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OPTICAL ABSORPTION AND STRUCTURAL ORDER IN SPUTTERED AMORPHOUS
PHOSPHORUS
R. Pomian, L. Pilione, J. Lannin
To cite this version:
R. Pomian, L. Pilione, J. Lannin. OPTICAL ABSORPTION AND STRUCTURAL ORDER IN
SPUTTERED AMORPHOUS PHOSPHORUS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-
873-C4-876. �10.1051/jphyscol:19814191�. �jpa-00220818�
7oZZoque C4, suppZSment au nOIO, Tome 42, octobre 1981 page C4-873
OPTICAL ABSORPTION AND STRUCTURAL ORDER I N SPUTTERED AMORPHOUS PHOSPHORUS
R.J. Pomian, L.J. Pilione and J.S. Lannin
Department of Physics, The Pennsylvania S t a t e University, University Park, PA 16802, U.S.A.
Abstract.-Optical a b s o r p t i o n measurements a r e r e p o r t e d f o r rf s p u t t e r e d a-P f i l m s prepared under c o n d i t i o n s o f v a r i a b l e s u b s t r a t e temperature.
V a r i a t i o n s i n t h e o p t i c a l gap w i t h Ts a r e s i m i l a r t o changes i n t h e Raman s c a t t e r i n g and x-ray d i f f r a c t i o n spectra. The r e s u l t s suggest t h a t i n t e r - mediate-range o r d e r p l a y s an important r o l e f o r t h e s t r u c t u r a l , v i b r a t i o n -
a l and o p t i c a l p r o p e r t i e s f o r more ordered a-P f i l m s . P h o t o s t r u c t u r a l e f f e c t s i n As-chalcogenides which e x h i b i t analogous behavior a l s o suggest m o d i f i c a t i o n s o f intermedi ate-range order.
Introduction.-Recent s t u d i e s i n t h i n f i l m a-P have emphasized t h a t i n t r i n s i c v a r i a t i o n s i n network s t r u c t u r e may a r i s e w i t h d e p o s i t i o n conditions. Both Raman s c a t t e r i n q and low-angle x-ray d i f f r a c t i o n measurements suggest t h a t s t a t i c and dynamic c o r r e l a t i o n e f f e c t s are present as a consequence o f intermediate-range order, i.e., order beyond t h e f i r s t c o o r d i n a t i o n s h e l l (1,2). Given these s t r u c - t u r a l and v i b r a t i o n a l r e s u l t s i t i s o f i n t e r e s t t o i n q u i r e i f t h e e l e c t r o n i c s t a t e s o f a-P a r e a l s o i n f l u e n c e d by s t r u c t u r a l c o r r e l a t i o n s o f i n t e r m e d i a t e range.
I n c o n t r a s t t o s t r u c t u r a l and v i b r a t i o n a l probes, o p t i c a l a b s o r p t i o n measurements a r e more d i f f i c u l t t o associate w i t h s t r u c t u r a l m o d i f i c a t i o n s . I n a d d i t i o n , e x t r i n s i c e f f e c t s associated w i t h voids o r m i c r o s t r u c t u r e i n general, may a l s o p l a y a r o l e t h a t r e q u i r e s consideration. I n t h e present study we focus on m o d i f i c a t i o n s o f t h e o p t i c a l spectra i n t h e near gap r e g i o n on s p u t t e r e d a-P f i l m s prepared under c o n d i t i o n s o f r e l a t i v e l y h i g h order as deduced from Raman s c a t t e r i n q and x-ray d i f f r a c t i o n measurements. The i n f l u e n c e o f short-range order and f i l m morphology on t h e o p t i c a l spectra w i l l be presented elsewhere. The r e s u l t s discussed below suggest t h a t i n ordered a-P f i l m s intermediate-range order a l s o m o d i f i e s t h e e l e c t r o n i c s t a t e s v i a s t r u c t u r a l c o r r e l a t i o n e f f e c t s . The a b s o r p t i o n spectra a l s o i n d i c a t e t h a t t h e Urbach slope i s n o t s e n s i t i v e t o i n t e r - mediate-range order, but may, i n c o n t r a s t t o e a r l i e r suggestions (3) vary i n e i e - mental semiconductors o f s i m i l a r coordination.
