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Thermally stimulated conductivity induced in SrF2:Tb by u.v. light
Solid State Comrnunications,Vol. 9, pp. 1291—1293, 1971.
Pergamon Press.
Printed in Great Britain.
THERMALLY STIMULATED CONDUCTIVITY INDUCED IN SrF
2 :Tb BY u.v. LIGHT
V. Kirsh and N. Kristianpoller Dept. of Physics and Astronomy, Tel-Aviv University, Ramat Aviv, Tel-Aviv, Israel
(Received 13 May 1971 by W. Low)
Thermally-stimulated-conductivity was excited in SrF2 :Tb crystals by non-ionizing u.v. light. The electrical glow curves were studied in the range 80°—300°K,and thermal activation energies were computed by various methods. A composed glow peak, with maxima at 157° and 169°K, is attributed to the two stages of thermal decay of VK centers.
THE IRRADIATION damage in alkaline-earthfluorides, has recently been investigated by EPR, 2 optical luminescencethemethods.” In almostabsorption all these and investigations, samples were irradiated by ionizing radiation, such as X-rays, y- rays, or high energy particles. Ewanizky reported also on the generation of color centers by u.v. light in CaF 3 Thermoluminescence (TL) excited by u.v.2 :Sm(Y) light in pure and doped SrF 2 .crystals has recently been studied in this lab-
80% optical transmitance were used. The samples were irradiated at 80°Kfor 1—30 mm by monochromatic u.v. light of various wavelengths in
oratory.’
Several TSC glow peaks appeared during the warm up, between LNT and RT. The first peak v~ascomposed of two components, centered at 157°Kand 169°K. Another peak appeared at 245°k, and an additional one above room temperature, which was mostly covered by the rise of the ionic conductivity. In Fig. 1, a typical u.v. excited TSC glow curve of SrF2 :Tb is given
In the present work, the effects of non-ionizing u.v. radiation on SrF2 :Tb has been studied by means of electrical conductivity measurements. Thermally-stimulated-conductivity (TSC) glowcurves were recorded, and the activation energies of the various electrical glow peaks were evaluated.
the range 205—225 nm. The u.v. light was obtamed from a 50W deuterium lamp, and an f/3.5 grating monochromator. The irradiated samples were then heated at a constant rate of 20°/mm. After each run the crystals were annealed at about 600 °K.
The thermal activation energies of the 157°, For the experiments, single SrF2 crystals, doped with 0.03, 0.1 and 0.3 at. % terbium, were used. These crystals were grown by the crystalsgrowing laboratory at the Hebrew University 2 cross section,of and Jerusalem. Specimenswere of 1 amounted cm about 2 mm thickness in a vacuum
169°, and 245°Kglow peaks were computed by the ‘initial rise’ method. In order to separate the 169°Kpeak from it~lower temperature component, the samples were heated to about energy 165°K,of and recooled to 80°K. The activation this peak was computed from the glow curve obtained during a consequent heating. For the computation of the activation energies of the 169°Kglow peaks also additional methods, as developed by
cryostat. The electrical circuit consisted of a 300V d.c. battery, and a Keithley 410 microammeter. For electrodes, nickel mashes of 1291
1292
CONDUCTIVITY INDUCED IN SrF
2 : Tb BY u.v. LIGHT
Vol. 9, No. 15
Table 1. Activation energies (in eV) of TSC glow peaks in SrF2 :Tb, computed by various methods. Tm The temperature at the maximum of the glow peak (in °K). IR Activation energies computed by the ‘initial rise’ method. 5 HB Activation energies energies computed computed by by Chen’s the method of Halperin and Braner. CH Activation method.6 — — — —
—
‘Geometrical factor’. 5,6 Activation Energies (eV)
Tm(°K) 157° 169° 245°
IR 0.19 ±0.02 0.30 ±0.02 0.65 ±0.03
80
120
HB
CH
—
—
0.31 ±0.03 0.68 ±0.04
160
200 T (‘iK)
0.295 ±0.025 0.66 ±0.04
240
0.410 0.405
280
FIG. 1. TSC glow curve of SrF 2 doped with 0.3 at. % Tb, obtained by u.v. irradiation (A 5 and by Chen~ were applied. Halperin and Braner These methods take into account the temperature of the maximum, as well as the half width of the peak, and enable us to draw conclusions concerning the order of the kinetics. The computed activation energies are summarized in Table 1. A good agreement was found between the values of the activation energies, obtained by the various methods, assuming first order kinetics. This assumption has also been supported by the values of 0.410 — 0.405 obtained for the geometrical factor ,a~. (/iQ = 6/&i, where 8 is the half width towards the falloff of the peak, and w is the total half intensity width; in case of first-order kinetics, ~g is expected to be 0.42 and for second order = 0.52).
