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PHOTOLUMINESCENCE STUDIES OF n-TYPE MODULATION DOPED AlGaAs-GaAs MULTIPLE QUANTUM WELLS IN TILTED MAGNETIC FIELDS
K. Lee, C. Perry, W. Zhou, J. Worlock
To cite this version:
K. Lee, C. Perry, W. Zhou, J. Worlock. PHOTOLUMINESCENCE STUDIES OF n- TYPE MODULATION DOPED AlGaAs-GaAs MULTIPLE QUANTUM WELLS IN TILTED MAGNETIC FIELDS. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-413-C5-417.
�10.1051/jphyscol:1987588�. �jpa-00226792�
JOURNAL DE PHYSIQUE
Colloque C5, supplement au noll, Tome 48, novembre 1987
PHOTOLUMINESCENCE STUDIES OF n-TYPE MODULATION DOPED AlGaAs-GaAs MULTIPLE QUANTUM WELLS IN TILTED MAGNETIC FIELDS
K.S. LEE, C.H. PERRY, W. ZHOU and J.M. WORLOCK*
Physics Department, Northeastern University, Boston, M A 02115, U.S.A.
e ell-core, Red Bank, NJ 00701, U.S.A.
A b s t r a c t
We have i n v e s t i g a t e d t h e e f f e c t o f t i l t e d magnetic f i e l d s on t h e energetics and t h e i n t e n s i t y o f t h e photoluminescence i n n-type modula- t i o n doped AlGaAs-GaAs m u l t i p l e quantum w e l l s t r u c t u r e s . Combined reso- nance e x c i t a t i o n s are observed; t h e data can be analyzed i n t o c o n t r i b u - t i o n s from f i e 1 d components para1 1 e l and perpendicular t o t h e surface normal. E f f e c t i v e masses i n t h e - t w o orthogonal d i r e c t i o n s can be de- duced. The o v e r a l l l u m i n e m n c e e f f i c i e n c y increases w i t h magnetic f i e l d f o r a l l o r i e n t a t i o n s b u t i S g r e a t e r f o r a x i a l f i e l d s than f o r r a d i a l f i e l d s .
1. I n t r o d u c t i o n
We r e p o r t photo1 uminescence s t u d i e s o f n-t3@ modul a t i o n doped A1 GaAs-GaAs mu1 t i p l e quantum we1 1 h e t e r o s t r u c t u r e s i n magnetic f i e l d s t i 1 te d away from t h e surface normal i n a x i a l magnetic f i e l d s . For
1 I1 8
t h e broad zero f i e l d l u m i - nescence centered aroarrd 12230 cm- (1.516eV) breaks i n t o a s e r i e s o f sharp fea- t u r e s a t t r i b u t e d t o t r a n s i t i o n s between conduction band (CB) and valence band (VB) Landau l e v e l s . The e n e r g e t i c s and t h e i n t e n s i t i e s o f these peaks has r e - ceived considerable a t t e n t i o n by t h i s group and (1-2) by o t h e r s ( 3 ) . The most prominant f e a t u r e s i n c l u d e a l a r g e increase i n t h e o v e r a l l photoluminescence i n t e n s i t y w i t h increasing magnetic f i e l d , i n t e n s i t y o s c i l l a t i o n s o f t h e lower Landau t r a n s i t i o n s t h a t a r e p e r i o d i c i n 1/B, t h e appearance o f a d d i t i o n a l peaks a t h i g h e r magnetic f i e l d s w i t h a concomitant t r a n s f e r o f t h e i n t e n s i t i e s ( 4 ) and t h e observation o f n o n - l i n e a r s t e p - l i k e behavior o f t h e lowest t r a n s i t i o n ( 5 ) . The mechanisms r e s p o n s i b l e f o r t h e magneto-1 umi nescence i n doped quantum w e l l s i s n o t f u l l y understood although various hypotheses have been p u t forward t o e x p l a i n t h e observed data. (2, 3, 5)2. Experimental
