PREPARATION AND CHARACTERIZATION OF STRAINED SUPERLATTICES STRUCTURES OF InGaAs/GaAs BY MBE

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PREPARATION AND CHARACTERIZATION OF STRAINED SUPERLATTICES STRUCTURES OF

InGaAs/GaAs BY MBE

L. Goldstein, M. Quillec, K. Rao, P. Hénoc, J. Masson, J. Marzin

To cite this version:

L. Goldstein, M. Quillec, K. Rao, P. Hénoc, J. Masson, et al.. PREPARATION AND CHARACTER- IZATION OF STRAINED SUPERLATTICES STRUCTURES OF InGaAs/GaAs BY MBE. Journal de Physique Colloques, 1982, 43 (C5), pp.C5-201-C5-207. �10.1051/jphyscol:1982524�. �jpa-00222243�

PREPARATION A N D CHARACTERIZATION O F S T R A I N E D S U P E R L A T T I C E S S T R U C T U R E S OF I ~ G ~ A S / G ~ A S BY M B E

I;. Goldstein, M. Quillec, K. Rao, P. Hbnoc, J.M. Masson and 3.Y. Marzin

Centre NationuZ dlEtudes des T 6 Z i c o m i c a t i o n s . 196, Rue de Paris, 92220 Bagnew, Frame

Resume : Des super reseaux sous-contrainte InxGal.,xAs/GaAs ont et6 epitaxie sur sub- X€FKGaAs avec des compositions voisines de x = 0,1, ce qui correspond & un dksaccord

de maille s a w contrainte de +7

-

et GaAs (200 A). La caracterisation a btk effectule au moyen de la microscopie blec- tronique, de diffraction X simple et double, de photoluminescence et absorption a

basse temperature. La periodicit6 peut Btre mesurle exactement a partir de la dif- fraction X. Le rendement de photoluminescence des couches quantiques est 1 a ^{2 ordres}

de grandeurs plus eleve que celui de couches de GaAs epaisses faites dans les mOmes conditions. La spectroscopie d'absorption indique une forte contribution excitonique

basse temperature dans les puits de InGaAs.

Abstract : Strained superlattices InxGal-xAs/GaAs have been epitaxially grown on GaAs substrates around the composition X = 0.1, which corresponds to a strain free lattice mismatch as large as +7. The films, growh by Molecular Beam Epitaxy (MBE), were made of successive alternating layers of InxGal-xAs (100 A) and GaAs (200 A). Charac- terization have been performed by transmission electron microscopy, X ray simp1 e and double diffraction, photo1 uminescence (P.L. ) and absorption at low temperature. Perio- dicity can be measured with high accuracy frow X ray diffraction profiles. P.L. effi- ciency of the quantum size InGaAs layers is 1 to 2 orders of magnitude higher than GaAs thick epilayers grown in the same conditions. Absorption spectroscopy shows a strong excitonic contribution in the InGaAs we1 1 s

.

1-Introduction

Misf it between an epitaxial film and its substrate can be accomodated by uniform elas- tic strain and by dislocations. Frank and Van der Merwe [1,2,3] have shown that a mis- fit smaller than about 7 percent will be accomodated by uniform elastic strain only until a critical thickness hc is reached. Beyond that limit, it i s energetically favo- rable to generate dislocations, and the misfit is shared between strain and disloca- tions. Experiments performed on various systems have generally resulted in a larger fraction of misfit accogodation attributed to strain than predicted by the theory.

(For instance hc = 350 A whereas 250 8 is predicted in the case GaAs,,P,, on GaAs [a]).

It is thus possible to grow epilayers very far from lattice matching to the substrate

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982524

C5-202 JOURNAL DE PHYSIQUE

provided that the layer thickness is small enough. Matthews and Blakeslee [43 tried to apply this idea to GaAsP/GaAs multilayers using VPE. They could not obtain good ma- terial unless they gradually accomodate the lattice parameter of the substrate to that of the multilayer taken as a whole. This way out was found unsatisfactory because the

g raded layer itself generated strain and dislocations.

In this paper, strained In ,Ga,,As/GaAs mutilayers grown by MBE are considered. If the layers were strain free, tke corresponding relative lattice mismatch would be 0.7 %.

The calculated critical thickness for a single layer would be, according to Franck and Van der Merwe's model, about 1000 F(. The thichness of each InGaAs layer will lie far below this expected critical thickness.

