PHOTOEMISSION STUDIES OF THE TRANSITION FROM AMORPHOUS TO MICROCRYSTALLINE SILICON

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PHOTOEMISSION STUDIES OF THE TRANSITION FROM AMORPHOUS TO MICROCRYSTALLINE

SILICON

H. Richter, L. Ley

To cite this version:

H. Richter, L. Ley. PHOTOEMISSION STUDIES OF THE TRANSITION FROM AMORPHOUS

TO MICROCRYSTALLINE SILICON. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-261-C4-

264. �10.1051/jphyscol:1981455�. �jpa-00220912�

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JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6ment au nO1O, Tome 42, octobre 1981 page C4-261

PHOTOEMISSION STUDIES OF THE TRANSITION FROM AMORPHOUS TO MICROCRYSTALLINE SILICON

H. R i c h t e r and L. Ley

Max-PZrmck-Institut fur Festk8~perforschng, Heisenbergstrasse 1, 7000 Stuttgart 80,

F. R. G.

A b s t r a c t .

-

We have s t u d i e d a s e r i e s o f samples spanning t h e range from p u r e l y amorphous t o m i c r o c r y s t a l l i n e s i l i c o n prepared b y chemical t r a n s p o r t i n a hydrogen plasma o r by s p u t t e r i n g i n a H / ~ r mixture. The f i r s t o r d e r Raman s p e c t r a show a s u p e r p o s i t i o n o f amorphous and c r y s t a l l i n e c o n t r i b u t i o n , show- 2 i n g some f e a t u r e s of w u r t z i t e - s i l i c o n . The e l e c t r o n i c d e n s i t y of s t a t e s , a s deduced from X-ray p h o t o e l e c t r o n - s p e c t r o s c o p y , shows a g r a d u a l change from m i c r o c r y s t a l l i n e s t r u c t u r e f o r samples p r e p a r e d by chemical t r a n s p o r t t o t h e amorphous DOS f o r samples s p u t t e r e d i n p u r e argon.

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

-

Thin f i l m m i c r o c r y s t a l l i n e s i l i c o n ( I l c S i ) i s i n some r e s p e c t s an i n t e r m e d i a t e phase between m o n o c r y s t a l l i n e s i l i c o n (c-Si) and amorphous s i l i c o n

(a-Si)

.

There have been a number of a t t e m p t s t o e x p l a i n p h y s i c a l p r o p e r t i e s of a-Si by means of m i c r o c r y s t a l models. It h a s been e s t a b l i s h e d t h a t t h e d i l u t i o n of SiH

-

s p e c i e s w i t h hydrogen i n t h e plasma f a v o u r s t h e formation of m i c r o c r y s t a l s even a f t e m p e r a t u r e s below 3 0 0 ~ ~ .

We p r e s e n t h e r e a new approach t o t h e p r e p a r a t i o n of PC-Si based o n t h e sput- t e r i n g p r o c e s s . By v a r y i n g t h e H - c o n c e n t r a t i o n i n a H / A r s p u t t e r g a s , we o b t a i n

2 2

SF-films a t 2 5 0 ~ ~ t h a t r a n g e from a-Si t o PC-Si, a s shown by t h e i r Raman s p e c t r a . T h i s continuous t r a n s i t i o n o f f e r s t h e unique p o s s i b i l i t y t o s t u d y t h e changes i n e l e c t r o n i c s t r u c t u r e of t h e s e specimens through & o t o e l e c t r o n spectroscopy.

P r e p a r a t i o n . - For t h e p r e p a r a t i o n o f t h e PC-Si sample, we used t h e following two methods:

1. Chemical t r a n s p o r t i n hydrogen plasma o n t o s u b s t r a t e s h e l d a t 3C0°C, a s d e s c r i b e d i n 1 1 3

2. r e a c t i v e DC-sputtering of a s i l i c o n - s i n g l e - c r y s t a l t a r g e t (n-type, 8.1019 cm 3, i n a mixture o f H2 and A r , t o t a l p r e s s u r e 0.1-0.2 mbar. The s u b s t r a t e s were h e l d a t 2 5 0 ~ ~ ; t h e t a r g e t d i a m e t e r was

30 mm a t a d i s t a n c e o f a b o u t 25 mm from t h e s u b s t r a t e ; t h e d i s - c h a r g e c u r r e n t was k e p t a 2mA and t h e v o l t a g e between 1 and 2 kV.

