An EXAFS study of the local structure of CeSiO amorphous thin films

Journal of Non-Crystalline Solids 110 (1989) 249-257 North-Holland, Amsterdam

249

AN E X A F S S T U D Y O F T H E L O C A L S T R U C T U R E O F C e - S i - O A M O R P H O U S T H I N F I L M S A. S I N G H , S.J. G U R M A N a n d E.A. D A V I S Department of Physics, University of Leicester, Leicester LE1 7RH, UK

Received 30 January 1989 Revised manuscript received 21 April 1989

An EXAFS study has been made on the structure of three composition ranges of Ce-Si-O amorphous thin films prepared by RF sputtering. The measurements, carried out on the K edge of silicon and the L 3 edge of cerium, reveal that in the stoichiometric oxygen films of the general formula (Ce, Si) 02, both cerium and silicon are four-coordinated by oxygen regardless of the O : Si ratio. In the oxygen-deficientfilms cerium remains four-coordinated by oxygen, but, around silicon, the oxygen atoms are progressivelyreplaced by silicon as the oxygen content of the films is reduced. In silicon-rich films which are very deficient in oxygen, the oxygen atoms prefer to remain coordinated with cerium, rather than silicon. A definite decrease in the Si-O distance with increase in Si-O coordination has been found. The effect is attributed to an increase in the charge of silicon with oxygen coordination, and supports a randomly bonded model for the structure. The total oxygen coordination, derived from a consideration of bond conservation, indicates that the film structures are probably SiO2-typecontinuous random networks.

1. Introduction T h i n dielectric films of the C e - S i - O ternary system are of interest because of their possible use in m e t a l - i n s u l a t o r - m e t a l ( M I M ) a n d electronic m e m o r y devices [1]. Previous studies of this system have been m a d e o n thin films prepared by the v a c u u m co-evaporation of SiO a n d CeO 2 using a technique described by H o g a r t h a n d Wright [2]. These studies i n c l u d e d optical b a n d g a p measurem e n t s [3], a n evaluation of spin densities employing E S R [4], a n d infrared [5] a n d defect structure [6] investigations. The last two studies revealed i n t r i g u i n g properties associated with the vibrational a n d electronic structure of these films, a n d p o i n t e d to the need for a complete structure analysis of a c o m p r e h e n s i v e range of films. W e have carried out E X A F S a n d other studies o n three c o m p o s i t i o n ranges of these films, prep a r e d by R F sputtering. T h e series comprise films (1) of stoichiometric oxygen content, i.e. films of c o m p o s i t i o n (Si, Ce) 02, a n d two series (2) a n d (3) of oxygen-deficient films, as indicated by the tie lines in the phase d i a g r a m shown in fig. 1. 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Si 1.StoichiQimetric 0 Series 2.Oxygen-deficient Series ich Series

Fig. 1. Composition ranges of Ce-Si-O thin films. Solid circles show values studied. The c o m p o s i t i o n s cover a wide range, a n d were chosen to span the whole region b e t w e e n wide b a n d g a p i n s u l a t i n g films o n the o n e h a n d to semic o n d u c t i n g films on the other.

2. Experimental details The films were prepared in a N o r d i k o - 2 0 0 0 T 8 - S E 1 sputtering system provided with a 13.56

