Application of ferroelectrics to create electroluminescent indicators of temperature
In this article, temperature dependences of brightness of thin film electroluminescent emitters (TFELE) based on metal-dielectric-semiconductor-metal (MDSM) structures with ceramic ferroelectric dielectric have been considered. Their comparable analysis with the temperature dependence of dielectr...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2012
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irk-123456789-1183032017-05-30T03:03:42Z Application of ferroelectrics to create electroluminescent indicators of temperature Boyko, V.G. Zayats, N.S. In this article, temperature dependences of brightness of thin film electroluminescent emitters (TFELE) based on metal-dielectric-semiconductor-metal (MDSM) structures with ceramic ferroelectric dielectric have been considered. Their comparable analysis with the temperature dependence of dielectric themself has been made. Literature data concerning this question have been briefly discussed. The conclusion about the possibility to use ferroelectrics (ceramics based on BaTiO₃, AlN, LiNbO₃) with a pronounced thermal dependence of capacitance characteristics to produce the temperature light indicators has been made. The design of a new temperature sensor can be realized in several ways. For example, in the form of the scales with a moving luminous column or separate lighting dots, depending on how clearly Curie peaks are observed on the temperature curves of dielectric permittivity. 2012 Article Application of ferroelectrics to create electroluminescent indicators of temperature / V.G. Boyko, N.S. Zayats // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 2. — С. 162-165. — Бібліогр.: 10 назв. — англ. 1560-8034 PACS 77.55.fe, hd; 77.80.B-, 78.60.Fi http://dspace.nbuv.gov.ua/handle/123456789/118303 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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In this article, temperature dependences of brightness of thin film
electroluminescent emitters (TFELE) based on metal-dielectric-semiconductor-metal
(MDSM) structures with ceramic ferroelectric dielectric have been considered. Their
comparable analysis with the temperature dependence of dielectric themself has been
made. Literature data concerning this question have been briefly discussed. The
conclusion about the possibility to use ferroelectrics (ceramics based on BaTiO₃, AlN,
LiNbO₃) with a pronounced thermal dependence of capacitance characteristics to
produce the temperature light indicators has been made. The design of a new temperature
sensor can be realized in several ways. For example, in the form of the scales with a
moving luminous column or separate lighting dots, depending on how clearly Curie
peaks are observed on the temperature curves of dielectric permittivity. |
| format |
Article |
| author |
Boyko, V.G. Zayats, N.S. |
| spellingShingle |
Boyko, V.G. Zayats, N.S. Application of ferroelectrics to create electroluminescent indicators of temperature Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Boyko, V.G. Zayats, N.S. |
| author_sort |
Boyko, V.G. |
| title |
Application of ferroelectrics to create electroluminescent indicators of temperature |
| title_short |
Application of ferroelectrics to create electroluminescent indicators of temperature |
| title_full |
Application of ferroelectrics to create electroluminescent indicators of temperature |
| title_fullStr |
Application of ferroelectrics to create electroluminescent indicators of temperature |
| title_full_unstemmed |
Application of ferroelectrics to create electroluminescent indicators of temperature |
| title_sort |
application of ferroelectrics to create electroluminescent indicators of temperature |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2012 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118303 |
| citation_txt |
Application of ferroelectrics to create electroluminescent
indicators of temperature / V.G. Boyko, N.S. Zayats // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 2. — С. 162-165. — Бібліогр.: 10 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT boykovg applicationofferroelectricstocreateelectroluminescentindicatorsoftemperature AT zayatsns applicationofferroelectricstocreateelectroluminescentindicatorsoftemperature |
| first_indexed |
2025-07-08T13:41:22Z |
| last_indexed |
2025-07-08T13:41:22Z |
| _version_ |
1837086365798367232 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 2. P. 162-165.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
162
PACS 77.55.fe, hd; 77.80.B-, 78.60.Fi
Application of ferroelectrics to create electroluminescent
indicators of temperature
V.G. Boyko, N.S. Zayats
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, phone: (38044) 525-62-81, e-mail: zayats@isp.kiev.ua
Abstract. In this article, temperature dependences of brightness of thin film
electroluminescent emitters (TFELE) based on metal-dielectric-semiconductor-metal
(MDSM) structures with ceramic ferroelectric dielectric have been considered. Their
comparable analysis with the temperature dependence of dielectric themself has been
made. Literature data concerning this question have been briefly discussed. The
conclusion about the possibility to use ferroelectrics (ceramics based on BaTiO3, AlN,
LiNbO3) with a pronounced thermal dependence of capacitance characteristics to
produce the temperature light indicators has been made. The design of a new temperature
sensor can be realized in several ways. For example, in the form of the scales with a
moving luminous column or separate lighting dots, depending on how clearly Curie
peaks are observed on the temperature curves of dielectric permittivity.
