654
ARTICLE
Studies of silver photodiffusion dynamics in Ag/GexS1−x (x = 0.2
and 0.4) films using neutron reflectometry1
Can. J. Phys. Downloaded from www.nrcresearchpress.com by University of Saskatchewan on 07/11/14
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Y. Sakaguchi, H. Asaoka, Y. Uozumi, Y. Kawakita, T. Ito, M. Kubota, D. Yamazaki, K. Soyama,
M. Ailavajhala, M.R. Latif, and M. Mitkova
Abstract: To better understand the dynamics of silver photodiffusion into amorphous chalcogenide (Ch) films, it is informative
to probe the time-dependent distribution of silver in the films while they are simultaneously exposed to visible light. Timeresolved neutron reflectometry is particularly well-suited to this purpose because it can follow time-dependent changes in
the multilayer structure (Ag/Ag–Ch/Ch) while excluding the possibility of probe beam induced changes. This paper reports the
results of time-resolved neutron reflectivity measurements of two Ag/GexS1−x (x = 0.2 and 0.4) films as they are exposed to a visible
light source. Analysis showed that silver diffusion occurs via two distinct processes: a fast diffusion that takes place during the
first 2 and 10 min of sample illumination for the x = 0.2 and 0.4 films, respectively; and a subsequent slower change that is
observed over the next 18 min (x = 0.2 film) and 107 min (x = 0.4 film). These results suggest the formation of a relatively stable
Ag-rich phase in the reaction layer followed by slower diffusion at the interface between Ag-rich and Ag-poor layers. Fourier
transform analysis shows that the position of the interface is essentially fixed — a conclusion that contradicts the “diffusion
front” model that has been previously postulated.
PACS Nos.: 82.50.Bc, 78.47.D, 61.05.fj.
Résumé : Afin de mieux comprendre la dynamique de la diffusion de Ag induite par radiation dans les films de chalcogénure
amorphe (Ch), il est instructif de sonder la variation dans le temps de la densité de Ag pendant l’irradiation en lumière visible.
La réflectométrie de neutrons à résolution temporelle est particulièrement bien adaptée à cette fin, parce qu’elle permet de
visualiser la variation dans le temps dans une structure multicouches (Ag/Ag–Ch/Ch), tout en excluant un effet dû à la sonde
elle-même. Nous présentons ici les résultats de nos mesures dans le temps sur deux couches Ag/GexSi1−x, pour x = 0.2 et 0.4,
lorsqu’exposées à la lumière visible. L’analyse montre que la diffusion de l’argent se produit selon deux mécanismes différents : une
diffusion rapide se produit pendant les 2 et 10 premières minutes pour les films x = 0.2 et 0.4 respectivement et une seconde
diffusion plus lente est observée durant les 18 et 107 minutes subséquentes pour les films x = 0.2 et 0.4 respectivement. Ces
résultats suggèrent la formation d’une phase riche en Ag relativement stable à la couche de réaction, suivie par une diffusion plus
lente à l’interface entre les couches riche et pauvre en Ag. L’analyse par transformée de Fourier montre que la position de l’interface
est essentiellement fixe, un résultat qui contredit le modèle du front de diffusion précédemment postulé. [Traduit par la Rédaction]
1. Introduction
Silver photodiffusion in amorphous (a-) chalcogenide films such
as As2S3 and GeSe2 has attracted much interest because of the
potential application of these films in, for example, photolithography [1, 2], memory devices [3], and fabrication of optical elements [4, 5]. This diffusion phenomenon is unique because, rather
than proceeding with a smooth variation in Ag distribution normal to the surface, it shows step-like changes in silver concentration with a distinct “diffusion front”. Until now, it has been thought
that the diffusion front in these systems advances continuously as
the sample is illuminated, stopping only when the silver layer is
exhausted or, when reaching the end of the film. This model of
diffusion is based on numerous previous studies, which used
mainly two types of experimental technique: the measurement of
time-dependent changes in properties, such as electrical resistivity [6] and optical reflectivity [7, 8]; and, the determination of
detailed concentration profiles using Rutherford backscattering
[5, 9–11]. Using this latter approach, Rennie et al. [10] conducted in
situ Rutherford backscattering measurements during illumination of the sample to eliminate the possibility of thermal diffusion. They observed the diffusion of silver into the film under the
influence of visible light and even reported a step-like distribution
of silver in the film during the process. However, no detailed Ag
concentration profile was given. It is likely that their measurements were statistically limited by the sensitivity of the samples
to the He+ beam and the lower current that was used as a result.
