IEEE TRANSACTIONS ON NUCLEAR SCIENCE
1
Structural and Material Changes in Thin Film
Chalcogenide Glasses Under Ar-Ion Irradiation
Tyler Nichol, Student Member, IEEE, Muhammad Rizwan Latif, Student Member, IEEE,
Mahesh S. Ailavajhala, Student Member, IEEE, Dmitri A. Tenne, Yago Gonzalez-Velo, Member, IEEE,
Hugh Barnaby, Senior Member, IEEE, Michael N. Kozicki, Member, IEEE, and Maria Mitkova, Senior Member, IEEE
Index Terms—CBRAM, chalcogenide glasses, ion beam radiation, memristor array fabrication, memristors, PMC, radiation-induced effects, TRIM simulation.
I. INTRODUCTION
C
of the amorphous network of chalcogenide glasses [6]. Using
deuterium ions, as well as
or
ions, surface structures
can be created [7], which by the application of appropriate materials and masks can lead to device preparation. This illustrates
the multidimensional scenarios for which it is important to understand the nature of the effects that could occur. Additional
interest towards this study arises due to the unique nature of
chalcogenide glasses, which are a part of the disordered polymer
system and their resultant response to ion bombardment should
be significantly different when compared to the interaction of
ions with crystalline materials [8]. One could expect scission or
expansion of the polymeric structure due to the diffusion and
incorporation of bombarding ions into it, ascribed to the free
volume within this type of structure. Consequently, this could
cause effects similar to the interaction of the chalcogenide matrix with other ionizing radiation such as or electron beam
[9]. Most importantly, ion irradiation can prove useful in replacing sophisticated microelectronic processes for device formation as demonstrated in [7], holding a potential for preparation of memory devices based on chalcogenide glasses. However, for the production of devices with particular performance
specifications, an in depth understanding the nature of all ion-induced processes is necessary, since ion irradiation, especially
with heavy ions, can cause substantial changes in the electrical
and optical properties of the chalcogenide thin films [10].
The above-mentioned properties of chalcogenide glasses are
a function of their unique electronic structure. Chalcogen elements possess within their outer electronic shell, and elecelectrons are deeper into the electronic structure
trons. The
and they do not participate in chemical reactions with other elements. Two of the electrons form a lone electron pair, and
the other two electrons contribute to the formation of chemical bonds with the surrounding elements. This electronic structure and the availability of the lone pair electrons contribute to
the fact that all chalcogen elements have relatively similar electronegativity (
), which leads to the formation of well-defined directional covalent bonds and satisfy the 8-N octet rule
[11]. The low connectivity of the chalcogen elements which can
lead to the formation of a two dimensional structure. However,
the addition of other elements which tend to have a higher coordination, for example As or Ge, forms the well-established
three dimensional network of the glass [12]. Here, we have to
point out that this type of bonding between hetero-atoms is preferred in the chalcogenide glass structure because it reduces the
structure’s enthalpy. The energetic ions that are present in the
ambient space have very high energies. However, due to the
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Abstract—We present results on structural and compositional
changes in
chalcogenide glasses under
ion irradiation as a function of fluence and ion energies. Energy dispersive
X-Ray spectroscopy (EDS) data obtained in this paper shows that
the interaction with ions results in the loss of Ge atoms in Se-rich
films. The compositional changes affect the structure of the films,
which was manifested in differences observed in the Raman
spectra. Ion interaction with of the films at the studied energies
does affect the surface properties. Simulation of the penetration
depth of the ions using Transport of Ions in Matter (TRIM)
ions with the
software shows that the interaction of incident
chalcogenide glass occurs within the top 5-nm film thickness, with
an etch rate for 450-eV ion energy of approximately 5 nm/s. We
suggest the application of this effect for the formation of Redox
Conductive Bridge Memory (RCBM) device arrays for which
electrical characteristics are presented and discussed.
HALCOGENIDE glasses are a segment of the amorphous
glass family, which have unique optical properties in the
IR region of the electromagnetic spectrum [1], [2]. These materials are highly transparent in this region and useful for many
commercial, military and space applications. In some of these
environments, primarily in space, these glasses can be exposed
to energetic ions [3]. This exposure can change the structure of
the glasses and alter the unique properties of these materials.
One example for the application of these IR transparent materials is the James Webb Space Telescope [4]. Furthermore, there
are many other conditions where chalcogenide glasses can interact with ions, for example ion implantation [5]. Studies have
shown that deuterium irradiation causes relaxation and ordering
Manuscript received July 10, 2014; revised September 22, 2014; accepted
October 31, 2014. This work was supported in part by the Defense Threat Reduction Agency under Grant HDTRA1-11-1-0055.
T. Nichol, M. R. Latif, M. S. Ailavajhala and M. Mitkova are with the Department of Electrical and Computer Engineering, Boise State University, Boise, ID
83725-2075, USA.
D. A. Tenne is with the Department of Physics, Boise State University, Boise,
ID 83725-1570, USA.
Y. Gonzalez-Velo, H. Barnaby and M. N. Kozicki are with School of Electrical, Computer and Energy Engineering, Arizona State University Tempe, AZ
85287-5706. USA.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNS.2014.2367578
0018-9499 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
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Fig. 2. SEM image of the RCBM array.
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Fig. 1. RCBM device functionality.
shielding and coatings that are inherently available, these energies can become significantly attenuated to lower energies. This
paper presents the effects of low energy ions on thin film chalcogenide materials, which could result after high energy ions penetrate through shielding and other materials.
In this paper, we present data about the effects of ion bombardment on thin films from the Ge-Se chalcogenide glass
system. This system, depending on the glass composition,
offers a large variety of structural unit organization, including
Se-Se chains (Ch), Ge-Se tetrahedral in corner-sharing (CS),
edge-sharing (ES), and ethane-like (ETH) as well as layered
structure (LS). Films with different compositions were exposed
to three different ion energies with different exposure times to
achieve different irradiation fluences. We have observed that
there is a close compositional dependence of the effects. The
studied structural and compositional changes arising in the
chalcogenide matrix, as well as the simulated ion interaction,
were applied to understand the ion-induced changes
in the
material.
