Kinetics of Photosensitivity in Ge-Sb-Se Thin Films
M. Olivier
1
, R. Boidin
1
, P. Hawlová
1
, P. Němec
1
and V. Nazabal
2,1
1
Department of Graphic Arts and Photophysics, Faculty of Chemical Technology, University of Pardubice,
Studenská 573, 53210 Pardubice, Czech Republic
2
Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, Equipe Verres et Céramiques,
Université de Rennes1, 35042 Rennes, France
Keywords: Amorphous Chalcogenides, Thin Films, Kinetics, Photosensitivity, Photodarkening, Photobleaching.
Abstract: Chalcogenide (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
thin films obtained using pulsed laser deposition are exposed to light
with energy close to band gap energy, in order to investigate kinetics of photoinduced phenomena. It
appears that a reversible transient photodarkening is observed. The metastable part of photodarkening,
which seems to be slower, is followed by photobleaching. A modelling of the evolution of transmission
during illumination suggests that each process has an independent effective time constant, and that
magnitude of photoinduced phenomena depends on various parameters, such as laser’s fluency, absorption
coefficient and composition.
1 INTRODUCTION
Photoinduced phenomena (appearing when the
material is illuminated with a light with energy close
to band gap energy) are among the most known
properties of amorphous chalcogenides.
Photoinduced phenomena in thin films are typically
linked with the changes of optical parameters,
mainly the refractive index (Kolobov, 2006; Němec
and Frumar, 2009; Petkov and Ewen, 1999; Todorov
et al., 2010) and the band gap energy (Němec and
Frumar, 2009; Petkov and Ewen, 1999). These
variations depend on composition (Kumar et al.,
2013; Todorov et al., 2010). The most notable effect,
observed in Ge-based amorphous thin films is photo-
bleaching (PB) (Yan et al., 2011). Photo-darkening
(PD) is also observed, mainly in As-based layers
(Antoine et al., 2009; Ganjoo et al., 2002). These
changes can be reversible or irreversible. Reversible
phenomena are observed in well annealed films
whereas irreversible phenomena are observed in as-
deposited films (Kolobov, 2006; Němec and Frumar,
2009).
PD, which is the most studied effect, is known to
presents a transient part (TPD) and a metastable part
(Antoine et al., 2009; Flaxer et al., 2009; Ganjoo et
al., 2002). TPD occurs only during illumination and
disappears when pump beam is switched off,
whereas metastable part can only be reversed by
annealing around glass transition temperature. Yan
recently studied photosensitivity in GeSe
2
films and
showed that PB and TPD can be observed for this
film (Yan et al., 2011). PB was linked to intrinsic
structural changes in the vitreous matrix, i.e. the
increase of Ge-Se bond density, and thus the
increase of the ordering of the local structure. The
most popular theory to explain the mechanism of
transient PD is the bond switching and atom
movement under illumination (Barik et al., 2013;
Kumar et al., 2013; Yan et al., 2011). This transient
state is present because of the existence of an
intermediate state of electron transitions between
ground and photoexcited state.
Kinetics of photosensitivity in more complicated
ternary systems are poorly studied and mainly
concern Ge-As-Se compositions (Barik et al., 2011;
Khan et al., 2012). Khan recently reported
coexistence of fast PD (presenting a transient part
and a metastable part), and slow PB in Ge
19
As
21
Se
60
and Ge
x
As
35-x
Se
65
glasses (Khan et al., 2014, 2012).
Khan reported that the light-induced response
depends on the Ge:As ratio and of the rigidity of the
glassy network (Khan et al., 2014). In this paper, we
report on kinetics of photosensitivity of (GeSe
2
)
100-
x
(Sb
2
Se
3
)
x
thin film for x=5 to 40. Previous study
showed, for these compositions, a PB effect after
irradiation of as-deposited films. Furthermore, it was
observed that when x increases, thin films were less
sensitive to irradiation.
67
Olivier M., Boidin R., Hawlová P., N
ˇ
emec P. and Nazabal V..
Kinetics of Photosensitivity in Ge-Sb-Se Thin Films.
DOI: 10.5220/0005332000670072
In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 67-72
ISBN: 978-989-758-092-5
Copyright
c
 2015 SCITEPRESS (Science and Technology Publications, Lda.)