250

A. Singh et al. / Local structure of C e - S i - O amorphous thin films

M H z crystal-controlled RF generator. The target consisted of a four-inch diameter SiO 2 disc covered with varying numbers of 0.5 cm diameter CeO 2 tablets for the first series of films, and a similar arrangement but with half the disc area covered with silicon wafer chips for the second series. The SiO 2 disc was replaced by a silicon one of similar size for the preparation of the third series. Depositions were carried out at room temperature under an argon pressure of 5 mTorr and a RF power of 150-250 W for periods of 6 - 9 h. The substrates were 2.5 × 5.0 cm 2 copper plates for the samples used for silicon K edge measurements, mylar sheets with deposition areas not less than 2.5 × 7.5 cm 2 for cerium L 3 edge measurements, and 2.5 × 2.5 cm 2 Corning or Suprasil glass plated for the optical absorption measurements. The silicon and cerium contents of the films were determined by electron microprobe analysis employed in the "energy-dispersive spectroscopy" mode. The oxygen content of the films was inferred from these measurements and the target composition. EXAFS measurements were carried out on the Synchrotron Radiation Source at the SERC Daresbury Laboratory, Warrington, UK. The silicon K edge was measured in the total electron yield mode at station 3.4 and the cerium L 3 edge in the transmission mode at station 7.1 of the facility. The useful range of the latter edge was limited to 400 eV above the L 3 edge by the presence of the L 2 edge at - 6050 eV. Optical absorption edges were measured over the 200-1500 nm range of wavelengths on a Perkin-Elmer 330 U V / visible spectrophotometer.

3. EXAFS studies

The raw experimental spectra were energycalibrated using the Daresbury EXCALIBV program. Pre-edge and post-edge background substraction was performed using the Daresbury EXBACKV program, which employs a polynomial form for the smoothly varying atomic absorption coefficient. The normalized EXAFS spectra obtained after the background subtraction process were analysed

using the fast curved-wave theory [7] as implemented in the EXCURV88 program at Daresbury. Electron scattering phase shifts were calculated within this program using atomic charge densities determined via a relativistic H a r t r e e - F o c k program held on the SRS EXAFS data bank. The phase shifts used related to close-packed elemental structures of the neutral-atom charge densities, charge overlap being treated using the Mattheiss prescription of overlapping charge densities and exchange using the Slater X a method with an exchange parameter of 2/3. The scattering phase shifts were checked by analysing EXAFS spectra of crystalline ceria, CeO 2 and of amorphous silicon and silica. Analysis _of these standards also provided an amplitude reduction factor due to multi-electron absorption (shake-up, shake-off factor) of 0.7 for the Si K edge and 0.8 for the Ce L 3 edge. The imaginary part of the photoelectron self-energy was set at - 1 eV to reproduce the elastic electron mean free path. The photoelectron energy zero E 0, was treated as a free parameter in the fit; it was found to lie in the range 10-15 eV, which is a physically reasonable range for insulating silicate glasses. The finite data range in our spectra leads to a correlation between some of the structural and other parameters. Examples are E 0 and the interatomic distances, which define the phase of the EXAFS function, and the coordination number and D e b y e - W a l l e r factor which fix its amplitude. The presence of this correlation increases the uncertainty in our fit parameters. We have used the statistical method of Joyner et al. [8], which is based on the F test, to obtain estimates of this uncertainty; the quality of fit is determined in terms of a fit index (defined in ref. [8]). When we plot the fit index as a function of the values of two correlated variables we obtain a contour map with the minimum fit index defining the best fit values. An example is shown in fig. 2. Joyner's method allows us to determine the size of the region around this minimum where the fit is not significantly worse; roughly speaking, this corresponds to a change in fit index of less than 5% for most EXAFS analyses. For the case of the spectra considered here, where the data range is rather small because of the strong disorder and light-atom

A. Singh et al. / Local structure of C e - S i - O amorphous thin films

nature of the scatterers, the region of significance is rather large. It leads us to put uncertainties of _+ 30% on coordination numbers (independent of their absolute value) and + 0.01 ,~2 D e b y e - W a l l e r factors (again independent of their absolute value). The corresponding uncertainties in E 0 and interatomic distance are _+2 eV and -+ 0.02 A, respectively. The results of our analysis of EXAFS spectra obtained from amorphous C e - S i - O films over a wide composition range are summarized in table 1. with typical fits being shown in figs. 3 to 5. The S i - O and Si-Si distances correspond to the appropriate chemical bond lengths in silica and silicon and hence we consider both to be nearestneighbour coordinations. Silicon is then found to be four-fold coordinated in all of our samples. No evidence was found in the Si K edge data for more distant neighbours. The Ce L 3 edge spectra show

