Journal of Non-Crystalline Solids 299–302 (2002) 1023–1027
www.elsevier.com/locate/jnoncrysol
Silver incorporation in Ge–Se glasses used in programmable
metallization cell devices
M. Mitkova *, M.N. Kozicki
Center for Solid State Electronics Research, Arizona State University, Tempe, AZ 85287-6206, USA
Abstract
We investigate the nature of thin films formed by the photodissolution of Ag into Se-rich Ge–Se glasses for use in
programmable metallization cell devices. These devices rely on ion transport in the film so produced to create electrically programmable resistance states. The way in which Ag incorporates into the chalcogenide film during photodiffusion is examined using Rutherford backscattering spectroscopy analysis and Raman spectroscopy. The results
suggest that an Ag-rich phase separates due to the reaction of Ag with free Se from the chalcogenide glass leaving a Gerich chalcogenide matrix. Ó 2002 Elsevier Science B.V. All rights reserved.
PACS: 73.61.J; 78.30.L; 85.25.H
1. Introduction
It has been known since the mid 1960s [1] that
silver can be photodissolved in chalcogenide glasses to form materials with interesting technological
properties. In the 35 years since, this effect has
been used in diverse applications such as the fabrication of relief images in optical elements [2],
micro photolithographic schemes [3], and for direct
imaging by photoinduced silver surface deposition
[4]. The introduction of silver into chalcogenide
glasses causes substantial changes in the electrical
properties of the material, decreasing the resistivity
by many orders of magnitude [5]. However, even
with metal concentrations as high as tens of atomic
percent, these ternaries have resistivities that are
*
Corresponding author.
E-mail address: mmitkova@asu.edu (M. Mitkova).
still many orders of magnitude higher than this of
thin films of the pure metal. Alloying Ag with the
Ge–Se glass leads to formation of a solid-state
layer in which a significant portion of the silver
ions are relatively mobile at room temperature. If
electrodes are formed in contact with this photodiffused layer, an anode which has oxidizable Ag
and an inert cathode, and a voltage is applied
between them, the positively charged metal ions
will migrate toward the cathode region. At small
applied bias (much less than 1 V) in structures
which are commensurate with state-of-the-art integrated device geometries, the ions will come out
of the photodiffused layer at the cathode to rapidly
form a stable metallic electrodeposit which may be
made to extend from the cathode to the anode.
The low resistance metal electrodeposit acts to
short-out the relatively high resistance glass and
hence the overall resistance of the structure can
be reduced by many orders of magnitude via this
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
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M. Mitkova, M.N. Kozicki / Journal of Non-Crystalline Solids 299–302 (2002) 1023–1027
non-volatile electrically stimulated deposition process. A reverse bias will cause electrodissolution
of the metal link, returning the device to a high
resistance state and this write–erase cycle may be
repeated many tens of millions of times. This reversible switching effect is the basis of the programmable metallization cell (PMC) memory
technology.
Fig. 1 depicts a schematic of a typical form of
PMC device. In this case, the Ag-photodiffused
Ge–Se glass layer is placed in a small geometry via
between two levels of metal, a configuration which
lends itself well to extreme device scaling. Typical
test devices are based on a layer of Si3 N4 deposited
by LPCVD which isolates the PMC devices (both
electrically and chemically) from the underlying
silicon substrate. The lower electrode (cathode) is
formed from an evaporated Ni film and the intermetal dielectric is also Si3 N4 . Following via formation in the nitride layer, the parent glass is
deposited in the via and then a layer of silver is
deposited on top of that. This bilayered structure
is then exposed to UV light to photodissolve the
silver and ‘saturate’ the glass but the relative layer
thicknesses are such that excess silver remains on
the surface to act as the anode. A connection is
made to this Ag layer by means of another layer of
Ni metallization. Representative dc characteristics
for a sub-micron device are shown in Fig. 2 and an
ac equivalent circuit of the electrical characteristics
is described in [6].
Fig. 2. Current–voltage characteristics of a PMC memory device. The off state is much greater than 1 MX but switches to
less than 20 kX at 0.18 V. The value of the on state depends on
writing conditions.
The key to the creation of a PMC device which
can be used in a high density memory lies with the
nature of the thin films of the Ag-photodiffused
Ge–Se glass. The aim of the work reported in this
paper is therefore to address the issues relating to
formation of such thin film ternaries by photodissolution of silver into the Ge–Se parent glass.
The effects of silver photodiffusion have been investigated by Raman spectroscopy, which yields
clues regarding the photoinduced changes in the
mesoscopic structure of the glass. The amount of
the diffused Ag in our samples has been verified
using Rutherford backscattering spectrometry
(RBS) methods. Interpretation of the experimental
results centers on the molecular structure of the
solid-state electrolytes and in particular the backbone structure of the ternary and how it relates to
the structure of the chalcogenide host.
