THE OPTICAL AND STRUCTURAL PROPERTIES OF CVD GERMANIUM CARBIDE

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THE OPTICAL AND STRUCTURAL PROPERTIES OF CVD GERMANIUM CARBIDE

D. Booth, K. Voss

To cite this version:

D. Booth, K. Voss. THE OPTICAL AND STRUCTURAL PROPERTIES OF CVD GER- MANIUM CARBIDE. Journal de Physique Colloques, 1981, 42 (C4), pp.C5-1033-C5-1036.

�10.1051/jphyscol:19814226�. �jpa-00220856�

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T H E O P T I C A L AND STRUCTURAL P R O P E R T I E S O F CVD GERMANIUM C A R B I D E

D.C. B o o t h and K.J. Voss

CpticaZ Sciences Center, U n i v e r s i t y of Arizona, Tucson, Arizona 85721, U.S.A.

A b s t r a c t .

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Germanium c a r b i d e f i l m s , GexC1-,, have been produced by c h e m i c a l vapor d e p o s i t i o n (CVD) f r o m acetylene-germane m i x t u r e s a t s u b s t r a t e tempera- t u r e s o f 475 C t o 550 C and {C2H21/{GeH4

+

C2H21 r a t i o s o f 0% t o 75%. O p t i - c a l and s t r u c t u r a l measurements o f t h e s e germanium c a r b i d e f i l m s a r e r e p o r t e d a s a f u n c t i o n o f t h e acetylene-germane r a t i o . We f o u n d a decrease o f t h e v a l u e o f t h e r e f r a c t i v e i n d e x and a s h i f t o f t h e p r o f i l e o f t h e a b s o r p t i o n c o e f f i c i e n t r e l a t i v e t o t h e v a l u e s o f pc-Ge as t h e a c e t y l e n e gas f r a c t i o n

i n c r e a s e s . X - r a y d i f f f r a c t i o n ( X R D ) measurements show s t r o n g p o l y c r y s t a l l i n e germanium peaks f o r l o w c a r b o n c o n t e n t , b u t t h e i n t e n s i t y o f t h e peaks de- c r e a s e s s i g n i f i c a n t l y as t h e a c e t y l e n e gas f r a c t i o n i n c r e a s e s . The GeXCl-, f i l m s t h u s appear t o be c e r m e t - l i k e compounds o f germanium and c a r b o n . The o p t i c a l a b s o r p t i o n p r o p e r t i e s o f t h e s e f i l m s suggest t h e i r use as a b s o r b e r

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

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

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F o r a s e l e c t i v e s u r f a c e t o c o n v e r t s o l a r r a d i a t i o n i n t o h i g h tem- p e r a t u r e h e a t e f f i c i e n t l y , i t must be c a p a b l e o f h i g h a b s o r p t i o n o v e r t h e s o l a r s p e c t r u m and l o w t h e r m a l e m i s s i o n l o s s e s i n t h e i n f r a r e d . I n t e r e s t was f i r s t d i - r e c t e d t o w a r d p o l y c r y s t a l l i n e s i l i c o n a b s o r b e r s , b u t t h e i n d i r e c t gap o f t h i s m a t e r - i a l r e s u l t s i n a s h a l l o w a b s o r p t i o n edge, r e q u i r i n g s i g n i f i c a n t t h i c k n e s s e s f o r t h e a b s o r p t i o n o f t h e s o l a r spectrum. F o r a g i v e n t h i c k n e s s , t h e s o l a r a b s o r p t a n c e o f amorphous s i l i c o n i s c o n s i d e r a b l y l a r g e r t h a n t h a t o f t h e p o l y c r y s t a l l i n e m a t e r i a l .

