Gamma radiation induced effects in floppy and rigid Ge-containing chalcogenide thin
films
Mahesh S. Ailavajhala, Yago Gonzalez-Velo, Christian Poweleit, Hugh Barnaby, Michael N. Kozicki, Keith
Holbert, Darryl P. Butt, and Maria Mitkova
Citation: Journal of Applied Physics 115, 043502 (2014); doi: 10.1063/1.4862561
View online: http://dx.doi.org/10.1063/1.4862561
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/4?ver=pdfcov
Published by the AIP Publishing
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JOURNAL OF APPLIED PHYSICS 115, 043502 (2014)
Gamma radiation induced effects in floppy and rigid Ge-containing
chalcogenide thin films
Mahesh S. Ailavajhala,1 Yago Gonzalez-Velo,2 Christian Poweleit,3 Hugh Barnaby,2
Michael N. Kozicki,2 Keith Holbert,2 Darryl P. Butt,4 and Maria Mitkova1
1
Department of Electrical Engineering, Boise State University, 1910 University Dr. Boise, Idaho 83725-2075,
USA
2
School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe,
Arizona 85287-9309, USA
3
Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA
4
Department of Material Science and Engineering, Boise State University, 1910 University Dr. Boise,
Idaho 83725-2090, USA
(Received 6 November 2013; accepted 3 January 2014; published online 22 January 2014)
We explore the radiation induced effects in thin films from the Ge-Se to Ge-Te systems
accompanied with silver radiation induced diffusion within these films, emphasizing two
distinctive compositional representatives from both systems containing a high concentration of
chalcogen or high concentration of Ge. The studies are conducted on blanket chalcogenide films or
on device structures containing also a silver source. Data about the electrical conductivity as a
function of the radiation dose were collected and discussed based on material characterization
analysis. Raman Spectroscopy, X-ray Diffraction Spectroscopy, and Energy Dispersive X-ray
Spectroscopy provided us with data about the structure, structural changes occurring as a result of
radiation, molecular formations after Ag diffusion into the chalcogenide films, Ag lateral diffusion
as a function of radiation and the level of oxidation of the studied films. Analysis of the electrical
testing suggests application possibilities of the studied devices for radiation sensing for various
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862561]
conditions. V
I. INTRODUCTION
Since their discovery in the mid 1950s, chalcogenide
glasses keep on attracting research attention due to the various material properties that are continuing to be a subject of
discovery more than 6 decades after their conception.
Throughout this period of time, the abundant and diverse
properties of chalcogenide glasses facilitated the use of these
materials as the foundational material for many important
technological milestones, such as electronic and optical
phase change memory, photoresist, applications related to
Ag photo diffusion within the glasses, nano-ionic redox conductive bridge memory, and waveguides for IR optics.1–3
Chalcogenide glasses result in well-expressed radiation
induced effects, which are well documented.4 However,
there is one type of material property, which has not been
thoroughly explored and offers a new frontier of opportunities and discoveries. This property refers to the radiation
induced effects within these materials and adapting these
changes in a manner to electrically measure these effects.
Indeed the relationship between radiation induced effects
and electrical response has been investigated in studies concerned with X-ray induced effects within these materials and
their application towards radiological imaging.5,6 However,
studies using c radiation induced effects and the corresponding electrical response are very scarce.
In general, the radiation induced effects have both a
dynamic and static nature. While the dynamic part vanishes
at the secession of radiation, the static effects remain stable
after the interruption of radiation exposure.7 The literature
data on the radiation induced electrical response are quite
0021-8979/2014/115(4)/043502/9/$30.00
contradictory—some authors report decreasing conductivity
in c radiated chalcogenide glasses,8,9 while others suggest a
radiation-induced decrease in the optical band gap resulting
in an enormous increase of the conductivity.10,11 There is a
universally accepted understanding that radiation causes the
formation of electron-hole (e-h) pairs, originating from photogeneration of defect pairs, which contribute to the reported
changes of the optical and electrical properties of the
glasses.4 Because these generated e-h pairs are charged particles and there is an abundance of charged defect sites available in the glass structure, the e-h pairs can easily recombine
at the defect sites, which is the main reason for the occurrence of the dynamic effects. At higher radiation doses and
for specific structures, bond reconfiguration can occur.4,7
This effect is significantly more stable than the dynamic
changes and is retained after the discontinuation of radiation.