I n these s e r i e s o f experiments, we measured t h e magneto-photoluminescence i n t h e GaAs m u l t i p l e quantum w e l l h e t e r o s t r u c t u r e s u s i n g a unique f i b e r - o p t i c s coupler
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987588
C5-414 JOURNAL DE PHYSIQUE
t h a t allowed continuous o r i e n t a t i o n o f t h e sample i n t h e magnetic f i e l d . The sample surface was b u t t e d up t o t h e p o l i s h e d end o f a 250
u
diameter step-index q u a r t z f i b e r which acted as b o t h an i n p u t / o u t p u t coupler. The system was i n s e r t e d i n t o a l i q u i d helium immersion c r y o s t a t . R o t a t i o n o f t h e sample-coupler combination could be accom- p l i s h e d using a motorized micrometer. T i l t angles o f t h e magnetic f i e l d between O < e < 90° w i t h r e s p e c t t o t h e surface normal could be accomodated w i t h o u t removal o f t h e sample from t h e c r y o s t a t o r decreasing t h e f i e l d . The sample o r i e n t a t i o n could be r e s e t t o b e t t e r than t lo. This arrangement i n s u r e d r e l i a b l e r e l a t i v e i n t e n s i t y s t u d i e s o f t h e various t r a n s i t i o n s t o be made as a f u n c t i o n o f f i e l d and ti 1 t angle.3. Results and Discussion
A t y p i c a l exampleAof the i n t e r b a n d Landau t r a n s i t i o n s as a f u n c t i o n o f a x i a l magnetic f i e l d s ([ I1 c , e = 0) i s shown i n F i g . l ( a ) . About 1 mw o f 2.41 eV e x i t a - t i o n energy (5145A) was i n c i d e n t on t h e sample i n these s e r i e s o f experiments. Such a h i g h photon energy r e s u l t s i n a r e l a t i v e l y h i g h e f f e c t i v e e l e c t r o n temperature Te.
This temperature was estimated t o be z 35K from a n a l y s i s o f t h e h i g h energy t a i l (exp. (-E/kTe) dependence) o f t h e zero f i e l d luminescence band. I t a l s o had t h e e f f e c t o f reducing t h e amplitude o f t h e i n t e n s i t y o s c i l l a t i o n s t h a t we have r e p o r t e d p r e v i o u s l y i n t h i s and o t h e r samples (5).At these h i g h e r e l e c t r o n temperatures t h e l i n e s d e p i c t i n g t h e interband Landau e x c i t a t i o n s remain l i n e a r even f o r t h e lower (L + L ) t r a n s i t i o n s . A t h i g h e r f i e l d s , a d i s c r e t e slope change o f t h e e n t i r e f a n occurs and an a d d i t i o n a l (although r a t h e r weak) l i n e aDpezrs between t h e ( 1 + 1) and t h e ( 2 -+ 2) t r a n s i t i o n . We have observed t h i s e f f e c t i n o t h e r samples and t h e l a t t e r has been attributes t o t h e coexistence o f both e x i t o n i c and i n t e r b a n d Landau t r a n s i t i o n s t h a t become allowed a t h i g h e r f i e l d s ( 4 ) . As t h e magnetic f i e l d i s t i l t e d w i t h respect t o t h e surface normal, t h e slopes ( i e t h e r a t e a t which t h e energy of these peaks changes w i t h t h e magnetic f i e l d ) o f t h e Landau t r a n s i t i o n s (L = 1, 2, 3
...
