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Physica Status Solidi A: Applications and Materials Science

Processes of silver photo-diffusion into Ge-chalcogenide probed by neutron reflectivitytechnique

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Manuscript Number: pssa.201533037R1

Full Title: Processes of silver photo-diffusion into Ge-chalcogenide probed by neutron reflectivitytechnique

Article Type: Original Paper

Section/Category: Amorphous and Nanocrystalline Semiconductors (ICANS26)

Keywords: amorphous chalcogenide; silver photo-diffusion; neutron reflectivity

Corresponding Author: Yoshifumi SakaguchiComprehensive Research Organization for Science and SocietyTokai, Ibaraki JAPAN

Corresponding Author SecondaryInformation:

Corresponding Author's Institution: Comprehensive Research Organization for Science and Society

Corresponding Author's SecondaryInstitution:

First Author: Yoshifumi Sakaguchi

First Author Secondary Information:

Order of Authors: Yoshifumi Sakaguchi

Hidehito Asaoka

Yuki Uozumi

Yobuyuki Kawakita

Takayoshi Ito

Masato Kubota

Dai Yamazaki

Kazuhiko Soyama

Gaurav Sheoran

Maria Mitkova

Order of Authors Secondary Information:

Abstract: We performed time-resolved neutron reflectivity meas-urement for stacks of Ag 500 /Ge25S75 1500 / Si sub-strate and Ge33S67 1500 / Ag 500 /Si substrate to clar-ifysilver photo-diffusion process into Ge-chalcogenide layer. For Ag 500 / Ge25S751500 / Si substrate stack, it was found that the silver layer dissolves into Ge-chalcogenide layer within 2 min by the light exposure, and Ag-doped reaction layerforms. However, after prolonged 70 min light exposure, two-layer structure withthicknesses of 700 and 1100 was established and it did not change to form onehomogeneous layer. For Ge33S67 1500 / Ag 500 / Si substrate stack, the silverlayer also dissolves within 2 min, followed by slow silver diffusion across Ag-richreaction layer/ Ag-poor reaction layer interface. Around 30 min after starting the lightexposure, the reflected intensity abruptly decreased and fringes in the reflectivityprofiles became unclear. The measurement of X-ray diffraction revealed that silverphoto-surface deposition occurred on the film by extended light exposure.

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Dear Guest Editors and pss Editor-in-Chief,

Please find attached revised manuscript entitled, Processes of silver photo-diffusioninto Ge-chalcogenide probed by neutron reflectivity technique by YoshifumiSakaguchi, et al.Among two reviewers reports, the reviewer #1 did not require any revision and thereviewer #2 required three revisions in the manuscript.1.To show the definition of d clearly, which appears in several figures.2.The qualities of printing of Figures 6a, 11, 13, 14 are very poor and they should berevised.3.In the references, all authors should be listed, instead of using et al..

For No.1, we found that use of z is appropriate, not d, according the definition in theequation (2).Therefore, we revised the letter in Figures 1, 2, 4, 5, 7, 8, 10, and 12.

For No.2, this probably occurred in the process of copy in the computer. We tried toimprove this situation. But please let us know if the quality does not change.

For No.3, all authors are listed in Ref. 6, 7, 8, 9, 10, and 13.

We hope that the manuscript has been revised according to the suggestions ofreviewer #2. We look forward to hearing from you at your earliest convenience.

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Yoshifumi Sakaguchi Ph.D.

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Processes of silver photo-diffusion into Ge-chalcogenide probed by neutron reflectivity technique Yoshifumi Sakaguchi *,1, Hidehito Asaoka2, Yuki Uozumi2, Yobuyuki Kawakita2, Takayoshi Ito1, Masato Kubota2, Dai Yamazaki2, Kazuhiko Soyama2, Gaurav Sheoran3, and Maria Mitkova3 1 Research Centre for Science and Technology, Comprehensive Research Organization for Science and Society, 319-1106 Tokai, Ja-

pan 2 Japan Atomic Energy Agency, 319-1195 Tokai, Japan 3 Department of Electrical and Computer Engineering, Boise State University Boise, ID 83725-2075, U.S.A.

Received ZZZ, revised ZZZ, accepted ZZZ Published online ZZZ (Dates will be provided by the publisher.)

