Chemical state effect in the auger (C KVV and Si LVV) spectra of hexamethyldisilane

Chemical Physics 129 (1989) 491-494 North-Holland, Amsterdam

CHEMICAL STATE EFFECT IN THE AUGER (C KW AND Si LW) SPECTRA OF HEXAMETHYLDISILANE

G.G.B. DE SOUZA”, R. PLATANIA b, A.C. DE A. E SOUZA a and%. MARACCIb a Universidade Federal do Rio de Janeiro, Institute de Quimica, Cidade Vniversitciria. 21910 Rio de Janeiro, Brazil b Consiglio Nazionale delle Ricerche, Istituto di Metodologie Avanrate Inorganiche, Area della Ricerca di Roma, Montelibretti, Via Salaria km 29.300, C. P. 10, 00016 Monterotondo Scala, Rome, Italy

Received 5 May 1988

Carbon KVV and Si LVV Auger spectra, produced by electron impact, have been measured for the hexamethyldisilane molecule Si2(CH3)6, in gas phase. A comparison with similar spectra obtained for the tetramethylsilane, Si(CH,)4, shows that while the Si L*,,VV spectrum reflects the difference in chemical environment surrounding the silicon atom in both molecules, the C KVV spectrum remains practically constant in the two compounds. The unsuccessful search for either L,VV Auger transitions and LILz,3V Coster-Kronig transitions may be associated to a very low intensity for both processes.

1. Introduction

The study of core-valence-valence (CVV) transitions in Auger spectroscopy has been shown to be a quite efficient way of getting information on local chemical environment in gas phase molecules [ 1,2]. These transitions can, in fact, originate from electronic levels which are both localized on a specific atom coupled to the valence electrons; they are thus ideal probes for the chemical environment which surrounds each component atom in a molecule. One specific point of interest concerns the bonding process in a molecule, as seen by individual participating atoms. By selectively regarding CVV Auger electron transitions centered in different atoms, one may be able to obtain useful information on their participation in the chemical bonding. An interesting example of this kind of study has been provided by Rye, Jennison and Houston, who measured the carbon KVV Auger spectrum of alkanes [ 3 1. On going from methane to hexane, it was observed that all CVV spectra were centered at about 249 eV. The shape of the spectrum showed a noticeable change on moving from methane to propane and remained the same thereafter. This could mean that the carbon atom in methane “felt” the substitution

of one hydrogen atom by a methyl group and that the chemical environment for the carbon atom remains practically constant in the other molecules. A very interesting chemical state effect can be observed, though, in the carbon CVV spectra of methane, ethylene and acetylene, pointing out that the Auger spectrum is indeed very sensitive to the different kinds of hybridization (sp3, sp2, and sp), present in these molecules [ 41. In the present work we address our attention to the study of a possible chemical state effect as observed in the Auger CVV spectra of the hexamethyldisilane molecule, Si2( CH3)6. In order to look into possible effects due to the Si-Si bonding, these spectra are compared to similar spectra of the tetramethylsilane molecule, Si ( CH3 ) 4. As compared to the alkanes, the hexamethyldisilane molecule shows the very interesting feature that one can in principle search for bonding effects from the point of view of two different atoms; in particular one would expect that the silicon atom should be quite sensitive to the chemical bonding. The ability of silicon to form bonds remains an actual problem, with marked differences to the carbon atom (for instance the reduced ability of the former

0301-0104/89/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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G.G.B. de Souza et al. /Auger spectra ofhexamethyldisilane

to form double bonds and the notably bond as compared to C-H [ 5 ] ).

