Transcript Slide 1

Studies of Gamma Radiation Induced
Effects in Ge-rich Chalcogenide Thin
Films
D. Nesheva1, M. Ailavajhala2, P. Chen2, D. A. Tenne3, H.
Barnaby4, M. Mitkova2*
1Institute
of Solid State Physics, Bulgarian Academy of Science, 1784 Sofia, Bulgaria
of Electrical and Computer Engineering, Boise State University, Boise, ID
83725, USA
3 Dept. of Physics, Boise State University, Boise, ID 83725-1570, USA
4 School of Electrical, Computer, and Energy Engineering, Arizona State University,
Tempe, AZ 85287-5706, USA
1Dept.
Outlines
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


Motivation
Experimental details
Raman scattering data
Dark and photoconductivity data
Conclusions
Chalcogenide glasses
These glasses are based on the chalcogen elements S, Se, and Te and are
formed by the addition of other elements such as Ge, As, Sb, Ga, etc.
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•
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•
They are:
generally transparent from the visible up to infrared;
show phase change memory effects;
can be doped by rare-earth elements and numerous applications
of active optical devices have been proposed;
are optically highly non-linear and could therefore be useful for
all-optical switching;
are sensitive to the absorption of electromagnetic radiation and
particle irradiation and show a variety of photoinduced effects as
a result of illumination: photodarkening, photobleaching,
photocrystallization, photoplastic effects, etc.
Photodiffusion in Chalcogenide Glasses
EC
EF
Ag film
Ag+
Ag film
EV
Glass
film
Radiation sensor, BSU February 24, 2011
Scheme of
electron/ion
generation at the
metal/metal-doped
chalcogenide
interface
Experimental details
• Synthesis of bulk Ge40Se60 glasses and thermal evaporation in vacuum,
film thickness 100 nm.
• EDS composition check by a LEO 1430VP Scanning Electron Microscope
with EDS accessory.
• Gamma irradiation in a closed cylindrical cavity by concentrically
established 60Co (average energy E = 1.25 MeV) radioisotope capsule at a
dose rate of 12 rad/sec. The applied γ-ray doses were ranging from 20
krad(Si) to 3 Mrad(Si).
• Raman scattering measured by a Horiba Jobin Yvon T64000 triple
monochromator, = 441.6 nm blue line of a He-Cd lase, power 60 mW on
~ 0.2 mm diameter circle area, Tm=100K.
• Electrical conductivity measured in the 293-423 K range; co-planar
aquadag (carbon) contacts on the top surface of the layers; the contacts
produce a gap cell with an active area of ~ (110) mm2.
There are two types of effects occurring in chalcogenide
glasses upon –irradiation*:
• reversible which decay after ceasing the radiation
• irreversible which are stable with the time.
The reported measurements have been carried out after
the radiation has been stopped, so that they can be
regarded as capturing only the irreversible changes
caused by radiation.
* O. I. Shpotyuk, in Semiconducting Chalcogenide Glass I: Glass formation, Structure and
simulated transformations in chalcogenide glasses, edited by R. Fairman and B. Ushkov
(Elsevier Academic Press, New York, 2004), p. 215.
Raman scattering
Ge40Se60
0.9
100 krad
20 krad
virgin
(a)
Ge40Se60
Normalized intensity
Normalized intensity
1.2
1.5
0.6
0.3
0.0
1.2
1.3 Mrad
200 krad
virgin
(b)
0.9
0.6
0.3
0.0
150
200 250 300
-1
Raman shift (cm )
350
150
200
250
300
350
-1
Raman shift (cm )
Raman scattering spectra (symbols) of virgin and -irradiated Ge40Se60 films:
(a) low-dose range, (b) high-dose range. The dashed lines represent the
deconvolution results for the samples irradiated with 100 krad (a) and 1.3
Mrad (b).
GeSe2 – main Raman modes
Symmetric, 150-230 cm-1
ETH units
Se3-Ge-Ge-Se3
178 cm-1
216 cm-2
Asymmetric 230-330 cm-1
Area ratio of ES/CS structure units
Raman scattering – symmetric
vibrations
Irradiation induced decrease of the
ethane-like units i.e. of the
weakest Ge-Ge bonds.
0.48
Ge40Se60
0.45
0.42
0.39
0.36
20 100 200 1300
0
Gamma irradiation dose (krad)
Dependence of the area ratio ES/CS
structural units for Ge40Se60 films at
different irradiation doses. Results
from different evaporation batches are
presented.
Raman scattering
1.5
Normalized intensity
Ge40Se60
1.2
1.3 Mrad
200 krad
virgin
(b)
0.9
0.6
0.3
0.0
Normalized intensity
Ge40Se60
0.6
100 krad
20 krad
virgin
0.3
0.0
(a)
250
300
350
-1
Raman shift (cm )
150
200
250
300
350
-1
Raman shift (cm )
Raman scattering spectra (symbols) of virgin and -irradiated Ge40Se60 films:
(a) low-dose range, (b) high-dose range. The dashed lines represent the
deconvolution results for the samples irradiated with 100 krad (a) and 1.3
Mrad (b).