I f d e p o s i t e d by t h e p y r o l y t i c d e c o m p o s i t i o n o f s i l a n e ( C V D ) , improved s t r u c t u r a l s t a b i l i t y o f t h e amorphous phase can be o b t a i n e d by t h e c o d e p o s i t i o n o f c a r b o n . ) 9 2

The a b s o r p t i o n edge o f amorphous germanium i s even b e t t e r matched t o t , h e s o l a r spec- t r u m . However, a l o w c r y s t a l l i z a t i o n t e m p e r a t u r e o f 300 C t o 400 C has l i m i t e d t h e use o f germanium as a s o l a r a b s o r b e r t h u s f a r . Our p r e v i o u s work on a - S i suggested an a t t e m p t a t s t r u c t u r a l s t a b i l i z a t i o n by a l l o y i n g c a r b o n i n t o t h e germanium m a t r i x . We p r e s e n t s t r u c t u r a l and o p t i c a l d a t a on a s - d e p o s i t e d CVD Ge:C f i l m s , and compare t h e i r s t r u c t u r a l and o p t i c a l p r o p e r t i e s w i t h t h o s e o f CVD a-Si:C.

Sample P r e p a r a t i o n . - GeXC1-, f i l m s were p r e p a r e d by t h e p y r o l y t i c d e c o m p o s i t i o n o f germane (GeH4) and a c e t y l e n e (C2H2) m i x t u r e s i n a h o r i z o n t a l , r a d i a t i o n h e a t e d , a t m o s p h e r i c p r e s s u r e r e a c t o r . ' The germane was p r e s e n t as a 5% m i x t u r e i n u l t r a - h i g h p u r i t y (UHP) He, w h i l e t h e a c e t y l e n e was p r e s e n t as a 1 % m i x t u r e i n UHP He.

F o r a p p r o x i m a t e l y one h a l f o f t h e f i l m s , commercial grade He was used as t h e c a r r i e r gas. F o r t h e r e m a i n d e r , an 8 . 5 % H2

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91.5% He (UHP) m i x t u r e was used as t h e c a r r i e r .

I n e i t h e r case, t h e c a r r i e r f l o w r a t e was 4 I/m. Net f l o w r a t e s f o r t h e germane and a c e t y l e n e v a r i e d f r o m 1 cc/m t o 50 cc/m, and t h e t o t a l system f l o w r a t e was 4.2

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^{5 l/m.} The s u b s t r a t e s were 25mm X 25mm X lmm f u s e d q u a r t z . The s u b s t r a t e t e m p e r a t u r e , TS, v a r i e d from 475 C t o 550 C, a c c u r a t e t o + 5 C . F i l m o t i c a l t h i c k - nesses were m ~ n i t o r e d ~ i n s i t u by a s i n g l e beam r e l a t i v e r e f l e c t o r n e t e r ; ' D e p o s i t i o n r a t e s v a r i e d from 20 A/m f o r t h e h i g h e s t a c e t y l e n e gas r a t i o s t o 800 A/m f o r t h e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814226

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C4- 1034 JOURNAL DE PHYSIQUE

Sample C h a r a c t e r i z a t i o n .

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F i l m t h i c k n e s s e s were d e t e r m i n e d by e t c h i n g s t e p s i n t h e f i l m s u s i n g a 10% HF

-

90% HNO3 s o l u t i o n . The s t e p s were measured i n s e v e r a l p l a c e s t o a n - a c c u r a c y o f + l 0 0

1

w i t h a Dektak p r o f i l o m e t e r , w i t h t h i c k n e s s e s r a n g i n g f r o m 1500 A t o 1.2 urn. F i l m adhesion was determined, i n d i r e c t l y , u s i n g t h e " s c o t c h tape"

t e s t d u r i n g s t e p e t c h i n g . F i l m s w i t h a l a r g e c a r b o n c o n t e n t were q u i t e s o f t and i n s e v e r a l cases p o r t i o n s o f t h e f i l m c o u l d b e p u l l e d o f f c l e a n l y f r o m t h e s u b s t r a t e . T h i s c o n t r a s t e d s h a r p l y w i t h b o t h a-Si:C and pc-Ge f i l m s w h i c h were i n a l l cases t i g h t l y a d h e r e n t .

