OPTICAL PROPERTIES OF POLYCRYSTALLINE THIN FILMS OF GaAs OBTAINED BY MBD

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OPTICAL PROPERTIES OF POLYCRYSTALLINE THIN FILMS OF GaAs OBTAINED BY MBD

C. Paparoditis, A. Rideau, G. Monnom, Ph. Gaucherel

To cite this version:

C. Paparoditis, A. Rideau, G. Monnom, Ph. Gaucherel. OPTICAL PROPERTIES OF POLYCRYS-

TALLINE THIN FILMS OF GaAs OBTAINED BY MBD. Journal de Physique Colloques, 1982, 43

(C1), pp.C1-159-C1-164. �10.1051/jphyscol:1982122�. �jpa-00221778�

JOURNAL DE PHYSIQUE

CoZZoque C l , supplément au n o 10, Tome 43, o c t o b r e 1982

O P T I C A L PROPERTIES OF POLYCRYSTALLINE T H I N F I L M S OF GaAs OBTAINED BY MBD

C. P a p a r o d i t i s , A . R i d e a u , G. Monnom a n d Ph. G a u c h e r e l

Laboratoire de Physique de l a Matière Condensée, L a b o r a t o i r e a s s o c i é au CNRS, N o 190, U n i v e r s i t é de Nice, Parc V a l r o s e , 06034 Nice Cedex, France

Résumé. Des couches minces p o l y c r i s t a l l i n e s d ' a r s ë n i u r e de g a l l i u m c a r a c t é r i -

- sees p a r un degré v a r i a b l e de d é s o r d r e o n t é t é préparées p a r l a méthode des j e t s m o l é c u l a i r e s dans un v i d e c l a s s i q u e . Des s u b s t r a t s de d i f f é r e n t e s n a t u r e s o n t é t é u t i l i s é s : pyrex, s i l i c e fondue, f l u o r i n e . Un d é s o r d r e c r o i s s a n t a pu ê t r e observé p a r l ' a b a i s s e m e n t de l a t e m p é r a t u r e du s u b s t r a t ( T s ) . Ces couches o n t p u ê t r e c a r a c t é r i s é e s à l ' a i d e d e l a d i f f r a c t i o n des r a y o n s X. Dans c e t r a v a i l , on p r é s e n t e l e s p r o p r i é t é s l e s p l u s marquantes obtenues p a r des mesures de t r a n s m i s s i o n e t de r é f l e x i o n o p t i q u e s , notamment dans l e s r é g i o n s du proche

U V

e t de 1 'i n f r a r o u g e l o i n t a i n , en l i a i s o n avec l e s c o n d i t i o n s de p r é p a r a t i o n ; l a t e m p é r a t u r e Ts e t l e r a p p o r t As/Ga de l ' i n t e n s i t é des j e t s é t a n t l e s paramè t r e s p r i n c i p a u x . Les é p a i s s e u r s des couches s ' é c h e l o n n e n t e n t r e 400 fi e t 2.6pm A b s t r a c t . Ga1 l i u m a r s e n i d e p o l y c r y s t a l l i n e t h i n f i lms of v a r i o u s degrees o f d i s o r d e r have been prepared by m o l e c u l a r beam d e p o s i t i o n i n a c o n v e n t i o n a l va- cuum system. D i f f e r e n t s u b s t r a t e s have been used : pyrex, f u s e d s i l i c a and f l u o r i n e . I n c r e a s i n g degree o f d i s o r d e r i s o b t a i n e d by l o w e r i n g t h e s u b s t r a t e temperature ( T s ) . C h a r a c t e r i z a t i o n was o b t a i n e d by means o f x -ray d i f f r a c t i o n . The most s t r i k i n g o p t i c a l p r o p e r t i e s o b t a i n e d by t r a n s m i s s i o n and r e f l e c t i o n measurements, m a i n l y i n t h e near-UV and f a r - i n f r a r e d range, a r e p r e s e n t e d i n r e l a t i o n w i t h t h e p r e p a r a t i o n c o n d i t i o n s , where he main parameters a r e Ts and

i

As/Ga beam r a t i o . Thicknesses range between 400 and 2.6 m.