Even though new discoveries are being made to help us
understand the nature of these effects, there is a significant
amount of detail that is still missing, which can relate the
structure, the composition of the chalcogenide glasses and
many other factors that may explain the observed changes
within these materials.
One of the effects that have not been investigated is the
behavior of fast moving ions within the glasses under radiation conditions, primarily silver. A silver atom, when introduced into the chalcogenide structure offers many unique
properties, of which photodoping in chalcogenide glass
when exposed to a bandgap light source has been thoroughly
studied.12 A couple of questions arise—does Ag diffuse and
behave in a similar manner when irradiated by c rays? If
yes, how does the effect of cinduced diffusion vary the
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V
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conductivity of the chalcogenide glass film? Indeed this
problem has never been addressed fully but it is important
when considering, for example, the response to high energy
irradiation of recently introduced redox conductive bridge
non-volatile memory (CBM) devices.8 It also can lead to the
development of a new type of radiation sensing device,13
which could be embedded in integrated circuits, and overcome the disadvantages of currently used p-intrinsic-n (PIN)
dosimeters14 and radiation sensing field effect transistors
(RADFETs).15
In order to provide answers to the above questions, radiation induced effects in thin films from the Ge-Se to Ge-Te
systems in the context of radiation induced Ag diffusion and
the evolution of their electrical conductivity were studied in
this work. These studies were accompanied with structural
and compositional characterization of the films in order to
identify correlations between all components affecting the
results and understand the nature of the obtained effects. We
have specifically chosen to compare floppy glasses from
both systems—Ge23Se77 and Ge11Te89, as well as, rigid
representatives—Ge44Se66 and Ge48Te52.
II. EXPERIMENTAL
The thin films were derived from a source material consisting of bulk chalcogenide glass, which was created using
the conventional melt quench technique from source elements
with 99.999% purity. Films were evaporated on an oxidized
silicon substrate using a Cressington 308R thermal evaporation system at 1 106 mbar pressure, using a semi Knudsen
cell crucible. The chalcogenide glass films for materials characterization and for device formation were deposited at the
same time to maintain uniformity between analyzed films and
electrically measured results. Fabricated devices were created
using conventional semiconductor photolithography techniques, which are described in detail in Ref. 16.
Films and devices were irradiated with 60Co gamma rays
using a Gammacell 220 irradiator, with a dose rate of
10.5 rad/s. After specific discrete doses, films and devices
were retrieved from the irradiator and then analyzed. Raman
spectroscopy was measured on bare films using an Acton 275
spectrometer with an Andor CCD back thinned detector. For
excitation, a parallel-polarized 514.5 nm line laser was
focused into a circular spot of 1 lm diameter at a laser beam
intensity of 25 lW. Although the laser wavelength is within
the absorption edge of the films, no illumination-induced
effects have been observed during and after several measurements due to the very low beam intensity.
X-ray diffraction spectra (XRD) 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
(k ¼ 1.5406 Å) through a G€obel mirror producing a collimated
beam. Further experimental details are given in Ref. 17.
Energy Dispersive X-ray Spectroscopy (EDS) has been
conducted with a Hitachi S-3400N-II. A beam of electrons
was generated from a tungsten filament to the electrons were
accelerated with 20 kV onto the sample. Interaction between
the electrons and the sample generates characteristic X-rays
J. Appl. Phys. 115, 043502 (2014)
corresponding to the elemental composition across the sample, which were analyzed using INCA software.
