THE VAPOR PHASE INTERACTION OF TRIMETHYLALUMINUM WITH GRAPHITE DURING OMVPE

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THE VAPOR PHASE INTERACTION OF TRIMETHYLALUMINUM WITH GRAPHITE

DURING OMVPE

D. Kisker, D. Stevenson, J. Miller, G. Stringfellow

To cite this version:

D. Kisker, D. Stevenson, J. Miller, G. Stringfellow. THE VAPOR PHASE INTERACTION OF TRIMETHYLALUMINUM WITH GRAPHITE DURING OMVPE. Journal de Physique Colloques, 1982, 43 (C5), pp.C5-221-C5-227. �10.1051/jphyscol:1982526�. �jpa-00222245�

Colloque C5, supplbment au n022, Tome 43, dlce&re 1982 page C5-221

THE VAPOR PHASE INTERACTION OF TRIMETHYLALUMINUM WITH GRAPHITE DURING OMVPE

** ***

D.W. ~isker*, D.A. Stevenson, J.N. Miller and G.B. Stringfellow

Dept. o f Materials Science, Stanford University, Stanford, CA 94305, U. S. A.

* * ~ e w Z e t t Packard Labs, 1501 Page M i l l Road, Palo Alto, CA 94304, U.S.A.

***Dept. o f Materials Science, University o f Utah, S a l t Lake C i t y , UT 84112, U. S. A.

RESUME: Nous avons Etudi6 l'interaction de (CH3)3A1 avec les surfaces de graphite en utilisant un spectrometre de masse 2 jet molgculaire. Nos r;sultats indiquent un effect d'adsorption important qui explique des phknomgnes observgs pendant la croissance organom$tallique de AlxGa,-xA~ comme des transitions lentes de la concentration d'oluminium, et la rgduction de l'incorporation d'oxyg;ne et de sglgnium.

ABSTRACT: We have used a molecular beam mass spectrometer to study the interaction of trimethylaluminum (TMA) with graphite surfaces. Our results indicate that a strong adsorption effect occurs which explains phenomena occuring during OMVPE growth of AlxGalbxAs, such as long A1 transients, oxygen gettering, and reduced incorporation of selenium.

I. INTRODUCTION: Highly purified, fine-grained graphite has often been a preferred construction material for vapor phase epitaxial reactors because of its thermal and chemical stability as well as the ease with which it can be formed into complex geometries. In addition, Stringfellow and Horn recently found that in the special case of orgonometallic vapor phase epitaxy (OMVPE) of AlxGa,-xAs, the use of

graphite baffles upstream of the reaction zone results in a dramatic increase in the observed photoluminescence, apparently by providing a surface where the heterogenous reaction between trimethylaluminum (TMA) and residual oxygen-containing species can occur, thus "gettering" them from the vapor phase 1

.

spectrometry (SIMS) results of Kisker et 01. confirmed this gettering effect of graphite baffles 2

.

Despite the advantages of using graphite baffles for reduction of residual oxygen, recent results have indicated that there are perhaps other, less desirable aspects of the chemical activity which results in this gettering ability. In particular,.during some attempts to produce double-heterostructure (DH) lasers, very long transients in aluminum concentration have prevented the attainment of very thin

*

Present address : Max-Planck Institiit - Heisenbergstr. 1,7000 Stuttgart 70 Fed.Rep. of Germany

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982526

JOURNAL DE PHYSIQUE

active layers 3 , and a reduction of the doping efficiency of selenium in A1 Ga As

4 x I-x

have been observed when graphite baffles are used

.

11. EXPERIMENTAL: For this work, we used a mass spectrometer specially designed for the study of vapor phase ambients such as might be found during VPE growth. In this system, which has been previously described in detail, the sampled gases are rapidly expanded through an orifice into a conical nozzle, where a molecular beam of the sampled species is formed and directed toward a quadrapole mass analyzer 5

.

sensitivity of the apparatus is of the order of a few parts per million for most species, and a particular advantage IS the rapid sampling time (-10-~sec) which results in the ability to easily study typlcal vapor phase transients, such as occur in conventional VPE systems.

