ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 866–872
www.elsevier.com/locate/jpcs
Ag-photodoping in Ge-chalcogenide amorphous thin films—Reaction
products and their characterization
M. Mitkovaa,, M.N. Kozickib
a
Department of Electrical & Computer Engineering, Boise State University, Boise, ID 83725-2075, USA
b
Center for Solid State Electronics Research, Arizona State University Tempe, AZ 85287-6206, USA
Abstract
We make a brief review on the effect of silver photodiffusion in Ge-chalcogenide glasses and report some of our recent results in this
aspect. Using Raman spectroscopy and X-ray diffraction analysis we demonstrate that the hosting backbone undergoes depletion in
chalcogen due to the specific conditions of photodiffusion and the diffusion products are silver chalcogenides. While in the Ge–Se system
preliminary binary Ag—chalcogenides are forming, in the Ge–S system formation of Ag2 GeS3 is evidenced. This effect is related to the
ability of the Ge–S glasses to form ethane—like structure at much lower Ge concentration than the Ge–Se glasses. For this type of
structures is known that Ag replaces Ge to form homogeneous material, hence formation of Ag-containing ternary occurs.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Chalcogenides; A. Nanostructures; C. Raman spectroscopy; C. X-ray diffraction
1. Introduction
2. Nature of the photodoping process
Silver photodiffusion—accelerated silver diffusion under
the action of light is a unique feature of chalcogenide
glasses. Ever since this effect was first encountered [1] it has
been profoundly investigated because it can be used in
many applications of chalcogenide glasses. We are using
this process to form the active material—the solid
electrolyte based on silver doped chalcogenide glass—for
the programmable metallization cell (PMC) two terminal
memory devices which have an electrode made from Ag
and the other being electrochemically inert (W, Ni Pt, etc.)
[2]. At application of a bias with positive voltage towards
the Ag electrode formation of a robust but reversible
conducting pathway by way of electrodeposition at low
voltage and current reduces the resistance of the electrolyte
by several orders of magnitude. In this manner non-volatile
memory is realized in elements that are highly electrically
and dimensionally scalable.
Our understanding is that the process of photodiffusion
is driven by the formation of charged defects in the
chalcogenide glass, which form by illumination with light
and create an electrical potential. The light that is critical
for metal photodissolution is absorbed at or near the
interface between the reacted and unreacted (doped and
undoped) chalcogenide layers [3,4]. In this process,
electrons are trapped by silver ions [5,6], while holes move
further into the chalcogenide film and are trapped there.
The electric field formed by the negatively charged
chalcogen atoms and positively charged silver ions can be
sufficient for the silver ions to overcome the energy barrier
at the interface. Therefore the penetration of the metal into
the chalcogenide during photodoping is due to the
difference in electrochemical potentials and the process
was considered to be similar to that occurring in a galvanic
cell, where the more electropositive metal is dissolved into
the electrolyte [7].
Kluge [8] 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
Corresponding author. Tel.: +1 208 426 3395; fax: +1 208 426 2479.
E-mail address: MariaMitkova@boisestate.edu (M. Mitkova).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.01.004
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M. Mitkova, M.N. Kozicki / Journal of Physics and Chemistry of Solids 68 (2007) 866–872
mixed with soft van der Waals interconnections. This type
of structure ensures formation of voids and channels where
the diffusing ions can migrate and can be hosted. 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:
suggest that the kinetics of the diffusion process is highly
influenced by free space availability in the chalcogenide
matrix and the chemical reaction between S and Ag in the
case of sulfur-rich glasses. When Ge rich glasses are
considered, the photodiffusion rate could be related to the
formation of the number of chemically active photo excited
localized centers [20].
(1)
This reaction describes the transition of an initially twofold
covalently bonded chalcogenide atom ðC02 Þ into a C
1
charged unit possessing only a single covalent bond and an
excess electron that establishes an ionic bond with
Agþ ðMþ Þ. Eq. (1) shows the importance of the potential
þ
in forming the new C
bonds of the intercalation
1M
product. The possible number of these bond-units is fairly
high as the chalcogenide glasses are capable of forming a
number of single C
1 centers under the influence of light
illumination. Once silver is introduced into the chalcogenide glass, its further migration into the chalcogenide glass
continues.
