Journal of Non-Crystalline Solids 377 (2013) 195–199
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/ locate/ jnoncrysol
Gamma ray induced structural effects in bare and Ag doped Ge–S thin films
for sensor application
M. Mitkova a, b, e,⁎, P. Chen a, M. Ailavajhala a, D.P. Butt b, e, D.A. Tenne c, H. Barnaby d, I.S. Esqueda d
a
Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725, USA
Department of Materials Science and Engineering, Boise State University, Boise, ID 83725, USA
Department of Physics, Boise State University, Boise, ID 83725, USA
d
Department of Electrical Engineering, Arizona State University, Tempe, AZ 85287, USA
e
Center for Advanced Energy Studies, 995 University Blvd., Idaho Falls, ID 83401, USA
b
c
a r t i c l e
i n f o
Article history:
Received 5 October 2012
Received in revised form 11 December 2012
Available online 31 January 2013
Keywords:
Chalcogenide glasses;
Radiation sensing;
Radiation effects;
Radiation induced Ag diffusion
a b s t r a c t
We present data on radiation-induced effects in chalcogenide glasses that also trigger radiation induced
structural reorganization contributing to silver (Ag) diffusion. To study these effects and silver diffusion,
depending on the radiation dose, films were prepared and analyzed using Raman spectroscopy, X-ray
diffraction and Energy Dispersion X-ray Spectroscopy. The results show a structural development occurring
in films containing 45.4 at.% Ge with increasing radiation dose defined by increase in the edge-sharing/
corner-sharing ratio, higher ethane-like unit values and rise of the amount of Ag diffused within the system.
Utilizing these effects, a resistance based radiation sensing device has been created. The I–V curves characterizing the sensor operation demonstrate decreased device resistance as a result of the radiation.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
One of the widely explored property of chalcogenide glasses are
the light induced effects in them, which manifest themselves by
bleaching or darkening of the material due to formation of electron–
hole pairs and even bond switching by the interaction with light [1].
Often these effects are reversible and can be “erased” by annealing
the material [2]. The photoinduced effects extend towards shorter
wavelengths, for example by radiation with γ rays [3] by which a
variety of radiation-induced phenomena occur, attributed to the freedom and flexibility associated with their atomic structure [4]. Studies
show that in the case of Se–S thin films, the band gap of the material
can decrease by 0.39 eV (from 2.25 eV to 1.86 eV) as a result of γ ray
dose of 500 kGy [3]. The traditional explanation of this effect is based
on the formation of electron–hole pairs created by radiation, which
aid in the formation of defects that introduce localized states within
the bandgap and contribute to a change in the effective Fermi level
due to an increase of carrier concentration. These effects usually
recombine shortly after cession of radiation because of the high availability of charged defects in the glass material. The radiation induced
effects can be accompanied by bond rearrangement resulting in
molecular rearrangements [4], which are time invariant.
Another important feature of chalcogenide glasses is related to
radiation induced diffusion of mobile metals (for example Ag or Cu)
within the host glass [5]. In this case, one can obtain a significant
increase in the radiation sensitivity by optical or electrical measurements, for example, Ag additives form states within the band gap of the
chalcogenide glasses thus changing the optical properties and conductivity of the glassy medium. Conductivity measurements of Ag doped
chalcogenide glasses show, that introduction of a very small amount of
Ag drastically changes the conductivity of the hosting glass [6].
The above-mentioned effects make chalcogenide glasses very
attractive for short wavelength radiation applications, for example
imaging in X-ray radiology [7] or dosimeters for gamma radiation
using radiation induced Ag diffusion. With respect to the latter, detailed
understanding of the relation between structure, composition and radiation induced effects is important since those are the main factors
influencing the performance of the sensor. In this work, we focused
our study towards understanding the structural effects of radiation
over a range of glass compositions from the Ge–S system as bare films
as well as Ag diffused films as a direct result of gamma radiation.