1. I n t r o d u c t i o n . Based on t h e t h r e e - t e m p e r a t u r e method [ 1 ! , m o l e c u l a r beam deposi- t i o n has proved, t h e s e r e c e n t years, t o b e a v e r y s u c c e s s f u l method i n t h e p r o d u c t - i o n o f h i g h q u a l i t y GaAs f i l m s , a t e l e v a t e d s u b s t r a t e temperatures (Ts). A t l o w e r temperatures ! 2 ] , p o l y c r y s t a l l i n e d e p o s i t s show a w i d e range o f p r o p e r t i e s . Recent- l y [ 3 ] an e f f o r t was developped towards t h e c o n t r o l o f s t o i c h i o m e t r y i n amorphous f i l m s . Among t h e d i f f e r e n t methods o f i n v e s t i g a t i o n i n t h i s area, x - r a y a n a l y s i s and o p t i c a l measurements a r e t h e most p o p u l a r . However, i n t h e former case, l i t t l e i n - f o r m a t i o n i s g a i n e d a t l o w Ts and l o w t h i c k n e s s , w h i l e i n t h e l a t t e r , most e f f o r t s have developed i n t h e o p t i c a l gap energy range where d e f e c t s and i n t e r f e r e n c e e f - f e c t s o f t e n obscure t h e s i t u a t i o n . Meanwhile e s s e n t i a l i n f o r m a t i o n c o n t a i n e d i n t h e o p t i c a l s p e c t r a i n t h e h i g h l y a b s o r b i n g r e g i o n [ 4 1 and i n t h e l a t t i c e v i b r a t i o n s i n t h e f a r - i n f r a r e d [ 51 i n r e l a t i o n w i t h d i s o r d e r has been r a r e o r m o s t l y o b t a i n e d on t h e amorphous s t a t e [6,7 ] . I n t h i s d i r e c t i o n , we want t o g i v e c o n c i s e e x p e r i m e n t a l evidence, as an e x t e n s i o n o f a s e p a r a t e paper [ 8 ] o n e f f e c t s o f d i s o r d e r , of how MBO i s capable o f r e p r o d u c i n g p r o g r e s s i v e v a r i a t i o n s i n t h e s e p r o p e r t i e s f o r t h e f u l l gamut o f p o l y c r y s t a l l i n e samples e x t e n d i n g f r o m t h e h i g h l y c r y s t a l l i n e s t a t e t o t h e amorphous s t a t e . T h i s b e i n g o b t a i n e d by l o w e r i n g Ts and v a r y i n g t h e As/Ga beam r a - t i o . O p t i c a l p r o p e r t i e s were i n v e s t i g a t e d by a b s o r p t i o n and r e f l e c t i o n measurements o v e r a wide s p e c t r a l range (2000 A - 50 ~ m ) i n c o m b i n a t i o n w i t h s t r u c t u r a l p r o p e r t - i e s as o b t a i n e b y x - r a y d i f f r a c t i o n . S e l e c t e d s p e c t r a l range i n t h i s work i s t h e 2000 R - 5000 1 r e g i o n c o v e r i n g e s s e n t i a l t r a n 3 i t i o n s abgve t h e fundamental gap a t t h e r e s t r a h l e n r e g i o n i n t h e i n f r a r e d (200 cm- - 400 cm- 1. Our u l t i m a t e aim, i n t h e f i r s t place, i s t o improve Our knowledge o f t h e o p t i c a l p r o p e r t i e s of p o l y c r y s t a l - l i n e f i l m s and, i n t h e second place, t o draw a t t e n t i o n on t h e s e r v i c e t h a t o p t i c a l i n v e s t i g a t i o n can o f f e r i n t h e p r e p a r a t i o n and c h a r a c t e r i z a t i o n o f p o l y c r y s t a l l i n e GaAs t h i n f i l m s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982122

