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PRESSURE DEPENDENCE OF DYNAMICAL CHARGES AND IONICITY OF SEMICONDUCTORS
M. Cardona
To cite this version:
M. Cardona. PRESSURE DEPENDENCE OF DYNAMICAL CHARGES AND IONICITY OF SEMICONDUCTORS. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-29-C8-39.
�10.1051/jphyscol:1984806�. �jpa-00224304�
JOURNAL DE PHYSIQUE
Colloque C 8 , supplément au n ° l l , Tome 45, novembre 1984 page C8-29
PRESSURE DEPENDENCE OF DYNAMICAL CHARGES A N D IONICITY OF SEMICONDUCTORS M. Cardona
Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304, U.S.A.
Résumé - Avec la cellule à enclumes de diamant il est devenu possible de mesurer très précisément les fréquences des phonons en fonction de la pression par spectroscopie Raman. Dans les m a t é r i a u x polaires on voit des paires de phonons LO-TO dont on peut mesurer l'êcartement en fonction de la pression. Ces mesu- res fournissent des renseignements sur la relation entre la char- ge dynamique e t , partant ,1'ionici té des l i a i s o n s , et la c o n s t a n - te de réseau. Des résultats pour les matériaux de structure blende sont discutés et comparés avec des calculs théoriques basés sur les méthodes de pseudo-potentiels et LCAO.
Abstract - The advent of the diamond anvil cell has made possi- ble very accurate determinations of the pressure dependence of phonon frequencies by means of Raman scattering. In polar mate- rials pairs of longitudinal and transverse phonons are observed.
The pressure dependence of the separation between these lines gives direct information on the dependence of the dynamic effec- tive charge on lattice constant which can be taken as a measure of the dependence of the bond-ionicities on a
0. We discuss expe- rimental data obtained for zincblende-type semiconductors in the light of the results of pseudopotential and LCAO calculations.
I - INTRODUCTION
The diamond anvil cell is an ideal instrument for performing optical m e a s u r e m e n t s , especially light scattering experiments ( 1 , 2 ) . Measure- ments of the Raman active phonons, and also of two-phonon critical p o i n t s , have yielded a great wealth of information about mode Gruneisen parameters (for a review see R e f . 2 ) . The available pressure ranges are so high (= 30 Gpa) that strong non-linearities in the pressure and/or lattice constant dependence can be observed. Whenever phase transitions occur, Raman scattering reveals them as discontinuous changes in the frequency and/or intensity of the phonon peaks.
Infrared (ir)-active modes show a splitting into a TO-doublet and a L 0 - singlet for wavevectors q" much larger than those which correspond to the polariton region in which transverse phonons and photons interact strongly. Such region corresponds to:
where n is the refractive index, u j
0the frequency of the transverse phonons, and c the speed of light in vacuum. Raman scattering experi- ments in semiconductors are usually performed in the backscattering configuration, for which the diamond anvil cell is most suited. For a laser frequency u
Lthe transfer in q-vector in a backscattering experi- ment is:
+
Permanent address : Max-Planek-Institut fur Festkorperforschung, Heisenbergstr. I, 7000 Stuttgart 80, F.R.G.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984806
J O U R N A L DE PHYSIQUE
h e n c e t h e c o r r e s p o n d i n g p h o n o n s s h o u l d s h o w t h e s t a n d a r d LO-TO s p l i t - t i n g g i v e n b y
w h e r e e T * i s t h e t r a n s v e r s e , o r d y n a m i c e f f e c t i v e c h a r g e o f t h e mode u n d e r c o n s i d e r a t i o n N t h e n u m b e r o f u n i t c e l l s p e r u n i t v o l u m e , p t h e r e d u c e d m a s s o f t h e u n i t c e l l , a n d
