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Photoemission study of local structure of amorphous GeSe
Journal of Non-Crystalline Solids 53 (1982) 195-199 North-Holland Publishing Company
195
LETTER TO THE EDITOR PHOTOEMISSION GeSe
STUDY OF LOCAL STRUCTURE OF AMORPHOUS
T. T A K A H A S H I and T. S A G A W A Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan
Received 28 June 1982 Revised manuscript received 22 July 1982
Photoemission (XPS and UPS) spectra have been measured for an amorphous GeSe film deposited onto a cooled substrate before and after thermal annealing of the film. The photoemission spectra show a remarkable change in the 4p and the Ge 4s region. A comparison of the experimental results with the recently presented electronic structure calculations reveals that an amorphous GeSe film deposited onto a cooled substrate has a 3-3 coordination character and relaxes into a chemically ordered 4-2 coordinated structure upon thermal annealing.
The local structure of a m o r p h o u s GeSe has been the subject of considerable controversy, although numerous structural studies [1-12] based on X-ray [2,4,6,12] and electron [1,7,8] diffraction, E X A F S [3,5], R a m a n spectra [9,11], and UPS [10] have been performed so far. The focal point of the controversy is whether the structure of a m o r p h o u s GeSe is 3 - 3 coordinated as the crystalline GeSe or 4 - 2 coordinated as amorphous GeSe 2. However, in view of the prolonged confusion, it might be possible that the local structure of a m o r p h o u s GeSe is not unique but in an intermediate state between the 3 - 3 and 4 - 2 coordinations. If that is the case, it would be quite interesting and meaningful to investigate which coordination, 4 - 2 or 3-3, the local structure of a m o r p h o u s GeSe relaxes u p o n an increase of the local order. This relates to the final problem, that is, what coordination n u m b e r "ideal a m o r p h o u s " GeSe has. In this letter, we present XPS (X-ray photoemission spectroscopy) and UPS (ultraviolet photoemission spectroscopy) spectra of the valence states of a m o r p h o u s GeSe before and after thermal annealing, and discuss the local structure of a m o r p h o u s GeSe referring to the recently presented electronic structure calculations [13] on model 3 - 3 and 4 - 2 coordinated networks of GeSe. Bulk GeSe used in this study was 99.999% pure. Photoemission spectra were measured with a V G A D E S 400 (for XPS and UPS) and a h o m e - m a d e (for UPS) photoelectron spectrometer. The pressure inside the spectrometers was kept at less than 3 x 10-10 Torr. The energy resolusion was 1.0 eV for XPS (Mg Ket, 1253.6 eV) and 0.15 eV for UPS (He I, 21.2 eV). A m o r p h o u s GeSe films were prepared in the spectrometers by vapor deposition onto a cooled 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
196
T. TakahashL T. Sagawa / Photoemission study of local structure of a - GeSe
( 7 7 - 1 5 0 K) substrate. The thickness of the films was about 200 .~. The composition of the deposited films was estimated by the relative intensity of the Ge 3d and Se 3d core peak [14], and was Ge0.48Se0.52 within the experimental error of 2%. Hereafter in this letter we call this Se-rich film an evaporated or deposited GeSe film. Photoemission spectra were measured before, during and after thermal annealing. N o compositional change of the film was observed. Fig. 1 shows XPS (Mg K a ) spectra of the valence states of an evaporated GeSe film before and after thermal annealing. The GeSe film was prepared on a substrate held at about 150 K, and was annealed at about 500 K for 1 h. According to the previous study [15], an amorphous GeSe film crystallizes at about 610 K, s o i t is expected that the GeSe film annealed up to 500 K is still amorphous. The spectrum of the annealed film was measured at 150 K to eliminate any temperature effects (surface adsorption of atmospheric gases etc.). As is shown in fig. 1, the two spectra have three main bands labelled by A, B and C. The b a n d A is due to the Ge and Se 4p states, and the bands B and C mainly due to the Ge 4s and the Se 4p state, respectively [16-18]. U p o n thermal annealing, the b a n d A is slightly stabilized and the band B decreases in intensity. O'Reilly et al. [13] have indicated that the most noticeable difference in the valence density of states between the 3 - 3 and 4 - 2 coordinated models lies in the Ge 4s region. Fig. 2 shows the results calculated by O'Reilly et al. The Ge 4s band (around 9 eV) of the 4 - 2 coordinated models is
A
Mg K~
I
( 1 2 5 3 . 6 eV)
C B
m E 3 o c.)
