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Dielectric properties of CsBr single crystals-effect of d.c. bias
Solid State Communications, Vol. 58, No. 2, pp. 137-142, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon Press Ltd.
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS - EFFECT OF d.c. BIAS K.V.S. Badarinath and A. Subrahmanyam Department of Physics, Indian Institute of Technology, Madras 600 036, India
(Received 4 November 1985 by R. Battle) The dielectric properties of CsBr single crystals, both virgin and T-irradiated have been measured as a function of frequency (10a--10 Hz), temperature (300-575 K) and d.c. bias ( 0 - 3 5 0 V cm -1). T-irradiation produces colour centres in these crystals and consequently an increase in space charge density is observed. D.c. bias has brought in considerable changes in dielectric constant (e') and loss (tanS) of these crystals particularly at low frequencies and high temperatures. An attempt has been made to understand these data qualitatively through barrier layer formation.
1. INTRODUCTION THE DIELECTRIC PROPERTIES of CsBr single crystals have been reported earlier along with the quantitative evaluation of polarisation mechanisms [1]. Dielectric dispersion is observed in these crystals at low frequencies (below lOSHz) and high temperatures (above 430K). It has been established that the space charge effects can be studied effectively by superposing a d.c. voltage during capacitance measurements [ 2 - 5 ] ; the present paper reports the dielectric data of CsBr single crystals both virgin and T-irradiated, under the influence of d.c. bias (0-350 Vcm-1). 2. EXPERIMENTAL METHODS CsBr single crystals are grown by the Kyropolous method using "Supra Pure" grade powder. The dimensions of the polished samples are about 1.0 x 0.5 x 0.1 cm a. A thin coating of Air Dry silver paint is applied on either side of the samples to serve as secondary electrodes. Samples are T-irradiated (0.4Megaradsh -1) for four hours. Dielectric measurements are taken on GR 1620 AP Capacitance Assembly (employing three electrode geometry) in the frequency range 10a--10 s Hz in the temperature region 3 0 0 - 5 7 5 K . The details of the sample holder are discussed elsewhere [6]. D.c. voltage is applied in series with the signal generator and bias is calculated. The accuracy in dielectric constant (e') value is 2% and in loss (tan 8), it is about 5%. Thermolumineseence glow peaks are recorded on a conventional set up [7] ; the rate ofheatingis maintained at 100 K min -x with an accuracy of + 2 K. 3. RESULTS The dielectric constant (e') of CsBr single crystals at 300K is measured to be 6.75 which is frequency
independent; loss tangent is of the order of 2 x 10 -4 at 10 a Hz. The variation of e' with temperature (Fig. I) and the calculated conductivity (o) are similar to the data reported earlier [ 1]. At room temperature (300K), bias has very little influence on e' and tan 8 values at any frequency. But as the temperature is increased, the effects are more pronounced, particularly at lower frequencies. It may be seen that at 10 a Hz a peak is exhibited in e' and tan 8 values at 165 and 7 5 V c m -1 respectively at 500K (Fig. 2(a) and (b)). The peak shifts towards lower field strengths with increasing temperature. Similar are the results at 104 Hz and 5 x 10 4 HZ but for the fact the peaks in e' and tan 8 are broadened (Figs. not presented). The dielectric constant (e') of 7-irradiated CsBr single crystals at 305K is measured to be 7.82 and it is frequency independent; loss tangent values are slightly higher and have exhibited a peak around 20 x I0 a Hz (Fig. 3). With increase in temperature, e' of T-irradiated crystals is found to increase up to 400 K. Above this temperature, rapid changes have been observed in e' which could not be followed up to 480K (due to manual balancing of the bridge). Beyond 480K, the variation in dielectric constant is similar to that of unirradiated crystal (Fig. not presented). The peak observed in loss tangent around 20 x I0 a Hz at 305 K shifts to higher frequencies with increasing temperature (Fig. 3). The variation of tan 8 with temperature, though rapid at low frequencies (similar to that of e'), could be measured at 10 s Hz (Fig. 4) in which a peak is observed at 425 K. D.c. bias is observed to bring in similar changes in dielectric constant values of T-irradiated CsBr single crystals as reported in virgin crystals. However, the effect of bias could be felt rather strongly at a relatively
137
138
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
.....
