Superlattices and Microstructures, VoL 5, No. 1, 1989 5

ATOMIC LA¥~ MBE {IROWTH AND CHARACTERIZATION OF AIAs/InAs STRAIIm[~ LAYER SUPERLATTICES OH (laAs.

L. Gonzalez, A. Ruiz, A. Mazuelas, G. Armelles, M. Recto and F. Briones. Centro Nacional de Microelectronlca, CSIC.

Serrano, I~4. 28006-Madrid. SPAIN

( R e c e i v e d 8 A u g u s t 1 9 8 8 )

A recent development of Molecular Beam Epitaxy (MBE)- the Atomic Layer Molecular Beam Epitaxy- has been used to grow AlAs/InAs strained layer superlattlces (SLS) on (001)GaAs at low growth teEperatuPes (T s < 400oc). The growth process basically consists on alternating group V and/or group III beams following an optimum timlng stablished by R ~ osc111ations observation during the monolayer fol~Dation sequence. This method allows t o grow at low substrate temperatures with excellent morphology, even for those systems which have extremely different optimtlm MBE growth condltlons and a severe lattice mismatch of 7Z like AIAs/InAs.

X-ray diffraction and optical characterization results for superlattices of different periodicities are presented. In particular, Raman spectra o~ these samples showing folded acoustic phonons demonstrate their quality.

Novel properties arisen w h e n d i f f e r e n t s e m i c o n ~ l c f o r m a t e r i a l s a r e stacked to ~orm a superlattice have brouSht new possibilities to the design o~ microelectronlc and optoelectronic devices. Most of the research worm has been done w~th lattice-matched systems because hlgh quality samples can be easlly obtalned and the electronic propertles are only affected by the artl~lcxal super-periodlclty, that already means a difflcult tasK.

Nevertheless, a new field appears when a superlattlce formed with lattice- mismatched materials is designed and grown, so that one or both of them are under slgnlflcant strain. The properties and applicatlons o~ this Kind o~ stralned layer superlattices (SLS) are still unexplored in many systems, mainly in those wlth extremely different lattlce parameters.

The maln battler found for the basic research in SLS wlth highly mismatched components is to obtaln good quality material wlth sharp interfaces, so that an ~ c t u a ] l y unlform superlattJce Js achzeved,

In the case of coherent epltaxlal systems, Molecular Beam Epitaxy (MBE) is the most widely u s e d growth technlque and it is w e l l known that atomically flat inter~aces can be obtained. However, ~n the case of heterostructures involving large lattice mlsmatch, successful growth by using conventional MBE is not so evldent, specially when

l a y e r s a r e v e r y t h i n . T h i s f a c t h a s p r o m o t e d t h e a p p e a r a n c e o f d e v e l o p m e n t s o f I ~ E t e c h n i q u e w i t h t h e a i m o f e x p a n d i n g i t s r a n g e o f a p p l i c a b i l i t i e s , K e e p i n g i t s i n h e r e n t a d v a n t a g e s , s u c h a s h i g h g r o w t h r a t e a n d t h e a b o v e m e n t i o n e d a t o m i c l a y e r s c a l e c o n t r o l o f interfaces,

One of this M~E variations, the Atomic Layer Molecular Beam ~pitaxy (ALMBE) I, 2 has been used %o grow good quality (AIAS)n/(InAs) m SLS on (100) GaAs substrates. In this letter we describe the growth procedure and present results on structural and optical characterization of this strained layer superlattices. Up to our K n o w l e d g e , this is the £irst time that results on this system are presented.

Pre-growth procedure is standard for MBE growth. RHEED pattern is obtained during growth on an At bacK-coated fluorescent screen ~ollowed with a high sensltlvity CCD camera and a video recording syste~ G r o w n structumes conslst of 0.3 ~m thick (InAs)m/(AIAs)n superlatt~ces directly grovnl on (001)GaAs substrates. In order to avoid oxldatlon a 10 monolayers (ml) InAs cap layer as grown on top as a protection. Surface morphology of the samples is checked with a NomarsKy interference microscope. No rough m o r p h o l o g y or detectable defects could be f o u n d J n these samples. Structural characterization o f the layers is performed after growth by X-ray

07494036/89/010005 + 05 $02.00/0 © 1989 Academic Press Limited

6 Superlattices and Microstructures, Vol. 5, No. 1, 198,9

diffractometry using Cu][~ radiation. Average composition of the superlattices has been confilnmed by Fulergy Dispersive X-ray Spectroscopy (EDAX). Detailed R~man spectroscopy has been performed on structures with 3<m<17 ml and l<n<5 ml.