Keywords amorphous chalcogenide, silver photo-diffusion, neutron reflectivity * Corresponding author: e-mail [email protected], Phone: +81-29-219-5300, Fax: +81-29-219-5311

We performed time-resolved neutron reflectivity meas-urement for stacks of Ag 500 / Ge25S75 1500 / Si sub-strate and Ge33S67 1500 / Ag 500 /Si substrate to clar-ify silver photo-diffusion process into Ge-chalcogenide layer. For Ag 500 / Ge25S75 1500 / Si substrate stack, it was found that the silver layer dissolves into Ge-chalcogenide layer within 2 min by the light exposure, and Ag-doped reaction layer forms. However, two-layer structure with thicknesses of 800 and 1100 was estab-

lished by a prolonged light exposure for 70 min and it did not change to form one homogeneous layer. For Ge33S67 1500 / Ag 500 / Si substrate stack, silver rapidly dis-solves into Ge33S67 layer leaving a thin silver layer in the first 2 min, and then, silver slowly dissolves from the sil-ver layer as the next reaction process. At approximately 25 min light exposure, an anomalous decrease in the neu-tron reflectivity, suggesting a formation of macroscopic surface roughness, was observed.


1 Introduction Silver photo-diffusion is one of the at-tractive phenomena observed in amorphous chalcogenide (a-Ch) films which exhibit various photo-induced changes related to their structural metastability [1]. Since silver photo-diffused film has a fast-ionic conductivity, it can be used as a solid electrolyte, and it has been actually demon-strated that programmable metallization cell devices, in which on-off switching in electrical resistance occurs by applying a bias, can be produced using such films [2]. From both fundamental research and application points of view, it is important to know how silver photo-diffusion takes place. Actually, there have been a number of reports on the phenomenon [3]. According to several studies using Rutherford backscattering (RBS), the photo-diffusion is unique to show a step-like profile of silver concentration in contrast to a usual diffusion in which the concentration gradually decreases as a function of distance from the sil-ver layer [4-7]. Although RBS is a powerful technique to clarify concentration profiles of constitutional elements, in situ studies under light exposure was not successful be-

cause silver diffusion occurred also by a strong He+ ion beam, which was used for time-resolved measurement to get enough statistics in a short time [5]. Therefore, in the previous studies, several films with a different light expo-sure time have been prepared, and studied by RBS. Using such ex situ technique, precise depth profiles as a function of exposure time have been obtained [6, 7]. However, it is desirable to use a proper probe beam which does not affect silver diffusion and to realize in situ studies.

Recently, we have carried out time-resolved neutron reflectivity measurements of Ag/Ge-S(Se) films using pulsed neutron beam with a time-of-flight technique [8-10]. Neutron and/or X-ray reflectivity are useful non-destructive technique to reveal the layer structure in a mul-ti-layer film. They also allow in situ studies under light ex-posure to be carried out. Since strong X-rays such as syn-chrotron radiation can induce silver diffusion, use of neu-trons is safer approach without inducing silver diffusion by the probe beam itself. In our previous time-resolved neu-tron reflectivity measurement for Ag 500/ a-Ge20S80

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2 Author, Author, and Author: Short title


1500/ Si substrate stack, it was found that there were a faster diffusion in the first 2 min and a slower diffusion for the next 20 min, after which the stack became homogene-ous. From the Fourier transform of the neutron reflectivity, it turned out that two reaction layers with different Ag-content are formed by the first 2 min silver diffusion, and that next slower diffusion takes place which almost did not change the two layers thicknesses. This observation con-tradicts previous model stating that the silver diffusion takes place by progression of the diffusion front, which is basically obtained by ex situ RBS experiment.

It is well-known that amorphous Ge-S(Se) consists of several building blocks, such as S(Se) chains, GeS(Se)4 tet-rahedra and S(Se)3-Ge-Ge-S(Se)3 ethane-like clusters, and the intensity of their appearance depends upon the glass composition [11, 12]. Therefore, it is easily expected that the structural changes, caused by composition differences, affect silver photo-diffusion process, and it is worth inves-tigating the kinetics by changing the composition. In our previous neutron reflectivity measurement for Ag 500/ a-Ge40S60 1500/ Si substrate stack, it was found that there was a faster diffusion in the first 10 min followed by a slower diffusion in the next 100 min. Apparently, these re-action rates were longer than those for Ge=20 at%. The fi-nal products of these processes were two Ag-diffused reac-tion layers with different Ag-content, and not one homoge-neous reaction layer. For a-Ge-Se films, the reaction rate was faster than within Ge-S films. However, only for Ge40Se60 1500/ Ag 500/ Si substrate stack, we observed that a microscopic surface roughness was formed by pro-longed light exposure even after forming one homogene-ous reaction layer [12]. From above experimental results, it is summarized that silver photo-diffusion depends marked-ly on Ge-composition and light exposure side and much more investigations on the dependence is required to un-derstand silver photo-diffusion processes through the whole system. In this paper, we report time-resolved neu-tron reflectivity measurements for Ag 500/a-Ge25S75 1500 / Si substrate stack and a-Ge33S67 1500 / Ag 500 /Si substrate stack, and provide extensive views of the de-pendence on silver photo-diffusion process.

