman, C.J, PoweU, J.W. Gadzuk, and D.W. Jennison.

References [ 1 ] T.E. Madey and J.T. Yates, Jr., J. Vacuum Sci. Technol. 8 (1971) 525. [21 R.R. Rye. T.E. Madey, J.E. Houston and P. Holloway, J. Chem. Phys. 69 (1978) 1504. I3] R.R. Rye, J.E. Houston, D.R. Jennison, T.E. Madey and P.H. Holloway, I&EC Product Res. Develop. 18 I1979) 18. [4] R.L. Ger!ach and A.R. DuCharme, Surface $ci. 32 (1972) 329. [5] P.A. Redhead, Vacuum 12 (1962)203. [6] T.E. Madey and J.T. Yates, Jr., Chem. Phys. Letters 51 (1977) 77. [7] W.L. Brown, L.J. Lanzerotti, J.M. Poate and W.M. Augustyniak, Phys. Rev. Letters 40 (1978) 1027. [8] D.R. Bates, Ed., Atomic and Molecular Processes (Academic Press, New York, 1962). [9] M. Isaacson and M. Utlaut, Optik 50 (1978) 213; M. lsaacson, Specimen Damage in the Electron Microscope, in: Principles and Techniques of Electron Microscopy, Vol. 7, Ed. M. Haya, (Van Nostrand-Reinhold, New York, 1977) p. 1. [ 10] J.P. Coad and J.C. Rivi~re, Faraday Disc. Chem. Soc. 60 (1975) 269. [ 11 ] See, for example, L.F. Thompson, J.P. Ballantyne and E.D. Feit, J. Vacuum Sci. Technol. 12 (1975) 1280. [ 12] R.H. Prince and (;.R. Flcyd, Chem. Phys. Letters 43 (1976) 326. [13] R.L. Hudson. M. Schiotani and F. Williams, Chem. Phys. Letters 48 (1977) 193. [ 14 ] H.H. Madden~ and D.G. Schreiner, Sandia laboratories Report No. SAN D76-0283 (1976), unpubfished. [15] H.H. Madden a~ ] J.E. Houston, J. Appl. Phys. 47 (1976} 3071. [ 16 ] R.R. Rye and J.E. Houston, unpublished data, June 1978. [ 17 ] K. Siegbahn, G. Nordling, G. Johansson, J. Hedman, P.F. Heden, K. Hamsin, U. Gelius, T.

P.H. Holloway et al. / Electron spectroscopy o f condensed muitilayers

! 37

Bergmark, L.O. Werme, R. Manne and T. Baer, ESCA Applierd tO Free Molecules (Elsevier, Amsterdam, 1969). [ 18] A. Barrie and C.R. Brundle, J. Electron Spectrosc. Related Phenomena $ (1974) 321. [ 19] W.E. Moddeman, T.A. Carlson, M.O. Krause, B.P. Publen, W.E. Bull and G.K. Schweitzer, J. Chem. Phys. 55 (1971) 2317. [20] J.S, Hussain and D.M. Newns, Solid State Commun. 25 (1978) 1 ~ 9 , . . . . . . . . . . . . [21 ] W.E. Moddeman, T.A. Carlson and M.O. Krause, Phys. Rev, A 1(1970)1406.

Surface Sci. 88 119791 138-152 O North-Holland Publishing Company

PHOTOELECTRON SFECTROSCOPY OF NICKEL ON ZINC OXIDE IN THE MONOLAYER AND SUBMONOLAYER RANGE D. SCHMEISSER and K. JACOBI Fritz-Haber-lns "irat der Max-Planck.Gesellschaft. Faradayweg 4 - 6. D- 1000 Berlin 33. German),

Received 13 March 1979" manuscript recdved in final form 5 June 1979

Nickel films of thicknesses between 0.05 and 5 monolayers were deposited by evaporation onto the polar ZnO(0001)Zn and ZnO(000i)O surfaces. The character and thickness of the Ni films were deteImined by Auger electron spectroscopy. The wod~. function, the bending of the substrate bands and the emission from the Ni 3d states were studied by UV photoelectron spectroscopy. After deposition of about 2 monolayers of Ni the O face showed an upward band bending whereas the Zn lace showed hardly any at all. After reaching a film thickness of about one monolayer the emission from the Ni 3d states becomes similar to that from the 3d band of bulk Ni. For a film thickness below 0.2 of a monolayer the emis:ion features at -4.3 eV and -6.0 eV were attributed to Ni atoms interacting with oxygen from the substrate and the features at -2.1 and - ! . 3 to Ni atoms and two-dimensional Ni clusters.

1. Ineroducfion Fhe properties of small metal aggregates like three-dimensional clusters [1] or tw,~-dimensional layers [2] have recei,:ed increased attention during the last few years. This is partly due to the fact that small metal particles are the active part of supported catalysts [3]. One of the most important u n k n o w n feature of such finely d!v~ded material is the density of states within the valence band region. This can, in principle, be probed with UV photoelectron spectroscopy (UPS). Recently [4] we have reported the condensation and characterization of Pd layers on the polar ZnOt0001 tZn (Zn face l and ZnO(000]i)O c ) face) surfaces. Here we report on Ni condensates deposited onto the same ZnO surfaces. For Pd we found in UPS an art: mic 4d level with a FWHM of 1.3 eV lying with its maximum 2.5 eV below the ~,ventual Fermi edge o f the thick Pd layer. This atomic Pd 4d-line develops into a 3 eV wide bulk Pd 4d band when m o n o i a y e r coverage is reached, in the meantime a sinlilar result was f\~und for Pd deposits on carbon (5]. For Ni the transition from an atomic 3d level to a bulklike 3d band should be observable in a similar way. On the od~er hand one must take the interactions between the reactive Ni atoms and the Zn and O surface atoms of the ZnO subs~rate into account. Recently UPS measurements on the Ni/carbon system have been reported I61, which are discussed ill the context of 1tile present results. 138