Vol. 58, No. 2
wrth bias ( 349.0 Vitro)
103Hz
13
/ I _
/
I
I
//
II
,4,03H
I0
/~/'
-
6 300
,'7
I 400
I 500 Tern p
~104Hz
/'~ 104Hz
I 600
k
Fig. 1. Variation of dielectric constant (e') with temperature at different frequencies (dashed curve represents similar data with d.c. bias (349 V cm-l)) of CsBr crystals. lower temperature as may be seen from the dashed curve (10 a Hz) in Fig. 2(a). The absolute values of tan 8 at lower frequencies and temperature are enhanced with bias not affecting the basic nature of the data as reported in Fig. 3 (Fig. not given). Similar are the results at higher frequencies (Fig. 4 - dashed curve). The thermoluminescence glow peak for -/-irradiated CsBr single crystals is observed around 438K (Fig. 5). F-centre absorption is recorded at 675 nm (with Cary14D Spectrophotometer) and the F-center concentration is calculated to be 8.1 x 1017/c.c. (Fig. not presented). 4. DISCUSSION The general ideas of space charge formation and behaviour in both virgin and 7-irradiated crystals have been discussed earlier in RbC1 single crystals [8] which may be applicable in the present investigation as well. The salient features of the observations in the present
investigation may be explained as follows: (i) The space charge concentration in virgin CsBr single crystals at 300 K is extremely small as indicated by the low dielectric loss (tan 8) (at low frequencies) and frequency independent e' values, (ii) With increase in temperature, space charge density increases due to thermal generation of carriers and exhibits itself above 400 K (Fig. 1). (iii) The increase in the frequency independent e' value of 1.07 at room temperature for the 7-irradiated CsBr crystals may be attributed to the eolour centres produced (8.1 x 1017]c.c.) [11]. Sharp peak in tan 6 values for these irradiated crystals at 305 K seem to indicate the formation of clusters of vacancies having dipolar nature. With increase in temperature these clusters may break into smaller clusters as may be seen by the shift (towards higher frequencies) in the peak position (Fig. 3) [11].
Vol. 58, No. 2
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
.....
139
If Irradiated
103Hz
IA
(~'J3
~
550K
•
~ --41,'---~.
. a ¢ " f~,.... J t'"
_
I
39OK
" lk
"'1)" ""
I~Ou
50
0
"4---e
1/05
J 200
I 250
I 300
i 350
Bios (Vitro)
103Hz
_ 550K
161 50OK
tan
~ o
446K
,o
I Bias
(VlCm)
Fig. 2. (a) Variation of dielectric constant (e') with bias at different temperatures at 10 3 Hz. Dashed curve represents similar data for "r-irradiated CsBr crystals. (b) Variation of tan 6 with bias at different temperatures at 10 3 Hz.
140
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
10-1 I
o 305K • 337K A. x
Vol. 58, No. 2
/
380K /-,0 5 K
tar
103
104
10b
Frequency Hz
Fig. 3. Variation of loss tangent as a function of frequency at different temperatures of T-irradiated CsBr single crystals.
(iv) The decrease of tan 8 values observed between 10 ~ and 104Hz (Fig. 3) up to 380K may be due to the partial d.c. behaviour of the migrational charges on the surface of the crystal [9]. (Detailed understanding of the phenomenon is beyond the scope of the present paper.) (v) The rapid changes observed in e' and tan 8 values in the temperature region 4 0 0 - 4 7 0 K in 7-irradiated CsBr crystals may be attributed to the thermoluminescence process [8] occurring around 438 K. The effect of d.c. bias may be understood by the formation of barrier layers. It is known that the slowness of the electrode reaction gives rise to the formation of barrier layers at the crystal-electrode interface [10].
With the application of (sufficient) d.c. bias, the loosely bound charges are released and au~aent the existing barrier layers. The accumulation of charges thus increases the interfacial charge density and correspondingly the e' values. Due to the incomplete blocking nature of the electrodes, there are always a finite number of charges leaking through the barrier leading to the conduction process, affecting tan 8 values. Since the space charge density in virgin CsBr crystals is quite small at room temperature, measurable changes in e' and tan 8 could not be observed with bias. At elevated temperatures, the space charge density is larger (due to thermal generation of carriers) hence bias could bring in corresponding observable changes in e' and tan 6
Vol. 58, No. 2
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
.....
With
141
Bias ( 3 4 9 . 0 V i t r o )
18'
cy
/
/
I
I
)(5 2
I !