All of the devices were wire bonded to device packages
with gold pins to reduce inconsistencies between measurements and ensure accurate measurements without external
influences. The probes and the test bench were tied to a common ground to minimize charge buildup, and high quality
gold probe tips were utilized. Current vs. voltage (I-V)
curves were measured using an Agilent 4156 C signal analyzer, using two Source Measuring Units (SMU) connected
to the device. A voltage sweep was applied from 0 V to 2 V
with a step size of 5 mV and the current was measured simultaneously. Further details about the IV measurements and
gamma source characteristics are given in Ref. 18.
III. RESULTS
A. Electrical measurements
From the perspective of a practical application, the most
important parameter is the film conductivity, measured as
current flowing through the device, as a function of the radiation dose. In this research, we focused on several typical
chalcogenide glass representatives—such as having floppy
or rigid structure from the systems containing Se or Te,
which could be used as a paradigm and give an idea of what
to expect for compositions in between these two extremes.
The data showing current as a function of the radiation dose
for devices with different compositions are presented in
Figures 1(a)–1(d). As one can see, there is a big difference in
the performance of the devices based on different
compositions.
B. Raman spectroscopy
The actual Raman spectra of different films on which
the devices are based, are presented in Figures 2(a)–2(d).
The spectra reveal the formation of a tetrahedrally coordinated structure for the majority of compositions. Besides
the mode around 250 cm1 related to the vibrations of the Se
chains, there are two broad bands centered around 202 and
219 cm1, associated to the occurrence of corner-sharing
(CS) and edge-sharing (ES) structures in the Ge-Se films. In
the rigid films besides these kinds of structural units, the
breathing mode of the ethane-like building blocks is present
at 179 cm1. For the system with Te, the presence of the respective structural units is demonstrated by the vibrations in
the bands around 118 and 131 cm1 (CS) and 147 cm1
(ES). For the Ge rich Ge-Te films, there is a very dominant
presence of a band at 88 cm1 linked to the availability of a
distorted rock salt structure, i.e., three fold coordinated Ge
and Te atoms. Indeed this mode is visible also in the spectra
of the Te rich films. Observing the Raman spectra, one can
see that radiation-induced changes are a function of the film
composition. More specifically, while the Ge20Se80 films do
not undergo structural changes caused by radiation, in the
Ge40Se60 films, structural reorganization occurs; the number
of the ES units increases, while the number of the CS building block decreases, which effectively opens up the
structure—Fig. 3.
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FIG. 1. Dependence of the current as a function of the radiation dose (the solid lines are guides to the eye only) for devices based on films with the following
compositions: (a) Ge25Se75, (b) Ge40Se60, (c) Ge10Te90, (d) Ge40Te60.
There is an interesting tendency for the Te containing
films, in which in both cases of floppy and rigid films, an
increase of the peak area of the distorted rock salt structure
grows with radiation dosage—Figures 3(b) and 3(c).
generally increases with increased radiation dose. However,
for the rigid Te containing films, the Ag diffusion distance
actually decreases at high dose levels.
D. X-ray diffraction spectroscopy
C. Ag lateral diffusion
The structure of the devices presents a good opportunity
to characterize Ag diffusion as a function of the radiation
dose through mapping of the Ag concentration between the
two inert electrodes on which the conductivity of the devices
is characterized.
The radiation field distribution during the experiments is
uniform around the radiation sensing devices, which results
in lateral Ag diffusion. The actual data collected for
Ge40Se60 films are shown in Figure 4(a). They resemble the
shape and the concentration distribution characteristic for all
data collected. Furthermore, Figures 4(b) and 4(c) represent
the particular concentration distribution, characteristic for
the center of the distance between the two inert electrodes
for devices based on floppy chalcogenide films containing Se
and Te (b) and rigid films containing Se and Te (c). For all
Ge-Se and floppy Ge-Te films, the distance of diffused Ag
The molecular composition and phases of the silver containing compounds are extremely important in understanding
the effect of the silver diffusion on the conductivity changes
observed in the electrical measurements. Some phases are
superconductors, while others are semiconductor in nature;
therefore, XRD was performed to reveal all the various
phases. The analyzed data are shown in Figures 5(a)–5(d)
What makes these results interesting is the fact that for
the Ge-Se system, there are traces from aAg2Se even in the
initial films. We regard the reason for this further in the discussion section of this work. For the Se rich phases, the ternary Ag8GeSe4 forms simultaneously with the bAg2Se.
