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Phys. Status Solidi C 9, No. 12, 2415–2419 (2012) / DOI 10.1002/pssc.201200197
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current topics in solid state physics
Simulation and process flow of
radiation sensors based on
chalcogenide glasses for in situ
measurement capability
Mahesh Ailavajhala1, Maria Mitkova*,1, and Darryl P. Butt2
1
2
Department of Electrical and Computer Engineering, Boise State University, 1910 University Dr., Boise, ID 83725-2075, USA
Department of Material Science and Engineering, Boise State University, 1910 University Dr., Boise, ID 83725-2090, USA
Received 29 May 2012, accepted 20 August 2012
Published online 16 November 2012
Keywords chalcogenide glasses, gamma radiation, radiation sensing, silver diffusion
* Corresponding author: e-mail mariamitkova@boisestate.edu
In this work we present data about electronic devices
based on a planar structure; inert electrode/nanophase
chalcogenide glass/inert electrode in close proximity with
a source of silver (Ag) oriented laterally over the chalcogenide glass film. The conductivity of the devices changes with radiation and it can be measured by contacting
the two inert electrodes. Spacing of the electrodes was
chosen after an in-depth investigation into the electric
field (E-field) and E-field energy displacement simulations with the aid of COMSOL multi-physics software.
Simulations have been used to enlighten a specific device
structure to perform in situ measurements. Bias voltages,
inert electrode material and thickness of the films were
used as standards for all different types of simulations
while only varying the spacing and the geometries to affect the E-fields. It has been established from the experimental results that a 1 V bias is the most appropriate
for the device performance and this is the voltage used in
the simulations. The key motivation for this research is to
find appropriate dimensions and geometry, which do not
cause a change in conduction as a direct result of the applied E-field, but rather effectively contribute to the establishment of only the radiation induced change of the
device conductivity. The process flow for the device fabrication is described and data from the device performance are presented as well. The radiation induced Ag
doping is mapped in the device volume using electron
dispersion X-ray spectroscopy (EDS).
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Chalcogenide glasses continue to attract a widespread interest in condensed mater science,
largely because they have opened new physics of disordered state and since they have made new applications
possible. Specifically, these amorphous semiconductors
exhibit a great variety of radiation-induced phenomena,
because of the freedom and flexibility associated with their
atomic structure. One of the interesting aspects in the chalcogenide glass performance are the radiation induced effects caused by ionizing radiation such as γ-quanta. Electronically these glasses are a type of semiconductors with
an abundance of states within the band gap due to lone-pair
electrons localized at chalcogen atoms and for that reason
they can be excited by a wide range of wavelengths including γ-rays. In addition, the excited carriers are effectively
localized in disordered and defective glass structures, and
the carriers undergo strong electron-lattice coupling. As a
result, the electrical properties of the glasses undergo remarkable change which can be utilized for radiation sensing. These effects have not been explored in depth, because
of the expectation that due to the lack of order in the structure of the glasses and the presence of a large number of
defects, fast recombination of radiation induced generation
of electron-hole pairs will also occur. This is in fact true
but never the less, transient effects [1] and more long-term
effects [2] on the fundamental absorption, ageing and other
properties have been reported [3,4]. Similarly, these effects
have been used in the x-ray radiology [5] where the occurring electron-hole pairs are captured by a transistor array
contributing to the formation of an image [6].
Structurally the chalcogenide glasses have moderate
atomic connectivity. This brings about the formation of
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Ailavajhala et al.: Simulation and process flow of radiation sensors
structure with different densities allowing for the diffusion
of metals. Silver (Ag) is a metal of great interest since it
has very high mobility; its diffusion can be accelerated by
photons to give rise to the effect of photodiffusion [7]. As
a result, a tremendous change in the electrical properties of
the hosting glass can occur even after the introduction of
few ppm Ag+, as shown for Ge-Se glass [8].
