SCATTERING STUDIES OF PHOTO-STRUCTURAL CHANGES IN CHALCOGENIDE GLASSES

HAL Id: jpa-00222399

https://hal.archives-ouvertes.fr/jpa-00222399

Submitted on 1 Jan 1982

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

SCATTERING STUDIES OF PHOTO-STRUCTURAL CHANGES IN CHALCOGENIDE GLASSES

S. Elliott, T. Rayment, S. Cummings

To cite this version:

S. Elliott, T. Rayment, S. Cummings. SCATTERING STUDIES OF PHOTO-STRUCTURAL

CHANGES IN CHALCOGENIDE GLASSES. Journal de Physique Colloques, 1982, 43 (C9), pp.C9-

35-C9-38. �10.1051/jphyscol:1982906�. �jpa-00222399�

SCATTERING STUDIES OF PHOTO-STRUCTURAL CHANGES I N CHALCOGENIDE GLASSES S.R. E l l i o t t , T. Rayment and S. Cummings

Dept. of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge, U. K.

+Institut Laue Langevin, 1£6X, F-38042 Grenoble, France

Résumé.- Ceci est la première étude par d i f f r a c t i o n de neutrons d'effets photostruc- turaux réversibles dans deux verres de chalcogénures, As2Se3 et As2S3. Les modifi- cations induites optiquement s'étendent aux valeurs Q élevées et donc correspondent aux changements dans, l'ordre à courte distance du verre. Les différences entre les FDR des échantillons éclairés et recuits sont notables au voisinage de l a distance séparant les seconds voisins. Le seul modèle compatible avec nos données consiste en une augmentation photo-stimulée des fluctuations de l'angle des liaisons de l ' o r - dre de 1°.

Abstract. - We report the f i r s t neutron scattering studies of the reversible photo- structural changes in two chalcogenide glasses, As2Se3 and As„S3. The o p t i c a l l y - induced changes extend to large values of Q, and are therefor! caused by changes in the short-range order of the glass. The differences in the RDFs between illuminated and annealed samples show marked changes i n the neighbourhood of the second-neighbour separation. The only model whic^ f i t s our data is that of a photo-induced increase in bond-angle fluctuations by M .

Introduction.- Chalcogenide semiconducting glasses, containing a large proportion of chalcogen atoms (S,Se or Te), are distinguished by the fact that illumination with l i g h t having an energy comparable to the band-gap induces a host of structural and concomitant opto-electronic changes. These photo-induced effects may be divided into two categories (1,2,3) : irreversible changes produced in as-deposited evaporated amorphous thin f i l m s , and reversible changes produced in well-annealed films or melt- quenched glasses which can be annealed out by heating the sample to the glass-trans- i t i o n temperature. The mechanism f o r the f i r s t effect is now well-understood. The structure of the as-deposited evaporated films owes much to the nature of the vapour species, which for As^S, are predominantly As.S* molecules, with the result that a s o l i d , consisting of molecular units weakly bonded together, is formed when the vapour strikes a cool substrate : optical i r r a d i a t i o n is thought to cross-link the molecular e n t i t i e s , forming a f u l l y connected network whose structure i s very similar to that of the bulk glass. The mechanism f o r the reversible photo-induced effects which are observed in well-annealed specimens cannot therefore be the same, and i t s precise nature is not known. Several models have been proposed to account for this behaviour, however; o p t i c a l l y induced fluctuations i n the chalcogen bond-angle ( 4 ) , a chalcogen atom bond-flip induced by changes in lone-pair interactions (3) and a self-trapped exciton involving the formation of a D ' - D~ charged-defect pair (5). However, d e t a i l - ed information concerning the reversible structural changes which take place upon i r - radiation has been lacking, and so i t has not proved possible to decide in favour of any given mechanism heretofore.