=
206nm; 15 mm; 80°K).
It has previously been found that X-irradiation at 80 °Kof alkaline-earth-fluorides doped with trivalent rare-earth ions, causes the reduction of the trivalent ions to a divalent state, and the creation of VK centers.”2 During the heating of the crystal, the VK centers become unstable, and the holes diffuse by hopping from site to site. In doped Sr~F 2 crystals, a linear motion of the VK centers predominates in the early stage of the thermal decay, while a random motion sets in at higher termperatures. Tzalmona and Pershan’ attributed a TL glow peak at 160°K(with a shoulder at 155°K)tothe radiative recombination of these VK centers, with electrons trapped at the sites of rare-earth ions. (The tempera2 lower may be due to ture reported by Beaumont et al.
Vol. 9, No. 15
CONDUCTIVITY INDUCED IN SrF
2 Tb BY u.v. LIGHT
the different rate.)other During the thermal decay of the heating VK centers, hole-centers,
1293
2 found by EPR methods for linear and 0.30 eV motions of the V~centers in doped SrF random 2.
such as V~and VH centers are formed. These centers were found to be stable up to higher temperatures. 1,2 In the present work it has been shown that TSC can be excited in SrF2 :Tb crystals with photons of about 6 eV, compared to a band gap of about 10 eV. The u.v. excited TSC peaks appeared about the temperatures TL peaks atobserved in same X-rayed doped SrF as the 2 by Tzalmona and Pershan’ It seems therefore that our electrical glow peaks at 157°and 169°K, like Tzalmona and Pershan’s 155° and 160°K luminescence glow peaks, are associated with the two stages of thermal decay of the V~ centers. Their 250°K TL peak has been attributed to the decay of ~ and VH centers. It appears that our 245°K TSC peak is associated with the same process. The thermal activation energies of 0.19 and 0.30 eV of our 157° and 169°KTSC peaks, fit the activation energies of 0.21 and
These findings indicate that 80°Kthe same type of hole centers was generated in doped SrF2 crystals by the non-ionizing u.v. lights, as by the ionizing X-radiation. The creation of VK centers by the non-ionizing radiation is probably connected with a transition of an electron from a F 3~),which ion to a nearby trivalent rare-earth ionof an corresponds to an excitation (Tb electron to an impurity level within the forbidden gap. In additional experiments presently carried out at our laboratory, photoconductivity induced in SrF2: Tb by non-ionizing u.v. light is also studied. In preliminary measurements, no photoconductivity could be detected at 80°Kbut a remarkable photoconductivity was obtained during the illumination of the crystals at temperatures above 200°K. Maximal efficiency was achieved by excitation with wavelengths of about 2lOnm.
REFERENCES 1.
TZALMONA A. and PERSHAN P.S., Phys. Rev. 182, 906 (1969).
2.
BEAUMONT J.H., HAYES W., KIRK D.L. and SUMMERS G.P., Proc. R. Soc., Lond.A.315, 69(1970).
3.
EWANIZKY T., Phys. Rev. 135, A221 (1964).
4.
KRISTIANPOLLER N. and GERSHENSON M. (to be published).
5.
HALPERIN A. and BRANER A.A., Phys. Rev. 117, 408 (1960).
6.
CHEN R., J. appi. Phys. 40, 570 (1969).
Thermisch stimulierte Leitfàhigkeit konnte in SrF2 :Tb Kristallen mit nicht-ionisierender u.v. Strahlung erregt werden. Die elektrischen Glowkurven wurden im Temperaturbereich 80°—300°Kuntersucht und die Aktivierungsenergien wurden nach verschiedenen Methoden berechnet. Em doppelter Glowpeak bei 157°Kund 169°Kwird den zwei Stufen des thermischen Verfalls von V~Zentern zugeschrieben.
Pergamon Press.
Printed in Great Britain.
THERMALLY STIMULATED CONDUCTIVITY INDUCED IN SrF
2 :Tb BY u.v. LIGHT
V. Kirsh and N. Kristianpoller Dept. of Physics and Astronomy, Tel-Aviv University, Ramat Aviv, Tel-Aviv, Israel
(Received 13 May 1971 by W. Low)
Thermally-stimulated-conductivity was excited in SrF2 :Tb crystals by non-ionizing u.v. light. The electrical glow curves were studied in the range 80°—300°K,and thermal activation energies were computed by various methods. A composed glow peak, with maxima at 157° and 169°K, is attributed to the two stages of thermal decay of VK centers.
