PHYSICAL REVIEW B
VOLUME 38, NUMBER 17
Reversible photoinduced
15 DECEMBER 1988-I
change of ac conduction in amorphous
As2S3 films
K. Shimakawa
Department
Department
of Physical
of Electronics,
Chemistry,
Gifu 501-11, Japan
Gifu University,
S. R. Elliott
of Cambridge,
Lensfteld Road, Cambridge, England
University
(Received 2 Iune 1988)
Prolonged exposure to strongly absorbed light increases the ac conductivity (o„) of well-annealed
amorphous As&S3 films, suggesting that new localized states are induced in the band gap. This
change is enhanced by low-temperature
illumination.
Although the change caused by roomtemperature illumination is removed by annealing near the glass temperature Tg, the change caused
(90 K) illumination is annealed out at around 200 K. This suggests that
by low-temperature
different centers, which depend on temperature, are induced by illumination. As the change in the
optical gap (Eo) induced by low-temperature illumination is removed by annealing at Tg (and is not
removed at 200 K), the change in Eo appears not to be directly related to this defect formation.
I. INTRODUCTION
exhibit a
chalcogenide semiconductors
Amorphous
number of interesting reversible changes in their physical,
chemical, and mechanical properties on illumination by
reversible
Such
photoinduced
band-gap
light. '
changes occur in vapor-deposited films annealed near the
T and in melt-quenched
glass transition temperature
glasses, in which case the changes can be removed by annealing to Tg.
There are many reports on photoinduced optical and
structural studies in amorphous chalcogenides'
and
several mechanisms have been proposed to account for
these photoinduced changes.
It is evident that photoinduced reversible changes accompany defect creation
Recent
by illumination and annihilation by annealing.
reports on reversible photoinduced changes in electrical
transport such as ac conductivity and photoconductivity'
show that new localized states are induced in the
band gap. However, at present, it is not clear whether
the reversible change in the optical gap Eo (photodarkening)' is directly related to defect formation or not.
In the present study, photoinduced changes in cr„and
Eo have been measured for amorphous As2S3 films which
are known to exhibit large photoinduced change in physical quantities such as the optical gap or volume expanis much larger than that obsion. ' The change in
served in Eo. It is demonstrated, through the difference
of annealing effects on induced changes in o.„and Eo,
that the photodarkening is not directly related to defect
'
"
'
tacts were made of aluminum deposited by thermal evaporation. The samples were illuminated with a mercury
lamp having an ir-cut filter (59 mW/cm ) through the
back indium-tin-oxide contact in an evacuated cryostat.
The ac conductivity was measured using a capacitance
bridge (Ando Denki TR-1C).
III. RESULTS AND DISCUSSION
Figure 1 shows the reversible changes of the ac conductivity and the optical gap by photoirradiation at 300
K (P) and thermal annealing at 170'C ( A and Az ) for a
film of thickness 0.7 pm. The value of o. measured at 1
kHz, increases by about 40% (state P) after illumination
(60 min). After 30 min annealing (state Az ), cr„returns
to its original value (state A, ). At the same time, Eo also
reveals reversible changes as indicated in Fig. 1. The
fractional change in Eo is 1.7%, which is much smaller
than that in o.
All these things confirm that we have
observed reversible changes of o.„and Eo between annealed and photoirradiated states. Overall features of the
reversible changes (o „and Eo ) are similar to those in the
previous paper.
&
„,
„.
3QP
-11
10
Eo= 2.41 eV
2. 37
K
2.41
o„
formation.
1
IE
V
LA
b
II. EXPERIMENTAL
11Q Hz
10
(-1 pm)
Thin films of amorphous AszS3
were prepared
by thermal evaporation of glassy AszS3 (99.999% pure)
onto indium-tin-oxide —coated glasses (evaporation rate 5
A/sec at 1 X 10 Torr and with a substrate temperature
300 K). The films were annealed at 170'C for 300 min in
a vacuum with a pressure of 1X10 Torr. Front con38
kHz
-12
10
13
P
A1
A2
FIG. 1. Reversible changes in the ac conductivity (110 Hz
and 1 kHz) measured at 300 K for an a-As2S3 film of thickness
0.7 pm. A& and A2 are the annealed states (170'C). P is the
photoirradiated (300 K) state. Values of the optical band gap
are shown for each state.
12 479
1988
The American Physical Society
K. SHIMAKA%'A AND S. R. ELLIOTT
12 480
Several mechanisms have been proposed to account for
It is evident
the reversible photoinduced changes.
that photoinduced reversible changes accompany defect
creation by illumination and annihilation by annealing.
The present experiments, showing an increase of o „(Fig.
1), also suggest the creation of defects by photoirradiation. Biegelsen and Street suggested from electron-spinresonance (ESR) measurements that the reversible change
in Ep is related to a bond-breaking
mechanism, where
self-trapped excitons (conjugate pairs of charged defects
D+
-are induced by illumination. A recent unified
model supports this model.