E,t h e i r d i e l e c t r i c c o n s t a n t . T h e Raman m o d e s o f t h e t e t r a h e d r a l s e m i c o n d u c t o r s o f t h e f o u r t h g r o u p
( G e , S i ) a r e i r - i n a c t i v e . T h e z i n c b l e n d e c o m p o u n d s , h o w e v e r , a r e p a r t l y p o l a r a n d t h u s i r - a c t i v e . T h e y e x h i b i t LO-TO s p l i t t i n g s w h i c h a r e n e v e r t h e l e s s , s m a l l ( - 2 5 c m - ' I d u e , i n p a r t , t o t h e l a r g e v a l u e s o f
E,w h i c h r e s u l t f r o m t h e s t r o n g c o v a l e n t c o m p o n e n t o f t h e b o n d i n g . I n t h i s c a s e t h e d y n a m i c a l c h a r g e e T * c a n b e t a k e n t o b e a m e a s u r e o f t h e i o n i - c i t y o r p o l a r i t y o f t h e c o m p o u n d / 3 , 4 / . T h u s , m e a s u r e m e n t s o f t h e d e p e n d e n c e o f e T * o n v o l u m e ( i . e . p r e s s u r e ) y i e l d i n f o r m a t i o n o n t h e d e p e n d e n c e o f t h e i o n i c i t y o f t h e c o m p o u n d o n l a t t i c e c o n s t a n t . T h e d e p e n d e n c e o f e T * o n p r e s s u r e c a n b e o b t a i n e d f r o m t h e m e a s u r e d d e p e n d e n c e o f w a n d
W To n p r e s s u r e p r o v i d e d t h e d e p e n d e n c e o f
E,o n p r e s s u r e ( o r v o i u m e ) i s e n o w n . T h e l i n e a r c o e f f i c i e n t B l n r , / a l n a o ( a o = l a t t i c e c o n s t a n t ) h a s b e e n m e a s u r e d f o r o n l y a f e w o f t h e m a t e r i a l s u n d e r c o n s i d e r a t i o n . I n c a s e s f o r w h i c h i t h a s n o t b e e n m e a s u r e d i t i s c u s t o m a r y t o t a k e t h e t h e o r e t i c a l v a l u e a l n c , / a l n a o = 3 /5/.
E a r l y a t t e m p s t o m e a s u r e t h e p r e s s u r e d e p e n d e n c e o f w O - W T O / 6 , ? / y i e l - d e d r a t h e r i n a c c u r a t e r e s u l t s a s t h e y w e r e p e r f o r m e d \ n c o n v e n t ~ o n a l l a r g e v o l u m e h i g h p r e s s u r e c e l l s a n d t h u s l i m i t e d t o a p r e s s u r e o f a b o u t 8 k b a r . T h e d i a m o n d a n v i l c e l l h a s made s u c h m e a s u r e m e n t s u p t o t h e p h a s e t r a n s i t i o n o f many s e m i c o n d u c t o r s p o s s i b l e / 1 , 4 , 8 - 1 1 / . I n a l l c a s e s f o r w h i c h r e l i a b l e d a t a e x i s t e T * i s f o u n d t o d e c r e a s e w i t h i n - c r e a s i n g p r e s s u r e , w i t h t h e o n l y e x c e p t i o n o f S i c / l o / . H e n c e o n e m u s t c o n c l u d e t h a t t h e b o n d i o n i c i t i e s ( a s o m e w h a t q u a l i t a t i v e c o n c e p t ) u s u a l l y d e c r e a s e w i t h d e c r e a s i n g b o n d l e n g t h .
I t i s a l s o p o s s i b l e , i n p r i n c i p l e , t o m e a s u r e t h e d e p e n d e n c e o f e T * o n u n i a x i a l s t r e s s . I n t h i s c a s e e T * ( o r a c t u a l l y e T * 2 ) b e c o m e s a s e c o n d r a n k t e n s o r w i t h t h r e e i n d e p e n d e n t c o m p o n e n t s w h ~ c h c o r r e s p o n d t o a h y d r o s t a t i c s t r e s s , a s t r e s s a l o n g [Ill], a n d a s t r e s s a l o n g [ l o o ] .
M e a s u r e m e n t s a r e l i m i t e d t o t h e y i e l d s t r e n g t h o f t h e m a t e r i a l , u s u a l l y b e l o w 2 Gpa. L i k e i n t h e c a s e o f h y d r o s t a t i c p r e s s u r e , t h e a c c u r a c y o b - t a i n e d i s t h e n m a r g i n a l f o r o b s e r v i n g t h e d e p e n d e n c e o f e T * o n s t r e s s / 1 2 / . A d i s c u s s i o n o f t h e t h e o r y o f t h e d e p e n d e n c e o f e T * o n u n i a x i a l s t r e s s a n d o f e x i s t i n g d a t a f o r z i n c b l e n d e - t y p e s e m i c o n d u c t o r s w i l l b e p u b 1 i s h e d e l s e w h e r e / 1 3 / .