Keywords: thin film electroluminescent structure (TFELS), ferroelectric, dielectric
permittivity, Curie point, temperature.
Manuscript received 06.02.12; revised version received 20.03.12; accepted for
publication 27.03.12; published online 15.05.12.
1. Introduction
The interest to development in the area of making the
thin film electroluminescent structures (TFELS), as it is
well known, is mainly related with the possibility of
their using in the miscellaneous type of information
displays. However, competition on the part of cathode
ray tubes, liquid-crystal displays, light-emitting diodes
and the other type screens [1], as well as specific lacks
of TFELS limit their mass consumption.
The analysis of necessary demands for TFELS
shows that these structures find and can find using not
only in screens, but also in more simple devices, namely:
light scales, point sources of light, signal indicators [1].
In particular, the authors of this paper propose light
indicators of the temperature. So, in this context, it is
very interesting to consider temperature characteristics
of EL emitters.
When studying temperature effects in thin-film
emitters of the type MDSDM (metal-dielectric-
semiconductor-dielectric-metal) under excitation by ac
current, it was ascertained that characteristics of these
structures change to higher or lower degree with
changing the temperature T, depending on material of
dielectric and semiconductor layers as well as methods
of their deposition. In particular, for the film system
Y2O3-ZnS-Y2O3 brightness is a monotonous decreasing
function of temperature [2].
Moreover, in the structure Al2O3-ZnS-SiO2 an
essential temperature shift of the electroluminescence
threshold with a simultaneous change in the capacitance
of dielectric layers was observed. These phenomena
were explained by temperature changes of electric
characteristics in SiO2 layer [3].
So, in each specific case it is necessary to
investigate the temperature dependences of
electroluminescence characteristics of whole TFELS and
electric parameters of its component parts.
The importance of these studies is conditioned by
the fact that changing the temperature can result in
redistribution of applied voltages between luminescent
and dielectric layers, because of the temperature change
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 2. P. 162-165.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
163
Table. Typical properties of ferroelectric ceramics.
Ceramic
marks
Composition,
wt.%
Dielectric
loss tangent
at 1 kHz,
tg()10-4
Operation
temperature
interval,
С
Curie
point,
С
Dielectric
permittivity
at 205 C,
Т-4500
ВаTiO3
CaZrO
Kaolin
ZnO
MnCO3
MgTiO
SrCO3
87.65
10.75
1.20
0.30
0.10
0.2-3.4
over 100
1.0
over 100
250 40…+85 4500
Т-11000
BaTiO3
CaSnO3
Kaolin
MnCO3
Nb2O5
88.84
10.81
0.25
0.1
0.5
over 100
250 40…+85 +10…+4
0
10000
in electric parameters of the latter that will influence the
electroluminescence excitation process. It should be
supposed that this effect in TFELS with dielectric layers
from ferroelectrics, which have high values of the
dielectric permittivity and their clearly expressed
dependence on temperature, is particularly pronounced.
So, in this work performed in a wide interval of
temperatures researched were electric parameters of the
ferroelectric substrates on which TFELS were formed
(in our case ZnS:Mn), and relationship of these
parameters with light-technical characteristics of this
structure as a whole was established. As a result of
analysis of the obtained data, there was offered the
possibility to make a new light type temperature sensor.
