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SECOND ORDER RAMAN SCATTERING IN
CRYSTALLINE SULPHUR SELENIUM AND
TELLURIM
P. Carroll, J. Lannin
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, supptdment
au
n o
12, Tome 42, dgcembre 1981 page C6-643SECOND ORDER RAMAN SCATTERING IN CRYSTALLINE SULPHUR SELENIUM AND
TELLURI UM
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WP.J. C a r r o l l and J.S. Lannin
Department of Physics, The Pennsylvania State University, University Park, Pennsy Zvania 16802, U. S. A.
Abstract.- The second o r d e r Raman spe'ctrum of orthorhombic(o-) S i s r e p o r t e d and compared w i t h t h e spectrum of monoclinic(m-) Se. The s p e c t r a of t h e s e r i n g systems e x h i b i t v e r y s i m i l a r s t r u c t u r e . Three r e g i o n s of s c a t t e r i n g a r e observed, two of which s u g g e s t a predominance of o v e r t o n e s c a t t e r i n g . The f e a t u r e s of t h e o t h e r r e g i o n a r i s e from combination processes. A com- p a r i s o n between t h e h i g h frequency bands of t r i g o n a l ( t - ) Te, t-Se, m-Se and o-S i s a l s o presented. D i f f e r e n c e s i n t h e s e s p e c t r a a r e q u a l i t a t i v e l y e x p l a i n e d i n terms of t h e molecular u n i t of t h e c r y s t a l ( r i n g o r c h a i n ) and changes i n i n t e r - u n i t and i n t r a - u n i t coupling.
The c r y s t a l l i n e forms of S and monoclinic(m-) Se a r e composed of t h e same b a s i c molecular u n i t , a n 8-membered-puckered r i n g . T r i g o n a l ( t - ) Se and t-Te a r e a l s o composed of s i m i l a r u n i t s , p a r a l l e l h e l i c a l c h a i n s , which a r e bonded t o g e t h e r i n a hexagonal l a t t i c e . Large i n t e r - u n i t t o i n t r a - u n i t d i s t a n c e r a t i o s i n t h e s e s o l i d s imply t h a t t h e dominant bonding i s w i t h i n t h e u n i t . However, i n t e r c h a i n bonding is more s i g n i f i c a n t t h a n t h e van d e r Waals coupling e x h i b i t e d between r i n g s and i s s t r o n g e r i n t-Te than i n t-Se.
The r o l e of t h i s i n t e r - u n i t bonding a s w e l l a s t h e i n f l u e n c e of t h e molecular u n i t i n t h e group V I elements of S, Se and Te a r e s t u d i e d h e r e through second o r d e r Raman s c a t t e r i n g . The second o r d e r Raman s p e c t r a of c r y s t a l s p r o v i d e s v a l u a b l e i n f o r m a t i o n a b o u t t h e one phonon d e n s i t y of s t a t e s under t h e c o n d i t i o n s of predominant overtone s c a t t e r i n g and m a t r i x element e f f e c t s which a r e a smooth f u n c t i o n of frequency. This i s observed t o be t h e c a s e i n p r e v i o u s s t u d i e s of group I V and V elements.' A dominance of overtone s c a t t e r i n g i s a l s o suggested i n t h e second o r d e r Raman s p e c t r a of t-Te and t-Se which have been p r e s e n t e d i n e a r l i e r I n t h e s e s p e c t r a t h e p o s i t i o n s of most of t h e f e a t u r e s a r e i n good agreement with twice t h e frequency of f e a t u r e s i n t h e phonon d e n s i t y of s t a t e s a s o b t a i n e d from t h e o r y and n e u t r o n s c a t t e r i n g .
The second o r d e r Raman spectrum of orthorhombic(o-) S i s d i s p l a y e d i n F i g u r e 1 w i t h , f o r t h e purpose of comparison, t h e spectrum of m-Se, which has been s t u d i e d i n d e t a i l previously.3 The s p e c t r a a r e s c a l e d s o t h a t t h e high frequency r e g i o n s a r e a l i g n e d . The dashed l i n e s i n d i c a t e t h e tails of t h e v e r y i n t e n s e f i r s t o r d e r peaks. The arrows i n t h e o-S spectrum i n d i c a t e twice t h e frequency of t h e k = 0 modes a t room temperature4 and a r e l a b e l e d w i t h t h e v i b r a t i o n a l
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P r e s e n t a d d r e s s : B e l l L a b o r a t o r i e s , Murray H i l l , N J 07974 Supported by National Science Foundation Grant DMR 7908390JOURNAL DE PXYSIQUE
mode assignments of an i s o l a t e d S 5 molecule a s c a l c u l a t e d by S c o t t e t a l .
