GROWTH OF PLASMA-TRANSPORT MICROCRYSTALLINE SILICON AS STUDIED BY IN-SITU RAMAN SPECTROSCOPY

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GROWTH OF PLASMA-TRANSPORT

MICROCRYSTALLINE SILICON AS STUDIED BY IN-SITU RAMAN SPECTROSCOPY

H. Richter, L. Ley

To cite this version:

H. Richter, L. Ley. GROWTH OF PLASMA-TRANSPORT MICROCRYSTALLINE SILICON AS

STUDIED BY IN-SITU RAMAN SPECTROSCOPY. Journal de Physique Colloques, 1982, 43 (C1),

pp.C1-247-C1-251. �10.1051/jphyscol:1982134�. �jpa-00221791�

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JOURNAL DE PHYSIQUE

CoZZoque CI, supple'ment au n D l 0 , Tome 43, octobre 1982 page C1-247

GROWTH OF PLASMA-TRANSPORT MICROCRYSTALLINE S I L I C O N AS STUDIED BY I N - S I T U RAMAN SPECTROSCOPY

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

Mm-PZanck-Institut ftir Festk6rperforschung, Heinsenbergstr. 1, 7000 S t u t t g a r t 80, F. R. G.

Resume.

-

Nous avons e t u d i e par spectroscopie Raman i n - s i t u l a croissance d e c h e s de s i l ic i u m m i c r e c r i s t a l l i n e s preparees par t r a n s p o r t a s s i s t e par un plasma,

a

des temperature? de s u b s t r a t d ' e n v i r o n 300°C. Nous trouvons qu'au debut, l e s i l i c i u m c r o l t en couche amorphe. Puis, quand l ' e p a i s s e u r excede 15 nm, des c r i s t a l l i t e s apparaissent e t une croissance a d d i t i o n n e l l e augmente l e r a p p o r t e n t r e materiau c r i s t a l l i n e t amorphe.

Une attaque p o s t e r i e u r e de l a couche montre que l a p a r t i e i n i t i a l e m e n t amorphe s ' e s t c r i s t a l l i s e e en cours de croissance.

Les v a l e u r s du r a p p o r t Antistokes/Stokes mesurees pendant l a croissance de l a couche m e t t e n t en evidence que l e s temperatures l o c a l e s sont au moins de 300°C i n f g r i e u r e s l a temperature de c r i s t a l l i s a t i o n de l a couche amorphe, d e t e r - minee separiment comme e t a n t de 700°C, l a meme que pour l e s couches amorphes

@paisses de s i l i c i u m . Nous en concluons que l a c r i s t a l l i s a t i o n ne peut s'ex- p l i q u e r par un processus de r e c u i t thermique.

Nos r e s u l t a t s sont compares avec l a c r i s t a l l i s a t i o n

a

basse temperature du s i l i c i u m amorphe dans un plasma d'hydrogene observee p a r B u s t a r r e t e t a l . ( l ) e t d i s c u t e s en tenant compte du modele de c r i s t a l l i s a t i o n propose par Veprek e t a1

.

(2).

A b s t r a c t .

-

Using i n - s i t u Raman spectroscopy, we have s t u d i e d the growth of m i c r o c r y s t a l 1 i ne s il i c o n f i l m s prepared by plasma-assi sted t r a n s p o r t a t sub- s t r a t e temperatures around 3 0 0 ~ ~ . Me f i n d t h a t i n i t i a l l y t h e m a t e r i a l grows as an amorphous f i l m . A t thicknesses over about 15 nm, c r y s t a l l i t e s are detected. F u r t h e r growth increases the r a t i o of c r y s t a l 1 in e t o amorphous m a t e r i a l i n t h e f i l m .

Subsequent e t c h i n g o f t h e f i l m r e v e a l s a c r y s t a l l i z a t i o n o f the i n i t i a l l y amorphous l a y e r d u r i n g t h e growth process.