2 Experimental 2.1 Neutron reflectivity measurements The neu-

tron reflectivity measurements were carried out on BL17 (SHARAKU) [13] at the Materials and Life Science Exper-imental 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 neu-tron source target with a repetition rate of 25Hz. The neu-tron flux is proportional to the power of the incident proton beam which was 300 kW in the present experiment. White light from a 300 W xenon lamp (MAX-303, ASAHI Spec-tra, 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. 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 relation-ships: = ht / mL, where is the neutron wavelength, h is Plancks 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 (scatter-ing angle). For every sample, two types of measurements were performed: static and transient. Before light exposure, TOF spectra at two different angles were measured and these were combined to give a static single spectrum over a wide Q range. During light exposure, the sample was fixed at one angle and the time evolution of the transient 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 neutron pulse number, time taken from a facility-wide standard clock, and TOF. From the full recorded data set, arbitrarily time sliced TOF spectra were obtained using the data reduction system of the facility.

X-ray diffraction was measured by X-ray diffractome-ter (Rigaku Ultima III) with Cu K radiation.

The samples were prepared by thermal evaporation us-ing a Cressington 308R coating system at 10 -6 mbar vacu-um. The thicknesses of the films were estimated using the output from a quartz crystal microbalance.

2.2 Analysis of neutron reflectivity result X-

ray/ neutron reflectivity is one of the applications of optics and the reflectivity is theoretically calculated by giving the scattering length density (SLD), (z), the thickness, the roughness, and Q, using Parratts recursive method or Abeles matrix method, where () = () , () is the number density of the i-th nucleus at the distance of z from the interface, bi is scattering length of the nucleus [14-16]. A model of SLD profile as a function of depth is verified by a fit of the calculated profile to the measured reflectivity profile.

In the kinematic approximation the specular neutron re-flectivity is given by [15]

() = 16

2

2 | () |

2 (1)

Furthermore, through integration by parts, the reflectivity is given by [14, 15]

() = 162

4 |

|2 (2)

The formula shows that R is related to the Fourier

transform of the derivative, d /dz, with respect to the depth,

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z. Actually, the thicknesses of layers are obtained by the positions of peaks in the Fourier transform of the reflectivi-ty [17-20].

3 Results and discussion 3.1 Ag/Ge25S75/Si substrate Figure 1 shows the

static neutron reflectivity profiles for Ag 500 / Ge25S75 1500 / Si substrate stack before and after 70 min expo sure of the sample to light from xenon lamp.

Figure 1 Neutron reflectivity profiles before and after a 70 min exposure by the xenon lamp. Cross marks show experimental da-ta and solid red curves show the fitted data, in which the parame-ters in the inset SLD depth profile have been used. The height of the cross mark indicate an error bar.

Figure 2 Fourier transforms of the reflectivity data shown in Fig.1. The inset shows the SLD profiles, which were obtained from the fitting to the reflectivity. The SLD profile of the film before the light exposure shows that the Ag layer is

basically preserved with the thickness of 400. This is confirmed in the Fourier transform of the reflectivity in Fig.2.There are a large peak at ~500 , which corresponds to a thickness of the pure Ag layer, and small humps around 1500 and 2000 ; thicknesses of the Ge25S75 layer and the total film, respectively. After 70 min light exposure, the neutron reflectivity profile is completely different from that before light exposure. In the Fourier transform in Fig.2, there are peaks at 800, 1100 and 1900 .These peaks are attributed to the thicknesses of two layers after silver pho-to-diffusion and the sum of them. According to the curve fitting, the film is composed of Ag-doped reaction layer with a thickness of 800 on top and a layer with a thick-ness of 1100 , underneath the reaction layer.

Figure 3 Time evolution of neutron reflectivity of Ag 500/ a-Ge25S75 1500/Si substrate stack under light exposure for 70 min. The time is measured from the moment of opening the shutter of the xenon lamp. The solid red curves are model fits, in which the SLD profiles in Fig. 5 are used.