2. Experimental
The investigated structures were made using the low-
frequency ferroelectric ceramics base on barium titanate
(BaTiO3) as dielectric layers. The ceramic substrates of
0.3 and 0.35 mm thick had high dielectric constants
11000 and 5000, respectively. The composition and
ceramic marks, applicable as substrates for fabrication of
EL emitters, and their basic characteristics are
summarized in Table.
The polished ceramic substrates (disks about 6 mm
in diameter and 0.3…0.35 mm thick) were manufactured
on the capacitor plant “Kation”.
The electrical properties were measured in the
metal-dielectric-metal (MDM) configuration with the Al
electrodes (with the diameter close to 5 mm and
thickness 1.5 m) were thermally deposited in vacuum
on both surfaces of the ceramic disks at the substrate
temperature 150 C.
The electroluminescence properties were measured
in the metal-dielectric-semiconductor-metal (MDSM)
geometry with the ZnS:Mn films (5 mm in diameter and
0.6 m in thickness) deposited directly on heated to
150 C ceramic substrates by the method of electron-
beam evaporation from ZnS:1% MnS pellets without
any post-heat-treatment after deposition. As upper
transparent electrodes, we used Au semitransparent
layers (diameter 5 mm) deposited by means of thermal
vacuum evaporation.
The capacitance and luminescence intensity were
measured with a digital LCR meter and photoamplifier,
respectively, by applying of 1-kHz sinusoidal excitation
voltages to the structures. All the temperature
measurements within the range 100…400 K were
carried out using an optical cryostat with a heater.
3. Results and discussion
Presented in Fig. 1 are the temperature dependences of
EL intensity in the emitters with substrates from
ceramics of different marks. Fig. 2 shows dependences
of the capacitance (or dielectric permittivity) of these
substrates on temperature.
From comparison of the data in Figs. 1 and 2, it is
seen that the temperature behavior of EL intensity and
dielectric permittivity correlates one to another and
differs by the maximum position of curves for different
types of these ceramics. I.e. in our case, when bulk
dielectric with the abrupt temperature dependence of
electric characteristics was used, the change of the
luminescence intensity in radiating elements with the
change of temperature can be planned for corresponding
substrates in accordance with their temperature
characteristics.
The electroluminescent structures are threshold
devices, and the light emission begins when a value of
the applied voltage reaches its threshold value (Uth).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 2. P. 162-165.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
164
From this time point, the device begins to radiate light in
the manner of pulses during each half-period of the
alternating voltage, since in absence of the luminescence
TFEL-element can be considered as a capacitor. It is
easy to show that under tg()<<1 (it is valid near the
threshold voltages, i.e. under low levels of the
brightness, when impedances of D and S layers are
basically defined by their capacitances) Uth (i.e., the
minimum voltage applied to the structure under which
all the operation area of a sample begins to evenly shine)
is related with the threshold field value Ea in the
luminescent layer by the following equation:
Uth (dа аdD /D)Eа, (1)
where Ea is the threshold electric field in the luminescent
layer, dа and dD are the thicknesses of the luminescent
and dielectric layers, respectively, а – dielectric
permittivity of the luminescent layer, D – dielectric
permittivity of ceramics [4].
100 150 200 250 300 350 400
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
E
L
in
te
n
si
ty
, a
rb
. u
n.
T, K
2
1
Fig. 1. Temperature dependence of EL-intensity of the
emitters on ceramic substrates of T-4500 (1) and T-
11000 (2).
100 150 200 250 300 350 400
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
C
, n
F
T, K
1 2
Fig. 2. Temperature dependence of the dielectric capacitance
of T-4500 (1) and T-11000 (2).