Measurements o f t h e A n t i stokes/Stokgs r a t i o d u r i n g f i l m growth i n d i c a t e t h a t l o c a l temperatures a r e a t l e a s t 300 C below the c r y s t a l l i z a t i o n tempegature o f t h e amorphous l a y e r , t h a t has been separately determined t o be 700 C, t h e same as t h a t o f t h i c k amorphous s i l i c o n f i l m s . We t h e r e f o r e conclude t h a t t h e c r y s t a l 1 i z a t i o n cannot be expl a i ned by a thermal annealing process.

The experimental f a c t s a r e compared w i t h t h e low temperature c r y s t a l l i z a t i o n o f amorphous s i l ic o n i n a hydrogen plasma observed by B u s t a r r e t e t a1

.

( 1 ) and they a r e discussed i n t h e l i g h t o f t h e c r y s t a l l i z a t i o n model proposed by Veprek e t a1 . ( 2 )

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

-

I n 1980 I q b a l e t a1 .(3) showedothat i t i s p o s s i b l e t o d e p o s i t c r y s t a l l i n e s i l i c o n a t temperatures as low as 200 C from the gas phase i n a plasma t r a n s p o r t process. Since then a number o f d i f f e r e n t methods have been developed t h a t g i v e a l s o m i c r o c r y s t a l l i n e s i l i c g n k c - S i ) w i t h t y p i c a l g r a i n sizes o f 10 nm a t d e p o s i t i o n temperatures around 300 C such as s p u t t e r i n g (4,5) and glow-dis- charge decomposition o f SiH4 (6,7,8)

.

T h i s new m a t e r i a l combines h i g h c o n d u c t i v i t y and doping e f f i c i e n c y ( 9 ) w i t h a pre- p a r a t i o n compatible w i t h t h a t o f amorphous

,

hydrogenated s i l icon (a-Si :H)

.

^{T h i s}

f a c t makes p c - S i a promi s i n g m a t e r i a l for appl i c a t i o n s 1 ik e h i g h c o n d u c t i v i t y e l e c - t r o d e s f o r a-Si:H s o l a r c e l l s (10).

The physical p r o p e r t i e s of c-Si such as s t r u c t u r e , o p t i c a l absorption, and elec-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982134

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JOURNAL DE PHYSIQUE

t r i c a l c o n d u c t i v i t y have been i n v e s t i g a t e d i n t e n s i v e l y (5-11). However, l i t t l e i s known as y e t about t h e d e t a i l s o f t h e growth process t h a t favours c r y s t a l l i z a t i o n a t tempe8atures several hundred degrees below t h e c r y s t a l l i z a t i o n temperature o f a-Si (%700 C).

In

t h i s paper we p r e s e n t the f i r s t i n - s i t u s t u d i e s o f t h e growth o f PC-Si f i l m s , u s i n g f i r s t o r d e r Raman spectroscopy. T h i s technique provides a f a s t a_nd con- v e n i e n t method t o determine t h e c r y s t a l l i n i t y o f a S i f i l m . A narrow (3.5 an ' ) l i n e a t 520 cm-I due t o t h e e x c i t a t i o n o f t h e

G=o

o p t i c a l phonons i s c h a r a c t e r i s t i c f o r c-Si, whereas-Raman spectra o f a-Si e x h i b i t a broad (50-80 cm ') hump centered around 430 cm

'.

T h i s hump a c t u a l l y r e f l e c t s t h e phonon d e n s i t y o f s t a t e s i n a-Si

( f o r d e t a i l s see f o r example (12) and references given t h e r e ) .

We have used t h e f i r s t o r d e r Raman-spectra t o monitor the r e l a t i v e c o n t r i b u t i o n s o f a-Si/uc-Si d u r i n g t h e growth o f t h e uc-Si f i l m s and a f t e r removing p o r t i o n s o f the f i l m s by plasma-etching o r s p u t t e r i n g .