We applied the combination of the two effects - radiation induced generation of electron-hole pairs and radiation
induced Ag diffusion in chalcogenide glasses to create elements for radiation sensing [9]. The proof of concept for
the sensing devices performance has been demonstrated by
the formation of a generic device shown in Fig. 1. It has a
lateral structure, consisting of a chalcogenide film deposited on an insulating substrate, two electrodes made from
electrochemically inert material, for example Al, W, Ni, or
ID
+
-
VD
Figure 1 General presentation of the radiation sensor.
or Pt and a source of Ag. The device has been characterized through measurement of its I-V characteristics at discrete radiation doses with the application of a DC voltage
sweep applied onto the inert electrodes and simultaneously
measuring the current. The initial pre irradiated state of the
devices can be recovered by the application of an E-field
between the Ag electrode being a cathode and the two inert
electrodes being anodes. Impedance measurements have
been performed to get exact data related to radiation induced Ag diffusion causing changes in the conductivity of
the material while excluding the influence of an applied
electrical field between the sensing electrodes [9]. One of
the most important characteristics of these devices is their
ability for real time radiation measurement. During the real
time measurements, the applied electrical field is constant
throughout the entire measurement; one needs such device
structure which can ignore the effects of the electrical field
induced Ag diffusion. To find out which device structure
will correspond to such requirement, we conducted device
simulations with the aid of COMSOL software and after illustrating the appropriate device structure, masks were
prepared and a specific process flow for the fabrication of
the device structure was developed. We tested the devices
and conducted materials characterization. These data are
presented below.
2 Experimental details COMSOL software was
used to model the devices for E-field and electric potential
that is generated due to the application of a constant DC
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
voltage biased. Two dimensional studies have been performed to study the dispersive effect of the electric field
and resultant E-field effects that occur when a 1 V bias is
applied between the two inert electrodes while leaving the
Ag pads at a floating voltage. Various geometries have
been simulated, to study which geometry will provide the
least disruptive E-field and therefore will result in measurements corresponding solely to radiation induced effects.
Each geometry was also simulated for a large device (1
mm spacing between the pad directly across from one another), small device (10 µm spacing between pad directly
across from one another) and most of the geometries were
also simulated with the case where the top and bottom pads
(simulating the Ag pads) were moved 5 µm away from the
previous location to study whether a specific ratio of Al
spacing to Ag spacing will result in reducing the E-field effects.
Materials data used for simulation were carefully selected to ensure the closest possible similarity to the thin
films used in the study. A large square was created to represent the large area for the thin chalcogenide film and the
material similar to it was the ternary Ge22As20Se58 since
this composition closely resembles the actual chalcogenide
glass composition and was preexisting within the materials
library. The contacts were simulated using material data
for aluminum and silver contacts.
Application of a potential bias causes an E-field which
has been simulated for each of the geometries. Same method has been applied for the amount of electrical energy that
is imparted into the film as a direct relation to the electric
potential. The electrical energy density provides evidence
of the amount of energy transferred to an ion in certain location, measured in Joules/m3.
The sizes of the shapes have been scaled to make them
proportional to the distance between the pads. The circle
pad sizes used in the simulation were 2 mm in diameter
with 1 mm spacing. Same was the length of the rectangles,
but the height of the triangles was 1 mm. The dimensions
of all shapes in certain simulations were the same to prevent any anomalies due to distorted shapes. The simulations also contained a parameter that accounts for the 100
nm film thicknesses, which is referred in COMSOL as
“Out of Plane Thickness” this corresponds to the film
thicknesses for the fabricated devices.
To verify the Ag ions distribution after the radiation,
mapping of the materials composition of the devices with
Hitachi S-3400N-II has been performed at the 20 keV
beam using energy dispersive X-ray spectroscopy (EDS)
operation mode.
3 Results Two representative results of COMSOL
simulation are shown in Fig. 1 whereas the results in the
Table 1 are the maximum values presented on the graphs.
The electrical potential (voltage) experienced by the top
and bottom Ag sources is approximately 0.5 V with slight
deviations in the areas around these electrodes due to the
geometries. From these graphs, the potential voltage exwww.pss-c.com
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Phys. Status Solidi C 9, No. 12 (2012)
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perienced by the Ag pads is going to be half of the applied
potential on one Al pad if the other Al pad is grounded.
The dark blue color indicates low values approximately 0
and the red – high values. Each topology has similar values
for the maximum electric field and energy density but the
regions that are affected vary depending on the geometry.
Visually, circle geometry provides a dispersive E-field as
well as large amounts of E-fields directly affecting the top
and bottom Ag sources. To improve this design, the top
and bottom pads were moved farther apart from each other
which substantially reduce the effective E-field. Further
reduction has been achieved by introducing the device design with antenna-type structure pointing to the center
point between the electrodes as shown in Fig. 2.
Based on the results from the COMSOL simulation, it
has been established that devices with the antenna type of
structure as shown in Fig. 2 confines the E-field and the
electric density effects to very small areas between the two
antennas. Another characteristic of these simulations is the
ability to create many devices of different dimensions
using a specific ratio of 1/1.5 which corresponds to Al
electrode spacing to Ag source spacing respectively. Using
the aforementioned ratio and with the aid of Visio software,
masks for these devices were created of which one such
device is shown in Fig. 3(a).