Figure 3 shows time evolution of neutron reflectivity of Ag 500 / Ge25S75 1500 / Si substrate stack under light ex-posure for 70 min. The neutron reflectivity profile drasti-cally changes by the light exposure in the first 20 min. The

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slope in Q=0.010-0.012 -1 became steeper from 0 (be-fore) to 8 min, and a broad peak around 0.014 -1 at 8-9 min changes in forming two small peaks at 10-12 min ex-posure. These features would indicate the disappearance of Ag layer and a new layer formation. On the contrary, the reflectivity does not change so much after 20 min and the silver diffusion seems to be saturated in the time region.

Figure 4 shows the time variation of the Fourier trans-form of the reflectivity data shown in Fig.3. The peak at ~500 , observed in the Fourier transform before light ex-posure, shifted to lower-side at 0-1 min, suggesting that Ag layer became thinner. Then, from 1-2 min to 3-4 min, the first peak position turned to higher-side and stayed around 600 , probably suggesting Ag-doped reaction layer for-mation. The formation of such metastable reaction layer is consistent with our previous results for Ag/Ge20S80/Si and Ag/ Ge40S60/ Si [8]. However, the peak disappeared and there were no specific peaks in the time region from 4 to 18 min. It is difficult to find what happened in the time re-gion. At 18-20 min, peaks appeared again around 800 and 2000 and these peaks were maintained even at 100-110 min exposure, suggesting that the silver diffusion was saturated in this time region. It is noted that the first peak position changed from 600 to 800 . This is different from our previous results for Ag/Ge20S80/Si and Ag/ Ge40S60/ Si, in which the first peak position was almost fixed and the peak height decreased with time.

In Figure 5, the time variation of the SLD profile is shown as a result of the fitting to the neutron reflectivity curves in Fig.3. The calculated curves in Fig.3 are fairly fitted to the reflectivity curves for 0-4 min, and 20-110 min exposure. In Fig.5, the Ag layer is described as a tall hat with SLD=3.47 x10-6 -2 and it rapidly disappeared within 1 min. After that, a new Ag-doped reaction layer was formed at 2-3 min exposure, in which the height of the hat became lower. For the time region from 4 to 18 min, fitting was not successful even if smooth parameters were used for fitting. From 20 to 110 min, the film is composed of two layers with thicknesses of about 800 and 1200 . Since the SLD profile did not change in this time region, the silver diffusion is supposed to be saturated without forming one homogeneous layer.

Although it was difficult to find the change in the layer structure in the time region from 4 to 12 min from model curve fitting and Fourier transformation, the change in the SLD, , of the stack could be roughly evaluated by the change around the critical edge of the reflectivity. In gen-eral, the critical wave vector, Qc, is theoretically calculated by optics, and is expressed as [14, 15]

= 4 (3) From (3), Qc of Ag is estimated to be 0.0132 (-1) while Qc of a-Ge25S75 is 0.0102 (-1). Actually, the experimental de-termination of the critical edge is not easy because the neu-tron reflectivity is very sensitive to Q-value around the critical edge. In the present analysis, we experimentally de-fined Qc of the stack during the light exposure as a position

Figure 4 Time variations of Fourier transforms of the reflectivity data shown in Fig.3.

Figure 5 Time variations of SLD profile obtained from fitting to the neutron reflectivity curve. The depths are corrected at the ox-ide layer position on Si substrate to be 1500 .

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Figure 6 Time evolution of RQ4 Q plots (a), and the time vari-ation of the critical edge (b).

where RQ4 drops from a Q4 curve (R=1); neutron reflec-tivity basically decays with Q-4 as shown in equation (2) and the curve should be on Q4 at the total reflection region. As shown in Fig.6 (a), there are two points where RQ4 drops in the RQ4-Q plots before light exposure, 0.010 and 0.017 (-1); Qc s of a-Ge25S75 layer and Ag layer, respec-tively. In the time variation (Fig.6 (b)), Qc at 0.017 (-1) disappeared at 2 min, suggesting that silver dissolved into a-Ge33S67 and disappeared. This is consistent with the re-

sults of Fourier transformation and the SLD profile ob-tained from curve fitting. After that, Qc was almost con-stant at 0.0100 (-1) until 7 min exposure. However, it rap-idly increased to 0.0105 at 8 min exposure. Considering eq.(3), the increase in Qc indicates an increase in ; in oth-er words, a scattering length of the compositional element,

i, or a mass (number) density, ni. In this experimental context, this implies a further participation of silver into the diffusion process. From the rapid change, we infer that transition-like silver introduction occurred on the surface layer.