+l-filL-~ /× !t t't o.o,,.h b t- t-+-t t/+ +/7 ! t, ,TAtT;l-d o.o,o17l / lt+ ! tl//f ++ o.o~s

-

i 4 "-+

0.012

--

,~

[

'

o

o.oot]17 ,t7 //-/ ///lY//tA

°°°,!!t!yX 0002

0.2

0.6

1.0

1.4

1.8

2.2

2.6

251

3.0

N~ Fig. 2. EXAFS error estimates for the first shell of sample T4 Si edge data. The contours are of the fit index for the coordination numbers N l and the D e b y e - W a l l e r factor Al. The heavy contour encloses the 95% significance region as defined by the F test, and its bounds yield the error range in A~ and N 1.

Table 1 Results for Si and Ce edge analysis. Note: The film compositions quoted do not show the concentration of argon which was incorporated during sputtering. In some cases, e.g. T2, this was considerable Sample composition (%)

Silicon coordination

Label

Elemerit

Si

Ce

O

Cerium coordination

Coord. No.

Shell radius

D-W factor

(_+30%)

(i, +0.03)

(iL _+0.0051

Element

Coord. No.

Shell radius

D-W factor

(_+30%)

(i,_+0.03)

(i, _+0.oo5)

2.32 3.04 2.39 3.17 2.38 3.06 2.39 3.05

0.054 0.068 0.033 0.064 0,025 0.088 0.05

-

Stoich-O series SI A

27

7

66

0

4.8

1.59

0.010

S3A

21

12

66

0

4.8

1.60

0.010

S4A

13

21

66

0

5.0

1.62

0.013

6

27

67

0

3.6

1.63

0.001

46

0 Si 0 Si 0 Si

1.3 2.8 1.2 2.6 2.1 2.4

1.63 2.35 1.65 2.38 1.63 2.37

0.011 0.015 0.003 0.017 0,017 0,020

0 (Si) 0 (Si)

3.9 10 5.0 8

2.38 3.10 2.39 3.07

0.032 0.094 0.034 0.079

0 Si 0 Si

0.5 3.0 1.6 2.6

2.35 1.63 2.37

0.012 0.010 0.018

0 (Si) 0 (Si)

4.8 6 5 6

2.39 3.07 2.39 3.06

0.036 0.068 0.036 0.068

$6

0 (Si) 0 (Si) 0 (Si) 0 (Si)

5.3 3.4 4.8 7 4.2 8 3.5

O-Deficient series $7

50

0

$9

22

17

56

S10A

18

20

58

O-Def, Si-rich series T2

63

9

18

T4

23

23

47

252

A. Singh et al. / Local structure of Ce-Si-O amorphous thin films Si EDGE

Si EDGE

Ce EDGE

20

Ce EDGE

1.2

1.5

08

1.0 ""

O-4

0.5

~o.o

reX' 0.0-

' ~ ~9~-

ix[

-0.5

-qo.6

-1.0 -1.5 -2 0

klA-1)

-1,0

k(/~')

"

-1.4 1.2

2.4

1.0

2.0

08 ~

1.6

,,"-0.6

I---1.1_ 1.2 0.8

0.4 0.2 ~

0.4

o.o

0.C

23456789

345678910

1

1 23

45

67

89101

R

Fig. 3. Typical EXAFS curves for the stoichiometric-oxygen series: sample S3A. Upper panels; EXAFS function. Lower panels: FT of the EXAFS function. - - , Experiment; . . . . . . , theory Si EDGE

Ce EDGE

2.0 "xe " 10 ~>< 00 1.0 u

-2.0 1.2 10

--~o8

1

,

U_ --0.6 0.4 0.2 0.0

1 2 3 4 567

8 9101

2 34

5 67

8 910

Fig. 4. Typical EXAFS curves for the second series: sample $9. Details as for fig. 3.

