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Study of light-induced creation of defects in a-Si:H by means of single and dual-beam photoconductivity
Journal of Non-CrystallineSolids 59 & 60 (1983) 397400 North-Holland Publishing Company
397
STUDY OF LIGHT-INDUCED CREATION OF DEFECTS IN a-Si:H BY MEANS OF SINGLE AND DUAL-BEAM PHOTOCONDUCTIVITY Daxing HANf and H. FRITZSCHE James Franck I n s t i t u t e , The University of Chicago, Chicago, I l l i n o i s
60637
The d i f f e r e n t response of photoconductive properties of a-Si:H to l i g h t exposure and annealing suggests that two kinds of metastable states can be produced. One kind decreases the m o b i l i t y - l i f e t i m e product ~ , the other increases the subbandgap absorption. I.
INTRODUCTION Infrared (IR) and thermal quenching of the photoconductivity ~p is believed
to be caused by IR or thermal e x c i t a t i o n of holes from class I I centers having small electron recombination rates via the valence band to class I centers having large electron recombination rates. 1
The s i m i l a r i t y of the spectral depend-
ence of IR quenching and of photo-induced IR absorption suggests that class I I centers are hole traps.
An additional center has been postulated to explain IR
enhancement above 30OK.
We report here single and dual beam Op measurements2
on an undoped a-Si:H sample a f t e r annealing and l i g h t exposure.
The l a t t e r
(Staebler-Wronski e f f e c t ) 4 is believed to produce metastable dangling bond defects via recombination. 2. EXPERIMENTAL RESULTS The 0.8 Im thick a-Si:H sample was glow-discharge deposited at 500K onto a Corning 7059 substrate which carried predeposited NiCr contacts 0.4 cm long and 0.05 cm apart.
The dark c o n d u c t i v i t y ~d of the annealed state A was activated
with Ea = 0.83 eV and a prefactor oo = 2 X 104 (ohm-cm)- I .
Fig. 1 shows the T-
dependences of od and op (with ~d subtracted) a f t e r four treatments:
A = an-
nealed at 430K f o r l h , A* = annealed at 480K f o r lh, B1 = exposed to AM1 for 3h at 30OK, and B2 = exposed to AM1 for 3h at 16OK. reproducible.
All states are reversible and
We were unable to reach state A* by annealing state A for l l h .
One observes the f a m i l i a r decrease of ~r due to photocreation of metastable recombination centers yet the T-dependences of the four states are noticeably different. fDepartment of Mathematics and Physics, Academia Sinica, B e i j i n g , China. Support provided by The National Science Foundation under Grant No. DMR8009225. 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
D. Han, H. Fritzsche /Study oflight-induced creation o f defects
398
-8
r
I
I
[
300 K
A
ffp ot h~:2eV
~iI\ E E v
-9
o
-
-
j
E
-i0
~-E
o
-
-g -25
-II I
~
[
4
L
I
~
103/T
I
6
5
7
I/Ual .o 48OK onoeol -aTUl//-r°
( K-~)
~/[
FIGURE 1 Temperature dependence of dark and p h o t o c o n d u c t i v i t y of annealed (A,A*) and exposed (BI,B2) a-Si:H
A x 430K onneol
I
Bf ~ 300K exposure 1.0
2.0 3.0 I.O photon energy hl/ (eV)
2.0
FIGURE 2 Spectral dependence of normalized photoconductivity The spectral dependences of the normalized ~p/F at 300K are shown on the l e f t of Fig. 2.
The incident photon f l u x F was changed such that Op and thus
the trap-quasi-Fermi level Etn remained constant over the spectral range.
On
the right the differences in ~T were eliminated by making a l l curves agree with A* at hv = 2 eV.
We interpret the shoulder below hv = 1.4 eV as subbandgap de-
fect-induced absorption ~ in agreement with Amer et a l . 5 although no independent m measurements were made.
An important r e s u l t of our studies is the f o l l o w -
ing. Although low T exposure (B2) reduces Hz as e f f i c i e n t l y
as 300K exposure
(BI) the subbandgap absorption increases much more by exposure at 300K than at 16OK. The dashed curve of Fig. 3 shows the decrease of hT as a f u n c t i o n of the increase in ~ at hv : 1 eV a f t e r i n c r e a s i n g l y longer l i g h t exposures.
We as-
sumed m to be p r o p o r t i o n a l to Op at 1 eV. 7 The increase in m occurs at longer exposures at 300K a f t e r the decrease in ~ e s s e n t i a l l y s a t u r a t e d . posure (B2) only the ~ decrease is observed.
s t a t e at 300K causes an increase in m towards Bl at constant ~ . in Fig. 3 shows the r e s u l t of stepwise annealing from B1 to A*. retrace of the dashed curve one finds t h a t ~
For 160K ex-
A d d i t i o n a l exposure of the B2 The f u l l
curve
Instead of a
recovers more r a p i d l y than m.