As will be explained later, growth mode consists of a periodic modulation of the impinging species on the sample by interrupting o r pulsing the group V flux, or alternating group III and V beams. Growth rates are typically 0. 5- 1.5 ml/s. Shutter operation is controlled by a desM-top computer wlth fast response time: 0. I seconds fop the magnetically operated shutter of In cell and even less for the As 4 and A1 cells, w h i c h a p e pne-m~tically operated. Special As 4 cell has been designed to provide short p~11ses (down to 0, I s. ) of group W molecules, and a large OR/OFF flux ratio of typically ~ ON/~ OFF = 150.

Rb~,* specular beam intensity oscillations are recorded d u r i n g g r o w t h w i t h a l o w noise blue sensitive photodiode facing the (00) spot on the TV screen. Intensity distributions along selected lines on the RHEED diagram are obtained b~ mechanical scanning the photodiode o v e r a single stationary image. Time evolution o f this distribution is obtained by repeating the scanning image by image on the recorded video. In this way, measuring the distance between appropriate streams, on each scan, we can ~ollow the evolution of the in-plane lattice parameter caused by lattice relaxation across interfaces through the growth process. A calibration of the camera length is provided by the GaAs substrate (00) and (I0) Pods distance.

TWO different ways o~ g r o w i n g by ALMBE can be used: either alternating group IlI and group V fluxes or supplying group III flux contlnuously while group V f l u x iS modulated periodically. In the first case, the tlme interval durin~ which A1 or In cells are opened tAl oH, tln OH As set to coincide with the period of RHEED intensity oscillations during a previous s t ~ m ~ d a r d M~E r u n . Time of arsenic cell opened, tAs OH, is usually set approximately the sl as the time d u r i n g g r o u p I I I c e l l s a r e o p e n e d , although i t is not critical. The effective growth rate i s consequently halved com@~ared with MBE. In the second case, As pulses are programmed to occur once per monolayer, so that its periodicity is also deter~ined by the growth rate. Pulses of 0.2 s are commonly used although their length is not relevant, provided the integral flux

o f g r o u p V b e a m i s e n o u g h t o c o m p l e t e o n e m o n o l a y e r .

A s s h o w n b e f o r e 3 , b o t h m e t h o d s a r e e q u i v a l e n t a n d f o r p r a c t i c a l c o n v e n i e n c e , m o s t o f t h e s a m p l e s h a v e been g r o w t h by only pulsing the arsenic beaum

The major improvement of ALM~E over conventional MBE resldes in the cycllc perturbation o f the growth front In synchronlsm w i t h the monolayer by monolayer growth sequence.

While in conventional MBE 2D growth (in competition with several other mechanisms such as steps propagation and ~D islands foz~mation) is only achieved over an optimized range of temperatures, V/III flux ratios and low denzity of surface steps, ALMBE process actually enhances the 2D growth simultaneously over the whole surface in a broad range of growth conditions.

Every time that one of the beams xs shuttered ON or OFF, the whole sul-face will be suddenly and si~mlltaneously separated from equ111brium (T S and flux dependent). The consequent temporal separation f r o m stoichiometry enhances the surface chemical reactivity simultaneously over the whole surface promotlng the formation of 2D nuclel. The detailed mechanlsm by which, under cyclic repetition 0 9 this process (even when the perlod Is not exactly a multiple o f the lnverse o f the growth rate) is too complex to be descrlbed in this context and will be published elsewhere.

As it has been mentloned, thls broadens considerably the range of growth conditions under which good epltaxlal layers can be grown. In particular, uniform 2D nucleation will reduce the need ~or long range mass exchange or surface mlgration and wll] allow to grow at lower substrate temperatures. This mares possible to g r o w short period heterostructures of alternating materials which have extremely different optimum MBE growth conditions.

We think that thls mechanism 0 9 e n h a n c e d u n i f o r m n u c l e a t i o n , ~ o r c i n g 2D growth, is also the reason for excellent growth morphology and confinement of lattlce relaxation to a narrow interfaclal reglon in the case o~ large mismatched structures.

