ELECTRIC FIELD EFFECTS ON EXCITONS IN Alx Ga1-x As QUANTUM WELLS AND THEIR APPLICATIONS TO OPTOELECTRONIC DEVICES

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ELECTRIC FIELD EFFECTS ON EXCITONS IN Alx Ga1-x As QUANTUM WELLS AND THEIR

APPLICATIONS TO OPTOELECTRONIC DEVICES

S. Tarucha, K. Ploog

To cite this version:

S. Tarucha, K. Ploog. ELECTRIC FIELD EFFECTS ON EXCITONS IN Alx Ga1-x As QUANTUM

WELLS AND THEIR APPLICATIONS TO OPTOELECTRONIC DEVICES. Journal de Physique

Colloques, 1987, 48 (C5), pp.C5-85-C5-92. �10.1051/jphyscol:1987514�. �jpa-00226718�

JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au n 0 l l , Tome 4 8 , novembre 1987

ELECTRIC FIELD EFFECTS ON EXCITONS IN AlxGa,-,As QUANTUM WELLS AND THEIR APPLICATIONS TO OPTOELECTRONIC DEVICES

S. TARUCHA'~' and K. PLOOG

Max-Planck-Institut fur Festkorperforschung, 0-7000 Stuttgart 80, F.R.G.

AaSTRACT

T h i s paper d e s c r i b e s o u r r e c e n t work on e l e c t r i c f i e l d e f f e c t s on e x c i - t o n s i n AlGaAs/GaAs quantum w e l l s . We d i s c u s s t h e l a r g e change o f o p t i c a l ab- s o r p t i o n a t t h e l a s i n g wavelength o f a m u l t i p l e - q u a n t u m - w e l l waveguide induced by t h e S t a r k e f f e c t , t h e a b s o r p t i o n s a t u r a t i o n o f e x c i t o n s c o n t r o l l e d by t h e S t a r k e f f e c t , and t h e quenching o f e x c i t o n i c luminescence induced by resonance e f f e c t s o f e l e c t r o n s . I n a d d i t i o n , t h e a p p l i c a t i o n s o f these f e a t u r e s t o f a b r i - c a t e a n o p t i c a l waveguide m o d u l a t o r m o n o l i t h i c a l l y i n t e g r a t e d w i t h a m u l t i p l e - quantum-well l a s e r d i o d e and a v o l t a g e - c o n t r o l l e d b i s t a b l e l a s e r a r e r e p c r t e d .

I

-

INTRODUCTION

E l e c t r i c f i e l d e f f e c t s on e x c i t o n s i n semiconductor quantum we1 l ^S (QWs) have r e c e n t - l y a t t r a c t e d much a t t e n t i o n due t o p o s s i b l e a p p l i c a t i o n s i n n o v e l p h o t o n i c d e v i c e s . I n t h i s paper we summarize o u r r e c e n t work i n t h i s f i e l d . An e l e c t r i c f i e l d a p p l i e d p e r p e n d i c u l a r t o QW l a y e r s p r o v i d e s two main e f f e c t s on e x c i t o n s , i.e. a S t a r k s h i f t o f t h e e x c i t o n energy / 1 / and a quenching o f t h e e x c i t o n i c photoluminescence (PL) i n t e n s i t y / 2 /

.

The S t a r k s h i f t o f e x c i t o n i c a b s o r p t i o n peaks i s observed a t a f i e l d as h i g h as 100 kV/cm because t h e h e t e r o j u n c t i o n p o t e n t i a l b a r r i e r s p r e v e n t t h e f i e l d i o n i z a t i o n o f e x c i t o n s . U s i n g t h i s e f f e c t , an o p t i c a l m o d u l a t o r / 3 / and an o p t i c a l b i s t a b l e d e v i c e c a l l e d s e l f - e l e c t r o o p t i c e f f e c t d e v i c e (SEED) / 4 / were r e a l i z e d . These d e v i c e s a r e o p e r a t e d w i t h l i g h t t r a n s m i t t e d p e r p e n d i c u l a r t o QW l a y - e r s . We have s t u d i e d t h e e l e c t r i c f i e l d e f f e c t s on t h e t r a n s m i s s i o n o f l i g h t t h r o u g h mu1 t i ple-quantum-well (MQW) waveguides, which a r e s u i t a b l e f o r monol i t h i c i n t e g r a t i c r w i t h MQW l a s e r diodes / 5, 6 /

.

We have f a b r i c a t e d a MQW waveguide m o d u l a t o r mono- l i t h i c a l l y i n t e g r a t e d w i t h a MQW l a s e r d i o d e / 7 / as w e l l as a v o l t a g e - c o n t r o l l e d b i s t a b l e l a s e r / 8 /

.

The quenching o f e x c i t o n i c luminescence i s induced by two main f a c t o r s , i . e . t h e s p a t i a l s e p a r a t i o n o f e l e c t r o n s and h o l e s i n t h e w e l l s / 9 / and t h e c a r r i e r t u n n e l i n g t h r o u g h t h e b a r r i e r s / 9

,

10 /

.