Such D+-D
close pairs could contribute to the ac
conduction particularly for glassy chalcogenides. ' Following E11iott, ' the ac conductivity in the correlated
barrier hopping (CBH) model is given by
38
9OZ
'
EO=2 4~ eV
/
2 2
)= N~e
12
X exp
8e 2
1-/+5 s-p
/
eV
/
/
/
tf)
/
/
E
1
kHz
/
/
1Q
/
V
/
/
/
/
D)'
(
2. 32
/
. 11Q Hz
A)
A2
FIG. 2. Reversible changes in
at 90 K. Ai is the well-annealed
room-temperature annealed state.
gap measured at room temperature
the ac conductivity measured
state (170'C) and A2 is the
Changes in the optical band
are shown for each state.
e8'M
SkT
k T tanh( b, o/2k
T),
where N is the spatial density of defects, e the eff'ective
dielectric constant, cu the angular frequency, 7 p a characteristic relaxation time, 8'M the maximum barrier height
for bipolaron hopping which is taken to be nearly equal
to the band gap, T the glass transition temperature, and
ho the maximum site-energy difference (broadening of
states in the gap).
The quantities
defect-energy
P=6kT/WM and 5=T/8Tg for AszS& are almost the
same (taking the values WM=2. 4 eV for annealed films
and 2.37 eV for irradiated films and Tg =450 K).
is
thus predicted to be proportional to frequency. By taking appropriate physical parameters, @=10 and Ap=100
meV, which are the same values used in the previous paper, the spatial density of defects N is estimated to be
1.8X10' cm
for the annealed film and 2.7X10' cm
for the illuminated film. The photoinduced density of defects seems to be smaller than that (10' —10 cm —3) estimated from the concentration of photoinduced spins at
30 K. Self-annealing and creation of defects could take
place simultaneously for room-temperature illumination.
The change in Ep is known to be enhanced by lowilLow-temperature
temperature
photoirradiation. '
lumination and measurement of o,, could provide more
detailed information.
Figure 2 shows the reversible
change in o. together with the change in Ep. Note that
The measureEp was measured at room temperature.
ment of o„(90 K) was made 3 h after cessation of illumination at 90 K. Large changes (about an order of
magnitude) in cr„a dFno were observed between 3, (before illumination; annealed at 170 C) and P (illuminated)
states. 32 corresponds to the state annealed at 300 K.
o. for state Az is the same as that for 3, . However, the
anchange in Ep is not removed by room-temperature
nealing. This suggests that the centers induced by illumination at 90 K, which contribute to the ac conduction, are annealed out at room temperature, although the
centers related to the change in Ep are not annealed out
at room temperature.
Photoinduced changes in Ep can
be removed by annealing to T . '
The frequency dependence of conductivity o. is shown
in Fig. 3 for the same sample stated above. o. is proportional to ~' with s =1.0. The frequency exponent s is unchanged by photoillumination,
suggesting that a change
in the density of D+-D close pairs dominates
The
other physical parameters, except for WM (Eo), appearing in Eq. (1) cannot be changed with illumination.
Again, by taking the same parameters
e, ho) and using 8'M=2. 32 eV, a value of N9p
1.7X10' cm
for
the 90-K-irradiated film is estimated. The photoinduced
density of N9Q K is consistent with that (10' —10 cm
)
„„
o„:
I
(T,
o„
300
K
„,
„
l
]p2
10
t (Hz)
FIG. 3. Frequency dependence of reversible ac conductivities
measured at 300 K (before and after illumination at 300 K) and
at 90 K (before and after illumination at 90 K).
..
REVERSIBLE PHOTOINDUCED CHANGE OF ac CONDUCTION.
38
deduced from the saturated photoinduced spins at 30 K.
Note that, although the spin-active center is the neutral
the density of charged defects (D+ and D ) can be
deduced from the saturated spin density. Note also that
new defects induced by photoirradiation
are annealed
above 200 K (annealing at room temperature restores the
initial state). It is thus expected that centers related to
o.
the same defect centers predicted by Biegelsen
and Street.
for
Figure 4 shows the temperature dependence of
annealed and photoirradiated states at frequency 1 kHz.
First, o for the annealed film is measured down to 90 K
and then photoirradiated.
The photoinduced O.
90 K
was made 3 h after cessation of illumination and then
measured up to room temperature.
It is noted that
temperature-dependent
annealing behavior is qualitatively the same at all frequencies. The photoinduced o„recovers to the initial state around 200 K, which is very
similar to the behavior of the ESR study for As2S3.
The photoinduced close D+-D pair for As2S3 can be
produced by the process shown in Fig. 5. Street' proposed, originally for the case of pure chalcogen glasses,
that an optically created exciton (electron-hole pair) can
lower its energy by nonradiative "self-trapping" instead
of direct (radiative) recombination; the self-trapping exciton (STE) state was envisaged to be a conjugate pair of
D+-D defects. In As2S3, the D+-D pair should correspond to P4+-C&, where P =As and C = S.