Cl-160 JOURNAL DE PHYSIQUE

2. P r e p a r a t i o n . Elemental g a l l i u m and a r s e n i c a r e evaporated f r o m two s e p a r a t e and r e g u l a t e d sources. A g r a p h i t e ce11 mounted w i t h a narrow chimney i s used f o r a r s e n i c ; i n t h e g r a p h i t e one f o r G a l l i u m , f r e e e v a p o r a t i o n i s a l l o w e d f o r t h i c k f i l m s , w h i l e a chimney i s adapted f o r f i l m s l e s s t h a n O.lwm t h i c k . T y p i c a l s o u r c e temperatures a r e 1200 "C f o r g a l l i u m and 300

f o r a r s e n i c . These a r e t h e v a l u e s subsequently r e f e r r e d t o as " l o w " As/Ga beam r a t i o , w h i l e " h i g h " As/Ga beam r a t i o r e f e r s t o a 10

i n c r e a s e i n t h e a r s e n i c source temperature. These w o r k i n g c o n d i t i o n s have,been determined e m p i r i c a l l y f o r Our e x p e r i m e n t a l s e t up. T y p i c a l growth r a t e s a r e 2 A/s f o r f i l m s 400 a t h i c k d e p o s i t e d on f u s e d s i l i c and f l u o r i n e s u b s t r a t e s f o r t r a n s - m i s s i o n measurements i n t h e near

range, 30 b s f o r t h i c k f i l m s d e p o s i t e d on py- r e x .

The background p r e s s u r e i n a c o n v e n t i o n a l vacuum system i s k e p t d u r i n g e v a p o r a t i o n below 3 x 1 0 - ~ ~ o r r b y k e e p i n g t h e w a l l a t l i q u i d n i tr o g e n temperature. R a d i a t i o n f r o m h o t spots towards t h e s u b s t r a t e s i s m i n i m i s e d by e f f i c i e n t screening, s h u t t e r s and passage o f t h e beams t h r o u g h narrow h o l e s i n t h e screens. The temperature range o v e r which t h e s u b s t r a t e t e m p e r a t u r e Ts was made t o Vary was 20

- 470

By v i - sua1 i n s p e c t i o n , t h e f i l m s were f a i r l y t o h i g h l y s p e c u l a r on t h e a i r s i d e . I n a few cases, d e p a r t u r e f r o m normal procedure gave m i l k y o r b l a c k aspect. A t t h e most e l e - v a t e d temperatures, f i l m s were d i s t i n c t l y g r a i n y and d i f f u s i v e . B u t i n a l 1 cases, i n s p e c t e d f r o m t h e back s i d e , r e f l e c t i o n was always s p e c u l a r . Besides, i n t h e o p t i - c a l i n v e s t i g a t i o n s d e s c r i b e d f u r t h e r , h i g h s t a b i l i t y was d i s p l a y e d o v e r t h e e n t i r e range o f d e p o s i t i o n temperature. Adherence i s e x c e l l e n t a t h i g h temperatures t o f a i r a t l o w t e m p e r a t u r e s on b a r e as w e l l on m e t a l ( A l , N i 1-coited s u b s t r a t e s .

Thickness was measured by T and R i n t e r f e r e n c e f r i n g e s and w i t h t h e Nomarsky i n t e r - f e r o m e t e r . E x c e l l e n t agreement and good measure a r e o b t a i n e d i n t h i c k f i l m s o t h e r t h a n amorphous. The e r r o r s a r e g r e a t e r w i t h t h e t h i n n e s t f i l m s due t o absence o f f r i n g e s . T h i c k amorphous f i l m s do g i v e i n t e r f e r e n c e f r i n g e s b u t c a l c u l a t i n g t h e t h i c k n e s s i s i m p o s s i b l e w i t h t h e c r y s t a l v a l u e s o f t h e i n d e x o f r e f r a c t i o n . T h i s inadequacy can be used as a c r i t e r i o n t o c h a r a c t e r i z e t h e d e p o s i t as n o n - c r y s t a l l i n e o r amorphous.