Table 1 HMDS: Si L,,,VV, main lines (upper main lines (lower part)

weaker Si-H

2. Experimental The apparatus used to obtain the electron-impactexcited Auger CVV spectra has been described previously [ 61. The spectra presented here were measured with a 1800 eV, 70 uA exciting beam. The energy resolution was 0.5 eV for the silicon spectrum and 1 eV for the carbon spectrum. The energy scale was calibrated using the Ar L2.3MM Auger transitions in the range 200-2 10 eV and Kr MNN Auger transitions in the range 50-60 eV according to the procedure of ref. [ 6 1. The accuracy of the scale is estimated to be about 0.3 eV. All the spectra were taken at an angle of 33” with respect to the incident electron beam, where the best signal-to-background ratio was found. All the data were subjected to background subtraction [ 6 1. 3. Results and discussion 3. I. Silicon Lz,3 VV spectrum The electron-impact-induced silicon L2,3VV Auger spectra of SiZ (CH,), and Si ( CH3)4 are shown in fig. 1, on a two-hole binding energy scale. The Auger spectrum of tetramethylsilane (TMS) has been obtained before by synchrotron radiation [ 71 and, in

1 3

1

I

t

E

I

-50

,

-45

I

-40

c

1

-35

1. I

I

-30

I

H

I

-25

IKE-IclleVl

Fig. 1. Silicon Lz,s Auger spectra for HMDS (broken line) and TMS (solid line), plotted on a two-hole binding energy scale (KE=kinetic energy; I,= core ionization potential).

Kinetic energy (eV)

HMDS’+

1 2 3

55.5 56.7 60.2 65.2 68.7 71.0 75.0 78.2 81.5

50.8 49.6 46.1 41.1 31.6 35.3 31.3 28.1 24.8

5 6 7 8 9

A B C D E F G H

TMS: Si L>JVV,

Peak

4

Peak

part);

Kinetic energy (eV)

TMS’+

ref. [7]

ref. [7]

69 76 80

ref. [8] 55.4 57.0 60.2 65.8 68.8 72.2 75.6 80.0

37.1 30.1

(eV)

(eV) ref. [8] 50.7 49.1 45.9 40.3 37.3 33.9 30.5 26.1

more detail, by electron impact [ 8 1. The spectrum of hexamethyldisilane (HMDS) has not, to our knowledge, been published before. In table 1 we present the energy of the main Auger lines and the energy of the corresponding double-ion states for the two molecules. The Si 2p,,, ionization energy and valence photon electron spectrum for the HMDS molecule have been measured very recently with the use of synchrotron radiation by de Souza et al. [ 91 who found 106.3 eV for the Si 2p-’ threshold. The valence photoelectron spectrum, which was found to be in good agreement with a previous one measured using a helium lamp by Bock and Ensslin [ lo], shows four bands, centered at about 8.7, 10.7, 14.0 and 21.5 eV binding energy. In the case of TMS, the Si 2p-’ threshold occurs at 106.1 eV and three bands, located at 10.7, 14.2 and 2 1.7 eV are observed in the valence photoelectron spectrum [ 7 1. Except for the lowest binding energy peak of HMDS, the shape and energy of the valence bands for the two molecules are very similar. This raises the point that they could be associated mainly to Si-C and C-H parentages. To the first band Bock and Ensslin associated a SiSi character. Indeed this was confirmed by the cal-

G.G.B.de Souza et al. /Auger spectraof hexamethyldisilane

493

culations of Berkovitch-Yellin et al., who assigned a strong Si-Si bonding character to the HOMO in HMDS (8a,,). An Auger process involving a final state with two holes in the 8a,, orbital should be responsible for the highest kinetic energy band in the Si L2.3VVspectrum of HDMS. The determination of U, the hole-hole interaction which includes correlation and relaxation effects, was based on the simple oneelectron model of Jennison [ 111, according to which the kinetic energy of the Auger lines, KE, can be given by the following expression: KE=I,-I,-I,-u,

where Z, is the core ionization energy, 4 and Ik are the ionization energies of the valence orbitalsj and k. Using this formula and the previous assignment for the most energetic peak of HMDS, we can estimate a U value of approximately 7.9 eV for this transition, This seems a bit surprising as one expects, in a simple model, that U would decrease with the increase in size of the molecule [ 2 1, and also by taking into consideration that an average value of 55 eV was found in TMS [7,8]. This high U value could probably be interpreted, within this simplified model, as an indication of localization due to the formation of Si-Si bonding. 3.2. Carbon KVVspectrum The electron impact KVV spectra of HMDS and TMS are shown in fig. 2, on a two-hole binding energy scale. There is only one published result on the KVV spectrum for TMS [ 8 ] ; the spectrum for HMDS is here presented for the first time. The kinetic energies and energies for the corresponding double ion states are presented in table 2. The C Is- I threshold in TMS is known to be at 289.8 eV [ 12,131 and the C 1s- ’ threshold in HMDS was assumed to be at the same value. A strong similarity is evident between the CVV spectrum of the two molecules; while the L2.3VVsilicon spectrum could be considered as a “fingerprint” for each molecule, the KVV carbon spectrum seems to be practically insensitive to the substitution, in TMS, of a methyl group by a Si ( CH3 )s group. 3.3. Silicon L, spectrum

