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DUAL LIGHT BEAM MODULATION OF
PHOTOCARRIER LIFETIME IN INTRINSIC a-Si:H
P. Persans, H. Fritzsche
To cite this version:
P. Persans, H. Fritzsche. DUAL LIGHT BEAM MODULATION OF PHOTOCARRIER LIFE- TIME IN INTRINSIC a-Si:H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-597-C4-600.
�10.1051/jphyscol:19814130�. �jpa-00220749�
DUAL LIGHT BEAM MODULATION OF PHOTOCARRIER L I F E T I M E I N I N T R I N S I C a - S i : H
P.D. ~ e r s a n s * a n d H. F r i t z s c h e
J a m s Frmck I n s t i t u t e and Department of Physics, University of Chicago, Chicago, IL 60637, U.S.A.
ABSTRACT
-
We present dual beam p h o t o c o n d u c t i v i t y measurements on i n t r i n s i c g1 ow discharge a-Si :H.Several recombination e f f e c t s have been is01 ated by v a r y i n g the wavelength and chopping frequency o f a secondary photon beam w h i l e pumping t h e sample w i t h a primary beam o f band gap photons and d e t e c t i n g t h e changes i n t h e photoconductivity. For T <200K i n f r a r e d quenching o f p h o t o c o n d u c t i v i t y w i t h a low energy t h r e s h o l d o f -0.6 eV i s found. Above 250K a d i f f e r e n t i n f r a r e d quenching process occurs w i t h a low energy thresh01 d o f -0.9 eV. . A t intermediate temperatures
(200K
<
T<
250K) a new p o s i t i v e modulation signal i s observed. Quenching r e s u l t s are i n t e r p r e t e d i n terms o f o p t i c a l e x c i t a t i o n o f e l e c t r o n s from the valence band i n t o recombination centers w i t h small e l e c t r o n capture c o e f f i c i e n t s . The p o s i t i v e modulation signal i s a t t r i - buted t o an o p t i c a l t r a n s i t i o n between l o c a l i z e d gap states.INTRODUCTION
-
The dual beam p h o t o c o n d u c t i v i t y modulation tech- nique i n v o l v e s the use o f a steady pump l i g h t beam o f interband photons and a chopped modulation beam which scans t h e energies w i t h i n t h e gap. The pump beam changes t h e occupation o f gap s t a t e s through which recombination takes place. The r e l a t i o n s h i p between gap s t a t e occupation and recom- b i n a t i o n o f photogenerated c a r r i e r s i s i n v e s t i g a t e d by m d u l a t j n g the occu- p a t i o n o f s p e c i f i c sets o f gap s t a t e s using t h e modulation beam andmeasuring t h e r e s u l t a n t changes i n t h e photoconductance. I n t h e f o l l o w i n g we describe and i n t e r p r e t some f e a t u r e s observed i n t h e dual beam spectra o f i n t r i n s i c a-Si:H. F u r t h e r data and a more complete a n a l y s i s are pre- sented e l sewherel
.
RESULTS AND DISCUSSION
-
Amorphous Si:H f i l m s were prepared by t h e rf plasma decomposition o f SiH4 mixed w i t h Ar b u f f e r gas i n a c a p a c i t i v e l y coupled system2 under c o n d i t i o n s described e a r l i e r . ]The temperature dependence o f t h e dc photoconductance GP o f a 2um t h i c k undoped a-Si:H sample e x c i t e d w i t h a pump beam f l u x o f -2 X
1016 photons cm-2 sec-l i s shown i n Figure 1. Three temperature regimes are denoted i n the f i g u r e corresponding t o c h a r a c t e r i s t i c temperature and i n t e n - s i t y dependence o f GP. I n r e g i o n I G i s n e a r l y independent o f temperature and depends on photon f l u x i n t e n s i t y
p
as G a FY w i t h 0.9<
y <1.0. I n r e g i o n 11 Gp~decreases w i t h i n c r e a s i n g T (tRerma1 quenching) and has a s u p r a l i n e a r i n t e n s i t y dependence w i t h 1.0<
y <l .l. I n r e g i o n I 1 1 GP increases w i t h i n c r e a s i n g T w i t h an a c t i v a t i o n energy o f -0.2 eV and y*
Present address: Exxon CRSL, P.O. Box 45, Linden, NJ 07036Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814130
C4-598 JOURNAL DE PHYSIQUE
decreases t o 0.59 a t 400K. GP would be expected t o increase monotonically w i t h i n c r e a s i n g T i f a l l recombination centers i n a-Si:H had s i m i l a r e l e c t r o n capture r a t e s and s i m i l a r hole capture rates. This i s because f o r a given l i g h t f l u x , t h e separation between the t r a p quasi-Fermi l e v e l s f o r e l e c t r o n s and f o r holes and hence t h e number o f e f f e c t i v e recombination centersdecreases as T increases.3 The thermal quench i n g and t h e s u p r a l i - near i n t e n s i t y dependence o f GP i n region I 1 as w e l l as the constancy o f GP i n r e g i o n I i n d i c a t e t h e presence o f a t l e a s t two d i s t i n c t classes o f recombination c e n t e r s . 4 ~ 5
I n order t o e x p l a i n these observations as w e l l as i n f r a r e d
quenching discussed l a t e r , we use t h e f o l l o w i n g simple model. We assume two k i n d s o f recombination centers: ( i ) r - c e n t e r s which have a l a r g e e l e c t r o n capture c o e f f i c i e n t and l i e p r i m a r i l y above the dark Fermi l e v e l EF (above midgap) and ( i i ) S-centers w i t h a small e l e c t r o n capture c o e f f i c i e n t which l i e predominantly below EF. The hole capture c o e f f i c i e n t s are assumed f o r s i m p l i c i t y t o be t h e same f o r r- and S-centers. The m a j o r i t y p h o t o c a r r i e r s are assumed t o be electrons.