Geometry
Electrical
Energy
Density
Electric
Field
Potential
Figure 2 Geometry, electrical energy density, E-field and potential of circular and triangle shapes with antenna structure.
Al
Al
Ag
(a)
Ag
(b)
Figure 3 (a) Mask arrangement example for devices with antennas. (b) Devices post fabrication process.
The process flow for device fabrication starts with a
photolithography, on top of an insulator, that creates openings for Al deposition followed by a secondary photolithography step to allow for the deposition of the Ag pads.
This step is followed by a third lithography which covers
the large pads of the Al and Ag pads with photoresist while
allowing for the deposition of a 100 nm chalcogenide glass
film to deposit over the entire structure with the exception
of the areas covered by the photoresist. This process ensures that the chalcogenide glass film is not exposed to the
basic solutions that are customary in the contemporary
photolithography processes since these glasses dissolve in
a basic solution. Following the Al deposition process, a
thin layer of Ag was deposited capping the Al pad, without
breaking vacuum to prevent the oxidation of Al. Devices
prepared using this process are shown in Fig. 3b.
The devices were placed in a 60Co camera and radiated
with a 3 Mrad dose of γ-rays after which electrical testing
and material characterization were performed.
1 0 0 .0 p
3 5 0 μ m A l sp a cin g
3 0 0 μ m A g sp a cin g
Table 1 Maximum data for the devices and fields used in the
simulation.
Circle
Energy
(J/m^3)
E_field
(V/m)
Rectangle
and
Triangle
Energy
(J/m^3)
E_field
(V/m)
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1mm
5.21E06
10mico
0.095
0.0777
0.0761
1084.8
1.46E+05
1.32E+05
1.31E+05
1mm
8.23E06
1348.8
20micro
C urrent (A)
5 0 .0 p
0 .0
D e v ic e
D e v ic e
D e v ic e
D e v ic e
30micro
P re R a d s w e e p 1
P re R a d s w e e p 2
P o s tR a d (3 M ra d ) s w e e p 2
P o s tR a d (3 M ra d ) s w e e p 1
- 5 0 .0 p
10 micro
Adjusted
0.0638
0.0638
1.18E+05
1.19E+05
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
V o lt a g e ( V )
Figure 4 I-V characteristics obtained by two sweeps before and
following radiation.
The difference in the current flowing through the devices before and after radiation is relatively low compared
to the big dot devices tested in [9] which we relate to the
lower supply of Ag in the antenna type devices in which
the source in the active device area has very limited dimensions and big spacing (Al at 350 μm and Ag at 300
μm).
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Ailavajhala et al.: Simulation and process flow of radiation sensors
electric displacement field, and μ magnetic permeability.
Then the total energy could be calculated as follows:
U=
1
( E ◊ D + H ◊ B)
2
(2)
U=
1 Ê 2 B2 ˆ
εE + ˜
μ¯
2 ËÁ
(3)
The total energy has been contemplated in the simulations which guarantee that all external field factors that additionally could affect Ag diffusion are considered.
When discussing Ag doping in chalcogenide glasses
Figure 5 EDS data of Ag distribution after radiation .
due to radiation induced effects, it is important to consider
Figure 5 shows the Ag distribution (black dots) after radia- that an intrinsic E-field is also generated. It occurs because
tion using EDS mapping technique which illustrates a large the thermally generated electrons in the doped region move
concentration of black dots that signify Ag atoms, as towards the non-doped part generating a field at the intershown on the Ag pads. From the figure one can see that face, which is limited by the equilibrium conditions, charAg has diffused into the area between the two inert elec- acteristic for diffusion and drift of the charge carriers. It
trodes which substantially changes the electrical conduc- contributes to the changes of the electrical conductivity of
tivity of the devices. These devices can be reversed using the film as a sub set of the change in conductivity during
radiation and is accompanying the Ag diffusion. After cesthe aforementioned procedure.
