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Phys. Status Solidi B, 1–7 (2013) / DOI 10.1002/pssb.201350188
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basic solid state physics
Thin Ge–Se films as a sensing material
for radiation doses
Mahesh S. Ailavajhala1, Tyler Nichol1, Yago Gonzalez-Velo2, Christian D. Poweleit3, Hugh J. Barnaby2,
2
4
,1
Michael N. Kozicki , Darryl P. Butt , and Maria Mitkova*
1
Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725, USA
School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ 85287-5706, USA
3
Department of Physics, Arizona State University, Tempe, AZ 85287-5706, USA
4
Department of Materials Science and Engineering, Boise State University, Boise, ID 83725, USA
2
Received 10 September 2013, revised 13 November 2013, accepted 26 November 2013
Published online 27 December 2013
Keywords chalcogenide glasses, radiation-induced Ag diffusion, radiation-induced effects
* Corresponding
author: e-mail mariamitkova@boisestate.edu, Phone: þ1 208 426 1319, Fax þ1 208 426 2470
This work focuses on the study of Ge rich phases in the Ge–Se
chalcogenide glass system. Radiation induced effects particularly related to Ag diffusion in the glasses under the influence of
different doses of g radiation are investigated and documented.
Raman spectroscopy, X-ray diffraction, energy dispersive
X-ray spectroscopy, scanning electron microscopy, and atom
force microscopy confirmed the occurrence of radiation-
induced Ag diffusion and oxidation of the hosting chalcogenide
thin films. This causes Ag surface deposition and structural
reorganization of the hosting backbone, and affects the
electrical performance of the films. It is suggested that the
sensing ability of the thin films can be substantially influenced
by the encapsulating the sensing elements to avoid the
oxidation of the chalcogenide film.
ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The research on chalcogenide glasses
has made substantial progress in identifying their properties
and relevance for various applications. One part of the
chalcogenide glass exploration, that is thoroughly studied is
related to photoinduced effects within these materials [1]. In
the process of understanding these effects, the importance of
the lone pair electrons associated with the chalcogen atoms
was revealed. They are usually non-bonding electrons whose
function is not obvious. Since the states related to these
electrons are situated on the top of the valence band, when
the glasses are illuminated by a light source, the lone pair
electrons can be easily excited producing photoinduced
effects. These photoinduced effects could be restricted to the
formation of electron hole pairs or contribute to a change of
the chalcogenide atoms’ coordination and play a role in the
formation of onefold or threefold coordinated chalcogens
(defects). In accordance with all these changes, two specific
effects have been reported: photo bleaching [2] or photo
darkening [3]. The light illumination can even have a dual
effect, contributing to a transition from one effect to
another [4, 5]. Furthermore, chalcogenide glasses have
various structural coordination ranging from floppy to rigid
structure [6], which allows for the generation of a variety of
defects and structural modifications in the presence of
radiation with visible light [7, 8] or shorter wavelength
electromagnetic waves. This created interest in studying the
influence of the X-rays over the chalcogenide glasses [9, 10]
as well as the effects caused by g radiation [11, 12].
One other effect, closely related to the above-mentioned
effects and depending upon the radiation-induced changes,
corresponds to the photo induced Ag diffusion within the
glasses [13]. Kluge [14] considered the process of photodiffusion of metals in chalcogenides as an intercalation
reaction. The main reason this can be realized in
chalcogenide glasses is the fact that they possess relatively
rigid covalent bonds, mixed with soft Van der Waals
interconnections. This type of structure ensures the
formation of voids and channels where the diffusing ions
can migrate and can reside [15]. The reaction can be efficient
when the reversible transport of ions and electrons can be
achieved, accompanied by formation of bonds with the host
matrix, according to the reaction:
þ
C02 þ e þ Mþ ! C
1M :
ð1Þ
This reaction describes the transition of an initially
twofold covalently bonded chalcogenide atom (C02 ) into a C
1
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M. S. Ailavajhala et al.: Thin Ge–Se films as a sensing material for radiation doses
charged unit possessing only a single covalent bond and an
excess electron that can establish an ionic bond with Agþ
(Mþ). Equation (1) shows the importance of the potential in
þ
forming the new C
1 M bonds of another compound – the
intercalation product. The possible number of these bondunits is fairly high as the chalcogenide glasses are able to
form a significant number of single C
1 centers under light
illumination. Indeed, this capability is a unique property
of the chalcogenides and thus explains the occurrence
of number of illumination-induced effects within these
materials. One can expect that similar effects will also occur
under X-ray and g radiation, which however have not been
studied so far.
