Gamma radiation induced effects in floppy and rigid Ge-containing chalcogenide thin … · 2020. 3....

Gamma radiation induced effects in floppy and rigid Ge-containing chalcogenide thinfilmsMahesh 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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Gamma radiation induced effects in floppy and rigid Ge-containingchalcogenide 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 Mitkova11Department of Electrical Engineering, Boise State University, 1910 University Dr. Boise, Idaho 83725-2075,USA2School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe,Arizona 85287-9309, USA3Department of Physics, Arizona State University, Tempe, Arizona 85287-1504, USA4Department 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

conditions. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4862561]

I. INTRODUCTION

Since their discovery in the mid 1950s, chalcogenide

glasses keep on attracting research attention due to the vari-

ous 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 con-

ductive 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 opportuni-

ties 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 con-

cerned 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 correspond-ing 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

contradictory—some authors report decreasing conductivity

in c radiated chalcogenide glasses,8,9 while others suggest aradiation-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 pho-

togeneration 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 par-

ticles and there is an abundance of charged defect sites avail-

able in the glass structure, the e-h pairs can easily recombine

at the defect sites, which is the main reason for the occur-

rence 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 radia-

tion conditions, primarily silver. A silver atom, when intro-

duced 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? Ifyes, how does the effect of c�induced diffusion vary the

0021-8979/2014/115(4)/043502/9/$30.00 VC 2014 AIP Publishing LLC115, 043502-1

JOURNAL OF APPLIED PHYSICS 115, 043502 (2014)


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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 over-

come 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, radi-

ation 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 con-

sisting 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 evapora-

tion system at 1� 10�6 mbar pressure, using a semi Knudsencell crucible. The chalcogenide glass films for materials char-

acterization 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 techni-

ques, 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 beamintensity of 25 lW. Although the laser wavelength is withinthe absorption edge of the films, no illumination-induced

effects have been observed during and after several measure-

ments 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(k¼ 1.5406 Å) through a G€obel mirror producing a collimatedbeam. 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

corresponding to the elemental composition across the sam-

ple, which were analyzed using INCA software.

All of the devices were wire bonded to device packages

with gold pins to reduce inconsistencies between measure-

ments and ensure accurate measurements without external

influences. The probes and the test bench were tied to a com-

mon 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 ana-

lyzer, 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 simul-

taneously. 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 radia-

tion 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 coor-

dinated structure for the majority of compositions. Besides

the mode around 250 cm�1 related to the vibrations of the Se

chains, there are two broad bands centered around 202 and

219 cm�1, 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 cm�1. For the system with Te, the presence of the re-

spective structural units is demonstrated by the vibrations in

the bands around 118 and 131 cm�1 (CS) and 147 cm�1

(ES). For the Ge rich Ge-Te films, there is a very dominant

presence of a band at 88 cm�1 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 build-

ing block decreases, which effectively opens up the

structure—Fig. 3.

043502-2 Ailavajhala et al. J. Appl. Phys. 115, 043502 (2014)


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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).

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

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

The molecular composition and phases of the silver con-

taining 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 theinitial films. We regard the reason for this further in the dis-

cussion section of this work. For the Se rich phases, the ter-

nary Ag8GeSe4 forms simultaneously with the b�Ag2Se.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 struc-

ture and also to the positive charge coupled with the Te

atoms, which repel the Ag ions diffusion. In the Ge-rich

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.



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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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Ge-Te films, due to irradiation, there is an increase in the for-

mation of Ge-Te crystals with increased radiation dose.

E. Radiation induced oxidation of theGe-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 oxida-

tion 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).

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.



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IV. DISCUSSION

When analyzing the electrical performance of the devi-

ces, 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 to-

gether or can cancel each other depending on the chalcoge-

nide backbone and the charges, generated by irradiation.

Because of all specifics related to the particular studied com-

positions, 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 irradiationsand 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 formationof 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 reor-

ganization 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 reg-

istered by the Raman studies—Figure 2(a), although the ma-

terial has been influenced by the c radiation. Because of theshort wavelength and high energy of the c rays, by theabsorption 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 magni-

tude—Figure 1(a), while the studies of Urena et al. orKawasaki et al.20,21 report a much stronger effect of Ag onthe 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 contrib-

uting 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 photoli-

thography 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 isnot supposed to be stable at room temperature. We assume

that this is due to the fact that the photolithographic proc-

esses 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 cubicpolymorphic phase of this molecule (aAg2Se) and also keepsthe 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 phasethat predominantly develops during c-radaition-induced Agdiffusion, as revealed by the XRD spectra. bAg2Se is semi-conductor with a low ionic conductivity component,23 which

FIG. 6. Development of oxygen inclusion in the chalcogenide glasses as a function of the radiation dose.



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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 recombi-

nation 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 dan-

gling bonds in Ge and therefore limits its reactivity to oxy-

gen as presented in Figure 6(a).

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 b�Ag2Se formswhose 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 estab-

lished, i.e., decrease of the band gap.

At higher radiation doses, the b�Ag2Se has quite limitedsize 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 con-

ductivity during irradiation has been reported as well by Zhao

et al.24 However, the obtained large drop in conductivity in aregion 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 sec-

ond 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 cre-

ated and the existence of the super conductive a-Ag2Se phaseaffect 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

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 pres-

ence of GeO2 is the major reason for the decrease of the con-

ductivity 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

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 sta-

ble 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 tran-

sient 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 suffi-cient 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 struc-

ture 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 tempera-tures 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—Figure5(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 cm�1 (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 tet-

rahedral 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 attrac-

tive for the positively charged diffusing Ag ions. It is for this

reason that in this film composition, the ternary Ag8GeTe6forms—Figure 5(d), where the covalent bonding require-

ments 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 conduc-

tivity in the system—Figure 1(d). What is left to affect, in a

limited manner, the conductivity in the system is the Ge oxi-

dation. 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 con-

ductivity appears—Figure 1(d). It is also related to the lower

diffusion distance of diffused Ag—Figure 4(b), which is re-

stricted 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 char-

acteristic for the Ge-Se system. The conductivity of the devi-

ces is a function of the availability of chalcogen elements in

which for the chalcogen rich materials, the c radiation affectsthe 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

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 linear-

ity. In the case of Se containing films increasing of the con-

ductivity 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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