I n t h e same r e a c t o r w e produced a-Si b y e i t h e r s p u t t e r i n g i n p u r e AK o r decomposing SiH i n a DC-plasma o n t o s u b s t r a t e s k e p t a t room t e m p e r a t u r e . A l l t h e samples were d e p o s i t e d on s t a i n l e s s s t e e l and on s i l i c o n - s i n g l e c r y s t a l s u b s t r a t e s , They were 4 t r a n s f e r r e d under vacuum i n t o a HP 5950 A XPS-spectrometer within a few minutes.

No contamination of oxygen, carbon o r o t h e r elements were d e t e c t e d i n any sample r e f e r r e d t o h e r e .

F i r s t - O r d e r Raman S p e c t r a .

-

The f i r s t o r d e r Raman spectrum p r o v i d e s a convenient method f o r d e t e r m i n i n g whetherla s i l i c o n f i l m i s c r y s t a l l i n e o r amorphous: a-Si -1 y i e l d s a broad hump a t 480 cm

,

while c - S i y i e l d s a s i n g l e sharp l i n e a t 522 cm

.

I n UC-Si (5 t o 20 nm l i n e a r dimension o f t h e c r y s t a l l i t e s l p r e p a r e d by chemical o r by l a s e r a n n e a l i n g o f a - S i 7 , t h e 522 cm-l l i n e i s broadened up t o a f a c t o r o f 2 and s h i f t e d t o lower e n e r g i e s by u p t o 7 cm with d e c r e a s i n g c r y s t a l - l i t e s i z e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981455

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C4-262 JOURNAL DE PHYSIQUE

These e f f e c t s have been e x p l a i n e d by a t h i n s l a b model8 o r by a q-vector r e l a -

+

x a t i o n model6 and w i l l n o t be d i s c u s s e d f u r t h e r h e r e , In F i g . 1 t h e Raman s p e c t r a f o r some of the f i l m s a r e p l o t t e d . A l l s p e c t r a were taken under the same c o n d i t i o n s : 0.2 W l a s e r power, 5145

2

wavelength, c y l i n d r i c a l o p t i c s was used t o avoid h e a t i n g by t h e i n c i d e n t l a s e r l i g h t , s p e c t r o m e t e r r e s o l u t i o n 4 cm (2 cm-l f o r samples a , b)

.

RAMAN -SHIFT (cm-l )

Fig. 1 :

First-Order-Raman-Spectra of d i v e r s e s i l i c o n sample.

The l e t t e r s r e f e r t o F i g . 2. For c l e a r - ness a l l t h e s p e c t r a (except g ) have been a l i g n e d a t t h e maximum. The energy s c a l e t h e r e f o r e r e f e r s o n l y to t h e c-Si- sample

.

I n e

'

and f ' t h e amorphous-contribution has been s u b t r a c t e d . The arrow marks t h e hump a t 495 cm-I c h a r a c t e r i s t i c f o r w u r t z i t e S i .

For convenience t h e peaks ( e x c e p t for a - S i ) are a l i g n e d a t 522 cm-l. I n s p e c t r a c , e , f two major f e a t u r e s can be d i s t i n g u i s h e d :

a r e l a t i v e l y s h a r p l i n e c e n t e r e d a t about 520 cml and a-long i n t e n s e tail s t r e t c h i n g t o 400 cm

'.

The sharp l i n e i s a t t r i b u t e d t o t h e s c a t t e r i n g from t h e m i c r o c r y s t a l l i t e

,

while t h e o t h e r p a r t is due t o amorphous-like m a t e r i a l between t h e c r y s t a l l i t e s

. '

From a comparison of t h e i n t e n s i - t i e s of t h e c r y s t a l l i n e and amorphous s i g n a l , t a k i n g t h e Raman c r o s s s e c t i o n s i n t o account

,

we e s t i m a t e t h a t t h e amorphous phase a m u n t s t o %20% i n sample c and %40%

i n sample d , e and f

.

We f i n d a t h i r d component w i t h a hump a t 495 cm-' i n curve f . This s t r u c t u r e i s enhanced i n c u r v e f ' , which i s t h e d i f f e r e n c e between f and g a f t e r g was s c a l e d t o be &he same a s f a t 400 cm-l. T h i s peak i s c h a r a c t e r i s t i c o f w u r t z i t e microcry- s t a l s . T h i s i s e v i d e n c e f o r w u r t z i t e - l i k e s t r u c t u r e s i n sample f and probably t o a l e s s e r e x t e n t i n t h e o t h e r specimens.