The incident electron energy used in our experi-

20

IKE-IcllrVI Fig. 2. Carbon KVV spectra for HMDS (broken line) and TMS (solid line), plotted on a two-hole binding energy scale (KE= kinetic energy; &=core ionization potential).

ment should also induce the ejection of 2s (L, ) electrons in the silicon atom (2~’ threshold is 157.31 eV in TMS [ 71) thus allowing the observation of the L, Auger spectrum. A careful search in the loo-140 eV kinetic energy range for the L,VV transitions and the 20-40 eV kinetic energy range for the L,L2.3V Table 2 HMDS : C KVV, main lines (upper part); TMS : C KVV, main lines (lower part) Peak

Kinetic energy (eV)

1 2 3 4 5 6 7 8 9

239.6 243.9 246.6 249.1 251.1 252.4 254.1 257.1 262.1

Peak

Kinetic energy (eV )

TM??+ (eV)

A B C D E F G H I

239.2 244.2 246.6 249.2 251.0 252.0 253.2 257.6 261.8

50.6 45.6 43.2 40.6 38.8 37.8 36.6 32.2 28.0

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G.G.B. de Souza et al. /Auger spectra ofhexamethyldisilane

Coster-Kronig transitions showed that the intensity of the two processes were very low and could not be distinguished from the background. In the case of the argon atom, it was shown [ 141 that the ratio between the electronic vacancies originated from primary ionization in the L, and L2,3shells was 1 : 4 and that the L, holes decay for about 96% [ 151 through the L,L2,3 process while the remaining part decays by other channels (LIVV channel included ). Extending this estimate to our case, we conclude that the overall intensity of the L,L2,3V Coster-Kronig transitions will be at best about 20% of the total intensity of the L2,3VV Auger transitions and that the total intensity of the L,VV transitions will constitute a very much lower fraction. This estimate is also supported by a comparison with the experimental results reported by Madden [ 161 who found the same order of intensity ratio for the LIL2.3V and L2.3VV transitions in solid silicon. It is also well known that the Lz,3 states originating from the parent process L,L2,3V will decay through the L2,3V-VVV channel and will produce satellites which will fall in the same energy region as L2.3VV transitions [ 17,181. We could then have an indirect evidence for the existence of the LIL2,3V process from a comparison between our data, obtained at 1 keV incident electron energy, and the experimental results obtained with synchrotron radiation at a photon energy below the L, silicon ionization threshold. This seems to be the case in TMS, in which for instance peak F (at 72.2 eV) kinetic energy in fig. 1 has not been observed in the synchrotron measurements ]7]. The same peak has not been predicted by the theoretical calculations of Cini et al. [ 81 what seems to corroborate our hypothesis. In the case of HMDS the same kind of analysis cannot be clearly carried out, due to the poor statistics of the only known synchrotron measurement [ 91 and to the absence of theoretical results.

4. Conclusions A comparison between the Auger spectra of the HMDS and TMS molecules has shown that while the carbon KVV spectra have basically the same shape

and intensities, the silicon L,,,VV spectra do reflect the change in chemical environment surrounding the silicon atom. The absence of direct experimental evidence of Si LIL2,3V Coster-Kronig transitions and Si L,VV Auger transitions in both molecules, shows that a very low cross section should be associated to these two processes.

Acknowledgement The authors acknowledge the support of International Scientific Grants from Conselho National de Desenvolvimento Cientifico e Tecnologico, CNPq, Brazil, and Cons&ho Nazionale delle Ricerche, CNR, Italy.

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