Wagner et al. [11] later used Rutherford backscattering to demonstrate detailed time-dependent changes in the Ag concentration
profile by examining a series of films with different illumination
times. This approach assumes, of course, that the progress of
diffusion is halted when the exposure of the sample to visible
light is stopped. It is desirable, therefore, to bypass the intrinsic
assumptions and limitations of these approaches and measure
the depth profile in situ during exposure of the sample to light.
X-ray and neutron reflectivity are powerful techniques that can be
used to “capture” such transient depth profiles. Of these, synchro-
Received 20 October 2013. Accepted 3 January 2014.
Y. Sakaguchi and T. Ito. Research Center for Neutron Science and Technology, Comprehensive Research Organization for Science and Society
(CROSS), Tokai, 319-1106, Japan.
H. Asaoka, Y. Uozumi, Y. Kawakita, M. Kubota, D. Yamazaki, and K. Soyama. Japan Atomic Energy Agency, Tokai, 319-1195, Japan.
M. Ailavajhala, M.R. Latif, and M. Mitkova. Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725-2075, USA.
Corresponding author: Yoshifumi Sakaguchi (e-mail: y_sakaguchi@cross.or.jp).
1This paper was presented at the 25th International Conference on Amorphous and Nanocrystalline Semiconductors (ICANS25).
Can. J. Phys. 92: 654–658 (2014) dx.doi.org/10.1139/cjp-2013-0593
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Sakaguchi et al.
tron radiation provides brilliant photon fluxes for time-resolved
X-ray reflectivity measurements and can readily discern small
changes in surface or interface structure. However, it is well
known that visible light and X-rays can induce silver diffusion in
chalcogenide materials [12] and the strong possibility of rapid
sample decomposition because of the synchrotron radiation
beam — even before any data can be collected — cannot be overlooked. Neutrons, on the other hand, offer a safer approach by
excluding the possibility of changes induced by the probe beam
and, the use of an intense pulsed neutron source is well-suited to
realize time-resolved measurements. This paper describes timeresolved neutron reflectivity measurements carried out on thin
film samples of Ag/GexS1−x (x = 0.2 and 0.4) while being exposed to
visible light and discusses the results in terms of the physical
picture of silver photodiffusion into a-GexS1−x films.
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Fig. 1. Neutron reflectivity profiles before and after a 117 min
exposure to the xenon lamp. Dots show experimental data and solid
curves show the fitted data, in which the parameters in the inset
table have been used: SLD, scattering length density in 10−6 Å−2;
d, thickness; and ␴, roughness.
σ
2. Experimental
The neutron reflectivity measurements were carried out on
BL17 (SHARAKU) [13] at the Materials and Life Science Experimental
Facility (MLF) of the Japan Proton Accelerator Research Complex
(J-PARC). At the MLF, intense pulsed neutrons are generated
through nuclear spallation reactions between a high-energy proton beam and the liquid-Hg neutron source target. The neutron
flux is proportional to the power of the incident proton beam,
which was 200 and 300 kW for the measurements of the Ag/
Ge0.4S0.6 and Ag/Ge0.2S0.8 films, respectively. White light from a
300 W xenon lamp (MAX-303, ASAHI Spectra, Co., Ltd.) was used as
an excitation light source and the exposure of the sample was
under computer control.
Neutron reflectivity, R, was obtained by R = I/ I0, where I is the
intensity of the reflected beam and I0 is the intensity of the incident beam as a function of neutron time-of-flight (TOF), t. The
value of I was obtained by measuring the intensity of the direct
beam without sample. The TOF was converted to the modulus of
the wave vector transfer, Q, using the relationships: ␭ = htmL,
where ␭ is the neutron wavelength, h is Planck’s constant, m is the
mass of a neutron, L is the length between the neutron source and
the detector, and Q = 4␲ sin ␪/␭, where ␪ = ␪i (incident angle) = ␪f
(scattering angle) [14]. In this work, two types of measurements
were performed: static and transient. In the static measurements,
TOF spectra at two different angles were measured and these were
combined to give a single spectrum over a wide Q range. In the
transient measurements, the sample was fixed at one angle and
the time evolution of the TOF spectrum was measured while exposing the sample to light from the xenon lamp.