In flgu/-e I we show the evolution of the lattice relaxation process between the substrate and the epltaxlal structure meas1~ed on the v~deo recording as described above. We observe that the in-plane lattice parameter increases during the ~irst InAs layer growth, reaching an intermedlate value

Superlattices and Microstructures, Vol. 5, No. 1, 1989 7

THICKNESS (monolayers) O 3 11 14 22

10 , , , , , I/

< g ~ 6

' 1 -

4

L.u 2 _o I - - -

._l 0 3C 0

F i g . I:

IA,As I InAs IA~AslInAs I O.3~m

2 on~o- o--o -o-o--o-//- oo o -

/ o /

o / 9/00 I I ~ ~ / / 10 2O 3O 40 50

TIME (seconds)

L a t t i c e m i ~ t c ~ to ~ a~ a fUlnCtion

of thidkr~ ~o~ ~ gro~ (Ir~s)7/(AIAs) 4 sope-]attice on (0Ol)(]~As sobst~a%e.

- Ga As (002 }

5 ~j - L-1

I L

I A L,2 L+3 L ]~ ~ l t ~1 ! L+Z,

x

22 2~ 26 28 30 32 34 36 38 ~0

2~Y(DEGREES)

F i g . 2: X-ray i n t e s ~ i t y d i s t r i ] m / t l ~ a ~ t l ~ (00a)GeAs s ~ a t e r e f l e c t i o n o f a O. 3 ~m t-hick (Iz~%s)IT/(AIAs)5 sopePlattice ~o~s on (oo1) GeAS stmbstrate.

D

L >-

LH

Z L]-I

>- < rY

x

i

22 2L 26 28 30

Ga As {002 )

Ltl

32 34 36 38 26'(DEGR EES)

F i g . 3: X - r a ~ i n t e n s i t y d i s t r i ] J t K i o n ~ t h e (002)C~As s ~ t r a % e r e f l e c t i o n o f O.3~m t_h ick (lzl~s)BCAIAs) I sm~pePlattice grown o~ C0(O)(3e~s

subsl~ate.

I ,A -1 m

g

O2

10 30 50 70

F R E Q U E N C Y SHIFT ( c m -1)

9O

Fig.4: lZ~n spectra of the (Ir~S)lTJCAiAs)5 saWple. The doal-act~l st i c LA doublet is clearly o]oserved ( kex ~ 488 rim).

between GaAs (or AIAs) and InAs. l)urlng

consecutive layers no lattice parameter

variatlon occurs. Values correspondin~

to the |ast perlod o~ the layer ape also shown zn the flgure.

As for ALMBE growth of InAs on GaAs 4

a fast lattlce relaxation is obse~wed in

the case o f (InAs)m/(AIAs) n supeFlattices. The constant walue o~ a H from the second period indicates that both AIAs and InAs in the superlatilce are under strain due to the dlfference between thelr b u l R ]attlce parameters

a n d t h e a v e r a g e s u p e r l a t t l c e l a t t i c e c o n s t a n t a c h i e v e d a f t e r t h e f i r s t p e r i o d ,

q~is means that pseudomorphic

superlattlces can be grown even for individual layer thicMnesses above the critical values calculated for the co~respondin~ lattice mismatch.

Figures 2 and B show X-ray diffrac%o~rams around GaAs(OO2)

reflections f o r (InAs)m/(AIAs)n with (m,n)= (17, 5) and (3, I) respectlve]y. The appearance of the satellite peaMs Is

8 Superlattices and Microstructures, Vol. 5, No. 1, 1989

LO

v >- l-

Z uJ

7

200 24.0 280 320 360 4,00

F R E Q U E N C Y SHIFT (cm -1)

Fig. 5: Baman spectra of the (~)Io~CAIAS)~ s a ~ l e . D i ~ IX) ~ a r e s e e ~ i n t h e I r ~ r e g i a n w ~ i c ~ c ~ ' r - e s p ( r ~ s t o ~ c~inex~ IX) ~ . T h e e ~ c i t a t i ~ n e n e r g y ( q ~ n m ) l i e ~ close to tl~ Irla_s El-tramsition.

a proof of the crystalline quality oT these structures. The total perlod o~ the superlattice ]s directly derived from the distance between the~ The average composltion o f the superlattice obtained from the Oth order pear has been confirmed by EDAX, being in good agreement with the as grown expected values.