In summary, we observed silver dissolution in the first 2 min from time evolution of the neutron reflectivity as we did for Ag/Ge20S80/Si stack before [8]. However, the slow silver diffusion, which should occur from metastable Ag-rich reaction layer to Ag-poor layer and was observed in Ag/Ge20S80/Si and Ag/Ge40S60/Si stacks before [8], was not observed in the present study for Ag 500 / Ge25S75 1500 / Si stack. Also, it is noted that two-layer structure was maintained by a prolonged light exposure for Ag 500 / Ge25S75 1500 / Si stack while Ag 500 / Ge20S80 1500 /Si stack became one homogeneous layer with 20 min light exposure although the silver layer was supposed to completely dissolve in both cases. At present, the reasons of these discrepancies from the previous results are not clear for us. However, the process of silver diffusion could depend on Ge-composition. According to Elliott [21], sil-ver content attains a limiting value while silver diffuses in-to chalcogenide layer. He pointed out for Ge-chalcogenide that the Ag concentration in the Ag-photo-doped Ge-chalcogenide layer is the same as that of Ge content. Ap-plying this idea to the present case, the difference of the layer structure can be understood by the difference of matching content of Ge atoms; silver could diffuse deeper in the Ge20S80 layer because of low Ge content due to the high availability of S which has strong affinity towards Ag, which is driving force for Ag diffusion, while silver diffu-sion could stop before reaching the end in the Ge25S75 layer because of higher Ge content. However, the explanation can only be applied with the assumption of the diffusion model of the progression of the diffusion front. Probably, in Ag 500 / Ge25S75 1500 / Si stack, a thicker metasta-ble reaction layer was formed and the diffusion stopped at the stage with some reasons. In any case, further analysis is required, especially, for the reflectivity in the time region from 4 to 18 min.

3.2 Ge33S67/Ag/Si substrate Figure 7 shows the static neu-tron reflectivity profiles for Ge33S67 1500 / Ag 500 / Si substrate stack before light exposure of the sample. The re-sult of curve fitting indicates that for Ge33S67/ Ag two layer structure is intact without silver diffusion. Fourier trans-form of the reflectivity in Figure 8 also confirms the intact two layer structure, with three peaks at 400 (Ag layer thickness), 1800 (Ge33S67 layer thickness), and 2200 (total thickness).

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Figure 9(a) shows the time evolution of neutron reflectiv-ity of Ge33S67 1500 / Ag 500 / Si substrate stack under light exposure to the sample. At the beginning, before light exposure, a large period (~0.02 -1) of the oscillation is clearly observed, suggesting a presence of silver layer with thickness of 500 . After that, the oscillation became un-clear with exposure time, suggesting the dissolution of sil-ver. However, anomalous change occurred in the reflectivi-ty after 30 min light exposure; the intensity in the total re-flection region (Q ~0.010 -1) decreased and the fringes became unclear. Figure 9(b) shows the time variation of the total reflected intensity over the whole TOF spectrum, which could indicates basic categories of the photo-induced reaction process. There are three time regions; a first rapid decrease in the intensity from 0 to 2 min, a sec-ond gradual decrease from 2 to 30 min, and a third rapid and unusual decrease in the intensity.

Figure 7 Neutron reflectivity of a-Ge33S67 1500/ Ag 500/ Si substrate stack before a light exposure.

Figure 8 Fourier transforms of the reflectivity data shown in Fig.7.

The first and second decrease in the intensity would be re-lated to the fast and slow silver diffusion, respectively,

which we observed in both Ag/ Ge20S80/ Si and Ag/ Ge40S60/ Si stacks [8]. The third rapid intensity decrease

Figure 9 Time evolution of neutron reflectivity of a-Ge33S67 1500/ Ag 500/ Si substrate stack under light exposure. The time is measured from the moment of opening the shutter of the xenon lamp (a), and the time variation of the total reflected inten-sity (b).

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Figure 10 Time variations of Fourier transforms of the reflectiv-ity data shown in Fig.9 (a).

Figure 11 Time variations of the position of peaks in the Fourier transform in Fig. 10.

Figure 12 Time variations of SLD profile obtained from fitting of the neutron reflectivity curve in Fig.9 (a).

Figure 13 Time variations of (a) SLD and (b) thickness of the layers used for the curve fitting of the reflectivity in Fig.9 (a).

