INTERFACE SPINODAL DECOMPOSITION IN LPE InxGa1-xAsyP1-y LATTICE MATCHED TO InP

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INTERFACE SPINODAL DECOMPOSITION IN LPE InxGa1-xAsyP1-y LATTICE MATCHED TO InP

F. Glas, M. Treacy, M. Quillec, H. Launois

To cite this version:

F. Glas, M. Treacy, M. Quillec, H. Launois. INTERFACE SPINODAL DECOMPOSITION IN LPE

InxGa1-xAsyP1-y LATTICE MATCHED TO InP. Journal de Physique Colloques, 1982, 43 (C5),

pp.C5-11-C5-16. �10.1051/jphyscol:1982502�. �jpa-00222221�

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MATCHED TO I n P

F. Glas, M.M.J. Treacy, M. Quillec and H. Launois

Centre NationaZ dlEtudes des Te'ZBcomunieations, 196 rue de Pa2.is, 92220 Bagneux, fiance

Resume

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Nous montrons en u t i l i s a n t un MEBT en modes image e t microanalyse X qu'un champde deplacement quasiperiodique e t une modulation de composition associee e x i s -

t e n t dans des couches de InGaAsP e p i t a x i e e s s u r s u b s t r a t InP o r i e n t 6 (001). Nous v&- r i f i o n s que c e t t e modulation n ' e s t pas un a r t e f a c t dO A des e f f e t s de c a n a l i s a t i o n . Le domaine de temperature e t de composition pour l e q u e l l a s t r u c t u r e quasiperiodique e s t observee e s t en bon accord avec l a p r e d i c t i o n theorique d'une zone d l i n s t a b i l i t &

spinodale dans l a phase s o l i d e .

Summary

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We show by STEM imaging and X-ray microanalysis t h a t a q u a s i p e r i o d i c s t r a i n m n d an associated composition modulation e x i s t i n fnGaAsP l a y e r s LPE-grown on

(001) InP. This modulation i s checked n o t t o be an a r t e f a c t due t o c h a n n e l l i n g e f - f e c t s . The composition and temperature range f o r which t h e q u a s i p e r i o d i c s t r u c t u r e i s observed i s i n good agreement w i t h t h e t h e o r e t i c a l p r e d i c t i o n o f a spinodal i n s - t a b i l i t y zone i n t h e s o l i d phase.

I

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I n t r o d u c t i o n

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The use o f InxGal-xA~yPl-y 111-V quaternary a l l o y s f o r t h e f a b r i - c a t i o n o f o p t o - e l e c t r o n i c devices f o r f i b r e - o p t i c systems has grown r a p i d l y over t h e l a s t few years. The a t t r a c t i o n o f these a l l o y s i s t h a t by choosing t h e composition x and y , t h e c r y s t a l l a t t i c e parameter and energy band gap may be s e l e c t e d independan- t l y . T h i s f l e x i b i l i t y i s i m p o r t a n t because (1) i t permits e p i t a x i a l growth o f rnGaAsP l a y e r s l a t t i c e matched t o s i n g l e c r y s t a l s u b s t r a t e s t o p r o v i d e t h e d e f e c t - f r e e i n t e r f a c e s necessary f o r h i g h q u a l i t y h e t e r o s t r u c t u r e devices and (2) i t a l l o w s a range o f devices t o be made o p e r a t i n g i n t h e wavelength range 1.1 t o 1.7 urn, where l o s s and d i s p e r s i o n i n SiO2 based o p t i c a l f i b r e s a r e a minimum.

