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Photoconductivity in γ-irradiated alkali halides
Volume 115, number 2
PHOTOCONDUCTIVITY H.J. ARNIKAR,, Depnnmenr
29 March 1985
CHEMICAL PHYSICS LETTERS
IN y-lRRADL4TED
Leela BAPAT,
of Chemurty,
Apama
ALKALI
V. DESHPANDE
HALIDES ’ and S.S. SARDESAI
Umuersrty of Poonn. Poonn, Zndrn
Recewed 19 September 1984; LIIfinal form 2 January 1985
y-n-ratiated alkah hahdes undergo bleaching under pxionged hght ~llummauon, hence the photocurrent decreases with tnne. We stud& the kmeucs of phomcurrenr decay of y-uxtdiated a&ah hahdcs under steady-slate tiummauon, and the spectral response of the photocurrent f,r various alkah hahdes From Lhedata on the vanation of the photocurrent W&I apphed electnc field and mtenslty of F light employed for Illumination. the value of the product of quantcm efficiency and mean electron range per unit field IScalculated at room temperature
1. Introduction
for KCl, NaCl, KBr and Ki. The results m the present study are compared with the results of other authors.
Alkali halides, when -y-irradiated, become intensely coloured owing to the formation of various colour centres [l-5]. There have been various measurements of the photocurrent in KCZ and KRr crystals that are additively coloured, mostly at low temperatures [6-g]_ There has been little work on the photoconductivity of y-irradiated alkali halides at room temperature_ ru-radiated alkali halides undergo bleaching under light illumination and the decay curves can be described accurately by an exponential term [lO,ll]. As y-irra&ated alkali hahdes and additively coloured alkali halides show differences m theu photobleaching properties, differences in photoconductivity are expected. only the photoconductlvity of adltively coloured KC1 and KBr cvstals has been studied, not that of the other alkali halides. Hence an attempt is made to investigate the photocurrent properties of r-irradiated alkali halides. The variation of photocurrent with intensity of illuminating light, dose and applied electric field is studied. From the variation of photocurrent with applied electric field, which is found to be linear, the product of the quantum efficiency (v) and the mean electron range per unit field (~0) is obtained
2. Experimental Single crystals of NaCl, KCI, KBr and KI (Harshaw Chemical Company) were used in these experiments. Thin slices of approximately 2.0 mm thick and 2 cm length and 2 cm breadth were cleaved from a large single crystal and were irradiated by a 2 kCi 6OCo source. Immediately after Irra&atlon, the absorption spectrum is recorded and subsequently the crystals were subjected to photoconductivity measurements. The photocurrent was measured on a digital picoammeter by using plane parallel electrodes. After trying electrodes of various metals, stainless-steel electrodes were chosen as they were found to give good noncorrosive contacts and constant, fiim pressure. The electrode system was enclosed in a copper box which was earthed to shield the system from any stray electromagnetic pick-up. A small window of approximately 2.5 X 2.5 cm2 was provided in the copper box through which the crystal could be illuminated by light of the desired wavelength. The incident beam was perpendicular to the electric field. The conductivity was measured prior to r-irradiation, immediately after r-irradiation in the dark and then the c.rystal was illuminated by the desired light. After each experiment,
’ Present address. Department of Chermcal Technology, Umverslty of Bombay. Matunga. Jnla
0 0092614/85/$03.30 0 Elsevier (North-Holland Physics Pubhshing
Science Publishers Divlslon)
B.V_
187
CHEMICAL PHYSICS LETTERS
Volume 115, number 2 the crystal was carefully 500°C. The same sample sets of experiments after the irutial reading under
3.2. Specrrul response of rhe photocurrent
annealed at a temperature of crystal was used for other ensuring reproduc5bLit.y of similar conditions.
3. Results 3.1. Time dependence of the photocurrent The magnitude of the current for a fresh, unirradiated crystal is about IO-l4 A when a voltage of the order of 2 V is applied. When the crystal IS r-irradiated, there is no appreciable rise in the conductivity III the dark, but when it is exposed to F light (absorbed by F centres), the current increases by a factor of about 1000. The nse time of the current IS very small compared to the response time of the electrometer, and hence can be considered as mstantaneous. Under steady illumination the photocurrent decays, initially at a fast rate and then slowly until it reaches the pre-irradiation value. The plot of log(photocurrent) versus time is shown in fig. 1 for KCI. The plots for other alkah halides are similar.