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SLD

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10

-6

-2)

2000150010005000z ()

Before

0-1 min

1-2

2-4

6-8

10-12

24-26

0

0

0

0

0

0

0

2

14-16

0

22-24

26-28

28-30

0

0

0

4

350

300

250

200

150

thic

knes

s (

)

2520151050time (min)

1720

1680

1640

Ag

a-Ch (reaction layer)

(b)

1.98

1.96

1.94

1.92

SLD

( x

10-6

-2

)

2520151050time (min)

3.47Ag

a-Ch (reaction layer)

(a)

60

50

40

30

20

10

0

roug

hnes

s (

)

2520151050time (min)

a-Ch (reaction layer)/ air

Ag/ a-Ch (reaction layer)

(c)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65



is unique to the Ge33S67/ Ag/ Si stack in Ge-S system. From the unusual weak intensity, we infer that the surface is no more specular and that there could be a macroscopic roughness on the surface as we observed in Ge40Se601 1500 / Ag 500 / Si stack before [9].

Figure 10 shows the time variation of the Fourier trans-forms of the reflectivity in Fig.9 (a). Time variations of the peak heights and the peak positions are plotted in Figure 11. As shown in the figures, the first peak rapidly changed in the first 2 min; the peak shifted to lower-side and be-came higher. The lower shift of the first peak position indi-cates that the silver layer became thinner due to silver dis-solution. The increase in the peak height indicates that the contrast of the SLDs at the interface became larger and this would be related to the sharp edge of Ag layer front as we discuss later. After the first 2 min light exposure, the lower shift in the first peak position became slower. As for the second and third peaks, they were clearly separated before light exposure (1800 and 2200 ). However, they almost coalesce into one peak and it is difficult to distinguish be-tween two peaks at 14-16 min exposure. It seems that the second peak position increased faster in the first 2 min and slower after 2 min, with the same manner as the first peak. The increase in the second peak position is ascribed to the volume expansion in a-Ch or reaction layer because an in-clusion of silver in the Ge-S network can lead to a volume expansion. After 25 minutes light exposure, the height of the united peak around 2000 decreased with time while the position was almost constant. The change would be re-lated to a dumping of the oscillation and anomalous de-crease in the reflectivity curve.

Figure 12 shows the time variation of the SLD profiles, which were obtained from curve fitting to the neutron re-flectivity in Fig. 9 (a). The used parameters (SLD, thick-ness and roughness) are plotted in Figure 13 (a), (b) and (c). In the model fitting, we first assumed that the metastable reaction layer is formed in the first fast diffusion process, in which silver diffuses from Ag layer to the metastable layer, and that the next slower diffusion occurs from the metastable Ag-rich reaction layer to Ag-poor originally a-Ch layer, as we did before [8]. However, the obtained SLD of the reaction layer was about 3.40 x 10-6 (-2), which was close to that of Ag, 3.47 x 10-6 (-2), and the features of the SDL profile for Ag-poor reaction layer/ Ag-rich reaction layer/ Si (two reaction layers) was basically the same as that for reaction layer/ Ag/ Si (one reaction layer). There-fore, we have chosen a simpler assumption of the latter one, without a metastable reaction layer (Ag-rich reaction layer). The results are shown in Fig.12 and 13. As shown in Fig.9(a), the fitting curves are in a good agreement with the experimental data, indicating that the assumption could be reasonable. In the first 2 min light exposure, the Ag lay-er became rapidly thinner from 350 to 250 . After that, the thickness of Ag decreased slowly. Interestingly, the edge of Ag layer (reaction layer/Ag interface) was very

sharp in the slow diffusion period. Such concentration pro-file is consistent with the previous RBS results as men-tioned in the introduction section. Probably, the fast diffu-sion process may be explained as a process to produce a sharp interface edge. Even when a metastable reaction lay-er was assumed in the curve fitting, the thickness of the metastable reaction layer changed in the same way; there were fast and slow processes, and the edge at the interface was sharp in the slow process. At approximately 25 minutes after starting the light exposure, the roughness at reaction layer/Ag interface abruptly increased. This implies that the formation of macroscopic structure could start from the instability at the reaction layer/ Ag interface.

Figure 14 X-ray diffraction pattern of Ge33S67 1500 / Ag 500 / Si substrate stacks. Dotted curve: no light exposure, solid curve: after light exposure.