I 1 - THEORY
T h e m o d e r n t h e o r y o f p h o n o n s i n s e m i c o n d u c t o r s i s n b a s e d o n t h e e l e c t r o - n i c e n e r g y b a n d s / 1 4 - 1 7 / . Two t y p e s o f s o - c a l l e d f i r s t p r i n c i p l e s "
c a l c u l a t i o n s a r e i n c u r r e n t u s e . One o f t h e m / 1 4 / i n v o l v e s t h e d i e l e c -
t r i c m a t r i x f o r m a l i s m . T h e p h o n o n f r e q u e n c i e s t h e n r e s u l t f r o m t h e
p l a s m a f r e q u e n c i e s o f t h e i o n c o r e s a p p r o p r i a t e l y s c r e e n e d o u t b y t h e v a l e n c e e l e c t r o n s o f t h e s e m i c o n d u c t o r . T h i s s c r e e n i n g i s c a l c u l a t e d f r o m any m a n a g e a b l e r e p r e s e n t a t i o n o f t h e e n e r g y b a n d s t r u c t u r e , u s u a l - l y t h e p s e u d o p o t e n t i a l m e t h o d , b y a p p l y i n g t h e d i e l e c t r i c m a t r i x f o r m a - l i s m ; , The o t h e r c z m p u t a t i o n a l t e c h n i q u e c u r r e n t l y i n u s e i s b a s e d on t h e f r o z e n p h o n o n c o n c e p t : one c a l c u l a t e s t h e t o t a l e n e r g o f t h e c r y s t a l i n t h e p r e s e n c e o f a p l a n e wave distorti-t t o a f r o z e n p h o n o n / 1 5 - 1 7 / . The s e c o n d d e r i v a t i v e o f t h i s e n e r g y w i t h r e s - p e c t t o t h e a m p l i t u d e o f t h e f r o z e n p h o n o n y i e l d s t h e e f f e c t i v e f o r c e c o n s t a n t o f t h e p h o n o n mode f r o m w h i c h t h e p h o n o n a m p l i t u d e i s e a s i l y o b t a i n e d .
The a p p r o a c h e s j u s t m e n t i o n e d r e q u i r e e x t e n s i v e n u m e r i c a l c a l c u l a t i o n s i n w h i c h t h e p h y s i c a l s i g n i f i c a n c e o f t h e v a r i o u s p a r a m e t e r s i n v o l v e d i s l o s t . F o r t u n a t e l y , i t i s p o s s i b l e t o o b t a i n a s i m p l i f i e d e x p r e s s i o n f o r t h e f r e q u e n c i e s o f t h e o p t i c a l p h o n o n s a t q=O i n z i n c b l e n d e t y p e s e m i c o n d u c t o r s , w h i c h c a n b e h a n d l e d w i t h a p o c k e t c a l c u l a t o r / 1 8 / . T h i s e x p r e s s i o n , b a s e d on t h e d i e l e c t r i c m a t r i x a p p r o a c h , i s , f o r t h e TO-phonons a t q=O:
+ + +
S ( G ) = 1 e x p [ i G - ( X 2 - x , ) ( 6 )
s t a r o f G
w h e r e i s th: r e d u c e d mass o f t h e p r i m i t i v e c e l l , a t h e v o l u m e o f t h i s c e l l , X 1 a n d x 2 a r e t h e i o n p o s i t i o n s i n t h i s c e l l , Z a a n d Z a r e t h e i o n - c o r e c h a r g e s o f a n i o n a n d c a t i o n , r e s p e c t i v e l y , V S ( G ) a n 8 I A ( G ) a r e t h e s y m m e t r i c a n d a n t i s y m m e t r i c p s e u d o p o t e n t i a l f o r m f a c t o r s , G t h e r e c i p r o c a l l a t t i c e v e c t o r s , a n d
E Gt h e G - d e p e n d e n t d i e l e c t r i c f u n c t i o n f o r w=O.