3. F i l m s t r u c t u r e . X - r a y d i f f r a c t i o n was used t o analyse t h e s t r u c t u r e o f a l 1 f i l m s . Fig-e p a t t e r n s o f f i l m s d e p o s i t e d on f u s e d s i l i c a a t t h r e e d i f f e r e n t temperatures. The t h i c k n e s s b e i n g r o u g h l y t h e same. T h i s f i g u r e shows t h r e e f e a t u - r e s : i ) a s t r o n g (111 1 t e x t u r e i i ) t h e decrease i n a m p l i t u d e a t i n t e r m e d i a t e tem- p e r a t u r e iii complete disappearance o f s t r u c t u r e a t 150

These t h r e e f e a t u r e s a r e c h a r a c t e r i s t i c o f a l 1 Our samples : whatever t h e n a t u r e o f t h e s u b s t r a t e , t h e d e p o s i t i o n t e m p e r a t u r e o r t h i c k n e s s , t h e (111) t e x t u r e i s dominant, and d i f f u s e p a t - t e r n appears around 150

Measurement of l i n e w i d t h shows an i n c r e a s e as Ts i s lowered, t h u s i n d i c a t i n g a decrease i n t h e s i z e of t h e c r y s t a l l i t e s . The e v o l u t i o n o f t h e a m p l i t u d e and o f t h e l i n e w i d t h o f a s e t o f samples (400 8 t h i c k ) d e p o s i t e d on f u s e d s i l i c a has been r e p r e s e n t e d on F i g u r e l b .

F i g . 1 : ( a ) X-ray d i f f r a c t i o n p a t t e r n s o f f F T m s d e p o s i t e d on f u s e d s i l i c a a t v a r i o u s temperatures T . (b)Ampl i t u d e and l i n e w i d t h o f t h e (71 11 l i n e as a.

f u n c t i o n o f Ts f o r f i l m s w i t h a 400 A

---- average t h i c k n e s s .

L .

.

60 40 2 0

2 0 Ts ( O C )

It shows t h e l i m i t a t 150

a t which d i f f u s e p a t t e r n s appear, g e n e r a l l y a s s o c i a t e d

w i t h t h e appearance o f an amorphous phase; i t shows symmetric t r e n d s i n t h e e v o l u -

t i o n o f a m p l i t u d e and l i n e w i d t h . The p r e c i p i t o u s d e t e r i o r a t i o n o f t h e c r y s t a l

s t r u c t u r e i n i t i a t e d a t around 300

down t o 250

t h e n a r e l a t i v e s t a b i l i s a t i o n

f u r t h e r down i n temperature. T h i s i s i n e x c e l l e n t accord w i t h Davey and Pankey's

work [ 2 ] . I t remains however t h a t x - r a y d i f f r a c t i o n i s u n a b l e t o t r a c e any m i c r o s -

t r u c t u r e below 150

i n Our work, 230

i n t h e l a t t e r .

4. O p t i c a l p r o p e r t i e s . Absorption and r e f l e c t i v i t y spectra of GaAs above t h e funda- menfal absorption edge up t o 6 eV c o n t a i n important s t r u c t u r e , w i t h two important peaks centered a t 3 eV ( E L 1 and 5 eV ( E l ) . Most o f t h i s s t r u c t u r e a r i s e s from nu- merous i d e n t i f i e d i n t e r b a n d t r a n s i t i o n s which are l o c a t e d a t c r i t i c a l p o i n t s i n

F i g . 2: Band s t r u c t u r e o f GaAs i n c l u d i n g s p i n - o r b i t cou- p l i n g , and some o f t h e r e l e v a n t o p t i c a l t r a n s i - t i o n s a t c r i t i c a l p o i n t s o f t h e B r i l l o u i n zone.

t h e B r i l l o u i n zone ( F i g . 2 ) . The existence o f these c r i t i c a l points i s c l o s e l y r e l a t - ed t o t h e c r y s t a l l i n e s t r u c t u r e and any d i s r u p t i o n i n t h e p e r i o d i c i t y o f t h e l a t t i c e must have d r a s t i c e f f e c t s o f t h e o p t i c a l p r o p e r t i e s .