In this work, we study the effect of Ag diffusion in
Ge40Se60 based on a device structure, which allows
investigating the influence of various radiation doses on
the material properties through the measurement of the
electrical performance of the devices. Our research focuses
on Ge rich glasses, since our previous studies on both Ge–Se
and Ge–S systems show that this type of material undergoes
structural changes under g radiation, which stabilize the
radiation-induced effects within them [16, 17]. Raman
spectroscopy, X-ray diffraction (XRD), atom force microscopy (AFM), scanning electron microscopy (SEM), and
energy dispersive X-ray spectroscopy (EDS) were used to
collect data about the effects occurring in the films after
being exposed to different radiation doses. Analysis of these
data was used to create understanding about the nature of
effects and provide an insight into the device performance.
2 Experimental The source material for film deposition was synthesized by using the conventional melt quench
technique to obtain bulk chalcogenide glasses. Thin films
were deposited at 1 104 Pa pressure, on oxidized silicon
wafer using thermal evaporation (PVD) with the aid of a
Cressington 308R evaporation system from a semi Knudsen
cell crucible. Multiple layers were deposited onto the
substrate without breaking vacuum to protect against the
introduction of contaminants between the films. Initially,
100 nm of Ge40Se60 film was deposited followed by a 50 nm
continuous film of Ag, after which a 300 nm film Ge40Se60
was evaporated. Part of the wafer was set aside after this step
to be used for the film study, while on the remaining portion
of the wafer radiation-sensing devices were created by
placing of non-diffusive aluminum (Al) electrodes. These
electrodes were thermally evaporated and selectively
deposited in specific regions of the wafer with the aid of
a circular mask. This mask generated a final device with
2 mm diameter circles used as contacting electrodes with
1 mm spacing. The cross-section of these devices is shown in
Fig. 1.
Devices and films were simultaneously irradiated by
60
Co g rays by a Gammacell 220 irradiator, with a dose
rate of 10.5 rad s1. Films and devices were retrieved as
measured at discrete doses.
Raman spectroscopy was performed using an Acton 275
spectrometer with an Andor CCD back thinned detector. For
ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1 Radiation sensing device structure.
excitation, a 514.5 nm line laser was focused into a circular
spot of 1 mm diameter. To avoid photo-induced changes in
the studied film, a laser power of 250 mW was used with
an accumulation time of 600 s. These measurements were
conducted at room temperature.
XRD patterns were obtained using a Bruker AXS D8
Discover X-Ray Diffractometer equipped with a Hi-Star
area detector. Beam conditions included a Cu anode at 40 kV
and 40 mA to produce Cu Ka1 radiation (l ¼ 1.5406 Å)
through a Göbel mirror producing a collimated beam.
Further experimental details are given in Ref. [18].
EDS has been conducted with a Hitachi S-3400N-II. A
beam of electrons was generated from a tungsten filament,
and the electrons were accelerated using a 20 kV bias, which
were then directed onto the sample. Interaction between the
electrons and the sample generates characteristic X-rays
corresponding to the elemental composition across the
sample, which were analyzed using INCA software.
Current versus voltage (I–V) curves have been measured
using an Agilent 4156C signal analyzer using two source
measuring units (SMU) connected to the device. A voltage
sweep was applied from 0 V to 200 mV with a step size of
5 mV and the current was measured simultaneously. These
experiments were similar to our previous studies where the
IV measurements and g source radiation are explained in
more details [17].