The temperature o f t h e growing f i l m w s deduced from t h e i n t e n s i t y r a t i o o f t h e An- t i stokes /Stokes c o n t r i b u t i o n s t o t h e Raman spectrum.

2. Experimental.

-

I n - s i t u Raman spectra o f vc-Si f i l m s were obtained i n a set-up t h a t combines a conventional Raman spectrometer w i t h o u r plasma-transport r e a c t o r (see f i g . 1, f o r d e t a i l s o f t h e preparation systen see r e f . ( 9 ) ) .

Raman spectra were thus taken a t d i f - f e r e n t stages o f d e p o s i t i o n w i t h o u t ex- posi ng t h e growi ng f i l m to atmosphered

m n o c h o m ~ w The d e p o s i t i o n temperature T was 350 C

except f o r some o f t h e tempepature mea- s u r e k n t s an the growing f i l m where T,

, was 250 C.

A 100 nm :nolybdenum f i l m evaporated on p a r t s o f a p o l i s h e d s i l i c o n (111) surface served as a s u b s t r a t e w h i l e t h e un- covered p a r t o f the S i was used f o r a l i g m e n t o f t h e o p t i c a l system.

1.10 was chosen as a s u b s t r a t e because 1 ) t h e r e i s very 1 ittl e i n t e r d i g f u s i o n

between 110 and Si up t o 1000 C, 2) No and Si do n o t form e u t e c t i c s t h a t

c r y s t a l l i z e below 8 0 0 ~ ~ (13), 3 ) i n the freqyency range between 400

and 600 cm

,

Flo has a s t r u c t u r e l e s s 1 ow s c a t t e r i n g background.

The f i l m s could also be etched i n a pure hydrogen plasma o r by Ar i o n sput- t e r i n g (150 PA/& a t 280 V) a t room temperature.

The Raman spectra were e x c i t e d w i t h an

]

Ar l a s e r u s i n ? 200-400 mW power, f o - cussed w i t h a c v l i n d r i c a l l e n s on the samples i n ord& t o a v o i d l o c a l heating.

2 2 0 ~ Between wavelength i n t h e range 457.9 nm

t o 514.5 nm o f t h e e x c i t i n g l a s e r , no F i g . 1 : Experimental set-up f o r i n - s i t u s i g n i f i c a n t d i f f e r e n c e s i n t h e Raman Ramanspectroscopy o f u c - S i f i l m s . spectra were found. A l l spectra shown

MI,

M2 p l a n a r m i r r o r s here are thus taken w i t h t h e 488 nm-la- L 1 150 m c y l i n d r i c a l l e n s s e r - l i n e w i t h a r e s o l u t i o n o f 10 cm

.

L2

135 mn f / 2 camera l e n s The p e n e t r a t i o n depth f o r t h i s wavelength i s o f the order o f 100 nm i n PC-Si ( 9 ) . The i n t e n s e and h i g h l y s t r u c t u r e d background o f v i s i b l e l i g h t emitted by t h e hydrogen plasma (14) made i t necessary t o take d i f f e r e n c e spectra i n those cases where t h e Raman spectrum was measured d u r i n g f i l m growth. T h i s was achieved by chopping t h e e x c i t i n g l a s e r l i g h t and b y s u b t r a c t i n g the " l a s e r off"-counts from the " l a s e r on"-counts i n m e d i a t e l y i n a multichannel analyser.

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3. Results and Discussion.

-

F i g . 2 shows a t y p i c a l sequence o f Raman spectra o f a S i f i l m taken a t v a r i o u s stages o f the d e p o s i t i o n . The thickness o f t h e f i n a l f i l m was measured w i t h a mechanical thickness gauge t o be 0.45 urn. Assuming a constant growth

r a t e o f 0.1 nm/s, t h e t h i c k n e s s o f t h e f i l m corresponding t o t h e spectra i n f i g . 2 can be estimated. It i s obvious from the spectrum taken a f t e r 80s, t h a t approximately

t h e f i r s t 8 nm o f t h e S i f i l m are p u r e l y amorphous. This i s t o o u r know1 edge the f i r s t experimental evidence t h a t PC-Si s t a r t s growing amorphous.