However, t h e r e i s evidence t h a t a s o l i d phase i n s t a b i l i t y occurs over a l a r g e p a r t o f the quaternary composition range a t the usual growth temperature (1,2,3). This i s i n apparent c o n t r a d i c t i o n t o t h e f a c t t h a t good-quality devices can be r e a d i l y f a - b r i c a t e d over the f u l l composition range. I t has been argued (1) t h a t t h e composition o f l a t t i c e - m a t c h e d quaternary l a y e r s grown w i t h i n t h e i m m i s c i b i l i t y zone a r e s t a b i - l i z e d by s t r a i n induced by the s u b s t r a t e . Q u i l l e c e t a l . ( 3 ) have shown t h a t f o r l a y e r s grown from t h e l i q u i d phase on i n t e n t i o n a l l y l a t t i c e mismatched (001) Gap substrates, phase s e p a r a t i o n does indeed occur f o r compositions grown w i t h i n the i m m i s c i b i l i ty?'For p e r f e c t l a t t i c e matching (& % a few 10-4), t h e e p i l a y e r s a r e homogeneous a t the micron scale. Henoc e t a t . (4) have shown by scanning transmission A e l e c t r o n microscopy (STEP!) t h a t quaternary l a y e r s grown by LPE, l a t t i c e matched t o

(001) InP w i t h i n t h e i m m i s c i b i l i t y zone d i s p l a y a q u a s i p e r i o d i c s t r a i n f i e l d w i t h an associated composition modulation. A s e r i e s o f d i f f e r e n t quaternary compositions LPE.grown a t 650°C were examined i n r e f e r e n c e ( 4 ) . I t was shown t h a t a l l compositions l y i n g w i t h i n t h e p r e d i c t e d spinodal isothermcurve (see f i g . 1 ) d i s p l a y e d " t w e e d M - l i ke c o n t r a s t ( f i l l e d squares i n f i g . 1 ) and those l y i n g o u t s i d e t h e curve (open squares) d i d n o t e x h i b i t "tweed". A l l specimens were l a t t i c e matched t o InP and lay on t h e i s o - l a t t i c e parameter l i n e a. = 0.587 m.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982502

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F i g . 1 : Composition representa- t i o n o f t h e InxGal-xA~yP1-y a1 l o y system showing the spinodal i s o - thermal curves c a l cu1 a t e d by De Cremoux (1) and t h e isoparameter (---) and isobandgap (.

.

^.) l i n e s .

I n t h i s paper we extend t h e preceeding STEM r e s u l t s on composition modulations : ( i ) complements a r e g i v e n on t h e STEM a n a l y s i s ; ( i i ) f o r g i v e n compositions ( e m i t t i n g wavelength 1.3 pm an 1.65 urn) we show t h a t t h e composition modulations a r e observed o n l y when the growth temperature l i e s i n t h e i m m i s c i b i l i t y domain.

I 1

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Experimental r e s u l t s

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A s e r i e s o f lattice-matched inGaAsP/InP specimenswere grown by LPE u s i n g e i t h e r e q u i l i b r i u m c o o l i n g o r step cooling. For examination by STEM, t h e specimens were thinned from t h e s u b s t r a t e s i d e t o a thickness o f about 30 pin, f i r s t by mechanical and then by chemical p o l i s h i n g , u s i n g Br2/CH30H s o l u t i o n . F i n a l t h i n n i n g was completed by e t c h i n g a h o l e i n the specimen using e i t h e r Br2/CH30H o r ion-beam t h i n n i n g . Specimens were glued onto n i c k e l s l o t g r i d s . T h i n areas near t h e edge o f t h e h o l e were examined i n a Vacuum Generators L t d . HB501 STEM equipped w i t h an energy d i s p e r s i v e X-ray analyzer.

11.1

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STEM images and m i c r o a n a l y s i s

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Fig.2 shows a STEM (220) dark f i e l d image o f an I n ~ a l s P l a y e r (1.3 urn l a s i n g wavelength) grown a t 650°C w i t h i n t h e expected im- m i s c i b i l i t y zone. The image shows t h e c h a r a c t e r i s t i c "tweed-like" c o n t r a s t propaga- t i n g i n t h e [100] and [ O l O ] d i r e c t i o n s w i t h a p e r i o d i c i t y o f about 200 nm. T h i s con- t r a s t i s q u i t e d i f f e r e n t from t h e much narrower c o n t r a s t due t o a m i s f i t d i s l o c a t i o n network such as i s observed i n the [I101 d i r e c t i o n s i n s i m i l a r compounds ( b u t very r a r e l y i n o u r own samples).

F i g . 2 : (220) STEM dark f i e l d image showing t h e quasi - p e r i o d i c c o n t r a s t

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caused by l o c a l changes i n

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g i t s e r f i s t o o weak t o adequately e x p l a i n t h e observed c o n t r a s t .