As a result of these studies we suggest a method for formation of Redox Conductive Bridge Memory (RCBM) device arrays [13] which can be integrated into neuromorphic or reconfigurable logic integrated circuits. The performance of these devices relies on the formation, low resistance state (LRS), and
dissolution, high resistance state (HRS), of a conductive bridge
between two electrodes, one of which is electrochemically inert
and the other is based on a metal with high mobility, usually Ag.
As a result of the diffusion of
between the two electrodes,
), Ag
following the electrochemical process (reduction of
atoms deposit on the inert electrode. These Ag atoms deposit on
this electrode until a Ag bridge between these two electrodes is
formed. Fig. 1. illustrates the different conditions of device operation, such as the LRS and HRS states, that remain after the
power to the RCBM device is removed, which classifies these
devices as nonvolatile memory.
II. EXPERIMENTAL
Bulk chalcogenide glasses
(
, 0.3, 0.4)
were prepared from high purity germanium and selenium using
a melt-quench method. Germanium and selenium were weighed
to the appropriate atomic weights for achieving the desired compositions and placed in a quartz ampoule. The quartz ampoule
was then placed under vacuum and sealed in preparation for the
melt. The elements were melted in a Lindberg/Blue M rocking
furnace reaching a temperature of 950 and quenched at 60
above the melting temperature of each composition. Thin film
stack systems of
(
, 0.3, 0.4) source
material, with the evaporated compositions fluctuating from this
composition, were prepared on a silicon substrate. First, 200 nm
of
was thermally grown on a Si
substrate,
followed by 100 nm of sputtered tungsten (W) using an AJA
ATC Orion 5 sputtering system, and lastly
m of
chalcogenide glass was thermally evaporated using a Cressmbar vacuum.
ington 308R coating system at
Stack samples from each composition were bombarded with
ions, having an initial ion energy of 150 eV, 300 eV,
particles cm s,
and 450 eV with a flux of
to achieve a fluence of
,
and
particles cm at each energy level. Ion bombardment was performed with a Veeco ME 1001 ion mill at an
angle normal to the surface of each sample using a 300-mA
beam current and chamber pressure of 0.2 mTorr.
A nickel mesh with holes of
nm
nm was placed on
a small area of the samples before ion bombardment, which
allowed ions to pass only through the opening in the mesh,
forming vias for RCBM devices. Using the same mesh, the
silver electrodes were selectively deposited at the base of the
vias by thermal evaporation using the Cressington 308R coating
system. The newly deposited silver functions as the active electrode and W layer as the inert electrode, with chalcogenide thin
film between them functioning as the active switching layer,
thus forming a
RCBM device array. Judging from the
values of the conductivity,
, depending
upon the materials’ composition [14], these glasses are insulators rather than semiconductors. In this manner, the vias become
naturally insulated by the chalcogenide glass between them,
which confines the devices in the array, Fig. 2.
Following ion bombardment, each sample was analyzed
to determine its composition, chemical bonding, and surface
roughness changes in the top chalcogenide layer and this data
was compared to the as-deposited films. Particularly, energy
dispersive X-Ray spectroscopy (EDS) was used to measure
compositional changes caused by the ion bombardment, and
also to determine the presence of Ar ions within the chalcogenide matrix, using a Hitachi S-3400N scanning electron
microscope (SEM) equipped with Oxford Instruments Energy+
X-ray detector. Sampling five different areas on the samples,
NICHOL et al.: STRUCTURAL AND MATERIAL CHANGES IN THIN FILM CHALCOGENIDE GLASSES UNDER AR-ION IRRADIATION
; (b)
; (c)
.
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Fig. 3. Film compositional analysis: (a)
3
Fig. 4. Raman analysis of the evolution of (a) Se-Se bonding in
radiation fluences with the respective energy.
. (b) Ethane-like bonding in
revealed that the average composition of the evaporated films
as follows:
,
and
.
Raman spectroscopy was performed on five different locations on each sample contributing to the average and standard
deviation values presented in Fig. 4(a)–(c). This spectroscopy
method is useful to study the changes in the structure of the
chalcogenide material. It was performed using a Horiba Jobin
Yvon T64000 Raman spectroscopic system in back scattering
mode. For excitation, a parallel-polarized 441.6-nm He-Cd laser
was focused onto a circular spot of
mm diameter at a
laser beam intensity of 60 mW at cryogenic temperature and
mTorr. Although the laser wavelength is within
vacuum of
the absorption spectrum of the films, no illumination-induced
effects were observed during and after several measurements,
which at the same time was enough for collecting a good Raman
signal. The absorption coefficient for
thin films is
cm for wavelength of 441.6 nm, and increases with
increasing germanium content [15]. The inverse of the absorption coefficient can be used to determine the penetration depth
of the excitation light, and subsequently the probing depth of
the Raman signal, which is approximately half of the penetration depth. For these measurements it is determined the measured Raman spectra originated from the top 28 nm of the irradiated samples. Previous investigations of one of us [16] show
that at the applied evaporation technique the composition and
the film’s structure in depth do not change, so that it is expected
.(c) Ethane-like bonding in
at various
that all differences found by the Raman and EDS studies will
originate from the interaction of the films with the ion beam.
The surface morphology of Ge-Se layer was studied using an
OTESPA probe with Veeco Dimensions 3100 Atom Force Microscopy (AFM) system equipped with a Nanoscope IV controller in tapping mode.
Since the interaction of the chalcogenide films with ions
results in energy loss, which limits the penetration depth of
the ions, ion bombardment simulations were performed using
Transport of Ions in Matter (TRIM) software, which provides
an insight into the depth of the ionization due to an incident
ion. The target material thickness was modeled using the same
thicknesses as the experimental films. Density of the film was
derived from the densities of chalcogenide glasses as measured
g/cm for
[17]. To achieve
by Ivanov et al. as
a higher statistical average, 10,000
atoms were used,
which gives a large certainty regarding the depth of penetration
and distance of maximum ionization.
III. RESULTS
The EDS results revealed that the Ge concentration in
the
films incurred a significant reduction with increasing ion beam exposure, while the film concentration in the
and
films remains fairly stable as shown
in Fig. 3(a)–(c).