Time Fig. 1. De-y 188
29 March 1985
For a fixed dose of +y-irralation and a fiied voltage and for a fixed intensity of illuminating light for all wavelengths, the initial photocurrent J,-, (corresponds to time t = 0) falls on both sides of the F peak. The spectral response of the photocurrent in KQ is as shown, together with the absorption spectrum of the crystal, in fig. 2. Other alkali hahdes show similar behaviour. It IS seen that both curves have the same peak and other characteristics indicating that the source of photoelectrons is the F centres. 3.3. The effect of dose of y-irradiation on photocuwent As the dose of y-irradiation is increased, the number of F centres produced in the crystal also increases unti a saturation value is reached, as seen in fig. 3. Fig. 3 gives a plot of absorbance at peak versus dose absorbed by the crystal. For a fiied field applied across the crystal and a tiEed intensity of F light, the initial photocurrent Jo also increased m a way sumlar to the absorbance until saturation, for all alkali halides studied. This curve is also plotted in fig. 3 for KCl. The photocurrent was found to obey a linear relationship with intensity of F light and apphed electnc
IS of photocurrent.
CHEMICAL
Volume 115. number 2
29 March 1985
PHYSICS LETTERS
KCI - 0 12
12-
-0
400
1
450
1
I
500
I
I
550
I
I
600
I
08
$’ c 8 b 2 Q
I
‘130
650
h /nm Fig. 2. Spectral response of photocurrent and absorbance.
I
#Cl
Dose
-1
I krad
Fig. 3. Vanatlon of photocurrent and absorbance with dose.
189
Volume
115, number 2
field ranging from
CHF%UCAL
10 to 1000
that there is no electron
V cm-l.
PHYSICS
This indicates in the crystal
multiplication
over the range studied.
4. Discussion From fig. 1, it is clear that -@-radiated alkali halides show a decay of photocurrent with time of exposure to hght, while the photocurrent is substantially constant in additively coloured alkali halides. The curve in fig. 1 can be resolved into two exponential components and the photocurrent value at time t = 0 is used to obtam the product nwo [12]. Comparison of absorption and photocurrent spectra shows that F centres are responsible for the photocurrent in r-irradiated alkali halides, as in additively coloured crystals. This is also confirmed by the doscvariation studies as seen in fig_ 3. The values of qwo are calculated from the slopes of the cilrves of photocurrent versus voltage for various alkali halides. They are given in table 1_ Jenkin et al. [9] have studied the photoconductivity of additively coloured crystals of KC1 and KBr at 4 K by exciting F centres in L bands. They have studied the trappmg time of electrons. The reported values of distance travelled by the electron before it gets trapped are of the order of lOA to 10m5 cm for F centre concentrations between 1015 and 10’7 cm-3. The concentration of F centres m the present study is of the order of 1Ol6 cmw3 immediately after y-irradiation. Wild and Brown [6] reported values of wo of the order of 10m6 cm2 V-l in additively coloured KC1 at 8.5 K for an F centre concentration of 1Ol5 cm-s. Nakazawa et al. [7] reported wo a 4 X 10-8 f-x a 1Ol6 cmm3 F centre concentration in additively coloured KC1 at 4 K. They also found that pF = 1 above 120 K. If this is correct, the values of qwo and w. would be same at room temperature.
At low temperature it 1s known that the values of the distance travelled by an eIectron before it gets trapped in KC1 and KBr are of the same order [9]. In the presenr study, the values of Two for all the potassium halide crystals studied are of the same order. Some authors [ 131 have studied photoconductivity and Hall mobility of additively coloured KBr at moderate temperatures and have reported that the photocurrent is lmear with applied field at 20°C. Our results also show a linear relationship between the photocurrent and applied field over the field range studied for ah alkali halides. They have also found that, as the temperature 1s increased from 20°C to 8O”C, the slope of the photocurrent versus voltage graph increases, i.e. there is an increase in qwo wrth temperature. The values of qw,-, in the present work at 300 K are higher than the values at low temperatures. Reported values of photocurrent are also comparable to, those in the present study. Critsenko et al. [ 141 reported the decay of photocurrent of electrolyticahy coloured NaCl crystals containing aggregate centres, with results similar to the decay of photocurrent found in the present work. The values of qwo 111the present study of potassrum halides are of the same order.whereas the value for NaCl 1s small compared to other halrdes. This may be due to a lower senstuvity of NaCl crystals to hght; m NaCl the traps are deeper compared to potassium halides.
References H-T. Am&ax.