This suggests t h a t the photo-induced centers which reduce p~ are not the ones which cause an increase in m , or more p r e c i s e l y , in the r e l a t i v e magnitude of
399
D. Han, H. Fritzsche / Study o f light-induced creation of defects Op/F below 1.4 eV a f t e r normalizing f o r differences in !~ at 2 eV.
IR quenching, which is observed below about 20OK, depends also on the states of anneal and exposure as shown in Fig. 4.
The f i n a l annealing between A and
A* increases quenching s i g n i f i c a n t l y whereas l i g h t exposure diminishes i t w i t h out changing the threshold near 0.42 eV. agree with those of Vanier et a l . 3 t i o n spectra. 6
The shapes of the quenching spectra
and are s i m i l a r to the photoinduced absorp-
Our r e s u l t s suggest t h a t l i g h t exposure diminishes the f r a c t i o n
of class I I center recombination by creating e i t h e r class I or a t h i r d kind of recombination centers. Above approximately 200K one observes 3
IR enhancement which means the dual beam
- -
,
i
~p is l a r g e r than the sum of the two s i n gle beam Op'S.
One cause f o r t h i s e f -
f e c t are the increased number of IR exE i_J
c i t a t i o n s i n t o the conduction band due to the r i s e of Etn in the presence of pump l i g h t .
The results shown in Fig.
5 are measured at 300K f o r the same Opump = 1.2 X 10-7 ohm-I cm-I and nearly the same Etn.
The four states of anneal
and exposure a f f e c t the magnitudes of enhancement and quenching in the oppos i t e manner.
Moreover, the B1 spectrum
shows a pronounced f l a t
region s i m i l a r
I0 v
=L /
o
)
'
~-o • e-
B1 / o
B2@ 2 4 6 8 leV Absorption e (orb. units)
to the B1 s i n g l e beam spectrum of Fig. 2.
There is a very small change in en-
hancement between states A and B2 despite the large change in ~T (see Fig. 3). This suggests t h a t the enhancement
FIGURE 3 Change of o D at h~ = 2 eV (~T) versus absorption at 1 eV as a function of exposure (dashed) and annealing ( s o l i d curve)
process is associated with the cause f o r increased absorption below 1.4 eV and not with the photoinduced recombination centers which decrease ~ . 3. CONCLUSIONS The e f f e c t s of l i g h t exposure at 160K and 300K and of stepwise annealing on single and dual beam Op spectra i n d i c a t e t h a t the metastable centers produced are of two kinds.
One a f f e c t s p r i m a r i l y the p h o t o c a r r i e r l i f e t i m e (~T), the
o t h e r produces an increase in op below 1.4 eV r e l a t i v e to Op at 2 eV (an increase in subbandgap a b s o r p t i o n ) .
This second kind disappears at higher anneal
temperatures than the f i r s t
The photocreated defects are not the sensi-
kind.
t i z i n g states (class I I ) which y i e l d low T quenching but instead they provide a
D. Han, H. Fritzsche / Study o f light4nduced creation o f defects
400
competing recombination channel. o #
300 K
/
O'pomp:L2x~O-7(ohm-cm)-I E
E
'E
TE - I
A'
~o I]OK 8 -I ~Oump:4.7xlO {ohm-crn)
-2
0.4
0:6
o:8
,.%
,.z
,.4
photon energy h~ (eV)
0.6
0.8
1.0
1.2
photon energy hZZ {eV)
FIGURE 4 Normalized infrared quenching at 130K
FIGURE 5 Normalized infrared enhancement at 300K
REFERENCES I) A. Rose, Concepts in Photoconductivity and A l l i e d Problems ( R. E. Krieger, Huntington, N.Y. 1978). 2) P. D. Persans and H. Fritzsche, J. de Physique 42 (1981) C4 - 597; P. D. Persans, Phil. Mag. 46 (1982) 435. 3) P. E. Vanier and R. W. G r i f f i t h ,
J. Appl. Phys. 53 (]982) 4.
4) D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31 (1977) 292. 5) W. B. Jackson and N. M. Amer, Phys. Rev. B 15 (1982) 5559. 6) P. O'Connor and J. Tauc, Solid State Commun. 36 (1980) 947. 7) We assumed ~ to be proportional to ap at hv = 1 eV a f t e r normalizing ~p for the differences in ~T at 2 eV as shown on the r i g h t hand side of figure 2.