The Raman spectrum affirms the disordered nature of the
studied films. The spectrum for Ge-Se system contains some
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Fig. 5. AFM 3D image of all films bombarded with
ion bombarded samples. (c)
(b)
particles cm Ar Ions at different ion energies. (a)
ion bombarded samples.
intrinsic structures that are composition dependent. In the
selenium-rich spectra, the structure is manifested through four
distinct peaks located at
cm ,
cm ,
cm
and
cm
corresponding to corner-shared structure
(Ge-Se-Ge), edge-shared structure (Ge-Se-Ge), selenium-selenium (Se-Se) vibrational bonds and the asymmetrically
stretching edge-shared structures, respectively [18], [19].
In the Ge-richer samples (
, and
),
ethane-like bonding (Se-Ge-Ge-Se) structure is exhibited in the
cm . The Raman spectra were fitted
Raman spectra at
with Gaussian peaks, examples of which are illustrated in the
insets of graphs in Fig. 4(a)–(c). Comparison of the areas of
each peak revealed a unique trend in the Se-Se bonding and the
ethane-like bonding, which are also presented in Fig. 4(a)–(c).
In the
films, there are two trends - first a decrease
in the Se-Se bonding followed by an increase with additional
ion fluence. In
and
films, the ETH-like
structures predominantly react and their reaction is very complex, as shown in Fig. 4(b)–(c), which will be discussed in the
following section.
After irradiation, the films were also characterized using
AFM to study their surface roughness as illustrated in
Fig. 5(a)–(c). The surface roughness after the lowest radiation fluence is similar to the unirradiated samples. The Ge-rich
samples that have been exposed to the largest ion energy for
the greatest fluence exhibit a large change in the surface roughness when compared to the as-deposited films. Selenium-rich
samples incurred a reduction in the surface roughness while the
surfaces for the other two samples were significantly rougher
after exposure as demonstrated in Fig. 6.
A Wyko NT1100 optical profiler was used to determine the
etch rate for ions with energy of 450 eV by measuring the step
profile after bombarding the material through a mask for 30 s.
The etch rate is approximately 5 nm/s which equates to etching
300 nm, 600 nm, and 900 nm at 60-, 120-, and 180-s exposures,
ion bombarded samples.
Fig. 6. AFM surface roughness for Ge-Se samples for different ion energies
particles cm Ar Ions.
bombarded with
respectively. The simulation result revealed that the maximum
ionization and interaction of the incident
ions with the
chalcogenide glass occurs within the top 5-nm film thickness.
This effect could be explained by the packing density within
the hosting chalcogenide glass, data for which is presented in
Fig. 7. [20]. Data regarding the molar volume was taken from
[21] and packing fraction data was calculated according to [22].
Fig. 2. shows an SEM image of the fabricated
-device
array with Ge-Se film isolating individual cells at the maximum
ion energy of 450 eV, which offers the highest throughput for
devices fabrication. Experimental measurements revealed that
the average thickness of remaining material at the base of the
via was around 100 nm, with a standard deviation of 20 nm.
A demonstration of the validity of this technology for fabricating arrays of devices has been achieved through their
current–voltage (IV) characteristics, presented in Fig. 8. The
Se-rich devices revealed a significant variability in the device
NICHOL et al.: STRUCTURAL AND MATERIAL CHANGES IN THIN FILM CHALCOGENIDE GLASSES UNDER AR-ION IRRADIATION
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which have the highest packing fraction when compared to the
films–Fig. 7. The introduction of
ions also affects the surface properties of the material related to the appearance of hillocks at the highest ion fluence. Hillocks are small
mounds or hills that are detected on the film surface. Greater
number and/or larger hillocks will cause different film thickness
in close proximity, which can alter the device performance from
one write cycle to the next. When we connect the introduction of
ions and appearance of hillocks with the results presented
in Fig. 6., the decrease in the hillock height for the
glasses at the highest ion energy confirms the role of interstitial
Ar contributing to the changes in surface morphology. This effect is not as expressed for the other compositions where there is
a limited amount of
incorporated within the chalcogenide
film matrix.
Comparing this data to the Raman structural information
in the
films, it reveals that at the lowest energy
(150 eV), structural reorganization predominantly occurs since
the increased material loss with ion flux is coupled with the
decrease of the Se-Se bonding. This means that the structure
is stabilizing after the interaction with ions through three-dimensional self-organization. Self-organization is an important
factor in disordered systems [23]. In the particular case, we suggest the decrease of Se chain vibrations coupled with decrease
of Ge in the film composition is related to the formation of 3D
tetrahedral structure (CS or ES) due to the breakage of some
Ge-Ge bonds, leaving free Ge valent states to react with Se.
Such structural variation has also been found with ion interaction in the Ge-Te system [24]. This tendency is sustained by the
irradiation with 300 eV ions up to
particles cm ,
after which an increase in the Se-Se bonding was manifested.
We relate the last fact to the substantial loss of Ge at the highest
ion fluence. In difference from [10], we obtained a red shift of
the Se-Se Raman modes–Fig. 9. due to the loss of Ge in the
films. These modes are very composition sensitive [25] and we
attribute this as the reason behind the development of the peak
shift at lower ion fluence. At higher ion fluences, due to increased accumulation of
into the chalcogen matrix, acting
as an external pressure over the chalcogenide network, the
observed shift in the Se-Se Raman mode at the lowest fluence
was sustained up to the highest fluence regardless of the ion
energy. Under these radiation conditions, this film behaves like
one with composition similar to Ge-rich compositions, even
though the film incurred a loss of Ge atoms due to sputtering.
In the films containing 33 at.% and 40 at.% Ge, the weakest
bond in this system is one connecting two neighboring Ge atoms
which are part of the ETH-like units. Ion irradiation produces
changes in these bonds as shown in Fig. 4(b). and (c). For the
composition, the saturation of all bonds and lack of
dangling bond defects, because of its stoichiometric composition (
), is an important factor for the reduced compositional variations demonstrated in Fig. 3(b). Similarly, the minimal changes to the areal intensity of the Ge-Ge bond are also
attributed to the near stoichiometric composition of these films.
With increasing amount of Ge, as in the
films, we suggest an effect similar to the swelling reported for amorphous Ge
films [26] which occurs in order to reduce the packing density
in the system, since it is very high for this composition [20].