Typical for the ferroelectric polar phase (i.e. the
temperature range, where T<TC – a Curie temperature)
Curie-Weiss’s law describing a temperature dependence
of the dielectric permittivity in the crystals looks as
follows:
= СC/2 (ТC – Т), (2)
where СC is the Curie-Weiss constant [5, 6]. Substituting
(2) into (1), one can obtain the expression describing the
dependence of the lighting threshold of the
electroluminescence on temperature in TFELS:
Uth = [da + 2adD (ТC – Т)/СC] Ea. (3)
Perceiving from the expression (3), one can
estimate the sensitivity of this structure with the BaTiO3
crystal. The change in Uth with varying the temperature
by one degree will look as:
Uth = 2adDEa/СC. (4)
For ZnS a = 9, Ea = 2106 V/cm [2], СC =
1.2105 K for BaTiO3 [9] and dD = cm103 2 , which
yields Uth = 0.9 V/degree. It is seen from the expression
(3) that changing the values of the Curie point for
substrates in such a manner that the value of ТC-T
remains constant, the value of Uth will remain constant,
too. I.e. Eq. (3) can be a basic characteristic for practical
realization of the proposed thermometer. In this relation,
mixed crystals can be of considerable interest [6]. The
mixed crystals can be of a ferroelectric type and not of
this one. Their Curie point can be shifted in wide
interval of temperatures.
If barium titanate is accomplished with the
ferroelectric possessing a Curie point higher than 120 С
(BaTiO3 Curie point), and the row of solid solutions is
formed between compounds, then the Curie point of the
latter lies above BaTiO3 Curie point. For instance, the
compounds raising the Curie point are PbTiO3, PbZrO3
and others. The compounds BaSnO3, BaZrO3, SrTiO3
and others form with BaTiO3 the continuous row of solid
solutions which lowers its Curie point. Position of the
Curie point is shown in Fig. 3 for the binary system
BaTiO3-SrTiO3.
Fig. 3. Position of Curie point for system ВаTiO3-SrTiO3.
-160
-140
-120
-100
-80
-60
-40
-20
0
20
40
60
80
100
120
BaTiO3,%
SrTiO3,%100
2030405060708090100
80706050403020
C
ur
ie
p
oi
nt
,
ºC
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 2. P. 162-165.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
165
The linear dependence for the Curie point on
composition overlaps the temperature range from –
160 С up to +120 С. Starting from the expression (3)
and Fig. 3 and taking into account that the dielectric
permittivity of AlN increases from 9 up to 100 at
frequencies 100…1000 Hz within the temperature range
20 С…500 С [7, 8], and LiNbO3 ferroelectric has the
Curie point 1483 K [9], one can expect that the most
expedient is to produce temperature EL indicators with
the accuracy 10 С, which can be used within the range
–160 С to +500 С. The upper temperature limit of EL
indicators can be set by temperature conditions of
excitation and quenching their luminosity [10] and also
by the Debye temperature of materials. Design of the
thermometer can be realized in several ways. For
instance, in the manner of the scales with a moving
luminous column or separate lighting points, depending
on how clearly Curie peaks are observed on the
temperature curves of dielectric constants.
4. Conclusions
In conclusion, it seems necessary to note that the basic
physical and technical feature of light temperature
indicators is that the dielectric layer is not a passive
element in the classical sense (an insulator), but an
active functional element (a ferroelectric material with
clearly pronounced temperature dependence of the
dielectric permittivity = f (T)) in the electroluminescent
structure, which effectively operates its luminescent
properties in a wide range of change of temperatures.
The considered indicators can be excited by the
network voltage 220 V at the frequency 50 Hz. At these
excitation conditions the brightness about 80 Cd/m2 was
reached. The main difficulty in fabrication of these
structures is a very complicated sintering technology of
ferroelectric ceramics with crystalline and
polycrystalline phases.
References
1. I.K. Vereschagin, B.A. Kovalev,
L.A. Kosyachenko, S.M. Kokin, Electro-
luminescent Light Source. Energoatomizdat,
Moscow, 1990 (in Russian).