The photodiffusion kinetics depends on a number of
factors such as light intensity [9], light wavelength [10],
temperature [11], pressure [12], external electric field [13],
composition of the hosting glass [14], and the atmosphere
in which the diffusion process is performed [15]. Many
details in this respect are given in the work of Kolobov and
Elliott [16]. In this review, we will specifically discuss the
data concerning Ge-chalcogenide systems.
3. Silver photodiffusion in the Ge–S system
3.1. Basic data
The most profound investigation of silver diffusion in
sulfur-rich Ge glasses has been made by Oldale and Elliott
[17]. They have found that the Ag photo-dissolution rate in
a-Ge29 S71 has no induction period and the process has 2
stages—phase 1, which is an acceleratory stage leading to a
maximum in the photodissolution rate, and phase 2, a final
deceleratory stage. As the time to develop the acceleratory
stage shows a spectral dependency, it is obvious that the
absorption of actinic radiation in the photodoped film is
responsible for this stage of the photodissolution kinetic
profile. Maruno and Ban [18] reported that when the silver
film is deposited on previously illuminated Ge30 S70 film, the
diffusion process proceeds very slowly. Speculations are
made upon the changes in the structure of the chalcogenide
film due to the light illumination. However, we assume that
oxidation of the chalcogenide film during the initial
illumination could also be responsible for the occurrence
of this effect. The defects that can be created by
illumination are indeed the driving force for the oxidation
process.
Kawaguchi and Maruno [19] obtained very important
data about the compositional dependences on the initial
photodoping rate and photodoping kinetics. Their data
3.2. XRD data generated in our research
We contributed with some research related to GeS2
photodiffused with Ag [21] towards further understanding
of what the diffusion products are and how diffusion
affects the hosting backbone, combined with data about
the influence of annealing at 150, 300 and 430 C since in
the real world of application, the glasses are usually
processed at similar temperatures. The X-ray diffraction
(XRD) spectra of photodiffused films are shown on Fig. 1
(a)–(d). As shown on the figure diffusion products are Ag2 S
and Ag2 GeS3 and only during annealing at 430 C
Ag8 GeS6 forms. The most impressive result of this
experiment is the fact of a fast growth of the diffusion
products during annealing. The studies of the density of
Ge–S glasses show that the stoichiometric composition
GeS2 is expected to have the highest density [22]. However,
the real composition of the backbone after introduction of
Ag is much more Ge rich because of the reaction of Ag
with the negatively charged defects and it is expected to
have much lower density. This easily allows formation of
channels, which, because of the low polymerization, can
offer substantial space where the Ag containing phase is
located. Numerical simulations of the structure also suggest
their existence [23]. Therefore, we believe this enables
introduction of high amount of Ag and the rapid growth
^
+
+
Intensity (Arb. units)
þ
C02 þ e þ Mþ ! C
1M .
867
430 C
300 oC
*
20
d
+
*
*
+
*
150oC
RT
*
* ^ *
o
+
* **
30
+
+
+
+
+
40
2 Theta (Deg.)
+
c
+
b
**
**
a
50
Fig. 1. XRD data for: (a) photodiffused Ge–S film; (b) photodiffused
Ge–S film annealed at 150 C; (c) photodiffused Ge–S film annealed at
300 C; (d) photodiffused Ge–S film annealed at 430 C; * denotes
appearance of Ag2 GeS3; þ denotes appearance of Ag2 S; ˆ denotes
appearance of Ag8 GeS6 . Figure taken from Balakrishnan et al. [21].