2. Experimental
Fig. 1. Active-in-via (AIV) configuration of a PMC memory
device between two levels of metal.
In order to perform the analysis of thin films of
the ternary, a standard sample type was prepared
as follows. Amorphous layers of the parent glass
were prepared by thermal evaporation, using a
previously synthesized Ge30 Se70 glass source and
utilizing a crucible with a special membrane structure [7]. Use of the membrane crucible assured
formation of films with excellent control over ma-
M. Mitkova, M.N. Kozicki / Journal of Non-Crystalline Solids 299–302 (2002) 1023–1027
terial stoichiometry. The evaporation rate was 2
nm/s and the final deposited film thickness was in
the order of 40 nm. Following this, a silver film 25
nm thick was deposited without breaking vacuum.
The wafers were then exposed to UV light at a
power density of 115 mJ=cm2 s to form the ternary.
The amount of photodiffused silver in the glass
was determined by RBS with a 1.7 MeV Heþ ion
beam at normal incidence to the sample and with
a backscattering angle of 160°. The samples were
found to be sensitive to the measurement technique for doses between 0.5 and 1.0 lC=mm2 and
so a charge limit of around 0.25 lC=mm2 was
used.
The structure of the films was determined using
Raman spectroscopy. The Raman scattering measurements were made in a backscattering configuration with the 647 nm line (typical power of
0.5 mW) of a Kr ion laser. The excitation light
was focused on the sample in a region of 0:1 0:1 mm2 . The temperature increase caused by the
laser irradiation was less than 20 °C as calculated
from the ratio of anti-Stokes to Stokes intensities
[8].
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Fig. 3. RBS spectra of Ge30 Se70 film photodiffused with Ag
after 5 min of illumination and after 10 min of illumination (the
dashed line).
3. Results
The RBS analysis confirmed that the amount of
the photodiffused Ag depends upon the time of
illumination but saturation of the glass can be
achieved for the conditions of our experiment after
10 min of illumination. Beyond this time, no
change in the amount of Ag introduced into the
glassy film was found. This saturation occurs when
33 at.% silver has photodiffused into the Ge30 Se70
thin films, as shown in the RBS spectrum of Fig. 3.
Fig. 4 represents the Raman spectrum of the
initial Ge30 Se70 thin film as well as the spectra of
the Ge30 Se70 film saturated with 33 at.% Ag and
the Ag2 Se film. As shown, the Ge30 Se70 composition consists of Se–Sen chains whose vibrational
mode appears at 260 cm1 in the Raman spectrum,
edge-sharing Ge-containing structural units giving
rise to a breathing mode at 216 cm1 , and cornersharing Ge-containing units with a feature at 199
cm1 . The strength of the latter substantially pre-
Fig. 4. Raman spectra of: thin film with composition Ge30 Se70 ,
thin film – composition, resulting after saturation of the initial
Ge30 Se70 film with Ag and Ag2 Se film.
vails over the edge-shared units as the number of
Se atoms is relatively high and connection due to
sharing at the corners of the tetrahedra is preferred. The spectrum of the film resulting after Ag
diffusion shows significant reorganization of the
structure.
4. Discussion
In principle, the structure of thermally evaporated films from the Ge–Se binary does not differ
considerably from that of bulk material [9]. As
in the bulk material, they possess a fragmented
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molecular structure but the size of Ge-rich and Serich clusters can be expected to be smaller than in
the bulk due to surface effects. Even though these
glasses possess a number of low co-ordinated Se
atoms, they are still quite rigid. The results of
previous work regarding the compositional variation of the glass transition temperature in the Ge–
Se system imply that the Ge30 Se70 composition can
be characterized as having medium range order in
which particular rigid moieties form [10], leading
to formation of a network with lower configurational entropy due to the occurrence of specific
chemical configurations that are energetically
preferred.
The great change of the Raman spectrum of the
photodiffused material suggests total reorganization of the films in which face-sharing units containing Ge–Ge bonds occur. These features are
characteristic of glasses with Ge concentration
>0.33 [10]. One can discern the scattering of the
180 cm1 mode of ethane-like units consisting of
one Ge–Ge and six Ge–Se bonds. So far investigations of the photoinduced changes in Ge–Se
glasses have not revealed the formation of Ge–Ge
bonds but we assume that our observation of this
effect is due to the consumption of free Se by the
photodiffused Ag and the subsequent formation of
Ag2 Se. The act of illumination of the film results in
the creation of electron–hole pairs and is accompanied by the formation of metastable states at the
chalcogen [11] that can react spontaneously with
the surrounding Ag ions. This reaction will be
preferred as the energy that it requires is about
three times less the energy for the Ge–Se bonding
(48.4 vs. 113 kcal/mol). Indeed the two compositions GeSe2 and Ag2 Se have very close heats of
formation )10.8 and 9.0 kcal/mol, respectively,
and this assures their co-existence in the system
leading to some dynamics by which Ag will slowly
dissolve from Ag2 Se into GeSe2 because of the
entropy of mixing [12] but will then react again
with the charged chalcogens. The fact that the
Ag2 Se modes do not interfere with those of the
Ge–Se backbone after Ag diffusion is one more
piece of evidence that no mixed ternary phase
arises as a result of Ag diffusion [13]. We assume
that the process reaches saturation because of the
strict number of free and under-co-ordinated Se
atoms that are available for reaction with Ag. As
the saturation of the Ge–Se films occurs with the
same amount of Ag as in bulk glasses [14], we used
these data to calculate the ratio of the thickness of
Ag film for saturation in a Ge30 Se70 film, for which
we obtained 1:3.62.