I n f l u e n c e o f t h e d e p o s i t i o n temperature (Ts)

R g h q u a l i t y p o l y c r y s t a l l i n e t i l m s are c u r r e n t l y obtained by MBD. R e f l e c t i v i t y and absorption s ~ e c t r a o f these f i l m s d e ~ o s i t e d a t h i q h temperature (400-450 OC) are i n a l 1 r e s p e c t i d e n t i c a l t o those r e p o r t e d f o r t h e b;lk m a t e r i a l . The g r a i n s i z e i s f a i r l y l a r g e as i n f e r r e d from t h e 1 in e w i d t h o f t h e predominant (1 11) 1 in e i n t h e x-ray d i f f r a c t i o n p a t t e r n s ( F i g . l b ) . However, a r a p i d e v o l u t i o n o f these spectra(Fig.3a) and p a t t e r n s (Fig.1) takes place as Ts i s lowered down t o near room temperature.

F i g . 3: ( a ) o p t i c a l d e n s i t y ( 0 D ) spectra vs Ts :

1) 425

21 250 "C 3) 184

4) 77

( b ) v a r i a t i o n o f 0D and back-si- de r e f l e c t a n c e a t 5 eV vs. Ts.

? 2 3 4 5 6 7

T,~==("c )

E n e r q y

Gross f e a t u r e s o f e v o l u t i o n are a decrease i n absorption above about 2.5 eV and a corresponding increase below t h i s energy i n t h e r e g i o n of t h e absorption edge. As shown h e r e a f t e r ( F i g . 6 and 7 s t r u c t u r e i s preserved down t o near RT and peaks and o t h e r f e a t u r e s i n t h e spectra (e.g.shoulders) remain f i x e d i n energy p o s i t i o n i n s p i t e o f s e l e c t i v e e v o l u t i o n o f each f e a t u r e . A s i m i l a r behaviour has been observed i n t h e e v o l u t i o n o f t h e r e f l e c t i o n spectra [ 8 1 . We have p l o t t e d (Fig.3b) t h e change i n t h e o p t i c a l d e n s i t y o f t h e E2 peak a t 5 eV as a f u n c t i o n o f Ts. A s t r i k i n g s i m i l a r i t y e x i s t s between t h i s curve and t h e e v o l u t i o n i n t h e ( l l 1 ) l i n e o f e i t h e r t h e amplitude o r l i n e w i d t h ( F i g . l b ) obtained by x-ray d i f f r a c t i o n . The e v o l u t i o n o f t h e amplitude and o f t h e l i n e w i d t h of t h e ( l l l 1 l i n e on one s i d e and o f t h e o p t i c a l den- s i t y and r e f l e c t a n c e of t h e E2 t r a n s i t i o n a t 5 eV o f f e r a s t r i k i n g s i m i l a r i t y . Sud- den changes occur a t t h e same temperatures which are i n agreement w i t h an e a r l i e r r e p o r t e d work [ 2 ] . The m o d i f i c a t i o n o f t h e o p t i c a l d e n s i t y spectra suggestsas a f i r s t approach two kinds of d i s o r d e r . The f i r s t , e s s e n t i a l l y r e l a t e d t o t h e g r a i n s i z e e f f e c t i n t h e 300

- ⁴⁵⁰

range,is seen i n t h e i n c r e a s i n g absorption, as TS i s lowered, due t o t h e i n c r e a s i n g s c a t t e r i n g a t t h e g r a i n boundaries. The second, set- t i n g i n p r e c i p i t o u s l y below 300 "C,is presumed t o be due t o t h e appearance of an am- orphous connective t i s s u e between m i c r o c r y s t a l s . This coexistence and therefore, t h e s u r v i v a l of long-range order, i s being detected down t o low temperatures, 77

a t least,beyond t h e d e t e c t i o n l i m i t o f x-ray d i f f r a c t i o n . Below 250

c e r t a i n l y import- ant m o d i f i c a t i o n s [2] must occur on a smaller dimensional scale. These are marked i n t h e absorption spectra by t h e trough around 250

and t h e s i n g u l a r behaviour a t 230

O C ,

f i r s t , then a t 130

where t h e f i l m growth i s enhanced presumably due t o increas-

ed arsenic s t i c k i n g c o e f f i c i e n t . These c r i t i c a l temperatures are n o t a c c i d e n t a l since

they have been s y s t e m a t i c a l l y . Q u i t e s i m i l a r i s t h e behaviour o f t h e r e f l e c t a n c e a t

E2 where, because o f t h e h i g h absorption, thickness has no e f f e c t .