etc.) continues t o have a 3:5:7 r a t i o b u t has a Bcose dependence.There i s a r i s e i n t h e base l i n e o f t h e f a n a B sine. The combined resonance t r a n - s i t i o n s a r e found t o approximately f o l l o w t h e r e l a t i o n :
Here L i s t h e Landau quantum number f o r t h e CBYB t r a n s i t i o n f o r e l e c t r o n s w i t h i n t h e l a y e r . e i s t h e t i l t angle o f t h e magnetic f i e l d H,with respect t o t h e sur- face normal. v$ and p: r e f e r t o t h e reduced e f f e c t i v e masses o f t h e
(CB)
elec- t r o n and t h e (VB) heavy-hole masses p a r a l l e l and perpendicular t o t h e two-dimen- s i o n a l e l e c t r o n gas l a y e r , r e s p e c t i v e l y . The s o l i d l i n e s i n Fig. I show t h e f i t s t o t h e Landau t r a n s i t i o n s . The dashed l i n e i n d i c a t e s t h e e f f e c t s of t h e sine/,,:term. The i n s e t i n F i g l ( c ) d i s p l a y s r e p r e s e n t a t i v e examples o f t h e i n t e n s i t y versus f i e l d scans f o r two d i f f e r e n t energies a t 0 and 54'. The peak h e i g h t i n t e n s i t i e s associated w i t h t h e 0 + 0 and t h e 1 + 1 t r a n s i t i o n s as a f u n c t i o n o f magnetic f i e l d f o r v a r i o u s angles between 0 and 90" are shown i n Fig. 2. The g i a n t increase i n the 0 + 0 t r a n s i t i o n i n t e n s i t y a t small t i 1 t angles (approx- i m a t e l y t e n - f o l d a t 8 Tesla f o r e = 0) dominates t h e e n t i r e spectrum. The
mechanism associated w i t h t h i s increase needs f u r t h e r i n v e s t i g a t i o n . As t h e f i e l d i s r o t a t e d away from t h e surface normal, t h e i n t e n s i t y diminishes b u t approaches a f i n i t e v a l u e w i t h t h e f i e l d p a r a l l e l t o t h e l a y e r . A t
e
= 90' i t i s a f a c t o r o f t h r e e ( a t 8T) l a r g e r than t h e i n t e n s i t y o f t h e z e r o peak f i e l d . I n t e n s i t y o s c i l l a - t i o n s a r e s t i l l v i s i b l e b u t t h e angular dependence needs f u r t h e r study. Lower e x c i - t a t i o n energies and hence lower Tels w i l l p r o v i d e more c o n c l u s i v e r e s u l t s on t h e i r behavior w i t h t i l t angle and f i e l d . The (1-1) t r a n s i t i o n i n t e n s i t y i n i t i a l l y r i s e s and then decreases w i t h i n c r e a s i n g f i e l d as e l e c t r o n s a r e depleted from t h e L = 1 s t a t e and f i l l t h e L = 0 s t a t e . The maximum (and minimum) i n t e n s i t y s h i f t s t oFig. 1 Sample 2-11-80; n = 4 x 1011 Te
-
35K. ( a ). e
= 0"; ( b )e . =
54';( c )
e
= 90' ~ n s e ? shows swept f i e l d scans a t two d ~ f f e r e n t energles f o re
= 0 and 5 8Fig. 2. (a) I n t e n s i t y o f t h e 0 + 0 t r a n s i t i o n ; ( b ) I n t e n s i t y o f t h e 1 + 1 t r a n s i t i o n as a f u n c t i o n of magnetic f i e l d f o r several angles between 0 - 904
C5-416 JOURNAL DE PHYSIQUE
h i g h e r f i e l d s w i t h t i l t - a n g l e as expected due t o t h e decrease i n slope o f t h e (1-1) t r a n s i t i o n . However, by comparing F i g . 2(a) w i t h 2(b) i t i s apparent t h a t t h e o v e r a l l luminescence i n t e n s i t y increases w i t h i n c r e a s i n g f i e l d . Even measurements made up t o 40 Tesla i n a pulsed f i e l d show t h a t t h e i n t e n s i t y i s s t i l l increasing.