In this work, we shall briefly summarize the results of our previous

measurements of the graphite-TMA interaction6 before turning to the more important aspects of the effect upon epitaxial growth. Our molecular beam mass spectrometer is situated such that it samples the effluent gas from o reoction chamber. This chamber can be heated and for the study of the interaction of TMA with graphite, strips of graphite are placed inside. We have found that with an empty reactlon chamber, the response of the system to a change in gas input concentration is limited only by the propagation of the concentration pulse through the reaction chamber. Diffusion of the gaseous species during this time results in "smearing" of a step change in gaseous concentration. The presence of adsorbing surfaces in the reaction chamber results in a lengthening of the increasing and decreasing

transients of a TMA pulse as shown in Fig. 1. In the following, we will derive a relationship between the "integrated desorption", represented by the shaded area of the desorption curve, and the total amount of adsorbed material at steady-state.

To begin, we assume that the desorption process is controlled by a Langmuir type process7, in which the rate of change in surface coverage is proportional to the amount of remaining surface coverage, 0 . In this case the desorbing flux, J d ,

EFFECT OF ADSORPTION ON TMA PULSE

2 0

5

--- WITH Ama(P1ION

s

0

0 6 on TMA pulse as measured by

o MBMS

.

$ O 4 5 0 2 W a

0 ON

TIME ^OFF

where 8 i s t h e s t e a d y - s t a t e f r a c t i o n o f s u r f a c e c o v e r a g e b e f o r e t h e b e g i n n i n g o f t h e d e s o r p t i o n t r a n s i e n t , k d i s a c o n s t a n t r e l a t e d t o t h e d e s o r p t i o n r a t e , NT i s t h e s u r f a c e d e n s i t y o f a d s o r p t i o n s i t e s and Nav i s A v o g a d r o ' s number. The

t i m e - d e p e n d e n t d e s o r p t i o n f l u x i s t h e d i f f e r e n c e between t h e t r a n s i e n t measured i n t h e p r e s e n c e o f g r a p h i t e o r o t h e r a d s o r b i n g s u r f a c e , a n d t h e r e f e r e n c e t r a n s i e n t measured,with n o g r a p h i t e p r e s e n t . Thus, a n e x p r e s s i o n f o r t h e shaded a r e a i n F i g .

1 i s o b t a i n e d by i n t e g r a t i o n o f Eq. 1 :

where I r e p r e s e n t s t h e i n t e g r a t e d d e s o r p t i o n . From t h i s e x p r e s s i o n , we o b t a i n a v a l u e f o r t h e amount o f s t e a d y - s t a t e a d s o r p t i o n , g i v e n b y BoNT. F o r l o w c o v e r a g e s , we e x p e c t

%

Then, from Eq. 2, t h e i n t e g r a t e d d e s o r p t i o n w i l l e x h i b i t t h e same e x p o n e n t i a l i n v e r s e t e m p e r a t u r e dependence.

111. RESULTS: We u t i l i z e d t h e MBMS s y s t e m t o o b s e r v e d e c r e a s i n g TMA t r a n s i e n t s w h i l e v a r y i n g t h e amount o f g r a p h i t e , t h e f l o w r a t e s and t h e t e m p e r a t u r e o f t h e

r e a c t i o n chamber. I n o r d e r t o be s u r e t h a t t h e o b s e r v e d r e s u l t s were a c t u a l l y caused by t h e a d s o r p t i o n t o TMA, we d i d s i m i l a r e x p e r i m e n t s a t v a r i o u s t e m p e r a t u r e s w i t h i n e r t gases and q u a r t z s t r i p s i n s t e a d o f g r a p h i t e . O n l y when TMA and g r a p h i t e were p r e s e n t t o g e t h e r d i d we o b s e r v e t h e l e n g t h e n e d t r a n s i e n t s c h a r a c t e r i s t i c o f a d s o r p t i o n . T y p i c a l r e s u l t s , i n F i g . 2, show t h e s t r o n g t e m p e r a t u r e dependence o f t h e d e s o r ~ t i o n p r o c e s s . I n F i g . 3, we p r e s e n t t h e r e s u l t s o f t h e c o m p l e t e s e t o f measurements o f t h e i n t e g r a t e d d e s o r p t i o n f r o m g r a p h i t e a s a f u n c t i o n o f i n v e r s e t e m p e r a t u r e a n d o b s e r v e t h e agreement t o t h e e x p e c t e d e x p o n e n t i a l dependence. From t h i s d a t a , a v a l u e o f t h e e n e r g y o f a d s o r p t i o n i s f o u n d t o b e 15.7 k c a l l m o l e . - T h i s v a l u e i s r e l a t i v e l y h i g h , i n d i c a t i n g t h e a t t a c h m e n t o f TMA t o g r a p h i t e i s o f a c h e m i c a l n a t u r e 7

.