It has to be pointed out that a relaxed, dislocation free InGaAs/GaAs mu1 tilayer taken as a whole, would have a lattice parameter in the direction parallel to the surface different to that of the GaAs substrate (see section 2.2 in reference 4,C). A critical thickness can thus be defined also for a given alternating multilayer structure. Be- yond that critical thickness, dislocation should be generated at the interface with

the substrate. Serrano and Chang 151 have shown that in case of MBE growth of O

InxGaf-xAs on GaAs, the misfit dislocations are confined close to the interface (300A) if x is lar e enough (x = .15). Besides, saw-tooth like graded regions 163 or super- lattices [7q have proved efficient for dislocation confining. So misfit dislocations, if any, between the In,,Gao,As/GaAs multilayer considered as a whole and the GaAs substrate are expected to be confined at the vicinity of the first heterointerface.

Experimental results show that this indeed occurs.

11-Experiment : sample preparation and growth procedure

The MBE vacuum system consists of a ionic and titanium sublimation pumped stainless steel bell jar equipped with a load-lock set up. The flux are directed nearly verti- calllv towards the substrate.

The GaAs substrates were etched using a 2 % Bromide. Methanol solution and mechano- chemically polished on a lens paper. At the end of the etching process, the substrate surface was flushed in water in order to passivate the clean surface with oxides.

Before growth, the substrate was heated at 630°C inder an As flux with a bacKground arsenic pressure of torr to remove oxide. During growth, the gallium and arsenic flux are kept constant. The indium cell shutter is periodically opened and shut down as to obtain the desired structure. Adjustment of proper compssition and growth rate are selected by choosing appropriate cell temperatures. Composition measurements of thick InkGa ,,As layers performed by electron microprobe show good homogeneity (x=0.1) with lateral compositional gradient less than I 1. A 1 em thick.buffer layer was first deposited on the substrate followed by alterning layers of 100 A thick In,,Ga,,As

wells and 100 thick GaAs barriers. In most cases the structure was Covered by a lpm GaAs caplayer.The substrate temperature was 520°C. A few experiments performed at higher substrate temperature (580°C) resulted in poor cristal lographic quality.

Three samples have been analysed :

They were made of 10 periods of a l t e r n a t i n g layers Ino lGaO gAs (thickness LT) and GaAs (thickness EB)

0

Sample a : LT = L B = 100 A

0 0

Sample b : LB = 200 A , LT = 100 A

0 0 0

Sample c : LB = 200 A , LT = 100 A, and a 1000 A Ino lGaO 9As layer was grown on top of t h i s mu1 ti 1 ayer.

a ) Transmission electron microscopy

The samples were chemically bevel led, then thinned from the substrate t o a1 low transmission microscopic observations. Figure 1 shows an optical image of such a bevel : the InGaAs/GaAs multilayer was chemically stained.

Fig. 1 : Chemical bevel showing 10 periods of a1 ternating layers GaInAs/GaAs (optical view X50)

Sample a (Fig.2) shows a high density of dislocations located mostly a t the f i r s t i n t e r f a c e but a l s o within the multilayer. In sample b, the GaAs layers were twice as thick. I t resulted i n no dislocation i n the niultilayer (as observed using STEM) and a low density of 1/2 (110) dislocations located a t the f i r s t i n t e r f a c e (Fig.3).

The typical distance between dislocations i s a few microns.

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Fig.2 : High d e n s i t y o f d i s l o c a t i o n s Fig.3 : Very low d i s l o c a t i o n d e n s i t y observed a t t h e f i r s t i n t e r f a c e o f sample observed a t t h e f i r s t i n t e r f a c e o f

a (TEM image) sample b (TEM image)

0

I n sample c, the 1000 A InGaAs l a g e r induced. a h i g h e r d e n s i t y of d i s l o c a t i o n s as sample b (Fig.4), s t i l l c o n f i n e d a t f i r s t i n t e r f a c e . T h i s 1000 A l a y e r simulates a t h i c k e r m u l t i l a y e r as f a r as s t r a i n i s concerned. So t h e m u l t i l a y e r thtckness i n samole b i s c l o s e to its c r ' i t i c a l value, whereas it i s b u m x i t h a t v a l u e i n s a m ~ l e c.