500K 150K/
I
0
. . . .
=
. . . .
5 Binding
I
. . . .
I
. . . .
]0 15 e n e r g y (eV)
20
Fig. 1. XPS (Mg Ko0 spectra of the valence states of an evaporated OeSe film before and after thermal annealing. (Subsidiary dashed lines are drawn at the bands A and B to make the differences clear.) The film was prepared on a substrate held at 150 K and was annealed at 500 K for 1 h. Both spectra were measured at 150 K. The binding energy on the abscissa is taken from the Fermi level of the substrate.
7~ Takahashi, T. Sagawa / Photoemission study of local structure of a-GeSe
(a)
197
3-3
He I
0
A1
( 21.2 eV)
A2 A3
m
(b) 4-2 C.O.
tn r U
0
48
;
290~/
"J
7 L K J _ : .. 0
4
8
12
Binding energy (¢V)
16
Binding e n e r g y (¢V)
Fig. 2. Calculated density of valence states of GeSe for (a) a 3-3 coordinated, (b) a chemically ordered (C.O.) and a randomly bonded (R.B.) 4-2 coordinated network model [13]. Fig. 3. He I photoemission spectral change of an evaporated GeSe film upon thermal annealing. The GeSe film was prepared on a substrate held at 77 K, and each spectrum was measured during thermal annealing. Annealing temperatures are indicated in each spectrum. Subsidiary dashed lines are drawn in the spectra of 77 and 480 K to indicate the top of the valence states.
c o n s i d e r a b l y r e d u c e d in intensity a n d spreads over a b r o a d e r range of energy c o m p a r e d with that of the 3 - 3 c o o r d i n a t e d model, p r o b a b l y because of the f o r m a t i o n o f G e - G e b o n d s a n d the sp 3 h y b r i d orbital. W h e n we c o m p a r e these c a l c u l a t e d results with the o b s e r v e d XPS spectral change, especially the thermally i n d u c e d r e d u c t i o n of the G e 4s b a n d (see fig. 1), we can tentatively c o n c l u d e that an a m o r p h o u s G e S e film d e p o s i t e d o n t o a cooled s u b s t r a t e has a 3 - 3 c o o r d i n a t i o n c h a r a c t e r a n d relaxes into the 4 - 2 c o o r d i n a t e d structure u p o n t h e r m a l annealing. F u r t h e r , this relaxed 4 - 2 c o o r d i n a t e d a m o r p h o u s G e S e m a y have the chemically o r d e r e d structure r a t h e r than the r a n d o m l y b o n d e d one, b e c a u s e no new s u b - b a n d s a p p e a r u p o n t h e r m a l a n n e a l i n g j u s t between the b a n d s B a n d C as shown in the c a l c u l a t e d density of states of the r a n d o m l y b o n d e d 4 - 2 c o o r d i n a t e d m o d e l (fig. 2 (c)). T h e a b o v e i n t e r p r e t a t i o n is given an a d d i t i o n a l c o n f i r m a t i o n b y the p r e s e n t U P S measurements. Fig. 3 shows the t h e r m a l l y i n d u c e d H e I p h o t o e m i s s i o n spectral change of a G e S e film d e p o s i t e d o n t o a s u b s t r a t e held at 77 K. These spectra were m e a s u r e d d u r i n g t h e r m a l annealing, a n d the t e m p e r a t u r e s of the m e a s u r e m e n t s are i n d i c a t e d on each spectrum. This spectral change was o b s e r v e d to be
198
T. Takahashi, T. Sagawa / Photoemission study of local structure of a-GeSe
irreversible. Owing to the high resolving power of UPS we can observe the detailed features of the valence states, although only the first valence band (corresponding to the band A in fig. 1) is seen because of the low exciting energy. As is shown in fig. 3, the spectrum at 77 K shows a broad feature and upon thermal annealing it transforms into three well-resolved sub-bands, A 1, A 2 and A 3. We also find that the top of the valence states becomes stabilized upon increase of the annealing temperature. According to the results of O'Reilly et al. shown in fig. 2 all the calculated densities of states (one 3-3 and two 4 - 2 coordinated models) have a relatively well-resolved first peak at about 1.5 eV, so it is almost impossible to discriminate these three models only by the appearence of the first sub-band A 1 (fig. 3). However, the 3-3 coordinated model has a broad second band around 3 eV in the lower 4p region, while the two 4 - 2 coordinated models have two well-resolved sub-bands at about 3 and 4.5 eV, respectively. This clear difference in the lower 4p region is very useful to differentiate between the 3-3 and 4-2 coordinated structure. As is shown in fig. 3, upon thermal annealing two distinct sub-bands (A 2 and m 3 ) appear in the lower 4p region. This spectral change suggests that the 4-2 coordination character has been intensified in the annealed GeSe film. Furthermore, we prefer the chemically ordered 4-2 coordinated structure to the randomly bonded one as the thermally relaxed structure of amorphous GeSe, because the two sub-bands in the lower 4p region of the chemically ordered structure is better resolved than those of the randomly bonded structure. This conjecture is given an additional support by the fact that the observed thermally induced stabilization of the top of the valence states (fig. 3) well coincides with the change of the calculated valence density of states in going from the 3-3 to the chemically ordered 4-2 coordinated model (fig. 2). In conclusion, the present photoemission study indicates that the amorphous GeSe film deposited onto a cooled substrate has a 3-3 coordination character and relaxes into a chemically ordered 4 - 2 coordinated structure upon thermal annealing. The authors ar.e very grateful to Prof. Y. Harada, The University of Tokyo, for his helpful discussion. They also thank Mr. H. Sakurai for his assistance in the XPS measurements. This work was supported by the Japan Securities Scholarship Foundation.
References [1] [21 [3] [4] [5]
A.G. Mikolaichuk and A.N. Kogut, Sov. Phys. Crystallogr. 15 (1970) 294. R.W. Fawcen, C.N.J. Wagner and G.S. Cargill III, J. Non-Cryst. Solids 8-10 (1972) 369. D.E. Sayers, F.W. Lytle and E.A. Stem, J. Non-Cryst. Solids 8-10 (1972) 401. A. Bienensteck, J. Non-Cryst. Solids 11 (1973) 447. D.E. Sayer, F.W. Lytle and E.A. Stem, Prec. 5th Int. Conf. Amorphous and Liquid Semiconductors, Garmisch-Partenkirchen, 1973, Eds. J. Stuke and W. Brenig (Taylor and Francis, London, 1974) p. 403.
T. Takahashi, T. Sagawa / Photoemission study of local structure of a- GeSe
199
[6] L. Cervinka and A. Hrub2¢, Proc. 5th Int. Conf. Amorphous and Liquid Semiconductors, Garmisch-Partenkirchen, 1973, Eds. J. Stuke and W. Brenig (Taylor and Francis, London, 1974) p. 431. [7] B.J. Molnar and D.B. Dove, J. Non-Cryst. Solids 16 (1974) 149. [8] O. Uemura, Y. Sagara and T. Satow, Phys. Status Solidi (a) 26 (1974) 99. [9] P. Tronc, M. Bensoussan, A. Brenac, G. Errandonea and C. Sebenne, J. Physique 38 (1977) 1493.
[10] [11] [12] [13] [14] [15] [16] [17] [18]
S. Hino, T. Takahashi and Y. Harada, Solid State Commun. 35 (1980) 379. H. Kawamura, M. Matsushita and S. Ushioda, J. Non-Cryst. Solids 35-36 (1980) 1215. P.H. Fuoss, W.K. Warburton and A. Bienenstock, J. Non-Cryst. Solids 35-36 (1980) 1233. E.P. O'Reilly, J. Robertson and M.J. Kelly, Solid State Commun. 38 (1981) 565. I.M. Band, Yu.l. Kharitonov and M.B. Trzhaskovskaya, At. Data Nucl. Data Tables 23 (1979) 443. D.J. Sarrach and J.P. de Neufville, J. Non-Cryst. Solids 22 (1976) 245. R.B. Shalvoy, G.B. Fisher and P.J. Stiles, Phys. Rev. BI5 (1977) 2021. A. Kosakov, H. Neumann and G. Leonhardt, J. Electron Spectrosc. Relat. Phenom. 12 (1977) 181. G.D. Davis, P.E. Viljoen and M.G. Lagally, J. Electron Spectrosc. Relat. Phenom. 21 (1980) 135.