1.4
397
STUDY OF LIGHT-INDUCED CREATION OF DEFECTS IN a-Si:H BY MEANS OF SINGLE AND DUAL-BEAM PHOTOCONDUCTIVITY Daxing HANf and H. FRITZSCHE James Franck I n s t i t u t e , The University of Chicago, Chicago, I l l i n o i s
60637
The d i f f e r e n t response of photoconductive properties of a-Si:H to l i g h t exposure and annealing suggests that two kinds of metastable states can be produced. One kind decreases the m o b i l i t y - l i f e t i m e product ~ , the other increases the subbandgap absorption. I.
INTRODUCTION Infrared (IR) and thermal quenching of the photoconductivity ~p is believed
to be caused by IR or thermal e x c i t a t i o n of holes from class I I centers having small electron recombination rates via the valence band to class I centers having large electron recombination rates. 1
The s i m i l a r i t y of the spectral depend-
ence of IR quenching and of photo-induced IR absorption suggests that class I I centers are hole traps.
An additional center has been postulated to explain IR
enhancement above 30OK.
We report here single and dual beam Op measurements2
on an undoped a-Si:H sample a f t e r annealing and l i g h t exposure.
The l a t t e r
(Staebler-Wronski e f f e c t ) 4 is believed to produce metastable dangling bond defects via recombination. 2. EXPERIMENTAL RESULTS The 0.8 Im thick a-Si:H sample was glow-discharge deposited at 500K onto a Corning 7059 substrate which carried predeposited NiCr contacts 0.4 cm long and 0.05 cm apart.
The dark c o n d u c t i v i t y ~d of the annealed state A was activated
with Ea = 0.83 eV and a prefactor oo = 2 X 104 (ohm-cm)- I .
Fig. 1 shows the T-
dependences of od and op (with ~d subtracted) a f t e r four treatments:
A = an-
nealed at 430K f o r l h , A* = annealed at 480K f o r lh, B1 = exposed to AM1 for 3h at 30OK, and B2 = exposed to AM1 for 3h at 16OK. reproducible.
All states are reversible and
We were unable to reach state A* by annealing state A for l l h .
One observes the f a m i l i a r decrease of ~r due to photocreation of metastable recombination centers yet the T-dependences of the four states are noticeably different. fDepartment of Mathematics and Physics, Academia Sinica, B e i j i n g , China. Support provided by The National Science Foundation under Grant No. DMR8009225. 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
D. Han, H. Fritzsche /Study oflight-induced creation o f defects
398
-8
r
I
I
[
300 K
A
ffp ot h~:2eV
~iI\ E E v
-9
o
-
-
j
E
-i0
~-E
o
-
-g -25
-II I
~
[
4
L
I
~
103/T
I
6
5
7
I/Ual .o 48OK onoeol -aTUl//-r°
( K-~)
~/[
FIGURE 1 Temperature dependence of dark and p h o t o c o n d u c t i v i t y of annealed (A,A*) and exposed (BI,B2) a-Si:H
A x 430K onneol
I
Bf ~ 300K exposure 1.0
2.0 3.0 I.O photon energy hl/ (eV)
2.0
FIGURE 2 Spectral dependence of normalized photoconductivity The spectral dependences of the normalized ~p/F at 300K are shown on the l e f t of Fig. 2.
The incident photon f l u x F was changed such that Op and thus
the trap-quasi-Fermi level Etn remained constant over the spectral range.
On
the right the differences in ~T were eliminated by making a l l curves agree with A* at hv = 2 eV.
We interpret the shoulder below hv = 1.4 eV as subbandgap de-
fect-induced absorption ~ in agreement with Amer et a l . 5 although no independent m measurements were made.
An important r e s u l t of our studies is the f o l l o w -
ing. Although low T exposure (B2) reduces Hz as e f f i c i e n t l y
as 300K exposure
(BI) the subbandgap absorption increases much more by exposure at 300K than at 16OK. The dashed curve of Fig. 3 shows the decrease of hT as a f u n c t i o n of the increase in ~ at hv : 1 eV a f t e r i n c r e a s i n g l y longer l i g h t exposures.
We as-
sumed m to be p r o p o r t i o n a l to Op at 1 eV. 7 The increase in m occurs at longer exposures at 300K a f t e r the decrease in ~ e s s e n t i a l l y s a t u r a t e d . posure (B2) only the ~ decrease is observed.
s t a t e at 300K causes an increase in m towards Bl at constant ~ . in Fig. 3 shows the r e s u l t of stepwise annealing from B1 to A*. retrace of the dashed curve one finds t h a t ~
For 160K ex-
A d d i t i o n a l exposure of the B2 The f u l l
curve
Instead of a
recovers more r a p i d l y than m.