A f i n e ( 2 . 15 nm) g r a n u l a r s t r u c t u r e i s a l s o v i s i b l e i n f i g . 3 . T h i s g r a n u l a r s t r u c - t u r e i s n o t e a s i l y c o r r e l a t e d between d i f f e r e n t d a r k f i e l d images, and assumes a s t r e a k y n a t u r e i n t h e g = (040) image. Granular c o n t r a s t i s v i s i b l e i n a l l specimens whether o r n o t they arT w i t h i n t h e i m m i s c i b i l i t y zone, and i s v i s i b l e e q u a l l y i n specimens which have been chemically o r ion-beam thinned. ThB o r i g i n o f t h i s c o n t r a s t i s n o t c l e a r , b u t i t may be r e l a t e d t o s h o r t range ordering.

The area shown i n f i g . 3 was microanalyzed i n t h e t h r e e d i f f r a c t i o n c o n d i t i o n s u s i n g t h e energy d i s p e r s i v e X-ray spectrom8ter. The nominal probe used-during m i c r o a n a l y s i s was about 1 nm, b u t because o f probe spreading w i t h i n the sample, about 100 nm

t h i c k , t h i s increases t o about 20 nm (5) a t t h e sample e x i t face. The r e s u l t s f o r the P, Ga and I n Ka emission energies (2.01, 9.22' and 24.00 keV r e s p e c t i v e l y ) a r e p l o t - t e d i n f i g . 4 r e l a t i v e t o t h e As peak a t 10.5lkeV.The Ga/As r a t i o changes by

+

^{4 %}

over the zone analyzed whereas t h e In/As r a t i o remains constant w i t h i n s t a t i s t i c a l e r r o r . The P/As r a t i o f o l l o w s the Ga/As r a t i o b u t t h e s t a t i s t i c s i s n o t as good.

The weakly e n e r g e t i c PK X-rays a r e s t r o n g l y absorbed i n these specimens and i f PK i s compared w i t h AsL peak (1.39 keV) which i s s t i l l more absorbed, we a l s o f i n d t h a t the P/As r a t i o i s s i m i l a r i n amplitude t o Ga/As. These r e s u l t s do n o t change s i g n i f i - c a n t l y on changing t h e specimen d i f f r a c t i o n c o n d i t i o n s , so we can d i s c o u n t t h e hypo- t h e s i s t h a t these c o n c e n t r a t i o n f l u c t u a t i o n s a r i s e through anomalous chan e l l i n g e f f e c t s caused by t h e q u a s i p e r i o d i c Bragg r e f l e c t i o n e x c i t a t i o n e r r o r g

a~ .

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^.iE

11.2

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Phase diagram e x p l o r a t i o n

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Many samples grown a t 650°C, and corresponding t o 1.3 urn e ~ e x a m i n e d I n a l l cases, t h e c ~ m p o s i t i o n . modulations a r e observed and correspond t o a constant valae o f x/y, i n agreement w i t h t h e observed t i e - l i n e d i r e c t i o n o f demixion (3). The composition range we f i n d f a l l s s h o r t o f t h a t observed by Q u i l l e c , and i s sample dependant. The h i g h e s t measured v a l u e i s shown by t h e arrow i n f i g . 1 .

We f i n d t h a t s t r o n g image c o n t r a s t i s r e l a t e d t o s t r o n g composition changes. Occa- s i o n a l l y , c o n t r a s t s suggestive o f lame1 l a r p r e c i p i t a t e s a r e observed (see fig.5.a).

Such areas show the s t r o n g e s t composition modulations ( f i g . 5-by c, d).

F u r t h e r connection of these composition modulation w i t h i n s t a b i l i t y o f t h e s o l i d phase i n these a l l o y s i s obtained by changing t h e growth c o n d t t i o n s . l . 3 Urn l a y e r s LPE-grown a t t h e e l e v a t e d temperature 750°C, t a k i n g t h e g ~ o w t h t b n d i t i o n s o u t s i d e t h e i m m i s c i b i l i t y zone, do n o t e x h i b i t t h e q u a s i p e r i o d i c tweed".contrast, although t h e g r a n u l a r c o n t r a s t p e r s i s t s . Composition p r o f i l e s along [100] d i r e c t i o n s show no s i g n i f i c a n t d e v i a t i o n from constant v a l u e (6).