ARTICLE IN PRESS
M. Mitkova, M.N. Kozicki / Journal of Physics and Chemistry of Solids 68 (2007) 866–872
Ge-Ge bond vibration
(ethane-like structure)
1.0
d
0.5
0.0
1.0
c
0.5
Intensity (Arb. units)
Intensity (Arb. units)
symmetric stretch of Ge(S1/2)4
tetrahedra
clu
mo ster
de ed
g
1.0
e
Sclu S str
ste etc
r e h fr
dg om
ed
im
ers
868
0.0
200
400
300
Rel. Wavenumber (cm-1)
0.5
0.0
1.0
b
0.5
500
Fig. 2. Raman spectra for pure Ge–S films at room temperature. Figure
taken from Balakrishnan et al. [21].
0.0
1.0
a
0.5
of the diffusion products through agglomeration of the
nanoclusters as established by the XRD data—Fig. 1(a)–(d).
3.3. Raman data generated in our research
The Raman results for pure Ge–S films Fig. 2 taken from
[21] show appearance of relatively high intensive mode at
343 cm1 of the A1 symmetric stretch of GeðS1=2 Þ4 [24]
combined with scattering at 370 and 427 cm1 from the
edge sharing structures. There is a well resolved peak at
252 cm1 corresponding to the vibrations coming from the
ethane like structures available in glasses containing more
than 33 at% Ge [25]. The deconvolution of the Raman
modes between 340 and 440 cm1 manifests formation of a
peak at 427 cm1 . We believe that this signal occurs from
the vibrations of the S chains available due to formation of
wrong bonds in these glasses. The variety of building
blocks forming this film is indication for the specific
structure that develops in the Ge–S system. Boolchand
et al. [26] have demonstrated that the formation of ethanelike structural units containing Ge–Ge bonds starts at the
stoichiometric composition GeS2 . On grounds of stoichiometry an equivalent number of S–S bonds are available.
This is a very rare case in chalcogenide glasses in which
almost all possible building blocks emerge in one composition. The implication of this effect is that upon illumination
metastable states [27] occur on sulfur atoms with different
surroundings.
When Ag is photodiffused in the Ge–S glass, well
expressed changes occur in the Raman activity of the newly
formed material, (Fig. 3(a)). One observes intensity growth
of the mode at 250 cm1 , indicating formation of a higher
number of ethane like structural units. Meanwhile, the
relative intensity of the mode at 343 cm1 is reduced, and
the vibrations at 370 and 400 cm1 strengthen. We
attribute the latter modes to development of thiogermanate
0.0
200
300
400
500
Rel. Wavenumber (cm-1)
Fig. 3. Raman spectra for: (a) photodiffused Ge–S film at room
temperature; (b) photodiffused Ge–S film annealed at 150 C; (c)
photodiffused Ge–S film annealed at 300 C; (d) photodiffused Ge–S film
annealed at 430 C. Solid lines are fitted results. Figure taken from
Balakrishnan et al. [21].
bonds ðGe2S Þ forming metathiogermanate tetrahedra
ðGeS2
3 Þ and dithiogermanate tetrahedra ðGeS2:5 Þ as
suggested by Kamitsos et al. [28]. Illumination with light
causes formation of defects not only on S atoms forming S
chains but also on S that is part of other structural units. It
is for this reason that we observe formation of both Ag2 S
and Ag2 GeS3 after Ag diffusion by which the Ag ions react
with the photoinduced defects in the chalcogenide glass as
suggested by Eq. (1). The growth of the mode at 250 cm1
indicates significant sulfur depletion of the initial composition (Fig. 3) of the hosting backbone after Ag is
photodiffused in the Ge–S film (Fig. 3) and a large number
of ethane-like structures are formed.