B o t h f a c t o r s have r e c e n t - l y been i n v e s t i g a t e d by t i m e - r e s o l v e d PL measurements / 9 - 1 1 /

.

We have s t u d i e d he f i e l d - i n d u c e d quenching o f e x c i t o n i c PL i n AlGaAs/GaAs s u p e r l a t t i c e s u s i n g time-

("on leave from NTT Electrical Communications Laboratorres. Tokyo 180. Japan

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987514

J O U R N A L D E PHYSIQUE

5

-

4 ^- -

3

-

2

-

1 ^-

r o o m temperature 0 ^-

1

800 820 840 860 880

WAVELENGTH

(

nm

)

F i g . 1 - PC spectrum o f a p - i - n MQW d i o d e s t r u c t u r e (8-nm GaAs and 5-nm AlGaAs, 16 p e r i o d s ) f o r d i f f e r e n t b i a s v o l t a g e s . XL i s t h e wavelength f o r l a s e r o s c i l l a t i o n .

r e s o l v e d p h o t o c u r r e n t (PC) as w e l l as s t a t i c PL and PC measurements. The t o p i c s p r e - sented h e r e i n c l u d e t h e S t a r k e f f e c t on e x c i t o n s i n AlGaAs/GaAs MQW waveguides a t t h e l a s i n g wavelength o f monol i t h i c a l l y i n t e g r a t e d MQW l a s e r s , t h e a b s o r p t i o n s a t u - r a t i o n o f e x c i t o n s under an e l e c t r i c f i e l d , and t h e f i e l d induced quenching o f e x c i - t o n i c PL i n S e c t i o n 11, and t h e a p p l i c a t i o n s o f these e f f e c t s t o n o v e l o p t i c a l d e v i - ces i n S e c t i o n 111.

I 1 - ELECTRIC FIELD EFFECTS ON EXCITONS I N QWS I 1

-

1 S t a r k E f f e c t i n MQW Waveguides

E x c i t o n s i n QWs e x h i b i t l a r g e a b s o r p t i o n peaks even a t room temperatures due t o t h e c o n f i n e m e n t e f f e c t / 12 /

.

The S t a r k e f f e c t has a l s o a s t r o n g impact on room-tempe- r a t u r e e x c i t o n s / 2 / so t h a t i t becomes a t t r a c t i v e f o r a p p l i c a t i o n s t o o p t o e l e c t r o - n i c d e v i c e s . We have s t u d i e d t h e o p t i c a l p r o p e r t i e s o f AlGaAs/GaAs MQW waveguides, which have s i m i l a r l a y e r s t r u c t u r e s as MQW l a s e r diodes. We have f o u n d t h a t t h e l a s e r o s c i l l a t i o n of t h e MQW waveguide o c c u r s a t t h e low-energy s i d e o f t h e e x c i t o - n i c a b s o r p t i o n peak o b t a i n e d f o r t h e same MQW waveguide under u n e x c i t e d c o n d i t i o n s / 5 / because o f t h e band gap s h r i n k a g e e f f e c t / ¹³ /

,

and t h a t t h e a b s o r p t i o n co- e f f i c i e n t a t t h e l a s i n g wavelength i s much s m a l l e r f o r t h e MQW waveguide than f o r t h e c o n v e n t i o n a l GaAs DH waveguide / 5 /

.

The f i r s t p r o p e r t y suggests t h a t a l a r g e e x c i t o n i c a b s o r p t i o n can be induced a t t h e l a s i n g wavelength by a p p l y i n g a v o l t a g e t o t h e MQW r e g i o n . The second f e a t u r e suggests t h a t MQW waveguides a r e f a v o r a b l e f o r i n t e g r a t i n g p a s s i v e o p t i c a l components r n o n o l i t h i c a l l y w i t h l a s e r s and o t h e r p a s s i v e o p t i c a l components because o f t h e l o w t r a n s m i s s i o n l o s s . These p r o p e r t i e s a r e con- f i r m e d by PC spectroscopy measurements. I n F i g . 1 we show t h e PC s p e c t r a f o r a p - i - n MQW l a s e r d i o d e s t r u c t u r e measured w i t h d i f f e r e n t b i a s v o l t a g e s (Vb). The PC spec- trum i s a r e p l i c a o f t h e a b s o r p t i o n spectrum f o r t h e i n t r i n s i c MQW r e g i o n . The peaks c l e a r l y observed on t h e spectrum f o r V c l V a r e due t o t h e a b s o r p t i o n peaks of t h e heavy h o l e ( h h ) e x c i t o n and l i g h t Ro7e ( l h ) e x c i t o n . These peaks s h i f t t o l o w e r e n e r g i e s w i t h d e c r e a s i n g Vb a c c o r d i n g t o t h e S t a r k s h i f t s . The l a s i n g wavelength of t h i s d i o d e i s i n d i c a t e d by

x L ,

which i s a b o u t 20 meV below t h e peak o f t h e hh e x c i - t o n . The d a t a o f t h e f i g u r e show t h a t a l a r g e change o f a b s o r p t i o n can b e induced a t XL b y s w i t c h i n g t h e a p p l i e d v o l t a g e , e.g., f r o m + l t o -1 V. The o p t i c a l waveguide m o d u l a t o r d e s c r i b e d i n I 1 1

