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TEMPERATURE DEPENDENCE OF EXCITON LIFETIMES IN GaAs/AlGaAs MULTI QUANTUM
WELLS
G. Peter, J. Feldmann, E. Göbel, K. Moore, P. Dawson, C. Foxon, R. Elliott
To cite this version:
G. Peter, J. Feldmann, E. Göbel, K. Moore, P. Dawson, et al.. TEMPERATURE DEPENDENCE OF EXCITON LIFETIMES IN GaAs/AlGaAs MULTI QUANTUM WELLS. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-517-C5-520. �10.1051/jphyscol:19875110�. �jpa-00226693�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au noll, Tome 48, novembre 1987
TEMPERATURE DEPENDENCE OF EXCITON LIFETIMES IN GaAs/AlGaAs MULTI QUANTUM WELLS
G. PETER, J. FELDMANN, E.O. GOBEL, K. MOORE*, P. DAWSON*, C. FOXON* and R. J. ELLIOTT* *
Philipps-Universitat, Fachbereich Physik. Renthof 5, 0-3550 Marburg, F.R.G.
"philips Research Lab., GB-Redhill, RH1 5HA, Surrey, Great-Britain
"'~epartrnent of Theor. Physics, Oxford University, GB-Oxford, OX1 3NP, Great-Britain
Abstract -The photoluminescence decay times in GaAsIAlGaAs multi quantum well structures with layer thickness between 2.5nm and 15nm are investigated i n the temperature range of 5K t o 100K. A pronounced increase of the decay times with increasing temperature is found for all samples. T h i s increase of the decay times i s attributed t o a re- duction of the free exciton transition strength d u e to an i n c r e a s e of the homogeneous linewidth by acoustic phonon scattering.
Excitons in quasi two dimensional (2D) semiconductor structures like GaAsIAlGaAs multi quantum wells (MQW) exhibit strongly modified op- tical properties a s compared to the bulk (3D) case d u e to confinement effects [l]. Most evident i s the increase of binding energy corre- sponding t o a decrease of the exciton Bohr radius. T h e r a d i a t i v e re- combination i n QW at low temperature i s governed by excitonic recom- bination, h o w e v e r , the excitonic luminescence i n thin Q W ( 5 10nm) generally i s red shifted with respect t o the absorption spectrum.
T h i s red shift of the photoluminescence (PL) has been attributed to localization of excitons within island like well width fluctuations and/or binding at interface defects [ 2 ] . Localization due to well width fluctuations l e a d s to inhomogeneous broadening of the exciton a s long a s the diameter of the islands are smaller than the diameter of the excitons [3]. I n s t e a d , separated exciton lines c a n be resolved-, if the island s i z e i s much larger [ 4 ] . It has been argued that this localization severely affects the radiative recombination probability of e x c i t o n s , similar t o the c a s e of 3D bound excitons.
I n this paper we report results o n the temperature dependence of the P L decay t i m e s , which demonstrate that the recombination strength of excitons i n MQW i s governed by t h e properties of the f r e e quasi 2D exciton.
T h e experimental results f o r the thickness dependence of the P L decay times for different s a m p l e s are summarized i n Fig.1. T h e P L measurements are performed by exciting the MQW s a m p l e s with pico- second pulses of a synchroneously pumped d y e laser ( h w < E g , ~ l ~ a ~ s ) '
The s a m p l e s are mounted i n a variable temperature cryostat. T h e spectrally integrated P L signal i s detected with a synchroscan streak camera with a time resolution of 20ps. We find an increase of the P L decay times T from about 0.311s for Lz=2.5nm to about 1.711s for
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875110
JOURNAL DE PHYSIQUE
Fig.1
Photolminescence decay t i m e s versus quantum well thickness LZ at T = 5 K 1.5-
m
1.0 P
0.5 0
Lz=15nm. An i n c r e a s e of the P L decay times i s i n f a c t predicted for
Ga AslAl Ga As MQW T=5K -
- 0 .
I I I
_
free as well a s bound (localized) excitons, d u e to the dependence of the 2 D Bohr r a d i u s on LZ [5]. The observed variation of T with LZ
0 5 10 15
L,
(nm)
actually i s e v e n slightly larger than a factor of 4 , which would be expected for a transition from a hydrogen l i k e 3D exciton with Bohr radius a, to an ideal 2D exciton with Bohr r a d i u s ao/2. A more realistic discription of the exciton i n Q W , h o w e v e r , has t o take in account the real valence band structure and its complicated dependence o n LZ.
A decision on the importace of localization effects o n the recombina- tion lifetimes cannot be d r a w n from the low temperature d a t a alone.