The surface characterization of the bare films was
studied using Bruker Dimensions 3100 AFM system. These
measurements were verified using a Hitachi field emission
scanning electron microscope (FESEM) capable of 5 nm
resolution with backscattering detector and Quartz imaging
capture software.
3 Results EDS has been performed on five locations
on each sample such that 25 points were used to determine
the uniformity of the film composition. The average
deposited film composition was Ge37.65Se62.35 with a
standard deviation of 0.93, which suggests that the overall
film composition is uniform.
Raman spectra of the sandwich structure (chalcogenide
glass/silver/chalcogenide glass), mode assignments and
corresponding structural units for characterized studied
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Phys. Status Solidi B (2013)
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structures are shown in Fig. 2a. Development of the spectra
as a function of the applied radiation shows a decrease in the
intensity of the peaks relating to the ethane-like (ETH) and
the edge-shared (ES) modes when compared to the cornershared (CS) mode. The spectra show the peaks located at
178, 195, and 219 cm1 which correspond to ETH, CS, and
ES structural units, respectively [19]. A close observation of
the area ratio between ES and CS modes demonstrates a nonETH CS
ES
ES
Counts (arb.)
14.82Mrad
7.60Mrad
3.19Mrad
1.58Mrad
Pre-radiation
150
200
250
300
350
400
450
500
-1
Wavenumber (cm )
a)
Area Ratio (arb.)
ES/CS
0.0
3.0M
6.0M
9.0M
12.0M
15.0M
b)
monotonous decrease in the ratio as shown in Fig. 2b. The
comparison of the areas of the fitted ETH structure shows a
continued decrease with increase in the radiation dose up
to 7.58 Mrad, after which the areal intensity of this mode
increases as shown in Fig. 2c.
XRD spectra for four radiation doses are shown in Fig. 3
and respective peaks have been assigned for the formation of
various diffusion products. The XRD data obtained at very
low radiation dose reflects a spectrum of an amorphous film,
as was also the non-radiated film, while the higher radiation
doses affirm the notion of silver diffusion and the formation
of Ag-containing compositions within the chalcogenide film.
There are three main peaks that are evident from the spectra,
which have been identified with JCPDS cards 04-0783, 71190, 24-1041, corresponding to pure Ag, Ag8GeSe6, and
bAg2Se respectively.
The electrical performances of the devices were
characterized by current versus voltage (I–V) measurements
obtained after discrete radiation dose steps. Data for one of
the irradiated and analyzed devices is represented in Fig. 4.
The trend observed from the device measurement reveals
that for low radiation doses (<7.59 Mrad), there is a
consistent increase in the current through the device with
increasing radiation dose, while over 7.59 Mrad, the current
through the device is significantly lower.
Analysis of the SEM images shows that silver surface
deposition occurs as a result of the Ag diffusion in the
chalcogenide film and Ag-containing clusters are visible.
The radius of the silver deposits and the distribution density
of the deposits are inversely related. Increasing the
radiation dose caused an increase in the silver surface
deposits and concurrently, a decrease in the density of
nucleation of the silver islands per unit area up to
7.59 Mrad. Above this radiation dose, the radius of the
deposits decreases while the density increases. Simultaneously, a second phase with a smaller size is also formed in
the higher radiation doses. The SEM images are shown in
Radiation Dose (rad)
2000
ETH
1.58Mrad
3.19Mrad
7.59Mrad
14.82Mrad
1800
1600
Intensity
Area (arb.)
1400 Ag GeSe
8
6
1200
Si
Ag8GeSe6
β−Ag2Se
1000
Ag
800
600
400
0.0
3.0M
6.0M
9.0M
12.0M
Radiation Dose (rad)
15.0M
c)
200
30
Figure 2 (a) Raman data and the corresponding mode assignment.