2 EXPERIMENTAL PART
Thin films are obtained using pulsed laser deposition
(PLD). Targets are 25 mm diameter chalcogenide
glasses from (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
system, where x
is varying from 5 to 40. Their synthesis was reported
elsewhere (Olivier et al., 2014). Targets were
ablated with a KrF excimer laser emitting at 248 nm.
Output pulse energy was 300±3 mJ with a pulse
duration of 30 ns and a repetition rate of 20 Hz.
Laser fluency is set at 2.6 J.cm
-2
. Pressure in the
vacuum chamber at the beginning of deposition is
about 3-4.10
-4
Pa. In order to obtain films with
uniform thickness, off axis PLD technique with
rotating substrates and targets was used. Substrates
were microscope glass (10 mm x 15 mm) and were
positioned parallel to the target at a distance of 5 cm.
The study of kinetics was carried out on as-
deposited films so in irreversible regime.
Photosensitivity was studied for exposures with laser
light in band gap region during 2 hours, with power
density of 160 mW.cm
-2
and a laser-sample distance
of 50 cm. The probe beam is a low intensity white
light operating in the 400 -1000 nm wavelength
range. The two beams were directed such as they
cross each other at the sample surface with the
pumping light completely overlapping the
probe light.
During illumination, transmission of thin layer is
recorded every second using a Stellar Inc EPP 2000
portable spectrophotometer, which can collect the
entire optical spectrum in 50 ms.
To avoid the oxidation of the films during
irradiation, the samples were maintained under
nitrogen flux. For these experiments, 5 different
laser wavelengths were available: 532 nm (2.33 eV),
593 nm (2.09 eV), 635 nm (1.95 eV), 690 nm (1.80
eV) and 808 nm (1.53 eV).
Thicknesses of thin films were extracted from
the variable angle spectroscopic ellipsometry data
measured with an automatic rotating analyzer
ellipsometer (VASE, J. A. Woollam Co., Inc.).
Measurement parameters were the following:
spectral region 300-2300 nm (i.e. 4.13-0.54 eV) with
wavelength steps of 10 nm and angles of incidence
of 50°, 60°, and 70°.
Figure 1: Typical transmission time dependences for (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
as-deposited films under different irradiation
wavelengths, (a) x = 5, (b) and (c) x = 30.
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68