2 3 4 5 67

8 910

(,~)

Fig. 5. Typical EXAFS curves for the third series: sample T4. Details as for fig. 3. two shells in general, a n oxygen shell at 2.4 ,~ a n d a second shell at a b o u t 3.1 ,~. The more d i s t a n t shell, whose c o o r d i n a t i o n n u m b e r a n d chemical type are very poorly defined, is interpreted, largely o n the evidence of i n t e r a t o m i c distance, as being m a d e up of silicon atoms. O n chemical g r o u n d s we consider only the oxygen a t o m s as nearest neighbours, leading to a most p r o b a b l e coordination of four for the Ce atoms, although the uncertainties are such that in some cases we c a n n o t rule out the presence of a fair p r o p o r t i o n of six-fold cerium. The second n e i g h b o u r s we consider to be silicon atoms, linked to the c e r i u m a t o m via its oxygen nearest neighbours. The general similarity between the n u m b e r s of oxygen a n d second n e i g h b o u r s supports this i n t e r p r e t a t i o n , as does the large difference in D e b y e - W a l l e r factors. The results s u m m a r i z e d in table l show that, within the stoichiometric oxygen series, silicon is always four-fold c o o r d i n a t e d boy oxygen alone at the usual S i - O distance of 1.6 A (fig. 3). N o more distant shells are clearly discernable, suggesting a high degree of structural disorder in the silicon e n v i r o n m e n t . C e r i u m is also always four-fold co-

A. Singh et a L / Local structure of Ce-Si 0 amorphous thin films

ordinated by oxygen in these films, the C e - O distance of 2.4 ,~ corresponding to the usual bond length. If we interpret the second shell as being made up of silicon atoms, then a combination of the Si K edge and Ce L 3 edge results give a C e - O Si bond angle of about 100 o. In the second series the cerium remains fourfold coordinated to oxygen alone. However, the silicon environment varies with film composition, although the total coordination of each silicon atom remains at four. As the S i / O ratio rises, oxygen is steadily replaced by silicon as the nearest neighbour of silicon, the Si-Si distance being the usual bond length of 2.4 .A. A similar process occurs in the third series, where it has progressed much further, so much so that, at the lowest oxygen content (T2) (fig. 5), there are no oxygen nearest neighbours of silicon, even though the cerium continues to maintain its full four-fold coordination by oxygen. These results suggest that the C e - O bond is stronger than the S i - O bond, a conclusion consistent with the electronegativities of these two elements. We may also combine the Si K edge and Ce L 3 edge data to investigate the total coordination of the oxygen atoms. From the S i - O and C e - O bond coordinations, we may calculate the O - S i and O Ce coordinations by use of the bond consistency condition.

where C, is the atomic concentration of species i and N,j is the number of j atoms coordinated with an i atom. The equation states that the number of ij bonds is the same whether we look from the i or j end of the bond. Using the results of table 1 we can calculate the O - S i and O - C e coordination for each film. With one exception, the sum of these (shown in table 4) is two, within our rather large errors bars, which implies that there are very few or no non-bridging oxygens in all but one of our films. The exception is sample $7, which is pure SiO, where the O - S i coordination is 1.3 + 0.5. The infra-red data of Singh and Hogarth [5] suggest that this sample contains a fair proportion of non-bridging oxygen and thus supports our EXAFS results. The same data shows that neither $9 nor SIOA contain non-bridging

166

~~

253

@

16a 1.62

1.60 11

i

i

i

i

i

2

3

~

5

6

--

Nsio

b

Fig. 6. Variation of S i - O distance with S i - O coordination.

oxygens in significant numbers; our EXAFS results give total oxygen coordinations of 1.7 _+ 1.0 and 2.4 _+ 1.0 for these samples. When we view the data of table 1 as a whole, we see that there is a definite downward trend in the S i - O distances with increasing S i - O coordination. This is shown in fig. 6. A similar but more pronounced trend was noted by Greaves et al. [9] in their EXAFS study of SiO~ amorphous films. As was pointed out by Greaves et al. such a trend is strong evidence that our films are single-phase materials rather than being made up of separate Si and SiO 2 regions. We interpret the trend as due to an increase in the charge on the silicon atom with increasing oxygen coordination arising from the partially ionic nature of the S i - O bond. This strengthens the bond and shortens the bond length. The change in frequency of the S i - O bond stretch mode observed in I R studies [10] is interpreted in the same way.