This is also the most abundant phase after Ag diffusion into
Ge40Se60. Initial introduction of Ag in the Ge-Te films has
not been established, which is clearly related to their structure and also to the positive charge coupled with the Te
atoms, which repel the Ag ions diffusion. In the Ge-rich
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J. Appl. Phys. 115, 043502 (2014)
FIG. 2. Raman spectra of the (a) Ge-Se and (b) Ge-Te films as a function of the radiation.
FIG. 3. Structure development in thin films after radiation for: (a) Ge40Se60; (b) Ge20Te80; and (c) Ge40Te60 films.
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J. Appl. Phys. 115, 043502 (2014)
FIG. 4. Lateral Ag diffusion in the radiation sensing devices (a) actual data Ag concentration distribution as a function of the radiation dose for (b) floppy and
(c) rigid films.
FIG. 5. XRD data after Ag diffusion as a function of the radiation dose for (a) Ge20Se80; (b)Ge40Se60; (c)Ge20Te80; and (d) Ge40Te60 thin films.
Ge-Te films, due to irradiation, there is an increase in the formation of Ge-Te crystals with increased radiation dose.
E. Radiation induced oxidation of the
Ge-chalcogenide films
When dealing with Ge-containing systems, oxidation is
a specific concern. This effect can be accelerated by
radiation due to the formation of plenty of dangling bonds
ready to react with oxygen. To investigate this, films oxidation measurements were performed as a function of radiation
dose. As expected, the Ge rich glasses are more perceptive to
oxidation, while those containing predominantly chalcogen
atoms are stable and the amount of oxygen included in them
remains almost unchanged with radiation, as presented in
Figures 6(a) and 6(b).
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J. Appl. Phys. 115, 043502 (2014)
FIG. 6. Development of oxygen inclusion in the chalcogenide glasses as a function of the radiation dose.
IV. DISCUSSION
When analyzing the electrical performance of the devices, one has to keep in mind that the effects occurring are a
consequence of two events. On one hand, the chalcogenide
material itself reacts upon the radiation, and on the other,
radiation-induced Ag diffusion occurs. They can work together or can cancel each other depending on the chalcogenide backbone and the charges, generated by irradiation.
Because of all specifics related to the particular studied compositions, this discussion will be made individually for each
type of composition. We would like to note here that we
were not able to capture all dynamic processes because we
could not make in situ measurements during the irradiations
and all characterizations were made immediately after the
exposure has been completed.
A. Se rich Ge-Se films
For these films, Se is the predominant element. Its two
specific characteristics are availability of lone pair electrons
and mixture of covalent and van der Waals bonding between
the Se atoms. One can expect that being an electromagnetic
wave in character, c radiation of Se will result in formation
of electron-hole pairs. This can affect the materials’ general
organization but should not influence the structure since, as
Tanaka19 has shown, flipping around a stable position in the
chalcogenide structure is possible without breaking and reorganization of the chemical bonding, because of the presence
of the Van der Waals bonding between the Se chains. Since
the Se-Se and Ge-Se bonds are relatively strong, they stay
intact and the charged carrier, occurring after the formation
of the electron-hole pairs recombines at the available defects.