2. R. Mach, G.O. Muller, Physical concepts of
high-field, thin-film devices electroluminescent
// Phys. Status Solidi, 69(a), No.11, p. 11-66
(1982).
3. S.I. Balyasnaya, L.I. Veligura, N.A. Vlasenko,
V.S. Khomchenko, On the nature of the
temperature dependence of the characteristics of
thin-film electroluminescent MDPDM-
structures based on ZnS:TbF3 // Optoelektronika
i poluprovodnikovaia tekhnika, 18, p. 81-85
(1990), in Russian.
4. N.M. Parfenov, S.M. Kokin, B.G. Bekker,
I.E. Ostriy, A.V. Lipoveckiy, A.D. Chromov,
The influence of dielectrics on the parameters of
thin-film electroluminescent structures //
Izvestiya vuzov. Fizika, No. 4, p. 119-120
(1985), in Russian.
5. Yu.M. Poplavko, Physics of Dielectrics.
Vyshcha shkola, Kiev, 1980 (in Russian).
6. A. Andrianov, N.P. Bogorodickiy,
Yu.V. Korickiy, Handbook on Electrotechnical
Materials. Gosenergoizdat, Moscow-Leningrad,
vol. II, 1960 (in Russian).
7. T.J. Kosolapova, T.B. Andreeva, T.S.
Bartnitskaya et al., Nonmetallic Refractory
Compounds. Metallurgiya, Moscow, 1985 (in
Russian).
8. V.G. Boyko, M.S. Zayats, S.M. Zayats et al.,
Electrophysical properties of AlN obtained by
high-frequency reactive magnetron sputtering //
Physics and technology of thin films and
nanosystems (ICPTTFN-XIII): XIII
International Materials Conference, 16-21 May
2011, Ivano-Frankivsk, Ukraine, V.1, p. 157
(2011).
9. V.G. Kolesnikov, Electronics, in: Encyclopedic
Dictionary. Sovetskaya entsiklopediya,
Moscow, 1991 (in Russian).
10. I.K. Vereshchagin, Electroluminescence of
Crystals. Nauka, Moscow, 1974 (in Russian).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 2. P. 162-165.
PACS 77.55.fe, hd; 77.80.B-, 78.60.Fi
Application of ferroelectrics to create electroluminescent
indicators of temperature
V.G. Boyko, N.S. Zayats
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, phone: (38044) 525-62-81, e-mail: zayats@isp.kiev.ua
Abstract. In this article, temperature dependences of brightness of thin film electroluminescent emitters (TFELE) based on metal-dielectric-semiconductor-metal (MDSM) structures with ceramic ferroelectric dielectric have been considered. Their comparable analysis with the temperature dependence of dielectric themself has been made. Literature data concerning this question have been briefly discussed. The conclusion about the possibility to use ferroelectrics (ceramics based on BaTiO3, AlN, LiNbO3) with a pronounced thermal dependence of capacitance characteristics to produce the temperature light indicators has been made. The design of a new temperature sensor can be realized in several ways. For example, in the form of the scales with a moving luminous column or separate lighting dots, depending on how clearly Curie peaks are observed on the temperature curves of dielectric permittivity.
Keywords: thin film electroluminescent structure (TFELS), ferroelectric, dielectric permittivity, Curie point, temperature.
Manuscript received 06.02.12; revised version received 20.03.12; accepted for publication 27.03.12; published online 15.05.12.
1. Introduction
The interest to development in the area of making the thin film electroluminescent structures (TFELS), as it is well known, is mainly related with the possibility of their using in the miscellaneous type of information displays. However, competition on the part of cathode ray tubes, liquid-crystal displays, light-emitting diodes and the other type screens [1], as well as specific lacks of TFELS limit their mass consumption.
The analysis of necessary demands for TFELS shows that these structures find and can find using not only in screens, but also in more simple devices, namely: light scales, point sources of light, signal indicators [1]. In particular, the authors of this paper propose light indicators of the temperature. So, in this context, it is very interesting to consider temperature characteristics of EL emitters.