-

1 u t i l i z e s t h i s change o f t h e e x c i t o n i c a b s o r p t i o n .

v

1 0 -1

BIAS VOLTAGE ( V

)

F i g . 2

-

P h o t o r e s p o n s i v i t y of a p - i - n MQW d i o d e s t r u c t u r e (8-nm GaAs and 5-nm AlGaAs, 16 p e r i o d s ) f o r d i f f e r e n t e x c i t a t i o n d e n s i t i e s . The i n s e t shows a schematic c o n f i g u r a t i o n o f t h e e x c i t i n g wavelength and t h e PC spectrum under n e a r l y f l a t band c o n d i t i o n .

I 1

-

2 A b s o r p t i o n S a t u r a t i o n o f E x c i t o n s i n E l e c t r i c F i e l d s

The a b s o r p t i o n s a t u r a t i o n o f e x c i t o n s i s an i m p o r t a n t p r o p e r t y f o r a p p l i c a t i o n s t o n o n l i n e a r o p t i c a l devices. Large a b s o r p t i o n s a t u r a t i o n o f e x c i t o n s i n GaAs QWs was r e p o r t e d b y M i l l e r e t a l . and Chemla e t a l . / 14 /

.

A four-wave m i x i n g t e c h n i q u e r e v e a l e d t h a t t h e a b s o r p t i o n s a t u r a t i o n o c c u r s most s t r o n g l y a t t h e low-energy s i d e o f t h e hh e x c i t o n peak / 14 /

.

The mechanism o f t h e a b s o r p t i o n s a t u r a t i o n i s a t t r i - b u t e d t o a s c r e e n i n g e f f e c t o f f r e e c a r r i e r s a t room temperature and t o t h e phase- space f i l l i n g e f f e c t i n a d d i t i o n t o t h e s c r e e n i n g e f f e c t a t l o w temperatures / 14 /.

I n t h e MQW waveguide under i n v e s t i g a t i o n A i s l o c a t e d a t t h e low-energy s i d e o f t h e hh e x c i t o n peak, where a l a r g e a b s o r p t i o n k a t u r a t i o n i s expected. I n a d d i t i o n , t h e magnitude o f t h e e x c i t o n i c a b s o r p t i o n ( i . e . t h e magnitude o f t h e a b s o r p t i o n s a t u r a -

t i o n ) a t A can be changed u s i n g t h e S t a r k e f f e c t . F i g u r e 2 shows t h e p h o t o r e s p o n s i - v i t y o f t h k p - i - n MQW d i o d e s t r u c t u r e as a f u n c t i o n o f a p p l i e d v o l t a g e (V ) o b t a i n e d w i t h d i f f e r e n t e x c i t a t i o n d e n s i t i e s a t 50 K. The p h o t o r e s p o n s i v i t y i s g i v k n by t h e PC d i v i d e d by t h e e x c i t a t i o n d e n s i t y . The e x c i t i n g l a s e r wavelength i s f i x e d j u s t below the a b s o r p t i o n peak o f t h e hh e x c i t o n under n e a r l y f l a t band c o n d i t i o n s , as s c h e m a t i c a l l y shown i n t h e i n s e t . The l a r g e r V v a l u e s ( l e f t hand s i d e ) correspond t o t h e low-energy s i d e o f t h e a b s o r p t i o n spectlum s i n c e t h e a b s o r p t i o n spectrum s h i f t s t o t h e low-energy s i d e w i t h d e c r e a s i n g V

.

The two peaks o f t h e s p e c t r a a r e due t o t h e hh e x c i t o n ( a t l a r g e r V ) and l h excbton ( a t s m a l l e r V ). No peak i s ob- s e r v e d on t h e photorespons i v i t y spkctrum ~i t h w h i t e l i g 5 t e x c i t a t b o n . When t h e e x c i

-

t a t i o n d e n s i t y i s i n c r e a s e d f r o m % 1 W/cm t o ^% 1 kW/cm

,

t h e hh e x c i t o n a s s o c i a t e d peak becomes s i g n i f i c a n t l y s m a l l e r i n i n t e n s i t y due t o t h e a b s o r p t i o n s a t u r a t i o n . On t h e o t h e r hand, t h e i n t e n s i t y o f t h e l h e x c i t o n a s s o c i a t e d peak i s h a r d l y reduced.

