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INVESTIGATION INTO THE CONDUCTION MECHANISM OF CVD AMORPHOUS AND
POLYCRYSTALLINE SILICON FILMS
Yu-Liang He, Hsiang-Na Liu
To cite this version:
Yu-Liang He, Hsiang-Na Liu. INVESTIGATION INTO THE CONDUCTION MECHANISM OF CVD AMORPHOUS AND POLYCRYSTALLINE SILICON FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-831-C4-834. �10.1051/jphyscol:19814183�. �jpa-00220809�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZe'msnt au nO1O, Tome 42, octobre 1981 page C4-831
INVESTIGATION INTO THE COWDUCTION MECHANISI,l O F CVD ANORPHOUS AND POLYCRYSTALLIilE SILICON F I LPIS
Yu-liang He and Hsiang-na Liu
Department of Phgsics Nanjing University, Nanjing, China
ABSTRTICT.
-
T h i s paper s t u d i e s t h e s t r u c t u r e p r o p e r t y and con- d u c t i o n mechanism of o r d i n a r y CVD amorphous s i l i c o n f i l m s de- p o s i t e d i n a range of temperature 550-7500C, around t h e c r y s t a l - l i z a t i o n temperature. I t i s found t h a t t h e t r a n s p o r t p r o c e s s e s of l o c a l i z e d s t a t e s a s w e l l a s t h e band t a i l e x i s t s t i l l i n t h e S i f i l m s d u r i n g t h e i r t r a n s i t i o n from amorphous t o p o l y c r y s t a l - l i n e one. It i s a l s o found t h a t a s t h e d e p o s i t e d temperature i n c r e a s e s , t h e g r a i n s i z e i n c r e a s e s and t h e t a i l i n g width nar- rows down. An e m p i r i c a l f u n c t i o n of g r a i n s i z e and t a i l i n g width has been given.INTRODUCTION
It is well known t h a t t h e conduction mechanism of band t a i l s and l o c a l i z e d gap s t a t e s i n o r d i n a r y CYD amorphous S i f i l m s obeys t h e Mott- Davis model, owing t o l a c k of l o n g range o r d e r (1). The p o l y c r y s t a l l i n e f i l m s a t h i g h e r temperatures i n CVD method have a high d e n s i t y of t r a p s which s e t up space-charge l a y e r s by a t t r a c t i n g t h e o p p o s i t e c h a r g e s a t t h e g r a i n boundaries. Therefore, t h e t r a n s p o r t p r o c e s s e s of p o l y c r y s t a l l i n e S i f i l m s obey t h e g r a i n boundary conduction model p r e s e n t e d by S e t , e t a l . ( 2 )
.
These two models a s mentioned above a r e d i s t i n c t l y d i f - f e r e n t . The purpose of t h i s paper i s t o p r e s e n t t h e change i n e l e c t r i - c a l t r a n s p o r t p r o c e s s e s a r i s i n g from t h e i n c r e a s i n g l a t t i c e o r d e r , while amorphous s t r u c t u r e is transformed t o p o l y c r y s t a l l i n e one.We prepared amorphous and p o l y c r y s t a l l i n e s i l i c o n films of d i f - f e r e n t g r a i n s i z e by c o n t r o l l i n g temperature s e l e c t e d around t h e cry- s t a l l i z a t i o n temperature of A-Si ranged from 5500C t o 7500C. The DC c o n d u c t i v i t y has been measured o v e r a wide range o f temperature, 100- 7 0 0 ~ ~ - We have analysed t h e i n f l u e n c e of t h e change i n g r a i n s i z e on t h e e l e c t r i c a l t r a n s p o r t processes.
EXPERIMENTAL RESULTS AND ANALYSIS
The samples were prepared i n a n o r d i n a r y CM system of thermal decomposition of s i l a n e c a r r i e d by H
.
Both amorphous and p o l y c r y s t a l - l i n e f i l m s were d e p o s i t e d on S i s u b s g r a t e s covered with 9102. The growing temperature was measured by an i n f r a r e d r a d i o pyrometer. DC c o n d u c t i v i t y was measured with ZC-36 t y p e extra-high r e s i s t a n s e meter.By a n a l y z i n g t h e h a l f h e i g h t of t h e X-ray d i f f r a c t i o n peaks and t h e p a t t e r n of scanning e l e c t r o n micrographs, we o b t a i n e d t h e g r a i n
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814183
C4-832 JOURNAL DE PHYSIQUE
s i z e of t h e f i l m s . Fig. 1 shows t h a t t h e g r a i n s i z e i n c r e a s e s continu- ously with i n c r e a s i n g temperatures. Fig. 2 is a p l o t of growing r a t e s
'600
860 I-Growing temperature (OC) Fig. 1 Grain s i z e v s growing
temperature f o r CVD S i f i l m s Fig. 2 Growing r a t e s v s r e c i - procal temperature f o r CYD S i f i l m s
of f i l m s a s a f u n c t i o n of r e c i p r o c a l temperature. It i n d i c a t e s t h a t t h e r e is an obvious abrupt change a t 6800C-+looc. According t o t h e S t a t i s t i c a l r u l e of t h e growth of f i l m s i n CVD system, t h e a c t i v a t i o n ,
energy of growth i s 1.78 ev above 6800C..and 0.44ev below 6800C. This abrupt change 0 4 temperature may be considered t o be *e c r y s t a l l i z a - t i o n temperature of A-91. It i s c o n s i s t e n t with t h e r e s u l t given by M.
Hirose e t a l e ( 3 ) , who suggested t h a t t h e microscopic crystaLlization occurs a t a temperature 675-6900C, confirmed by an abrupt change in r e f r a c t i v e index.