(b) Scattering strength ratio between edge-shared and corner-shared
modes. (c) Area analysis of the scattering strength for the ethanelike structural units.
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40
50
60
70
2-Theta
Figure 3 XRD spectra of the films after discrete radiation doses,
depicting the formation of Ag2Se and Ag8GeSe6 phases.
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M. S. Ailavajhala et al.: Thin Ge–Se films as a sensing material for radiation doses
140.0p
0 krad
787 krad
1.577 Mrad
2.381Mrad
7.229 Mrad
12.890 Mrad
120.0p
Current (A)
100.0p
80.0p
60.0p
40.0p
20.0p
0.0
0.00
0.02
0.04
0.06
0.08
0.10
Voltage (V)
Figure 4 Current versus voltage device measurements at various
radiation doses. The conductivity of the film rises with initial
exposure to radiation followed by a decrease at high radiation
doses.
Fig. 5 and the analysis of these images is summarized
in Fig. 6.
The AFM study revealed that the height of these deposits
decreases with increasing radiation doses. This trend is
opposite to the mean radius of the deposits as illustrated
through the SEM analysis mentioned above. Another aspect
that was studied using AFM was the topological roughness.
The surface roughness of the films was measured by
excluding the areas occupied by the deposits to study
roughness of the film attributed to the presence of smaller
silver deposits and the radiation-induced changes due to
structural reorganization. The AFM scans were performed
on the same areas where the SEM images were taken to
maintain consistency between the two types of studies.
Analysis of the AFM films is presented in Fig. 7.
Detailed inspection of the EDS spectra revealed the
presence of oxygen within the films. Based on this analysis,
it was revealed that the oxygen content in the films increases
with radiation dose (Fig. 8).
4 Discussion First, it is important to distinguish the
type of films that were characterized in these studies. This
Figure 5 SEM images of radiation-induced Ag surface deposition at various radiation doses: (a) 1.58 Mrad, (b) 3.19 Mrad, (c) 7.59 Mrad,
and (d) 14.82 Mrad.
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0.19
480
440
0.16
420
0.15
Se
18.6
100
18.4
2
Oxygen Content (arb.)
0.17
O
Counts (arb.)
Mean Radius (nm)
460
Deposits per unit area (N/ μm )
0.18
Ge
1.58 Mrad
3.19 Mrad
7.59 Mrad
14.82 Mrad
200
18.2
18.0
17.8
17.6
17.4
0.0
3.0M
6.0M
9.0M
12.0M
Oxygen Content
15.0M
17.2
0.0
distinction can be made through XRD, EDS, and Raman
inspections. Based on the XRD spectra, it can be stated that
the films are amorphous in nature. Additionally, the EDS
analysis revealed that the films consisted of 37.65% Ge
content, categorizing them to be germanium rich when
comparison to GeSe2 composition. The peak at 178 cm1 in
the Raman spectra corresponding to the ETH structure, also
affirms the claim that these films are germanium rich [20,
21]. This structure arises due to lack of bonding sites on
neighboring chalcogen atoms and the bond between two
germanium atoms forms the ETH structure. This bond is
indeed the weakest in the system. One can expect that g
radiation will cause a destruction of these bonds and
consequently the areal intensity of the ETH structures will
decrease. The destruction of the Ge–Ge bonding creates
defect sites located on the germanium atom, which can be
influenced by the presence of oxygen. Since the radiation
experiments have been carried out at ambient environment,
oxidation can easily occur as revealed by the EDS data
presented in Fig. 8. Even though the presence of oxygen in
540
520
115
500
110
480
105
460
100
440
Deposit Height (nm)
Surface Roughness (nm)
120
420
95
6.0M
9.0M
12.0M
15.0M
Radiation Dose (rad)
Figure 7 Dependence of the surface roughness and deposit heights
upon the radiation dose – AFM analysis.