It is for this reason that no structural changes have been registered by the Raman studies—Figure 2(a), although the material has been influenced by the c radiation. Because of the
short wavelength and high energy of the c rays, by the
absorption of this type of radiation, excitation of electrons
positioned at the deeper level is possible. This is expected to
introduce high structural changes without changes to the
band gap. The results of Raman spectroscopy suggest that
the energy absorbed was not sufficient for the occurrence of
such effects or their level was under the resolution of
measurement.
Silver diffusion generally increases with radiation dose
and this affects the conductivity of the devices. However, the
change of the conductivity is only two orders of magnitude—Figure 1(a), while the studies of Urena et al. or
Kawasaki et al.20,21 report a much stronger effect of Ag on
the conductivity of Se rich glasses. It is known that in the
case of Se rich Ge-Se glasses phase separation occurs,22
which de-polymerizes the initial glassy structure. Parts of the
introduced Ag atoms take positions in the band gap contributing to an increase in the level of defects and creation of
charges. The XRD studies show the molecular formula of
the Ag containing phases—Figure 5(a). There are some
of them even in the pre-irradiated structures, which we
assume, have been introduced in the films during the photolithography processes for devices formation, since the films
are illuminated for 20 s with a light source with 436 nm
wavelength. This wavelength is within the absorption edge
of the films and is completely absorbed by the chalcogenide
glass film, which can cause photoinduced effects. As such,
the processing of the devices could initiate some Ag photo
diffusion within the films. It is very interesting that there is a
low amount of a-Ag2Se with a very fine structure, which is
not supposed to be stable at room temperature. We assume
that this is due to the fact that the photolithographic processes have been attempted immediately after the films were
deposited before they relax, so that their structure is very
stressed, which does not allow natural development of the
bAg2Se crystals. This stabilizes the smallest in volume cubic
polymorphic phase of this molecule (aAg2Se) and also keeps
the size of the crystals to a minimum dimension. The size of
these crystals diminishes after irradiation, presumably due to
a polymorph transition to a b phase. This is also the phase
that predominantly develops during c-radaition-induced Ag
diffusion, as revealed by the XRD spectra. bAg2Se is semiconductor with a low ionic conductivity component,23 which
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does not drastically contribute to the conductivity of the Ag
radiation diffused films. Increased radiation dose absorbed
by the samples naturally results in a formation of a high
amount of e-h pairs, which, however, makes their recombination easier since they are quite close and hence we observe
a saturation in the radiation induced conductivity at about
5 Mrad, although the amount of diffused Ag is higher. It
should not be affected by oxidation that is limited in this
composition since Ge atoms are strongly connected to the
excess of Se atoms, which reduces the appearance of dangling bonds in Ge and therefore limits its reactivity to oxygen as presented in Figure 6(a).
J. Appl. Phys. 115, 043502 (2014)
radiation dose, though the oxidation process develops very
rapidly because of the high concentration of defects forming
which makes the material oxidation susceptible. In fact, the
presence of the much shorter Ge-O bonds replacing the Ge-Se
bonding causes formation of the much smaller crystals of the
Ag containing compositions as revealed by the XRD data. All
this forms the entire grid of interrelated effects that complete
the picture of the studied result in which we suggest the presence of GeO2 is the major reason for the decrease of the conductivity of the studied devices. However, this would not be
the case in encapsulated devices, which we will study next.
C. Te rich Ge-Te glasses
B. Ge rich Ge-Se glasses
The general trend described above is valid for these films
as well but there are some specifics related to the availability
of Ge-Ge bonds in them. Because of the low bond strength of
these bonds, they react easily to radiation, which contributes
to the higher radiation sensitivity of these films. Such strongly
expressed reaction of Ge rich films towards irradiation has
been established also by Donghui et al.24 In the studied case,
increase in the number of ES structural units—Figure 3(a)
obtained through transformation of the CS structural units
opens up the structure and allows greater Ag diffusion into the
films. The relatively low density of the films offers space
availability, because of which primarily bAg2Se forms
whose crystals grow with increasing radiation dose up to
3.615 Mrad as depicted by the XRD studies—Figure 5(b).
This affects positively the conductivity of the films and it
grows almost 5 orders of magnitude—Figure 1(b). Indeed
such good sensitivity has been found out by the studies of the
optical properties as well25 in which red shift has been established, i.e., decrease of the band gap.
