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ELECTRON TRANSPORT IN HYDROGENATED AMORPHOUS SILICON SCHOTTKY BARRIERS
AND DEEP LOCALIZED STATES KINETICS
S. Deleonibus, D. Jousse
To cite this version:
S. Deleonibus, D. Jousse. ELECTRON TRANSPORT IN HYDROGENATED AMORPHOUS SILI-
CON SCHOTTKY BARRIERS AND DEEP LOCALIZED STATES KINETICS. Journal de Physique
Colloques, 1981, 42 (C4), pp.C4-487-C4-490. �10.1051/jphyscol:19814103�. �jpa-00220718�
CoZZoque C4, supptdment au nolO, Tome 42, octobre 1981
ELECTRON TRANSPORT I N HYDROGENATED AMORPHOUS S I L I C O N
S. Deleonibus and D. Jousse (b)
*
Laboratoire de Physique des Composants d Semiconducteurs, ERA CNRS 659, ENSER, 23 rue des Martyrs, 38031 GrenobZe, France
*
G q o de Energia Institute de Fisica, Universidade EstaduaZ de Ccmzpinas, Campi- nas, BraziA b s t r a c t : We present a method f o r a n a l y s i n g t r a n s p o r t i n Schottky,diodes on hydrogenated amorphous s i l i c o n (aSiH). Experimental r e s u l t s show a c o r r e l a t i o n between t h e r e d u c t i o n o f c a r r i e r c o l l e c t i o n v e l o c i t y , the r e d u c t i o n o f b a r r i e r h e i g h t f o r reduced space-charge r e g i o n s . I n t h e same way, t h e k i n e t i c s o f
"midgap" l o c a l i z e d s t a t e s i s slowered as b u l k s t a t e s l o c a t e d near t h e Pt-aSiH c o n t a c t c o n t r i b u t e t o capacitance.
I n t r o d u c t i o n .
-
It i s o f t e n assumed t h a t t r a n s p o r t i n b c h o t t k y diodes on amorphous m a t e r i a l s 3s l i m i t e d by a d i f f u s i o n process i n t h e space-charge region. The mean reason p u t foward i s an account f o r t h e low c a r r i e r m o b i l i t y i n these types o f m a t e r i a l s . Nevertheless, t h e thermionic theory has o f t e n been supposed t o h o l d t o measure t h e d e n s i t y o f s t a t e s (D.O.S.) i n t h e gap o f hydrogenated amorphous s i l i c o n(aSiH) by the C(V) technique [ l ] ,C21, C31. I n o r d e r t o a v o i d t h e doubt about t r a n s p o r t , several authors C41, i51, C61 c a r r y o u t capacitance measurements a t zero b i a s versus frequency and ( o r ) temperature.
In
t h i s paper, we p r e s e n t a method o f analysing r e s u l t s o f current-v01 tage I ( V ),
zero bias low frequency capacitance C(w) c h a r a c t e r i s t i c s versus temperature which must l e a d t o a b e t t e r understanding o f t h e a c t u a l n a t u r e o f extended s t a t e e l e c t r o n t r a n s p o r t through Schottky b a r r i e r s on aSiH. We p u t i n evidence t h e i n f l u e n c e o f t h e extension o f the space-charge r e g i o n on t h e c o l l e c t i o n v e l o c i t y o f c a r r i e r s a t t h e metal-aSiH i n t e r f a c e . An analogous treatment i s used f o r t h e a n a l y s i s o f t h e k i n e t i c s o f l o c a l i z e d s t a t e s around t h e Fermi l e v e l w i t h e x p e r i m e n t a l l y determined parameters d e r i v e d from C(w) measurements.
The samples i n v e s t i g a t e d were o b t a i n e d by glow discharge decomposition o f s i l a n e a t t h r e e s u b s t r a t e temperatures : 350, 400 and 450°C. The back c o n t a c t i s a 0 . 1 ~ ~ t h i c k n' l a y e r (PH3/SiH,,=5%) deposited on a t a n t a l um-alumina s u b s t r a t e . The Schottky c o n t a c t i s obtained by s p u t t e r i n g a 50R-100R t h i c k P t l a y e r on t o p o f t h e i n t r i n s i c ( ~ l p m t h i c k ) aSiH. The diodes achieved on the 400°C m a t e r i a l e x h i b i t more than 2% power e f f i c i e n c y C51.