CI-162 JOURNAL DE PHYSIQUE

Reflectance and surface e f f e c t s - While absorption spectra are e s s e n t i a l l y a b u l k e f f e c t , r e f l e c t a n c e spectra i n t h e h i g h absorption r e g i o n a r e a s u r f a c e property.

When r e f l e c t a n c e measurements are c a r r i e d o u t on t h e back ( s u b s t r a t e l - s i d e , El and E2 r e f l e c t i v i t y peaks behave i n t h e same way as i n absorption (Fig.3a) where E2 de- creases w i t h Ts. However when t h e measurements a r e c a r r i e d o u t on a i r - s i d e , Ep r e f l e c - t i o n peak i s i n v a r i a b l y l o s t , when f i l m s are deposited on f l u o r i n e (Fig.4) o r fused s i l i c a , whereas on pyrex E2 i s conserved.

F i g . 4: R e f l r c t a n c e spectra o f f i l m s deposited a t various Ts on

1- f l u o r i n e w i t h a " h i g h " As/Ga beam r a t i o , obtained on t h e a i r - i n t e r f a c e as compared t o one, a t Ts

184

measured on t h e back-side. Base l e v e l has been taken a t 0.2pm.

Note t h a t l o s s o f E, i s obtained f o r a l 1 values of Ts up t o 450

X ( p m l

Possible candidates f o r t h i s behaviour might be : d i f f e r e n t surface s t r u c t u r e i 2 ] g r a t i n g and p o l a r i t y ( 1 11 ) e f f e c t s [ 91 , d i sordered s u r f a c e 1 ayer, d i f f e r e n t surface phase o r s t r a i n s . These e f f e c t s can u l t i m a t e l y be b u l k - d e r i v e d or/and substrate-derived. S t r a i n s have been shown t o e x e r t a p o s i t i v e e f f e c t . Deposited on metal ( A l o r Ni)-coated pyrex, f i l m s undergo an even lowering o f t h e E l and E2 l e v e l s and a d r a s t i c f a l l - o f f o f t h e 5.3 eV t r a n s i t i o n . Therefore t h e n a t u r e o f t h e substra- t e has a d i r e c t e f f e c t on t h e f i l m through t h e s t r a i n s . Bulk-derived e f f e c t s are presumed t o be r e s p o n s i b l e f o r p e c u l i a r o p t i c a l p r o p e r t i e s a t two s i n g u l a r temperatu- r e s Ts (130°C and 230°C) where r e f l e c t a n c e s p e c t r a a t b o t h i n t e r f a c e s ( a i r and sub- s t r a t e ) a r e i d e n t i c a l . Thus combined i n v e s t i g a t i o n o f t h e Ep r e g i o n by r e f l e c t i o n and transmission measurements g i v e unique i n f o r m a t i o n on b o t h b u l k and s u r f a c e p r o p e r t i e s I n f l u e n c e o f t h e As/Ga beam r a t i o - Important m o d i f i c a t i o n s observed i n t h e absorption

181 as w e l l as i n t h e r e f l e c t i v i t y spectra, i n d i c a t e d r a s t i c e f f e c t s i n t h e b u l k as w e l l as on t h e s u b s t r a t e - f i l m i n t e r f a c e . This can be seen by comparing spectra o f f i l m s prepared w i t h " l o w U (Fig.5a) and "high" r a t i o (Fig.5b) r e s p e c t i v e l y . It i s r e - minded t h a t "high" r a t i o corresponds t o a 10°C increase i n t h e temperature o f t h e arsenic source.

F i g . 5: Back-side r e f l e c t a n c e spectra o f f i l m s de- p o s i t e d on f l u o r i n e a t various temperatu- r e s f o r ( a ) "low" and ( b ) " h i g h " As/Ga r a t i o . Base l e v e l has been taken a t 0.2pm.