These effects are studied utilizing Raman spectroscopy and X-ray diffraction (XRD). Energy Dispersion X-ray Spectroscopy (EDS) elucidates
the amount of Ag introduced during the gamma radiation. Examples of
the electrical performance of structures with non-diffusive metal electrodes and source of Ag are presented in order to illustrate both the potential of the material and the suggested structure for radiation sensing.
2. Experimental
⁎ Corresponding author at: Department of Electrical and Computer Engineering, Boise
State University, Boise, ID 83725, USA. Tel.: +1 206 426 1319; fax: +1 208 426 2470.
E-mail address: mariamitkova@boisestate.edu (M. Mitkova).
0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jnoncrysol.2012.12.031
Using thermal evaporation (PVD) with Cressington evaporation
system and applying a crucible with a semi-Knudsen cell structure
M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) 195–199
to establish pressure equilibrium, 60 nm thin films were evaporated
from the Ge20S80, Ge30S70, Ge33S67 and Ge40S60 glasses. These films
were studied before and after radiation either as-prepared or covered
with a 30 nm film of Ag. After radiation, excess topological silver was
dissolved with a 0.1 mol/L solution of Fe(NO3)3 for further compositional and structural studies.
To determine the exact elemental composition of the thin films,
EDS studies were performed on a LEO 1430VP Scanning Electron
Microscope with EDS. Several points (usually 5) were measured
across the wafer for each sample to study uniformity.
The Raman spectroscopy was accomplished in macro Raman
mode using a Horiba-Jobin Yvon T64000, triple spectrometer and
macro-backscattering configuration. For excitation a 441.6 nm line
of a He–Cd laser was used with 60 mW power, focused into a circular
spot of ~1 mm diameter. The samples were measured in an evacuated
cryostat (at 1 × 10−6 Torr) at 100 K as described in [8]. To give a proof
of the concept for electrically measuring radiation induced effects, a
basic lateral device was created, build up by two inert electrodes and
a Ag source in close proximity.
X-ray diffraction patterns were obtained using a Bruker AXS D8
Discover X-ray Diffractometer equipped with a Hi-Star area detector.
The typical setting is 2 frames, 300 s per frame, the X-ray tube and the
area detector scan axes are coupled starting from 15° with step size of
20°. The final XRD spectra are integrated along Chi. Beam conditions
included a Cu anode at 40 kV and 40 mA to produce Cu Kα1 radiation
(λ = 1.5406 Å) through a Göbel mirror producing a collimated beam.
Further experimental details are given in [8].
DC current-voltage measurements were performed on fabricated
sensors using an Agilent 4146 parameter analyzer with the aid of
Labview software to control the sweep parameters. Conditions of
the sweep were adjusted such that voltage was varied from 0 to 1 V
with 5 mV steps, while simultaneously measuring the current using
a Source Monitoring Unit (SMU).
3. Results
The results show that the films are quite homogenous since EDS
in different areas and different samples yielding a variation of ~ 1%.
Evaporated films tend to be much more Ge-rich than the synthesized
bulk glass and the Ge/S ratio does not change with dose or Ag diffusion since gamma rays do not cause structural transmutation. The
compositional results for pure and Ag-diffused films are summarized
in Table 1.
Raman spectra for pure chalcogenide glass films, the mode assignments, cross-section of Ge–S and corresponding structural units are
summarized, and their positions are represented in Fig. 1. The figure
shows a standard development of the glass structure depicting an
increase in the intensity of edge-sharing modes (ES) and ethane-like
modes (ETH) compared to corner-sharing modes (CS) as the samples
become more Ge-rich. For samples with greater than 33.3 at.% Ge, the
signature of the distorted rock salt double-layer structure (RL) is registered. This specific trend for the GeS2 glass corresponds to the Raman
results for the bulk glass with the same composition [9] and is an
evidence for films of good quality. The difference in the intensity of
Ge32.8S67.2
CS
Ge41.0S59.0
Ge-S Ch
100 krad ETH
ES
Si
200 krad
20 krad
Sulfur
Virgin
Virgin
CS
ES Ge-S Ch
Si
ES
Ge34.7S65.3
CS
1.3 Mrad ETH
ES
Raman Intensity (arb. units)
196
RL
Ge45.4S54.6
Ge-S Ch
ETH
ES Ge-S Ch
CS
ES
1.3 Mrad ETH
1.3 Mrad
ES
Si
ES
Si
200 krad
200 krad
RL
Sulfur
Virgin
500
200
Virgin
200
300
400
300
400
500
Wavenumber(cm-1)
Fig. 1. Raman data for the studied films at different radiation doses.