I
I i
%
IJ I
I
i
,8 3 _--
/
I
! _
I
tanfo
/I
/
J
16 4
,6sl 300
I
I
I
400
500
600
Temp*K
Fig. 4. Variation of tan 8 with temperature at lO s Hz for ~,-irradiated CsBr crystal. Dashed curve represents similar data for -/-irradiated CsBr crystal with bias (349 V cm -1).
values at lower frequencies. At lower d.c. field strengths, the released carrier may be trapped in the corresponding vacant lattice sites and hence a decrease in tan 8 (and e') is observed [5]. In y-irradiated CsBr single crystals, the initial concentration of defects (space charge) itself being
higher, the effect of bias seems to have been felt at a relatively low temperature. With the present data it may not be possible to explain the difference in the value of bias at which peak in e' and tan 8 is observed at any frequency and temperature.
142
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
Vol. 58, No. 2
12
~438 K 10 °~
e-
=>, 2 .13 i..
0
6
._c 4-,"
c-
4
r=-i
2
0 300
i
,
,
I
i
i
400
t
I 500
t
I
i
I
600
Temperature (K) Fig. 5. Glow curve for 3,-irradiated CsBr single crystal. REFERENCES 1. 2. 3. 4. 5. 6.
K.V. Rao, A. Subrahmanyam & A.K. Gupta, Ind. J. Pure and AppL Phy~ 17, 445 (1979). A. Subrahmanyam, K.V. Rao & H.N. Bose, Ind. J. Phy~ 53A, 30 (1979). M. Rapos & J.H. Calderwood, Proc. lEE 125, 162 (1978). A. Jonova & J.H. Calderwood, Acta. Phy~ Slov. 29, 4 (1979). A. Jonova, Acta Phy~ Slov. 29, 264 (1979). C. Ramasastry & Y. Syamasundara Rao, J. Phy~ C. Solid State Phy~ 13, 887 (1980).
7. 8. 9. 10. 11.
S.B.S. Sastry & S. Sapru, Phyz Status Solidi (b) 94, K149 (1979). A. Subrahmanyam & K.V.S. Badarinath, Phys. Status Solidi (a) 84, K93 (1984). A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, (1983). J.H. Beanmont & P.W.M. Jacobs, J. Phy~ Chem. Solids 28, 657 (1967). A. Subrahmanyam, Phys. Status Solidi (a) 69, 773 (1982).
0038-1098/86 $3.00 + .00 Pergamon Press Ltd.
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS - EFFECT OF d.c. BIAS K.V.S. Badarinath and A. Subrahmanyam Department of Physics, Indian Institute of Technology, Madras 600 036, India
(Received 4 November 1985 by R. Battle) The dielectric properties of CsBr single crystals, both virgin and T-irradiated have been measured as a function of frequency (10a--10 Hz), temperature (300-575 K) and d.c. bias ( 0 - 3 5 0 V cm -1). T-irradiation produces colour centres in these crystals and consequently an increase in space charge density is observed. D.c. bias has brought in considerable changes in dielectric constant (e') and loss (tanS) of these crystals particularly at low frequencies and high temperatures. An attempt has been made to understand these data qualitatively through barrier layer formation.
1. INTRODUCTION THE DIELECTRIC PROPERTIES of CsBr single crystals have been reported earlier along with the quantitative evaluation of polarisation mechanisms [1]. Dielectric dispersion is observed in these crystals at low frequencies (below lOSHz) and high temperatures (above 430K). It has been established that the space charge effects can be studied effectively by superposing a d.c. voltage during capacitance measurements [ 2 - 5 ] ; the present paper reports the dielectric data of CsBr single crystals both virgin and T-irradiated, under the influence of d.c. bias (0-350 Vcm-1). 2. EXPERIMENTAL METHODS CsBr single crystals are grown by the Kyropolous method using "Supra Pure" grade powder. The dimensions of the polished samples are about 1.0 x 0.5 x 0.1 cm a. A thin coating of Air Dry silver paint is applied on either side of the samples to serve as secondary electrodes. Samples are T-irradiated (0.4Megaradsh -1) for four hours. Dielectric measurements are taken on GR 1620 AP Capacitance Assembly (employing three electrode geometry) in the frequency range 10a--10 s Hz in the temperature region 3 0 0 - 5 7 5 K . The details of the sample holder are discussed elsewhere [6]. D.c. voltage is applied in series with the signal generator and bias is calculated. The accuracy in dielectric constant (e') value is 2% and in loss (tan 8), it is about 5%. Thermolumineseence glow peaks are recorded on a conventional set up [7] ; the rate ofheatingis maintained at 100 K min -x with an accuracy of + 2 K. 3. RESULTS The dielectric constant (e') of CsBr single crystals at 300K is measured to be 6.75 which is frequency