T h i s phenomenon i s a t t r i b u t e d p a r t l y t o t h e s m a l l e r a b s o r p t i o n s a t u r a t i o n o f l h ex- c i t o n s and p a r t l y t o t h e r e d u c t i o n o f t h e c a r r i e r l i f e t i m e w i t h enhanced e l e c t r i c f i e l d . The s h i f t s o f t h e peaks a r e caused by t h e f i e l d s c r e e n i n g due t o t h e p h o t o - gene5ated c a r r i e r s . A n a r r o w i n g o f t h e hh e x c i t o n a s s o c i a t e d peak observed a t

-

¹⁰⁰

W/cm i s p r o b a b l y due t o a k i n d o f feedback mechanism a s s o c i a t e d w i t h t h e f i e l d screening. I n a MQW waveguide w i t h 13-nm w e l l s , t h e a b s o r p t i o n s a t u r a t i o n accompani- ed by t h e s m a l l e r s c r e e n i n g e f f e c t i s achieved w i t h a l o w e r e x c i t a t i o n d e n s i t y . The peak observed i n t h e p h o t o r e s p o n s i v i t y c a n n o t be compared d i r e c t l y t o t h e peaks of t h e e x c i t o n i c s p e c t r a because a f i e l d induced change o f t h e e x c i t o n i c a b s o r p t i o n spectrum and an enhancement o f t h e magnitude o f t h e PC s h o u l d be taken i n t o account.

Most r e c e n t l y Iwamura e t a l . have o b t a i n e d a s i m i l a r s a t u r a t i o n b e h a v i o u r by o p t i c a l a b s o r p t i o n measurements / 15 /

.

The s a t u r a t i o n b e h a v i o u r observed h e r e i m p l i e s

JOURNAL

DE

PHYSIQUE

EXTERNAL FIELD

(

kV1cm)

Fig. 3

-

S t a t i c PC and s p e c t r a l l y i n t e g r a t e d PL i n t e n s i t y versus e x t e r n a l f i e l d measured f o r a p - i - n diode w i t h a AlGaAs/GaAs super- l a t t i c e i n t r i n s i c r e g i o n (12-nm GaAs and 5.8-nm AlGaAs, 100 p e r i o d s ) . E x c i t a t i o n d e n s i t y 5 ~/cm2.

t h a t the magnitude o f the a b s o r p t i o n s a t u r a t i o n o f e x c i t o n s a t an e x c i t i n g l a s e r wa- velength below t h e hh e x c i ton peak can be changed by t h e a p p l i e d v01 tage. T h i s fea- t u r e i s u t i l i z e d i n t h e v o l t a g e - c o n t r o l l e d b i s t a b l e l a s e r described i n Section I 1 1

-

^2.

I 1

-

3 Resonance Induced Quenching o f E x c i t o n i c Luminescence

Photoexcited c a r r i e r s i n QWs s u b j e c t t o an e l e c t c r i c f i e l d generate e i t h e r PL o r PC / 11, 16 /

.

A t low temperatures the PL a t moderate e x c i t a t i o n l e v e l s i s mostly gene- r a t e d by e x c i t o n i c recombination, and the PC by c a r r i e r s t u n n e l i n g through the b a r - r i e r s . The time-resolved PL measured f o r i s o l a t e d GaAs QWs a t 5 K e x h i b i t e d an en- hancement of the l i f e t i m e of e x c i t o n s a t low f i e l d s , which i s due t o t h e s p a t i a l charge separation, and a r e d u c t i o n a t h i g h f i e l d s , which i s due t o the t u n n e l i n g from the i n s i d e o f the w e l l t o the o u t s i d e o f t h e QW / 9, 10 /

.

I n s u p e r l a t t i c e s t u n n e l i n g occurs mostly between t h e adjacent r e s p e c t i v e w e l l s . I n AlInAs/GaInAs and AlGaAs/GaAs s u p e r l a t t i c e s , an excess PC due t o sequential resonant t u n n e l i n g was observed i n s t a t i c PC-bias v o l t a g e c h a r a c t e r i s t i c s / 17, 18

/ .

We have s t u d i e d the f i e l d induced quenching o f t h e e x c i t o n i c luminescence i n AlGaAsfGaAs s u p e r l a t t l c e s i n c o r p o r a t e d i n p - i - n diode s t r u c t u r e s using time-resolved PC as w e l l as s t a t i c PL and PC measurements. F i g u r e 3 shows the f i e l d dependences o f the s t a t i c PL i n t e n s i t y and o f t h e PC measured under cw e x c i t a t i o n c o n d i t i o n . The observed PL i s associated w i t h hh e x c i t o n s / 11 /

.