P.S. Dan&,
B-D. Chaure and B-KM_
Naturwssensch. 57 (1970) [2] H.J. Amikar, CD. *and
Table 1 crystal
wo(an2
KQ KBr KI Nail
7.154 4.807 1.196
1.360
V--l) x 1O-3 x 10-3 x 10-S x 1O-s
1985
There are some (rather old) reports on photoconductivity of additively coloured KC1 at room temperature. The reported values of wo at low temperatures cannot be compared duectIy with the values in the present study for 3L’O K. However one can compare the trend of results.
[l]
190
29 March
LETTERS
[3] [4]
Chem. 14A (1976) 1009. HJ. Amkx, A.V. Khedekar Phys. 74 (1977) 19.
S.S. Sardesai, and A.S.
Banejl,
A.V. Khedekar, R. Ramam, S.S. Sardeti Nud. Instr. Methods 184 (1981) 577.
[S] L. Bapat, I. Radioanal. 273.
Rao.
541.
NucL
Chem.
Indian J. J. Chim.
and C D. Kalkar
Letters
85 (1984)
Volume [6] [7]
115, number
2
CHEMICAL
R.L. Wdd and F.C. Brown, Phys. Rev. 121 (1961) 1X6 F. Nakazawa and H. Kanzak~. J. Phys. Sot. Japan 22 (1967) 844. 18 J E.N. Bereznyakovsti and Y-1. Gntsenko. Soviet Phys. Sobd State 17 (1975) 3653. [9] C T. Jenkm. D.W. Staccy, J G. Crowder and J-W. Hodby, J. Phys. Cl1 (1978) 1841. [IO] HJ. Anukar, B.S.M. Rao, M.A. ClJare and S.S. Sardea, J. Glum. Phys. 72 (197.5) 654.
PHYSICS
LETTERS
29 March
1985
[ 111 A.V. Deshpande, M.R. Chandratike, 1. Hamblett, G.W.A. Newton, S.F. Patil and VJ. Robmson, J. Chem. Sot Faraday Trans II 77 (1981) 135. [ 121 R.S. wn Heynmgen and F.C. Brown, Phys. Rev. 111 (1958) 462. [13] IS. At-Saffar, J.H. Calderwood and K C. Kao, 1. Phys. Chem. SoUs 36 (1975) 213. [14]
Y.I. Gritsenko and E.V. Pisareva, Soviet Phys state 19 (1977) 681.
Sohd
191
PHOTOCONDUCTIVITY H.J. ARNIKAR,, Depnnmenr
29 March 1985
CHEMICAL PHYSICS LETTERS
IN y-lRRADL4TED
Leela BAPAT,
of Chemurty,
Apama
ALKALI
V. DESHPANDE
HALIDES ’ and S.S. SARDESAI
Umuersrty of Poonn. Poonn, Zndrn
Recewed 19 September 1984; LIIfinal form 2 January 1985
y-n-ratiated alkah hahdes undergo bleaching under pxionged hght ~llummauon, hence the photocurrent decreases with tnne. We stud& the kmeucs of phomcurrenr decay of y-uxtdiated a&ah hahdcs under steady-slate tiummauon, and the spectral response of the photocurrent f,r various alkah hahdes From Lhedata on the vanation of the photocurrent W&I apphed electnc field and mtenslty of F light employed for Illumination. the value of the product of quantcm efficiency and mean electron range per unit field IScalculated at room temperature
1. Introduction
for KCl, NaCl, KBr and Ki. The results m the present study are compared with the results of other authors.