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9.0M
12.0M
15.0M
0
Figure 6 Cluster size and number of deposited clusters on the film
surface as a function of the radiation dose.
3.0M
6.0M
Radiation Dose (rad)
Radiation Dose (rad)
0.0
3.0M
0.4
0.6
0.8
1.0
1.2
1.4
Energy (keV)
Figure 8 EDS analysis confirming the oxidation in the studied
system. Inset shows the development of the oxygen concentration
with radiation.
the Ge rich glasses is well documented, there are plenty of
discussions as how exactly it reacts with them. Indeed, the
problem persists, since there is not easy to give a direct proof
for the formation of chalcogenide oxides, and because of
this, secondary data like film shrinking and weight loss have
been used in support of the formation of gaseous chalcogenoxide products leaving the system. In some cases, shrinking
of the films has been observed [22] in support of the idea that
the chalcogen atoms are oxidizing, while other authors [23,
24] reported direct evidence, studying the infrared spectra,
for the appearance of Ge–O bonds. Considering the standard
potential data for the formation of the particular bivalent
oxides E0/En2, it turns out that germanium is much easier
to oxidize with potential VGe ¼ 0.23 compared to that for
selenium (VSe ¼ 0.35). Consequently, after the radiation and
formation of defects on germanium sites, even if Se defect
sites exist, germanium will be oxidized first. As a result of
this, one can expect that the oxygen atoms will replace part
of the chalcogen atoms bonded to Ge. In this manner, the
number of selenium atoms ready to build structural units
with germanium increases and formation of CS units, which
consume the highest number of chalcogen atoms grows. In
other words, the Ge:Se ratio will decrease giving rise to the
formation of units, characteristic for compositions richer in
selenium. It is for this reason that there is increase of the areal
intensity of the CS units in the system, which otherwise are
not expected to appear with such intensity in the initially
regarded system. One other evidence of this fact is the light
shift of the CS peak to lower wave numbers from 202.82
to 201.86 cm1. We have to point out that due to its low
intensity, the Ge–O bonding is not detectable by Raman
scattering.
Bonding strength of Ge–O is 6.83 eV, which is
significantly greater than the Ge–Se 2.38 eV and Ge–Ge
1.92 eV and this stabilizes the presence of oxygen in the
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M. S. Ailavajhala et al.: Thin Ge–Se films as a sensing material for radiation doses
films. The bond length of the Ge–O bond is much shorter –
1.62 Å, compared to 2.35 Å and 2.45 Å for Ge–Se and Ge–
Ge, respectively. The significantly strong Ge–O bond and
the shorter bond length create a constriction of the films
limiting the amount of passages and free volume for silver to
diffuse within the films. Due to these limiting factors, XRD
only exhibits very small crystalline phases and the nucleation
sites of the surface deposited silver dendrites decrease in size
with increased radiation dose. There is one more effect that
we can relay to oxygen – at low radiation doses the ternary
Ag8GeSe6 forms, while at higher radiation due to the
reduced amount of Ge to react and form the Ag-containing
diffusion products, formation of Ag2Se is documented on the
XRD spectra. However, both types of products are Raman
silent and not visible on the Raman spectra.
The nucleation and growth of Ag clusters on the surface
of the film up to a radiation dose of 7.59 Mrad coincides with
the data discussed by Kawaguchi and Maruno [25] for the
Ag surface deposition in Ag–As–S glasses and can be related
to the increased Ag diffusion with increase of the radiation.