XPS-Valence Band-Spectra and E l e c t r o n i c D e n s i t y of S t a t e s ,

-

The XPS-valence band (VB) s p e c t r a ( e x c i t e d w i t h hv = 1486 eV) r e f l e c t s t h e e l e c t r o n i c d e n s i t y o f s t a t e s (DOS) of t h e VB, a s i d e from p h o t o e l e c t r o n i c c r o s s s e c t i o n v a r i a t i o n s . By f o l d i n g t h e c a l c u l a t e d DOS w i t h t h e spectrometer r e s o l u t i o n f u n c t i o n , t h e s p e c t r a show a v e r y good agreement between t h e o r y and experiment e s p e c i a l l y f o r S i and &.lo

An advantage of u s i n g XPS i n t h e p r e s e n t c o n t e x t i s t h a t t h e c r o s s s e c t i o n of hydrogen induced s t a t e s i s n e g l i g i b l e compared t o t h a t o f t h e ~ i - s t a t e s . " Therefore t h e S i p a r t i a l d e n s i t y i s measured.

I n F i g . 2 t h e XPS-VB-spectra f o r s e v e r a l S i samples a r e shown, ranging from s i n g l e c r y s t a l a t t h e t o p t o a-Si and a-Si:H a t t h e bottom. The s p e c t r a a r e a l i g n e d a t t h e t o p o f t h e VB.

S p e c t r a a ( c - S i ) and b(!.ic-Si, chemical t r a n s p o r t ) e x h i b i t t h e t h r e e peak s t r u c - t u r e t h a t i s c h a r a c t e r i s t i c f o r t h e DOS of c - S i . l 4 T h i s means t h a t w i t h i n our reso- l u t i o n (0.6 eV) t h e r e i s no d i f f e r e n c e between t h e DOS o f s i n g l e c r y s t a l S i and of

~ c - S i p r e p a r e d by chemical t r a n s p o r t .

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F i g . 2:

XPS-Valence band-Spectra o f d i v e r s e S i l i c o n - samples

a : c-Si f i l e d i n UHV

b: PC-Si p r e p a r e d by chemical t r a n s p o r t c-g: S i prepared by s p u t t e r i n g i n H ~ / A ~ -

-

atmosphere

:

^H- c o n c e n t r a t i o n 50%,

lo%,

^5%,2.5%,

-

²

m ^0%

+

h: a-Si: H prepared by X-glow-discharge

Z i n SiH a t room-temperature

3 4 .

0 f o r d e t a l l s of p r e p a r a t i o n s e e t e x t .

0 A l l t h e s p e c t r a have been smoothed by using

t h e same F o u r i e r - f i l t e r procedure. There is no c o r r e c t i o n f o r i n e l a s t i c e l e c t r o n back- ground.

Spectrum g (a-Si) and h(a-Si:H) a r e v i r t u a l l y i d e n t i c a l . They a r e c h a r a c t e r i s t i c f o r t h e IDS o f a-Si in which peaks I1 + I11 have merged t o form a s i n g l e broad hump. The i n c o r p o r a t i o n of hydrogen along a p p a r e n t l y

15 10 5 0

does n o t l e a d to a s u f f i c i e n t r e l a x a t i o n of t h e amorphous network t o a l t e r t h e DOS s i g -

BINDINGENERGY ( e v )

n i f i c a n t l v . Reve sz

'

deduced from X-ray s c a t t e r i n g d a t a t h a t a - S i should undergo a t r a n s i t i o n from a diamond-like s t r u c t u r e t o a BC-%-like s t r u c t u r e when about 10% hydrogen i s i n c o r p o r a t e d . This change i n s t r u c t u r e , i f it o c c u r s , does n o t af f e c t t h e VB-spectrum. It remains i n doubt, how- e v e r , whether t h e DOS of t h e two s t r u c t u r e s ( d i a m n d and BC-8) a r e s u f f i c i e n t l y d i f - f e r e n t t o be o b s e r v a b l e w i t h t h e r e s o l u t i o n of 0.6 eV achieved h e r e .

