Auger electron spectroscopy of insulating silicon compounds

Auger electron spectroscopy of insulating silicon compounds

Auger electron compounds* received spectroscopy of insulating silicon 10 July 1972 B Carriere, J-P Deville and S Goldsztaub, 1 rue Blessig, 67 St...

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Auger electron compounds* received

spectroscopy

of insulating

silicon

10 July 1972

B Carriere, J-P Deville and S Goldsztaub, 1 rue Blessig, 67 Strasboorg, France

Universite

Louis

Pasteur,

Laboratoire

de Minkalogie

et de PBtrographie,

The difficulties which are to be encountered when studying insulators by means of Auger electron spectroscopy are reviewed. Examples of charging-up phenomena occurring if insulators are struck by an electron beam are taken among Auger spectra of silicon compounds, particularly silicates. These phenomena are discussed and investigations have been made to measure or to limif these charge problems. They have shown that great care must be taken during fhe AES study of chemical shifts or valence spectra of insulating materials. Routine investigations require less care and may be carried out on a number of industrial silicates like glasses.

Introduction

Auger electron spectroscopy is a powerful technique for surface analysis but until now it has not been much used to investigate insulators. This is because practical and theoretical difficulties are to be encountered if using an electron beam as a probe to study surfaces. We have tried to find out if it is possible to overcome these difficulties and how this can be done. We shall report here only the results which were obtained with silicon compounds, although we have studied many other insulators like alumina, lithium fluoride. Experimental techniques The analyzer. A home-made,

conventional four-grid optics of a LEED apparatus was used as a retarding field analyzer in the manner described by Weber and Perial. A capacitance neutralizer and part of the electronics are Vacuum Generators’ devices. As all experiments must be carried out in ultra-high vacuum, the electron optics is set in a bakeable, ion-pumped stainless steel vessel where typical pressures of 1O-8 to lo-lo torr can be achieved. Figure 1 shows a sketch of the LEED-Auger system. The retarding potential is applied on grids G, and G,. G1 and G, are earthed so that, firstly, field-free conditions are insured between the sample and the analyzer and, secondly, capacitive induction is reduced between the suppressor grids and the fluorescent screen where electrons are collected. The collector current is then sent in a lock-in amplifier and dN(E)/dE curves are recorded. All measurements were made at normal incidence with primary electron beams the energy of which was comprised between 400 eV and 1200 eV. The cathode current which gives reliable information about the number of electrons impinging on the surface was maintained at moderate or low values (l-100 PA). Some experiments were done with a coaxial cylindrical Auger spectrometer to compare its possibilities to those of the retarding field analyzerZ. In that case it was possible to work at glancing or normal incidence. The samples. The insulating silicon compounds under examina*Paper No 24. Vacuum/volume

PPlnumber

10.

Pergamon

Press

LtdlPrinted

Figure 1. Sketch of the LEED-AUGER

system.

tion were micas (muscovite, phlogopite, biotite and lepidolite), the formula of which is K, X,_, (Si,Al), O,, (OH),, with X= Al, Mg, Fe, Li, feldspars, fused silica, quartz and glasses from two systems: Na,O-SiOa and Li,O-SiOZ. Besides, several comparisons were made with non-insulating silicon compounds such as pure monocrystalline silicon and iron-silicon alloys. All samples were cleaved or broken under ultra-high vacuum just before investigation to avoid surface contamination. Results and discussion Evolution of Auger spectra of insulators. It is a well known fact

that the Auger transitions of a given element in a given compound should always have the same energy. But, in the case of insulators, we noted the unexpected fact that, if the energy and current density of the primary beam were low, the Auger spectrum was shifting in course of time towards higher energies. For a given sample the shift was the same for every Auger peak, from whatever element it arised, and it stopped after a certain time as if an equilibrium value was reached. If the incident beam was very energetic this final state was observed at once. This was the case for Poppa and Elliot3 who studied micas. For in Great Britain

485

B Carriere, J-P Deville and S Goldszfaob:

Auger electron

spectroscopy

reasons of convenience, mainly due to the time constant of our apparatus, we chose to study only the Auger peaks due to the silicon atoms. Auger spectrum of silicon in silicates. The evolution of the silicon Auger spectrum of a quartz sample is shown in Figure 2. This evolution and the shape of the spectrum are similar for all other silicates. Just after having broken the sample, a series of three peaks is observed (time t,: 10 mn); these three peaks, E,, Ez and Es are located respectively at 70 eV, 54 eV and 45 eV. They have decreasing heights and the E, peak, which is the largest, will be called the main peak. After about 3 h bombardment (1000 eV, 40 ,uA) the peaks are shifted towards higher energies, respectively 78 eV, 62 eV and 52 eV (time t4). Besides, during the electron bombardment a new peak E’, appears at an energy which is 12 eV higher than that of the main peak. For example, at time tz (40 mn) E’, is located at 85 eV and at time I, it is at 90 eV.

of insulating

silicon compounds

dN(E XT t,

3Omn

ta

60mn

170pA

9OOeV

f

9Omn

15Omn

2OOpA

200pA

900eV

900eV

2OOpA

dN dE

t,

ta 70

40mn

85 tl

Figure 3. Evolution of the silicon Auger spectrum of biotite mica.