This suggests t h a t the photo-induced centers which reduce p~ are not the ones which cause an increase in m , or more p r e c i s e l y , in the r e l a t i v e magnitude of
399
D. Han, H. Fritzsche / Study o f light-induced creation of defects Op/F below 1.4 eV a f t e r normalizing f o r differences in !~ at 2 eV.
IR quenching, which is observed below about 20OK, depends also on the states of anneal and exposure as shown in Fig. 4.
The f i n a l annealing between A and
A* increases quenching s i g n i f i c a n t l y whereas l i g h t exposure diminishes i t w i t h out changing the threshold near 0.42 eV. agree with those of Vanier et a l . 3 t i o n spectra. 6
The shapes of the quenching spectra
and are s i m i l a r to the photoinduced absorp-
Our r e s u l t s suggest t h a t l i g h t exposure diminishes the f r a c t i o n
of class I I center recombination by creating e i t h e r class I or a t h i r d kind of recombination centers. Above approximately 200K one observes 3
IR enhancement which means the dual beam
- -
,
i
~p is l a r g e r than the sum of the two s i n gle beam Op'S.
One cause f o r t h i s e f -
f e c t are the increased number of IR exE i_J
c i t a t i o n s i n t o the conduction band due to the r i s e of Etn in the presence of pump l i g h t .
The results shown in Fig.
5 are measured at 300K f o r the same Opump = 1.2 X 10-7 ohm-I cm-I and nearly the same Etn.
The four states of anneal
and exposure a f f e c t the magnitudes of enhancement and quenching in the oppos i t e manner.
Moreover, the B1 spectrum
shows a pronounced f l a t
region s i m i l a r
I0 v
=L /
o
)
'
~-o • e-
B1 / o
B2@ 2 4 6 8 leV Absorption e (orb. units)
to the B1 s i n g l e beam spectrum of Fig. 2.
There is a very small change in en-
hancement between states A and B2 despite the large change in ~T (see Fig. 3). This suggests t h a t the enhancement
FIGURE 3 Change of o D at h~ = 2 eV (~T) versus absorption at 1 eV as a function of exposure (dashed) and annealing ( s o l i d curve)
process is associated with the cause f o r increased absorption below 1.4 eV and not with the photoinduced recombination centers which decrease ~ . 3. CONCLUSIONS The e f f e c t s of l i g h t exposure at 160K and 300K and of stepwise annealing on single and dual beam Op spectra i n d i c a t e t h a t the metastable centers produced are of two kinds.
One a f f e c t s p r i m a r i l y the p h o t o c a r r i e r l i f e t i m e (~T), the
o t h e r produces an increase in op below 1.4 eV r e l a t i v e to Op at 2 eV (an increase in subbandgap a b s o r p t i o n ) .
This second kind disappears at higher anneal
temperatures than the f i r s t
The photocreated defects are not the sensi-
kind.
t i z i n g states (class I I ) which y i e l d low T quenching but instead they provide a
D. Han, H. Fritzsche / Study o f light4nduced creation o f defects
400
competing recombination channel. o #
300 K
/
O'pomp:L2x~O-7(ohm-cm)-I E
E
'E
TE - I
A'
~o I]OK 8 -I ~Oump:4.7xlO {ohm-crn)
-2
0.4
0:6
o:8
,.%
,.z
,.4
photon energy h~ (eV)
0.6
0.8
1.0
1.2
photon energy hZZ {eV)
FIGURE 4 Normalized infrared quenching at 130K
FIGURE 5 Normalized infrared enhancement at 300K
REFERENCES I) A. Rose, Concepts in Photoconductivity and A l l i e d Problems ( R. E. Krieger, Huntington, N.Y. 1978). 2) P. D. Persans and H. Fritzsche, J. de Physique 42 (1981) C4 - 597; P. D. Persans, Phil. Mag. 46 (1982) 435. 3) P. E. Vanier and R. W. G r i f f i t h ,
J. Appl. Phys. 53 (]982) 4.
4) D. L. Staebler and C. R. Wronski, Appl. Phys. Lett. 31 (1977) 292. 5) W. B. Jackson and N. M. Amer, Phys. Rev. B 15 (1982) 5559. 6) P. O'Connor and J. Tauc, Solid State Commun. 36 (1980) 947. 7) We assumed ~ to be proportional to ap at hv = 1 eV a f t e r normalizing ~p for the differences in ~T at 2 eV as shown on the r i g h t hand side of figure 2.
1.4