When studying temperature effects in thin-film emitters of the type MDSDM (metal-dielectric-semiconductor-dielectric-metal) under excitation by ac current, it was ascertained that characteristics of these structures change to higher or lower degree with changing the temperature T, depending on material of dielectric and semiconductor layers as well as methods of their deposition. In particular, for the film system Y2O3-ZnS-Y2O3 brightness is a monotonous decreasing function of temperature [2].
Moreover, in the structure Al2O3-ZnS-SiO2 an essential temperature shift of the electroluminescence threshold with a simultaneous change in the capacitance of dielectric layers was observed. These phenomena were explained by temperature changes of electric characteristics in SiO2 layer [3].
So, in each specific case it is necessary to investigate the temperature dependences of electroluminescence characteristics of whole TFELS and electric parameters of its component parts.
The importance of these studies is conditioned by the fact that changing the temperature can result in redistribution of applied voltages between luminescent and dielectric layers, because of the temperature change in electric parameters of the latter that will influence the electroluminescence excitation process. It should be supposed that this effect in TFELS with dielectric layers from ferroelectrics, which have high values of the dielectric permittivity and their clearly expressed dependence on temperature, is particularly pronounced.
So, in this work performed in a wide interval of temperatures researched were electric parameters of the ferroelectric substrates on which TFELS were formed (in our case ZnS:Mn), and relationship of these parameters with light-technical characteristics of this structure as a whole was established. As a result of analysis of the obtained data, there was offered the possibility to make a new light type temperature sensor.
2. Experimental
The investigated structures were made using the low-frequency ferroelectric ceramics base on barium titanate (BaTiO3) as dielectric layers. The ceramic substrates of 0.3 and 0.35 mm thick had high dielectric constants 11000 and 5000, respectively. The composition and ceramic marks, applicable as substrates for fabrication of EL emitters, and their basic characteristics are summarized in Table.
The polished ceramic substrates (disks about 6 mm in diameter and 0.3…0.35 mm thick) were manufactured on the capacitor plant “Kation”.
The electrical properties were measured in the metal-dielectric-metal (MDM) configuration with the Al electrodes (with the diameter close to 5 mm and thickness 1.5 (m) were thermally deposited in vacuum on both surfaces of the ceramic disks at the substrate temperature 150 (C.
The electroluminescence properties were measured in the metal-dielectric-semiconductor-metal (MDSM) geometry with the ZnS:Mn films (5 mm in diameter and 0.6 (m in thickness) deposited directly on heated to 150 (C ceramic substrates by the method of electron-beam evaporation from ZnS:1% MnS pellets without any post-heat-treatment after deposition. As upper transparent electrodes, we used Au semitransparent layers (diameter 5 mm) deposited by means of thermal vacuum evaporation.
The capacitance and luminescence intensity were measured with a digital LCR meter and photoamplifier, respectively, by applying of 1-kHz sinusoidal excitation voltages to the structures. All the temperature measurements within the range 100…400 K were carried out using an optical cryostat with a heater.
3. Results and discussion
Presented in Fig. 1 are the temperature dependences of EL intensity in the emitters with substrates from ceramics of different marks. Fig. 2 shows dependences of the capacitance (or dielectric permittivity) of these substrates on temperature.
From comparison of the data in Figs. 1 and 2, it is seen that the temperature behavior of EL intensity and dielectric permittivity correlates one to another and differs by the maximum position of curves for different types of these ceramics. I.e. in our case, when bulk dielectric with the abrupt temperature dependence of electric characteristics was used, the change of the luminescence intensity in radiating elements with the change of temperature can be planned for corresponding substrates in accordance with their temperature characteristics.
