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Electronic and ionic processes influence on
electrical properties of TlBr crystals
J. Vaitkus
Institute of Materials Science and Applied Research (IMSAR),
Vilnius University, Lithuania
Co-authors:
V. Gostilo, S.Zatoloka Baltic Scientific Instruments, Latvia;
A.Mekys, J.Storasta, A.Žindulis IMSAR, Vilnius University, Lithuania
J.Banys, Faculty of Physics, Vilnius University, Lithuania;
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Basic:
The interest in TlBr crystal is due to its high average atomic number (Tl: 81,
Br: 35), high density (7.5 g/cm3) and wide bandgap (2.7 eV).
The photon stopping power of TlBr crystal is greater than any of the
semiconductors discussed. Therefore this material is promising for Xand γ- ray detector applications. K.S.Shah et al. IEEE Trans.Nucl.Sci. (1989) v.39(1).
Problem:
The stability of TlBr radiation detector is not good,
the investigation of degradation phenomena and improving the
properties are important for the future of detectors
-9
10
0.92 eV
0.50 eV
TlBr dark current (dc)
multiple heating
-11
II
8
A cycle of dark current and
mobility vs temperature
2
I, A
10
10
,cm /Vs
-10
10
12
-12
10
-13
10
0.71 eV
6
4
0.94 eV
III
2
I
3.2
3.6
J.Vaitkus
4.0
4.4
-1
1000/T, K
3.0
4.8
3.2
3.4
3.6
3.8
-1
1000/T, K
4.0
4.2
5.2
IWORID6, Glasgow, 2004.06.25-29
Outline:
1. Investigation of photoconductivity spectra &
electrical conductivity (at different frequencies and temperature)
2. The contacts degradation phenomena.
3. A fractal approach.
The schematic view on the samples: the true photos of the
crystals but not the contacts
A fresh crystal
J.Vaitkus
A “tired” crystal
IWORID6, Glasgow, 2004.06.25-29
Spectral Dependencies of Photoconductivity
1
Shows the edge of
intrinsic PC at 2.75 eV
and
a deep level at 2.63 eV
Sample M5
If / If max
0.1
0.01
The spectra shows PC is related with the space charge regions,
and they changes with bias voltage and a sign
0
T=60 C U=+20V
0
T=20 C U=+20V
420
430
440
450
460
470
480
, nm
Sample M14
1.0
If / If max
If / If max
1.0
0.5
Sample P3 (1.2)
0.5
+10V
-10V
U=+20V
U= -20V
RT
RT
0.0
0.0
420
430
440
450
, nm
J.Vaitkus
460
470
480
420
430
440
450
460
, nm
IWORID6, Glasgow, 2004.06.25-29
470
The dielectric spectroscopy
The complex dielectric permittivity * = ’ - i“ was measured by
a capacitance bridge HP4284A in the frequency range 20 Hz - 1 MHz.
Temperature dependence of
frequencies.
the real part
and of
the imaginary part of dielectric permitivity at different
At low frequencies dielectric losses increase with increasing temperature and cause increase of the real part of the dielectric permittivity.
If can be caused by the big ionic conductivity as it was already shown previously [Secco, R.A., Secco, E.A. and Chen, Q. Defects and ionic
conductivity in TlCl, TlBr and TlI at high pressure and temperature. Journal of solid state chemistry 141 (1998) 462-465p.].
At low frequencies, the conductivity phenomena dominate in the dielectric spectra. With such a high value of
conductivity the contacts and barrier regions can play an important role.
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Electrical conductivity at high temperature
The electric conductivity:
 = 0".
 = DC + As,
where DC is the DC conductivity and As
is the AC conductivity.
(A.K.Joncher, Dielectric relaxation in solids,
London, Celsea Dielectric Press (1983).)
 = 0exp(EA/kT).
EA = 0.8 eV,
 0 = 6,7  10-15 S/m.
Conductivity are caused by Tl ions,
which can move in the crystal
lattice.
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Electric modulus
Frequency dependence of the real part and the imaginary part of electrical modulus at different temperatures.
The conductivity of mobile ions can be related to the electrical modulus: M*() = 1/*() = M'() + M"().
The low frequency value of M' is zero and represents a lack of the restoring force for the electric field induced mobile Tl ions. As frequency
increases, each ion moves a shorter and shorter distance until finally the electric field changes so rapidly that the ions only oscillate within the confinements of their
potential energy wells. As a result, M' increases to a maximum asymptotic value M() = 1/().
The spectra of M" show a slightly asymmetric peak centered in the dispersion region of M’.
The region where the peak occurs ( = 1) is indicative of the transition from long-range (left) to short-range (right) ion
mobility and the peak frequency corresponds to the average electric field (or conductivity) relaxation time, 1/. The broadening in
the modulus spectra indicates a cooperative motion of mobile ions, especially in the higher frequency range.
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Dark current frequency dependence
1E-5
1E-6
E=0,80eV
0,80eV
,  m
-1
1E-7
f, kHz
100
1000
-1
-1
,  m
-1
1E-5
1E-8
1E-9
f, kHz
1,2
3,7
7,8
35
241
667
11,4
2,6
5,4
17
100
480
1000
1E-6
1E-7
2.4
1E-10
2.7
3.0
3.3
3.6
3.9
4.2
4.5
4.8
5.1
5.4
-1
2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0
1000/T, K
1000/T, K
-1
Dark current vs temperature and vs frequency.
Peculiarities related to the percolation character of ion drift and, probably, the specific
features related to ion oscillations and space charge effects.
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Tl-TlBr-Au
-11
6x10
-11
4x10
dI/dt, A/h
330
I, nA
325
2.0
320
1.6
-11
T, K
310
0
-2x10
315
1.2
-11
2x10
-11
-4x10
0
100
305
0.8
200
300
derivative of Furje spectrum without a noise
0
100
200
300
400
500
phase, deg
0.0
0.4
500
time, h
300
295
400
0.2
500
0
-500
-1000
0,042
time, h
-11
Electric current time dependence in TlBr in system Al-Tl-TlBr-Al and
applied 30V DC voltage. Total charge 2,33mC (1,45·1016 particles).
A deposited Tl contact disappears due to
ionic conductivity and characteristic
instabilities (spikes) are observed.
J.Vaitkus
Amplitude
1x10
-12
5x10
0,055
0
0.00
0.05
0.10
0.15
0.20
0.25
frequency, 1/h
IWORID6, Glasgow, 2004.06.25-29
0.30
A new result was found by direct Tl+ ions transport:
diffusion – limited aggregation is responsible for a dendritic structures
which could be grown by Tl+ or electrode ion electrodiffusion.
-9
2.4x10
Proposed: spikes appear
during a growth of a dendrit
or creation of cluster
-9
I, A
2.1x10
-9
1.8x10
A fractal analysis approach
seems very promising.
-9
1.5x10
200 225 250 275 300
time, h
A part from the current time dependence
Typical fractal system:
Cu electrode and mineral water
The current vs time dependence
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Conclusions:
1. Photoconductivity spectra demonstrates the
electric field redistribution in the sample and an
existence of the deep centres.
2. Frequence dependence of conductivity allows to
measure the ionic conductivity and demonstrates the
regions of ionic instabilities. T < 250 K is promising for
improve a stability of detectors.
3. Tl+ ion current time dependence shows the fractal
behaviour of ion migration.
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29
Fractal in the natural colours
Thank you for your attention !
J.Vaitkus
IWORID6, Glasgow, 2004.06.25-29