This high compactness of the structure significantly increases
Fig. 7. Molar volume and packing fraction of Ge-Se glasses [20].
threshold voltage, while increasing the germanium content in
the active film compositions resulted in devices demonstrating
a consistent decrease in this variability. The devices with the
highest Ge-content resulted in the most uniform threshold
voltage and IV sweeps, presented in the inset of Fig. 8(c).
IV. DISCUSSION
The EDS data suggests that there is a loss of Ge-content from
the
films at 150 eV, which intensifies with the increase
of the ion energy. The incident ions cause recoils within the
system. The probability that a Se atom is recoiled is higher in
the
because of the abundance of Se-Se chains, when
compared to the other two compositions, as well as their lower
surface binding energy compared to Ge atoms, which are mostly
found in four-fold coordinated tetrahedral structures. The recoiled Ge atoms have less kinetic energy than Se atoms while
moving through the material, due to their higher lattice binding
energy, which allows the Se atoms to migrate deeper into the
material through collision cascades. Due to the low energy of
the incident ions, many recoiled atoms lose their energy before
sputtering an atom from the target. Therefore, the largest decrease in the Ge content is exhibited in these films since more
Se atoms are recoiled deeper into the films, resulting in more
Ge atoms being sputtered from the surface. The
films
a minimal change in composition, except when exposed to ions
with the highest energy. The change in composition is not as exaggerated as the
due to the reduction of Se-Se chains,
which is also evident in the lack of compositional changes in
the
films, as verified in the Raman spectra described
in detail later.
The EDS data also revealed a presence of interstitial Ar atoms
in the film. Statistically the number of absorbed
is higher
in the
films, diminishes in the
films and almost vanishes in the
composition. We suggest that
the accommodation of
into the chalcogenide matrix depends upon the network packing fraction, since we noticed a reand
films,
duced presence of
into the
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IEEE TRANSACTIONS ON NUCLEAR SCIENCE
device. (b)
device. (c)
Device.
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Fig. 8. IV curves in different cells of the 9th row for the fabricated RCBM array. (a)
Fig. 9. Peak locations of the
stretch of the Se-Se Raman active
(a) Fluence ( Particle cm ). (b) Fluence ( Particle cm ).
stretch combining the contributions of
the stress within the films. It is for this reason that the areal intensity of the Ge-Ge bond Raman vibrations increases, and also
the reduction of packing through the formation of CS structures.
The TRIM simulations suggest that the ion-film interaction
occurring on the surface, which should affect the surface roughness of the films since the highest total ionization of electrons
occurs in this region. The surface roughness of the chalcogenide
films is an important factor for the performance of the RCBM
devices since it corresponds to the effective distance for the
silver ions building the conductive bridge. The results of the
AFM studies show that the sputtering of individual atoms does
change the surface morphology. Additionally, the restructuring
due to relaxation of the network to reduce stress caused by interaction with the ions, also contributes to the observed surface
changes.
The material characterization data shows that the Ge richest
composition (
) presents the best stability, and the
electrical performance characteristics of these devices formed
by ion bombardment confirm this observation. Overall, as presented in Fig. 8(b).and (c), the devices based on composition
containing over 33 at.% germanium exhibit good stability, and
their performance is similar to those reported for devices obtained by conventional lithographic methods [27]. We attribute
the similarity to the fact that incorporated
are not electronically active dopants affecting the electrical performance of the
devices, and the surface damage caused by the ions does not
rings and Se chains in
Raman spectra.
critically affect the device performance. This data supports the
viability of this method for the fabrication of memory device
(memristors) arrays.
V. CONCLUSION
inIn this paper, we present a study of low energy
teraction with chalcogenide glass thin films. Our data of
irradiated chalcogenide glasses, ranging from chalcogen-rich to
chalcogen-poor, demonstrate that the chalcogen-richest glasses
are most sensitive to Ar ion irradiation, towards structural and
compositional changes. These effects can be related to the
extraction of Ge atoms out of the chalcogenide matrix during
irradiation, and structural reorganization to accommodate the
stress caused by the introduction of Ar. An increase of the Ge
content in the films leads to higher compactness and rigidity
of the structure. This data was confirmed with AFM imaging
and TRIM simulation. The energies applied are tailored for
the needs of device formation and performance due to their
relatively low effect over the chalcogenide matrix, and are representative of possible ambient space conditions that devices
could experience while under cover from shielding. At the
same time, these energies provide a well-established sputtering
velocity, yielding defined RCBM device arrays with excellent
and switching
device characteristics, such as uniform
behavior. Integrated circuits based solely on memristor arrays
NICHOL et al.: STRUCTURAL AND MATERIAL CHANGES IN THIN FILM CHALCOGENIDE GLASSES UNDER AR-ION IRRADIATION
show a great potential for new circuit solutions as demonstrated
by Cali et al. [28].
ACKNOWLEDGMENT
The authors would like to thank J. Reed of DTRA for his
support. They would also like to recognize the Surface Science
lab at Boise State University for AFM use and P. Davis for his
assistance in performing AFM studies.
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[3] Jursa, Handbook of geophysics and the space environment, A. S. Jursa,
Ed. Springfield, VA, USA: Air Force Geophysics Laboratory, 1985.
[4] J. P. Gardner, J. C. Mather, M. Clampin, R. Doyon, M. A. Greenhouse,
H. B. Hammel, J. B. Hutchings, P. Jakobsen, S. J. Lilly, and K. S.
Long, “The James Webb Space Telescope,” Space Sci. Rev., vol. 123,
pp. 485–606, 2006.
[5] T. Tsvetkova, S. Balabanov, B. Amov, A. Djakov, and I. Wilson,
“Surface morphology changes in ion implanted chalcogenide films
after annealing,” Nucl. Instrum. Meth. Phys. Res., Sect. B, vol. 80, pp.
1264–1267, 1993.
[6] I. Ivan, S. Szegedi, L. Daroczi, I. Szabo, and S. Kokenyesi, “Deuteron
irradiation induced changes in amorphous AsSe films,” Nucl. Instrum.
Meth. Phys. Res. Sect. B: Beam Interactions with Materials and Atoms,
vol. 229, pp. 240–245, 2005.