Figure 14 shows the X-ray diffraction patterns of the Ge33S67 1500 / Ag 500 / Si substrate stacks, with and without light exposure, measured by -2 scan (out-of-plane configuration). Apparently, there are diffraction peaks from Ag crystal for both stacks. Such peaks were not observed in the Ag 500 / Ge25S75 1500 / Si stack after silver photo-diffusion. Considering the ratio of the peak heights, the thickness of Ag layer is roughly estimated to be 100 , and it is comparable to the Ag thickness just be-fore anomalous decrease in the neutron reflectivity intensi-ty, 150 , which was obtained by model fitting in Fig.13. Therefore, the Ag peak in Fig.14 could be contributed from a remaining Ag layer in the stack, and our assumption of reaction/Ag two-layer structure, without the metastable re-action layer, would be plausible.

Figure 15 shows the image of an optical microscope Ge33S67 1500 / Ag 500 / Si substrate stack after light exposure. The image clearly shows that the anomalous de-crease in the neutron reflectivity after 25 min light expo-sure is attributed to the macroscopic inhomogeneity com-posed of the mountains-valleys-like pattern. It is sure that there are topographical variations on the surface because the focal point depends on the mountain we observe. The

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57

pattern seems to be similar to that observed in Ge40S60 1500 / Ag 500 / Si stack [9], but this looks more com-plicated. As of now, we cannot answer the structural origin of the change. However, we can speculate that the inho-mogeneous pattern could be formed by a specific status of the photo-generated layer. For instance, the excess intro-duction of silver into Ge33S67 layer may lead to a phase separation and such macroscopic pattern may be produced as a result of the phase separation. The possibility for this very high since there are no free S atoms, which to react with the diffusing Ag. This is also the main reason that there is not glass formation in the ternary Ge-Se(S)-Ag system exactly in the region of introduction of Ag into the Ge-Se(S) matrix of the stoichiometric GeSe(S)2 composi-tion [22, 23]. Also, there is another possibility that the macroscopic pattern formation based on topographic height variations arises from dewetting of liquid film from a substrate [24]. In fact, it is well known that photo-induced fluidity effect can occur in chalcogenide glasses [25]. Therefore Ag-doped Ge33S67 film may also have flu-idity by light exposure with specific conditions, and form such macroscopic patterns as a result of a specific interac-tion between the liquid-like film and the substrate. Further studies are required to conclude the structural origin of the macroscopic inhomogeneous pattern formation.

Figure 15 Image of an optical microscope of Ge33S67 1500 / Ag 500 / Si substrate stack after light exposure.

4 Conclusions The processes of silver photo-diffusion into amorphous Ge-chalcogenide films have been investigated by time-resolved neutron reflectivity meas-urements for Ag 500/ a-Ge25S75 1500/ Si and Ge33S67 1500/ Ag 500 / Si stacks. It was found from the meas-urements of Ag 500/ a-Ge25S75 1500/ Si stack that dis-solution of silver was observed in the first 2 min. It seems that silver completely dissolved into a-Ge25S75 and a meta-stable reaction layer was formed on surface after the disso-lution. However, it did not change to be one homogeneous layer and two-layer structure was maintained even by a prolonged light exposure for 70 min. This was in contrast

to Ag 500 / a-Ge20S80 1500 / Si stack, which changed to be one homogeneous layer by prolonged light exposure for 20 min. For Ge33S67 1500 / Ag 500 / Si stack, the fast dissolution of silver was also observed in the first 2 min. However, the dissolution was not completed and a part of the silver layer was left at this reaction stage. The edge of the silver layer front became sharp in the SLD profile and silver dissolution continued slowly in the next reaction stage. At approximately 25 min light exposure, an anoma-lous decrease in the neutron reflectivity intensity was ob-served, which suggests a formation of macroscopic surface roughness. We do not know the structural origin of the macroscopic change at this moment. However, this must occur in a specific status of the Ag-doped Ge-chalcogenide and can also be regarded as a photo-induced change unique to chalcogenide film. Overall, the composition-sensitive behaviours are attributed to the structural flexibility in the system and it is interesting to study further the diversified changes.

Acknowledgements This work was supported by JSPS Grant-in-Aid for Scientific Research (C) Grant No. 25400435. The neutron reflectivity measurements were performed on BL17 (SHARAKU) in J-PARC MLF under Project No. 2013B0159 and 2014B0178. We would like to thank N. Miyata, K. Akutsu, and S. Kasai (CROSS) for technical support on the neutron reflectivity instrument, and R. Maruyama (JAEA) for technical support on X-ray diffraction measurements. This work has been also funded by the Defense Threat Reduction Agency under grant no: HDTRA1-11-1-0055.