I n conclusion, we have demonstrated t h a t i n n-type modulation doped AlGaAs-GaAs m u l t i p l e quantum w e l l h e t e r o s t r u c t u r e , t h e e f f e c t on t h e luminescence o f t i l t i n g an a p p l i e d magnetic f i e l d i s t o c r e a t e a combined resonance e x c i t a t i o n . The e x c i t a t i o n s c o n s i s t o f c o n t r i b u t i o n s from t h e f i e l d components p a r a l l e l and perpen- d i c u l a r t o t h e s u r f a c e normal. The former corresponds t o interband Landau t r a n s i - t i o n s w h i l e t h e l a t t e r corresponds t o e l e c t r o n i c motion perpendicular t o t h e 1 ayers.
The r e l a t i v e i n t e n s i t y of t h e t r a n s i t i o n s can be monitored as a f u n c t i o n o f t i l t angle and f i e l d . T h i s i n t e n s i t y i s several times s t r o n g e r i n an a x i a l f i e l d com- pared w i t h a r a d i a l f i e l d w i t h r e s p e c t t o t h e surface normal t o t h e two-dimensional e l e c t r o n 1 ayer.
4. Acknowledgements
The authors a r e guest s c i e n t i s t s a t t h e Francis B i t t e r N a t i o n a l Magnet Laboratory, MIT, Cambridge, MA USA. We appreciate t h e cooperation and hos- p i t a l i t y provided by L. Rubin, B. Brandt, and t h e s t a f f a t National Magnet Laboratory. We would a l s o l i k e t o thank A. C. Gossard and W. Wiegmann f o r p r o v i d i n g t h e samples and f o r t h e i r continued i n t e r e s t i n t h i s work. T h i s work was supported by t h e N a t i o n a l Science Foundation Grant #DMR-8604706.
NOTE ADDED IN PROOF
A more complete analysis of the magneto-photoluminescence spectra at diff- erent electron temperatures on a variety of modulation-doped WQW samples in tilted magnetic fields is currently underway. Such a paper is cle;!rly beyond the scope of this contribution where data on only one sample are presented;
these more detailed results will be published elsewhere.
Most relevant to the data reported here are the theoretical and experimen- tal work of AND0 et al. (6), WAAN (7) and co-workers (8), TURBERFIELD et a1 (9), and ALLEN et a1 (10).
The calculated angular dependance of the magnetic field of the eigen-freque- ncies of the Landau levels in a parabolic potential by NAAN (7) provides a simple soluble campasite oscillator model for comparing our experimental results.
5. References
(1) fl. C. Smith, A. Petrou, C. H. Parry, J. R. Worlock, and R. L. Aggarwal, 17th Int. Conf. on Physics of Semiconductars, San Fransisco, 1984 (Eds. J. 0. Chadi and W. A. Harrison, Springer Uerlag, N Y 1984) p. 547 (2) R. C. Smith, A. Petrou, C. H. Perry and 3. M. Warlack, Surface Science
174,
136 (1986)
(3) F. Weseguer, J. C. Waan, and K. Ploog, Phys. Rev.
835,
2505 (1987) (4) C. H. Perry, 3. N. Worlock, W. C. Smith and A. Petrou: Springer Series inSolid-State Sciences 7 l , 202 (1987)
(5) C. H. Perry, A. Petreu, N. C. Smith, J. W. Worlock, and R. L. Aggarwal, 3. of Luminescence
31
&32
(1984) 491(6) T. Ando, A. 8. Fowler and F. Stern, Rev. mod. Phys.,
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437 (1982) (7) 3 . C. Wman, Springer Series in Solid-State Sciences, 53, 183 (1984) (8) G. Belle, J. C. flasn, and G. Weimann, Solid State Commun., 56, 65 (1985);Surface Science,
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612 (7986)( 9 ) A . 3. T u r b e r f i e l d , J. F. Ryan and J. fl. U o r l o c k , S u r f a c e S c i e n c e ,
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( 1 0 ) S . 3 . A l l e n J r . , T . D u f f i e l d , R . B h a t , N. Koza, N. C . Tamorgo, J. F . Harbison F . DeRosn, D . N . Hwang, P. Grabbe and K. Pl. Rush, S p r i n g e r S e r i e s i n S o l i d - S t a t e S c i e n c e s , 7 l , I 8 4 (1987).