Experiment.-To s y s t e m a t i c a l l y study v a r i a t i o n s i n t h e p h y s i c a l p r o p e r t i e s o f a-P f i l m s t h e rf plasma d e p o s i t i o n c o n d i t i o n s i n a p l a n a r diode s p u t t e r i n g system were f i x e d and o n l y t h e s u b s t r a t e temperature modified. The temperature o f t h e f i l m s was determined d i r e c t l y by a t h i n f i l m Ag-A1 thermocouple. An Ar pressure o f 60 P and a cathode-anode separation o f 2" were employed t o t h e r m a l i z e t h e i n c i d e n t P species and y i e l d ordered films. The t a r g e t employed was a 2" diameter p i e c e o f p o l y c r y s t a l l i n e black P. Reflectance and t r a n s m i t t a n c e measurements were performed on a Cary 14 system f o r 0.5-15 p t h i c k f i l m s . The absorption coef- f i c i e n t and index o f r e f r a c t i o n were determined by s o l v i n g t h e a p p r o p r i a t e f i l m p l u s s u b s t r a t e equations (4) u s i n g a microcomputer.
Results and Discussion.-In Fig. 1 t h e o p t i c a l absorption c o e f f i c i e n t o f s p u t t e r e d a-P f i l m s deposited a t d i f f e r e n t s u b s t r a t e temperatures, Ts, i s shown. The values of Ts = 8, 64, 126 and 180 C f o r f i l m s A, 8, C and D, r e s p e c t i v e l y , were chosen t o y i e l d approximately 60 C i n t e r v a l s i n TS. Although Fig. 1 i n d i c a t e s a r e l - a t i v e l y s i m i l a r form f o r each o f t h e f i l m s , t h e s h i f t o f t h e absorption spectra a r e c l e a r l y n o n l i n e a r i n TS.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814191
C4-874 JOURNAL DE PHYSIQUE
F i q u r e 1 a l s o i n d i c a t e s , w i t h t h e exception o f a small deqree o f t a i l i n g
lo6-
,
-
-
,
--
- - -
:
-
-
I -
-
- -
-
1 0 ~ ~
i0
~ 2 1 2i4
216 3 0below -1.7 eV, an exponential a b s o r p t i o n edqe whose Urbach parameter,
r,
has a r e l a t i v e l y constant value o f 8.2+
0.8 ev-1. T h i s value i s approximztely 30% l o w e r than t h a t o f bulk a-As o f 11.9 ev-1 (3). Previous observations i n elemental amorphous semiconductors have l e d some observers t o suggest t h a t t h e Urbach edge i s p r i m a r i l y a f u n c t i o n o f c o o r d i n a t i o n number (3,5). The r e s u l t s i n a-P as w e l l as recent measurements ( 6 ) i n l i q u i d ( I - ) S which i n d i c a t e an Urbach parameter which d i f f e r s consid- e r a b l y from t h a t o f a-Se and a-Te imply t h a t o t h e r f a c t o r s , such as t h e degree o f covalency, may s u b s t a n t i a l l y modify
r.
The s m a l l e r value o fr
i n a-P r e l a - t i v e t o a-As i m p l i e s a l a r g e r a c t i v a t i o n energy f o r e l e c t r o n delocal i z a t i o n i n a-P. The r e l a t i v e constancy o fr
w i t h TS f o r t h e a-P f i l m s s t u d i e d here con- t r a s t s w i t h changes i n t h e Raman scat- t e r i n g and low-angle x-ray d i f f r a c t i o n s p e c t r a which i n d i c a t e i n c r e a s i n g d i s o r -hu(eV) d e r a t lower Ts values. A s i m i l a r
r e s u l t i s observed i n 1 - S , where
r
i s e s s e n t i a l l y independent o f Ts across t h e Fig. 1. O p t i c a l a b s o r p t i o n i n rf temperature range t h a t i n c l u d e s t h e s p u t t e r e d a-P as a f u n c t i o n o f Ts. p o l y m e r i z a t i o n t r a n s i t i o n (6). The TA = 8 C, TB = 64 C, TC = 126 C, r e s u l t s i n a-P and 1-S suqgest t h a t r and TD = 180 C. may be i n s e n s i t i v e t o l o n g e r ranges t r u c t u r a l c o r r e l a t i o n s and i s p r i m a r i l y
8 r 8 . . - n n
-
-
a f u n c t i o n o f short-range order. Con- s i d e r a b l e s i m i l a r i t y i n t h e Raman s p e c t r a o f b u l k a-P and t h e h i g h
- Ts = 180 C f i l m f u r t h e r p r e d i c t com- p a r a b l e values o f
r
and Eo f o r t h e bulk6 - - m a t e r i a l . Microscopic inhomogeneities
i n b u l k a-P samples examined t o date have, however, prevented c o n f i r m a t i o n o f - t h i s point.