Figure 6 shows the configurational-coordinate
diagram
for the process illustrated in Fig. 5. 8'is the barrier to
self-trapping and V is the potential-well depth of a metastable D+-D pair. The values of 8'and V, which depend on the strength of the electron-phonon
coupling,
may be distributed around certain values.
The rate equation for inducing the STE can be written
as
Q
p
(As)
~
C
(5)
D,
12 481
„are
u„
„
C1
„at
f
FIG. 5. Self-trapped
(P =As and C =S).
exciton (STE) mechanism
dN; /dt =Gkp(NT
N, )
for As2S3
—k„—
N, ,
and
k
=vexp(
—WlkT)
and
k„=vexp(
—VlkT),
where N, is the density of STE, XT is the participating total site density, G is a constant that depends on illumination intensity, k is the rate for surmounting the potential
barrier W, and k„ is the rate for surmounting the barrier
V (recovery reaction toward the ground state). v is a
characteristic frequency.
The solution of Eq. (2) is given by
P
I'
This predicts that the STE with weak electron-phonon
coupling [small V and W, Fig. 6(a)j can be induced dominantly for low-temperature illumination; the probability
of inducing STE's with larger V and W [Fig. 6(b)] is very
=1 kHz
Exci ton
1012
o
after
~
before
i
I
lurnina t ion
1013
Energy
|(
= q
Conf iguration
200
100
T
300
(v)
FIG. 4. Temperature dependence of ac conductivities (1 kHz)
for well-annealed ( A, ) and 90-K-illuminated (P) states.
FIG. 6. Configurational-coordinate
diagram for the process
illustrated in Fig. 5. W is the barrier to self-trapping and V is
the potential-well depth of a metastable D -D pair. (a) and
(b) are STE centers with weak and strong electron-lattice coupling, respectively.
K. SHIMAKAWA AND S. R. ELLIOTT
12 482
small. The ratio k l(k +k„) must be larger for lowiltemperature illumination than for high-temperature
lumination and hence X, for 90-K illumination is suggested to be higher than that for room-temperature illumination. Furthermore, due to the small value of V, STE's induced for 90-K illumination might disappear at around
200 K.
IV. CONCLUSIONS
It has been demonstrated that prolonged exposure to
strongly absorbed light increases the ac conductivity of
well-annealed amorphous As2S3 films. This photoinduced
change is enhanced by exposure at low temperatures. Although the change caused by room-temperature illumination is removed by annealing near the glass-transition
temperature, the change induced by 90-K illumination
recovers at around 200 K.
The ac conduction is believed to be dominated by close
D+-D pairs (STE's) induced by photoirradiation. It is
'Ka. Tanaka, J. Non-Cryst. Solids 35436, 1023 (1980).
2S. R. Elliott, J. Non-Cryst. Solids 59469, 899 (1983).
A. J. Lowe, S. R. Elliott, and G. N. Greaves, Philos. Mag. B
'54, 483 (1986).
4Ke Tanaka, Jpn. J. Appl. Phys. 25, 779 (1986).
5S. R. Elliott, J. Non-Cryst. Solids 81, 71 (1986).
D. K. Biegelsen and R. A. Street, Phys. Rev. Lett. 44, 803
(1980).
7R. Grigorovici, A. Vancu, and L. Ghita,
598560, 909 (1983).
J. Non-Cryst.
Solids
38
suggested that STE centers, with strong and weak
electron-phonon
are induced
highcoupling,
by
illumination,
respectemperature and low-temperature
tively. These STE's may be annihilated at different temperatures. The change in tlte optical gap (photodarkenillumination.
ing) is also enhanced by low-temperature
This photoinduced change at low temperatures is not removed at room temperature. Annealing to the glass transition temperature is required to remove photodarkening,
leading to the conclusion that the photodarkening cannot
be directly related to defect creation.
ACKNOWLEDGMENTS
We would like to thank H. Hashimoto for his help in
the experiments. K.S; wishes to thank the Science and
Engineering Research Council (SERC) in the U. K. and
the Japan Society for the Promotion of Science (JSPS) for
financial support.
M. Frumar, A. P. Firth, and A. E. Owen, Philos. Mag. B 50,
463 (1984).
K. Shimakawa, K. Hattori, and S. R. Elliott, Phys. Rev. B 36,
7741(1987).
'
K. Shimakawa, Phys. Rev. B 34, 8703 (1986).
"H. Naito, T. Teramine, M. Okuda, and T. Matsushita,
Non-Cryst. Solids 078r98, 1231 (1987).
'2R. A. Street, Solid State Commun. 24, 363 (1977).
' S. R. Elliott, Adv. Phys. 36, 135 (1987).
J.