After annealing of Ag diffused films, we realize that their
structure keeps the initial character and the intensity of the
ethane like structures increases with annealing temperature
(Fig. 3(b)–(d)). The decreasing ratio between the intensity
of the mode at 252 cm1 and the mode at 334 cm1 could
be related to some continuing reaction between the three
elements at annealing. Note that the peak at 252 cm1 is
very stable due to a higher rigidity of the structure and
better filling of the intercluster space with introduction of
Ag in the film. This prevents incidence of intercluster
changes. In fact, due to the low dimensional nature of the
clusters, the Ge–S host is expected to be less strained
ARTICLE IN PRESS
M. Mitkova, M.N. Kozicki / Journal of Physics and Chemistry of Solids 68 (2007) 866–872
compared to its isoionic counterpart, Ge–Se glass. This,
along with the bond strength of the covalent bonding in the
studied glass accounts for the lower polymerization relative
to the Ge–Se system. As a result a more relaxed structure is
formed where the Ag-containing products experience much
lower pressure from the surrounding backbone and only
the low-temperature forms of the respective compositions
occur that are known to have larger volume than the hightemperature forms. The dramatic decline in the scattering
intensity of the edge and corner sharing tetrahedra during
annealing at 430 C (Fig. 3(d)) can be related to formation
of a new ternary composition—Ag8 GeS6 (Fig. 1(d)). For
this composition the structure is formed by isolated GeS4
tetrahedra as well as S atoms that are not bonded to the Ge
atoms [29]. The anion parts GeS4 and S are connected by
Ag atoms to form 3D structure. The Ag atoms are bi-,
three- and fourfold coordinated with S [30]. In other words,
formation of Ag8 GeS6 brings about a serious depolymerization of the structure and hence decreases the intensity of
the modes related to particular structural coordination of
the Ge–S tetrahedra. This effect can be also accompanied
with high concentration of Ag on the surface at the highest
annealing temperature that would be the most natural
effect considering the highest number of wrong bonds that
are related to the surface defects. The scattering coming
from this Ag rich medium with a narrow band gap will
significantly reduce the scattering from the Ge–S host.
However, note that the second order Si substrate mode at
303 cm1 yields almost consistent intensity in all the fits,
indicating that the laser beam reaches with ample energy
the studied material and the sample penetration depth
through the films is not influenced by Ag clustering near
the surface.
4. Silver photodiffusion in the Ge–Se system
4.1. Basic data
Most extensive data about Ag diffusion in Ge–Se glasses
have been reported by Kluge et al. [31]. They have found
that the diffusion kinetics and the total amount of diffused
Ag are very closely related to the composition of the
hosting backbone. While for a backbone containing
75290 at% Se there is almost no induction period for the
diffusion process, at lower Se concentration this period
grows with decreasing Se content. This is somewhat
correlated with the depth profile of the Ag diffusion. At
high Se concentration, for GeSe5:5 , a step-like profile is
found [32,33] by which a Ag-depleted layer lies over the
Ag-enriched layer situated just above the substrate. Leung
et al. [34] also confirm that surface diffusion is much
smaller than the bulk diffusion. Indeed, some irregularities
and discontinuity of the silver doped film with formations
of islands have been found also by Rennie et al. [35]. For
GeSe2 glasses, the process is slower and follows the
classical distribution [36]. Wagner et al. [37] established a
significant difference in the diffusion profiles of laterally
869
diffused Ag in Ge20 Se80 and Ge40 Se60 glasses, with sharp
edge diffusion for the Se-rich glass and a classical diffusion
profile for the Ge-rich glass. Their interpretation of this
effect is related to the existence of two glass-forming
regions in the Ge–Se–Ag system. While for the Ge-rich
glasses the diffusion process goes through compositions
characteristic only for one of the sub regions, in the case of
Se-rich glasses the composition of the diffused product
resembles those of the two regions. This requires some
structural rearrangements that affect the diffusion profile.
Structural investigations on the photodiffused material
reveal formation of a heterogeneous structure after
photodiffusion [38,39]. Chen and Tai [38] report formation
of bcc Ag2 Se, Ag2 SO4 and small amounts of orthorhombic
Ag2 Se when Ag is diffused into GeSe2 glass and bcc Ag2 Se
and traces of free Ag when the diffusion process is
conducted in Ge0:1 Se0:9 glass. We assume that these
differences in the diffusion processes are closely related to
the availability of space and channels for the diffusion of
Ag in the particular hosting glasses. Formation of Ag2 Se
has been submitted also by Zembutsu [39]. Considering the
existing results, Kawaguchi et al. [40] proposed a schematic
model for the evolution of the structure of the chalcogenide
glasses during Ag diffusion that depict the formation of the
two phases—Fig. 4.