-IN,
B
- The o p t i c a l gap f o r t h e f i l m s o f- Fig. 1 have been determined by t h e
. conventional ( ~ ~ h v ) l / ~ vs. h v p l o t shown i n Fig. 2. The e x t r a p o l a t e d Eo(Ts) values a r e shown i n Fig. 3, where t h e s o l i d l i n e i s merely t o i n d i c a t e t h e
- t r e n d a t h i g h TS t o an approximate
- asymptotic value o f Eo
-
2.1 eV. This- suggests, f o r t h e s p u t t e r i n g c o n d i t i o n - employed, t h a t h i g h Ts f i l m s tend toward
_
an anneal s t a b l e state. The r e s u l t s a r e t h u s s i m i l a r t o those i n a-Ge which a l s o i n d i c a t e t h a t h i g h s u b s t r a t e o r an- o neal i n g temperature f o r evaporated f i l m shv(eV) y i e l d a s a t u r a t i o n o f Eo (7).
Fig. 2. (ahv)1/2 vs. hv f o r t h e Evidence t h a t Eo i s m o d i f i e d by f i l m s o f Fig. 1. s t r u c t u r a l o r d e r i s shown i n Fig. 4,
where t h e r a t i o o f x-ray d i f f r a c t i o n
t h e o p t i c a l gap, Eo, w i t h Ts. The s o l i d l i n e i s m e r e l y a v i s u a l aid.
i n t e n s i t i e s , 11/12, i s compared t o t h e o p t i c a l gap. The i n t e n s i t i y , I 1 i s t h a t o f t h e l o w - a n g l e d i f f r a c t i o n peak a t k
-
1.1 A-1, whose presence i n a-As, a-P and a-As(Ge)-chalcogenides has been i n t e r p r e t e d i n terms o f i n t e r m e d i a t e - range o r d e r between 10-30 A (8,9). 12 i s t h e i n t e n s i t y o f t h e ' n o r m a l ' d i f - f r a c t i o n peak i n a-P a t k = 2.3 A-1.F o r a l a y e r - l i k e model suggested f o r t h e s t r u c t u r e o f a-P f i l m s , 11/12 i s a measure o f t h e f r a c t i o n o f m a t e r i a l t h a t i s l o c a l l y p a r a l l e l (1 , l o ) . As such, t h i s r a t i o w i l l be a f u n c t i o n o f b o t h
I- -r/
i n t e r l a v e r and i n t r a l a v e r s t r u c t u r a l1.8 @--- I c o r r e l a i i o n s . The v a A a t i o n o f Eo w i t h
1 . 7
1.81.9 2.0 2:1
i n c r e a s i n g 11/12 i n d i c a t e s t h a t i n t e r --
,. ..
mediate-ranae o r d e r m o d i f i e s t h e e l e c -~ ~ l e v l t r o n i c s t a t e s and o p t i c a l p r o p e r t i e s o f o r d e r e d a-P f i l m s . F u r t h e r e v i d e n c e i s Fiq. 4. V a r i a t i o n o f t h e o p t i c a l gap a l s o p r o v i d e d by t h e v a r i a t i o n o f t h e w i t h x - r a y d i f f r a c t i o n i n t e n s i t y r a t i o . Raman s p e c t r a w i t h Eo. That i n t e r - The r e c t a n g u l a r d a t a p o i n t i s o b t a i n e d mediate-range o r d e r may s u b s t a n t i a l l y f r o m t h e measured 11/12 ( r e f . 1 ) and m o d i f y t h e e l e c t r o n i c w a v e f u n c t i o n s o f a n e s t i m a t e o f Eo f r o m Fig. 3. The amorphous s o l i d s i s reasonable, g i v e n s o l i d l i n e i s a v i s u a l a i d only. r e c e n t t h e o r e t i c a l s t u d i e s ( 1 1 ) i n a-Si
which suggest t h a t t h e d i h e d r a l a n g l e d i s t r i b u t i o n and r i n g s t a t i s t i c s may i n f l u e n c e t h e o p t i c a l gap. The r e l a t i v e l y l a r g e r s h i f t o f Eo i n Fig. 4 f o r t h e l o w e s t Ts suqqests, however, t h a t o t h e r s t r u c t u r a l f a c t o r s i n v o l v i n g s h o r t - r a n g e o r d e r m o d i f i c a t i o n s may p l a y a r o l e w i t h i n c r e a s i n g d i s o r d e r . T h i s i s a l s o c o n s i s t e n t w i t h weaker v a r i a t i o n s i n t h e Raman i n t e n s i t y f o r s e l e c t e d s p e c t r a l f e a t u r e s i n t h e d e p o l a r i z e d component i n l o w e r Ts f i l m s (1).