The structural data reported in the l i t e r a t u r e pertaining to the reversible photo- structural (PS) effect is very l i m i t e d . Essentially i t consists of the changes ob- served in the f i r s t diffuse peak ifi the X-ray scattering intensity which occurs at a value of the scattering vector k=lA (3). This peak decreases in height and shifts to a s l i g h t l y higher value of k after i r r a d i a t i o n ; the effect is most marked i f illumination takes place at low temperatures. Although these data i n - dicate that a change in the structure of the glass is induced by l i g h t , the amount of direct information i t affords is negligible. Diffraction data must be measured to

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982906

C9-3 6 JOURNAL DE PHYSIQUE

h i g h values o f k ( o r 8 ) i n o r d e r t h a t F o u r i e r t r a n s f o r m a t i o n may be a r i e d o u t t o y i e l d real-space i n f o r m a t i o n : e x i s t i n g data extending o n l y t o k=1. S & - I make t h i s task impossible. Furthermore, although un oybtedly l a r g e changes i n the d i f f r a c t i o n p a t t e r n do occur i n t h e f i r s t peak a t k = l i

,

t h e s t r u c t u r a l o r i g i n o f t h i s f e a t u r e remains obscure and i t i s t h e r e f o r e impossi b l e t o draw any f i r m conclusions concerning the p r e c i s e e f f e c t s o f i r r a d i a t i o n on the s t r u c t u r e from t h i s i n f o r m a t i o n alone.

With these considerations i n mind, we have undertaken t h e f i r s t neutron s c a t t e r - i n g measurements on t h e PS e f f e c t i n chalcogenide glasses, thereby a c q u i r i n g s c a t t e r i n g data t o s u f f i c i e n t l y h i g h values o f k t o enable t h e real-space r a d i a l distribution f u n c t i o n (RDF) t o be calculated, and so o b t a i n t h e f i r s t d i r e c t s t r u c t u r a l evidence f o r t h e changes t h a t accompany the photo-induced e f f e c t s .

Experimental.- The archetypal chalcogenide glasses, As S and As Se

,

were chosen f o r study, and manufactured from 6N As and Se (M.C5P.) and 4N S ( E O C ~ L i g h t ) s t a r t i n g m a t e r i a l s , r e a c t i o n t a k i n g p l a c & i n evacuated (10 T o r r ) sealed ampoules placed i n a r o t a t i n g tube furnace a t 0,700 C; t h e glasses were formed by a i r quenching. They were then annealed t o t h e i r r e s p e c t i v e g l a s s - t r a n s i t i o n temperatures, and f i n e l y pow-

dered. The samples were sealed under vacuum and i l l u m i n a t e d using a 1kW Xe lamp. F o r As2S3, i r r a d i a t i o n was through a 500 nm i n t e r f e r e n c e f i l t e r (band pass = 50 nm)

f o r a p e r i o d o f 93 hours, w h i l s t f o r As Se3 no narrow band-pass f i l t e r was used, b u t i n s t e a d h e a t f i l t e r s were employed a1 18wing 1 ig h t , i ( -700nm, E21.75eV, onto the sample f o r a p e r i o d o f 89 hours; i n both cases, t h e powdered samples were con- t i n u o u s l y s t i r r e d w h i l e being h e l d a t 77K. The samples (2.19 As Se and 1.49 As2S3) were t r a n s f e r r e d t o a narrow-wall vanadium c o n t a i n e r a f t e r warmiKg $hem t o room temperature.

A l l neutron s c a t t e r i n g experiments were performed a t room temperature on t h e D2 powder d i f f r a c t o m e t e r a t the ILL, Grenoble. A f t e r measuring the s c a t t e r i n g i n t e n s i t y f o r 4 0 h r s i n each case f o r t h e i l l u m i n a t e d samples, they were each then annealed i n s i t u a t t h e i r r e s p e c t i v e Tg's, and the s c a t t e r i n g data f o r the samples i n t h e ' r

-

a n n m d s t a t e were then measured. The wavelength o f the neutrons used was 1.228, and the maxim mlattainable value o f the s c a t t e r i n g v e c t o r u s i n g t h i s instrument was t h e r e f a r e 8 . 7 k

.