Raman scattering – asymmetric
vibrations
(a)
(b) 3.5
I230-330/ICS+IES
2
3
1.8
2.5
1.6
2
0
40 80
0 800 1600
Gamma irradiation dose (krad)
Relative variation of the
integrated intensity of the
scattered light in the 230-330
cm-1 range with respect to
the sum of the integrated
intensities of the cornershared (ICS) and edgeshared (ICS) modes. Each
group of symbols
corresponds to different
evaporation batch.
Dark and photoconductivity
Ip, virgin
Ge40Se60
Ip, 3Mrad
-9
10
Id, virgin
-1
-1
Conductivity ( cm )
Id, 3Mrad
-10
10
dark current, Id
Ead(virgin)=0.75 eV
-11 Ead(3Mrad)=0.74 eV
10
photocurrent, Ip
Eap(virgin)=0.26 eV
-12 Eap(3Mrad)=0.28 eV
10
2.1
2.4
2.7
3
3.0
-1
10 /T (K )
3.3
Temperature dependences
of dc dark conductivity and
steady-state
photoconductivity of virgin
and -irradiated (dose of 3
Mrad) Ge40Se60 thin films.
The solid lines represent a
linear fit of the experimental
points (symbols).
Conclusions
 In Ge40Se60 thin films the gamma irradiation induces :
o decrease of the weakest Ge-Ge bonds;
o increase in the ES/CS ratio and at high irradiation doses
an overall disorder decrease. Both effects could be
important for Ag diffusion in the films;
 The disorder decrease is similar to the effect of the
temperature treatment but it is related to strong electronphonon interaction rather than to sample heating.
Interaction of “pure”Chalcogenide glasses
with electromagnetic radiation
Creation of electron-hole pairs
Separation of
electron-hole pair,
thus contributing
to electrical
response of the
material Photoconductivity
Radiative
recombination
of the
electron-hole
pair Photolumines
cence
Non-radiative
recombination of the
electron-hole pair - this
brings material in a
different state than the
non-illuminated one
Radiation sensor, BSU February 24, 2011
15
Gamma-induced optical changes in GeSe thin films
Absorption spectra of (1,2) gammairradiated and (3) unirradiated
a-GeSe films (1.25 m) (a) before
and (b) after storage for a year;
D= (1)1 MGy and (2) 10 kGy.
(1, 2) Eopt and (3, 4) n as functions of
gamma dose for a-GeSe films (2, 3)
before and (1, 4) after storage for a year.
R. R. Romanyuk et al., Inorganic Materials, 2007, Vol. 43, No. 6, pp. 584–587.
Chalcogenide glasses
•
•
•
•
These glasses are based on the chalcogen elements S, Se, and Te and are formed by the addition
of other elements such as Ge, As, Sb, Ga, etc. They
are generally transparent from the visible up to infrared
can be doped by rare-earth elements and numerous applications of active optical devices
have been proposed.
are optically highly non-linear and could therefore be useful for all-optical switching.
are sensitive to the absorption of electromagnetic radiation and particle irradiation and show
a variety of photoinduced effects as a result of illumination: photodarkening,
photobleaching, photocrystallization, photoplastic effects, etc.
3D AFM picture (a) and a cross-section (b)
of the pattern obtained on 0.5 lm thick a-Se
layer irradiated with 60 keV Xe24+ ions.
3D AFM picture (a) and a cross-section (b) of the
pattern recorded on Se/As2S3 nanolayered film with
total thickness d = 900 nm, illuminated through a
microgrid with a 17 mW, 532 nm diode laser for 30 min.
S. Kokenyesi et al. / Journal of Non-Crystalline Solids 353 (2007) 1470–1473.
Cross Section of a Pixel of a X-ray Imaging
Array
S. A. Kasap, J.A. Rolands, IEE Proc. Circuits Devices Syst., 149 (2002) 85.
Radiation sensor, BSU February 24, 2011
Solid electrolyte
The solid state electrolyte material is formed by Ag diffusion in Ge Chalcogenide glasses (ChG). This electrolyte allows the movement of silver
ions under the influence of an electric field. The electrically stimulated injection
and removal of metal in the solid electrolyte at very low voltage is the basis of
Programmable Metallization Cell memory PMCm devices technology.
Schematic of (a) PMCm device modeled as an electrochemical cell. a. Unwritten device. (b) Device
during WRITE: A very small voltage rapidly “injects” into the electrolyte excess silver ions, which are
reduced by the electron current to form highly stable silver dendrites within the electrolyte. (c)
Written Device: The electrondeposition process stops when a conducting link is formed. Metallic link
reduced the resistance of the structure by many orders of magnitude.(d) Device during ERASE: A
very small reverse voltage (a few hundred mV) removes excess silver from the electrolyte. Excess
silver is backed on the silver electrode by an easily reversible reaction.