195
LETTER TO THE EDITOR PHOTOEMISSION GeSe
STUDY OF LOCAL STRUCTURE OF AMORPHOUS
T. T A K A H A S H I and T. S A G A W A Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan
Received 28 June 1982 Revised manuscript received 22 July 1982
Photoemission (XPS and UPS) spectra have been measured for an amorphous GeSe film deposited onto a cooled substrate before and after thermal annealing of the film. The photoemission spectra show a remarkable change in the 4p and the Ge 4s region. A comparison of the experimental results with the recently presented electronic structure calculations reveals that an amorphous GeSe film deposited onto a cooled substrate has a 3-3 coordination character and relaxes into a chemically ordered 4-2 coordinated structure upon thermal annealing.
The local structure of a m o r p h o u s GeSe has been the subject of considerable controversy, although numerous structural studies [1-12] based on X-ray [2,4,6,12] and electron [1,7,8] diffraction, E X A F S [3,5], R a m a n spectra [9,11], and UPS [10] have been performed so far. The focal point of the controversy is whether the structure of a m o r p h o u s GeSe is 3 - 3 coordinated as the crystalline GeSe or 4 - 2 coordinated as amorphous GeSe 2. However, in view of the prolonged confusion, it might be possible that the local structure of a m o r p h o u s GeSe is not unique but in an intermediate state between the 3 - 3 and 4 - 2 coordinations. If that is the case, it would be quite interesting and meaningful to investigate which coordination, 4 - 2 or 3-3, the local structure of a m o r p h o u s GeSe relaxes u p o n an increase of the local order. This relates to the final problem, that is, what coordination n u m b e r "ideal a m o r p h o u s " GeSe has. In this letter, we present XPS (X-ray photoemission spectroscopy) and UPS (ultraviolet photoemission spectroscopy) spectra of the valence states of a m o r p h o u s GeSe before and after thermal annealing, and discuss the local structure of a m o r p h o u s GeSe referring to the recently presented electronic structure calculations [13] on model 3 - 3 and 4 - 2 coordinated networks of GeSe. Bulk GeSe used in this study was 99.999% pure. Photoemission spectra were measured with a V G A D E S 400 (for XPS and UPS) and a h o m e - m a d e (for UPS) photoelectron spectrometer. The pressure inside the spectrometers was kept at less than 3 x 10-10 Torr. The energy resolusion was 1.0 eV for XPS (Mg Ket, 1253.6 eV) and 0.15 eV for UPS (He I, 21.2 eV). A m o r p h o u s GeSe films were prepared in the spectrometers by vapor deposition onto a cooled 0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
196
T. TakahashL T. Sagawa / Photoemission study of local structure of a - GeSe
( 7 7 - 1 5 0 K) substrate. The thickness of the films was about 200 .~. The composition of the deposited films was estimated by the relative intensity of the Ge 3d and Se 3d core peak [14], and was Ge0.48Se0.52 within the experimental error of 2%. Hereafter in this letter we call this Se-rich film an evaporated or deposited GeSe film. Photoemission spectra were measured before, during and after thermal annealing. N o compositional change of the film was observed. Fig. 1 shows XPS (Mg K a ) spectra of the valence states of an evaporated GeSe film before and after thermal annealing. The GeSe film was prepared on a substrate held at about 150 K, and was annealed at about 500 K for 1 h. According to the previous study [15], an amorphous GeSe film crystallizes at about 610 K, s o i t is expected that the GeSe film annealed up to 500 K is still amorphous. The spectrum of the annealed film was measured at 150 K to eliminate any temperature effects (surface adsorption of atmospheric gases etc.). As is shown in fig. 1, the two spectra have three main bands labelled by A, B and C. The b a n d A is due to the Ge and Se 4p states, and the bands B and C mainly due to the Ge 4s and the Se 4p state, respectively [16-18]. U p o n thermal annealing, the b a n d A is slightly stabilized and the band B decreases in intensity. O'Reilly et al. [13] have indicated that the most noticeable difference in the valence density of states between the 3 - 3 and 4 - 2 coordinated models lies in the Ge 4s region. Fig. 2 shows the results calculated by O'Reilly et al. The Ge 4s band (around 9 eV) of the 4 - 2 coordinated models is
A
Mg K~
I
( 1 2 5 3 . 6 eV)
C B
m E 3 o c.)