At the MLF, neutron data are acquired using an event recording
system in which every detected neutron is tagged with (i) neutron
pulse number, (ii) time taken from a facility-wide standard clock,
and (iii) TOF. The TOF spectrum is obtained by making a histogram
where the start, end, and interval of the TOF are designated. The
clock time region can also be specified. Using the data reduction
system, arbitrarily time sliced spectra can be obtained from the
full recorded data set.
The Ag 500 Å/a-Ge40S60 1500 Å/Si substrate and Ag 500 Å/
a-Ge20S80 1500 Å/Si substrate samples were prepared using thermal evaporation. The thicknesses were estimated using the output from a quartz crystal microbalance.
3. Results and discussion
3.1. Ag 500 Å/a-Ge40S60 1500 Å/Si substrate
Figure 1 shows the static neutron reflectivity profiles for this
sample before and after a 117 min exposure of the sample to light
from the xenon lamp. Curve-fitting results indicate that the Ag/
a-Ge40S60 two-layer structure is intact (i.e., there is no silver diffusion) before exposure to the xenon lamp. There are three peaks in
the Fourier transform [15] of the reflectivity profile, shown in
Fig. 2. These peaks correspond to 400 Å (Ag), 1200 Å (a-Ge40S60),
Fig. 2. Fourier transforms of the reflectivity data shown in Fig. 1.
and 1600 Å (the total thickness) and confirm a two-layer structure.
After exposure to the xenon lamp, the reflectivity profile has
changed indicating some degree of silver diffusion. However, the
Fourier transform indicates that a two-layer structure is preserved
and it can be deduced, therefore, that a new two-layer structure
has been formed. In fact, the result of curve fitting shows that
there are two Ag-doped reaction layers with different Ag content.
Figure 3 shows the time evolution of the neutron reflectivity
profile. Although the reflectivity changes with time, it is difficult
to ascribe a physical meaning to these changes based on this
observation alone. Figure 4 shows the time variation of the Fourier transform of the reflectivity data shown in Fig. 3. From the
figure, the position and height of the first peak are plotted as a
function of time in Fig. 5. Two diffusion processes are observed: a
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Fig. 3. Time evolution of neutron reflectivity of Ag 500 Å/a-Ge40S60
1500 Å/Si substrate film under light illumination. The time is measured
from the moment of opening the shutter of the xenon lamp.
Can. J. Phys. Vol. 92, 2014
Fig. 5. Time variation of (Œ) the position and (●) the height of the
first peak in the Fourier transform.
Fig. 6. Neutron reflectivities of Ag 500 Å/a-Ge20S80 1500 Å/Si
substrate film before and after a 71 min light exposure.
σ
Fig. 4. Time variations of Fourier transform of the reflectivity data
shown in Fig. 3.
fast change observed in the first 10 min after starting light exposure and a second, slow, change observed after 107 min of light
exposure. Even after turning off the light source, the observed
reflectivity profiles continue to change.
3.2. Ag 500 Å/a-Ge20S80 1500 Å/Si substrate
Figure 6 shows the static neutron reflectivity profiles of the
Ag 500 Å/a-Ge20S80 1500 Å/Si substrate film before and after a 71 min
light exposure. In this case, silver has already diffused into the
a-Ge20S80 layer before exposure began. Conceivably, this could
occur thermally in the process of film synthesis or during storage
at room temperature. An additional peak at ⬃100 Å in the Fourier
transform (Fig. 7) confirms the presence of the reaction layer.
After exposure to light from the xenon lamp for 71 min, the reflectivity profile has changed. Curve-fitting shows that silver
diffusion is complete and that the film has become one homogeneous layer. In the Fourier transform, there is only one sharp peak
around 1500 Å. This supports the conclusion that the film is composed of one layer. Figure 8 shows the time evolution of the neutron reflectivity that changes significantly in the first 10 min after
starting the light exposure. Similar rapid change is observed in
the time evolution of the Fourier transform (Fig. 9). The position
of the first and the second peaks, and the height of the first peak
are plotted as a function of time (Fig. 10). Two diffusion processes
are evident: a fast change observed in the first 2 min after starting
a light exposure, and a slower change that has essentially finished
after 20 min; at which point, the film is one homogeneous layer.