4.2. XRD data generated in our research
We studied [41] the diffusion products in the Ge–Se
system and their growth during annealing at 85, 110, 125
and 150 C in hosting materials with composition Ge20 Se80 ,
Ge30 Se70 , Ge33 Se67 and Ge40 Se60 . Fig. 5 gives representative curves of the XRD spectra of the photodiffused glasses
for an initial composition of Ge33 Se67 . In all cases, the
hosting Ge–Se glass remained amorphous during the
annealing while the silver containing species formed
nanocrystals. We found orthorhombic, bAg2 Se, as well as
cubic, aAg2 Se with Ag8 GeSe6 appearing only when Ag is
introduced in a host containing 33 at% and higher
concentration of Ge. The crystals forming after diffusion
a
b
c
Ag-poor phase
Ag-rich phase
d
e
f
Ag particles
Fig. 4. Schematic illustration of the change of structure of
ðGex SeðSÞ1x Þ1y Agy deposited films with increasing Ag content: (a)
shows the initial glass in which the amount of Ag gradually increases
(b)–(e) until Ag particles phase separate (f). Figure taken from Kawaguchi
et al. [40].
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M. Mitkova, M.N. Kozicki / Journal of Physics and Chemistry of Solids 68 (2007) 866–872
870
*
+
+ + ^
+
* *
d
Intensity (Arb. units)
*
*
*
+ + ^
*
+
*
+
+
+ +
^+
+
+ ^ + +
* *
*
c
*
b
* *
30
50
40
2 Theta, Deg.
60
b
Ge30Se70
c
Ge33Se67
d
Ge40Se60
e
after Ag diffusion
Film resulting after photodiffusion
a
100
20
Ge20Se80
a
Intensity, Arb. Units
*
70
Fig. 5. Representative XRD plots of Ge33 Se67 glass photodiffused with
Ag annealed (a) at 85 C for 15 min, (b) at 85 C for 120 min, (c) at 150 C
for 15 min and (d) at 150 C for 120 min; * peaks characteristic for
Ag8 GeSe6 ; ˆ peaks characteristic for aAg2 Se; þ peaks characteristic for
bAg2 Se. Some peaks were reduced to fit on a single graph. Figure taken
from Mitkova et al. [41].
are relatively small because they can only form in the free
interspaces available in the matrix of the hosting glass.
Although in the case of Ge20 Se80 glass the initial structure
is floppy, following the initial silver inclusion and formation of Ag2 Se, the glass structure becomes depleted in Se
and stiffer. The internal space limitation produces the same
effects as elevated pressure, stabilizing some clusters in the
high temperature form which has the closest packing. With
Ge-enrichment of the hosting backbone, the intensity of
the peaks of aAg2 Se becomes higher, suggesting reflectance
from a larger number of planes. At the same time,
Ag8 GeSe6 clusters are formed and we assume that these
occur at terminal defects on the Ge–Se tetrahedra in the
case of the Ge33 Se67 host or develop within the volume of
the films when Ag is diffused in a Ge40 Se60 host. Indeed,
Mössbauer spectroscopy definitely shows that replacement
of Ge by Ag occurs in Ge-rich glasses [42] so a combined
effect could be the reason for the development of the
ternary composition.
4.3. Raman data generated in our research
Raman features of initial hosts closely match those of
bulk materials with the same composition, as illustrated in
Fig. 6(a)–(d). However, after diffusion, the spectra of all
samples show a vibrational band at 180 cm1 and a higher
frequency band at 200 cm1 independent of the composition (Fig. 6(e)), suggesting the formation of a structure
containing ethane-like units with Ge–Ge bond as well as
the Ge–Se tetrahedra. These spectra remained unchanged
following the moderate annealing.
200
300
400
500
Raman shift, cm-1
Fig. 6. Raman spectra of the undoped Ge–Se glasses and spectrum of the
photodiffused material. Compositions are noted in the figure. Figure
taken from Mitkova et al. [41].