The s t r u c t u r e of t h e films deposited i n v a r i o u s temperatures had been analyzed by X-ray spectrometer of RigaKu 3015 type. X-ray d i f - f r a c t i o n p a t t e r n s of both 55OOC and 6000C deposited samples produce obvious non-crystal peak i n t h e low-angle range (2-10-250~)~ a s shown i n F i g . 3 ( s ) . The non-crystal peak of 6300C deposited sample decreases apparently I n t h e p a t t e r n of 670% deposited sample, a (111) peak r e v e a l s a t 2 W = 28.40, b u t t h e non-crystal peak vanishes. The r e s u l t s i n d i c a t e s t h a t t h e s t r u c t u r e of CM f i l m s deposited b e l w 600% a r e amorphous mainly, b u t t h e amorphous component decreases a s t h e cry- s t a l l i t e s e x i s t i n t h e f i l m s deposited i n 600-680% range. The in- c r e a s i n g (110) and(311) peak a s regard t o 7000C deposited samples [Fig.
3 ( b ) l l e a d us t o conclude t h a t the f i l m has been transformed t o poly- c r y s t a l l i n e one. This conclusion is confirmed by t h e r e s u l t of trans- mission e l e c t r o n d i f f r a c t i o n (TED)
.
Conductivity v s temperature (100-7000K) have been p l o t t e d i n two samples grown below 6800C, a s shown i n Fig. 4. Each p r e s e n t s f o u r dif- f e r e n t conducting processes, which, according t o Mott-Davis t r a n s p o r t model, may be described as:
where E = Ec- EF i s the a c t i v a t i o n energy of extended s t a t e s t r a n s p o r t .
Et
= EA-
EF + Wh i s the a c t i v a t i o n energy i n l o c a l i z e d s t a t e s . E p is t e t r a n s p o r t a c t i v 8 t i o n energy of s t a t e s which l o c a t e d a t Ex i n energy gap. The l a s t term r e l a t e s t o v a r i a b l e range hopping i n lower tempera- t u r e .The conduction band t a i l i n g width may be derived form t h e experi- mental curves a s follows:
Fig.3 X-ray d i f f r a c t i o n pattern of CVD s i l i c o n f i l m s
F i g . 4 DC c o n d u c t i v i t y v s r e c i p r o c a l temperature f o r A - S i .
JOURNAL DE PHYSIQUE
By c o n s i d e r i n g s t i l l band t a i l s and l o c a l i z e d s t a t e c o n t r i b u t i o n s i n p o l y c r y s t a l l i n e s i l i c o n w i t h a g r a i n s i z e of s e v e r a l hundreds A n g s t r o m s , we o b t a i n e d t h e r e l a t i o n between t a i l i n g width and g r a i n s i z e , a s shown i n l i s t 1 and F i g . 5 , which can e m p i r i c a l l y be ex- p r e s s e d a s f o l l o w s : (where x i s g r a i n s i z e i n micrometer)
If x a p p r o a c h e s z e r o , t h e t a i l i n g width would r e a c h a maxium v a l u e (Ec
-
EA)max = 0.51ev; i f t h e band t a i l e l i m i n a t e s , t h e g r a i n s i z e would approach 700 A , which, a c c o r d i n g t o o u r work* should r e l a t e d t o t h a t grown i n 774OC ( d e r i v e d from F i g . 1 ) .L i s t 1 The change of t a i l i n g w i d t h a s a f u n c t i o n of g r a i n s i z e
Growth Nucleated a t 650°C
t e m p e r a t u r e ( O C ) 560+10 610f10 660*10 grown a t 8 0 0 " ~ 750f 10
Average grain
s i z e ( 8 ) 70-80 130-140 230-250 300 500
Tailing
width (eV) 0.365~0.01 0.33*0.01 0.3420.02 0.29+0.02 0.20+0.02
F i g . 5 T a i l i n g width as a func- t i o n of g r a i n s i z e f o r CVD s i l i c o n f i l m s
r 0% 710 ;OS
Grain s i z e
(1)
Summary:l. The 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 of A-Si should be 6800C?100C.
2. The t r a n s p o r t p r o c e s s e s i n pseudo-amorphous s i l i c o n which has i n d i v i d u a l c r y s t a l l i t e s l e s s t h a n 700A s t i l l f o l l o w t h e Mott-Davis model, t h e t a i l i n g width still e x i s t s . T h e t r a n s p o r t p r o c e s s e s should transformed t o g r a i n boundary c o n d u c t i o n when t h e film e x h i b i t e s a p p a r e n t polycry- s t a l l i n e s t r u c t u r e w i t h g r a i n s i z e x
>
700 A.3. During t h e t r a n s i t i o n from amorphous t o p o l y c r y s t a l l i n e s t a t e , t h e t a i l i n g width d e c r e a s e s w i t h i n c r e a s i n g g r a i n s i z e * t h e i r r e l a t i o n should f o l l o w an e m p i r i c a l f u n c t i o n : EC
-
EA = 6.e4-
6.33 e l . l o x I f t h e t a i l i n g width v a n i s h e d , t h e g r a i n s i z e should be>
700 A.R e f e r e n c e s : 1. N-F .Matt, E .A.DaviS " E l e c t r o n i c P r o c e s s e s i n Non-crystal- l i n e M a t e r i a l s " P.219 Oxford second e d i t i o n (1979) 2. Y .W.Seto J. Appl Phys 46 No. 1 2 (1975) 5247
Y. Osaka Jap. J. AppL Phys 1 7 No. 6 (1978) 985 3 - M.Hirose e t a l . J . Appl Phys 50 No. 1 (1979) 377 ACKNOWLEDGEMENT: The a u t h o r s would l i k e t o thank M r . Zhou Heng-nan f o r X-ray d i f f r a c t i o n measurements and h e l p f u l d i s c u s s i o n s .
