Investigation of photodoping process in Ag/GeS2 double layer films by using neutron beam
Yoshihisa Murakami (Tsukuba Univ. of Tech.) Shibuya Takehisa, Wakaki Moriaki (Tokai Univ.)
Sakaguchi Yoshifumi (CROSS)
1. Introduction
When metal layers like Ag and Cu deposited on the chalcogenide glass films such as As2S3, As2Se3,GeS2, and GeSe2, are illuminated by light, the metal diffuses abnormally into the chalcogenide glasslayer. The photon energy of the light required for the doping should be larger than that of the
optical gap of the chalcogenide glass. This abnormal diffusion of metal is called photodoping
discovered by Kostyshin et al in 1965. The photodoping process using several methods, for example,optical transmittance, AFM, ellipsometry, RBS, and ATR have been investigated. The purpose of
our research is to make clear the behavior of Ag in the photodoping phenomenon. Especially,
information on the time dependence of the Ag doped layer is not fully understood through thevarious techniques. As an alternative method to know the detailed behavior of the doping process,we selected the neutron reflectometry in this time.
2. Experiment
Sample preparation : GeS2 films were deposited on thin Si single crystals (30 x 30mm) with thethickness of 100-200nm by the vacuum evaporation at the vacuum less than 6 x 10-5 torr. The Aglayer of 10-70nm thick was deposited onto the GeS2 film under shielding from the outside light toprevent the extra photodoping before experiments.
Experimental procedure : The experimental setup is shown in Figs. 1, 2 and 3. The sample film
(30 x 30mm) is set on a sample holder, which is conventionally used on BL17. A laser diode (405nm)
with a beam expander is prepared by applicants and the exposing light area is adjusted to the rightposition where neutron beam is irradiated. Since the original beam size of the laser diode is 1mm indiameter, the beam is expanded to 30mm in diameter at the sample position using a beam expander.
The laser beam covers the squared area of 20mm x 20mm, where neutron beams are irradiated in
the experiment.

Fig. 1. Experimental setup in BL17

Fig. 2. Experimental setup

Fig. 3. Laser Setup

Data analysis method : From a full set of event data, a histogram of Time-of-Flight (TOF) spectrum is madeconsidering a proper time resolution in the TOF (, which is related to Q-resolution), and a slicing time-region (,which depends on photoresponse in the reflectivity and is related to S/N ratio). The obtained reflectivity profile iscompared to a simulated profile (model calculations), and are fitted, providing modified parameters such as
scattering length density, layer thickness, and roughness. Time evolution of the reflectivity curve is also analyzed1by Fourier transformation, to evaluate changes in thickness. In addition, the experimental results obtained without
neutron irradiation are analyzed and compared with the irradiated results to confirm the influence of neutron
irradiation on photodoping.
3. Results
The following four samples such as a Si substrate, an a-GeS2 film deposited on a Si substrate, anAg film deposited on a Si substrate and an Ag/a-GeS2 (30nm/200nm) film deposited on a Si
substrate were measured. The time evolution of the neutron reflectivity curves for the two layered
sample were obtained as shown in Fig.4 together with the fitting curves. The derived parameters
corresponding to the thickness, densities and roughness of respective layers are shown in Table 1.
The behavior of Ag photodoping was obtained through the Fourier analysis for the reflectivity
curves and compared with the results of proceeding results using optical transmittance, AFM,ellipsometry, RBS, and ATR. As an example of the result, final ratio between Ag and doped layer
was almost similar to the value estimated optical transmittance data measured with aspectrophotometer at Tokai University shown in Fig. 5.
Table 1Thickness(nm) SLD
Roughness(nm)13.23.47
3.44
11.7
2.81
0.388
258.5
2.06
0.376
1.5
3.47
0.3
2.07
0.4

Ag
Ag/GeS2
a-GeS2
SiO2
Si
10090

80

70

60
T(%)

Ag/GeS2(30nm/200nm)

50

GeS2(200nm)
480min600min40

1000min

3020

100
1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

2100

2200

2300

2400

2500

Wavelemgth(nm)Fig. 4. Time dependence of neutron reflectance.

Fig.5. Time dependence of spectral transmittance.

Black line: measured, red line: fitting.
4. Conclusion
The time evolution of the neutron reflectivity curves for the two layered sample were characterized
and parameters corresponding to the thickness, densities and roughness of respective layers werederived. Through this experiment, it is suggested the neutron reflectivity method is effective tool toelucidate the mechanism of the photodoping phenomenon. We appreciate the cooperation of J-PARCand CROSS. As a further research, many samples with different compositions should be studied
through enough machine time. In this time, net machine time was largely limited due to thesystem shut down of the beam line.
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