Consistent with the Ag/a-Ge40S60 result, two diffusion processes
were observed. The only difference is the reaction rate: it is faster
in the Ag/a-Ge20S80 film than in the Ag/a-Ge40S60 analogue. This
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Fig. 7. Fourier transforms of the reflectivity data shown in Fig. 6.
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Fig. 9. Time variations of Fourier transform of the reflectivity data
shown in Fig. 8.
Fig. 8. Time evolution of neutron reflectivity of Ag 500 Å/a-Ge20S80
1500 Å/Si substrate film.
Fig. 10. Time variation of the positions of the first (Œ) and the
second peaks (o) and the height of the first peak (●).
compositional dependence is consistent with the structural flexibility of the Ge–S system, which is often explained by a floppy–
rigid transition model [16]. The faster reaction rate in Ag/a-Ge20S80
is attributed to such structural flexibility.
3.3. Silver diffusion process
Overall, two silver diffusion processes — one fast and one
slow — were observed in each sample. Two diffusion processes
were also observed in modulated optical reflectivity measurements of silver photodiffusion in the Ag/Ag–S system [8]. In addition, the result is consistent with that reported by one of the
authors (MM) in which modeling of Ag transport in Ge–Se glass
showed the presence of both slow and fast moving Ag ions [17].
The observation of two diffusion processes implies the presence
of a comparatively stable (metastable) Ag-rich state in the Agdoped GexS1−x layer. It is deduced that, in the first stage of diffusion, a Ag-rich reaction layer is formed and the Ag layer is
exhausted. Considering that the two-layer structure is basically
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658
preserved and that the position of the interface is almost fixed
during the first diffusion process, it follows that the second silver
diffusion process involves diffusion from the Ag-rich reaction
layer to the Ag-poor reaction layer via a mechanism in which
there is a potential barrier between the two layers. Such a mechanism predicts that both layers maintain their homogeneity during the second diffusion process and explains why fringes in the
reflectivity profile are clearly observed even after the first silver
diffusion process is complete. This mechanism of silver diffusion
contradicts the previously proposed model in which a diffusion
front progresses through the film. Such a mechanism predicts
shrinkage of the Ag layer, expansion of the Ag-doped reaction
layer and, shrinkage of the a-Ge40S60 layer as the reaction progresses. Such changes in thickness were not, however, observed in
the Fourier transform results obtained in the current study. This
discrepancy may be due to a difference in sample (Ge–S versus
As–S) or the different measurement technique. It should be noted
that the conclusions of the current study are based on the results
obtained from a model-free analysis (Fourier transformation) and
therefore, it would be interesting to perform time-resolved neutron reflectivity measurements on other systems, such as As–S
and Ge–Se to compare or contrast these findings.
4. Conclusion
The effect of exposure to visible light on thin film samples of
Ag/GexS1−x (x = 0.2 and 0.4) have been studied using time-resolved
reflectivity. These measurements support a diffusion model in
which (i) there is a metastable reaction layer formed in the first
silver diffusion process and that (ii) the following silver diffusion
takes place via a mechanism in which the position of the interface
between the Ag-rich reaction layer and the Ag-poor reaction layer
is almost fixed. The difference in reaction rate between the two
diffusion processes is attributed to a difference in the reaction
potential barrier between the Ag/Ag-rich reaction layer interface
and that of the Ag-rich reaction layer and Ag-poor reaction layer
interface. Further, it is proposed that changes in the composition
of the film (Ge20S80 versus Ge40S60) also affect the size of these
potential barriers and a clear difference in the reaction rates was
observed as a result.
Can. J. Phys. Vol. 92, 2014
Acknowledgements
This work was supported by JSPS Grant-in-Aid for Scientific Research (C) Grant No. 25400435 and by Defense Threat Reduction
Agency under Grant HDTRA 1-11-1-0055R. YS thanks the International Materials Institute for New Functionality in Glass NSF,
grant No. DMR–0844014 for the financial support of the research
exchange visit to Boise State University, Boise, Ida. The neutron
reflectivity measurements were performed on BL17 in J-PARC MLF
under project Nos. 2012A0068 and 2013A0203. We would like
to thank G. Foran (CROSS) for valuable discussions and late
K. Kunigihara (CROSS) for technical support.
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