At higher radiation doses, the bAg2Se has quite limited
size and there is a drastic reduction in the conductivity. We
suggest two reasons for this. One could be a coupling of a
high number of defects generated by radiation, which are
positioned so close to each other that they immediately
recombine, thus reducing the conductivity. We believe that
this effect if related to the optical properties of the glass films,
will be similar to the transformation from photodarkening to
photobleaching under illumination with visible light, which
recently has been described.9,26 A reduction in the film conductivity during irradiation has been reported as well by Zhao
et al.24 However, the obtained large drop in conductivity in a
region of the radiation dose range, where silver within the
films continues to diffuse needs additional explanation and to
find more reasonable justification for this effect, we checked
the oxygen content in the films, which we consider as a second reason for the conductivity reduction. As the data show,
the oxygen amount increases immediately after the radiation
starts but since at this moment, the number of the defects created and the existence of the super conductive a-Ag2Se phase
affect the conductivity therefore the effect of the oxidation
does not influence the electrical performance of the films. At
a higher radiation dose, saturation in the oxidation is effective
due to the mixture of the effects of the defect formation and
introduction of Ag into the glassy matrix. At the highest
As depicted by the Raman spectra—Figure 2(c), the
structure of these films is mainly represented by tetrahedral
units in which Te is twofold coordinated and Ge is fourfold
coordinated.27 In this case, the lone pairs located on Te do not
participate in the bonding. They rather split into a filled band
and occupy a band location higher than the location of the
bonded electrons, therefore, forming the top of the valence
band. Due to the arrangement of electrons, this material is
considered as a lone pair (LP) semiconductor in nature, which
also due to the density of states in the band gap, has p-type
conductivity as suggested by Chopra28 and is a narrow gap
semiconductor. Radiation introduces plenty of long-term stable transient effects such as the rise of rock salt structure, as
observed by the Raman spectroscopy Figure 2(c). There is an
interesting tendency, showing that with increasing radiation
dose, dynamically destroying and transformation from ES
structures to rocksalt units and vice versa can occur by which
the area of the peak related to the rock salt structural units
fluctuates between 0.62 arb.un. and 0.78 arb. un. Such transient effects have been described as raising because of defect
formation on some metastable states and then again reordering
of the system. The energy, possessed by c radiation is sufficient for overcoming the barrier for generating these effects,
leading to the formation of new structures, not characteristic
for this particular composition.29 Note that the rocksalt structure contains dative bonds, where both—Ge and Te appear
three fold coordinated with the two electrons for the dative
bind supplied by the Te atom. In this manner, Te becomes
polarized with a high negative charge on it. This regrouping
of the structure contributes to phase separation in the system.
Indeed Jovari et al.27 report on two glass transition temperatures for samples, very close to this composition even without
radiation. The crystalline phases appearing from these two
glasses were identified to be Te and Ge-Te crystals. We
believe that this phase separation is the reason for formation
of bAg2Te, which we obtained by the XRD studies—Figure
5(c). Its formation deals with the reduction of some charged
defects and an upward shift of the Fermi level, which could be
the reason for the decline in conductivity—Figure 1(c). We
suggest that the dipole model proposed by Khurana et al.30
would be applicable in the studied case.