independent; loss tangent is of the order of 2 x 10 -4 at 10 a Hz. The variation of e' with temperature (Fig. I) and the calculated conductivity (o) are similar to the data reported earlier [ 1]. At room temperature (300K), bias has very little influence on e' and tan 8 values at any frequency. But as the temperature is increased, the effects are more pronounced, particularly at lower frequencies. It may be seen that at 10 a Hz a peak is exhibited in e' and tan 8 values at 165 and 7 5 V c m -1 respectively at 500K (Fig. 2(a) and (b)). The peak shifts towards lower field strengths with increasing temperature. Similar are the results at 104 Hz and 5 x 10 4 HZ but for the fact the peaks in e' and tan 8 are broadened (Figs. not presented). The dielectric constant (e') of 7-irradiated CsBr single crystals at 305K is measured to be 7.82 and it is frequency independent; loss tangent values are slightly higher and have exhibited a peak around 20 x I0 a Hz (Fig. 3). With increase in temperature, e' of T-irradiated crystals is found to increase up to 400 K. Above this temperature, rapid changes have been observed in e' which could not be followed up to 480K (due to manual balancing of the bridge). Beyond 480K, the variation in dielectric constant is similar to that of unirradiated crystal (Fig. not presented). The peak observed in loss tangent around 20 x I0 a Hz at 305 K shifts to higher frequencies with increasing temperature (Fig. 3). The variation of tan 8 with temperature, though rapid at low frequencies (similar to that of e'), could be measured at 10 s Hz (Fig. 4) in which a peak is observed at 425 K. D.c. bias is observed to bring in similar changes in dielectric constant values of T-irradiated CsBr single crystals as reported in virgin crystals. However, the effect of bias could be felt rather strongly at a relatively
137
138
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
.....
Vol. 58, No. 2
wrth bias ( 349.0 Vitro)
103Hz
13
/ I _
/
I
I
//
II
,4,03H
I0
/~/'
-
6 300
,'7
I 400
I 500 Tern p
~104Hz
/'~ 104Hz
I 600
k
Fig. 1. Variation of dielectric constant (e') with temperature at different frequencies (dashed curve represents similar data with d.c. bias (349 V cm-l)) of CsBr crystals. lower temperature as may be seen from the dashed curve (10 a Hz) in Fig. 2(a). The absolute values of tan 8 at lower frequencies and temperature are enhanced with bias not affecting the basic nature of the data as reported in Fig. 3 (Fig. not given). Similar are the results at higher frequencies (Fig. 4 - dashed curve). The thermoluminescence glow peak for -/-irradiated CsBr single crystals is observed around 438K (Fig. 5). F-centre absorption is recorded at 675 nm (with Cary14D Spectrophotometer) and the F-center concentration is calculated to be 8.1 x 1017/c.c. (Fig. not presented). 4. DISCUSSION The general ideas of space charge formation and behaviour in both virgin and 7-irradiated crystals have been discussed earlier in RbC1 single crystals [8] which may be applicable in the present investigation as well. The salient features of the observations in the present
investigation may be explained as follows: (i) The space charge concentration in virgin CsBr single crystals at 300 K is extremely small as indicated by the low dielectric loss (tan 8) (at low frequencies) and frequency independent e' values, (ii) With increase in temperature, space charge density increases due to thermal generation of carriers and exhibits itself above 400 K (Fig. 1). (iii) The increase in the frequency independent e' value of 1.07 at room temperature for the 7-irradiated CsBr crystals may be attributed to the eolour centres produced (8.1 x 1017]c.c.) [11]. Sharp peak in tan 6 values for these irradiated crystals at 305 K seem to indicate the formation of clusters of vacancies having dipolar nature. With increase in temperature these clusters may break into smaller clusters as may be seen by the shift (towards higher frequencies) in the peak position (Fig. 3) [11].
Vol. 58, No. 2
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
.....
139
If Irradiated
103Hz
IA
(~'J3
~
550K
•
~ --41,'---~.
. a ¢ " f~,.... J t'"
_
I
39OK
" lk
"'1)" ""
I~Ou
50
0
"4---e
1/05
J 200
I 250
I 300
i 350
Bios (Vitro)
103Hz
_ 550K
161 50OK
tan
~ o
446K
,o
I Bias
(VlCm)
Fig. 2. (a) Variation of dielectric constant (e') with bias at different temperatures at 10 3 Hz. Dashed curve represents similar data for "r-irradiated CsBr crystals. (b) Variation of tan 6 with bias at different temperatures at 10 3 Hz.
