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SMALL-ANGLE NEUTRON SCATTERING STUDY
OF IRREVERSIBLE PHOTOSTRUCTURAL
EFFECTS IN OBLIQUELY DEPOSITED GeSe3 FILMS
C. Spence, S. Elliott
To cite this version:
Résumé - Les films amorphes GeSe-j évaporés obliquement ont une structure colonnaire très marquée. La contraction induite par photo-irradiation a été étudiée par diffusion des neutrons aux petits angles et microscopie électro-nique à balayage. Il apparaît que la densification vient de l'élimination de trous dans les colonnes sans que la microstructure colonnaire soit pour autant perturbée.
SMALL-ANGLE NEUTRON SCATTERING STUDY OF IRREVERSIBLE PHOTOSTRUCTURAL EFFECTS IN OBLIQUELY DEPOSITED G e S e3 FILMS
C.A. Spence and S.R. Elliott
Department of Physical Chemistry, University of Cambridge, Lens field Road, Cambridge, U. K.
Abstract - Obliquely evaporated amorphous GeSe3 films have a marked columnar microstructure. The photo-induced contraction of a-GeSe3 films has been
studied with small-angle neutron scattering and scanning electron microscopy. The results show that whilst the density change may be accounted for by the collapse of voids within the columns, the gross columnar microstructure per-sists after illumination.
INTRODUCTION
Vapour deposited amorphous chalcogenide films, and particularly those deposited at oblique incidence to the film normal, show large changes in several physical and chemical properties on illumination with light of energy greater than the band gap /l/. Changes in density of up to 16% have been observed in Ge-Se films /l/. These films show a marked columnar microstructure, as is clearly seen frcm the SHI pict-ures (fig.l). Computer modelling of the growth of obliquely deposited films sug-gest that they contain voids enclosed in the bulk of the material as well as having a columnar microstructure /2/. On illumination the films densify, and a previous small-angle neutron scattering (SANS) study /3/ showed that this was due to the collapse of the voids. This paper complements that study by extending the range of Q(=47rsin8/X) to smaller values, thus enabling the behaviour of larger voids to be studied.
Fig. 1 - Scanning electron micrographs. Edge on SEM of 80° before (a) and after (b) illumination.
JOURNAL DE PHYSIQUE
The measuranents reported i n t h i s paper were performed on t h e D l 1 SANS d i f f r a c t meter a t t h e I n s t i t u t e h u e laogevin, Orenoble, using neutrons of wavelength
101;
with t h e detector positioned a t 1Qn (giving a Q range of 5x10-~ t o 1-1) and a t 2n (2.5~10-2<Q<
1.5~10-11-11
; counting times per run were two hours and f i f t e e n minutes, respectively. The Dl1 d i f f r a c t m e t e r has an area detector which greatlyf a c i l i t a t e s the study of anisotropic s t r u c t u r a l inhanogeneities.
Films of amorphous GeSeg were produced by evaporating powdered bulk GeSe3 onto t h i n (0.- thick) sapphire substrates. The substrates were held a t a variety of angles t o t h e vapour source, on a water-cooled j i g . The films were between 1 and 2 thick.
Sane of t h e films were illuminated with broad-band l i g h t f r a n a Xe a r c lamp passed through a heat-cut f i l t e r . The i n t e n s i t y of t h e l i g h t on t h e sample w a s 2.1~/&
(integrated over a l l frequencies) and t h e period of illumination w a s twenty minutes.