X r a y double d i f f r a c t i o n

Double d i f f r a c t i o n was performed u s i n g t h e (400) symmetrie r e f l e c t i o n w i t h InP as a f i r s t c r y s t a l . The p e r i o d i c a l s e t o f peaks (Fig.5) is t y p i c a l of t h e d i f f e r e n t r e - f l e c t i o n orders of a s u p e r l a t t i c e . FSg.5 i s a p r o f i l e obtained on sample br S i m i l a r p r e f i l e s were obtained from samples showing a good tnorphology. Samples showing a h i g h d e n s i t y o f dislocat4ons'did n o t e x h i b i t these s a t e l l i t e peaks. The s u p e r l a t t i c e p e r i o d i c i t y can be determined w i t h h i g h accuracy from t h e X r a y p r o f i l e : 318 A i n t h i s case.

P

GaAs

-1 ,

-0,7 -0.6 -0,5 -Q4 -0.3 -Q2 -Ql 0 8()deg)

Fig.5 : Double X ray d i f f r a c t i o n profile obtained on sample b.

Photoluminescence and absorption.

Absorption measurements are useful here because the contribution of the InGaAs we1 1 s i s separated from absorption due t o the Gats substrate.

Low temperature absorption performed on sample b (Fig 6a) shows a strong exci tonic contribution . T h i s r e s u l t is important because single thick

As layers did not show such

t:#tb!'i~t?on, clue t o t h e i r poor purity. -:

!de believe t h a t 2D excitons contribute 2:

e f f i c i e n t l y t o absorption.

4

cs W6 140 144 138 152

Fig 6. Absorption and photolumines-

cence spectra obtained on sample b atuK6K ; c V) Z W

z C

1.36 1.40 1.44 1.48 1.52

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Photoluminescence a t t h e same temperature (Fig.6b) occurs a t the same energy ; e x c i - t o n i c recombinations a r e probably i n v o l v e d a l s o i n th+s emission,Photoluminescence was performed on sample b along t h e bevel (Fig.7), on t h e GaAs b u f f e r l a y e r (1) and on t o p o f t h e s u p e r l a t t i c e (2). The s t r i k i n g r e s u l t i s t h a t I n As shows a 1 uminercence & f f i c i e n c y far l a r g e r than t h e GaAs b u f f e r l a y e r ?&k::~b~ a reference Fig.7.

Fig.7 : Photoluminescence spectra obtained a t 77OK along a chemical bevel performed on sample b .

A t 77OK7 t h e r e i s a two orders o f magnitude r a t i o between t h e InGaAs emission a t hv = 1.372 eV and t h e GaAs a t 1.50 eV. This c l e a p l y shows t h a t m i n o r i t y induced a a r r i e r s d i f f u s e and recombine v e r y e f f i c i e n t l y i n the InGaAs wells; On sample c comparison was made w i t h emission From the 1000 A t h i c k InGaAs l a y e r . We d i d n o t observe a s i g n i f i c a n t wavelength s h i f t o f t h e InGaAs peak between t h a t l a y e r and t h e s u p e r l a t t i c e . B u t t h e difference i n energy gap between InGaAs and GaAs i s r e l a - t i v e l y small (0.13 eV) add a q u a n t i f i c a t i o n e f f e c t i n t h e energy w e l l s , woSld probably n o t be n o t i c a b l e i n t h a t case.

CONCLUSION

S t r a i n s u p e r l a t t i c e Tn Ga As/GaAs have been s u c c e s f u l l y grown by MBE. They show good m o r p ~ o l o g i c a ~ ~ ~ u a ? i 8 i e s (from X r a y d i f f r a c t i o n ) and very e x c i t i n g o p t i c a l p r o p e r t i e s , r e l a t i v e l y - h f g h luminescent e f f i c i e n t y and s t r o n g e x c i t o n i c a b s o r p t i o n a t 1 ow temperature.

This proves t h a t i t i s p o s s i b l e t o b b t a i n v e r y h i g h q u a l i t y m a t e r i a l f a r from l a t t i c e matching ho t h e s u b s t r a t e .

1 - F.C. Frank and J.H. Van Der Merwe Proc Roy Soc (London) A 198 (1949;2 216

2 - J.H. VanX3Der Merwe, J. Appl. Phys 34 (1963) 117

3

-

a) J. o f Cryst. Growth 32 (1976) 265 b) J. o f Cryst. Growth 27 (1974) 118 c ) J. o f Cryst. Growth 29 (1975) 273

5 - ^C.M. Serrano and Chin-An Chang, Appl . Phys. L e t t . 39 (1981) 808

6 . S. Hiyamitzu, T. F u j i i , K. Nanbu, S. Maekawa and T. Hisatsugu, Surf. Science 86 (1979> 137

7 - ^Y.G. Chai and R. Chow, J. Appl. Phys. 52 (1982) 1229