[7] S. Kokenyesi, I. Iván, V. Takáts, J. Pálinkás, S. Biri, and I. Szabo,
“Formation of surface structures on amorphous chalcogenide films,”
J. Non-Cryst. Sol., vol. 353, pp. 1470–1473, 2007.
[8] E. H. Lee, “Ion-beam modification of polymeric materials–fundamental principles and applications,” Nucl. Instrum. Meth. Phys. Res.,
Sect. B, vol. 151, pp. 29–41, 1999.
[9] M. S. Ailavajhala, Y. Gonzalez-Velo, C. Poweleit, H. Barnaby, M. N.
Kozicki, K. Holbert, D. P. Butt, and M. Mitkova, “Gamma radiation
induced effects in floppy and rigid Ge-containing chalcogenide thin
films,” J. Appl. Phys., vol. 115, pp. 043502-1–9, 2014.
[10] M. S. Kamboj, G. Kaur, R. Thangaraj, and D. Avasthi, “Effect of heavy
ion irradiation on the electrical and optical properties of amorphous
chalcogenide thin films,” J. Phys. D: Appl. Phys., vol. 35, p. 477, 2002.
[11] N. F. Mott and E. A. Davis, Electronic Processes in Non-Crystalline
Materials. London, U.K.: Oxford Univ. Press, 2012.
[12] M. F. Thorpe, “Networks, flexibility and mobility in,” in Encyclopedia
of Complexity and Systems Science. New York, NY, USA: Springer,
2009, pp. 6013–6024.
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1
Structural and Material Changes in Thin Film
Chalcogenide Glasses Under Ar-Ion Irradiation
Tyler Nichol, Student Member, IEEE, Muhammad Rizwan Latif, Student Member, IEEE,
Mahesh S. Ailavajhala, Student Member, IEEE, Dmitri A. Tenne, Yago Gonzalez-Velo, Member, IEEE,
Hugh Barnaby, Senior Member, IEEE, Michael N. Kozicki, Member, IEEE, and Maria Mitkova, Senior Member, IEEE
Index Terms—CBRAM, chalcogenide glasses, ion beam radiation, memristor array fabrication, memristors, PMC, radiation-induced effects, TRIM simulation.
I. INTRODUCTION
C
of the amorphous network of chalcogenide glasses [6]. Using
deuterium ions, as well as
or
ions, surface structures
can be created [7], which by the application of appropriate materials and masks can lead to device preparation. This illustrates
the multidimensional scenarios for which it is important to understand the nature of the effects that could occur. Additional
interest towards this study arises due to the unique nature of
chalcogenide glasses, which are a part of the disordered polymer
system and their resultant response to ion bombardment should
be significantly different when compared to the interaction of
ions with crystalline materials [8]. One could expect scission or
expansion of the polymeric structure due to the diffusion and
incorporation of bombarding ions into it, ascribed to the free
volume within this type of structure. Consequently, this could
cause effects similar to the interaction of the chalcogenide matrix with other ionizing radiation such as or electron beam
[9]. Most importantly, ion irradiation can prove useful in replacing sophisticated microelectronic processes for device formation as demonstrated in [7], holding a potential for preparation of memory devices based on chalcogenide glasses. However, for the production of devices with particular performance
specifications, an in depth understanding the nature of all ion-induced processes is necessary, since ion irradiation, especially
with heavy ions, can cause substantial changes in the electrical
and optical properties of the chalcogenide thin films [10].
The above-mentioned properties of chalcogenide glasses are
a function of their unique electronic structure. Chalcogen elements possess within their outer electronic shell, and electrons. The
electrons are deeper into the electronic structure
and they do not participate in chemical reactions with other elements. Two of the electrons form a lone electron pair, and
the other two electrons contribute to the formation of chemical bonds with the surrounding elements. This electronic structure and the availability of the lone pair electrons contribute to
the fact that all chalcogen elements have relatively similar electronegativity (
), which leads to the formation of well-defined directional covalent bonds and satisfy the 8-N octet rule
[11]. The low connectivity of the chalcogen elements which can
lead to the formation of a two dimensional structure. However,
the addition of other elements which tend to have a higher coordination, for example As or Ge, forms the well-established
three dimensional network of the glass [12]. Here, we have to
point out that this type of bonding between hetero-atoms is preferred in the chalcogenide glass structure because it reduces the
structure’s enthalpy. The energetic ions that are present in the
ambient space have very high energies. However, due to the
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Abstract—We present results on structural and compositional
changes in
chalcogenide glasses under
ion irradiation as a function of fluence and ion energies. Energy dispersive
X-Ray spectroscopy (EDS) data obtained in this paper shows that
the interaction with ions results in the loss of Ge atoms in Se-rich
films. The compositional changes affect the structure of the films,
which was manifested in differences observed in the Raman
spectra. Ion interaction with of the films at the studied energies
does affect the surface properties. Simulation of the penetration
depth of the ions using Transport of Ions in Matter (TRIM)
ions with the
software shows that the interaction of incident
chalcogenide glass occurs within the top 5-nm film thickness, with
an etch rate for 450-eV ion energy of approximately 5 nm/s. We
suggest the application of this effect for the formation of Redox
Conductive Bridge Memory (RCBM) device arrays for which
electrical characteristics are presented and discussed.
HALCOGENIDE glasses are a segment of the amorphous
glass family, which have unique optical properties in the
IR region of the electromagnetic spectrum [1], [2]. These materials are highly transparent in this region and useful for many
commercial, military and space applications. In some of these
environments, primarily in space, these glasses can be exposed
to energetic ions [3]. This exposure can change the structure of
the glasses and alter the unique properties of these materials.
One example for the application of these IR transparent materials is the James Webb Space Telescope [4]. Furthermore, there
are many other conditions where chalcogenide glasses can interact with ions, for example ion implantation [5]. Studies have
shown that deuterium irradiation causes relaxation and ordering
Manuscript received July 10, 2014; revised September 22, 2014; accepted
October 31, 2014. This work was supported in part by the Defense Threat Reduction Agency under Grant HDTRA1-11-1-0055.
T. Nichol, M. R. Latif, M. S. Ailavajhala and M. Mitkova are with the Department of Electrical and Computer Engineering, Boise State University, Boise, ID
83725-2075, USA.
D. A. Tenne is with the Department of Physics, Boise State University, Boise,
ID 83725-1570, USA.