the characteristic vibrations of the structural units in the radiated
samples is approximately 2–3%, which is within the error of the fitting
method, so it is difficult to say whether these effects are real. However,
well expressed difference has been registered for samples with composition Ge45.4S54.6 where a clear tendency towards increase of the ES/CS
area ratio has been well expressed as shown in Fig. 2.
The Raman spectra of films that were doped with Ag, due to radiation, show that the CS, ES, ETH and RL modes remain. We suggest
that these are Ge–Sbr modes since the sulfur atoms are two-fold coordinated, connecting two Ge atoms. Adding Ag atoms to the backbone
structure tends to break sulfur bridges and form Ag cations terminated
by S anion pairs. The tentative mode assignments of the Ge–St modes
are shown in Fig. 3. For these samples, the spectra decreases in counts
and shows a sloped background by increasing the Ag content. There
are a number of terminal Ge–S modes, which progressively grow in
scattering strength with higher radiation dose. This further proves
that when silver enters the network, it preferably breaks sulfur bridges
instead of Ge\Ge bonds, leading to a predominant increase of ETH
modes compared to other modes.
The reaction products forming after Ag diffusion at room temperature are studied by XRD and the results are presented in Fig. 4. In
general, the films are amorphous and there are no strongly expressed
crystalline molecular peaks. Using the JCPDS card 14-0072, we could
identify the binary composition Ag2S which is only present in the
spectra for Ge32.8S67.2 films. There are some peaks which could be
associated with the presence of Ag2GeS3 (JCPDS card 83-1247) but
they are wide and with a small intensity indicating that the crystalline
Table 1
EDS results on GeS and Ag/GeS samples.
Bulk glass composition
Ge20S80
Ge30S70
Ge33S67
Ge40S60
Evaporated non-Ag samples, GexS1-x
Ag-containing samples, Agy(GexS1-x)1-y
Ge (x)
Ge (x)
32.8 ± 0.1%
34.7 ± 0.3%
41.0 ± 1.0%
45.4 ± 0.8%
33.4 ± 0.1%
35.1 ± 0.2%
43.0 ± 0.4%
46.1 ± 1.7%
Ag (y)
Virgin
20 krad
100 krad
200 krad
1.3 Mrad
0.7%
0.8%
1.1%
0.7%
3.2%
3.8%
–
4.0%
3.4%
3.5%
–
4.6%
–
3.2%
4.5%
5.7%
–
4.0%
6.0%
7.2%
Ag (200)
Ag (220)
197
Ag (111)
Ge45.4S54.6
1.5
Ag PDF 04-0783
(a)
Ag/Ge S
Ge41.0S59.0
Ge34.7S65.3
0.5
0.0
100 krad
45.4 54.6
1.0
Ge32.8S67.2
20
100
200
1.3
krad
krad
krad
Mrad
Virgin
Fig. 2. Dependence of the ES/CS Raman modes ratio for the studied films at different
doses.
XRD intensity (Arb. units)
Area ratio of ES/CS
structure units
M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) 195–199
20 krad
Virgin
(b)
Ag/Ge S
100 krad
34.7 65.3
20 krad
Virgin
(c)
Ag/Ge S
32.8 67.2
clusters related to them are very small and the structure is predominantly amorphous. There are also some small peaks that have been
identified as pure Ag (JCPDS card 04-0783) and we assume that
those are traces of non-dissolved Ag clusters from the surface of the
samples.