We also have used Raman spectroscopy as a more powerful technique for characterizing SLSs.

T w o Kinds o f information are available from the Ramn spectra: first, the period o~ the superlattlce can be obtained from the energy position of the LA phonons pears (folded acoustic phonons) and second, the strain of the two constituent m~terials can be deduced from the energy shl{t of their LO phonons peaKs (provided that the layers of the two materials are wide enough to neglect the confinement effect on those phonons).

In {igures 4 and 5 we p~esent two typical Raman speclra o { ~hese samples.

The spectra were obtained at room temperature in the backscatterln~

configuration. The scattered light was analyzed with a computer-controlled 0.85m double monoch~omator equipped with holographic gratings and detected with standard photon-countlng techniques. In order to avoid scattering from the air, the samples where located inside a vacuum chamber several lines o{ an Ar + laser were used as incident beam.

The spectra o f the long period samples present the characteristic doublet of the folded LA phonons. We were not able to resolve such a doublet in the short period samples. We suggest that the poorer crystalline quality (interface roughness) of the short period samples is responsible ~or such different behavior. Similar results were obtained from the X-ray spectra.

In table I we present the calculated position o{ the ~irst doublet LA phonons o f these samples using the Rytov model 5 The values of the parameters used in our calculation are the unmodl~ted bulk ones6:

VInAs : 383608 cm/s, VAIAs : 565403 cm/s Pln~s = 5. 66 gr/cm ~, " ~AIAs : 3. 75

gr/cm ~. 'fhe agreement between these values

and the experimental ones Is quite good considering that we have not taken into account the possible modification of those parameters due to strain.

The strain o{ the InAs layers~ as

deduced from the energy shi{t of the LO I

phonon 7, is sample dependent, and correspond to an in-plane lattice parameter around b. 89 ~ which agrees well with the results o { RHEED measurements.

To summarize, we have obtained a novel Mind of strained layer superl attice (InAs) m/(AlAs) n on (OOi) GaAs thanks to a recently developed growth process, the so called ALMBE. Raman spectroscopy characterization

together with X-ray di{{raction demonstrate the qual£ty o{ these structures.

Table I: ~ E D t a i am c a ~ a t ~ p ~ s i t i ~ of ~ f i r s t doublet LA tl6iI]~l ~ Rytov Iii13del. ~ I ~ a g l e t ~ ~ i n ot~ ca lcu la t i c l l

are give~1 in the te~,.

(Izi~S) m (AIAs)n Fxpe~" tmexltal ( cm -I ) Calcu/atecl( em -I )

w_ I w~l w_% %4÷i

R 1 119.3 113.5 118.5 q i 95.4 ~.5 93.5 8 B 45 ~0 q5

10 4 31.7 36.9 3! B6 17 5 19.5 ~4.6 I 8 .5 R~

Superlattices and Microstructures, Vol. 5, No. 1, 1989 9

Acl4nowledgement- The authors want to thank M. A b a l l e for EDAX analysis at CE]IIM-CSIC.

REFEREHCES:

i. F. Briones, D. Golmayo, L. Gonzalez, M. Recio, A. Rulz and d. P. Silveira, Proc. of the "l~%h Int. Symp. on Galllum Arsenide and Related Compounds" Heraklion, Crete. September, 1987.

2. F. Briones, A. Rulz and L. Gonzalez, Proc. o f the European MRS Symp. o n "Photon, Beam and Plasma Assisted

P r o c e s s i n g . F u n d a m e n t a l s a n d D e v i c e T e c h n o l o g y " . S t r a s b o u r g , F r a n c e . May, 1 9 8 8 .

3 . F . B r i o n e s , L. G o n z a l e z , M. R e c t o a n d M. Vaz~lez) Jpn. J. Appl. Phys. 26 (1987) L1225.

4. A. Rulz, L. Gonzalez, A. Mazue]as and F. Brionez, submitted to App!. Phys. Left.

5. S. M. Rytov, Soviet Phys]cs-Acoustics 2 (1956) 67.

6. Landolt-BoFnsteln Vol 17a Physlcs o~ Group IV elements and III-V Compounds Edltor O. Modelun~. SprlnEer-Verlag 1982.

7. F. Cerdeira, C. V. Buchenaver, F. H. Poll ak and M. Cardona, Phys. Rev. B 5 (1972), 580.