E l e c t r i c a l c h a r a c t e r i z a t i o n .
-
Current-voltage I ( V ) c h a r a c t e r i s t i c s are performed i n t h e range o f temperature C-9O0C, +18O0C;.
C51, C71.
They f i r s t behave exponen- t i a l l y ( f i g . 2 ) f o r forward b i a s lower than the b u i l t - i n p o t e n t i a l Vso ( f i g . 1 ) f o l l o w i n g t h e r e l a t i o n :(a) Work supported by COMES
(b) On leave from L a b o r a t o i r e de Physique des Composants
a
Semiconducteurs ERA CYRS 659.*
Samples achieved by 6. BOURDON, CRCGE ilarcoussis, FranceArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814103
JOURNAL DE PHYSIQUE
I ( V ) = where 10 = 1 0 0 exp(- '@B) ( 2 ) i d e a l i t y f a c t o r o f t h F d i o d e .
b4-L
O w t
Fig.1 : Band diagram o f s c h o t t k y diode
on aSiH
i
g i v e*)
-
1J ( 1 )the b a r r i e r h e i g h t +B ( f i g - l )
I )
F d i o d e s on m a t e r i a l s
,
B i s thep-repared a t : a)350°C, b ) 400°C, c ) 450°C A t h i g h e r voltage, we o b t a i n a l i n e a r dependence o f c u r r e n t w i t h voltage. The s l o - pe o f t h e c h a r a c t e r i s t i c s allows us t o determine a mean value o f t h e b u l k conductance GB o f t h e m a t e r i a l . I t s a c t i v a t i o n energy w i l l g i v e t h e mean e l e c t r o n quasi-Fermi l e v e l d i s t a n c e (Ec-EF) from t h e conduction band (Fig.1). Thus we have :
GB
-
UB S w i t h ag = a. exp[-
C$]!where ag i s the mean b u l k c o n d u c t i v i t y , S i s t h e surface o f the P t contact, t t h e thickness o f the i n t r i n s i c aSiH l a y e r , a, i s given by : GO = q Ncii ( 3 )
Nc being the e f f e c t i v e d e n s i t y o f s t a t e s a t Ec. I.I t h e e l e c t r o n n o b i l i t y , q t h e e l e c t r o n charge.
I n t h e same range o f temperature we perform low-frequency C(w) measurements ClHz, lOHzl (see f i g . 3 ) f o r 1Hz capacitance). For temperatures h i g h e r than Tresp the capacitance s t a r t s t o r i s e : t h e s t a t e s around t h e Fermi l e v e l a t t h e n e u t r a l b u l k boundary-W ( f i g . l ) begin t o respond t o t h e modulation.
I n t h e same way, when the capacitance s a t u r a t e s a t a temperature T s a t the b u l k s t a t e s around EF near t h e Pt-aSiti i n t e r f a c e c o n t r i b u t e t o capacitance ( n e g l e c t i n g the response o f surface s t a t e s ) . The s a t u r a t i o n value o f low frequency capacitance ( t y p i - c a l l y 1Hz f i g . 3 ) C s a t allows us t o deduce t h e D.O.S. around t h e midgap i n the case
o f a u n i f o r m d i s t r i b u t i o n N : N =
-
(Table I).Thus, the e q u i l i b r i u m value o f the space-charge w i d t h w i l l be' : W = L g ( ) (Table I )
The e l e c t r i c a l p o t e n t i a l V(x) having an exponentialshape : V(x) = Vs0 exp
- $
LD being t h e Debye l e n g t h associated t o t h e u n i f o r m 3.O.S. around t h e midgap : 'U
Lo,
=(&l
~ a b i e 1L
Results ( e l e c t r i c a l c h a r a c t e r i z a t i o n )- , .