4. XANES The XANES of tetrahedrally coordinated atoms possess two characteristic post-absorption peaks (called multiple scattering resonances or MSR), their size depending on the backscattering strength of the first shell neighbours [11]. The Si edge spectra of the stoichiometric oxygen series possess very similar XANES, showing the presence of such MSR peaks (fig. 7). The post-edge peaks

254

A. Singh et aL

/

Local structure o f C e - S i - O amorphous thin f i l m s

of the silicon atom in the first series, and the increasing presence of Si-Si bonds with a reduction in oxygen content.

Stoichiometric Oxygen series ~ ~ ~ ~ S 3 A

S1A

~Oxygen - deficient selils

I--

Z of. < Z 0m i--

n cc. o t13 <

I 0

i

I I I I 20 Z,0 ENERGY ( e V )

~ I 60

l

Fig. 7. XANES spectra of the films. Absorption thresholds are omitted because of unpredictable small shifts in monochromator positions between runs.

centred at - 2 0 eV above the edge are probably MSR due to backscattering from oxygen atoms, as are similar peaks in the second series. In the most silicon-rich specimen, a new peak appears at - 10 eV below the original edge position. Re-examination of the oxygen-deficient X A N E S structures reveals a pre-edge shoulder at approximately the same position as this new peak. It is thus probably due to backscattering from silicon neighbours, its strength being weakened in the second series because of the lower number of nearest-neighbour silicons. It is completely absent in the stoichiometric series where there are no Si-Si bonds. The XANES results are thus generally seen to support those derived from EXAFS, indicating very similar tetrahedral environments

5. Optical bandgaps Preliminary values of optical bandgap ( E opt) derived from optical absorption edge measurements [12] on the films show that, whereas for the stoichiometric oxygen series these are clustered around 3.2 eV, within the oxygen-deficient second series they tend to remain around 1.7 eV, while for the low-oxygen members of the third series they are even lower (fig. 8). The bandgaps may be related to the silicon and cerium environments through the valence and conduction band densities of states in the following manner. In as much as these densities of states are formed from a linear superposition of the broadened bonding, antibonding and non-bonding energy bands associated with the Si-O, C e - O , Si-Si bonds and the oxygen 2P lone-pair orbitals, the size of the gap will depend on the presence or otherwise of such bonds, i.e. the respective coordinations. Figure 9 may then be used as a schematic guide to assess the relative sizes of the bandgaps.

}(Sl -S6) 3.0-

2.0o(Sg) o (S7)

o(S1OA)

O(T4} uJ 1.0 T2

0-0

. . . .

I

. . . .

I

. . . .

I

. . . .

[

1 2 3 4 Si - 0 Coordination (Nsio) " Fig. 8. Variation of optical bandgaps Eopt, with Si-O coordination Nsio.

A. Singh et aL / Local structure of Ce-Si-O amorphous thin films Si-Oo-m

6. Structural interpretation of data and conclusions

Ce-Oo.m

D--

re <

Si-

v

(.9 rr Ld Z L~

Si0""

Z)Lp

Si- Si0"

Si- 0o"

255

Ce-Oo"

Fig. 9. Schematic b a n d g a p systematics for C e - S i - O thin films.