Y. Gonzalez-Velo, H. Barnaby and M. N. Kozicki are with School of Electrical, Computer and Energy Engineering, Arizona State University Tempe, AZ
85287-5706. USA.
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNS.2014.2367578
0018-9499 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
2
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
Fig. 2. SEM image of the RCBM array.
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Fig. 1. RCBM device functionality.
shielding and coatings that are inherently available, these energies can become significantly attenuated to lower energies. This
paper presents the effects of low energy ions on thin film chalcogenide materials, which could result after high energy ions penetrate through shielding and other materials.
In this paper, we present data about the effects of ion bombardment on thin films from the Ge-Se chalcogenide glass
system. This system, depending on the glass composition,
offers a large variety of structural unit organization, including
Se-Se chains (Ch), Ge-Se tetrahedral in corner-sharing (CS),
edge-sharing (ES), and ethane-like (ETH) as well as layered
structure (LS). Films with different compositions were exposed
to three different ion energies with different exposure times to
achieve different irradiation fluences. We have observed that
there is a close compositional dependence of the effects. The
studied structural and compositional changes arising in the
chalcogenide matrix, as well as the simulated ion interaction,
were applied to understand the ion-induced changes
in the
material.
As a result of these studies we suggest a method for formation of Redox Conductive Bridge Memory (RCBM) device arrays [13] which can be integrated into neuromorphic or reconfigurable logic integrated circuits. The performance of these devices relies on the formation, low resistance state (LRS), and
dissolution, high resistance state (HRS), of a conductive bridge
between two electrodes, one of which is electrochemically inert
and the other is based on a metal with high mobility, usually Ag.
As a result of the diffusion of
between the two electrodes,
), Ag
following the electrochemical process (reduction of
atoms deposit on the inert electrode. These Ag atoms deposit on
this electrode until a Ag bridge between these two electrodes is
formed. Fig. 1. illustrates the different conditions of device operation, such as the LRS and HRS states, that remain after the
power to the RCBM device is removed, which classifies these
devices as nonvolatile memory.
II. EXPERIMENTAL
Bulk chalcogenide glasses
(
, 0.3, 0.4)
were prepared from high purity germanium and selenium using
a melt-quench method. Germanium and selenium were weighed
to the appropriate atomic weights for achieving the desired compositions and placed in a quartz ampoule. The quartz ampoule
was then placed under vacuum and sealed in preparation for the
melt. The elements were melted in a Lindberg/Blue M rocking
furnace reaching a temperature of 950 and quenched at 60
above the melting temperature of each composition. Thin film
stack systems of
(
, 0.3, 0.4) source
material, with the evaporated compositions fluctuating from this
composition, were prepared on a silicon substrate. First, 200 nm
of
was thermally grown on a Si
substrate,
followed by 100 nm of sputtered tungsten (W) using an AJA
m of
ATC Orion 5 sputtering system, and lastly
chalcogenide glass was thermally evaporated using a Cressmbar vacuum.
ington 308R coating system at
Stack samples from each composition were bombarded with
ions, having an initial ion energy of 150 eV, 300 eV,
particles cm s,
and 450 eV with a flux of
to achieve a fluence of
,
and
particles cm at each energy level. Ion bombardment was performed with a Veeco ME 1001 ion mill at an
angle normal to the surface of each sample using a 300-mA
beam current and chamber pressure of 0.2 mTorr.
nm
nm was placed on
A nickel mesh with holes of
a small area of the samples before ion bombardment, which
allowed ions to pass only through the opening in the mesh,
forming vias for RCBM devices. Using the same mesh, the
silver electrodes were selectively deposited at the base of the
vias by thermal evaporation using the Cressington 308R coating
system. The newly deposited silver functions as the active electrode and W layer as the inert electrode, with chalcogenide thin
film between them functioning as the active switching layer,
thus forming a
RCBM device array. Judging from the
values of the conductivity,
, depending
upon the materials’ composition [14], these glasses are insulators rather than semiconductors. In this manner, the vias become
naturally insulated by the chalcogenide glass between them,
which confines the devices in the array, Fig. 2.
Following ion bombardment, each sample was analyzed
to determine its composition, chemical bonding, and surface
roughness changes in the top chalcogenide layer and this data
was compared to the as-deposited films. Particularly, energy
dispersive X-Ray spectroscopy (EDS) was used to measure
compositional changes caused by the ion bombardment, and
also to determine the presence of Ar ions within the chalcogenide matrix, using a Hitachi S-3400N scanning electron
microscope (SEM) equipped with Oxford Instruments Energy+
X-ray detector. Sampling five different areas on the samples,
NICHOL et al.: STRUCTURAL AND MATERIAL CHANGES IN THIN FILM CHALCOGENIDE GLASSES UNDER AR-ION IRRADIATION
; (b)
; (c)
.
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Fig. 3. Film compositional analysis: (a)
3
Fig. 4. Raman analysis of the evolution of (a) Se-Se bonding in
radiation fluences with the respective energy.
. (b) Ethane-like bonding in
revealed that the average composition of the evaporated films
as follows:
,
and
.
Raman spectroscopy was performed on five different locations on each sample contributing to the average and standard
deviation values presented in Fig. 4(a)–(c). This spectroscopy
method is useful to study the changes in the structure of the
chalcogenide material. It was performed using a Horiba Jobin
Yvon T64000 Raman spectroscopic system in back scattering
mode. For excitation, a parallel-polarized 441.6-nm He-Cd laser
was focused onto a circular spot of
mm diameter at a
laser beam intensity of 60 mW at cryogenic temperature and
vacuum of
mTorr. Although the laser wavelength is within
the absorption spectrum of the films, no illumination-induced
effects were observed during and after several measurements,
which at the same time was enough for collecting a good Raman
thin films is
signal. The absorption coefficient for
cm for wavelength of 441.6 nm, and increases with
increasing germanium content [15]. The inverse of the absorption coefficient can be used to determine the penetration depth
of the excitation light, and subsequently the probing depth of
the Raman signal, which is approximately half of the penetration depth. For these measurements it is determined the measured Raman spectra originated from the top 28 nm of the irradiated samples. Previous investigations of one of us [16] show
that at the applied evaporation technique the composition and
the film’s structure in depth do not change, so that it is expected
.(c) Ethane-like bonding in
at various
that all differences found by the Raman and EDS studies will
originate from the interaction of the films with the ion beam.