Although c o r r e c t i o n s were made f o r t h e s c a t t e r i n g due t o t h e con- t a i n e r and f o r m u l t i p l e s c a t t e r i n g , due t o the l i m i t e d time a v a i l a b l e i t was n o t poss- i b l e t o measure the s c a t t e r i n g from a c a l i b r a t i o n standard such as a V rod i n o r d e r t o convert a l l the s c a t t e r i n g i n t e n s i t i e s t o an absolute s c a l e . For t h i s reason, therefore, a l l s c a t t e r i n g data had the constant background I ( Q ), s u b t r a c t e d and were normalized t o t h i s background; t h e r e f o r e the s c a t t e r i W B x i n t e n s i t i e s and t h e

F o u r i e r transformed RDFs are i n a r b i t r a r y u n i t s . However, t h i s l i m i t a t i o n does n o t a f f e c t t h e d i f f e r e n c e s o f t h e data whicn we w i ' l l present.

Results.- We show i n f i g . l ( a ) t h e reduced s c a t t e r i n g i n t e n s i t y f o r t h e annealed a- As2Seg sample. The raw s c a t t e r i n g i n t e n s i t y data has been smoothed by means of a cubic s p l i n e passing through t h e data p o i n t s . Shown i n f i g . 1 ( b ) i s t h e d i f f e r e n c e (illuminated-annealed) o f t h e reduced s c a t t e r i n g i n t e n s i t i e s f o r the same sample o f

Se having the same u n i t s f o r the v e r t i c a l scale as f i g . 1 ( a ) . I t i s immediately en2 t h a t photo-induced changes a r e n o t r e s t r i c t e d s o l e l y t o t h e f i r s t peak a t

as t h e o n l y data i n t h e l i t e r a t u r e x d i c a t e (3), b u t extend as f a r o u t i n Q- space as our p r e s e n t experiments reach. This i n i t s e l f i s t h e f i r s t d i r e c t e x p e r i - mental p r o o f t h a t the r e v e r s i b l e p h o t o - s t r u c t u r a l e f f e c t s i n v o l v e changes i n the short-range order o f t h e glass, i n v o l v i n g perhaps f i r s t and second nearest n e i ghbonrs.

e o serve y neutron d ' f r a c t i o n o f glassy As2Se photo-induced changes i n t h e f i r s t

!eakb( "pre-Eeak") a t % 1 k f s i m i l a r i n form and maani tude t o those observed by X-ray d i f f r a c t i o n i n annealed evaporated f i l m s o f a-As S3 a t one temperat r ( 3 ) , a broaden- i n g o f the peak, accompanied by a fil l i n g - i n o f ?.he minimum a t ~1 . 5 k T . However, the most marked changes a r e seen i n t h e peaks a t h i g h e r Q-values where reductions i n peak h e i g h t s o f %2% occur. The data f o r the sample o f a-As S3 were q u a l i t a t i v e l y s i m i l a r b u t the changes were somewhat smaller, r e f l e c t i n g i n o 6 r view, the s m a l l e r photon f l u x t h a t reached the As S through the narrow band-pass f i l t e r than was i n c i d e n t on the sample o f a-As2Se3, ?l?uminated w i t h o u t such a f i l t e r .

- 0 2 -0 4

0 6 - 0 8

-1 0

Irradtated - annealed - 0 0 5

-006"

,

' ^{5 6 7} I ⁸ ' ^Q

^(a4)

I Fig.2 a ) Reduced RDF o f annealed a-AsZSej b) D i f f e r e n c e i n reduced RDF a f t e r F i g . 1 a ) educed s c a t t e r i n g i n t e n s ? ty of i 1 lumination. The r e g i o n enclosed i n the annealed a-As Se3; b ) D i f f e r e n c e i n box i s discussed i n d e t a i l i n the t e x t . s c a t t e r i n g ingensi t y a f t e r i l l u m i n a t i o n .

D i r e c t s t r u c t u r a l i n f o r m a t i o n i s o b t a i n a b l e from t h e reduced RDF G(r) c a l c u l a t e d by F o u r i e r t r a n s f o r m a t i o n o f t h e reduced s c a t t e r i n g i n t e n s i t y shown i n f i g . 1 (a), taken t o be p r o p o r t i o n a l t o F(Q), t h e c z r r e c t l y scaled reduced s c a t t e r i n g i n t e n s ~ t y .