1 - During f u r t h e r d e p o s i t i o n the c r y s t a l l i n e Raman l i n e benins t o show up. A f t e r about 4000s a f i n a l spectrum evolves t h a t does n o t change w i t h f u r t h e r d e p o s i t i o n . The r a t i o o f " c r y s t a l l i n e " ( a t 520 cm

'

⁾

versus "amorphous. ( a t 480 cm ' ) Raman i n t e n s i t y , Ic/Ia, i s p l o t t e d i n f i g . 3.

F i g . 3 : I n t e n s i t y o f c r y s t a l l i n e v e r - sus amorphous Raman 1 in e Ic/Ia versus

LOO LLO LBO 520 560 f i l m thickness.

RAMAN SHIFT (cm-')

X d u r i n g growth o f the f i l m . The t h i c k - ness i s c a l c u l a t e d from t h e d e p o s i t i o n Fig. 2 : Raman spectra o f s i l i c o n f i l m s t i m e assuming a constant growth r a t e .

taking

a f t e r successive deposi t i o n . 0 d u r i n g Hz-plasma-etching o f the f i l m . t i s t h e accumulated d e p o s i t i o n time.

dep The thickness o f the i n i t i a l f i l m i s

The spectra o f f i l m s w i t h t

'

^300s

a r e normalized t o 1, ~ p e c t r $ ~ ~ 1 6 0 s and estimated from the d e p o s i t i o n t i m e and t h e r a t i o Ic/Ia. A f t e r e t c h i n g 80s a r e the same s c a l e a s spectrum

300s. From t h e thickness o f t h e f i n a l the t h i c k n e s s of t h e f i l m i s estimated f i l m o f 0.45 um we c a l c u l a t e a growth from the t o t a l Raman i n t e n s i t y . r a t e o f 0.1 nm/s.

The s a t u r a t i o n value o f I /I i s determined by t h e b u l k r a t i o o f c r y s t a l l i t e s and amorphous i n a t e r i a l . It vaFie!! w i t h d e p o s i t i o n parameters (11).

I n order t o study whether o r n o t an amorphous l a y e r remains near t h e s u b s t r a t e a f t e r f u r t h e r d e p o s i t i o n , we deposited a PC-Si f i l m o f approximately 40 nm thickness under t h e same d e p o s i t i o n c o n d i t i o n s . When t h i s f i l m was etched under t h e i n f l u e n c e of a hydrogen plasma, we measured t h e sequence o f Raman spectra p l o t t e d i n f i g . 4 .

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JOURNAL DE P H Y S I Q U E

( ~ 3 0 0 sec). This i m p l i e s t h a t t h e r a t i o Ic/Ia i n f i g . 3 represents the f r a c t i o n o f c r y s t a l 1 i n e t o Fig. 4 : Raman spectra o f s i l i c o n f i l m s amorphous m t e r i a1 throughout t h e t a k e n a f t e r successive H2-pl a s m etching. f i l m a t each stage. We can, a t A1 1 s p e c t r a are p l o t t e d on the same scale. present, n o t decide whether the i n -

crease i n Ic/Ia r e f l e c t s an i n - crease i n t h e number o f c r y s t a l T i t e s o r i n t h e i r s i z e . T h i s decisfon would be i n p r i n c i p l e p o s s i b l e a s e d on t h e p o s i t i o n and w i d t K 3 the 520 cm- component o f t h e Raman spectrum (12). However, t h e h i g h s e n s i t i v i t y needed i n these experiments pre- vented t h e necessary s p e c t r a l r e s o l u t i o n (Q cm I ) .