TS values: ( a ) 1 ) 405 "C 2 ) 184 "C 3 ) 77

4 ) 20 "C ( b l 1 ) 184 IC 2) 155 "C

3 ) 77

~ s a b s o r ~ t i o n [8] , back-side r e f lectance s p e c t r a remain p r a c t i c a l l y unmodified as temperature decreases f r o m 190°C t o 77°C. The l e v e l on t h e E peak remains h i g h w i t h o n l y a s l i g h t s h i f t t o lower energies, probably on account

t h e l o s s o f t h e h i g h e r i n energy t r a n s i t i o n s . I n r e f l e c t a n c e spectra on t h e a i r - s i d e , on t h e o t h e r hand, as i n t h e "low" r a t i o f i l m s , E2 peak i s l o s t . These observations l e a d us t o two conclu- sions. The f i r s t , a general one, i s t h a t back-side r e f l e c t a n c e o n l y corresponds t o t h e b u l k o f t h e f i l m and t h a t a i r - s i d e r e f l e c t a n c e i s a p r o p e r t y o f t h e a i r - s u r f a c e only. The second i s t h a t an enhancement o f t h e p o p u l a t i o n o f arsenic molecules f a l l -

i n g on t h e surface, and an eventual excess i n t h e d e p o s i t f a v o r s c r y s t a l l i s a t i o n a t low temperature. Unambiguous i n t e r p r e t a t i o n o f mechanims (nucleation, growth, etc.. - 1 and d e t a i l e d s t r u c t u r e i s h a r d l y p o s s i b l e a t t h i s stage.

D e t a i l e d s t r u c t u r e o f t h e absorption spectra - D e t a i l e d s t r u c t u r e o f t h e o p t i c a l spec-

r r a are remarkably preserved down t o low temperatures. The s e l e c t i v e modification,as

Ts decreases, i n t h e absorption o f d i f f e r e n t neighbouring o r overlapping t r a n s i t i o n s

can, p a r a d o x i c a l l y , make these be b e t t e r observed on f i l m s w i t h increased d i s o r d e r .

F i g . 6: O p t i c a l . d e n s i t y spectra i n t h e r e g i o n o f t h e El t r a n s i t i o n showing the 0.22 eV-wide s p i n - o r b i t s o l i t t i n g .

Such i s t h e case o f t h e El t r a n s i t i o n ( F i g . 6) where t h e s p i n - o r b i t s p l i t components are observed a l 1 through d i f f e r e n t d e p o s i t i o n temperature down t o 77

i n t h e ab- s o r p t i o n spectra. The 0.22 eV separation i s remarkably unchanged and so i s t h e e - nergy p o s i t i o n o f each component.

F i g u r e 7 represents t h e d i f f e r e n t spectra o f t h e Ep region. Many o f t h e and E2 t r a n s i t i o n s can be i d e n t i f i e d and f o l l o w e d as Ts decreases. Again these remain f i x e d i n energy p o s i t i o n and t h e s h i f t below Ts

150

i s o n l y due t o t h e acceleratedde- crease o f t h e E2 ( L) t r a n s i t i o n .

F i g . 7 : O p t i c a l d e n s i t y s p e c t r a o f t h e Ed and E2 main t r a n s i t i o n s f o r f i l m s deposited on fused s i l i c a a t d i f f e r e n t Ts.

5. L a t t i c e v i b r a t i o n s - F a r - i n f r a r e d r e f l e c t i v i t y spectra i n t h e r e s t s t r a h l e n r e g i o n have been obtained f o r d i f f e r e n t thicknesses and d i f f e r e n t values o f Ts.

Figure 8 shows t h e normalised TO phonon spectra of f i l m s prepared above 400

as a f u n c t i o n o f thickness. I n t e r f e r e n c e e f f e c t s are important f o r f i l m s above 1 pm i n thickness. Below t h i s value, down t o 400 a, where t h e l i n e w i d t h i s 6 cm-,l, t h e TO

1

1

F i g . 8: Near-normal incidence r e f l e c t a n c e due t o t h e TO phonon normalised i n amplitude f o r f i l m s of d i f f e r e n t thickness deposited above 400

\ I

phonon l i n e s are a t 268.2 cm-' i n accordance w i t h previous measurements [ 5 ] . The LO phonon l i n e , 6 cm-1 i n l i n e w i d t h , c o u l d be obtained on 0.3 ~ m t h i c k f i l m s by t h e

' E

amorphous

F i g . 9: S h i f t o f t h e TO and LO r e f l e c t a n c e l i n e s as a f u n c t i o n o f d e p o s i t i o n temperature Ts.