We assume that as in the case of Ge–Se glasses, the
illumination with light causes formation of charge defects
that can react with Ag and form the diffusion product. This
fact has important consequence since some Se is extracted
from the initial Ge–Se backbone to react with the diffused
Ag. So the remaining chalcogenide glass backbone
becomes Se deficient, as demonstrated by the appearance
of a Raman signature that is characteristic of a Ge-rich
glass, independent of the initial composition of the host. In
this composition the underlying molecular phase consists
of face-sharing quasi one-dimensional ethane-like
Ge2 ðSe1=2 Þ6 chain fragments whose presence is manifested
on the Raman spectra by the appearance of the mode at
180 cm1 [43] depicted in Fig. 6(e). The Raman spectrum of
the resulting material shows a lower intensity ratio between
the modes at 180 cm1 and the mode of the Ge-tetrahedral
units at 200 cm1 when compared to the intensity ratio of
these modes for a Ge40 Se60 initial glass film indicating that
the number of ethane like units is lower than in Ge40 Se60
glass. However this structure still contains Ge–Ge bonds.
They are the result of the spontaneous reaction of Ag with
charged metastable states on the chalcogen initiated by
light illumination and with charged defects occurring at
bond conversion [44]. This reaction will be preferred since
the energy that it requires is less than the energy for the
Ge–Se bonding (48.4 vs. 113 kcal/mol). We suggest that
this, together with space organization in the material is the
reason for the extraction of some Se from the Ge–Se
backbone for the formation of Ag2 Se in addition to the
reaction of Ag with the initially available free Se chains.
The structure of Ge–Se backbone formed after photodoping is depolymerized to some extent due to the extraction of
Se and formation of crystalline products. It is for this
reason that the organization of the photodiffused hosting
glass does not change with the moderate annealing applied,
ARTICLE IN PRESS
M. Mitkova, M.N. Kozicki / Journal of Physics and Chemistry of Solids 68 (2007) 866–872
as happens with pure Ge–Se films [45] where the local
stressed configurations with a high free energy relax
through breaking of the Ge–Ge bonds and formation of
Ge–Se corner-sharing units due to reaction with Se–Se
wrong bonds.
5. Conclusions
In this work we gave a brief review with extended
references of the published results about the Ag photodiffusion in Ge–S(Se) chalcogenide glasses and combined
them with our recent results. We demonstrate that the
photodiffusion effect can be well characterized by XRD
method which gives direct evidences about the diffusion
products that are crystalline and by Raman spectroscopy
which supplies data about the structure of the hosting
Ge–S(Se) backbone. We found out differences in the
photoinduced effects in the investigated systems which
can be summarized as follows:
For the Ge–S system:
The diffusion products are nanocrystals of Ag2 S and
Ag2 GeS3 which grow via agglomeration with increasing
the annealing temperature. At 430 C Ag8 GeS6 forms
which is product of reaction of agglomerated Ag2 S with
the hosting backbone.
The Raman data about the hosting Ge–S backbone
show that it becomes more rigid and Ge-rich after the
act of Ag photodiffusion in it.
The intensity of the Raman mode characterizing the
formation of Ge–Ge bond after introduction of Ag
grows with the annealing temperature up to 430 C. At
this temperature the intensity characterizing the Ge–S
tetrahedra decreases drastically because of structural
rearrangement and formation of Ag8 GeS6 ternary which
is build up by isolated GeS4 tetrahedra and this
essentially affects the structure of the host.
For the Ge–Se system:
Regardless of the initial composition of the hosting
glass, the photodiffused material shows Raman features
characteristic for Ge-rich material. The glassy component becomes Se-deficient due to consumption of Se in
the formation of the diffusion products.
The diffusion products are nanocrystalline regions
dispersed into the glassy matrix and their composition
is dependent upon the hosting glass composition and
develops from Ag2 Se to a combination of Ag2 Se and
Ag8 GeSe6 with enrichment of the host in Ge.
The cluster size of the crystalline products depends on
the molar volume of the host in close relation to its
rigidity.
Isothermal annealing at moderate temperatures results
in diffusion limited slow growth of the Ag2 Se clusters
and homogeneous growth of the Ag8 GeSe6 clusters.
871
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