G(r) = h r ( p ( r ) - p o ) = - , f ~ ( ~ ) s i n ~ r d ~ 2

75 ^{( 1}

1

Shown i n f i g . 2(a) i s G(r) obtained from t h e data i n f i g . l ( a ) , m u l t i p l i e d by a Lorch f u n c t i o n t o reduce t e r m i n a t i o n r i p p l e due t o the f i n i t e value o f Q used (8.78

'

⁾

Since the neutron s c a t t e r i n g f a c t o r s i n v o l v e d i n F(Q) a r e independ&% of Q, eqn. ( 1 ) gives a good approximation t o the RDF even f o r a b i n a r y a l l o y ; we d i d n o t undertake d e t a i l e d peak-shape analyses i n view o f the l i m i t e d r e s o l u t i o n a v a i l a b l e . The d i f f e r - ence i n G(r) (illuminated-annealed) i s shown i n f i g . 2(b). The most s i g n i f i c a n t r e s u l t i s t h e ~ 5 % decrease i n t h e h e i g h t o f the second peak i n G(r) on i l l u m i n a t i o n . There are a l s o smaller, b u t s i g n i f i c a n t , changes i n the v i c i n i t y o f t h e o t h e r peaks.

Discussion.- The d i f f e r e n c e AG(r)

6

lluminated-annealed) i n f i g . 2 ( b ) shows d i r e c t l y t h a t t h e r e a r e changes i n t h e short-range order. Both t h e As Se and As S samples showed the same behaviour o f AG(r) i n t h e v i c i n i t y o f t h e se?on?! peak i$ g ( r ) , v i z . a negative peak a t the average p o s i t i o n o f the second peak, w i t h two p o s i t i v e peaks on e i t h e r s i d e . There i s some evidence f o r a change i n t h e f i r s t peak a t 2.48 i n As Se b u t t h i s was n o t observed i n As S

.

Changes i n the r e g i o n o f t h e t h i r d peak at2~53;(8 are observed i n b o t h cases, bu2 d i f f e r q u a l i t a t i v e l y . However, since an understanding o f c o r r e l a t i o n s a t t h i s distance depend on a knowledge o f the d i h e d r a l - angle d i s t r i b u t i o n , which i s unknown i n the absence o f modelling studies, we w i l l n o t consider these changes n o r those i n t h e f i r s t peak f u r t h e r , and concentrate i n s t e a d on the changes i n the second peak which appear t o be a u n i v e r s a l f e a t u r e . Note t h a t

the second peak i n G ( r ) i s made up from two c o n t r i b u t i o n s , namely As-As c o r r e l a t i o n s

(J -As) and Se-Se c o r r e l a t i o n s ( J ) i n v o l v i n g the Se and As bond-angles, respect- v&y, and the l a c k of resolved s t k t b r e i n the second peak i m p l i e s t h a t

eSd

^OAs.

Two p o s s i b l e explanat'ons can be advanced f o r t h e c e n t r a l minimum and two f l a n k - i n g maxima centred a t ~ 3 . 7

A

i n G(r). The f i r s t i n v o l v e s the s e l f - t r a p p e d e x c i t o n model (5). A decrease i n t h e h e i g h t o f t h e second peak i n G(r) r e s u l t s i f atomic con-

* i g u r a t i o n s corresponding t o t h e average bond-angle are a l t e r e d on i l l u m i n a t i o n , pro-

C9-38 JOURNAL DE PHYSIQUE

ducing configurations having d i f f e r e n t bond-angles. Tk,e detailed changes ( 2 ) t h a t occur upon exciton self-trapping a r e t h a t normally bonded As o r Se atoms both having an average bond-angle of "101 -4 areotrapsformed anto+charged dangling-bond centres (C,7) which have bond-angles of "-109 fP4) o r %90 ( C o r P i ) . Therefore i t i s expec- ted t h a t a decrease in t e heig t ur m e s cond peak ahould be accompanied by increas- ed contributions a t % 3 . 4

k

^(0.90

B

⁾ nd13.9k @=lo9 ) . However, the two peaks in f i g . 2(b) a r e observed t o f a l l a t "3.15! and %4.3h, values which are inconsistent w i t h reasonable bond-angles a t the photo-induced defects ( 8 ) .