I n the low T r e g i o n I under t h e high pump c o n d i t i o n s o f Figure 1, most r - c e n t e r s are occupied and most S-centers are empty due t o t h e d i f - ference i n e l e c t r o n capture c o e f f i c i e n t s . GP i s r e l a t i v e l y l a r g e because t h e r-centers, which are t h e most e f f e c t i v e recombination centers, c o n t a i n a r e l a t i v e l y small number o f holes compared t o S-centers. As T i s r a i s e d ( r e g i o n 11) an i n c r e a s i n g number o f sdcenters come i n t o thermal e q u i l i b r i u m w i t h the valence band r a i s i n g the number o f f r e e holes a v a i l a b l e f o r cap- t u r e i n t o t h e more e f f e c t i v e r - c e n t e r s and decreasing GP (thermal
quenching). Thermal quenching o f p h o t o c o n d u c t i v i t y has r e c e n t l y been observed i n i n t r i n s i c a-Si:H samples prepared under a v a r i e t y o f condi t i o n s l . 6 and has been c o r r e l a t e d w i t h i n f r a r e d quenching of.
p h o t o c o n d ~ c t i v i t y . l ~ ~ I n f r a r e d quenching i s t h e r e s u l t o f o p t i c a l e x c i t a - t i o n s o f e l e c t r o n s from the valence band t o empty S-centers and t h e sub- sequent capture o f some o f t h e f r e e holes i n t o r-centers.
The dual beam modulation spectra Pm(hym) i n Figure 2 c l e a r l y demonstrate i n f r a r e d quenching o f photoconductivity. S o l i d l i n e s i n the f i g u r e i n d i c a t e negative modulation photoconductance. Thus a t T=100K t h e t o t a l photoconductance o f t h e sample i s decreased upon the a d d i t i o n o f t h e beam f o r 0.6 eV
<
hy,,<
1.5 eV. . N o t e t h a t Pm i s t h e rms change i nG normalized l i n e a r l y t o t h e modulation beam photon f l u x and t h a t t h e cRange i n GP i s a small p e r t u r b a t i o n upon the t o t a l photoconductance.
I n an e a r l i e r paper1 it was shown t h a t t h e low energy o p t i c a l t h r e s h o l d f o r i n f r a r e d quenching o f GP i s 0.58
+
0.05 eV and i s temperatureindependent and t h e same for many samples. T h i s i n d i c a t e s t h a t t h e s- centers are d i s t r i b u t e d i n energy above 0.6 eV above t h e valence band.
From the s a t u r a t i o n o f the magnitude o f t h e i n f r a r e d quenching e f f e c t w i t h i n c r e a s i n g m d u l a t i o n beam i n t e n s i t y we deduce an e l e c t r o n capture coef- f i c i e n t f o r S-centers o f -4 X 10-13 cm3 sec-l a t 1 3 3 ~ . 1 This value suggests charged centers because t y p i c a l ca t u r e c o e f f i c i e n t s f o r n e u t r a l
!
centers i n c r y s t a l s are about 10-8 cm3 sec-
.
The low T quenching e f f e c t disappears when T i s increased above 210K as t h e t r a p quasi-Fermi l e v e l f o r holes passes through t h e S-center d i s t r i b u t i o n .In
temperature r e g i o n I 1 1 a d i f f e r e n t i n f r a r e d quenching signal i s observed w i t h a t h r e s h o l d energy o f -0.9 eV, as shown by the curve l a b e l e d 300K i n Figure 3. A negative modulation e f f e c t w i l l occur whenever t h e r e i s an o p t i c a l exchange o f charge from a center w i t h a small capture coef- f i c i e n t t o one w i t h a l a r g e one as long as both l e v e l s are enclosed by t h e i r r e s p e c t i v e t r a p quasi-Fermi l e v e l s . Supral i n e a r i t y w i l l only bePhotoconductance G, o f an i n t r i n s i c a-Si:H sample as a'function o f temper- ature. Band g p photon f l u x i s c 2 X
1016 cm-2 sec-
B .
photon energy (eV)
F i q u r e 3
Same as i n Fig. 2 except T=200 K and 300 K.