The electroluminescent structures are threshold devices, and the light emission begins when a value of the applied voltage reaches its threshold value (Uth). From this time point, the device begins to radiate light in the manner of pulses during each half-period of the alternating voltage, since in absence of the luminescence TFEL-element can be considered as a capacitor. It is easy to show that under tg(()<<1 (it is valid near the threshold voltages, i.e. under low levels of the brightness, when impedances of D and S layers are basically defined by their capacitances) Uth (i.e., the minimum voltage applied to the structure under which all the operation area of a sample begins to evenly shine) is related with the threshold field value Ea in the luminescent layer by the following equation:
Uth ( (dа ( (аdD /(D)Eа,
(1)
where Ea is the threshold electric field in the luminescent layer, dа and dD are the thicknesses of the luminescent and dielectric layers, respectively, (а – dielectric permittivity of the luminescent layer, (D – dielectric permittivity of ceramics [4].
100
150
200
250
300
350
400
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
EL intensity, arb. un
.
T, K
2
1
Fig. 1. Temperature dependence of EL-intensity of the emitters on ceramic substrates of T-4500 (1) and T-11000 (2).
100
150
200
250
300
350
400
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
C, nF
T, K
1
2
Fig. 2. Temperature dependence of the dielectric capacitance of T-4500 (1) and T-11000 (2).
Typical for the ferroelectric polar phase (i.e. the temperature range, where T<TC – a Curie temperature) Curie-Weiss’s law describing a temperature dependence of the dielectric permittivity in the crystals looks as follows:
( = СC/2 (ТC – Т),
(2)
where СC is the Curie-Weiss constant [5, 6]. Substituting (2) into (1), one can obtain the expression describing the dependence of the lighting threshold of the electroluminescence on temperature in TFELS:
Uth = [da + 2(adD (ТC – Т)/СC] Ea.
(3)
Perceiving from the expression (3), one can estimate the sensitivity of this structure with the BaTiO3 crystal. The change in Uth with varying the temperature by one degree will look as:
(Uth = 2(adDEa/СC.
(4)
For ZnS (a = 9, Ea = 2(106 V/cm [2], СC = 1.2(105 K for BaTiO3 [9] and dD =
cm
10
3
2
-
´
, which yields (Uth = 0.9 V/degree. It is seen from the expression (3) that changing the values of the Curie point for substrates in such a manner that the value of ТC-T remains constant, the value of Uth will remain constant, too. I.e. Eq. (3) can be a basic characteristic for practical realization of the proposed thermometer. In this relation, mixed crystals can be of considerable interest [6]. The mixed crystals can be of a ferroelectric type and not of this one. Their Curie point can be shifted in wide interval of temperatures.
If barium titanate is accomplished with the ferroelectric possessing a Curie point higher than 120 (С (BaTiO3 Curie point), and the row of solid solutions is formed between compounds, then the Curie point of the latter lies above BaTiO3 Curie point. For instance, the compounds raising the Curie point are PbTiO3, PbZrO3 and others. The compounds BaSnO3, BaZrO3, SrTiO3 and others form with BaTiO3 the continuous row of solid solutions which lowers its Curie point. Position of the Curie point is shown in Fig. 3 for the binary system BaTiO3-SrTiO3.
Fig. 3. Position of Curie point for system ВаTiO3-SrTiO3.
The linear dependence for the Curie point on composition overlaps the temperature range from –160 (С up to +120 (С. Starting from the expression (3) and Fig. 3 and taking into account that the dielectric permittivity of AlN increases from 9 up to 100 at frequencies 100…1000 Hz within the temperature range 20 (С…500 (С [7, 8], and LiNbO3 ferroelectric has the Curie point 1483 K [9], one can expect that the most expedient is to produce temperature EL indicators with the accuracy 10 (С, which can be used within the range –160 (С to +500 (С. The upper temperature limit of EL indicators can be set by temperature conditions of excitation and quenching their luminosity [10] and also by the Debye temperature of materials. Design of the thermometer can be realized in several ways. For instance, in the manner of the scales with a moving luminous column or separate lighting points, depending on how clearly Curie peaks are observed on the temperature curves of dielectric constants.
