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DYNAMICS OF LOCALIZED EXCITONS IN MIXED
SEMICONDUCTING CRYSTALS
A. Nakamura, M. Shimura, M. Hirai, S. Nakashima
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C7, supplement au n°10, Tome *6, octobre 1985 page C7-179
DYNAMICS OF LOCALIZED EXCITONS IN MIXED SEMICONDUCTING CRYSTALS
A. Nakamura, M. Shimura, M. H i r a i and S. Nakashima
Department of Applied Physios, Faculty of Engineering, Tohoku University, Sendai 980, Japan
Department of Applied Physics, Faculty of Engineering, Osaka University, Suita 565, Japan
Résumé - Nous présentons une étude expérimentale de la dynamique des excitons localisés dans les alliages semiconducteurs (Cd Zn^_x Te : x = 0.32) par spectroscopie subnanoseconde aux températures comprises entre 1.8 K et 21 K. Des excitons sont localisés dessous "d'un seuil effectif" et mobiles au-delà. Sous excitation résonante, nous avons mis en évidence, pour la première fois, la transition continue de diffusion Raman résonant dans la luminescence des alliages semiconducteurs en fonction des énergies incidentes.
Abs t r a c t - We r e p o r t an e x p e r i m e n t a l s t u d y o f d y n a m i c a l b e h a v i o r o f l o c a l i z e d e x c i t o n s i n Cd Z n . _ Te m i x e d s e m i c o n d u c t i n g c r y s t a l b y s u b - n a n o s e c o n d s p e c t r o s c o p y a t t e m p e r a t u r e s 1. 8K t o 21K. I t i s s h o w n t h a t e x c i t o n s a r e e f f e c t i v e l y l o c a l i z e d b e l o w " a n e f f e c t i v e e d g e " and m o b i l e a b o v e i t , h a v i n g a h i g h e r t r a n s f e r r a t e i n t o t h e t a i l s t a t e . W i t h r e s o n a n t e x c i t a t i o n , we f i n d f o r t h e f i r s t t i m e t h e c o n t i n u o u s t r a n s i t i o n o f r e s o n a n t Raman s c a t t e r i n g t o l u m i -n e s c e -n c e i -n m i x e d c r y s t a l s a s a f u -n c t i o -n o f i -n c i d e -n t e -n e r g i e s . I -INTRODUCTION
Recently, there has been considerable interest in the energy transfer between localized states in glasses / l / , mixed molecular crystals / 2 / , and mixed semiconducting crystals / 3 , 4 / . Random p o t e n -tial fluctuations which arise from random compositional disorder in mixed semiconducting crystals produce a band edge smearing and introduce a tail state of excitons in the forbidden gap. It is interesting to investigate relaxation dynamics of Mott-Wannier excitons with a radius of the order of 50 A within an inhomogeneously broadened band and a relationship between resonant Raman scattering and luminescence from localized excitons in the tail state by time-resolved spectroscopy / 5 / . We report an experimental study of dynamical behavior of excitons in A B._ C type semiconductors CCd Zn._ Te :x=0. 32) by sub-nanosecond spectroscopy with band-to-band excitatixon and resonant excitation. We will show the experimental evidence of the exciton transfer between random potential wells and the continuous transition of resonant Raman scattering to luminescence as a function of energies of incident light.
II - EXPERIMENTAL
A cw mode-locked Ar laser and a synchronously pumped dye laser were used for excitation of samples. Luminescence from the C110) surface of Cd Zn. Te crystal was colected by the backward scattering
C7-180 JOURNAL DE PHYSIQUE g e o m e t r y . The s p e c t r u m i s a n a l y s e d b y d o u b l e - m o n o c h r o m a t o r e q u i p e d w i t h a c o o l e d p h o t o m u l t i p l i e r . We e m p l o y e d a t i m e - c o r r e l a t e d s i n g l e p h o t o n c o u n t i n g m e t h o d f o r m e a s u r i n g d e c a y b e h a v i o r o f e m i s s i o n i n t e n s i t i e s . The f u l l w i d t h s a t h a l f maximum o f t h e t i m e r e s p o n s e o f t h e s y s t e m w e r e 425 p s a n d 350 p s f o r t h e p u l s e o f t h e m o d e - l o c k e d ~ r - l a s e r a n d t h a t o f t h e s y n c h r o n o u s l y pumped d y e l a s e r , r e s p e c t i v e l y . The t i m e r e s o l u t i o n o f a b o u t 5 0 p s f o r d e c a y t i m e s was o b t a i n e d b y u s i n g a c o n v o l u t i o n a n a l y s i s .