At this point we should like to mention one
more detail – we expect that there will be a slight
difference in the Ag co-ordination when it is alloyed with Se to form a glass by fast quenching
compared to when it is introduced by photodiffusion at room temperature. In fact, this difference
in the Ag co-ordination has been enumerated by
Oldale et al. [15] in their extended X-ray absorption fine structure (EXAFS) analysis. We suggest
that this is due to the fact that at quenching of the
alloyed glass because of the high rate of solidification, the solid phase that forms has a short range
order very close to the structure of the high temperature form – aAg2 Se while in the diffusion
process the phase that forms has a structure
analogous to bAg2 Se which is stable at room
temperature. However the results of Chen and Tai
[16] and our XRD experiments confirm the presence of some bcc Ag2 Se as well and this could be
due to the fact that Ag diffuses in an organized
solid-state matrix in which the formation of the
Ag2 Se structures is restricted due to space limitations. This perhaps suppresses to a great extent the
crystallization process of the Ag2 Se formed so that
the size of the crystalline particles remains in the
. This stabilizes the doped film
vicinity of several A
against Ag crystallization.
5. Conclusions
Our investigation of Ag photodoping in Se-rich
Ge–Se glass has been driven by our desire to understand the properties of thin films of solid solutions for use in PMC devices. Photodiffusion of
Ag causes significant reorganization of the structure of the initial films as revealed by Raman
spectroscopy. This effect is due to the photoinduced reaction between the Ag ions and the ‘free’
Se which forms a phase close to Ag2 Se and also
causes the formation of a backbone with Ge–Ge
bonding features. The photodissolution process
M. Mitkova, M.N. Kozicki / Journal of Non-Crystalline Solids 299–302 (2002) 1023–1027
saturates when 33 at.% Ag is introduced into the
Ge–Se matrix of Ge30 Se70 glass. The saturation
effect means that the photodiffusion process is selfcontrolled and highly repeatable.
Acknowledgements
The authors would like to acknowledge Barry
Wilkens for performing the RBS investigations
and David Wright for his technical assistance.
Work supported by Axon Technologies Corp.
References
[1] M.T. Kostyshin, E.V. Mikhailovskaya, P.F. Romanenko,
Sov. Phys. (Solid State) 8 (1966) 451.
[2] J. Hajto, P.J.S. Ewen, R.E. Belford, A.E. Owen, Thin Solid
Films 200 (1991) 229.
1027
[3] M.N. Kozicki, S.W. Hsia, A.E. Owen, P.J.S. Ewen,
J. Non-Cryst. Solids 137&138 (1991) 1341.
[4] T. Kawaguchi, S. Maruno, S.R. Elliott, J. Non-Cryst.
Solids 212 (1997) 166.
[5] A. Pradel, G. Taillades, C. Cramer, M. Ribes, Solid State
Ion. 105 (1998) 139.
[6] W.C. West, K. Sieradzki, B. Kardynal, M.N. Kozicki,
J. Electrochem. Soc. 145 (1998) 2971.
[7] T. Petkova, M. Mitkova, Thin Solid Films 205 (1991) 205.
[8] K. Murase, in: P. Boolchand (Ed.), Insulating and Semiconducting Glasses, World Scientific, Singapore, 2000,
p. 415.
[9] H. Takeuchi, O. Matsuda, K. Murase, J. Non-Cryst. Solids
238 (1998) 91.
[10] P. Boolchand, W.J. Bresser, Philos. Mag. B 80 (2000) 1757.
[11] K. Shimakawa, A. Kolobov, S.R. Elliott, Adv. Phys. 44
(1995) 475.
[12] J.C. Phillips, J. Non-Cryst. Solids 64 (1984) 81.
[13] P. Boolchand, W. Bresser, Nature 410 (2001) 1070.
[14] M. Mitkova, Yu. Wang, P. Boolchand, Phys. Rev. Lett.
83 (1999) 3848.
[15] J.M. Oldale, J. Rennie, S.R. Elliott, Thin Solid Films 164
(1988) 467.
[16] C.H. Chen, K.L. Tai, Appl. Phys. Lett. 37 (1980) 605.