Trends in quantum well devices

Surface Science 168 (1986) 847-851 North-Holland, Amsterdam

T R E N D S IN Q U A N T U M J P

WELL DEVICES

NOBLANC

Centre Natumal d'Etude~ de~ Tel~communt~atton~,

196 Rue de Part~, 9222(I Bagneu~, Fran(e

The progress of epitaxlal methods hke M B E or M O C V D associated with the investigation of the physical properties of quantum well (QW) structures introduces new potentialities for devices both for optoelectronlcs and microelectronics As a matter of tact, m optoelectronlcs several key performances of devices are greatly improved by the effect of quantum wells Concerning integrated optoelectromcs, the reahzat]on of low current operating lasers compatible with the field effect or bipolar transistor drivers is a crucial point Moreover, the thermal behawour of semiconductor lasers is an important feature m considering potential appllcaUons and especially integrated optoelectronics The threshold current density ot double heterostructure lasers exhibits a minimum for a typical thickness of the laser actwe region in the range of 0 1 1~m Below this figure, the optical wave ~s no longer confined m the actwe region (fig 1 T

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848

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T h e i n t r o d u c t i o n ot q u a n t u m well layers a r e d u c h o n ot the t h r e s h o l d c u r r e n t density t r e d u c t i o n is due to the c h a r a c t e r i s t i c s of the structures with the c o n s e q u e n c e that a lowei reach the s a m e g a m m q u a n t u m well struct~ DH structures T w o types of Q W s t r u c t u r e s offer the bt S Q W G R 1 N S C H (single q u a n t u m well grad h e t e r o s t r u c t u r e ) and the m u l t i p l e q u a n t u m ' W e have r e a h z e d by M B E A 1 G a A s - G a A c u r r e n t d e n s i t y of 270 A / c m : Stripe b u r i e d 1, sed with an o p e r a t i n g c u r r e n t as ' o w as 5 m

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J index s e p a r a t e d c o n f i n e m e n t J1 s t r u c t u r e s (fig 2) S Q W G R I N S C H lasers with ,i er structures have b e e n proces(fig 3)

J P Noblanc / Trends m quantum ~4ell devtces

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On the other hand, the temperature variation ot the laser threshold current density is currently expressed by Jth = J(, exp(T/T0),

where T, is a phenomenologlcal parameter which gives a measurement of the threshold thermal variation The T0 parameter is associated to several factors non-radiative recombinations, optical and electrical leakage at the hererolonctlons So the T0 values are in the range of 150 K for G a A s - A I G a A s D H structures and much more lower, in the range of 40-60 K for I n G a A s P lasers In this case, the Auger recombination explains partially the low T(~value relatively to G a A s - A 1 G a A s lasers So the reduction of carrier density for reaching a given gain in the laser of quantum well structures, reducing the Auger effect, Is a factor favormg an increase of the T, value for quaternary laser In microelectromc devices, the effect of heterojunctlons Is extensively used m A 1 G a A s - G a A s and I n G a A s - I n P heterojunctlon bipolar transistors (HBT) and in the two-dimensional electron gas field effect transistors ( T E G F E T ) In the first case the static performances like current gain are greatly influenced by the graduahty of the emitter-base heterojunction With a gradual variation of AI composition, which can be carried out by liquid phase epltaxy, very high current gain can be obtained However, in order to get fast response the emitter base heterojunctlon must be abrupt with the consequence of lower current gain The reduction ot the current gain is increased by the spike appearing with heterojunctlons reahsed by MBE With A I G a A s - G a A s abrupt HBT, gain in the order o! 5 is currently obtained On the other hand, in order to reduce the effect of the spike, a superlattlce with very short period

850

J P N o b l a n c / Trends m quat

(10-20 ,~) has b e e n c a r r i e d o u t at the A I G a A s - G a A s H B T with a significant incre at p r e s e n t (fig 4) In the case of the T E G F E T , the c u r r e n t c m e n s l o n a l e l e c t r o n gas ( 2 - D E G ) d e n s i t y Id~ 2 - D E G c a r r i e r c o n c e n t r a t i o n in o r d e r to increase both transconductance and curren s h e e t c a r r i e r d e n s i t y in c o n v e n t i o n a l T E G a p p r o a c h to increase the s h e e t c a r r i e r dens~ tlon T E G F E T with h e t e r o j u n c t l o n on b o t h d u c t o r c h a n n e l Significant i n c r e a s e of the th~s c o n f i g u r a t i o n with typtcal value ol Gm p a r e d with 200 m S / m m for c o n v e n t i o n a l T [ T E G F E T offers a t t r a c t i v e p o t e n t l a h t l e s esp d e m o n s t r a t e d by R o c k w e l l , of m a x i m u m ctm m , m o r e t h a n twice the m a x i m u m channt M E S F E T s (fig 5)

m~ well d e v u e s

n l t t e r - b a s e i n t e r f a c e in M B E se o f t h e c u r r e n t gain, up to 15 nslty is g o v e r n e d by the two-dilly, one w o u l d hke to have high duce source resistance a n d to Jrive T h e typical value of the E T is a b o u t 1 x 1012 cm 2 A n is to use a d o u b l e h e t e r o j u n c des o f the u n d o p e d G a A s con"ansconductance ~s o b t a i n e d in t a b o u t 30() m S / m m to be c o m ~FET Moreover, multlchannel .lally for p o w e r a p p l i c a t i o n s , as nnel c u r r e n t values of 80(1 m A / c u r r e n t ot typical G a A s p o w e r

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J P Noblanc / Trends zn quantum well dewces

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So the field of apphcatton of multlheterojunctlon concepts opens new areas m the potentmlltles of III-V devices both m optoelectronlcs and mlcroelectromcs Therefore, basic studies both on the physical properties and on the metallurgical b e h a w o u r of the interfaces are of crucml ~mportance for the development of these devices