50

a7

,p

t, 48

120mn , o~Y&~

73

H

74

210mn ,04030%

5.9

78

F

99 El

El

, 40

60

80

E

100

Figure 2. Evolution of the silicon Auger spectrum of quartz. An experiment with biotite, an iron-magnesium mica, is shown in Figure 3. The shift of the silicon spectrum from time t, to time t, is clearly demonstrated. At time t, the position of the electron beam on the crystal has been changed so that a new area is struck. The position of the main peak is then 73 eV and at time t4 after 1 h bombardment the final state is reached on this new area. The silicon Auger spectrum of silicates can be then characterized by four peaks which are related each one to another in the following manner:

E;-El=12

eV, E,-E,=16

eV, El--E,=25

eV.

Discussion of the results. The E’, peak which appears during the electron bombardment and which is located in the neighbourhood of the main Auger peak of pure silicon has been 486

2OOpA

1 obsoe~

54

45

200mn

related to a reduction of the silicon atoms located in the SiO$ tetrahedra4. These tetrahedra build the structural skeleton of the silicon compounds under investigation. This new peak illustrates drastic changes in the valence spectrum of silicon and shows that AES may be used for an analysis of the chemical bonding. This point, being beyond the scope of this paper, will not be discussed in detail. The shift of the whole spectrum has been attributed to charges which arise at the surface of insulators. A substance, if struck by a beam of charged particles can release excess charges in one of two ways : either by conduction to earth or by the secondary emission process. In the case of an insulator the second process alone is possible. We know that 6, the secondary electron emission yield, is a function of V, the energy of the incident beam, and that it is lower than 1 below a given threshold V, and above another one V, (Figure 4). Between these two

d, .

2~

2-

1.4 -

l-

I I I

I i

, I

I

j”, I

55

Iv2 350

800

1000

_ VP

Figure 4. Secondary electron emission yields of gold and mica vs energy of the primary electron beam.

B Carriere, J-P Devil/e and S Goldsztaub:

Auger electron

spectroscopy

thresholds 6 is higher than 1. VI and V, depend on the material and, for micas, they are about 40 and 2400 eV. In our working conditions, 6 is always higher than 1 so that more electrons leave the surface than arrive on it. For insulators this leads to a local positive charge which speeds up the electrons leaving the sample. So it appears that charging-up problems can change completely the energies of Auger peaks and lead to misinterpretations. The measure of surface charging-up of insulators

The idea was to measure the surface potential by means of a calibration with a conductive material. A thin film of gold, the Auger peaks of which are well known, was deposited onto a freshly cleaved mica surface so that we had three distinct areas: the first one consisted of a gold film on mica, the gold film being grounded through the specimen holder; the second area was mica without any gold and this area insulated the third area from the first one, this third area being gold-plated mica. On the first area the Auger peak of gold located at 69 eV did not shift during the electron bombardment whereas on the third area this peak shifted from 69 to 73 eV. So, a 4 eV shift was measured on insulated gold whilst during the same time of bombardment 8 eV was measured for the silicon peak of mica. This calibrationmethod, which ascertains the existenceof superficial charges, is not accurate because of the differences between the secondary electron emission yields of gold and mica; 6 is higher for mica than for gold (Figure 4) and that explains a more positive charge for mica. The calibration method might be improved by vaporizing only l/10 or l/5 of a monolayer of any conductive material on an insulator. Of course this very thin film should be deposited inside the LEED-Auger system. Because of the importantwidth of the incident beam the secondary electron emission yield would be averaged and then the measure of the surface potential of an insulator might be possible. How to limit charging-up problems with insulators

As the calibration method is rather complicated to carry out we tried to find out how to limit charge problems. A method has been sometimes used by various authors. It consists of biasing the first grid so that low energy secondaries are repelled and go back to the sample. Then a discharge occurs but with this method a great deal of resolution is lost, specially at low energies, and of course the exact potential of the surface is never known.

of insulating

silicon

compounds

The best way of limiting the charging-up of the insulating sample is to study areas which are in the neighbourhood of the conductive earthed specimen holder. It has been done for glass samples and very good results were obtained. We noted in that case that the silicon Auger spectrum was located at higher energies for areas in the centre of the sample than for areas near the specimen holder. Finally, for some alkaline glasses, which are important industrial materials, it is possible to avoid completely all these problems by heating the sample up to 500°C. The conductivity of these glasses increases largely with temperature and we actually noted that the silicon Auger spectrum was shifting towards lower energies when glass samples were heated. At about 5OO”C, the main peak is located at 70 eV which is the value for the non-charged specimens. Conclusions

We have shown that it is possible to use AES to study insulators. If the elements which are to be analyzed give well known Auger peaks and if these peaks do not suffer important chemical shifts or drastic changes in the valence spectrum, charge problems will not be actually very troublesome. Shifts due to the charging-up of the sample will occur but the difference between

the various

Auger peaks

will be always

the same. So,

instead of looking at the position of the Auger peaks as it is generally done, we shall have to study the differences between these Auger peaks. It will be possible in that case to have a finger-print of an element in an insulator and it will be sufficient for routine analysis. If we need to study chemical shifts or valence spectra on an insulator we are bound to have serious difficulties. Particularly it will be of importance to use primary beams having low current densities and to sweep the Auger spectrum slowly enough to be sure that any shift is due to physical or chemical reasons related to the sample and not to the electronics of the analyzer. In any case it will always be interesting to carry out measurements using various techniques to get some idea of the surface potential and to avoid misinterpretations of the AES results. References 1 R-T Weber and W-T Peria, J appl Phys, 38, 1967,435s.

a A Eberhardt, Thkse de Doctorat de SpCcialitC,Strasbourg (1971). 3 H Poppa and A G Elliot, Surface Sci, 24, 1971, 149. *B Carriere, J-P Deville and S Goldsztaub, CR hebd Acad Sci, Paris, B274,1972,415.

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