The surface morphology of Ge-Se layer was studied using an
OTESPA probe with Veeco Dimensions 3100 Atom Force Microscopy (AFM) system equipped with a Nanoscope IV controller in tapping mode.
Since the interaction of the chalcogenide films with ions
results in energy loss, which limits the penetration depth of
the ions, ion bombardment simulations were performed using
Transport of Ions in Matter (TRIM) software, which provides
an insight into the depth of the ionization due to an incident
ion. The target material thickness was modeled using the same
thicknesses as the experimental films. Density of the film was
derived from the densities of chalcogenide glasses as measured
by Ivanov et al. as
g/cm for
[17]. To achieve
atoms were used,
a higher statistical average, 10,000
which gives a large certainty regarding the depth of penetration
and distance of maximum ionization.
III. RESULTS
The EDS results revealed that the Ge concentration in
the
films incurred a significant reduction with increasing ion beam exposure, while the film concentration in the
and
films remains fairly stable as shown
in Fig. 3(a)–(c).
The Raman spectrum affirms the disordered nature of the
studied films. The spectrum for Ge-Se system contains some
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
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4
Fig. 5. AFM 3D image of all films bombarded with
(b)
ion bombarded samples. (c)
particles cm Ar Ions at different ion energies. (a)
ion bombarded samples.
intrinsic structures that are composition dependent. In the
selenium-rich spectra, the structure is manifested through four
distinct peaks located at
cm ,
cm ,
cm
and
cm
corresponding to corner-shared structure
(Ge-Se-Ge), edge-shared structure (Ge-Se-Ge), selenium-selenium (Se-Se) vibrational bonds and the asymmetrically
stretching edge-shared structures, respectively [18], [19].
, and
),
In the Ge-richer samples (
ethane-like bonding (Se-Ge-Ge-Se) structure is exhibited in the
Raman spectra at
cm . The Raman spectra were fitted
with Gaussian peaks, examples of which are illustrated in the
insets of graphs in Fig. 4(a)–(c). Comparison of the areas of
each peak revealed a unique trend in the Se-Se bonding and the
ethane-like bonding, which are also presented in Fig. 4(a)–(c).
In the
films, there are two trends - first a decrease
in the Se-Se bonding followed by an increase with additional
and
films, the ETH-like
ion fluence. In
structures predominantly react and their reaction is very complex, as shown in Fig. 4(b)–(c), which will be discussed in the
following section.
After irradiation, the films were also characterized using
AFM to study their surface roughness as illustrated in
Fig. 5(a)–(c). The surface roughness after the lowest radiation fluence is similar to the unirradiated samples. The Ge-rich
samples that have been exposed to the largest ion energy for
the greatest fluence exhibit a large change in the surface roughness when compared to the as-deposited films. Selenium-rich
samples incurred a reduction in the surface roughness while the
surfaces for the other two samples were significantly rougher
after exposure as demonstrated in Fig. 6.
A Wyko NT1100 optical profiler was used to determine the
etch rate for ions with energy of 450 eV by measuring the step
profile after bombarding the material through a mask for 30 s.
The etch rate is approximately 5 nm/s which equates to etching
300 nm, 600 nm, and 900 nm at 60-, 120-, and 180-s exposures,
ion bombarded samples.
Fig. 6. AFM surface roughness for Ge-Se samples for different ion energies
bombarded with
particles cm Ar Ions.
respectively. The simulation result revealed that the maximum
ions with the
ionization and interaction of the incident
chalcogenide glass occurs within the top 5-nm film thickness.
This effect could be explained by the packing density within
the hosting chalcogenide glass, data for which is presented in
Fig. 7. [20]. Data regarding the molar volume was taken from
[21] and packing fraction data was calculated according to [22].
Fig. 2. shows an SEM image of the fabricated
-device
array with Ge-Se film isolating individual cells at the maximum
ion energy of 450 eV, which offers the highest throughput for
devices fabrication. Experimental measurements revealed that
the average thickness of remaining material at the base of the
via was around 100 nm, with a standard deviation of 20 nm.
A demonstration of the validity of this technology for fabricating arrays of devices has been achieved through their
current–voltage (IV) characteristics, presented in Fig. 8. The
Se-rich devices revealed a significant variability in the device
NICHOL et al.: STRUCTURAL AND MATERIAL CHANGES IN THIN FILM CHALCOGENIDE GLASSES UNDER AR-ION IRRADIATION
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which have the highest packing fraction when compared to the
films–Fig. 7. The introduction of
ions also affects the surface properties of the material related to the appearance of hillocks at the highest ion fluence. Hillocks are small
mounds or hills that are detected on the film surface. Greater
number and/or larger hillocks will cause different film thickness
in close proximity, which can alter the device performance from
one write cycle to the next. When we connect the introduction of
ions and appearance of hillocks with the results presented
in Fig. 6., the decrease in the hillock height for the
glasses at the highest ion energy confirms the role of interstitial
Ar contributing to the changes in surface morphology. This effect is not as expressed for the other compositions where there is
a limited amount of
incorporated within the chalcogenide
film matrix.
Comparing this data to the Raman structural information
in the
films, it reveals that at the lowest energy
(150 eV), structural reorganization predominantly occurs since
the increased material loss with ion flux is coupled with the
decrease of the Se-Se bonding. This means that the structure
is stabilizing after the interaction with ions through three-dimensional self-organization. Self-organization is an important
factor in disordered systems [23]. In the particular case, we suggest the decrease of Se chain vibrations coupled with decrease
of Ge in the film composition is related to the formation of 3D
tetrahedral structure (CS or ES) due to the breakage of some
Ge-Ge bonds, leaving free Ge valent states to react with Se.
Such structural variation has also been found with ion interaction in the Ge-Te system [24]. This tendency is sustained by the
irradiation with 300 eV ions up to
particles cm ,
after which an increase in the Se-Se bonding was manifested.