I V . DISCUSSION: Our r e s u l t s have shown t h e e x i s t e n c e o f a s t r o n g a d s o r p t i o n e f f e c t o f TMA t o g r a p h i t e . T h i s p r o v i d e s a s o u r c e and s i n k f o r TMA d u r i n g OMVPE g r o w t h , r e s u l t i n g i n s l o w A1 c o n c e n t r a t i o n t r a n s i e n t s i n t h e s o l i d l a y e r s and p r o v i d i n g a

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F i g . 2 : E x p e r i m e n t a l measurement o f v a r i a t i o n o f d e s o r p t i o n t r a n s i e n t s a s a f u n c t i o n o f g r a p h i t e t e m p e r a t u r e .

F i g . 3 : Adsorbed2TMA vs. 1000IT. ( * ) d a t a t a k e n w i t h 150 cm o f g r a p h i t e i n r 1 a c t i o n chamber; ( 4 ) d a t a t a k e n w i t h 300cm p r e s e n t .

s u r f a c e where g e t t e r i n g r e a c t i o n s can o c c u r . I n t h e f o l l o w i n g , we i n v e s t i g a t e t h e i m p a c t o f t h e s e phenomena on OMVPE AlxGal-xAs l a y e r s .

The c a l c u l a t e d e f f e c t o f t h e a d s o r p t i o n and d e s o r p t i o n o f TMA f r o m g r a p h i t e o n one A l - p r o f i l e dependent p r o c e s s , t h e a c t i v e l a y e r g r o w t h o f a DH l o s e r , i s shown i n F i g . 4. I n t h e case t h a t t h e r e i s no TMA a d s o r p t i o n , t h e TMA f l o w i s reduced and t h e t r a n s i e n t can o c c u r s u f f i c i e n t l y q u i c k l y s o t h a t t h e a c t i v e l a y e r A1

c o n c e n t r a t i o n can be c o n t r o l l e d t o any chosen v a l u e . I n a c t u a l g r o w t h systems w i t h g r a p h i t e b a f f l e s , t h e minimum a t t a i n a b l e a c t i v e l a y e r A1 c o n c e n t r a t i o n may be r e l a t i v e l y h i g h due t o s l o w TMA t r a n s i e n t s as a l s o shown i n F i g . 4. P r e v i o u s l y measured A1 c o n c e n t r a t i o n p r o f i l e s o f l a y e r s grown u n d e r such c o n d i t i o n s appear q u a l i t a t i v e l y v e r y much t h e same o s i n t h i s c a l c u l a t i o n 3

.

u n i n t e r r u p t e d g r o w t h , an aluminum " s p i k e " i s o b s e r v e d a t t h e b e g i n n i n g o f t h e l a y e r . We can e x p l a i n t h i s o b s e r v a t i o n a s shown i n F i g . 5. The TMG and TMA, a f t e r b e i n g t u r n e d o f f t o g e t h e r , f o l l o w t h e i r own decay t r a n s i e n t s . S i n c e TMG does n o t a d s o r b t o g r a p h i t e 6 , i t s t r a n s i e n t i s much f a s t e r , t h u s c a u s i n g a s p i k e i n t h e f r a c t i o n o f vapor-phase TMA and t h e r e f o r e , i n t h e A1 c o n c e n t r a t i o n o f t h e s o l i d .

I n a d d i t i o n t o t h e i r a b i l i t y t o a c t as a s o u r c e and s i n k o f TMA d u r i n g AlxGol-xAs g r o w t h , t h e r e a r e o t h e r e f f e c t s o f g r a p h i t e b a f f l e s . I n t h e i r e a r l i e r work, K i s k e r e t 01. p r o p o s e d t h a t t h e oxygen g e t t e r i n g e f f e c t s w h i c h t h e y o b s e r v e d

+ ACTIVE

LAYER

*

-

; $

LAVER

-TMG

-

--TMA

-

1MG.TM.A TMG TMA

O F F ON ON

T I M E ( s e c l

F i g . 4 : E f f e c t o f a d s o r p t i o n o n DH l a s e r a c t i v e l a y e r . (- ) c a l c u l a t e d p r o f i l e w i t h n o TM.4 a d s o r p t i o n ; (-*-) c a l c u l a t e d p r o f i l e assuming TMA a d s o r p t i o n o c c u r s ; (---) SIMS measurement when g r o w t h i n t e r r u p t i o n i s used.