The e x t e r n a l f i e l d , given by ( b u i l t - i n - v o l t a g e

+

a p p l i e d v o l t a g e ) d i v i d e d by t o t a l thickness o f t h e i n t r i n s i c region, i s shown a t the lower h o r i z o n t a l a x i s . A d d i t i o n a l l y , t h e c a l i b r a t e d f i e l d as evaluated from t h e Stark s h i f t o f t h e PL emission peak energy i s shown a t t h e upper h o r i z o n t a l a x i s / 1 1

/ .

The discrepancy between c a l i b r a t e d and e x t e r n a l f i e l d s i s due t o $he f i e l d screening by photogenerated c a r r i e r s under t h e cw and f a i r l y h i g h (a 5 W/cm ) e x c i t a t i o n con- d i t i o n s employed here. The PL i n t e n s i t y i s s t r o n g l y quenched a t the f i e l d s i n d i c a t e d by C and D. I n good agreement w i t h these PL quenchings an excess PC appears a t t h e p o i n t s l a b e l e d by c and d, which i s caused by t h e f i e l d induced d i s s o c i a t i o n of excitons. These PL quenchings a r e a t t r i b u t e d t o resonance e f f e c t s o f e l e c t r o n s b e t - ween t h e f i r s t e x c i t e d s t a t e and the ground s t a t e (quenching C) and between the se- cond e x c i t e d s t a t e and the ground s t a t e (quenching D) o f adjacent w e l l s . The c o r r e s - ponding c a l i b r a t e d f i e l d s a r e we1 l p r e d i c t e d from the c a l c u l a t i o n o f t h e e l e c t r o n subband l e v e l s . The existence o f resonance e f f e c t s i s confirmed by the time-resolved PC measurements. When t h e e x c i t a t i o n power d e n s i t y i s reduced, t h e peaks of t h e PC s h i f t t o lower f i e l d s because o f t h e s m a l l e r f i e l d screening. The e x t e r n a l f i e l d s 2 o f t h e peaks c and d measured w i t h a reduced e x c i t a t i o n d e n s i t y down t o 0.05 W/cm

Fig. 4

-

Peak value (PC . . - t i r

re!

f i e l d measured w i t h t h e 200 ps p u l s e d u r a t i o n a peak power d e n s i t y . The

2 - 2 2 * L . . .l

1 ana aecay _{785n1-n putse} excitation

ne

constant ^{( T - )} o f tffaxtime-

;olved PC as a ' f u n c t i o n o f e x t e r n a l W 20 K

e x c i t a t i o n 2 0 f

a

^0.01

.,

nd ^arrows

-

^{1 W/cm}

^{! k} 1 ^"

irlaicarea m e peaKs o r PC and t h e ^{I I I I I I I I I I}

d i p s o f -ri. max 20 40 60 80 100 120 l40

EXTERNAL FIELD ( kV/cm)

a r e 44 and 88 kV/cm, r e s p e c t i v e l y . These values a r e very c l o s e t o the c a l i b r a t e d f i e l d s o f t h e peaks c and d i n the f i g u r e . Another small PL quenching i n d i c a t e d by A i s p o s s i b l y due t o resonance e f f e c t s o f holes according t o t h e c a l c u l a t i o n o f the h o l e subband l e v e l s . T h i s peak i s n o t discussed f u r t h e r because t h e resonance e f f e c t i s n o t confirmed by time-resolved PC due t o t h e t o o small PC i n t e n s i t y . An excess PC i n d i c a t e d by b i s p o s s i b l y d i f f e r e n t from the i n t r i n s i c e f f e c t because i t accompa- n i e s no PL quenching.

I n F i g . 4 we show the decay time constant T~ and t h e peak value PC, o f t h e PC mea- sured a f t e r s h o r t o p t i c a l p u l s e e x c i t a t i o n . The e x c i t i n g pulse has

$6

e r r o r - f u n c t i o n - l i k e time-resolved p r o f i l e w i t h 200 ps d u r a t i o n . The value o f T . = 60 ps i s t h e lower l i m i t imposed by the time-resolved f a l l p r o f i l e o f t h e

exciting

pulse. r

.

ex- h i b i t s a s t r o n g r e d u c t i o n a t the e x t e r n a l f i e l d s o f 46 and 85 kV/cm, where t h &

PC e x h i b i t s an excess i n t e n s i t y . These two r e d u c t i o n s o f T . c o n f i r m the r e d u c t i o n ofm%e t u n n e l i n g time induced by resonance e f f e c t s o f e l e c t r o a s between t h e f i r s t two e x c i t e d s t a t e s and the ground s t a t e as described before. The e x t e r n a l f i e l d s ob- served f o r the r e d u c t i o n o f ^T~ a r e lower than those o f the peaks c and d, and they a r e close t o the c a l i b r a t e d f i e l d s (see F i g . 3) because the f i e l d screening e f f e c t

i s s m a l l e r under $he e x c i t a t i o n c o n d i t i o n s o f a s h o r t p u l s e and a smaller peak power d e n s i t y (* 1 W/cm ).