Ts ('Cl

JOURNAL DE PHYSIQUE

Berreman t e c h n i q u e [IO], by d e p o s i t i n g 0.3 pm t h i c k f i l m s on m e t a l - c o a t e d p y r e x s u b s t r a t e s a n d measuring t h e r e f l e c t i v i t y o f t h e P - p o l a r i s e d r a d i a t i o n . The measured wavenumber o f t h e LO phonon i s 291.2 cm-1 as p r e v i o u s l y o b t a i n e d L5J when Ts i s above 400

F i g u r e 9 shows t h e s h i f t o f t h e two o p t i c a l phonons as Ts decreases f r o m 450 OC t o room-temperature. F i r s t , a s l i g h t p r o g r e s s i v e s h i f t , t w i c e more i m p o r t a n t i n LO t h a n i n TO, i s observed as Ts reaches t h e l o w e r l i m i t o f 50 OC, t h e n a t room-temperature TO q g j c k l y moves t o 250 cm-1 as a s i n g l e peak, whi l e LO develops a new peak a t some 17 cm f r o m i t s c r y s t a l value. The s m a l l s h i f t i n b o t h TO and CO a r e t h o u g h t t o b e due t o t h e decrease o f t h e g r a i n s i z e w h i l e t h e b i g s h i f t i s a t t r i b u t e d t o amorphous s t a t e . T h i s e v o l u t i o n i s accompaqied by a r a p i d decrease o f t h e a m p l i t u d e o f b o t h phonon l i n e s , b u t more q u i c k l y w f o r LO. The p l o t o f t h e a m p l i t - ude o d i f i c a t i o n o f t h e TO phonon l i n e has been o b t a i n e d as a f u n c t i o n o f Ts f o r t h e 400 ! t h i c k s e r i e s , whichshows a s t r i k i n g s i m i l a r i t y w i t h t h e r e p o r t e d o b s e r v a t i p n s on t h e E2 a b s o r p t i o n peak ( f i g . 3 b ) . The TO phonon i s observed f o r Ts-77 OC, however i t s h i f t s t o 250 cm-1 a t t h e h i g h e r c r i t i c a l Ts o f 130

Thus a narrow range o f temperature f a v o u r i n g amorphous d e p o s i t s seems t o b e supported by f a r - i n f r a r e d i n v e s t - i g a t i o n .

6. C o n c l u s i o n : O p t i c a l s p e c t r a o f GaAs t h i n f i l m s o b t a i n e d by MBD i n t h e near-UV and t h e f a r - i n f r a r e d have been shown t o be v e r y s e n s i t i v e t o p r e p a r a t i o n c o n d i t i o n s . Coherent and r e p r o d u c i b l e r e s u l t s a r e o b t a i n e d showing t h e conve r g i n g and comple- mentary i n f o r m a t i o n o b t a i n e d f r o m t h e e l e c t r o n i c t r a n s i t i o n s and l a t t i c e v i b r a t i o n s .

I n c o n j u n c t i o n w i t h x - r a y a n a l y s i s and TEM, i n v e s t i g a t i o n o f o p t i c a l p r o p e r t i e s s h o u l d be a v e r y u s e f u l means f o r t h e c h a r a c t e r i z a t i o n o f p o l y c r y s t a l l i n e t h r o u g h t o amorphous f i l m s .

Acknowledgments. We t h a n k PIROES f o r i t s f i n a n c i a l s u p p o r t i n t h i s r e s e a r c h . We t h a n k P r o f . TUKCO f o r f a c i l i t i e s i n x - r a y d i f f r a c t i o n , Dr. KOFMAN f o r f a c i l i t i e s i n

t h e measurementof r e f l e c t i v i t y i n t h e near-UV and P r o f . RICHARD f o r u s e f u l d i s c u s s - i o n s . We a l s o thank GILLON and MAMO f o r t e c h n i c a l a s s i s t a n c e .

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0 0 1 BERREMAN D.W., Phys. Rev. 130 (1963) 21V3 -