We relate the last fact to the substantial loss of Ge at the highest
ion fluence. In difference from [10], we obtained a red shift of
the Se-Se Raman modes–Fig. 9. due to the loss of Ge in the
films. These modes are very composition sensitive [25] and we
attribute this as the reason behind the development of the peak
shift at lower ion fluence. At higher ion fluences, due to increased accumulation of
into the chalcogen matrix, acting
as an external pressure over the chalcogenide network, the
observed shift in the Se-Se Raman mode at the lowest fluence
was sustained up to the highest fluence regardless of the ion
energy. Under these radiation conditions, this film behaves like
one with composition similar to Ge-rich compositions, even
though the film incurred a loss of Ge atoms due to sputtering.
In the films containing 33 at.% and 40 at.% Ge, the weakest
bond in this system is one connecting two neighboring Ge atoms
which are part of the ETH-like units. Ion irradiation produces
changes in these bonds as shown in Fig. 4(b). and (c). For the
composition, the saturation of all bonds and lack of
dangling bond defects, because of its stoichiometric composi), is an important factor for the reduced composition (
tional variations demonstrated in Fig. 3(b). Similarly, the minimal changes to the areal intensity of the Ge-Ge bond are also
attributed to the near stoichiometric composition of these films.
With increasing amount of Ge, as in the
films, we suggest an effect similar to the swelling reported for amorphous Ge
films [26] which occurs in order to reduce the packing density
in the system, since it is very high for this composition [20].
This high compactness of the structure significantly increases
Fig. 7. Molar volume and packing fraction of Ge-Se glasses [20].
threshold voltage, while increasing the germanium content in
the active film compositions resulted in devices demonstrating
a consistent decrease in this variability. The devices with the
highest Ge-content resulted in the most uniform threshold
voltage and IV sweeps, presented in the inset of Fig. 8(c).
IV. DISCUSSION
The EDS data suggests that there is a loss of Ge-content from
the
films at 150 eV, which intensifies with the increase
of the ion energy. The incident ions cause recoils within the
system. The probability that a Se atom is recoiled is higher in
the
because of the abundance of Se-Se chains, when
compared to the other two compositions, as well as their lower
surface binding energy compared to Ge atoms, which are mostly
found in four-fold coordinated tetrahedral structures. The recoiled Ge atoms have less kinetic energy than Se atoms while
moving through the material, due to their higher lattice binding
energy, which allows the Se atoms to migrate deeper into the
material through collision cascades. Due to the low energy of
the incident ions, many recoiled atoms lose their energy before
sputtering an atom from the target. Therefore, the largest decrease in the Ge content is exhibited in these films since more
Se atoms are recoiled deeper into the films, resulting in more
Ge atoms being sputtered from the surface. The
films
a minimal change in composition, except when exposed to ions
with the highest energy. The change in composition is not as exaggerated as the
due to the reduction of Se-Se chains,
which is also evident in the lack of compositional changes in
films, as verified in the Raman spectra described
the
in detail later.
The EDS data also revealed a presence of interstitial Ar atoms
in the film. Statistically the number of absorbed
is higher
in the
films, diminishes in the
films and almost vanishes in the
composition. We suggest that
into the chalcogenide matrix dethe accommodation of
pends upon the network packing fraction, since we noticed a reduced presence of
into the
and
films,
6
IEEE TRANSACTIONS ON NUCLEAR SCIENCE
device. (b)
device. (c)
Device.
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Fig. 8. IV curves in different cells of the 9th row for the fabricated RCBM array. (a)
Fig. 9. Peak locations of the
stretch of the Se-Se Raman active
(a) Fluence ( Particle cm ). (b) Fluence ( Particle cm ).
stretch combining the contributions of
the stress within the films. It is for this reason that the areal intensity of the Ge-Ge bond Raman vibrations increases, and also
the reduction of packing through the formation of CS structures.
The TRIM simulations suggest that the ion-film interaction
occurring on the surface, which should affect the surface roughness of the films since the highest total ionization of electrons
occurs in this region. The surface roughness of the chalcogenide
films is an important factor for the performance of the RCBM
devices since it corresponds to the effective distance for the
silver ions building the conductive bridge. The results of the
AFM studies show that the sputtering of individual atoms does
change the surface morphology. Additionally, the restructuring
due to relaxation of the network to reduce stress caused by interaction with the ions, also contributes to the observed surface
changes.
The material characterization data shows that the Ge richest
composition (
) presents the best stability, and the
electrical performance characteristics of these devices formed
by ion bombardment confirm this observation. Overall, as presented in Fig. 8(b).and (c), the devices based on composition
containing over 33 at.% germanium exhibit good stability, and
their performance is similar to those reported for devices obtained by conventional lithographic methods [27]. We attribute
the similarity to the fact that incorporated
are not electronically active dopants affecting the electrical performance of the
devices, and the surface damage caused by the ions does not
rings and Se chains in
Raman spectra.
critically affect the device performance. This data supports the
viability of this method for the fabrication of memory device
(memristors) arrays.
V. CONCLUSION
In this paper, we present a study of low energy
interaction with chalcogenide glass thin films. Our data of
irradiated chalcogenide glasses, ranging from chalcogen-rich to
chalcogen-poor, demonstrate that the chalcogen-richest glasses
are most sensitive to Ar ion irradiation, towards structural and
compositional changes. These effects can be related to the
extraction of Ge atoms out of the chalcogenide matrix during
irradiation, and structural reorganization to accommodate the
stress caused by the introduction of Ar. An increase of the Ge
content in the films leads to higher compactness and rigidity
of the structure. This data was confirmed with AFM imaging
and TRIM simulation. The energies applied are tailored for
the needs of device formation and performance due to their
relatively low effect over the chalcogenide matrix, and are representative of possible ambient space conditions that devices
could experience while under cover from shielding. At the
same time, these energies provide a well-established sputtering
velocity, yielding defined RCBM device arrays with excellent
device characteristics, such as uniform
and switching
behavior. Integrated circuits based solely on memristor arrays
NICHOL et al.: STRUCTURAL AND MATERIAL CHANGES IN THIN FILM CHALCOGENIDE GLASSES UNDER AR-ION IRRADIATION
show a great potential for new circuit solutions as demonstrated
by Cali et al. [28].
ACKNOWLEDGMENT
The authors would like to thank J. Reed of DTRA for his
support. They would also like to recognize the Surface Science
lab at Boise State University for AFM use and P. Davis for his
assistance in performing AFM studies.
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