a l l - a n g l e s c a t t e r i n g (SAS) w a s observed t o be greatest and most anisotropic f r a n t h e 800 films. A simple inspection of t h e s c a t t e r i n g i n t e n s i t y (see i n s e t t o f i g . 2) showed t h a t only t h e 800 films (both illuminated and unilluminated) showed much s c a t t e r i n g when t h e detector w a s a t lQn, whereas with t h e detector a t 2 n anisotropic SAS w a s observed from t h e 700 unilluminated film as well. I t w a s a l s o observed t h a t t h e SAS i n t e n s i t y ( t o t a l counts a t t h e detector) from t h e 80° films with t h e detect- or a t 1Qn is s l i g h t l y greater f o r t h e illuminated film than f o r t h e unilluminated film (though t h i s may be because they a r e d i f f e r e n t films and so may have been deposited a t s l i g h t l y d i f f e r e n t angles). With t h e detector a t 2n, however, t h e SAS is seen t o decrease dramatically on illumination. This suggests t h a t t h e sllaller voids a r e destroyed on illumination whilst leaving t h e larger voids e s s e n t i a l l y in- t a c t .
The data were corrected f o r substrate s c a t t e r i n g and nonnalised t o water scattering, and then sector averaged. A s can be seen from' t h e r a w data, a t low Q t h e r e is a problem with beam spillage, i . e . t h e beam stop did not canpletely mask t h e s t r a i g h t - through beam. To allow f o r t h i s c e l l s with ananalously high counts were excluded f r a n t h e sector averaging.
Fran SANS measuranents it is possible t o assign s i z e s t o t h e voids responsible f o r t h e scattering. However a p l o t of log i n t e n s i t y vs ~2 ( a Guinier p l o t ) is not l i n - ear with respect t o
@
over any portion of Q measured, and so a single radius of gyration cannot be assigned t o t h e voids. In order t o assign dimensions t o t h e voids, and t o estimate t h e i r number, t h e scattering w a s modelled as i n a previous study /3/. In t h a t paper t h e s c a t t e r i n g was assumed t o be due t o independently s c a t t e r i n g e l l i p s o i d s . These e l l i p s o i d s have equal minor axes Al=Az, and t h e dist- ribution of each length, about an average A?, is a Gausian. This model gives t h e follawing expression describing t h e -11-angle scattering:\mere
is
t h e neutron s c a t t e r i n g length per u n i t volumeCP =
(Q#l)2 + + ((&A312a
6 is t h e polydispersity.
ing t h e &el described in t h e t e x t . The i n s e t shows a 3D p l o t of t h e raw d a t a .
F i t s t o sane of t h e d a t a a r e shown i n f i g u r e 2 ; f o r t h e l e s s obliquely deposited f i l m s t h e s c a t t e r i n g was t o o weak t o be modelled. The u p p e m s t
set
of d a t a p o i n t s give information about t h e s i z e d i s t r i b u t i o n of t h e minor a x i s . These p o i n t s aref i t t e d f i r s t , and then t h e long a x i s A3 is varied t o obtain t h e b e s t f i t s f o r t h e o t h e r s e c t o r s . The r e s u l t s of t h e f i t s are s h a m i n t a b l e 1.
Table 1
Sample
~111
6 Detector positionlm80 u n i l l 185 575 0.50 10 80 ill 330 845 0.40 10 8 0 u n i l l 70 130 0.40 2 80 ill 280 580 0.40 2 70 u n i l l 55 110 0.50 2 r e f /3/ 80 u n i l l 60 120 0 . 4 0 1.76
Notice f i r s t t h a t t h e void s i z e seans t o change when t h e p o s i t i o n of t h e detector is
JOURNAL
DE
PHYSIQUE Samplepetector Position/m 80 u n i l l /10 80 ill /10 80 u n i l l / 2 80 ill /2 70 u n i l l /2 Table 2 Number of Voids 1 . 5 l d 4 2.7 x 1d3 2.2 x l d 6 1.0x
ld14 1 . 2 x 1016 %Void Volume 1 . 2 2 . 3 7.2 2.7 2.3 DISCUSSIONThe d a t show t a t f o r t h e 80° f i l m s most of t h e density d e f i c i t is due t o voids of s i z e 701 by
Id.