I~-+,,
= tic us exp(*) expW (pm) 0.28 0.67 0.115 E -E
" ( 0 ~ ) 9' ( e v ; " ( e ~ ) C ~ ( e v ) " ~ ~ (V) 6 10 a D
N
E l e c t r o n c o l l e c t i o n v e l o c i t y .
-
I n a general way, the dark f r e e e l e c t r o n c u r r e n t I , , m f r a m r t o t h e metal can be e x p e s s e d by :( ~ m - ~ e V - l ) 1x1Ol6 2 . 5 ~ 1 0 "
350
,
400 4501.72 1.79 1.71
(n-lcm-':
1 . 3 ~ 1 0 ~ ~ 5 ~ 1 0 ' ~ 3 x 1 0 2
1.9x102 0 . 8 1
1.02 0.72
[ ~ c m - 2 ) 1 . 1 ~ 1 0 ' 1.54Y.3x105
2.7x104 1.2
1.16 0.62
0.66 0.5P
0.13 0.36 0.30
us = !JFS = vds where Fs i s t h e f i e l d a t the i n t e r f a c e given by :
F = ( 5 ) (Table 11).
D I f t h e thermionic t h e o r y h01 ds, then us i s t h e metal c o l l e c t i o n v e l o c i t y and w i l l have t h e thermal v e l o c i t y v t h as a maxinun value C81.
Thus, we w i l l have according t o r e l a t i o n ( 2 ) : 1 0 0 = q NCuS
.
( 5 ) . From r e l a t i o n s (3) and (5) we deduce : IOQ- -
.-A USThus from r e l a t i o n ( 4 ) : 1 0 0 u 00 11
.
m = & .
The r a t i o uS/vds can then be evaluated from e x p e r i m e n t a l l y d e r i v e d parameters (Table 11).
Dynamic behaviour o f midgap s t a t e s .
-
When t h e s t a t e s l o c a t e d a t W s t a r t c o n t r i b u t i n ~ t o capacitance t h e i r response time ~ r i s given by S.R.11 s t a t i s t i c s : ~ s ~E -EF
Tresp = to^ exP
+ k .
Where T O F = 1.
( 6 ) .When t h e capaciBSnEe s a t u r a t e s we expectFa;fh;%E:l::~~s~ocated a t a d i s t a n c e q @ ~ from the conduction band t o c o n t r i b u t e p l a i n l y t o capacitance C51 w i t h a response
~ Q B
time : T s a t = TGB exp .-.- where 1
~ T s a t '@B.= *@B Vth2 N EC S ( 7 ) .
I n ( 6 ) and (7) : OF an6 are t h e electronmc capture cr~ss)::cttons o f t h e s t a t e s around t h e Fermi l e v e l r e s p e c t i v e l y a t W and a t the i n t e r f a c e , v t h l and vth2 a r e t h e thermal v e l o c i t i e s a t Tresp and Tsat
,
!{(Ec) i s the d e n s i t y o f s t a t e s a t Ec.We thus consider t h a t t h e b u l k s t a t e s a t t h e d i s t a n c e @B d o n ' t n e c e s s a r i l l y obey t h e same k i n e t i c s as t h e s t a t e s a t (Ec-EF) even i f t h e energy d i f f e r e n c e qVs0 i s small as i t i s t h e case f o r 350°C and 450°C samples (Table I ) .
For a ~ i v e n w o r k i ? ~ frequency vw must have a response temperature and a s a t u r a t i o n tenperature and : v
'
= Tresp = 'sat (see f i g . 3 f o r 1Hz capacitance).I n p r a c t i c e , we observe a l i n e a r 6ependence o f Log v as a f u n c t i o n o f (Tres ) - l
( f i s . 4 ) . I n f i g u r e 4 t h e a c t i v a t i o n energy given e x p e r i m e n t a l l y i s ( E ~ - E F ) : t R a t shows a p o s t e r i o r i t h a t t h e b u l k Fermi l e v e l determined from t h e l i n e a r p a r t o f I ( V ) c h a r a c t e r i s t i c s i s c o r r e c t . From t h e pre-exponential f a c t o r o f t h e curves i n f i g . 4 we can determine Q F (Table 11).