The other model i s t h a t of photo-induced randomization of the chalcogen bond- angle, proposed generally in connection with vibrational spectra ( 4 ) and a l s o as a r e s u l t of an atom f l i p ( 3 ) . We assume t h a t i t is the chalcogen atom t h a t s u f f e r s the change since i t i s probable t h a t the high-lying chalcogen lone-pair electrons a r e photo-exci ted. Thus only JAs-As (determined by OSe) i s affected, b u t t h i s i s present

2 2

only in the r a t i o 2b sJAs-As: 3bsf:ae-Se ( i .e. 1 : 2.94); we further assume t h a t both J cat? be represen as Gaussians with the same width o. Our model i ~ ~ d i E ; t : k ? f u m i n a t i o n . J has become wider , b u t 7 remai ns unchanged.

Thus, the difference AG(r) i s a mb%fi?e only of t h m f e r e n c S e ~ ; i e ~

,

which f o r two Gaussians of d i f f e r e n t widths, i s characterized by a central mia?mfli% flanked by two symnetrical maxima, q u a l i t a t i v e l y as in f i g . 2(b). F i t t i n g the width and height of the 3 s-A gives oan =0.388; a good f i t t o aG(r) can only be achieved f o r a i l - 0.418 (8y. Shese valuee produce a good match t o the depth of the minimum and rebro- duce the position and height of the two maxima. The incrgase in width of the second peak i n G ( r ) , o

- g ^,

corresponds to an increase of *1 i n the spread in the bond- angle d i s t r i b u t l a i . ?fie difference of two symmetrical Gaussians must i t s e l f be sym- metrical, and i t i s to be noted from f i g . 2(b) t h a t the two maxima are of unequal heights. This may be due to a variety of reasons : 1 ) e i t h e r o r both of the second peaks i n G(r) may be asymmetrical; 2) there may be a s h i f t in average bond-angle on illumination i n addition to a uniform broadening; 3) there may be difference i n aver- age density P between the two forms (an increase in volume, giving a decrease i n density, whica would explain the asymmetry, has been reported a f t e r i 1 lumination ( 3 ) , but the magnitude of the change "0.5% would seem t o be too small). This question can only be r e a l l y answered when higher resolution experiments become available.

Conclusions.- Ue have undertaken the f i r s t neutron s c a t t e r i n g studies of the reversib- l e photo-structural changes which occur i n chal cogeni de glasses. We have demonstrated

f o r the f i r s t time t h a t the optically-induced changes extend t o high values of Q and are therefore caused by changes in the short-range s t r u c t u r e of the glass. Our meas- urements extend to large enough values of Q to enable a real-space RDF to be obtained, and i t i s evident t h a t the l a r g e s t change occurs in the second neighbour distance. The only reasonable f i t to the difference between illuminated and annealed G( ) in the region of the second peak i s t h a t given by a photo-induced increase (0.03

k

^{) i n the}

width of a Gaussian d i s t r i b u t i o n of As-As negt-nearest ~ e i g h b o u r s , corresponding t o an increase in bond-angle fluctuations of "1

.

Acknowledgements.

-

We are grateful t o the Ruther.ford and Appleton Laboratories and the Royal Society f o r financial assistance.

References.

1 . De Neufville J.P. i n "Optical Properties of Solids", ed. B.O. Seraphin (North- Holland : 1976)

2. De Neufville J.P., Moss, S.C., and Ovshinsky, S.R., J.Non-Cryst.So1.

2

(1973/4) 191.

3. Tanaka K . J . Non-Cryst. Sol.

35-36

(1980) 1023.

4. Utsugi Y & Mizushima Y . , J.App1 .Phys.

2

(1978) 3470.

5. S t r e e t R.A. Sol. Sc. Comm. 24, (1977) 363.

6. Mott N.F., Davis E . A . and s t r e e t R.A. Phil .Mag. 32, (1975) 961 7. Kastner M, Adler D and Fritzsche H . Phys.Rev.LettYX, (1976) 1504 8. E l l i o t t S.R. and Rayment T.

-

To be published.