The s t r o n g PL quenching shown i n F i g . 3 and t h e s t r o n g r e d u c t i o n o f 7 . shown i n Fig.

4 a r e c o n s i s t e n t l y understood by comparing the e x c i t o n i c recombinatioa time ^T and the t u n n e l i n g time T

.

^{7 .} i s 1.5 ns a t low f i e l d s , which was independently deegrmin- ed by time-resolved

PL

m&asurements / 11 /

.

The s t r o n g r e d u c t i o n o f 7 . means t h a t t h e ^{T .} values measured under resonance c o n d i t i o n s a r e t o t a l l y d e t e r m i d d by t h e c a r r i & r tunnel ing. These T~ ( Q T ~ ) values a r e s u f f i c i e n t l y small t o quenc,h t h e ex- c i t o n i c luminescence as compared w i t h ,,,T which i s expected t o be enhanced by a fac- t o r o f two o r more a t t h i s f i e l d ( i . e . T,

;

3 n s ) due t o the s p a t i a l charge sepa- r a t i o n e f f e c t / 9 /

.

I n c o n t r a s t , a t the rower f i e l d s , where t h e s t a t i c PL i s a l - most i n v a r i a n t , ^{T .} (% l ns) i s comparable t o ^T ( -1.5 ns) so t h a t i t i s dominated by t h e e x c i t o n i c ?ecombination. Our r e s u l t exp?$ins t h e f i e l d induced c o m p e t i t i o n

JOURNAL DE PHYSIQUE

a . . A , - * - - Laser

n-contact v

F i g . 5

-

M o n o l i t h i c i n t e g r a t i o n o f a waveguide m o d u l a t o r and a l a s e r diode.

o f e x c i t c n i c r e c o m b i n a t i o n and c a r r i e r t u n n e l i n g i n s u p e r l a t t i c e s c o n s i s t e n t l y w i t h r e s p e c t t o t h e i r dynamical and s t a t i c a s p e c t s .

111.- APPLICATIONS TO OPTICAL DEVICES

111 - 1 Waveguide M o d u l a t o r M o n o l i t h i c a l l y I n t e g r a t e d w i t h MQW Laser

As d e s c r i b e d i n S e c t i o n I 1

-

1, MQW waveguides a r e a t t r a c t i v e f o r o p t i c a l m o d u l a t o r s t o b e m o n o l i t h i c a l l y i n t e g r a t e d w i t h MQW l a s e r diodes because o f t h e l o w l o s s as w e l l as t h e l a r g e change o f t h e e x c i t o n i c a b s o r p t i o n a t t h e l a s i n g wavelength. F i g - u r e 5 s c h e m a t i c a l l y shows t h e m o n o l i t h i c d e v i c e f a b r i c a t e d i n t h e p r e s e n t work. The m o d u l a t o r and t h e l a s e r d i o d e a r e s e p a r a t e d by a narrow gap f a b r i c a t e d by r e a c t i v e

i o n beam e t c h i n g . The gap i s deep enough t o s e p a r a t e t h e m o d u l a t o r and t h e l a s e r d i - ode b o t h o p t i c a l l y and e l e c t r i c a l l y . From t h e measured l i g h t o u t p u t v e r s u s i n j e c t i o n c u r r e n t c u r v e ( L - I c u r v e ) t $ e a b s o r p t i o n l o s s c o e f f i c i e n t o f t h e m o d u l a t o r waveguide i s e s t i m a t e d t o be 2. 60 cm a t t h e l a s i n g wavelength. T h i s v a l u e c o n f i r m s t h e l o w l o s s o f t h e waveguide as d e s c r i b e d i n S e c t i o n I 1 - 1 . O p t i c a l m o d u l a t i o n o f t h i s de- v i c e i s achieved by i n j e c t i n g a c o n s t a n t c u r r e n t i n t o t h e l a s e r d i o d e and a p p l y i n g a m o d u l a t i o n v o l t a g e t o t h e m o d u l a t o r . The a c h i e v e d m o d u l a t i o n depth i s 7 dB f o r a d r i v i n g v o l t a g e o f 2.3 V, and t h e c u t - o f f f r e q u e n c y i s 0.88 GHz. The c u t - o f f f r e - quency i s l i m i t e d by t h e RC t i m e c o n s t a n t , w h i c h i s improved by r e d u c i n g t h e d e v i c e c a p a c i t a n c e . We e x p e c t a c u t - o f f frequency beyond 10 GHz by r e d u c i n g t h e waveguide w i d t h as w e l l as i n s u l a t i n g t h e l a y e r s under t h e w i r i n g pads, e.g., by p r o t o n implan- t a t i o n .