Curves f t o c show t h e VB-Spectra of s p u t t e r e d S i - f i l m s with i n c r e a s i n g H- c o n c e n t r a t i o n s i n t h e s p u t t e r gas. I n c r e a s i n g H-concentration l e a d s t o a continuous t r a n s i t i o n towards t h e spectrum of c-Si. These c h a r g e s r e f l e c t m o d i f i c a t i o n s i n t h e S i p a r t i a l d e n s i t y o f s t a t e s s i n c e t h e H I s s t a t e s have n e g l i g i b l e p h o t o e l e c t r i c c r o s s s e c t i o n s , a s mentioned above. S t a r t i n g w i t h spectrum c , peak I1 d e c r e a s e s i n i n t e n s i t y u n t i l i n t h e a-Si sample, it h a s n e a r l y d i s a p p e a r e d , Simultaneously t h e minimum between I11 and I1 i s f i l l e d i n . Peak I v a r i e s o n l y a l i t t l e ; t h e VB- edge s t e e p e n s from c-Si t o a-Si. The g r a d u a l d i s a p p e a r a n c e o f I1 and t h e s t e e p e n i n g of I cannot be e x p l a i n e d by a s u p e r p o s i t i o n of t h e c r y s t a l l i n e and t h e a m r p h o u s s p e c t r a a l o n e . I n samples d t o f t h e amount of a m r p h o u s m a t e r i a l i s about t h e same, a s judged from Raman s p e c t r a , while t h e VB-spectra change c o n s i d e r a b l y .

Peak I1 i s caused by a v e r y s h a r p maximum i n t h e DOS o f c-Si due t o f l a t bands near L and X i n t h e B r i l l o u i n zone. A l o s s i n long range o r d e r broadens and d i m i n i s h e s t h e s h a r p maximum i n t h e ~ 0 s . l ~ I n t h i s s e n s e , t h e g r a d u a l l o s s of p a k

I1 r e f l e c t s a d e c r e a s e i n t h e s i z e o f t h e c r y s t a l l i t e s .

Changes i n t h e s h o r t o r i n t e r m e d i a t e range o r d e r have keen c o n s i d e r e d by Joan- nopoulos e t a1.14 u s i n g c r y s t a l l i n e polymorphs o f S i a s m d e l s f o r a-Si. They ar- gued t h a t t h e p r e s e n c e o f 5 and 7-fold r i n g s i n a-Si i s t h e m i n cause o f t h e merg- i n g of peak I1 and 111.

While t h i s miqht be t h e c a s e i n a-Si, we would l i k e t o s u g g e s t t h a t t h e i n c r e a s e d d e n s i t y of w u r t z i t e l i k e s t r u c t u r e elements seen i n t h e Ramn s p e c t r a of t h e s p u t t e r e d f i l m s c o n t r i b u t e s t o a r e d u c t i o n i n peak 11.

Fig. 3 compares t h e broadened DOS of t h e diamond and w u r t z i t e s t r u c t u r e s o f S i . I n t h e w u r t z i t e s t r u c t u r e , peak I1 i s s m a l l e r r e l a t i v e t o peak I and t h e v a l l e y between 11 and 111 i s a l s o reduced.

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C 4 - 2 6 4 JOURNAL DE PHYSIQUE F i g . 3:

C a l c u l a t e d DOS f o r d i a m n d - l i k e - S i ( 1 ) a n d w u r t z i t e - l i k e - S i ( 2 ) t a k e n from1' folded w i t h a G a u s s i a n - d i s t r i b u t i o n of 1.6 e V FWHM.

C o n c l u s i o n . - We have s h o w n that S i - f i l m s w i t h properties intermediate b e t w e e n those of PC-Si and a - S i c a n be m a d e by varying t h e H ~ / A ~ r a t i o i n a s p u t t e r a t m o s p h e r e . T h e R a m a n s p e c t r a are explaiwd by a s u p e r p o s i t i o n o f a m o r p h o u s and c r y s t a l l i n e p a r t s . T h e l a t t e r s h o w s o m e of t h e characteristics of t h e w u r t z i t e - s t r u c t u r e .

15 10 5 0

The VB DOS progresses f r o m t h e 3-peak struc-

BINDINGENERGY (eV

) ^{t u r e}^{of c-Si}^{t o}the 2 p a k - s t r u c t u r e s of a - S i as preparation c o n d i t i o n s are altered. T h i s c h a n g e can be explained by i n c r e a s i n g p o r t i o n s of a - S i i n t h e m a t e r i a l as w e l l as i n c r e a s i n g w u r t z i t e - l i k e regions.

A c k n o w l e d g e m e n t s .

-

We thank Z . P . W a n g f o r help w i t h t h e R a m a n m e a s u r e m e n t s , and W.

N e u f o r h i s t e c h n i c a l a s s i s t a n c e during t h e f i l m p r e p a r a t i o n .

L i t e r a t u r e .

-

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