Alkali halides, when -y-irradiated, become intensely coloured owing to the formation of various colour centres [l-5]. There have been various measurements of the photocurrent in KCZ and KRr crystals that are additively coloured, mostly at low temperatures [6-g]_ There has been little work on the photoconductivity of y-irradiated alkali halides at room temperature_ ru-radiated alkali halides undergo bleaching under light illumination and the decay curves can be described accurately by an exponential term [lO,ll]. As y-irra&ated alkali hahdes and additively coloured alkali halides show differences m theu photobleaching properties, differences in photoconductivity are expected. only the photoconductlvity of adltively coloured KC1 and KBr cvstals has been studied, not that of the other alkali halides. Hence an attempt is made to investigate the photocurrent properties of r-irradiated alkali halides. The variation of photocurrent with intensity of illuminating light, dose and applied electric field is studied. From the variation of photocurrent with applied electric field, which is found to be linear, the product of the quantum efficiency (v) and the mean electron range per unit field (~0) is obtained
2. Experimental Single crystals of NaCl, KCI, KBr and KI (Harshaw Chemical Company) were used in these experiments. Thin slices of approximately 2.0 mm thick and 2 cm length and 2 cm breadth were cleaved from a large single crystal and were irradiated by a 2 kCi 6OCo source. Immediately after Irra&atlon, the absorption spectrum is recorded and subsequently the crystals were subjected to photoconductivity measurements. The photocurrent was measured on a digital picoammeter by using plane parallel electrodes. After trying electrodes of various metals, stainless-steel electrodes were chosen as they were found to give good noncorrosive contacts and constant, fiim pressure. The electrode system was enclosed in a copper box which was earthed to shield the system from any stray electromagnetic pick-up. A small window of approximately 2.5 X 2.5 cm2 was provided in the copper box through which the crystal could be illuminated by light of the desired wavelength. The incident beam was perpendicular to the electric field. The conductivity was measured prior to r-irradiation, immediately after r-irradiation in the dark and then the c.rystal was illuminated by the desired light. After each experiment,
’ Present address. Department of Chermcal Technology, Umverslty of Bombay. Matunga. Jnla
0 0092614/85/$03.30 0 Elsevier (North-Holland Physics Pubhshing
Science Publishers Divlslon)
B.V_
187
CHEMICAL PHYSICS LETTERS
Volume 115, number 2 the crystal was carefully 500°C. The same sample sets of experiments after the irutial reading under
3.2. Specrrul response of rhe photocurrent
annealed at a temperature of crystal was used for other ensuring reproduc5bLit.y of similar conditions.
3. Results 3.1. Time dependence of the photocurrent The magnitude of the current for a fresh, unirradiated crystal is about IO-l4 A when a voltage of the order of 2 V is applied. When the crystal IS r-irradiated, there is no appreciable rise in the conductivity III the dark, but when it is exposed to F light (absorbed by F centres), the current increases by a factor of about 1000. The nse time of the current IS very small compared to the response time of the electrometer, and hence can be considered as mstantaneous. Under steady illumination the photocurrent decays, initially at a fast rate and then slowly until it reaches the pre-irradiation value. The plot of log(photocurrent) versus time is shown in fig. 1 for KCI. The plots for other alkah halides are similar.
Time Fig. 1. De-y 188
29 March 1985
For a fixed dose of +y-irralation and a fiied voltage and for a fixed intensity of illuminating light for all wavelengths, the initial photocurrent J,-, (corresponds to time t = 0) falls on both sides of the F peak. The spectral response of the photocurrent in KQ is as shown, together with the absorption spectrum of the crystal, in fig. 2. Other alkali hahdes show similar behaviour. It IS seen that both curves have the same peak and other characteristics indicating that the source of photoelectrons is the F centres. 3.3. The effect of dose of y-irradiation on photocuwent As the dose of y-irradiation is increased, the number of F centres produced in the crystal also increases unti a saturation value is reached, as seen in fig. 3. Fig. 3 gives a plot of absorbance at peak versus dose absorbed by the crystal. For a fiied field applied across the crystal and a tiEed intensity of F light, the initial photocurrent Jo also increased m a way sumlar to the absorbance until saturation, for all alkali halides studied. This curve is also plotted in fig. 3 for KCl. The photocurrent was found to obey a linear relationship with intensity of F light and apphed electnc
IS of photocurrent.
CHEMICAL
Volume 115. number 2
29 March 1985
PHYSICS LETTERS
KCI - 0 12
12-
-0
400
1
450
1
I
500
I
I
550
I
I
600
I
08
$’ c 8 b 2 Q
I
‘130
650
h /nm Fig. 2. Spectral response of photocurrent and absorbance.
I
#Cl
Dose
-1
I krad
Fig. 3. Vanatlon of photocurrent and absorbance with dose.
189
Volume
115, number 2
field ranging from
CHF%UCAL
10 to 1000
that there is no electron
V cm-l.
PHYSICS
This indicates in the crystal
multiplication
over the range studied.