Fig. 5 shows the DC current–voltage characteristic of lateral device,
pre and post irradiation. There is a decrease of the resistivity of the material after radiation credited to Ag diffusion. This effect is best expressed
for the Ge45.4S54.6 based devices, which corresponds with the introduction of a highest amount of Ag as shown in the compositional analysis.
100 krad
20 krad
Virgin
(d)α-Ag2 S,
PDF 14-0072
(e) Ag8GeS6, PDF 83-1247
20
24
28
32
36
40
44
48
52
56
60
64
68
2θ (deg.)
4. Discussion
The major Raman peak for the S rich glasses is located at around
340 cm −1 which represents the breathing mode of the Ge(S1/2)4
Fig. 4. XRD data for the studied films at different radiation doses.
corner sharing tetrahedra in which Ge is fourfold coordinated and S
is twofold coordinated. Its dominance fades with increasing the relative amount of Ge over 41.0 at.% when the edge sharing tetrahedra,
Ag/Ge32.8S67.2
Ge32.8S67.2
Ge-St
ETH
dipyro- metaGeS-2.5
GeS2GeS33
3.5
0.010
(a)
0.008
Ge34.7S65.3
0.006
Ag/Ge34.7S65.3
Ge34.7S65.3
RL ETH
Difference spectrum
pyro- meta- di-
Ge41.0S59.0
0.000
0.010
(b)
0.008
Ge45.4S54.6
0.006
RL
Difference
spectrum
150
Resistance decrease after
100 krad γ-ray irradiation
ΔR/R = 24%
0.002
Ge-St
Ag/Ge41.0S59.0
100
pre rad
post rad (100k)
0.004
ID (A)
Raman Intensity (arb. units)
Difference spectrum
200
Resistance decrease after
100 krad γ-ray irradiation
ΔR/R = 86%
Ge-St
ETH
0.004
pyro- meta- di-
250
300
350
400
450
pre rad
post rad (100k)
0.002
500
550
600
Wavenumber (cm-1)
Fig. 3. Raman spectra of Ag containing films after 100 krad irradiation compared to
non-Ag containing films and their difference spectra.
0.000
0.0
0.2
0.4
0.6
0.8
1.0
1.2
VD (V)
Fig. 5. I–V curves of radiation sensing structures before and after 100 krad γ irradiation.
198
M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) 195–199
ethane-like structures and outrigger raft structure concurrently
develop. It is worth mentioning that the frequency of the corner
sharing mode that appears at 340 cm −1 for the Ge32.8S67.2 composition undergoes a small shift to 345 cm −1 with increase of the Ge concentration, which is derived from intertetrahedral couplings [10]. This
factor is an important feature in the interaction of the glasses with
radiation. The coupling with the nearby tetrahedra creates a high connectivity of the system with high concentration of S keeping it intact
and the radiation at the conditions of this study does not result in significant bond breaking that could be detected.
Based on our radiation data we suggest that for the S rich glasses,
formation of electron–hole pairs as a result of radiation is the major
mechanism of their reaction to radiation. It is empowered by the high
concentration of chalcogen atoms that contain lone pair electrons.
This creates internal electric fields produced by non-equilibrium, radiation induced effects such as C1+ and C3+ centers [11]. They are the reason
for the reduction of the optical band gap reported by Xia et.al. [12] and
hence increased conductivity. At the conditions of our experiments
there is no significant bond breaking and structural reorganization for
chalcogen-rich glasses. However structural reorganization has been
obtained for sulfur-rich Ge–Sb–S glasses that have been radiated with
a 7.7 mGy dose which is much higher than the dose, used in our experiments [13].