We can dismiss thermal c r y s t a l l i z a t i o n of t h e i n i t i a l l y amorphous 8 nm o f t h e f i l m s f o r t h e f o i l o w i n g reasons. The macroscopic temperature o f t h e growing f i l m never ex- ceeded 380 C as measured by a thermocouple attached t o the s u b s t r a t e surface. A 1 oweri ng o f t h e c r y s t a l 1 iz a t i o n temperature due t o i n t e r n a l stresses f o r example was n o t observed. Taking an nm amorphous l a y e r through a s e r i e s o f annealing steps f o r two minutes a t each temperature c r y s t a l l i z a t i o n s e t i n a t 7 3 0 ' ~ i n agreement w i t h t h e c r y s t a l l i z a t i o n o f t h i c k amorpholts f i l m s . The two minutes a r e equal t o t h e t i m e needed t o observe t h e f i r s t s i g n o f c r y s t a l l i t e s i n t h e growing f i l m (compare F i a . 21.

The r e d u c t i o n i n Raman i n t e n s i t y w i t h e t c h i n g time i n d i c a t e s t h a t the PC-Si specimen i s thinned under t h e a c t i o n o f the hydrocpn p l asma. Note, however,

I- t h a t t h e etch r a t e i s two t o t h r e e

times t h a t o f t h e d e p o s i t i o n r a t e des- p i t e the f a c t t h a t t h e plasma d e n s i t y i s reduced by a f a c t o r 0.04 compared t o t h e d e p o s i t i o n o f f i g . 2.

Also and m s t important the f i l m r e t a i n s a c r y s t a l l i n e component down t o t h e l a s t few nanometers and t h e r e i s no s i g n o f an increase i n t h e amorphous compo- n e n t as we approach t h e s u b s t r a t e . I n f a c t , t h e r a t i o Ic/Ia remains v i r t u a l l y constant through t h e e t c h i n g s e r i e s as i n d i c a t e d i n F i g . 3 b y t h e open c i r c l e s . I n general, our data i n d i c a t e t h a t t h e r a t i o Ic/Ia remains c o n s t a n t w i t h e t c h i n g , regardless where we s t a r t on t h e d e p o s i t i o n curve o f F i g . 3.

lcle i n t e r p r e t these r e s u l t s i n t h e f o l -

a - I -

The macroscopic temperature measurement does n o t exclude a s t r o n g l o c a l heating i n t h e f i l m . We have t h e r e f o r e s t u d i e d the f i l m temperature d u r i n q t h e arowth o f PC-Si

0 2

0'

by measuring t h e r a t i o o f t h e Stokes RS and Antistokes RAS

ama an

i n t e n s i t y . T h i s r a t i o RS/RAS i s given by (15)

f$/RAS = C

.

exp (hwo/KT),

LOO 440 480 520 560 i t takes t o grow t h e f i r s t 30 nm RAMAN SHIFT (cm-1) which were n o t i nvestiqated

l o w i n g way:

145s ( i ) The i n i t i a l l y amorphous p o r t i o n o f

the f i l m c r y s t a l l i z e s as the f i l m

where hw i s t h e phonon frequency, T i s t h e absolute temperature, C i s determined by absorfhion c o e f f i c i e n t s , frequencies, and cross-sections o f the Stokes asd A n t i

-

stokes-1 i g h t . From a measurement of f$/RAS a t a known temperature (here 250 C, w h i l e t h e plasma was o f f ) the preexponenti a1 C has been determined. From measure-

180s

, I l l , l

grows.

(ii ) The f i l m i s homogeneous a t each stage except p o s s i b l y f o r t h e time

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merits o f RS/RAS d u r i n g f i l m growth we obtained a temperature r i s e o f 40

+

3 0 ' ~ t o

- . .-

290

+

3 0 ' ~ d u r i n g t h e d e p o s i t i o n process i n agreement w i t h t h e thermocouple measurements. A temperature r i s e approaching c r y s t a l 1 i z a t i o n temperature even i n a 10 nm s u r f a c e l a y e r could be excluded as w e l l f r c m a d e t a i l e d a n a l y s i s o f t h e shape and po- s i t i o n o f spectra.