4. Discussion From fig. 1, it is clear that -@-radiated alkali halides show a decay of photocurrent with time of exposure to hght, while the photocurrent is substantially constant in additively coloured alkali halides. The curve in fig. 1 can be resolved into two exponential components and the photocurrent value at time t = 0 is used to obtam the product nwo [12]. Comparison of absorption and photocurrent spectra shows that F centres are responsible for the photocurrent in r-irradiated alkali halides, as in additively coloured crystals. This is also confirmed by the doscvariation studies as seen in fig_ 3. The values of qwo are calculated from the slopes of the cilrves of photocurrent versus voltage for various alkali halides. They are given in table 1_ Jenkin et al. [9] have studied the photoconductivity of additively coloured crystals of KC1 and KBr at 4 K by exciting F centres in L bands. They have studied the trappmg time of electrons. The reported values of distance travelled by the electron before it gets trapped are of the order of lOA to 10m5 cm for F centre concentrations between 1015 and 10’7 cm-3. The concentration of F centres m the present study is of the order of 1Ol6 cmw3 immediately after y-irradiation. Wild and Brown [6] reported values of wo of the order of 10m6 cm2 V-l in additively coloured KC1 at 8.5 K for an F centre concentration of 1Ol5 cm-s. Nakazawa et al. [7] reported wo a 4 X 10-8 f-x a 1Ol6 cmm3 F centre concentration in additively coloured KC1 at 4 K. They also found that pF = 1 above 120 K. If this is correct, the values of qwo and w. would be same at room temperature.
At low temperature it 1s known that the values of the distance travelled by an eIectron before it gets trapped in KC1 and KBr are of the same order [9]. In the presenr study, the values of Two for all the potassium halide crystals studied are of the same order. Some authors [ 131 have studied photoconductivity and Hall mobility of additively coloured KBr at moderate temperatures and have reported that the photocurrent is lmear with applied field at 20°C. Our results also show a linear relationship between the photocurrent and applied field over the field range studied for ah alkali halides. They have also found that, as the temperature 1s increased from 20°C to 8O”C, the slope of the photocurrent versus voltage graph increases, i.e. there is an increase in qwo wrth temperature. The values of qw,-, in the present work at 300 K are higher than the values at low temperatures. Reported values of photocurrent are also comparable to, those in the present study. Critsenko et al. [ 141 reported the decay of photocurrent of electrolyticahy coloured NaCl crystals containing aggregate centres, with results similar to the decay of photocurrent found in the present work. The values of qwo 111the present study of potassrum halides are of the same order.whereas the value for NaCl 1s small compared to other halrdes. This may be due to a lower senstuvity of NaCl crystals to hght; m NaCl the traps are deeper compared to potassium halides.
References H-T. Am&ax.
P.S. Dan&,
B-D. Chaure and B-KM_
Naturwssensch. 57 (1970) [2] H.J. Amikar, CD. *and
Table 1 crystal
wo(an2
KQ KBr KI Nail
7.154 4.807 1.196
1.360
V--l) x 1O-3 x 10-3 x 10-S x 1O-s
1985
There are some (rather old) reports on photoconductivity of additively coloured KC1 at room temperature. The reported values of wo at low temperatures cannot be compared duectIy with the values in the present study for 3L’O K. However one can compare the trend of results.
[l]
190
29 March
LETTERS
[3] [4]
Chem. 14A (1976) 1009. HJ. Amkx, A.V. Khedekar Phys. 74 (1977) 19.
S.S. Sardesai, and A.S.
Banejl,
A.V. Khedekar, R. Ramam, S.S. Sardeti Nud. Instr. Methods 184 (1981) 577.
[S] L. Bapat, I. Radioanal. 273.
Rao.
541.
NucL
Chem.
Indian J. J. Chim.
and C D. Kalkar
Letters
85 (1984)
Volume [6] [7]
115, number
2
CHEMICAL
R.L. Wdd and F.C. Brown, Phys. Rev. 121 (1961) 1X6 F. Nakazawa and H. Kanzak~. J. Phys. Sot. Japan 22 (1967) 844. 18 J E.N. Bereznyakovsti and Y-1. Gntsenko. Soviet Phys. Sobd State 17 (1975) 3653. [9] C T. Jenkm. D.W. Staccy, J G. Crowder and J-W. Hodby, J. Phys. Cl1 (1978) 1841. [IO] HJ. Anukar, B.S.M. Rao, M.A. ClJare and S.S. Sardea, J. Glum. Phys. 72 (197.5) 654.
PHYSICS
LETTERS
29 March
1985
[ 111 A.V. Deshpande, M.R. Chandratike, 1. Hamblett, G.W.A. Newton, S.F. Patil and VJ. Robmson, J. Chem. Sot Faraday Trans II 77 (1981) 135. [ 121 R.S. wn Heynmgen and F.C. Brown, Phys. Rev. 111 (1958) 462. [13] IS. At-Saffar, J.H. Calderwood and K C. Kao, 1. Phys. Chem. SoUs 36 (1975) 213. [14]
Y.I. Gritsenko and E.V. Pisareva, Soviet Phys state 19 (1977) 681.
Sohd
191