sion of the radiation, the relaxation of the radiation induced
electron is associated with ion relaxation of the diffused
4 Discussion The effect of Ag migration in chalco- Ag. This intrinsic field dominates at low external voltage
genide glasses and specifically Ge containing glasses under that is used to measure the conductivity in the device. Simthe influence of electrical field is well known [10]. The ilarly, as stated in [12] and accordingly to our simulation
electric field also affects the Ag photodoping [11]. It is ob- results, there is an external threshold voltage of 10.8 V,
vious, that application of an E-field during the testing of above which the radiation induced field, that is present unthe radiation sensing devices can influence their diffusion der the radiation doping, is strongly affected, i.e. massive
parameters and introduce E-field related effects. Based on Ag diffusion is induced through this field. Our tests are
the modeling, we realized that for in situ measurements, limited at a lower voltage to avoid this effect. The EDS rethe best results can be achieved with the devices shown in sult showing homogeneous distribution of Ag between the
Figs. 2b and 3a, where the Ag diffusion is unaffected by device electrodes is a proof that at the conditions of the
the E-field since its value within the diffusion region is measurement, the external electrical field did not affect the
very limited or close to zero. It was proved to be the least Ag diffusion process.
At the applied voltage and fields in the modeling secinvasive method of applying a bias without ion displacement, while some of the other device models with robust tion (Table 1) it is obvious that in the case shown in Fig. 2a
contact areas in close proximity allow better Ag diffusion there would be very strong Ag migration due to the electrical field applied. We experimented with a large number of
when used towards measuring at discrete radiation doses.
Another important aspect for consideration, included shapes and electrodes orientation but because of space limin the software, is the amount of energy imparted into a itation, in this work, only two cases have been illustrated.
given space or medium as a direct result of the electric bias. The devices that have an antenna type of electrodes, disThis is also known as the energy density due to electric and tributed on a distance of 10 mm to 250 µm at applied bias
a magnetic field, of which the electric field is proportional of 1 V can be applied in real time tests.
to the permeability and the E-field as shown in the follow5 Conclusions Applying COMSOL software the oping equation:
erational conditions of radiation sensing devices were sim-
ηE =
energy 1 2
= εE
volume 2
(1)
Here ηE is the energy density due to electric field, ε the
dielectric constant of the material. It is critical to measure
this energy since electron–hole pair (EHP) could be generated as a direct result of the E-field rather than radiation.
Complete energy equation also considers the role the magnetic fields plays in the system, with U being the total energy, B the magnetic field, H the magnetizing field, D the
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ulated. It is shown that the structure of the devices strongly
affects the dependence of their performance upon the applied external field. Devices with antenna structure of their
electrodes with various spacing ranging from 10 mm to
250 µm and external field of 1 V bias are shown to ensure
stable performance of the sensors for real time measurement. At these conditions the internally forming field due
to radiation induced Ag diffusion dominates the devices
performance. The data about Ag distribution between the
source and inert electrodes support this conclusion.
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Acknowledgements The authors thank to P. Chen for the
high resolution post fabrication devices image and D. Oleski for
the electrical test measurement. This work is supported by a grant
from Battelle Energy Alliance under Blanket Master Contract No.
41394.
References
[1] T. Minami, A. Yoshida, and M. Tanaka, J. Non-Cryst. Solids 7, 328 (1972).
[2] V. Balitska, A. Kovalsky, O. Shpotyuk, and M. Vakiv, Radiat. Meas. 42, 941 (2007).
[3] A. Kovalskiy, Radiat. Eff. Defects Solids 158, 391 (2003).
[4] R. Golovchak, A. Kozdras, S. Kozyukhin, and O. Shpotyuk,
Nucl. Instrum. Methods Phys. Res. B 267, 2958 (2009).
[5] K. Shimakawa, K. Fukami, H. Kishi, G. Belev, and S. Kasap, Jpn. J. Appl. Phys. 46, L192 (2007).
www.pss-c.com
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[6] K.S. Karim, A. Nathan, J.A. Rowlands, and S.O. Kasap,
IEEE Proc. Circuits Devices Syst. 150, 267 (2003).
[7] M. Frumar and T. Wagner, Curr. Opin. Solid State Mater.
Sci. 7, 117 (2003).
[8] M. Ribes, E. Bychkov, and A. Pradel , J. Opt. Adv. Mater. 3, 665
(2001).
[9] P. Chen, M. Ailavajhala, M. Mitkova, D. Tenne, I. Esqueda,
and H. Barnaby, IEEE WMED, 22 April (2011), p. 1.
[10] V. Kh. Kudoyarova, J. Non-Cryst. Solids 90, 593 (1987).
[11] M. Mitkova, I. Iliev, V. Boev, and T. Petkova, J. Non-Cryst.
Solids 227-230, 748 (1998).
[12] T. W. Kang, C. Y. Hong, C. S. Chong, and T. W. Kim, J.
Mater. Sci. 27, 5620 (1992).
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