111

-

2 V o l t a g e - C o n t r o l l e d B i s t a b l e Laser

O p t i c a l b i s t a b i l i t y i s one o f t h e most e s s e n t i a l f u n c t i o n s f o r s i g n a l p r o c e s s i n g i n an o p t i c a l computer. A b i s t a b l e l a s e r i s a t t r a c t i v e as an a c t i v e element f o r t h e o p t i c a l b i s t a b i l i t y . T h i s d e v i c e which has a r e g i o n o f a b s o r p t i o n s a t u r a t i o n i n t h e c a v i t y was o r i g i n a l l y proposed by Lasher / 19 /

,

and i t s o p e r a t i o n was l a t e r demon- s t r a t e d i n b u l k InGaAsP / 20 / and GaAs / 21 / DH l a s e r diodes. These b i s t a b l e l a - s e r s p r o b a b l y u t i l i z e d a b s o r p t i o n s a t u r a t i o n a s s o c i a t e d w i t h t h e b a n d - f i l l i n g e f f e c t . Our b i s t a b l e l a s e r u t i l i z e s t h e a b s o r p t i o n s a t u r a t i o n o f e x c i t o n s i n QWs, whose mag- n i t u d e can be changed by t h e S t a r k e f f e c t . The d e v i c e has t h e same waveguide s t r u c - t u r e as shown i n F i g . 5. The d i f f e r e n c e i s t h a t t h e waveguide i s separated i n t o two segments o n l y e l e c t r i c a l l y b u t n o t o p t i c a l l y . The e l e c t r i c a l s e p a r a t i o n o n l y i s r e a - l i z e d by Ga focused i o n beam i m p l a n t a t i o n i n s t e a d o f r e a c t i v e i o n beam e t c h i n g . We show t h e b i s t a b l e o p e r a t i o n by i n j e c t i n g c u r r e n t i n t o one segment A ( I A ) w h i l e a p p l y - i n g a v o l t a g e t o t h e o t h e r segment B (VB

:

b u i l t - i n - v o l t a g e ) . F i g u r e 6 shows t h e c o n t i n u o u s l y o p e r a t i n g L - i A c u r v e s w i t h d i f f e r e n t VB v a l u e s a t 77 K. The h y s t e r e s i s

F i g . 6

-

Continuously o p e r a t i n g L - I Fig. 7

-

R e l a t i v e w i d t h o f t h e h y s t e r e s i s curves o f a b i s t a b l e MQW l a s e r measfired loop shown i n Fig.6 as a f u n c t i o n o f bfas

a t 77 K. v01 tage.

Inject~on Cunent. I. I m A )

Fig. 8

-

Room-temperature

cw

o p e r a t i o n of a b i s t a b l e MQW l a s e r .

l o o p i s observed i n t h e VB range from 0 t o 1.2 V. The w i d t h . o f the loop changes w i t h the a p p l i e d v o l t a g e V as shown i n F i g . 7. Room-temperature b i s t a b l e o p e r a t i o n of t h i s device i s a1 so aphieved under pulsed e x c i t a t i o n c o n d i t i o n s . The l a s i n g wave- l e n g t h i n t h e b i s t a b l e o p e r a t i o n i s a t t h e low-energy side o f the e x c i t o n absorption peak. More r e c e n t l y , we have obtained cw b i s t a b l e o p e r a t i o n a l s o a t room temperature, u s i n g a more s o p h i s t i c a t e d s t r u c t u r e ( F i g . 8 ) / 22 /

.

I n s p e c t i o n o f Fig. 7 r e v e a l s t h a t the b i s t a b i l i t y o f our device i s c o n t r o l l e d by an a p p l i e d v o l t a g e V smaller than the b u i l t - i n - v o l t a g e so t h a t no c u r r e n t i s i n j e c t e d t o the segment

B.

T h i s con- d i t i o n i s f a v o r a b l e f o r low power consumption t o c o n t r o l the b i s t a b l e operation.

C5-92 JOURNAL

DE

PHYSIQUE

I V

-

CONCLUSION

We have p r e s e n t e d s e v e r a l i n t e r e s t i n g p r o p e r t i e s o f e x c i t o n s i n AlGaAs/GaAs QWs sub- j e c t t o an e l e c t r i c f i e l d f r o m t h e view p o i n t o f b a s i c p h y s i c s as w e l l as d e v i c e a p p l i c a t i o n . A l a r g e change of o p t i c a l a b s o r p t i o n i s induced a t t h e l a s i n g wave- l e n g t h o f a MQW waveguide b y t h e S t a r k e f f e c t . The magnitude o f t h e a b s o r p t i o n s a t u - r a t i o n o f e x c i t o n s i s changed b y t h e S t a r k e f f e c t . A s t r o n g quenching o f e x c i t o n i c luminescence i s induced by resonance e f f e c t s o f e l e c t r o n s . Using these p r o p e r t i e s we have f a b r i c a t e d novel o p t i c a l devices, i n c l u d i n g a waveguide m o d u l a t o r m o n o l i t h i c a l - l y i n t e g r a t e d w i t h a MQW l a s e r d i o d e and a v o l t a g e - c o n t r o l l e d b i s t a b l e l a s e r . ACKNOWLEDGEMENT

The a u t h o r s a r e g r a t e f u l t o Dr. H. Iwamura, Or. Y. Hirayama, and Mr. T. Saku f o r t h e i r c o l l a b o r a t i o n s i n growing t h e samples and f a b r i c a t i n g t h e devices. They a l s o thank P r o f . K. 'von K l i t z i n g f o r v a l u a b l e d i s c u s s i o n on resonance e f f e c t s i n super- l a t t i c e s and H. Schneider f o r c r i t i c a l r e a d i n g o f t h e manuscript. P a r t o f t h i s work was sponsored b y t h e S t i f t u n g Volkswagenwerk and b y t h e Bundesministerium f u r Forschung und Technologie.

