Surface modifications of crystalline Sio2 and Al2O3 induced by energetic heavy ions

Nuclear Instruments North-Holland

and Methods

in Physics

SURFACE MODIFICATIONS HEAVY IONS

F. JOLLET ‘I Secrron

‘), J.P. DURAUD

Research

B46 (1990) 125-127

OF CRYSTALLINE

‘), C. NOGUERA

SiO,

des Surjuces,

AND

Ai,O,

2), E. DOORYHEE

CEN Suclay, 91191 Gif SW Ywtte, .‘I Luhoratowe de Physique du S&de, 91400 0rsu.v. Frmce ” CSNSM. 91406 Or.wy, France d’Erude

125

INDUCED

BY ENERGETIC

j) and Y. LANGEVIN

j)

Frmce

Surface modifications induced by high energy heavy ions on quartz (SiO,) and sapphire (A1,O1) have been studied by means of photoelectron spectroscopy and extended X-ray absorption fine structure spectroscopy. Results are interpreted in the frame of density of states calculations performed using a semiempirical tight binding method. Under oxygen ion irradiation, it appears that the induced damage consists mainly of lattice distortion plus a population of point defects identified as oxygen vacancies.

1. Introduction Previous studies have shown that high energy heavy ions (E > 1 MeV/amu) produce stable modifications of the geometrical and electronic structure of silicates as well as quartz [1,2], whereas in Al ,O, no bulk defect has been detected. Using X-ray scattering, electron spin resonance (ESR) and track etching, we showed that the formation of defects is linked to electronic energy losses and not to direct collisions. However, a better characterization of the damage produced by such ions is needed to understand the processes involved. This work deals with surface modification induced by irradiation of the samples with high energy heavy ions, available at GANIL (Caen, France), on quartz and sapphire. Using photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure spectroscopy (EXAFS), we observed a disordered crystalline quartz state due to lattice distortion arising from angle variations between adjacent tetrahedra. A calculation of density of states (DOS) using the semiempirical tight binding approximation enables us to interpret the experimental results.

2. Experimental and data analysis Irradiations have been performed at GANIL, Caen (oxygen. krypton and xenon). We used a low flux (< IO9 ions/cm2 s) so as to minimize temperature effects during irradiation. Both crytalline SiO, and Al 20, have been irradiated with krypton, oxygen and xenon ions of about 10 MeV/amu and with fluences ranging from 8 X 10” to 10 I4 ions/cm2. 0168-583X/90/$03.50 (North-Holland)

Before

irradiation,

the

samples

,T Elsevier Science Publishers

were B.V.

baked at about 1000” C and the surface structure has been studied using low energy electron diffraction (LEED). Reference samples were placed in the irradiation chamber, so that they have the same “history”. without being irradiated. XPS experiments were carried out in an ultrahigh vacuum chamber (lo-‘” mbar) where a monochromatized X-ray source with a twin anode (AI-Mg) was available. In these experiments, the Al-Ka anode was used (12 keV, 20 mA), giving a photon energy hv = 1486.6 eV. Electrons emitted under the photon beam were energy filtered in a Mac 2 analyser (Riber), with a constant resolution A E = 1.3 eV, as checked on a gold sample. Calibration of the spectrometer was performed using the Au 4f,,, photoelectron line ( Ehlndlng= 84 eV). The experimental accuracy when determining energy positions was estimated to be better than kO.2 eV. Detection was done by counting; data could be stored and processed using a microcomputer. EXAFS experiments were performed on a beam line of the Daresbury synchrotron light source in the total electron yield detection mode. The incident beam intensity I,, was measured in an ionization chamber. The total secondary electron current I was detected with a channeltron (Galileo) in an ultrahigh vacuum chamber. I/I,, was plotted as a function of E (photon energy) above the Si K threshold. The ability of the EXAFS technique to yield local structural information has been reviewed extensively. Essentially, the technique measures the oscillations observed on the high energy side of an X-ray absorption edge. The amplitude and frequency of these oscillations are functions of the number, type, and distance of neighbouring atoms to the atom involved in the X-ray absorption process. II. CRYSTALLINE

OXIDES

,’ CERAMICS

F. Jollet et al. / Surface modrficutions ofc~~stalline StO, und Al,O<

126 In the case

of a K edge, the variation

of absorption

x(k)=

CA,(k)

II

I

El III

is given by [3]

:

sin(2kR,+$,(k)). 3

where k = q’( E - E,, ) 2m/h is the electron wave vector and E, is the edge energy. This function is a superposition of the contributions of the different coordination shells, R, in the average coordination distance from the absorbing atom to the neighboring atoms in the jth coordination shell, and +,(k) represents the phase shift. A,( k ). the amplitude function for the jth shell, is given

>’

cd

ev

-_~

lo

.\

A -,

i,..,,_:

by

:.,.;

I__, , A,(k)

= $f,(k)

exp(-2k*o,‘)

exp(-2R,/h),

F(R)

=(~jxW(k)k’

e21kRx(k)

dkl.

where W(k) is the Kaiser-Bessel window. A shell is then selected using a smooth window on F(R), and the back-Fourier transform to k space yields EXAFS spectra, which can be fitted using the theoretical phase and amplitude functions, leading to R,, o, and N,. It is important to note that 0, and N, are correlated parameters.