E q u a t i o n 4 c l e a r l y e x h i b i t s t h e i o n i c p l a s m a f r e q u e n c y t e r m , 4 n Z a Z c / 3 , a n d t h e e l e c t r o n i c s c r e e n i n g t e r m E . The m a g n i t u d e o f E f o r a t y p i c a l I I I - V compound i s a b o u t h a l f t h a t o f t h e p l a s m a f r e q u e n c y c o n t r i b u t i o n . We d e f i n e t h e mode G r u n e i s e n p a r a m e t e r o f a p h o n o n o f f r e q u e n c y as:
d I n
Y = - d l n ~ ( 7 )
The f a c t o r Q - ' i n E q . 4 w o u l d l e a d t o a G r u n e i s e n p a r a m e t e r o f 1 / 2 ,
a s s u m i n g t h a t t h e s c r e e n i n g t e r m E i s i n d e p e n d e n t o f v o l u m e . A c t u a l l y ,
t h e s c r e e n i n g does d e p e n d on v o l u m e t h r o u g h t h e t e r m s n2, G 2 , V S , V A ,
a n d
EG.T h e t e r m n2 d o m i n a t e s , a f a c t w h i c h l e a d s t o a n i n c r e a s e i n t h e
G r u n e i s e n p a r a m e t e r . T a k i n g i n t o a c c o u n t o n l y t h e r e c i p r o c a l l a t t i c e
v e c t o r s ( 2 0 0 1 ( f o r { I l l } t h e s t r u c t u r e f a c t o r S(G) v a n i s h e s 1, o n e f i n d s
a G r u n e i s e n p a r a m e t e r f o r t h e TO modes a t q=O a p p r o x i m a t e l y e q u a l t o
one f o r a l l I I I - V a n d I I - V I z i n c b l e n d e - t y p e s e m i c o n d u c t o r s /11/, i n
a g r e e m e n t w i t h t h e e x p e r i m e n t a l f i n d i n g s /I/. The p s e u d o p o t e n t i a l f o r m
f a c t o r s V ( G ) r e q u i r e d f o r t h e e v a l u a t i o n o f Eq. 5 c a n be o b t a i n e d
e i t h e r f r o m s e m i e m p i r i c a l f i t s t o b a n d s t r u c t u r e d a t a / 1 0 , 1 9 / o r f r o m
t h e t h e o r e t i c a l e x p r e s s i o n /19/:
JOURNAL DE PHYSIQUE
w h e r e Rc,a r e p r e s e n t t h e c o r e r a d i i o f t h e c a t i o n o r a n i o n , r e s p e c t i v e - l y .
S i m i l a r l y , o n e c a n o b t a i n a n a n a l y t i c e x p r e s s i o n f o r t h e t r a n s v e r s e d y n a m i c a l c h a r g e e T * w h i c h , a c c o r d i n g t o Eq. 3 , d e t e r m i n e s t h e LO-TO
s p l i t t i n g . T h e e x p r e s s i o n i s :
w h e r e A Z = ~ ( Z - I c ) , w h i c h i s p o s i t i v e , i s o v e r c o m p e n s a t e d b y t h e t e r m c o n t a i n i n g tie p s e u d o p o t e n t i a l f o r m f a c t o r s . T h e l a t t e r g i v e r i s e t o t h e v o l u m e d e p e n d e n c e o f e ~ * . E q u a t i o n 9 y i e l d s f o r m o s t 1 1 1 - V com- p o u n d s v a l u e s o f e ~ * c l o s e t o 2, i n r e a s o n a b l e a g r e e m e n t w i t h e x p e r i - m e n t a l o b s e r v a t i o n s ( s e e T a b l e I 1 o f R e f . 1 1 ) .
T h e p s e u d o p o t e n t i a l t e c h n i q u e h a s b e e n v e r y s u c c e s s f u l f o r t h e i n t e r - p r e t a t i o n o f b a n d s t r u c t u r e p h e n o m e n a a n d t h u s h a s b e c o m e w i d e l y a c c e p - t e d d u r i n g t h e p a s t 2 5 y e a r s . I t h a s t h e a d v a n t a g e o f a l l o w i n g d i f f e - r e n t d e g r e e s o f s o p h i s t i c a t i o n , f r o m a n e a r l y f i r s t p r i n c i p l e s , s e l f c o n s i s t e n t a p p r o a c h / 1 5 , 1 7 / t o a s e m i e m p i r i c a l m e t h o d / 2 0 / . M o r e r e - c e n t l y a m e t h o d b a s e d on t h e t i g h t b i n d i n g o r LCAO ( l i n e a r c o m b i n a t i o n o f a t o m i c o r b i t a l s ) t e c h n i q u e h a s e s t a b l i s h e d i t s e l f o n t h e b a s i s o f i t s s i m p l i c i t y / 3 , 2 1 / . A c r u c i a l r o l e i n t h i s t e c h n i q u e i s p l a y e d b y t h e p o l a r i t y a p d e f i n e d a s :
2 2