X-ray d i f f r a c t i o n measurements were made on a General E l e c t r i c XRD-5 u s i n g a h i g h i n t e n s i t y Cu t a r g e t , a 3O s o u r c e s l i t , a 0 . 2 ~ d e t e c t o r s l i t , and a N i f i l t e r . A p o l y c r y s t a l l i n e CVD Ge f i l m was used f o r a r e f e r e n c e and c a l i b r a t i o n s t a n d a r d t o c o r r e c t f o r machine d r i f t induced e r r o r s . I n g e n e r a l , t h r e e pc-Ge XRD peaks ( ,

<220>, <311>) were d e t e c t e d i n a l l f i l m s p r e p a r e d . A l t h o u g h a d d i t i o n a l peaks were d e t e c t e d i n t h o s e f i l m s w i t h s m a l l carbon c o n c e n t r a t i o n s , comparisons were made o n l y on t h e b a s i s o f t h e s e t h r e e peaks. I n a d d i t i o n t o n o r m a l i z i n g t h e XRD i n t e n s i t i e s o f each sample a g a i n s t t h e r e f e r e n c e , t h e i n t e n s i t i e s o f t h e XRD peaks o f each sam- p l e were n o r m a l i z e d t o t h e maximum i n t e n s i t y o f t h e p a r t i c u l a r sample. I n t h i s man- n e r i t was p o s s i b l e t o d e t e r m i n e i f any p r e f e r e n t i a l o r i e n t a t i o n e x i s t e d i n t h e s e p o l y c r y s t a l l i n e samples.

O p t i c a l a n a l y s i s was p e r f o r m e d u s i n g a PE 450 (0.35 - 2.5 um) and a PE 137 (2.5

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15 urn) d o u b l e beam s p e c t r o p h o t o m e t e r , u s i n g an aged aluminum r e f e r e n c e f o r r e f l e c - t a n c e measurements and an a i r r e f e r e n c e f o r t r a n s m i t t a n c e measurements. S p e c t r a l c u r v e s were reduced u s i n g a w e l l known e q u a t i o n i n v o l v i n g t h e t r a n s m i t t a n c e , t h i c k - ness, and a i r - f i lm, f i lm - s u b s t r a t e , and s u b s t r a t e - a i r r e f l e c t a n c e s .

E x p e r i m e n t a l R e s u l t s .

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The p r e p a r a t i o n o f CVD GexC~-, f i l m s i s u n i q u e compared t o t h a t o f a-Ge, a - S i , o r a-Si:C f i l m s . A f o r m o f GeC analagous t o S I C i s n o t known t o e x i s t as a n a t u r a l l y o c c u r r i n g c r y s t a l , and t h e r e a r e o n l y a few r e p o r t s o f G ~ , C I - ~ f i l m s produced by non-CVD methods such as g l o w d i s c h a r g e . Such g l o w d i s - charge f i l m s have been p r e p a r e d a t a m u c h l o w e r s u b s t r a t e t e m p e r a t u r e t h a n t h a t used f o r C V D ~ , ~ , and a r e t h u s expected t o c o n t a i n s i g n i f i c a n t amounts o f hydrogen.

I n s i l i c o n - c a r b o n f i l m s p r e p a r e d by CVD, i t was p r e v i o u s l y c o n c l u d e d t h a t t h e S i and C e x i s t e d i n a homogeneous m a t r i x w i t h o u t any s u b s t a n t i a l C-C b o n d i n g , l as m i g h t be i n d i c a t e d by XRD and o p t i c a l measurements. These same measurements show a much d i f f e r e n t b e h a v i o r f o r CVD GexC1-,. X-ray d i f f r a c t i o n measurements showed, as a f u n c t i o n o f carbon c o n t e n t , t h a t t h e m a t e r i a l was n e i t h e r an amorphous n o r a c r y s t - a l l i n e compound. A l l f i l m s e x h i b i t e d pc-Ge XRD peaks, b u t t h e number o f s t r o n g peaks and t h e i r i n t e n s i t i e s decreased m a r k e d l y as t h e c a r b o n c o n t e n t i n c r e a s e d . F i g u r e 1 shows t h e XRD i n t e n s i t i e s f o r a number o f GeXC1-, samples n o r m a l i z e d t o t h e peak i n t e n s i t i e s o f a CVD pc-Ge s t a n d a r d . The t h r e e s t r o n g XRD l i n e s a r e t h e ,