S i m i l a r effects a r e observed i n t h e lattice-matched t e r n a r y l a y e r I n

When grown a t 650°C, i t does n o t show t h e tweed, b u t when grown a t

t ~ e ~ 2 Z ~ 8 t 4 4 ~ ~ / ~ ~ ~ '

temperatures o f 550°C and 490°C, t h e "tweed" c o n t r a s t appears w i t h equal and opposite I n and Ga c o n c e n t r a t i o n f l u c t u a t i o n s o f ir 5 %. Demixion has been observed 'at 550°C f o r mismatched t e r n a r y compounds (P. Henoc e t M.C. Joncour, p r i v a t e communication).

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F i g . 3 and 4 :

STEM

dark f i e l d s images o f t h e same area taken under t h r e e d i f f e r e n t d i f f r a c t i o n c o n d i t i o n s , and t h e r e s u l t s o f X-ray microanalyses a t t h e p o i n t s i n d i c a t e d on t h e micrographs.

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1 I 1 I

0 loo 200 300nm)

Also examined was a s e r i e s o f 1.3 um quaternary e p i l a y e r s grown a t 650°C by step c o o l i n g w i t h v a r i o u s degrees o f supersaturation. A1 1 e x h i b i t "tweed" c o n t r a s t , a1

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though r a t h e r weak. Samples thinned so as t o bbserve t h e quaternary l a y e r near the InGaAsP/TnP i n t e r f a c e were severely b e n t and d i f f i c u l t t o examine under f i x e d d i f - f r a c t i o n c o n d i t i o n s .

The development of these composition modulations i n l a t t i c e matched l a y e r s i s as y e t n o t c l e a r l y understood. D i f f u s i o n c o e f f t c i e n t s i n 111-V a l l o y s a r e too small t o account f o r composition modulations on a 200 nm scale. Annealing o f a 1.3 pm specimen a t the e p i t a x y temperature f o r 15 hours produced no n o t i c e a b l e m o d i f i c a t i o n t o t h e s t r u c t u r e . This s t r o n g l y suggests t h a t the modulation happens a t the s o l i d - l i q u i d i n t e r f a c e .

W i t h i n a f a c t o r o f two, t h e p e r i o d i c i t y . o f t h e tweed does n o t depend on composition, growth c o n d f t i o n s and p a r a l l e l mismatch o f t h e l a y e r s ( w i t h i n e l a s t i c l i m i t ) . I t i s l i k e l y t h a t t h e s o l i d s o l u t i o n f r e e energy i s minimized by coherent unmixing.

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In summary, we have shown t h a t lattice-matched In53G?47A~ and InxGal-xAsyP1-y layers grown on (001) InP exhibit a quasiperiodic composition modulation w i t h an associated s t r a i n f i e l d , when the growth temperature l i e s i n the immiscibility domain. For growth temperature above t h i s domain, the same composition layers do not show these composition modulations, and the sol i d solution i s homogeneous. Further d e t a i l s on the growth dynamics of the lagers a r e given in reference ( 6 ) .

References :

1 De Cremoux B, Hirtz P , Ricciardi J . , GaAs and related compounds, Vienna 1980, edited by H.W Thim ( I n s t i t u t e of P h y s k s , London, 1981)

2 Stringfellow G.B., J . of Crystal Growth

5

(1982) 194

3 Quillec,M., Daguet C . , Benchimol JL., Launois H. Appl. Phys. Lett.

40,

(1982) 325

4 HCnoc P . , Izrael A . , Quillec M . , Launois H . , Appl

.

Phys. Lett.

40,

(1982) 963 5 Goldstein J . I . , Costley JL., Lorimer GW., SEM 1977, edited by O.~Johari

(IITRI, Chicago, 1977) 315

6 Glas F., Treacy MMJ., Quillec !I., Launois H., GaAs and related compounds, A1 buquerque 1982, t o be pub1 ished.