F i g . 5 : E f f e c t o f g r o w t h i n t e r r u p t i o n on a c t i v e l a y e r g r o w t h o f DH l a s e r . were due t o oxygen s p e c i e s r e a c t i n g w i t h p r e - a d s o r b e d TMA o n t h e s u r f a c e o f g r a p h i t e b a f f l e s L . Our work c o n f i r m s t h e a d s o r p t i o n o f TMA, i t s a p p r o x i m a t e s u r f a c e

c o n c e n t r a t i o n , ond t h e l a c k o f o d s o r p t i o n t o q u a r t z w h i c h e x p l a i n s t h e absence o f a n oxygen g e t t e r i n g e f f e c t w i t h q u a r t z b a f f l e s . The l a c k o f oxygen g e t t e r i n g b y Sic

b o f f l e s 2 is p r o b a b l y due t o t h e same e f f e c t , s i n c e l l k e q u a r t z , i t i s e x p e c t e d t o be c o n s i d e r a b l y more i n e r t t h a n g r a p h i t e .

A n o t h e r e f f e c t w h i c h can be e x p l o i n e d by o u r TMA-adsorption model i s t h e m o d i f i e d i n c o r p o r a t i o n o f s e l e n i u m 4 when i t i s used a s a n n - t y p e d o p a n t d u r i n g OMVPE. Because o f t h e c h e m i c a l s i m i l a r i t y o f t h e g r o u p V I e l e m e n t s Se a n d S and t h e i r compounds t o t h e compounds o f oxygen, we m i g h t e x p e c t a g e t t e r i n g e f f e c t s i m i l a r t o t h a t f o r oxygen t o o c c u r when t h e s e atoms a r e used as d o p a n t s . ( N o t e f o r example t h a t aluminum f o r m s s t a b l e compounds w i t h Se and S a s w e l l a s w i t h o x y g e n 8 . )

I n F i g . 6, we p r e s e n t t h e r e s u l t s o f SIMS measurements on t w o l a y e r s grown t o i n v e s t i g a t e t h i s e f f e c t . A f t e r two c y c l e s o f H Se p u l s e s w i t h g r o w t h i n t e r r u p t i o n s ,

2

t h e TMA wos t u r n e d o f f a n d GaAs was grown. We make two o b s e r v a t i o n s o b o u t t h e l a y e r shown i n 60, grown w i t h S i c b a f f l e s : i ) t h e s t e a d y - s t a t e Se c o n c e n t r a t i o n i s t h e same i n &As a s i n AlxGa,-xAs, and i i ) t h e r e i s no e v i d e n c e o f a n y aluminum s p i k e s r e s u l t i n g f r o m t h e g r o w t h i n t e r r u p t i o n s .

I n c o n t r a s t t o t h e r e s u l t s f o r S i c b a f f l e s , t h e r e s u l t s shown i n F i g . 6b, f o r a l a y e r grown w i t h g r a p h i t e b a f f l e s c l e a r l y show e v i d e n c e o f t h e TMA-graphite-H2Se i n t e r a c t i o n . F i r s t , t h e A1 c o n c e n t r a t i o n s p i k e s a t t h e g r o w t h i n t e r r u p t i o n s a r e p r e s e n t , a s d e s c r i b e d above. Of g r e a t e r i n t e r e s t however, i s t h e l a r g e d i f f e r e n c e

C5-225 JOURNAL DE PHYSIQUE

S I C BAFFLES loZ= . . . . .

INTERRUPT

GRAPHITE BAFFLES INTERRUPT

Se INPLANT

0 0.5 1.0 1.5 2.0 2.5 3.0

F i g . 6 : SIMS measurement o f Se i n c o r p o r a t i o n i n A1

0 . 3 ~ ~ 0 7AS

grown i n t h e presence o f SIC and g r a p h i t e b a f f l e s . The v e r t i c a l l i n e a t about lum r e p r e s e n t s t h e p o i n t a t which t h e TMA was t u r n e d o f f . The Se i m p l a n t was used t o c a l i b r a t e t h e SIMS measurement.