<220>, and <31 l > , and t h e s e were used f o r comparison among t h e samples measured. We see i n t h i s f i g u r e t h a t f o r a l l t h r e e XRD peaks, t h e n o r m a l i z e d i n t e n s i t y decreases w i t h i n c r e a s i n g c a r b o n c o n t e n t t o a f i n a l r e d u c t i o n t o 10% t o 15%, f o r t h e sample w i t h t h e h i g h e s t c a r b o n c o n c e n t r a t i o n . The decreases appear s t r o n g e r f o r t h e <220>

and <311>, a r e s u l t s u p p o r t e d by F i g u r e 2. I n t h i s f i g u r e , t h e XRD i n t e n s i t i e s o f a g i v e n sample were n o r m a l i z e d t o t h e maximum v a l u e w i t h i n t h a t sample, n o r m a l l y t h e t h e peak. We n o t e t h a t as t h e a c e t y l e n e gas f r a c t i o n i n c r e a s e s , t h e f i l m s show more p r e f e r r e d o r i e n t a t i o n i n t h e d i r e c t i o n . We a l s o p o i n t o u t t h a t t h e CVD pc-Ge r e f e r e n c e sample had XRD i n t e n s i t i e s whose r a t i o s f o r t h e t h r e e peaks o f

i n t e r e s t were v e r y c l o s e t o t h o s e expected f o r a random p o l y c r y s t a l l i n e sample( i .e., powder sampl e )

.

The o p t i c a l p r o p e r t i e s show s y s t e m a t i c v a r i a t i o n s i n t h e i r v a l u e s w i t h changes i n a c e t y l e n e gas f r a c t i o n . F i g u r e 3 shows t h e s h i f t o f t h e a b s o r p t i o n c o e f f i c i e n t p r o f i l e . F o r a s u b s t r a t e t e m p e r a t u r e o f 500 C and a He c a r r i e r , t h e p r o f i l e s h i f t s s y s t e m a t i c a l l y , a l t h o u g h s l o w l y , t o l o w e r e n e r g i e s w i t h i n c r e a s i n g a c e t y l e n e gas f r a c t i o n . The magnitude and d i r e c t i o n o f s h i f t a r e q u a l i t a t i v e l y s i m i l a r t o t h o s e observed f o r a-Si:C a l l o y s p r e p a r e d a t 630 C . 1 ' 2 Because o f t h e much h i g h e r absorp- t i o n o f t h e GeXCl-, f i l m s , i t was n o t p o s s i b l e t o d e t e r m i n e i f a low energy t a i l e x i s t s s i m i l a r t o t h a t f o u n d i n a-Si:C a l l o y s . I f Ts i s i n c r e a s e d t o 550 C , t h e n we n o t e a s h i f t o f t h e a b s o r p t i o n c o e f f i c i e n t p r o f i l e t o h i g h e r e n e r g i e s , p a r t i c u -

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F i g u r e 1. XRD peak i n t e n s i t i e s as a F i g u r e 2. Increased o r i e n t a t i o n f u n c t i o n o f a c e t y l e n e gas f r a c t i o n . w i t h i n c r e a s i n g a c e t y l e n e gas f r a c t i o n .