4. Conclusions
In conclusion, it seems necessary to note that the basic physical and technical feature of light temperature indicators is that the dielectric layer is not a passive element in the classical sense (an insulator), but an active functional element (a ferroelectric material with clearly pronounced temperature dependence of the dielectric permittivity ( = f (T)) in the electroluminescent structure, which effectively operates its luminescent properties in a wide range of change of temperatures.
The considered indicators can be excited by the network voltage 220 V at the frequency 50 Hz. At these excitation conditions the brightness about 80 Cd/m2 was reached. The main difficulty in fabrication of these structures is a very complicated sintering technology of ferroelectric ceramics with crystalline and polycrystalline phases.
References
1.
I.K. Vereschagin, B.A. Kovalev, L.A. Kosyachenko, S.M. Kokin, Electroluminescent Light Source. Energoatomizdat, Moscow, 1990 (in Russian).
2.
R. Mach, G.O. Muller, Physical concepts of high-field, thin-film devices electroluminescent // Phys. Status Solidi, 69(a), No.11, p. 11-66 (1982).
3. S.I. Balyasnaya, L.I. Veligura, N.A. Vlasenko, V.S. Khomchenko, On the nature of the temperature dependence of the characteristics of thin-film electroluminescent MDPDM-structures based on ZnS:TbF3 // Optoelektronika i poluprovodnikovaia tekhnika, 18, p. 81-85 (1990), in Russian.
4.
N.M. Parfenov, S.M. Kokin, B.G. Bekker, I.E. Ostriy, A.V. Lipoveckiy, A.D. Chromov, The influence of dielectrics on the parameters of thin-film electroluminescent structures // Izvestiya vuzov. Fizika, No. 4, p. 119-120 (1985), in Russian.
5.
Yu.M. Poplavko, Physics of Dielectrics. Vyshcha shkola, Kiev, 1980 (in Russian).
6.
A. Andrianov, N.P. Bogorodickiy, Yu.V. Korickiy, Handbook on Electrotechnical Materials. Gosenergoizdat, Moscow-Leningrad, vol. II, 1960 (in Russian).
7.
T.J. Kosolapova, T.B. Andreeva, T.S. Bartnitskaya et al., Nonmetallic Refractory Compounds. Metallurgiya, Moscow, 1985 (in Russian).
8. V.G. Boyko, M.S. Zayats, S.M. Zayats et al., Electrophysical properties of AlN obtained by high-frequency reactive magnetron sputtering // Physics and technology of thin films and nanosystems (ICPTTFN-XIII): XIII International Materials Conference, 16-21 May 2011, Ivano-Frankivsk, Ukraine, V.1, p. 157 (2011).
9.
V.G. Kolesnikov, Electronics, in: Encyclopedic Dictionary. Sovetskaya entsiklopediya, Moscow, 1991 (in Russian).
10.
I.K. Vereshchagin, Electroluminescence of Crystals. Nauka, Moscow, 1974 (in Russian).
Curie point, ºC
20
30
40
50
60
70
80
100
90
80
70
60
Table. Typical properties of ferroelectric ceramics.
�
�
Ceramic
marks�
Composition,
wt.%�
Dielectric
loss tangent
at 1 kHz,
tg(()(10-4�
Operation
temperature
interval,
(С�
Curie point,
(С�
Dielectric
permittivity
at 20(5 (C,
(�
�
Т-4500�
ВаTiO3
CaZrO
Kaolin
ZnO
MnCO3
MgTiO
SrCO3
�
87.65
10.75
1.20
0.30
0.10
0.2-3.4
over 100
1.0
over 100�
250�
(40…+85�
(�
4500�
�
Т-11000�
BaTiO3
CaSnO3
Kaolin
MnCO3
Nb2O5�
88.84
10.81
0.25
0.1
0.5
over 100�
250�
(40…+85�
+10…+40�
10000�
�
50
40
30
20
0
10
SrTiO3,%
BaTiO3,%
120
100
80
60
40
20
0
-20
-40
-60
-80
-100
-120
-140
-160
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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