DEPTH Illrn)

i n Se i n c o r p o r a t i o n between A l Go As and GoAs i n t h e presence o f g r a p h i t e b a f f l e s . x 1-x

A l t h o u g h t h e same vapor phase c o n c e n t r a t i o n s as f o r t h e l a y e r i n F i g . 60 were used, t h e s t e a d y - s t a t e Se i n c o r p o r a t i o n i n AlxGal-xAs i s 100 t i m e s l e s s t h a n when S i c b a f f l e s o r e used! F u r t h e r , though t h e r e i s c l e a r l y an i n c o m p l e t e t r a n s i e n t e f f e c t i n t h e &As l a y e r , t h e Se i n c o r p o r a t i o n i n &As i s a t l e a s t 10 t i m e s h i g h e r t h a n i n AlxGol-xAs. We a l s o n o t e t h a t even when t h e H2Se i s t u r n e d o f f , t h e Se

c o n c e n t r a t i o n does n o t reach i t s l o w e s t v a l u e u n t i l t h e TMA i s a l s o t u r n e d o f f . These r e s u l t s i n d i c a t e t h a t o l t h o u g h t h e H2Se i s g e t t e r e d by t h e b a f f l e s , it must

-

somehow be r e - e v o l v e d d u r i n g t h e s e p e r i o d s .

I n a d d i t i o n t o t h e r e s u l t s r e p o r t e d i n t h i s work, i t i s p o s s i b l e t o s p e c u l o t e t h a t o t h e r e f f e c t s due t o t h e TMA-graphite i n t e r a c t i o n s h o u l d a l s o be observed. One such p o s s i b l e e f f e c t would be a g e t t e r i n g e f f e c t f o r s u l f u r - c o n t a i n i n g compounds l i k e those observed f o r oxygen- and s e l e n i u m - c o n t a i n i n g compounds. To t h e b e s t o f o u r knowledge, such a r e s u l t has n o t y e t been r e p o r t e d . S i m i l a r l y , a l t h o u g h r e p o r t s o f s t r o n g v a r i a t i o n o f g r o w t h c h a r o c t e r i s t i c s w i t h temperoture have n o t been made, becouse o f t h e l a r g e energy o f a d s o r p t i o n , such e f f e c t s s h o u l d be expected, and may r e s u l t i n r u n - t o - r u n o r r e a c t o r - t o - r e a c t o r v a r i a b i l i t y . F i n a l l y , as o t h e r OM compounds a r e used f o r sources o f m e t a l l i c components i n semiconductor m a t e r i a l s , e i t h e r as a m a j o r component o f a compound semiconductor, o r as a dopant, i t may be p o s s i b l e f o r o t h e r s p e c i e s t o p l a y a g e t t e r i n g r o l e s i m i l a r t o t h a t o f TMA i n t h e presence o f g r a p h i t e .

V. CONCLUSIONS: I n t h i s work, we have used a mass s p e c t r o m e t e r w i t h a n o l e c u l a r beam sampling a p p a r a t u s t o s t u d y t h e i n t e r a c t i o n of TMA w i t h g r a p h i t e as a f u n c t i o n o f temperoture and g r a p h i t e area. A v a l u e f o r t h e energy o f a d s o r p t i o n o f

15.7kcal/mole was found. The r e s u l t i n g s t r o n g a d s o r p t i o n o f TMA o n t o g r a p h i t e b a f f l e s p r o v i d e s t h e b a s i s f o r a model which e x p l a i n s t h e oxygen g e t t e r i n g , selenium g e t t e r i n g , aluminum c o n c e n t r a t i o n s p i k e s and l o n g t r a n s i e n t t i m e s f o r aluminum c o n c e n t r a t i o n c h m g e s observed d u r i n g OMVPE. The b a f f l e s con t h u s a c t as n o t o n l y a

t o o c c u r . SIMS r e s u l t s c o n f i r m e d t h e e x i s t e n c e o f t h e s e e f f e c t s . F i n a l l y , o t h e r p o s s i b l e e f f e c t s o f t h i s i n t e r a c t i o n and model were c o n s i d e r e d .

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