140
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
10-1 I
o 305K • 337K A. x
Vol. 58, No. 2
/
380K /-,0 5 K
tar
103
104
10b
Frequency Hz
Fig. 3. Variation of loss tangent as a function of frequency at different temperatures of T-irradiated CsBr single crystals.
(iv) The decrease of tan 8 values observed between 10 ~ and 104Hz (Fig. 3) up to 380K may be due to the partial d.c. behaviour of the migrational charges on the surface of the crystal [9]. (Detailed understanding of the phenomenon is beyond the scope of the present paper.) (v) The rapid changes observed in e' and tan 8 values in the temperature region 4 0 0 - 4 7 0 K in 7-irradiated CsBr crystals may be attributed to the thermoluminescence process [8] occurring around 438 K. The effect of d.c. bias may be understood by the formation of barrier layers. It is known that the slowness of the electrode reaction gives rise to the formation of barrier layers at the crystal-electrode interface [10].
With the application of (sufficient) d.c. bias, the loosely bound charges are released and au~aent the existing barrier layers. The accumulation of charges thus increases the interfacial charge density and correspondingly the e' values. Due to the incomplete blocking nature of the electrodes, there are always a finite number of charges leaking through the barrier leading to the conduction process, affecting tan 8 values. Since the space charge density in virgin CsBr crystals is quite small at room temperature, measurable changes in e' and tan 8 could not be observed with bias. At elevated temperatures, the space charge density is larger (due to thermal generation of carriers) hence bias could bring in corresponding observable changes in e' and tan 6
Vol. 58, No. 2
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
.....
With
141
Bias ( 3 4 9 . 0 V i t r o )
18'
cy
/
/
I
I
)(5 2
I !
I
I i
%
IJ I
I
i
,8 3 _--
/
I
! _
I
tanfo
/I
/
J
16 4
,6sl 300
I
I
I
400
500
600
Temp*K
Fig. 4. Variation of tan 8 with temperature at lO s Hz for ~,-irradiated CsBr crystal. Dashed curve represents similar data for -/-irradiated CsBr crystal with bias (349 V cm -1).
values at lower frequencies. At lower d.c. field strengths, the released carrier may be trapped in the corresponding vacant lattice sites and hence a decrease in tan 8 (and e') is observed [5]. In y-irradiated CsBr single crystals, the initial concentration of defects (space charge) itself being
higher, the effect of bias seems to have been felt at a relatively low temperature. With the present data it may not be possible to explain the difference in the value of bias at which peak in e' and tan 8 is observed at any frequency and temperature.
142
DIELECTRIC PROPERTIES OF CsBr SINGLE CRYSTALS
Vol. 58, No. 2
12
~438 K 10 °~
e-
=>, 2 .13 i..
0
6
._c 4-,"
c-
4
r=-i
2
0 300
i
,
,
I
i
i
400
t
I 500
t
I
i
I
600
Temperature (K) Fig. 5. Glow curve for 3,-irradiated CsBr single crystal. REFERENCES 1. 2. 3. 4. 5. 6.
K.V. Rao, A. Subrahmanyam & A.K. Gupta, Ind. J. Pure and AppL Phy~ 17, 445 (1979). A. Subrahmanyam, K.V. Rao & H.N. Bose, Ind. J. Phy~ 53A, 30 (1979). M. Rapos & J.H. Calderwood, Proc. lEE 125, 162 (1978). A. Jonova & J.H. Calderwood, Acta. Phy~ Slov. 29, 4 (1979). A. Jonova, Acta Phy~ Slov. 29, 264 (1979). C. Ramasastry & Y. Syamasundara Rao, J. Phy~ C. Solid State Phy~ 13, 887 (1980).
7. 8. 9. 10. 11.
S.B.S. Sastry & S. Sapru, Phyz Status Solidi (b) 94, K149 (1979). A. Subrahmanyam & K.V.S. Badarinath, Phys. Status Solidi (a) 84, K93 (1984). A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, (1983). J.H. Beanmont & P.W.M. Jacobs, J. Phy~ Chem. Solids 28, 657 (1967). A. Subrahmanyam, Phys. Status Solidi (a) 69, 773 (1982).