The void volume calculated here is smaller than t h a t observed by Chopra e t . a l . / I / , who found a density d e f i c i t of 16%; t h i s may r e f l e c t a r e a l difference i n t h e samples, o r may j u s t be due t o t h e s m a l l range of Q sampled. On i l l m i n a t i o n , it is clear t h a t t h e smaller voids are t h e f i r s t t o collapse; indeed t h e l a r g e r voids d o not appear t o change (perhaps longer illumination periods would r e s u l t i n a change and t h i s is c u r r e n t l y being s t u d i e d ) . These findings a r e consis- t e n t with SEM which shows t h e columnar s t r u c t u r e t o be l i t t l e a f f e c t e d by t h i s amount of illumination ( s e e f i g . 1 ) .
The f a c t t h a t t h e l o g I vs log ~2 p l o t s are not l i n e a r over any p a r t of t h e range of Q shows t h a t we
are
not dealing with a mono-disperse systen. W t h e n n o r e t h e f a c t t h a t t h e p l o t s a t d i f f e r e n t detector distances y i e l d d i f f e r e n t c h a r a c t e r i s t i c s i z e s f o r t h e s c a t t e r i n g p a r t i c l e s shows t h a t t h e p a r t i c l e s have a wide range of s i z e s . Ccnnbining t h e d a t a (although t h e regions do not q u i t e overlap, and t h e d a t a is a l i t t l e noisy a t l a r g e r Q) would g i v e a d i f f e r e n t estimate of t h e s i z e of t h e s c a t t - e r i n g p a r t i c l e s but t h i s might not be helpful. Since SEM shows t h a t t h e columnar microstructure is still present a f t e r illumination it is possible t h a t t h e r e are two types of s c a t t e r e r , i.e. i n t e r - , and intra-columnar voids. With t h i s model t h e d a t a could be i n t e r p r e t e d as showing t h a t t h e material within t h e columns collapses on illumination, w h i l s t t h e void space between t h e coLumns is l i t t l e a f f e c t e d by l i g h t ; t h i s d i f f e r e n t i a t i o n would be "smeared by canbining t h e d a t a sets.Another p o s s i b l e i n t e r p r e t a t i o n of t h e p a r t i c u l a r type of s c a t t e r i n g which has been observed can be made using a f r a c t a l approach / 5 / , i n which t h e s c a t t e r i n g from l a r g e pores with rough i n t e r i o r s is considered. I n our case t h i s m u l d correspond t o s c a t t e r i n g f r a n t h e rough-walled inter-columnar regions ( t h e r e might not be any intra-columnar s c a t t e r e r s ) . However t h e present d a t a do not appear t o p l o t l i n e a r l y on a log I v s l o g Q b a s i s , expected f o r f r a c t a l behaviour /5/. B e t t e r data over a l a r g e r range of Q w i l l enable a more unambiguous choice t o b e made.
These d a t a suggest t h a t t h e photo-induced contraction observed i n obliquely deposit- ed a-GeSe3 f i l m s is due t o t h e collapse of voids within t h e columns r a t h e r than just
t h e collapse of t h e g r o s s columnar microstructure. However it is not possible t o make any deductions f r m t h e s e d a t a a s t o why t h e voids should c o l l a p s e i n t h e f i r s t place.
The authors would l i k e t o thank Dr A.F. Wright f o r h i s a s s i s t a n c e with t h e neutron s c a t t e r i n g experiment, and Dr T. Rayment f o r h e l p f u l discussions. One of u s (CAS) would l i k e t o thank Pilkington Bros. f o r t h e i r i n t e r e s t and f i n a n c i a l a s s i s t a n c e through a case award.
/I/ S. Rajagopalan, K.S. Harshavaxdhan, L.K. Malhotra and K.L. Chopra, J.N.C.S. 50 1982 641.
/2/ D. Henderson, M.H. Brodsky, P. Chaudhari, Appl. Phys. Letters 25 1974 641. /3/ T. Rayment, S.R. E l l i o t , Phys. Rev. B 28 1983 1174.-