l a r l y f o r t h e h i g h e r a c e t y l e n e gas f r a c t i o n s . Even i n t h i s case, however, t h e p r o f i l e s l i e f u r t h e r t o lower e n e r g i e s than do those o f a-Si o r a-Si:C. T h i s h i g h e r a b s o r p t i o n ( f o r a g i v e n t h i c k n e s s ) g i v e s these GeXCl-, f i l m s a s t r o n g appeal as ab- s o r b e r m a t e r i a l s i n photothermal s o l a r energy conversion. We should n o t e t h a t s i n c e t h e GexCl-x f i l m s a r e a l l p o l y c r y s t a l l i n e t o some degree, t h e y may n o t r e q u i r e s t r u c t u r a l s t a b i l i z a t i o n as was t h e case f o r a-Si i n o r d e r t o m a i n t a i n t h e i r f a v o r - a b l e a b s o r p t i o n c h a r a c t e r i s t i c s .

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The r e f r a c t i v e index a l s o showed s t r o n g and s y s t e m a t i c changes as a f u n c t i o n o f a c e t y l e n e gas f r a c t i o n , as shown i n F i g u r e 4. I n c o n t r a s t t o changes i n t h e absorp- t i o n c o e f f i c i e n t , t h e r e f r a c t i v e index always decreases w i t h i n c r e a s i n g a c e t y l e n e , even f o r h i g h e r Ts. The r e f r a c t i v e index p r o f i l e s do n o t change shape markedly, b u t o n l y s h i f t t o lower values. At t h e h i g h e r Ts o f 550 C , t h e index decreases f a s t e r than f o r 500 C, t h e a c e t y l e n e gas f r a c t i o n b e i n g h e l d c o n s t a n t . T h i s t y p e o f decrease o f t h e index w i t h i n c r e a s i n g carbon c o n t e n t f o r GeXC1-, f i l m s has a l s o been observed i n CVD a-Si:C a l l ~ y s . l + ~

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D i s c u s s i o n and Conclusions.

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The s t r u c t u r a l r e s u l t s seem t o i n d i c a t e t h a t t h e G~,CI-~ f i l m s a r e c e r m e t - l i k e , w i t h pc-Ge c l u s t e r s d i s t r i b u t e d i n a m a t r i x o f GeyCz m a t e r i a l . Whereas a-Si:C f i l m s were amorphous f o r Ts 900 C and f o r a l l carbon c o n c e n t r a t i o n s ! t h e GexCj-x f i l m s show pc-Ge XRD l ines. The i n t e n s i t y and l ine- w i d t h o f these peaks i n d l c a t e m i c r o c r y s t a l l i t e g r a i n s i z e s g r e a t e r than 100

-

200 f o r l a r g e a c e t y l e n e gas f r a c t i o n s , and p r o g r e s s i v e l y l a r g e r g r a i n s i z e s f o r s m a l l e r gas f r a c t i o n s . I f indeed t h e germanium c a r b i d e f i l m s a r e c e r m e t - l i k e , then i t should be p o s s i b l e t o e x p l a i n t h e r e d s h i f t s o f t h e a b s o r p t i o n c o e f f i c i e n t by a d i l u t i o n e f f e c t . I n such a case, o n l y t h e pc-Ge c l u s t e r s i n t e r a c t w i t h t h e l i g h t

r e s u l t i n g i n a b s o r p t i o n , w h i l e t h e amorphous Ge:C r e g i o n s a r e r e l a t i v e l y t r a n s p a r - e n t . T h i s argument does n o t , however, h o l d f o r t h e r e f r a c t i v e index. A d i l u t i o n argument r e q u i r e s t h a t t h e s h i f t o f t h e r e f r a c t i v e index be i n t h e o p p o s i t e d i r e c - t i o n t o t h a t e x p e r i m e n t a l l y observed. I t t h u s seems more l i k e l y t h a t composition i s p l a y i n g t h e dominant r o l e i n d e t e r m i n i n g t h e o p t i c a l p r o p e r t i e s . and t h a t chemical

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C4-1036 JOURNAL DE PHYSIQUE

F i g u r e 3. A b s o r p t i o n c o e f f i c i e n t as a F i g u r e 4. R e f r a c t i v e i n d e x as a f u n c t i o n f u n c t i o n o f a c e t y l e n e gas f r a c t i o n and o f a c e t y l e n e gas f r a c t i o n and T s .