I 1 1
-
RESULTS AND DISCUSSIONSbe tween 6. 6 K and 21K. It ,is clear that these decay curves are not exactly exponential. In order to obtain decay times, we calculated decay kinetics by convoluting exponential functions with instrumental response. Assuming that a decay curve consists of three components: a fast decay component without a rise time, a fast decay component with a finite rise time and a slow component, we used the following kine tic function:
where r f , r , cr, al, a2, a3 are introduced as adjustable parameters. The convolutgd curves g ~ v i n g the best fits to the experimental ones are shown in Fig.
2
by open circles. Above 21K. we can get best fits by taking rr=O. The fast components are dominant and the decay time, 7 f, decreases with increase of temperatures. From the arguement of rate equations, we find that the fast component without a rise time corresponds to the decay of excitons which are initially distributed in the tail state by the exciting pulse. The fast component with a rise time is due to excitons transferred between random potential wells and the rise time corresponds to the average time of transfer. From the temperature dependence of 7f,. we can obtain activation energies of excitons. The Arrhenius plot gives us two kinds of acti- vation energies,E
andEa2.
In fig.3 are summarizedEal
andEa2
as a function of exalton energies. Smaller activation energy.E
<open circles) decreases li early with exciton energy intercepting tfie
-P
E
=O axis at oer16830 cm.
Larger one (crosses) is independent of eeciton energy and its value is in good agreement with the binding energy of free excitons in CdxZnl-xTe. Below we, excitons have an activation energy depending on the energy, whereas excitons above oehave no activation energy within our experimental errors, indicating that they are delocalized. We note that
the activation energy of the
exc iton at the energy w x = 0.32
coincides well with the energy +
difference between o and the o
luminescence energy
6
of this exc i ton. Thus, exc i ton states below o are effectivelylocal izgd and their activation energies correspond to the localization energies from oe.
When the temperature is raised, these localized
exc i tons are thermally 16700 16800 16900 activated into the hopp ina WAVENUMBER (cm-I)
states above o
.
We cbnciude Fig. 3-
Activation energies that there exigts "an as a function of exciton energies. effective edge" of exciton Emission spectrum at 1. 8K
is alsotransfer above which excitons shown in the figure. are mobile and have a hiah
-
transfer rate into the tail state.
111.2 Transformation of R R S into luminescence in a second order optical process
C7-182 JOURNAL DE PHYSIQUE
e n e r g i e s - p f p u r e CdTe a n d ZnTe c r y s t a l s , r e s p e c t i v e l y / 5 , 6 / . When o . S
o f t h e l a s e r p u l s e . A s s u m i n g t h a t d e c a y c u r v e s c a n b e f i t t e d by a sum o f two e x p o n e n t i a l ~ l f u n c t i o n s ( t h r e e e x p o n e n t i a l f u n c t i o n s f o r wi= 1 6 6 8 1 a n d 1 6 6 2 1 cm
> ,
we h a v e d e t e r m i n e d d e c a y t i m e s o f s h o r t - l i v e d c o m p o n e n t . s a n d l o n g - 1 i v e d c o m p o n e n t . r L f r o m t h e c o n v o l u t i o n a n a l y s i s . Bhe b e s t f i t o f t h e s h o r t - l i v e d c o m p o n e n t s c o u l d b e o b t a i n e d w i t h v a l u e s o f z R l e s s t h a n 50 p s . S i n c e t h e t i m e r e s p o n s e o f t h e s h o r t - l i v e d c o m p o n e n t i s a s s h o r t a s t h a t o f t h e l a s e r p u l s e , t h i s c o m p o n e n t c a n b e a t t r i b u t e d t o RS. The l o n g - 1 i v e d componemts, h o w e v e r , e x h i b i t a n e x p o n e n t i a l d e c a y a n d t