References [1] T. Wagner and M. Frumar, in: Photo-Induced Metastability

in Amorphous Semiconductors edited by A. V. Kolobov (Wiley-VCH, Berlin, 2003), p. 160.

[2] M. Mitkova and M. N. Kozicki, J. Non-Cryst. Solids 299-302, 1023 (2002).

[3] Review: A. V. Kolobov and S. R. Elliott, Adv. Phys. 40, 625 (1991).

[4] Y. Yamamoto, T. Itoh, Y. Hirose and H. Hirose, J. Appl. Phys., 47, 3603 (1976).

[5] J. Rennie, S. R. Elliott, and C. Jeynes, Appl. Phys. Lett. 48, 1430 (1986).

[6] T. Wagner, V. Peina, M. Vlek, M. Frumar, E. Rauhala, J. Saarilahti and P. J. S. Ewen, J. Non-Cryst. Solids, 212, 157 (1997).

[7] T. Wagner, G. Dale, P. J. S. Ewen, E. Owen and V. Perina, J. Appl. Phys., 87, 7758 (2000).

[8] Y. Sakaguchi, H. Asaoka, Y. Uozumi, Y. Kawakita, T. Ito, M. Kubota, D. Yamazaki, K, Soyama, M. Ailavajhala, M. R. Latif and M. Mitkova, Can. J. Phys. 92, 654 (2014).

[9] Y. Sakaguchi, H. Asaoka, Y. Uozumi, Y. Kawakita, T. Ito, M. Kubota, D. Yamazaki, K. Soyama, M. Ailavajhala, M. R. Latif, K. Wolf, M. Mitkova and W. A. Skoda, J. Phys: Conf. Ser. 619, 012046 (2015).

[10] Y. Sakaguchi, H. Asaoka, Y. Uozumi, Y. Kawakita, T. Ito, M. Kubota, D. Yamazaki, K. Soyama, M. Ailavajhala, K.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65



Wolf, M. Mitkova, M. W. A. Skoda, JPS Conf. Proc. 8, 031023 (2015).

[11] P. Boolchand, J. Grothaus, M. Tenhover, M.A. Hazle, and R. K. Grasselli, Phys. Rev. B 33, 5421 (1986).

[12] Y. Sakaguchi, D. A. Tenne and M. Mitkova, J. Non-Cryst. Solids, 355, 1792 (2009).

[13] M. Takeda, D. Yamazaki, K. Soyama, R. Maruyama, H. Hayashida, H. Asaoka, T. Yamazaki, M. Kubota, K. Aizawa, M. Arai, Y. Inamura, T. Itoh, K. Kaneko, T. Nakamura, T. Nakatani, K. Oikawa, T. Ohhara, Y. Sakaguchi, K Sakasai, T. Shinohara, J. Suzuki, K. Suzuya, I. Tamura, K. Toh, H. Yamagishi, N. Yo-shida, T. Hirano, Chi. J. Phys. 50, 161 (2012).

[14] C. Fermon, F. Ott and A. Menelle, in: X-ray and Neutron Reflectivity Principles and Applications, edited by J. Daillant and A. Gibaud, Springer-Verlag, Berlin, Heider-berg (2009).

[15] D. S. Sivia, Elementary Scattering Theory For X-ray and Neutron Users, Oxford University Press, Oxford (2011).

[16] Jens Als-Nielsen and Des McMorrow, Elements of Modern X-ray Physics, Second edition, John Wiley & Sons Ltd., West Sussex (2011).

[17] F. Bridou and B. Pardo, J. Phy. III France 4, 1523 (1994). [18] F. Bridou, J. Gautier, F. Delmotte, M.-F. Ravet, O. Durand

and M. Modreanu, Appl. Sur. Sci. 253, 12 (2006). [19] K. Sakurai and A. Iida, Adv. in X-ray Ana. 35, 813 (1992). [20] K. Sakurai, M. Mizusawa and M. Ishii, Trans. Mat. Res.

Soc. Jpn. 33[3], 523 (2008). [21] S. R. Elliott, J. Non-Cryst. Solids 130, 85 (1991). [22] M. Mitkova, Yu Wang, P. Boolchand, Phys. Rev. Lett. 83,

3848 (1999). [23] T. Kawaguchi, S. Maruno, S. R. Elliott, J. Appl. Phys. 79,

9096 (1996). [24] P.G. de Gennes, F. Brochard-Wyart and D. Qur, Gouttes,

bulles, perles et ondes, Belin, Paris (2005). [25] H. Hisakuni and K. Tanaka, Science 270, 974 (1995).

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