In films where both cerium and silicon have only oxygen as their nearest neighbours, we expect the valence band edge to consist of oxygen 2P lone pair states and the conduction band to be made up of S i - O a n d / o r C e - O antibonding levels. As oxygen is replaced by silicon in the silicon environment, the introduction of Si-Si bonds would mean that, whereas the valence band edge remains unchanged, the conduction band edge would be progressively replaced by the lower energy Si-Si antibonding states, resulting in a reduction of Eopt. With a further reduction in oxygen content, the number of oxygen lone pairs should diminish sufficiently and the number of Si-Si bonds become large enough for them to dominate optical absorption. The bandgap would then be determined by the bonding and antibonding states of this bond, leading to further lowering of Eom to a value close to that observed in amorphous Si (about 1.5 eV). The observed general reduction in Eop t with the lowering of the oxygen content of the films correlates well with the expected bandgap narrowing outlined above, and lends support to the coordination picture derived from EXAFS.

Our EXAFS results show that silicon is always four-fold coordinated in the amorphous films and we therefore assume that each silicon atom has its usual tetrahedral covalently bonded environment. Both S i - O and Si-Si bonds occur in these films, the relative numbers being fixed by the composition and by the observed requirement that the cerium atoms remain coordinated by oxygen alone. We now consider the role played by cerium in these amorphous films. Two competing structural regimes are possible. In the first we consider cerium to act as a network modifier, bonding ionically with a large number of oxygen nearest neighbours. Assuming Ce to be tetravalent, the structure of crystalline silicates suggests that the number of oxygen neighbours should be eight (as in zircon) or possibly six. This model yields structures resembling the modified random networks (MRNs) described by Greaves [13]. In the case of the stoichiometric oxygen series the local structure of the network should be similar to the structure of crystalline silicates, where the degree of polymerization of the SiO 4 units depends on the O : Si ratio as described by Bragg and Claringbull [14] and by West [15]. Examples are listed in table 2. This situation is typified by the structure of CaSiO 3 glass as determined by Geere et al. [16] using EXAFS and by Yin et al. [17] using X-ray diffraction. The similarity in size and electronegativity, though not valence, between Ca and Ce shown in table 3 leads us to consider Ce as a network modifier. In CaSiO 3 glass, the calcium ion is coordinated by about eight oxygen atoms, the distribution of distances being highly disordered.

Table 2 T y p i c a l silicate structures a n d total c o o r d i n a t i o n of oxygens O : Si Ratio

Type of silicate anion

Example

Total o x y g e n coordination

4 :1 3.5 : 1 3:1

SiO 4 (Si 207 )6- d i m e r s (SiO3)n 2" chains (Si3Og)n 2 " - rings ( S i 2 O s ) n 2 " - sheets SiO z 3-D f r a m e w o r k

Mg2SiO 4 Ca3Si 207 Na2SiO 3 CaSiO 3 NazSi20 s SiO z

4 3.7 5.3 3.3 4 2

2.5:1 2:1

A. Singh et al. / Local structure of C e - S i - O amorphous thin films

256

Table 3 Bonding-determining parameters for selected elements Element

Ce

Atomic radius

Ca

Si

O

1.81

1.97

1.32

-

1.11(3+)

0.99(2+)

0.41(4+) 1.40(2-)

1.0

1.8

(A) lonicradius

(A)

1.01 (4 + )

Electronegativity M - O bond % ionic character

1.1

76

79

3.6

51

-

In the second model we consider cerium to covalently bond to oxygen, thus acting as a network former. In this model the Ce atom effectively substitutes for a Si atom, and has four oxygen nearest neighbours (in line with the valence). The structure would then be a continuous random network, very similar to that of amorphous S i O 2 in the stoichiometric series and probably akin to the amorphous silicon sub-oxides in the oxygen-deficient films. Our data clearly allows us to assign our films to the second of these two models; in our C e - S i - O amorphous films, cerium acts as a network former. The measured cerium coordination is close to four and is made up entirely of oxygen atoms, with a second shell probably of silicon. The two distances give a C e - O - S i bond angle of 100 o which is not unreasonable for covalent bonding, although it is

Table 4 Actual and scaled total oxygen coordinations No for films Sample

Experimental O coord. (No)