3 RESULTS AND DISCUSSION
3.1 Experimental Results
As-deposited (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
thin layers were
homogeneous according to SEM observations. The
films show a good planarity with a smooth surface
and neither cracks nor breaks. Table 1 detailed
thicknesses of chalcogenide layers, determined from
VASE measurements. To study the effect of pump
beam illumination, transmission spectra were
previously recorded in dark condition (Initial
transmission). Then the pump beam was turned on
and transmission was recorded as a function of time
during 2 hours. Figure 1 presents typical evolution
of transmission with time at a wavelength close to
the band gap for various compositions.
First experimental results clearly reveal the
existence of fast PD phenomena. Regarding
irradiation of as-deposited films, the fast PD step is
followed by a slow PB process for x=5, 10, and 20.
For x=20, at irradiation wavelength of 593 nm,
absorption coefficient is higher (45 000 cm
-1
, instead
of ~20 000 cm
-1
for the 2 first samples).
In that case, PB seems to be even slower. For
x=30, when α~66 000 cm
-1
no slow phenomenon is
observed after 2 hours irradiation; when α~32 000
cm
-1
PB is observed but is not saturated after 2 hours
irradiation.
For x=20 irradiated at 635 nm and x=30
irradiated at 690 nm, one can observe a slow PD
effect, occurring after the fast PD and before the
beginning of the increase of transmission. The PB
process seems to start only after saturation of the
slow PD process (Khan et al., 2012). For x=40, slow
PB is observed and seems to saturate after 2 hours
irradiation.
Kinetics of PB depends on absorption coefficient
of the layer at irradiation wavelength (Ganjoo et al.,
2006, 2000) and the phenomenon is fastest for 15
000 cm
-1
<α<25 000 cm
-1
. It thus could be of interest
to work with a tunable laser to precisely choose
irradiation wavelength.
Long exposure during 8 hours at 690 nm of a
film (x=30) was performed. Fig.2 presents the
evolution of the transmission at 640 nm with time
for this film.
A PB effect occurs after the fast PD observed
during the first seconds, as observed in Figure 1.
Nevertheless, it seems that kinetic of PB is
particularly slow and that PB does not saturate, even
after 8 hours irradiation.
The kinetic of PD and PB depends on fluence of
pump beam (Khan et al., 2012), absorption
coefficient at irradiation wavelength (Ganjoo et al.,
2006), and temperature (Barik et al., 2011) but in
case of (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
system, it also depends
on vitreous composition and when x increase, PB
magnitude becomes lower.
Figure 2: Evolution of transmission at 640 nm for an as-
deposited film (x=30) under 690 nm irradiation light (8
hours exposure).
PB effect in GeSe
2
can be related to either
photooxidation (Spence and Elliott, 1989; Tichý et
al., 1995; Yan et al., 2011) or structural ordering.
Irradiations here are performed under inert
atmosphere and observed PB can thus be related to a
decrease in the density of localized states at the edge
of the conduction band, which enlarges the band
gap, leading to its blue shift (Nang et al., 1979).
Photosensitivity of samples seems to decrease
with x (by adding Sb
2
Se
3
). In a similar system (Ge-
As-Se), several authors show that photosensitivity
decrease with rigidity (mean coordination number)
of the network (Calvez et al., 2010; Khan et al.,
2014). This is not the case in Ge-Sb-Se system
because photosensitivity increases with rigidity.
Another mechanism should take place.
Disorder is mainly due to homopolar Ge-Ge and
Se-Se bonds which are found in as-deposited films.
PB process is thus proposed as the increase of
heteropolar Ge-Se bonds density (Ganjoo et al.,
2006; Kotsalas et al., 1998; Yan et al., 2011).
Considering these assumptions, one can explain the
decrease of PB magnitude when x increases with the
lower number of homopolar Ge-Ge and Se-Se bonds
in films.
3.2 Modeling
To model the kinetics of photoinduced response, a
combination of stretched exponential functions that
describe PB and PD can be used (Ganjoo et al.,
KineticsofPhotosensitivityinGe-Sb-SeThinFilms
69

Table 1: Fitting parameters obtained from equation (3) for as-deposited films from (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
at various
irradiation wavelengths. Absorption coefficient is given for each film at irradiation wavelength.
x
Irradiation
wavelength (nm)
Thickness
(nm)
± 1nm
Absorption
coefficient
(cm
-1
)
C
d
τ
d
(sec)
β
d
ΔT
sd
τ
b
(sec) β
b
ΔT
sb
5 593 595 18434 11.9 72.5 0.68 41.9 1372.8 0.50 11.9
10 593 690 22582 4.9 77.3 0.67 7.01 1441.9 0.49 9.3
20 593 778 45385 0.9 207.3 0.68 10.1 786000 0.5 5.1
635 778 25800 5.1 531.5 0.68 19.9 7181.2 0.48 7.9
30 593 827 66073 1.4 16.5 0.68 11.4 1.3.10
7
0.50 3.07
690 827 32589 1.2 912.3 0.68 14.4 74000 0.50 4.0
40 690 1567 30981 1.0 16.5 0.72 8.3 1.10
7
0.50 3.6
808 1567 7526 0.6 12.7 0.73 9.4 1.10
7
0.50 1.6
2002; Khan et al., 2012). PD is usually described as
follows:
∆TC