h e i r d e c a y t i m e s d e p e n d o n o . . The v a l u e s o f r L c o i n c i d e w e l l w i t h t h e l i f e t i m e s o f t h e l o c a f i z e d e x c i t o n s a t t h e e n e r g y o . , w h i c h w e r e d e t e r m i n e d u n d e r b a n d - t o - b a n d e x c i t a t i o n . T h e r e f o r e , t i e l o n g - l i v e d c o m p o n e n t o f t h e LO2 l i n e s i s a t t r i b u t e d t o t h e LO p h o n o n - a s s i s t e d l u m i n e s c e n c e o f t h e l o c a l i z e d e x c i t o n s t h a t a r e s e l e c t i v e l y e x c i t e d a s a r e a l s t a t e a t t h e e n e r g y o i . The e m i s s i o n-
f p e c t r a i n f i g . 4 c a n b e c h a r a c t e r i z e d a s f o l l o w s : f o r a . S 1 6 7 4 7 cm,
b o t h l u m i n e s c e n c e a n d RS p r o c e s s e s c o n t r i b u t e t o t h e " d a m a n - l i k e " l i n e s i n t h e s e c o n d a r y e m i s s i o n s p e c t r a . When o. i s t u n e d o n t o t h e l u m i n e s c e n c e p e a k o f l o c a l i z e d e x c i t o n s , s e c o n d o 8 d e r o p t i c a l p r o c e s s i s t r a n s f o r m e d i n t o t h e l u m i n e s c e n c e p r o c e s s k e e p i n g t h e s p e c t r a l s h a p e u n c h a n g e d . The c o n t i n u o u s t r a n s i t i o n o f RRS i n t o l u m i n e s c e n c e i n t h e m i x e d s e m i c o n d u c t i n g c r y s t a l c a n b e c o m p a r e d t o t h e e x p e r i m e n t o n i o d i n e m o l e c u l e s 7 The c o n t i n u o u s t r a n s i t i o n h a s b e e n a l s o o b s e r v e d i n I 2 b u t t h e i n t e n s i t y r a t i o o f l u m i n e s c e n c e t o RS b e c o m e s c o n s t a n t a t l a r g e v a l u e s o f o f f - r e s o n a n c e e n e r g y . T h i s r e s u l t h a s b e e n i n t e r - p r e t e d b y t h e t h e o r y /8/ i n t h e m o t i o n a l n a r r o w i n g l i m i t . I n o u r c a s e , h o w e v e r , t h e i n t e n s i t y r a t i o c o n t i n u e s t o d e c r e a s e when o f f - r e s o n a n c e e n e r g y i s i n c r e a s e d . T h e r e f o r e , t h i s t h e o r y c a n n o t a c c o u n t f o r o u r r e s u l t i n t h e o f f - r e s q n a n c e r e g i o n ; t h i s s u g g e s t s t h a t t h e non- m o t i o n a l n a r r o w i n g e f f e c t a s was p o i n t e d o u t b y A i h a r a /9/ i s s e e n i n t h e s e c o n d o r d e r o p t i c a l p r o c e s s i n t h e m i x e d s e m i c o n d u c t i n g c r y s t a l . I V . SUMMARY We h a v e m e a s u r e d t h e t i m e r e s p o n s e o f t h e s e c o n d a r y e m i s s i o n i n Cd Znl-xTe c r y s t a l s i n s u b - n a n o s e c o n d t i m e r e g i o n b y b a n d - t o - b a n d e x z i t a t i o n a n d r e s o n a n t e x c i t a t i o n . From t h e t e m p e r a t u r e d e p e n d e n c e a n d t h e e n e r g y d e p e n d e n c e o f e x c i t o n d y n a m i c s , we h a v e f o u n d t h e e f f e c t i v e e d g e o f e n e r g y t r a n s f e r i n t h e t a i l s t a t e o f e x c i t o n s . E x c i t o n s b e l o w t h e e d g e a r e e f f e c t i v e l y l o c a l i z e d , w h e r e a s a b o v e t h e e d g e t h e y a r e d e l o c a l i z e d . The t e m p o r a l b e h a v i o r o f t h e e m i s s i o n i n t e n s i t y u n d e r r e s o n a n t e x c i t a t i o n a l l o w e d u s t o f i n d . f o r t h e f i r s t t i m e , t h e c o n t i n u o u s t r a n s i t i o n f r o m RRS t o r e s o n a n t l u m i n e s c e n c e i n t h e m i x e d s e m i c o n d u c t i n g c r y a t a l a s a f u n c t i o n o f i n c i d e n t e n e r g i e s . REFERENCES / I / H e g a r t y . J . , H u b e r , D. L . , Yen, W. M., P h y s . Rev.B23
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