F u r t h e r e v i d e n c e f o r t h e r o l e o f i n t e r m e d i a t e - r a n g e o r d e r on t h e o p t i c a l gap i s suggested by p a r a l l e l b e h a v i o r observed d u r i n g p h o t o s t r u c t u r a l t r a n s f o r m a t i o n o f a-As-chalcogenide b u l k g l a s s e s (12). I n p a r t i c u l a r , d u r i n g t h i s t r a n s f o r - m a t i o n b o t h Eo and 11/12 decrease i n a manner q u a l i t a t i v e l y analogous t o t h a t ob- s e r v e d i n a-P f i l m s w i t h d e c r e a s i n q Ts. Thouqh t h e d e t a i l e d 11/12 r a t i o has n o t been p u b l i s h e d i n a-AspS3, t h e s p e c t r a suggest a somewhat s i m i l a r r e l a t i v e v a r i - a t i o n w i t h Eo t o t h a t observed i n a-P f i l m s . I n a d d i t i o n , r e c e n t n q r measurements
C4-876 JOURNAL DE PHYSIQUE
(13) i n a-As-chalcogenides have shown t h a t t h e l i n e shape i s unchanged d u r i n g p h o t o s t r u c t u r a l m o d i f i c a t i o n . T h i s i m p l i e s , g i v e n t h e l o c a l s e n s i t i v i t y o f n q r measurements t o s h o r t - r a n g e order, t h a t t h e p h o t o s t r u c t u r a l e f f e c t must be asso- c i a t e d w i t h v a r i a t i o n s i n o r d e r o v e r an i n t e r m e d i a t e ranqe. The r e s u l t s i n a-P and t h e r e l a t e d b e h a v i o r o f b u l k a-As-chalcogenides suggests a common o r i g i n t h a t i n v o l v e s t h e r o l e o f i n t e r m e d i a t e - r a n g e o r d e r on' t h e e l e c t r o n i c s t a t e s near t h e band edge o f amorphous semiconductors. The analogy i s strengthened by t h e obser- v a t i o n t h a t t h i n f i l m a-P a t h i g h Ts values i s an anneal s t a b l e s t a t e analogous, f r o m a f r e e energy p o i n t o f view, t o t h a t o f t h e b u l k chalcogenide glass. Under such c o n d i t i o n s i t i s most reasonable t h a t i n t e r m e d i a t e - r a n g e o r d e r may i n f l u e n c e t h e p r o p e r t i e s of n o n c r y s t a l l i n e s o l i d s .
"Supported by NSF Grant DMR 79-08390 References
( 1 ) Shanabrook B. V. and Lannin J. S., Phys. Rev. B ( t o be published).
( 2 ) L a n n i n J. S., B u l l . APS 26 (1981 ) 313.
( 3 ) Greaves G. N., Davis E. and Bordas J., P h i l . Mag. 34 (1976) 265.
( 4 ) Cody G. D., Abeles B., Wronski C. R. and Brooks B., JFNon-Cryst.
S o l i d s 35-36 (1980) 463.
( 5 ) M o t t ~.=nd Davis E. A., E l e c t r o n i c Processes i n N o n c r y s t a l l i n e M a t e r i a l s , ( O x f o r d U n i v e r s i t y Press, 1979).
( 6 ) Z a n i n i M. and Tauc J., J. Non-Cryst. S o l i d s 23 (1977) 349.
( 7 ) Thbye M. L., O p t i c a l P r o p e r t i e s o f s o l i d s - ~ e w ~ e v e l o p m e n t s , ed. Seraphin B.O.
( N o r t h Holland, Amsterdam, 1976) 353.
( 8 ) L e a d b e t t e r A. J. and A p p l i n g A. J., Proc. 5 t h I n t . Conf. Amorphous and L i q u i d Semicon., ed. Stuke J. and B r e n i g W. ( T a y l o r and F r a n c i s , London, 1974) 457.
( 9 ) P h i l l i p s J. C., Beevers C. A. and Gould S. E. B., Phys. Rev.
821,
(1980) 5724.(10) Bishop S. G. and Shevchik N. J., S o l i d S t a t e Commun.
15
(1974) 629.( 1 1 ) Cohen M. H., F r i t z s c h e H., Singh J. and Yonezawa F., Proc. 1 5 t h I n t . Conf.
Phys. Semicond., J. Phys. Soc., Japan 49, Suppl. A (1980) 1775.
(12) Tanaka K., J. Non-Cryst. S o l i d s 35-36 v 9 8 0 ) 1023.
(13) T a y l o r P. C., Strom U., P o n t u s c h k a M., Treacy D. J., Proc. 1 5 t h I n t . Conf.
Phys. Semicond., J. Phys. Soc., Japan