exp
t
τ






ΔT

(1)
Then, PB can be written as:
∆TΔT


1exp
t
τ






(2)
In equations (1) and (2), subscripts d and b stand
for darkening and bleaching, respectively. ΔT
represents the change in transmission occurring
during irradiation. C
d
is a temperature dependent
constant and corresponds to the total amount of
transient PD. This value is obtained during the
experimental data fitting. t is the illumination time in
seconds. τ is the effective time constant, and β is the
dispersion parameter (0≤β≤1). ΔT
s
is the saturated
value of ΔT (i.e. the metastable part of the changes).
The whole kinetics of photosensitivity in case of
as-deposited film is a summation of equations (1)
and (2):
∆TC


exp
t
τ






ΔT

ΔT


1
exp
t
τ






(3)
The experimental data fit well to the stretched
exponential functions forms detailed in equation (3)
as shown in Figure 1 (red curves). Fitting parameters
calculated from theoretical equations are listed in
Table 1 for as-deposited films.
Our results confirm previous studies (Khan et al.,
2012; Yan et al., 2012) showing that PD is faster
than PB. Behavior of thin films under irradiation is
nevertheless different depending on their
composition. For x=5 and 10, PD has a time constant
of around 75 seconds and PB has a time constant of
1400 seconds.
For x=20 and 30, a slow PD is observed after the
fast PD process, occurring during the first minute.
That is the reason why the time constants obtained
from the fits are larger (531 sec and 912 sec). We
thus highlight existence of two different kinetics for
the PD, which may be relevant for TPD and
metastable part of PD. Regarding this results, it is
nevertheless difficult to say if PB starts after
saturation of the PD process or if there is a
competition between the two phenomena from the
beginning of irradiation.
When x=40, only the fast PD is observed, with a
time constant of around 15 seconds, followed by a
slight increase of the transmission which seems to
saturate after 2 hours of irradiation.
In order to differentiate the transient and
metastable part of PD, pump beam was turned on
and off alternatively, every 5 minutes during 2
hours. On turning the pump beam off after a first
irradiation, the transmission increases further and
saturates quickly (Fig. 3), but does not reach the
initial transmission value, revealing the metastable
part of PD. When illumination is subsequently
turned on, transmission decreases to reach the value
before illumination was switched off. The on/off
cycles of pump beam are relevant for the TPD.
Indeed, TPD occurs only during illumination and
disappears when pump beam is turned off. This
phenomenon is probably of the same nature as the
TPD observed in As-based chalcogenide films and is
due to bond switching and atomic movement. Fig. 3
demonstrates that TPD as well as metastable part of
PD occur at short illumination time, but metastable
part accumulates with each successive illumination.
This effect was already observed in As-based thin
films (Flaxer et al., 2009).
PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology
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Figure 3: Observation of reversible transient PD in
(GeSe
2
)
90
(Sb
2
Se
3
)
10
PLD thin films irradiated at 593 nm
during 2 hrs. Dotted red line corresponds to pump beam
turn on and blue line to pump beam turn off.
4 CONCLUSIONS
Our experiments clearly demonstrate that TPD is an
instantaneous process, which systematically occurs
during irradiation of (GeSe
2
)
100-x
(Sb
2
Se
3
)
x
PLD thin
films, and is associated with a slower PD process,
corresponding to the metastable part of the
phenomenon. Irradiation of as-deposited films
induces PB, with a higher effective time constant,
which starts after saturation of both TPD and PD.
This phenomenon does not systematically saturate
after 2 hours exposure and its kinetic depends on
fluence of irradiation beam, absorption coefficient of
the film at the irradiation wavelength. Its magnitude
depends also on composition, as for high Sb
2
Se
3
content, PB appears to be very weak.
ACKNOWLEDGEMENTS
Czech Science Foundation (Project No. 13-05082S),
Ministry of Education, Youth, and Sports of the
Czech Republic (Project CZ.1.07/2.3.00/30.0058
“Development of Research Teams at the University
of Pardubice“ and 7AMB13FR039) and the CNRS
PICS (Projet International de Cooperation
Scientifique) program financially supported this
work.
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