F i q u r e
Normalized modulation photoconductance Pm (hum1 f o r several T measured i n ohase w i t h t h e modulation beam chopped a t ' 5 Hz.
S o l i d l i n e s denote n e g a t i v e and dashed l i n e s denote p o s i t i v e modulation s i g n a l .
photon energy (eV1 Fiqure 4
Normalized modulation photoconductance Pm (hvm) a t 220 K measured 90' o u t o f phase w i t h t h e modulation beam chopped a t 4 Hz.
C4-600 JOURNAL DE PHYSIQUE
observed when the small c o e f f i c i e n t center l i e s below and the l a r g e coef- f i c i e n t center above EF. We designate these small e l e c t r o n capture coef- f i c i e n t centers as X-centers. Since supral i n e a r i t y i s not observed, e i t h e r
( i ) t h e number o f X-centers i s small o r ( i i ) the X-centers l i e above t h e dark Fermi l e v e l . The h i g h t h r e s h o l d energy o f 0.9 eV supports t h e l a t t e r suggestion. I f t h e r a t i o o f e l e c t r o n capture c o e f f i c i e n t s o f r- and x- centers were constant i n T one would expect t o observe the high t h r e s h o l d quenching process not o n l y f o r
T
>250 b u t a l s o a t lower T. The absence of t h i s e f f e c t a t lower T suggests t h a t the X-center capture c o e f f i c i e n t decreases w i t h respect t o t h e sr-center c o e f f i c i e n t as T increases. One p o s s i b l e mechanism i s t h e capture o f e l e c t r o n s i n t o X-centers through an i n t e r m e d i a t e t r a p o r e x c i t e d state. We note here t h a t modulation spectra above 200K are sample dependent, however, o n l y two classes o f behavior are observed i n i n t r i n s i c ~ a r n ~ l e s . 8We t u r n now t o an i n t e r e s t i n g and as y e t unexplained modulation f e a t u r e which i s mast apparent a t 220K f o r a pump f l u x o f 2 X
1016 cm-2 sec-l. The Pm(hwn) spectrum measured i n phase w i t h the 5 Hz chopped modulation beam i s l a b e l e d 220K i n Figure 3. The shape o f the spectrum suggests a p o s i t i v e bump a t 1.0 eV superposed upon the normal p o s i t i v e background due t o e x c i t a t i o n from the band t a i l . The s p e c t r a l shape o f the bump can be i s o l a t e d by n o t i n g t h a t t h e spectrum i n t h i s r e g i o n i s composed o f a slow ( T W 32 msec) and a f a s t response time ( T
<
1.4 msec) component; removal o f the f a s t background i s accomplished by t u n i n g t o t h e signal which i s 90' out o f phase w i t h t h e modulation e x c i t a - t i o n a t a low chopping speed o f 4 Hz. The r e s u l t i n g spectrum i n Figure 4 c l e a r l y shows t h e peaked nature o f t h i s new feature. Peaked behavior i s i n d i c a t i v e o f a d i r e c t l o c a l i z e d - t o - l o c a l i z e d s t a t e t r a n s i t i o n between two bands o f s t a t e s which a f f e c t s t h e recombination k i n e t i c s . Further work i s necessary t o c l a r i f y the o r i g i n o f t h i s e f f e c t .
Dual beam p h o t o c o n d u c t i v i t y modulation i s a valuable t o o l f o r elu- c i d a t i n g t h e nature o f gap s t a t e s i n amorphous semiconductors. We have e s t a b l i s h e d the existence o f a t l e a s t t h r e e d i s t i n c t types o f gap s t a t e i n a-Si:H which are d i s t r i b u t e d i n w e l l - d e f i n e d energy ranges and which c o n t r o l the l i f e t i m e o f f r e e c a r r i e r s . We thank E. A. S c h i f f f o r h e l p f u l discussions. This work was supported i n p a r t by t h e NSF-MRL program a t The U n i v e r s i t y o f Chicago and by NSF Grant DMR-8009225.
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1. Persans, P. D., and Fritzsche, H., AIP Conf. Proc. o f I n t ' l . Conf. on
" T e t r a h e d r a l l y Bonded Amorphous Semiconductors" Carefree, AZ, March 1981, and Persans, P. D., t o be published.
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2041, 1979.3. Simmons, J. G. and Taylor, G. W., Phys. Rev. 84, 502, 1971.
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Non-Cryst. S o l i d s 35/36, 391, 1980.
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