For the glasses with 45.4 at.% Ge, due to the reduced amount of
nearby tetrahedra, restructuring of the system is possible. In this
case, Ge 2+ can be regarded as a modifier in the system which contributes to breaking up the bridging sulfur. It is for this reason that radiation induces formation of a higher number of edge sharing structural
units in the Ge-rich films by breaking some of the existing bonds. This
has the important consequence of opening the entire structure of the
films. For the Ge-rich glasses, the disconnection of the network and
decrease of the S bridging atoms makes the rigid structure more susceptible to bond reorganization. Zhao et al. [14] also have reported an
increase of the sensitivity with increase of the Ge concentration.
When Ag film is in contact with the chalcogenide film in the
presence of radiation, radiation induced diffusion takes place. Once
introduced into a non-crystalline or glassy phase, Ag could form
stoichiometric solids and could be included as an additive in the
base network [15]. These additives can either segregate [15,16] as
separate phases or uniformly mix [15] with the base glass to form
homogeneous solid electrolyte glasses. The fate of Ag strongly depends upon the matrix in which it is introduced. As revealed by the
XRD studies for the case when Ag diffuses in Ge32.8S67.2 film, part of
its phase separates and forms Ag2S reacting with S from the S chains
and rings. This Ag2S forms big clusters which can be sensed with the
XRD resolution, but are not visible on the Raman spectra since Ag2S is
Raman silent. However, we suggest that there is only a small fraction
of Ag which forms Ag2S since there is a large change in the Raman
spectrum of the hosting glass — Fig. 3a, suggesting reorganization of
the chalcogenide network to accommodate Ag that did not form
Ag2S. There are several specifics that we want to point out: (i) there
is intensity growth of the mode at 250 cm −1 indicating the formation
of higher number ETH structural units and significant sulfur depletion
of the initial composition of the hosting backbone. The number of ETH
units is limited in the case of host with 32.8 at.% Ge because of the
partial consumption of Ag atoms in formation of Ag2S. In the case of
glasses with higher concentration of Ge, the growth of this mode is
better expressed because of the higher consuming of the network
building blocks for Ag incorporation and the higher concentration of
Ag introduced in the glass-phases (Table 1). (ii) The relative intensity
of the mode at 343 cm−1 is reduced while the vibrations at 370 cm−1
and 400 cm−1 are strengthened. This tendency develops with the
increased Ag concentration which was traced by the EDS results. We
attribute the latter modes to the development of thiogermanate bonds
(GeS−) forming pyro- (GeS3−3.5), meta- (GeS2−3) and di- (GeS−2.5)
thiogermanate tetrahedra as suggested by Kamitsos et al. [17]. Note
the dominance of the metathiogermanate tetrahedra, which after
accommodation of Ag forms the stoichiometry that is specific for this
system — the Ag2GeS3 ternary. It is not visible on the XRD spectra
because it becomes part of the amorphous network.
A question can arise inquiring the reasoning behind the high concentration of introduced Ag in films with the highest concentration of
Ge since there is not enough S to attract Ag. The EDS data show that
indeed the real composition of this film is Ge45.4S54.6. For this composition, we [5] have demonstrated the formation of a layered structure
in which both Ge and S are threefold coordinated through formation
of dative bonds. The possibility of a threefold coordination of both Ge
and the chalcogen (Se) has been recently confirmed as well for liquid
GeSe2 by first-principle molecular dynamic calculation [18]. The
dative bonds are very weak and easily destroyable by radiation. This
results in a high negative charge concentrated on the chalcogen
atoms, which together with the channeled structure characteristic
for this composition [5], is a big driving factor for Ag + diffusion into
this glass. In addition, the demonstrated opening of the structure
through reorganization of CS to ES structural units helps to further
the introduction of Ag into the glassy matrix. It is for this reason
that the conductivity increases much more for this particular glass
compared to the cases with higher concentration of S. Regarded in
the context of radiation sensing elements, this particular structure
will result in a better expressed difference in their electrical performance during a radiation event. The large reduction in resistance in
the Ge-rich devices, shown in Fig. 5, is supported by the structural
reorganization and the increase in the Ag + diffusion in these films.