Ts.

b o n d i n g e f f e c t s must be t a k e n i n t o account t o p r o p e r l y d e s c r i b e t h e o p t i c a l r e - sponse o f t h e s e m a t e r i a l s .

The a r e a o f t h i n f i l m a l l o y s , m i x t u r e s , c e r m e t s , and so f o r t h has become o f g r e a t i n t e r e s t . Chemical v a p o r d e p o s i t i o n i n p a r t i c u l a r has g r e a t promise as a method p r e e m i n e n t l y s u i t e d f o r t h e p r e p a r a t i o n o f such m a t e r i a l s . W h i l e CVD d e p o s i t i o n o f p c - S i and e p i t a x i a l c - S i has l o n g been a m a i n s t a y o f t h e semiconductor i n d u s t r o n l y l a t e l y has t h e t e c h n i q u e been used t o p r e p a r e a - S i and a - S i a l l o y s . 1 , 2 , 6 , Y 7 Such t h i n f i l m a l l o y s / c e r m e t s a r e o f i n t e r e s t because t h e y r e p r e s e n t a c l a s s o f m a t e r i a l s f o r w h i c h t h e c o m p o s i t i o n a l , o p t i c a l , and e l e c t r i c a l p r o p e r t i e s can o f t e n be v a r i e d a t w i l l . T h i s c a p a b i l i t y a l l o w s f o r t h e p o s s i b i l i t y o f t a i l o r i n g a m a t e r i a l t o f i t t h e p a r t i c u l a r a p p l i c a t i o n a t hand. We have r e c e n t l y shown t h a t a-Si:C f i l m s a r e s t r u c t u r a l l y s t a b l e w h i l e m a i n t a i n i n g t h e f a v o r a b l e o p t i c a l p r o p - e r t i e s o f a-Si T h i s paper, r e p o r t i n g on a s i m i l a r s t u d y o f Ge-based a l l o y s , shows t h a t GexC1-x f i l m s can a l s o be made w i t h a v a r i e t y o f o p t i c a l , s t r u c t u r a l , and (presumably) c o m p o s i t i o n a l p r o p e r t i e s , depending on t h e e x a c t c h o i c e o f depo- s i t i o n p a r a m e t e r s .

Acknowledgments. - T h i s work s u p p o r t e d by t h e U.S. Department o f Energy, O f f i c e o f B a s i c Energy Sciences, under c o n t r a c t ER-78-S-02-4899.

References.

( 1 ) Booth, D.C., Ph.D. D i s s e r t a t i o n , U n i v e r s i t y o f A r i z o n a ( 1 9 8 0 ) .

( 2 ) Booth, D.C., D.D. AI l r e d , and B.O. S e r a p h i n , Sol Energy Mat

2

(1979) 107.

( 3 )

Connel l , G.A.N., R.J. Temkin, and W. Paul, Adv Phy 22 (1973) 643.

(4) Anderson, D.A. and W.E. Spear, P h i l Mag (1977)

K

( 5 ) Wieder, H . , M. Cardona, and C.R. G u a r n i e r i , Phy S t a Sol

B92

(1979) 99.

( 6 ) S o l , N., D. Kaplan, D. Dieumegard, and D . D u b r e u i l , J Non-Cryst Sol

35/36

(1980) 291.

(7) H i r o s e , M., M, T a n i g u c h i , T. N a k a s h i t a , Y . Osaka, T. S u z u k i , S. Hasegawa, and T. Shimizu, J Non-Cryst Sol

35/36

(1980) 297.