Fig. 1. Valence spectra of (a) reference

and (b) oxygen- and (c) krypton-irradiated cu-quartz. (“0: lOI ions/cm’, E = 10 MeV/n; X4Kr: 8 X 10” ions/cm’. E = 10 MeV/n).

possible by mean of DOS calculations that we have performed using a tight-binding method. to attribute the maximum I to non-bonding 0 2p electrons, II to the bonding 0 2p-Si 3p and III to bonding 0 Zp-Si 3s on the reference sample (fig. 2a). The spectra reveal a weakening of the structure III after irradiation, the more so as the fluence is high. Such a weakening is different from what is observed when going from crystalline to amorphous SiO, [5]. We may conclude that irradiation induces local disorder on n-quartz. Moreover. this disorder may induce internal stresses in the structure so that, it is possible to break the sample when the fluence is high enough, as was certainly the case under xenon irradiation at high fluence (5 x 1012 ions/cm2. E = 10 MeV/n). Indeed, no temperature increase was measured during the experiment and the current intensity was kept low enough to minimize heating effects (q I 10’ ions/cm2 s). A DOS calculation of the quartz valence band under hydrostatic pressure ( p = 50 kbar) is presented in fig. 2b. It also reveals modifications of the valence band. EXAFS results support these conclusions. Indeed they show (fig. 3) that the major changes occur in the second coordination shell and concern both variations of the Si-Si distances and static disorder in this coordi-

n(E)

n(E)

I

3. Results and discussion 7

Fig. 1 shows valence spectra of reference and oxygenand krypton-irradiated a-quartz obtained by photoelectron spectroscopy (XPS). SiO, has been pulverized under xenon irradiation (5 X 1012 ions/cm*). It has been

:. .._._ /

(d

/ where N, is the number of atoms in the j th shell at the average distance R,; f,(k) is the backscattering amplitude. h is the mean free path of a photoelectron; 0,’ represents the variance at distance R, and contains the contribution from thermally induced vibrational displacements within a shell of neighbors. Phase shifts and backscattering amplitudes were determined using Clementi-Roetti wave functions for Si and 0 [4]. The value x(k) was obtained from the expression x(k) = (p - po)/po; p is obtained from I/I,. pi, is the continuous background, fitted on the experimental curve (above the edge) by means of polynomial spline functions (means least-squares fitting). From x(k) it is possible to derive a pseudoradial distribution function around the excited atom. F(R), which contains a series of peaks whose positions, R,. are related to the coordination distances. The position of the peaks is shifted with respect to the real distances. because of the phase shifts $I ,( k). The distribution F(R) is given by the modulus of the Fourier transform of k’x(k):

lb)

(a) *

AL .

E

10

Fig. 2. DOS calculation

i

0

of (a) reference

10

E (eV) 0

and (b) stressed quartz.

of c~vstallrnc~ SiO,

F. J&et et al. / SurJace modificatrons

SLO P

1

I

r!

h

02468

02468

(b)

(a)

Fig. 3. Pseudoradial function distribution of (a) reference and (b) oxygen-irradiated quartz. It can be noted that the second corresponding irradiation

to the Si-Si distances is modified. parameters are the same as in fig. 1.

The

nation shell. Data handling gives a mean distance value between silicon atoms of - 2.992 A instead of 3.06 A in the reference sample (the accuracy of EXAFS is about 2/100 A) and an increase of the static disorder. These values can be expressed in terms of angle between adjacent tetrahedra. It leads to a mean value of 136” for the Si-0-Si bond angle with a static disorder in this angle

of about

127

These results are confirmed by infrared spectroscopy where shifts in frequency are observed on samples after irradiation. The mechanism that induces local disorder in quartz under heavy ion irradiation may be the relaxation of self-trapped excitions. as proposed by Itoh for defect formation in quartz under a laser beam [6]. Such a trend is not detected on crystalline Al,O,. This supports the well-known results according to which there is no stable defect produced in alumina under electronic excitation. However. we have found that the Auger parameter of oxygen in a c-cut AllO, has changed after Xe irradiation. the more so as the detection angle between the electron analyser and the sample is low during analysis. This proves that defects in crystalline Al,O, can be produced under certain surface orientations and that they are located in the topmost atomic layers.

L...

I

peak

urtd Al,O_,

+ 7 O. For

the reference

quartz

References

we found

144O for the Si-0-Si bond angle in good agreement with X-ray diffraction values reported previously. These results can be interpreted as being due to local stresses and justify the DOS calculation reported above. However, a thorough analysis of the EXAFS results reveals a weak change in the coordination number of the first O-shell (- 10%) although no change can be measured in the SiLO distance. Therefore, the damaged quartz appears to be consituted of a distorted lattice plus point defects.

[l] J.P. Duraud, F. Jollet. Y. Langevin and E. Instr. and Meth. B32 (198X) 248. [2] E. Dooryhee. These, Paris X1 (1987). [3] D.E. Sayers. E.A. Stern and F.W. Lyttle, 27 (1971) 1204. [4] B.K. Teo and P.A. Lee. J. Am. Ceram. 2815. [5] G. Hollinger. Ph. D. Thesis. Lyon (1979). [6] N. Itoh and T. Nakayama. Nucl. Instr. (1986) 550.

II. CRYSTALLINE

OXIDES

Dooryhee.

Nucl.

Phys. Rev. Lett. Sot.

101 (1979)

and

Meth.

B13

/ CERAMICS