This brings about the further growth and agglomeration of
the existing nuclei, which reduces the number of the Agcontaining sites. However, the radius of these clusters
increases. One could ask about the reason behind the lack of
continuation of these processes with increased radiation over
7.59 Mrad, i.e., why does the radius of the deposits decreases
beyond this radiation dose? We believe the reason is that at
that point, the majority of Ag is reacting to form bAg2Se,
which depletes the formation of Ag8GeSe6 clusters. The
resolution of our EDS system does not allow distinguishing
the elements embedded in the big and small crystals visible
on the SEM image but we suggest that the small crystals that
appear at radiation with 3.19 Mrad and higher, are those of
bAg2Se. Their nucleation is restricted at lower radiation
doses and therefore these crystals have not been registered
by the XRD system. At radiation with a dose of 7.59 Mrad
and over, their appearance is obvious with the high
number of nucleation sites increasing with radiation,
which suppresses the growth of the Ag8GeSe6 clusters. In
accordance to the bAg2Se nucleation on the surface, its
roughness increases as shown on Fig. 7. The formation of
bAg2Se contributes to a new depletion of the hosting film of
Se and because of this a new increase of the aerial intensity
of the ETH units occurs at radiation of 14.82 Mrad. We
suggest that it is also the reason behind the occurrence of the
ES/CS ratio saturation and even a small increase in the areal
intensity of the ES units. We noticed that plenty of effects go
through an inflection point at a radiation dose of 7.59 Mrad.
It seems that this dose is a threshold one, for many of the
studied processes.
The last and indeed the most important, from the
application point of view, is how to understand the device
performance. The largest increase in the conductivity occurs
between pre-radiation condition and 2 Mrad radiation, where
the structural changes (observed by the Raman spectroscopy) and silver incorporation (as exhibited by the XRD)
are dominant. After 2 Mrad, the oxygen-induced effects
ß 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
begin to dominate the device performance. To reduce the
consequences of oxygen on the device performance, the
radiation sensor can be encapsulated in a vacuum environment and in this manner; the silver and structural dominant
region of the device performance can be enhanced creating a
sensor that can sense large spectrum of radiation doses.
However, there could be another hypothesis. Considering
the chalcogenide film, there is published data about the dual
role of light within the chalcogenide systems [26]. These
effects are related to changes in the band gap of the glasses
and thus would contribute to an initial increase followed by a
decrease of the conductivity. In the studied case, Ag and
oxygen are introduced in the system, which considerably
change the situation and we suggest that their influence is
stronger than this of the effects, intrinsic to the chalcogenide
glasses. To check which influence (the one of the effects,
occurring in the chalcogenide films or the presence of Ag)
will prevail, studies in completely encapsulated devices are
in progress. Their data will confirm or reject the applicability
of the structure presented in this work for radiation sensing.
5 Conclusions Sandwich structures of Ge37.65Se62.35
glass–silver–Ge37.65Se62.35 glass with Al electrodes on top
of them are studied in order to understand the nature of the
effects occurring in them under radiation with different doses
of g radiation. It is shown that under radiation, the
chalcogenide films undergo structural changes related to
increase of the CS structural units and decrease of the ETH
and ES structural units up to radiation dose of 7 Mrad. This
effect is connected to the reaction of the chalcogenide matrix
with Ag diffusing within the films and the oxidation from the
environment in which the experiments have been conducted.
The diffusion product of the reaction of Ag with the
chalcogenide matrix is initially Ag8GeSe6 with development
of a second phase – bAg2Se once the amount of oxygen,
reacting with the chalcogenide matrix, increases due to
radiation. The Ag diffusion in the chalcogenide matrix
results in silver surface deposits, which are built initially by
clusters from Ag8GeSe6 whose growth at high radiation
doses is retarded due to formation of a new phase – bAg2Se
surface nucleates. The introduction of Ag and the intrinsic
radiation-induced effects in the chalcogenide matrix lead
initially to increasing of the conductivity of the structures,
which later due to the dominant role of the oxidation
decreases.
Acknowledgements This work has been funded by the
Defense Threat Reduction Agency under grant no: HDTRA1-11-10055. The authors would like to thank Dr. James Reed of DTRA for
his support. The authors would also like to acknowledge the Surface
Science Lab at Boise State University for AFM use and Dr. Paul
Davis for assistance in performing AFM and Muhammad Rizwan
Latif for his help with the SEM and the AFM studies.
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