Further insight into the recombination m e c h a n i s m , however, can be ob- tained by investigating the temperature d e p e n d e n c e of the decay ki- netics. T h e decay times are expected to be independent of temperature for excitons bound at impurities or localized within well width fluc- tuations and o n l y the i n t e n s i t y should decrease according t o thermal ionization of bound or localized excitons. I n c o n t r a s t , the free exciton radiative decay time should depend o n temperature a s illus- trated briefly i n the following:
T h e oscillator s t r e n g t h per unit c e l l of a f r e e quasi 2D exciton i s given by
where Mcv i s the dip01 matrixelement, n i s the volume of the unit cell and a(LZ) i s the " i n plane" exciton Bohr radius. If the K=O exciton s t a t e i s considered o n l y -which would correspond t o a 6-like absorption or emission line- all N unit cells of the entire QW con- tribute t o the optical transition. T h i s l e a d s to the transition strength
where A i s the QW area given by N.n/LZ and f, contains the remaining
f a c t o r s of Eq.(l). I n a realistic case, however, the exciton line- s h a p e i s not 6-like but i s intrinsically determined by the homogene- o u s linewidth A(T), due basically to acoustic phonon scattering in the temperature r a n g e below 8 0 K [ 6 ] . An effective oscillator strength for the e x c i t o n may then be obtained by sharing the K=O oscillator strength Eq.(l) equally amongst all states within the spectral width A(T). F u r t h e r m o r e w e finally have to consider that only a f r a c t i o n r(T) of e x c i t o n s within d(T) a s determined by their thermal distribu- tion can contribute t o recombination. Using Boltzmann s t a t i s t i c s f o r the distribution f u n c t i o n of excitons and taking into account the energy independent 2D density of states we obtain a temperature dependent effective transition strength
- 4 . n
for free e x c i t o n s , where EB= i s the binding energy of the p - a (LZ)
e x c i t o n , l/p=l/me+l/mh i s the reduced m a s s and M=m,+mh the c e n t e r of gravity mass. The variation of the e x c i t o n linewidth d(T) with temperature f o r Q W with different L Z h a s b e e n recently measured by Schultheis et al. 161 and a linear temperature dependence according t o d(T)=p-T with p=lOpeV/K has been obtained f o r the homogeneous con- tribution. Taking t h i s i n t o a c c o u n t Eq.(3) predicts a linear depend- ence of T on T since T i s inversely proportional t o F
X'
Fig.2
Temperature dependence of the photoluminescence decay times f o r multi quantum w e l l s a m p l e s wdth different LZ
C5-520 JOURNAL DE PHYSIQUE
T h e results f o r the temperature dependence of T for MQW s a m p l e s with different LZ are summarized in Fig.2. The decay times in fact i n - c r e a s e almost linearly with temperature i n a l l samples i n accordance with the predictions of Eq.(3). W e therefore c o n c l u d e that the radia- t i v e recombination in MQW at l o w temperatures i s governed by the properties of the free quasi-2D exciton.
I n summary w e h a v e studied the dependence of carrier lifetimes o n temperature f o r GaAs/AlGaAs MQW with different LZ. We find a pronounced i n c r e a s e of the P L d e c a y times for all s a m p l e s which i s attributed t o the increase of the homogeneous linewidth of the exciton. We c o n c l u d e that the excitonic decay i n MQW a t l o w tempera- t u r e s ( T < 1 0 0 K ) i s governed by the properties of t h e f r e e 2D exciton.
Acknowledgment -We would like to thank G.Noll and R.Fischer for help- f u l discussion and M - P r e i s for expert technical assistance. T h i s work h a s been partly supported by the Deutsche Forschungsgemeinschaft within the Sonderforschungsbereich SFB 185.
R e f e r e n c e s
[l] D.S.Chemla, Helv. Phys. Acta 56 (1983) 607.
121 G - B a s t a r d , C - D e l a l a n d e , H - M - M e y a n d i e r , P.M.Frijlink, M.Voos, Phys. Rev. B 29 (1984) 7042.
[3] C - W e i s b u c h , R - D i n g l e , A.C.Gossard,W.Wiegmann Solid S t a t e Commun. 4 9 (1982) 930.
[4] B.Deveaud, J.Y.Emery, A - C h o m e t t e , B - L a m b e r t , M - B a u d e t , Superlattices & Microstructures 1 (1985) 205.
[5] G - B a s t a r d , E.E.Mendez, L.L.Chang, L - E s a k i , Phys. Rev. B 2 6 (1982) 1974.
[61 L - S c h u l t h e i s , A.Honold, J.Kuh1, K - K o h l e r , C.W.Tu Phys. Rev. B 3 4 , 9 0 2 7 (1986) 9027.