500K 150K/
I
0
. . . .
=
. . . .
5 Binding
I
. . . .
I
. . . .
]0 15 e n e r g y (eV)
20
Fig. 1. XPS (Mg Ko0 spectra of the valence states of an evaporated OeSe film before and after thermal annealing. (Subsidiary dashed lines are drawn at the bands A and B to make the differences clear.) The film was prepared on a substrate held at 150 K and was annealed at 500 K for 1 h. Both spectra were measured at 150 K. The binding energy on the abscissa is taken from the Fermi level of the substrate.
7~ Takahashi, T. Sagawa / Photoemission study of local structure of a-GeSe
(a)
197
3-3
He I
0
A1
( 21.2 eV)
A2 A3
m
(b) 4-2 C.O.
tn r U
0
48
;
290~/
"J
7 L K J _ : .. 0
4
8
12
Binding energy (¢V)
16
Binding e n e r g y (¢V)
Fig. 2. Calculated density of valence states of GeSe for (a) a 3-3 coordinated, (b) a chemically ordered (C.O.) and a randomly bonded (R.B.) 4-2 coordinated network model [13]. Fig. 3. He I photoemission spectral change of an evaporated GeSe film upon thermal annealing. The GeSe film was prepared on a substrate held at 77 K, and each spectrum was measured during thermal annealing. Annealing temperatures are indicated in each spectrum. Subsidiary dashed lines are drawn in the spectra of 77 and 480 K to indicate the top of the valence states.
c o n s i d e r a b l y r e d u c e d in intensity a n d spreads over a b r o a d e r range of energy c o m p a r e d with that of the 3 - 3 c o o r d i n a t e d model, p r o b a b l y because of the f o r m a t i o n o f G e - G e b o n d s a n d the sp 3 h y b r i d orbital. W h e n we c o m p a r e these c a l c u l a t e d results with the o b s e r v e d XPS spectral change, especially the thermally i n d u c e d r e d u c t i o n of the G e 4s b a n d (see fig. 1), we can tentatively c o n c l u d e that an a m o r p h o u s G e S e film d e p o s i t e d o n t o a cooled s u b s t r a t e has a 3 - 3 c o o r d i n a t i o n c h a r a c t e r a n d relaxes into the 4 - 2 c o o r d i n a t e d structure u p o n t h e r m a l annealing. F u r t h e r , this relaxed 4 - 2 c o o r d i n a t e d a m o r p h o u s G e S e m a y have the chemically o r d e r e d structure r a t h e r than the r a n d o m l y b o n d e d one, b e c a u s e no new s u b - b a n d s a p p e a r u p o n t h e r m a l a n n e a l i n g j u s t between the b a n d s B a n d C as shown in the c a l c u l a t e d density of states of the r a n d o m l y b o n d e d 4 - 2 c o o r d i n a t e d m o d e l (fig. 2 (c)). T h e a b o v e i n t e r p r e t a t i o n is given an a d d i t i o n a l c o n f i r m a t i o n b y the p r e s e n t U P S measurements. Fig. 3 shows the t h e r m a l l y i n d u c e d H e I p h o t o e m i s s i o n spectral change of a G e S e film d e p o s i t e d o n t o a s u b s t r a t e held at 77 K. These spectra were m e a s u r e d d u r i n g t h e r m a l annealing, a n d the t e m p e r a t u r e s of the m e a s u r e m e n t s are i n d i c a t e d on each spectrum. This spectral change was o b s e r v e d to be
198
T. Takahashi, T. Sagawa / Photoemission study of local structure of a-GeSe
irreversible. Owing to the high resolving power of UPS we can observe the detailed features of the valence states, although only the first valence band (corresponding to the band A in fig. 1) is seen because of the low exciting energy. As is shown in fig. 3, the spectrum at 77 K shows a broad feature and upon thermal annealing it transforms into three well-resolved sub-bands, A 1, A 2 and A 3. We also find that the top of the valence states becomes stabilized upon increase of the annealing temperature. According to the results of O'Reilly et al. shown in fig. 2 all the calculated densities of states (one 3-3 and two 4 - 2 coordinated models) have a relatively well-resolved first peak at about 1.5 eV, so it is almost impossible to discriminate these three models only by the appearence of the first sub-band A 1 (fig. 3). However, the 3-3 coordinated model has a broad second band around 3 eV in the lower 4p region, while the two 4 - 2 coordinated models have two well-resolved sub-bands at about 3 and 4.5 eV, respectively. This clear difference in the lower 4p region is very useful to differentiate between the 3-3 and 4-2 coordinated structure. As is shown in fig. 3, upon thermal annealing two distinct sub-bands (A 2 and m 3 ) appear in the lower 4p region. This spectral change suggests that the 4-2 coordination character has been intensified in the annealed GeSe film. Furthermore, we prefer the chemically ordered 4-2 coordinated structure to the randomly bonded one as the thermally relaxed structure of amorphous GeSe, because the two sub-bands in the lower 4p region of the chemically ordered structure is better resolved than those of the randomly bonded structure. This conjecture is given an additional support by the fact that the observed thermally induced stabilization of the top of the valence states (fig. 3) well coincides with the change of the calculated valence density of states in going from the 3-3 to the chemically ordered 4-2 coordinated model (fig. 2). In conclusion, the present photoemission study indicates that the amorphous GeSe film deposited onto a cooled substrate has a 3-3 coordination character and relaxes into a chemically ordered 4 - 2 coordinated structure upon thermal annealing. The authors ar.e very grateful to Prof. Y. Harada, The University of Tokyo, for his helpful discussion. They also thank Mr. H. Sakurai for his assistance in the XPS measurements. This work was supported by the Japan Securities Scholarship Foundation.
References [1] [21 [3] [4] [5]
A.G. Mikolaichuk and A.N. Kogut, Sov. Phys. Crystallogr. 15 (1970) 294. R.W. Fawcen, C.N.J. Wagner and G.S. Cargill III, J. Non-Cryst. Solids 8-10 (1972) 369. D.E. Sayers, F.W. Lytle and E.A. Stem, J. Non-Cryst. Solids 8-10 (1972) 401. A. Bienensteck, J. Non-Cryst. Solids 11 (1973) 447. D.E. Sayer, F.W. Lytle and E.A. Stem, Prec. 5th Int. Conf. Amorphous and Liquid Semiconductors, Garmisch-Partenkirchen, 1973, Eds. J. Stuke and W. Brenig (Taylor and Francis, London, 1974) p. 403.
T. Takahashi, T. Sagawa / Photoemission study of local structure of a- GeSe
199
[6] L. Cervinka and A. Hrub2¢, Proc. 5th Int. Conf. Amorphous and Liquid Semiconductors, Garmisch-Partenkirchen, 1973, Eds. J. Stuke and W. Brenig (Taylor and Francis, London, 1974) p. 431. [7] B.J. Molnar and D.B. Dove, J. Non-Cryst. Solids 16 (1974) 149. [8] O. Uemura, Y. Sagara and T. Satow, Phys. Status Solidi (a) 26 (1974) 99. [9] P. Tronc, M. Bensoussan, A. Brenac, G. Errandonea and C. Sebenne, J. Physique 38 (1977) 1493.
[10] [11] [12] [13] [14] [15] [16] [17] [18]
S. Hino, T. Takahashi and Y. Harada, Solid State Commun. 35 (1980) 379. H. Kawamura, M. Matsushita and S. Ushioda, J. Non-Cryst. Solids 35-36 (1980) 1215. P.H. Fuoss, W.K. Warburton and A. Bienenstock, J. Non-Cryst. Solids 35-36 (1980) 1233. E.P. O'Reilly, J. Robertson and M.J. Kelly, Solid State Commun. 38 (1981) 565. I.M. Band, Yu.l. Kharitonov and M.B. Trzhaskovskaya, At. Data Nucl. Data Tables 23 (1979) 443. D.J. Sarrach and J.P. de Neufville, J. Non-Cryst. Solids 22 (1976) 245. R.B. Shalvoy, G.B. Fisher and P.J. Stiles, Phys. Rev. BI5 (1977) 2021. A. Kosakov, H. Neumann and G. Leonhardt, J. Electron Spectrosc. Relat. Phenom. 12 (1977) 181. G.D. Davis, P.E. Viljoen and M.G. Lagally, J. Electron Spectrosc. Relat. Phenom. 21 (1980) 135.