NO re-scaled with Nsi = 4, Nc~ = 4

NO re-scaled with Nsi = 4, Nce=6

S1A S3A HA $6 $7 $9 S10A T2 T4

2.6 2.4 2.3 1.7 1.4 1.7 2.4 2.4 3.3

2.2 1.9 1.8 1.4 1.4 1.7 2.0 2.0 2.8

2.4 2.3 2.7 2.7 1.4 2.4 2.6 3.0 3.8

smaller than the values of 140 ° in a-SiO 2 and 130 o in a-GeO 2. Also the oxygen total coordination calculated from the experimental coordination numbers as outlined in sect. 3 and then re-scaled by assuming Nsi = 4, N c e = 4, is very close to two in all our samples (except the pure SiO) as shown in table 4. Even if we assume that our Ce edge data give C e - O coordinations which are far too low, and scale them to six (table 4, last column) the total oxygen coordination is still a good deal lower than that observed in crystalline silicates containing ionically bonded metal atoms. A twofold total coordination of oxygen in these thin films, together with fourfold Si and Ce, implies that the bonding is covalent, obeying Mott's [12] 8 - N rule. The structure of the films would then be akin to amorphous SiO 2 or silicon suboxides, depending on the oxygen content, a conclusion further supported by the variation in S i - O distances with a Si-O coordination number which is much like that observed in the silicon suboxides [9]. We therefore conclude that cerium acts as a network former in our samples. We wish to thank R.J. Newport, A.M. Edwards and M.C. Fairbanks of the University of Kent for their generosity in allowing us to share their beam-time allocation at the Daresbury SRS and assistance in the measurements. One of us (AS) would also like to thank the Association of Commonwealth Universities for the scholarship enabling him to carry out this research.

References [1] Z.T. A1-Dhhan and C.A. Hogarth, Int. J. Electronics 63 (1987) 707. [2] C.A. Hogarth and L.A. Wright, Proc. Int. Conf. on Physics of Semiconductors, Moscow 2 (1968) 1274. [3] C.A. Hogarth and Z.T. AI-Dhhan, Phys. Stat. Sol. (b) 137 (1986) K157. [4] A. Razzaq, C.A. Hogarth and K.A. Lon, Phys. Stat. Sol. (b) 141 (1987) K67. [5] A. Singh and C.A. Hogarth, J. Mater. Sci. 23 (1988) 1090. [6] lbid 23 (1988) 1758. [7] S.J. Gurman, N. Binsted and I. Ross, J. Phys. C 17 (1984) 143. [8] R. Joyner, K.J. Martin and P. Meehan, J. Phys. C 20 (1987) 4005.

A. Singh et a L / Local structure of C e - S i - 0 amorphous thin films [9] G.N. Greaves, X.L. Jiang, R.N. Jenkins, E. Holzenkampfer and S. Kalbitzer, J. de Phys. C8 (1986) 47. [10] G. Lucovsky, Sol. St. Commun. 29 (1979) 571. [11] A. Bianconi, DL/SCI/R17, SERC Daresbury Laboratory, Warrington, UK P13. [12] N.F. Mott and E.A. Davis, Electronic Processes in Non-Crystalline Materials (Oxford Univ. Press., 1979). [13] G.N. Greaves, J. Non-Cryst. Solids 71 (1985) 203. [14] L. Bragg and G.F. Claringbull, The Crystalline State, Vol. 4 (Bell, London, 1965) p. 167.

257

[15] A.R. West, Solid State Chemistry and its Applications (Wiley, New York, 1984) pp. 259, 609. [16] R.G. Geere, P.H. Gaskell, G.N, Greaves, J. Greengrass and N. Binsted, EXAFS and Near-Edge Structure III, ed. K.O. Hodgson (Springer, Berlin, 1983). [17] C.D. Yin, M. Okuno, H. Morikawa, F. Marunu and T. Yamanaka, J. Non-Cryst. Solids 80 (1986) 167.