5. Conclusions
We have studied the effect of radiation on various compositions of
GexS100−x in bare and radiation induced silver doped films, as well as
devices that couple the radiation induced effects and silver diffusion.
Bare GexS100−x and Ag doped films have been characterized using EDS,
Raman and XRD, which show two characteristic behaviors depending
on the composition: in systems that are chalcogen-rich, we did not
observe a significant structural change. While in the Ge-rich films, an
increase in ES/CS ratio has been registered including a tendency to incorporate more Ag with an increase in radiation dose. The electrical performance of the devices fabricated from the studied glass films show a
decrease in resistance after radiation. This is primarily attributed to Ag
diffusion which is assisted by radiation induced changes presented in
this paper.
Acknowledgment
This work is supported by a grant from Battelle Energy Alliance
under Blanket Master Contract No. 41394. The Raman and XRD instruments have been supported by National Science Foundation (NSF)
DMR-1006136 and MRI-0619795, respectively. We would also like to
acknowledge Brian Jaques for the help with XRD measurements.
References
[1] A.V. Kolobov, S.R. Elliott, Adv. Phys. 40 (1991) 625–684.
[2] A.V. Kolobov, K. Tanaka, Semiconductors 32 (1998) 801–806.
[3] O.I. Shpotyuk, in: R. Fairman, B. Ushkov (Eds.), Properties of Chalcogenide
Glasses, Elsevier Acad. Press, 2004, pp. 215–260.
[4] M. El-Hagary, M. Emam-Ismail, E.R. Shaaban, A. El-Taher, J. Radiat. Phys. Chem. 81
(2012) 1572–1577.
[5] T. Kawaguchi, S. Maruno, S.R. Elliott, J. Appl. Phys. 79 (1996) 9096–9104.
[6] M. Ribes, E. Bychkov, A. Pradel, J. Optoelectron. Adv. Mater. 3 (2001) 665–674.
[7] S.O. Kasap, J.A. Rowlands, IEEE Proceedings — Circuits, Devices and Systems, 149,
2002, pp. 85–96.
[8] M. Mitkova, Y. Sakaguchi, D. Tenne, S.K. Bhagat, T.L. Alford, Phys. Status Solidi A
207 (2010) 621–626.
[9] L. Cai, P. Boolchand, Philos. Mag. Part B 82 (2002) 1649–1657.
[10] X. Feng, W.J. Bresser, P. Boolchand, Phys. Rev. Lett. 78 (1997) 4422–4425.
[11] H. Fritzsche, in: P. Boolchand (Ed.), Insulating and Semiconducting Glasses, World
Scientific, 2000, pp. 653–690.
M. Mitkova et al. / Journal of Non-Crystalline Solids 377 (2013) 195–199
[12] F. Xia, S. Baccaro, D. Zhao, M. Falconieri, G. Chen, Nucl. Inst. Methods Phys. Res. B
234 (2005) 525–532.
[13] A. Kovalskiy, H. Jain, A.C. Miller, R.Y. Golovchak, O.I. Shpotyuk, J. Phys. Chem. B
110 (2006) 22930–22934.
[14] D. Zhao, H. Wang, G. Chen, S. Baccaro, A. Cecilia, M. Falconieri, L. Pilloni, J. Am.
Ceram. Soc. 89 (2006) 3582–3584.
199
[15] C.A. Angell, K.L. Ngai, G.B. McKenna, P.F. McMillan, S.W. Martin, J. Appl. Phys. 88
(2000) 3113–3157.
[16] K.L. Ngai, S.W. Martin, Phys. Rev. B 40 (1989) 10550–10556.
[17] E.I. Kamitsos, J.A. Kapoutsis, G.D. Chryssikos, G. Taillades, A. Pradel, M. Ribes,
J. Solid State Chem. 112 (1994) 255–261.
[18] M. Micoulaut, S. Le Roux, C. Massobrio, J. Chem. Phys. 136 (2012) 224504–224506.