For a given m a t e r i a l , i f t h e response k i n e t i c s o f t h e s t a t e s around EF were t h e same a t W and a t t h e i n t e r f a c e then t h e q u a n t i t y AT = (Ts
-
Tresp) should be p r o p o r t i o n a l t o Vs0 accor"ing t o S.R.H s t a t i s t i c s . For s m ? f values o f VSo, AT i s t o o l a r g e (Table 11). For t h e sane samples, t h erea at
d i f f e r e n c e between T Q F andT Q @ B shows t h a t t h e k i n e t i c s i s slowered as t h e b u l k s t a t e s a t @B c o n t r i b u t e t o capacitance i n m a t e r i a l s w i t h a t h i n space-charge r e g i o n .
I n the sane way, t h e r a t i o uS/vds gets s m a l l e r (Table 11) and the b a r r i e r h e i ~ h t @B decreases as W decreases.
Possible i n t e r p r e t a t i o n .
-
We are i n r i g h t t o imagine t h a t t h e s t a t i c and dynamic b e h a v i o u r y diode have t h e same o r i g i n . The p o s s i b i l i t y o f c a r r i e r c o l l e c t i o n by i n t e r m e d i a t e t r a p p i n g i n l o c a l i z e d b a n d - t a i l s t a t e s i n t h e space- charge r e g i o n must n o t be excluded. Thus, the r a t i o us/vds decreases as W decreases.JOURNAL DE PHYSIQUE
- l o o
1o
IOO T(oE): C versus T f o r m a t e r i a l s d e f i n e d i n f i g . 2 a t 1 Hz.
V ( l!:)
102
-
10
1
F i g u r e 4 : Log v versus (Tresp) - l
C l a s s i c a l l y , we would have expected the c o n t r a r y i f t r a n s p o r t were d i f f u s i o n l i m i t e d . As a m a t t e r of f a c t , t h e e f f e c t i s nore pronounced as t h e band-bending a t the
Pt-aSiH c o n t a c t i s more i m p o r t a n t : (case o f 350 and 450°C m a t e r i a l s ) : us i s s m a l l e r than vds by a f a c t o r o f 10'.
The dynamic behaviour f o l l o w s t h e s t a t i c behaviour. (Table 11). The communication between deep s t a t e s a t i n t e r f a c e and the conduction band i s probably e s t a b l i s h e d v i a i n t e r m e d i a t e t r a p p i n g o f t h e e l e c t r o n s i n b a n d - t a i l s t a t e s . Thus ro$a can be h i g h e r than r o ~ by a f a c t o r o f 102.
Conclusion.
-
We have presented a method o f a n a l y s i s o f I ( V ) , C(w) c h a r a c t e r i s t i c s versus temperature t h a t can l e a d t o a b e t t e r understanding o f t r a n s p o r t i n Pt-aSiH Schottky diodes as w e l l as t h e k i n e t i c s o f deep l o c a l i z e d gap s t a t e s i n aSiH. We have proposed a p o s s i b l e i n t e r p r e t a t i o n f o r the r e d u c t i o n o f c o l l e c t i o n v e l o c i t y of extended s t a t e s e l e c t r o n s a t t h e metal-aSiH Schottky c o n t a c t . T h i s phenomena i s accompanied by an enhancement o f thermal r e l e a s e time o f t h e deep b u l k s t a t e s a t t h e surface Fermi l e v e l and a r e d u c t i o n of t h e b a r r i e r h e i g h t . That r e f l e c t s a l s o the i n d i r e c t i n f l u e n c e o f t h e D.O.S. around t h e midgap on t h e entended s t a t e s e l e c t r o n t r a n s p o r t .References
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.
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.
Appl.
- 14 (1579).C31 P. V i k t o r o v i t c h and D. Jousse
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( 9 ) (Sept. 1980).C71 S. Deleonibus, D. Jousse, R. Basset, Presented a t ESSDERC'80 York (Ga) C81 S.M. Sze, Physics o f Semiconductor devices, Wiley, !.dew-York (1969).