REFERENCES

/ 1 / Mendez, E.E., Bastard, G., Chang, L.L., Esaki, L., Morkoc, M., and Fischer,R., Phys. Rev. 826, (1982) 7101.

/ 2 / M i l l e r , D . A T , Chemla, D.S., Damen, T.C., Gossard, A.C., Wiegmann, W., Wood, T.H., and Burrus, C.A., Phys. Rev. L e t t .

53,

(1984) 2173.

/ 3 / Wood, T.H., Burrus, C.A., M i l l e r , D.A.B., Chemla, O.S., Damen, T.C., Gossard, A.C., and Wiegmann, W . , Appl. Phys. L e t t . 44, (1984) 16.

/ 4 / M i l l e r , D.A.B., Chemla, D.S., Oamen T.C., G s s a r d , A.C., Wiegmann, W., Wood, T.H., Burrus, C.A., Appl. Phys. L e t t . 45, (1984) 13.

/ 5 / Tarucha, S., H o r i k o s h i , Y., and O k a m o t c H., Jpn. J. Appl. Phys. 22, (1983) L482. Iwamura, H., Tarucha, S., Saku, T., H o r i k o s h i , Y., and Okamzo, H., Jpn. J. Appl. Phys. 22, (1983) L751.

/ 6 / Tarucha, S., IwamuraTH., Saku, T., and Okamoto, H., Jpn. J . Appl. Phys. - 24, (1985) L442.

/ 7 / Tarucha, S., and Okamoto, H., Appl. Phys. L e t t . 48, (1986) 1.

/ 8 / Tarucha, S., and Okamoto, H., Appl. Phys. L e t t .

m,

(1986) 543.

/ 9 / Polland, H.-J., S c h u l t h e i s , L., and Kuhl, J., P h E . Rev. L e t t .

55,

(1985) 2610.

/ 10 / Kash, A., Mendez, E.E., and Morkoc, H., Appl., Phys. L e t t . 46 (1985) 173.

/ 11 / Masumoto, Y., Tarucha, S., and Okamoto, H., Phys. Rev. 8 3 3 , 7 1 9 8 5 ) 5961.

/ 12 / I s h i b a s h i , T., Tarucha, S., and Okamoto, H., I n t . Symp. GaAs and R e l a t e d Compounds, Oiso, 1981, I n s t . Phys. Conf. Ser. 63, (1982) 587.

/ 13 / Tarucha, S., Kobayashi, H., H o r i k o s h i , Y., andakamoto, H., Jpn. J. Appl.

Phys. 23, (1984) 874.

/ 14 / Miller,-D.A.B., Chemla, D.S., Smith, P.W., Gossard, A.C., and Tsang, W.T., Phys. Rev. B28 (1982) 96. Chemla, D.S., M i l l e r , D.A.B., Smith, P.W., Gossard, A.C., and WEmann, W., IEEE J. Quantum E l e c t r o n .

QW-20,

(1984) 265. S c h m i t t -

Rink, S., Chemla, D.S., and M i l l e r , O.A.B., Phys. Rev. B32 (1985) 6601.

/ 15 / Iwamura, H., and Okamoto, H., S p r i n g M e e t i n g Jpn. Soc.

El.

Phys. (1987).

/ 16 / H o r i k o s h i , Y., F i s c h e r , A., and Ploog, K., Phys. Rev.

831,

(1985) 7859.

/ 17 / Capasso, F., Mohammed, K., and Cho, A.Y., Appl

.

Phys. L e t t . 48, (1986) 478.

/ 18 / F u r u t a , T., Hirakawa, K., Yoshino, I., and Sakaki

,

H., Jpn. Appl

.

Phys.

25,

(1986) L151.

/ 19 / Lasher, G.L., S o l i d - S t a t e E l e c t r o n . 7, (1964) 707.

/ 20 / Kawaguchi, H. and Iwane, G., E l e c t r o n . L e t t . 17 (1981) 167.

/ 21 / Harder, Ch., Lau, K.Y., and Y a r i v , A., Appl. m y s . L e t t .

40

(1982) 124.

/ 22 / Okamoto, H., Jpn. J. Appl. Phys.

6

(1987) 315.