D. Ge rich Ge-Te glasses
The main structural characteristic of these compositions
differentiating them from the previous compositions is the
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formation of distorted rocksalt structure with a vibrational
band below 100 cm1 (Ref. 31) containing a dative bond. It
is interesting to note that in a dative bond, the length of this
type of bond is equal to any of the other covalent bond
between Ge-Te atoms up to the third decimal digit (2.77 Å),
which makes the nature of this bonding indistinguishable
from other bonds within the system.32 Radiation aids in the
formation of such bonds, since this type of bonding satisfies
all requirements within a glass composition. Ideally, each Ge
atom would prefer bonding with 4 Te atoms to create the tetrahedral structure, the corner sharing building block, but due
to the lack of free Te atoms in this composition to fulfill this
type of bonding, dative bonds are formed instead of the
covalent bond. This structure is characteristic with coupling
of a negative charge on the Te atoms, which is very attractive for the positively charged diffusing Ag ions. It is for this
reason that in this film composition, the ternary Ag8GeTe6
forms—Figure 5(d), where the covalent bonding requirements of Ag are completely satisfied and there is minimal
structural rearrangement necessary for the creation of this
new molecule. Since there are no dangling bonds formed,
the introduction of Ag does not contribute to the generation
of donor or acceptor states in the band gap and hence there is
only a minimal influence that it could cause over the conductivity in the system—Figure 1(d). What is left to affect, in a
limited manner, the conductivity in the system is the Ge oxidation. It starts with a limited oxidation rate, which increases
at higher radiation dose due to formation of some surface
defects and reaches saturation in the middle of our studied
dose values—Figure 6(d). At higher doses due to formation
of an abundance of defects, as well as the formation of the
ternary Ag8GeTe6, a number of defects are formed, which
enhance the oxidation processes and formation of GeO2,
which is a dielectric. As a result, a small decline in the conductivity appears—Figure 1(d). It is also related to the lower
diffusion distance of diffused Ag—Figure 4(b), which is restricted by the densification of the structure by the presence
of O2 forming shorter bonding with Ge within the films.
V. CONCLUSIONS
In this work, the effect of the radiation dose over the
electrical performance of radiation sensing devices has been
characterized through device testing and materials studies,
based on floppy and rigid examples from the Ge-Se to Ge-Te
chalcogenide glass systems. Rich number of effects has been
tracked related to the chemical bonding in the two systems
by which due to the weaker chemical bonding in the system
Ge-Te with high Te concentration structural transitions
CS—distorted rock salt structure occurs, which are not characteristic for the Ge-Se system. The conductivity of the devices is a function of the availability of chalcogen elements in
which for the chalcogen rich materials, the c radiation affects
the lone pair electrons resulting in high sensitivity towards
the radiation dose. However, depending upon the byproducts
related to the introduction of Ag through radiation induced
diffusion and the specific structural effects in both systems,
the conductivity of the material in the Ge-Se system
increases with radiation, while decrease is registered for the
J. Appl. Phys. 115, 043502 (2014)
Ge-Te films. The Ge rich samples undergo larger, well
expressed on the Raman spectra, structural transformation
due to formation of weaker Ge-Ge bonds caused by the lack
of chalcogen elements to form the tetrahedral coordinated
structural units, characteristic for these glasses. This affects
the conductivity in the Ge-Se system, which raises more than
four orders of magnitude but undergoes annealing effect at
about 3 Mrad radiation due to the abundant number of
defects forming in the system and (further) oxidation, while
the conductivity of the Ge rich Ge-Te films remains almost
unchanged by the radiation due to formation of Ag8GeSe6,
lack of formation of states in the band gap and oxidation.
From the point of view, how the studied films could be used
for radiation sensing, both effects characteristic for the
chalcogen rich materials could be applied due to their linearity. In the case of Se containing films increasing of the conductivity will be related to the increased dosage of radiation,
while at application of Te rich films conductivity decrease
will be related to increased radiation doses. In the cases,
when high sensitivity for low radiation doses is necessary,
the Ge rich Se containing films are very appropriate due to
their high sensitivity at relatively low radiation doses.
ACKNOWLEDGMENTS
This work had been funded by the Defense Threat
Reduction Agency under Grant No. HDTRA1-11-1-0055.
The authors would also like to thank Dr. James Reed of
DTRA for his support.
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