Three r e g i o n s of t h e second o r d e r s c a t t e r i n g can be observed i n Figure 1 c o n t a i n i n g s i m i l a r s t r u c t u r e i n b o t h s p e c t r a . The r e g i o n which extends
-1 -1
from 255 cm t o 420 cm i n t h e o-S spectrum c o n t a i n s f e a t u r e s which a r e n e a r twice t h e frequency of E2 and E 1 f i r s t o r d e r modes. This s u g g e s t s t h a t o v e r t o n e s c a t t e r i n g o c c u r s h e r e from phonon branches corresponding t o t h e s e lower o p t i c modes. A second r e g i o n , between 500 cm-' and 760 cm-l, e x h i b i t s f e a t u r e s which can o n l y be i n t e r p r e t e d a s o r i g i n a t i n g from combination pro- c e s s e s between high and low frequency o p t i c modes. Notice t h a t t h e m-Se F i g u r e 1. The second o r d e r Raman spectrum h a s v e r y s i m i l a r s t r u c t u r e h e r e s p e c t r a of o-S and m-Se.
which i s n o t a s w e l l r e s o l v e d . A high frequency band c o n s i s t i n g of second o r d e r s c a t t e r i n g from t h e h i g h e r o p t i c modes i s t h e t h i r d region. The main f e a - t u r e s a t 822 cm-l, 876 cm-' and 937 cm-' a r e c e n t e r e d v e r y n e a r t o t w i c e t h e frequency of k = 0 modes s u g g e s t i n g t h a t t h e s e t h r e e peaks a r e due t o o v e r t o n e s c a t t e r i n g . This band may consequently be a good i n d i c a t i o n of t h e d e n s i t y of s t a t e s f o r t h e h i g h e r o p t i c modes. The narrowness of t h e s e peaks i n d i c a t e s t h a t r e l a t i v e l y l i t t l e d i s p e r s i o n o c c u r s i n t h e s e phonon branches. The s i m i l a r i t y of t h e h i g h frequency bands of m-Se and o-S a l s o s u g g e s t s a r e v i s e d mode assignment f o r t h e f i r s t o r d e r peaks i n m-Se and i s d i s c u s s e d elsewhere. 6
A comparison of t h e second o r d e r Raman s p e c t r a of t h e high frequency bands of t-Te, t-Se, m-Se, and o-S i s shown i n F i g u r e 2. A s c a l i n g f a c t o r of 1.64 i s used t o a l i g n t h e 257 cm-' peak of t h e t-Te spectrum t o t h e 441 cm-' peak of t h e t-Se spectrum and a f a c t o r of 1.77 was used t o s c a l e t h e most i n t e n s e peaks of o- S and m-Se. These e m p i r i c a l f a c t o r s a r e g r e a t e r t h a n t h e s q u a r e r o o t of t h e mass r a t i o s of t h e r e s p e c t i v e elements which i n d i c a t e s a t r e n d of weaker i n t r a - u n i t bonding i n t-Te t h a n i n t-Se and i n m-Se t h a n i n o-S. I n a d d i t i o n , t h e s h i f t t o lower f r e q u e n c i e s of t h e t-Se band w i t h r e s p e c t t o t h e m-Se band i s a t t r i b u t e d t o weaker bonding w i t h i n a t-Se c h a i n t h a n w i t h i n a m-Se r i n g . T h i s demonstrates t h a t i n t r a - u n i t c o v a l e n t bonding i n c r e a s e s a s i n t e r - u n i t bonding d e c r e a s e s i n t h e s e group - - V I elements.
t h i s may a l s o be t r u e of a system of i s o l a t e d r i n g s and t h e r e f o r e of two-fold c o o r d i n a t i o n i n g e n e r a l . The f a c t t h a t t h e t-Te spectrum does n o t have a double peaked s t r u c t u r e demon- s t r a t e s t h a t t-Te should n o t be viewed a s a
system of i s o l a t e d c h a i n s w i t h r e s p e c t t o i t s
v i b r a t i o n a l p r o p e r t i e s , a s i n t e r c h a i n i n t e r - a c t i o n s a r e important. Although v e r y s i m i l a r , t h e f e a t u r e s i n t h e o-S spectrum a r e s p r e a d o u t more t h a n i n m-Se s o t h a t a high f r e - quency f e a t u r e t h a t a p p e a r s a s a s h o u l d e r i n m-Se i s a peak i n o-S. A d d i t i o n a l f e a - t u r e s a r e a l s o observed i n t h e d e n s i t y o f valence e l e c t r o n s t a t e s of o-S'compared t o m-Se a s o b t a i n e d by x-ray photoemission spectra.' Thus b o t h t h e e l e c t r o n i c and v i b r a - t i o n a l d e n s i t i e s of s t a t e s i n d i c a t e g r e a t e r molecular c h a r a c t e r i n o-S a s i s expected s t r u c t u r a l l y .
I n g e n e r a l , it i s observed t h a t s i g n i - f i c a n t changes occur i n t h e phonon d e n s i t y of s t a t e s a s o b t a i n e d from second o r d e r Raman s c a t t e r i n g a s a f u n c t i o n of atomic weight i n
Figure 2. !rhe h i g h frequency t h e group V I elements These changes can be second o r d e r Raman bands of t-Te, q u a l i t a t i v e l y e x p l a i n e d i n terms of t h e molec- t-Se, m-Se and o-S.
u l a r u n i t and t h e r e l a t i v e s t r e n g t h o f i n t e r - u n i t and i n t r a - u n i t f o r c e s .
J. S. Lannin and B. V. han nab rook, i n Physics of Semiconductors, 1978, ed. by B. L. Wilson ( I n s t . of Physics, B r i s t o l and London, 1979) p. 643, and
r e f s . t h e r e i n .
P. J. C a r r o l l and J. S. Lannin, i n Proc. 7 t h I n t . Conf. Raman Spectroscopy, ed by W. F. Murphy (North Holland, NY, 1980) p. 66.
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