4. Conclusions.

-

The r e s u l t s presented here a l l o w the f o l l o w i n g conclusions t o be made about t h e growth o f PC-Si from the plasma t r a n s p o r t process. The f i l m s s t a r t t o grow i n i t i a l l y w i t h no d e t e c t a b l e c r y s t a l 1 i n i t y up t o a thickness o f about 8 nm. Fur- t h e r growth proceeds i n such a way t h a t m i c r o c r y s t a l s are formed and the r a t i o o f c r y s t a l 1 in e t o amorphous m a t e r i a l increases u n t i 1 a c r y s t a l 1 i n i t y i s reached t h a t depends on t h e d e p o s i t i o n parameters. The e t c h i n g experiments show t h a t a t each

stage t h e c r y s t a l 1 i n i t y o f the f i l m i s homogeneous i n c l u d i n g t h e i n i t i a l l y amorphous 8 nm. T h i s r e q u i r e s a c r y s t a l 1 i z a t i o n process t h a t i s n o t l i m i t e d to the growing surface alone. Temperature measurements u s i n g Raman techniques d u r i n g f i l m growth and annealing s t u d i e s exclude a thermal c r y s t a l 1 i z a t i o n process o f t h e amorphous 1 ayer.

These observations are corrgborated by p o s t d e p o s i t i o n c r y s t a l l i z a t i o n o f CVD a-Si i n a hydrogen plasma a t 400 C by B u s t a r r e t e t a1

.

(1). C r y s t a l l i t e s a r e detected t o a depth o f a t l e a s t 500 nm and t h e i r s i z e increases w i t h exposure t o the plasma up t o 10 nm.

There i s l i t t l e doubt, t h e r e f o r e , t h a t a h i g h d e n s i t y hydrogen plasma plays t h e es- s e n t i a l r o l e i n these low temperature c r y s t a l l i z a t i o n phenomena. Veprek e t a l . (2) suggest t h a t p a r t o f the energy released b y the recombination o f hydrogen atoms a t t h e surface t o form H molecules provides the " a c t i v a t i o n energy" f o r n u c l e a t i o n and growth o f c r y s t a l l i t & by non-thermal p p u l a t i o n o f zone boundary phonons. Our r e s u l t s and those o f B u s t a r r e t e t a l

.

r e q u i r e , however, t h a t t h e energy i s t r a n s - m i t t e d up t o 500 nm i n t o t h e f i l m . I t i s a t present n o t c l e a r whether such an energy t r a n s f e r could take p l a c e w i t h o u t a t h e r m a l i z a t i o n of t h e phonons t h a t would show up i n t h e Raman spectrum.

5. References.

( 1 ) BUSTARRET E., B. RANCHOUX, H. HMDI, A. DENEUVILLE, S. HUANT, and P. DEPELSE- NAIRE, Proc. 16th I n t

.

^Conf

.

on t h e Physics o f Semi cond

. ,

^Montpell i e r , 1982 ( i n press).

( 2 ) VEPREK, S., Z . IQBAL, H.R. OSWALD, F.A. SAROTT, and J.J. WAGER, J . de Phys.

42, Sup. C4 (1981) 251.

( 3 ) ITBAL Z., A.P. WEBB, and S. VEPREK, Appl. Phys. L e t t . 36 (1980) 163.

( 4 ) HIRAKI A., T. IMURA, K. MOGI, and M. TASHIRO, J. de Ph??.

--

42

,

Sup. C4 (1981) 277.

( 5 ) RICHTER H. and L. LEY, J. de Phys. 42, Sup. C4 (1981) 261.

(6) HAMASAKI T., H. KURATA, M. HIROSE, 2 n d Y. OSAKA, Appl

.

Phys. L e t t .

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