THE IRRADIATION damage in alkaline-earthfluorides, has recently been investigated by EPR, 2 optical luminescencethemethods.” In almostabsorption all these and investigations, samples were irradiated by ionizing radiation, such as X-rays, y- rays, or high energy particles. Ewanizky reported also on the generation of color centers by u.v. light in CaF 3 Thermoluminescence (TL) excited by u.v.2 :Sm(Y) light in pure and doped SrF 2 .crystals has recently been studied in this lab-
80% optical transmitance were used. The samples were irradiated at 80°Kfor 1—30 mm by monochromatic u.v. light of various wavelengths in
oratory.’
Several TSC glow peaks appeared during the warm up, between LNT and RT. The first peak v~ascomposed of two components, centered at 157°Kand 169°K. Another peak appeared at 245°k, and an additional one above room temperature, which was mostly covered by the rise of the ionic conductivity. In Fig. 1, a typical u.v. excited TSC glow curve of SrF2 :Tb is given
In the present work, the effects of non-ionizing u.v. radiation on SrF2 :Tb has been studied by means of electrical conductivity measurements. Thermally-stimulated-conductivity (TSC) glowcurves were recorded, and the activation energies of the various electrical glow peaks were evaluated.
the range 205—225 nm. The u.v. light was obtamed from a 50W deuterium lamp, and an f/3.5 grating monochromator. The irradiated samples were then heated at a constant rate of 20°/mm. After each run the crystals were annealed at about 600 °K.
The thermal activation energies of the 157°, For the experiments, single SrF2 crystals, doped with 0.03, 0.1 and 0.3 at. % terbium, were used. These crystals were grown by the crystalsgrowing laboratory at the Hebrew University 2 cross section,of and Jerusalem. Specimenswere of 1 amounted cm about 2 mm thickness in a vacuum
169°, and 245°Kglow peaks were computed by the ‘initial rise’ method. In order to separate the 169°Kpeak from it~lower temperature component, the samples were heated to about energy 165°K,of and recooled to 80°K. The activation this peak was computed from the glow curve obtained during a consequent heating. For the computation of the activation energies of the 169°Kglow peaks also additional methods, as developed by
cryostat. The electrical circuit consisted of a 300V d.c. battery, and a Keithley 410 microammeter. For electrodes, nickel mashes of 1291
1292
CONDUCTIVITY INDUCED IN SrF
2 : Tb BY u.v. LIGHT
Vol. 9, No. 15
Table 1. Activation energies (in eV) of TSC glow peaks in SrF2 :Tb, computed by various methods. Tm The temperature at the maximum of the glow peak (in °K). IR Activation energies computed by the ‘initial rise’ method. 5 HB Activation energies energies computed computed by by Chen’s the method of Halperin and Braner. CH Activation method.6 — — — —
—
‘Geometrical factor’. 5,6 Activation Energies (eV)
Tm(°K) 157° 169° 245°
IR 0.19 ±0.02 0.30 ±0.02 0.65 ±0.03
80
120
HB
CH
—
—
0.31 ±0.03 0.68 ±0.04
160
200 T (‘iK)
0.295 ±0.025 0.66 ±0.04
240
0.410 0.405
280
FIG. 1. TSC glow curve of SrF 2 doped with 0.3 at. % Tb, obtained by u.v. irradiation (A 5 and by Chen~ were applied. Halperin and Braner These methods take into account the temperature of the maximum, as well as the half width of the peak, and enable us to draw conclusions concerning the order of the kinetics. The computed activation energies are summarized in Table 1. A good agreement was found between the values of the activation energies, obtained by the various methods, assuming first order kinetics. This assumption has also been supported by the values of 0.410 — 0.405 obtained for the geometrical factor ,a~. (/iQ = 6/&i, where 8 is the half width towards the falloff of the peak, and w is the total half intensity width; in case of first-order kinetics, ~g is expected to be 0.42 and for second order = 0.52).
=
206nm; 15 mm; 80°K).
It has previously been found that X-irradiation at 80 °Kof alkaline-earth-fluorides doped with trivalent rare-earth ions, causes the reduction of the trivalent ions to a divalent state, and the creation of VK centers.”2 During the heating of the crystal, the VK centers become unstable, and the holes diffuse by hopping from site to site. In doped Sr~F 2 crystals, a linear motion of the VK centers predominates in the early stage of the thermal decay, while a random motion sets in at higher termperatures. Tzalmona and Pershan’ attributed a TL glow peak at 160°K(with a shoulder at 155°K)tothe radiative recombination of these VK centers, with electrons trapped at the sites of rare-earth ions. (The tempera2 lower may be due to ture reported by Beaumont et al.
Vol. 9, No. 15
CONDUCTIVITY INDUCED IN SrF
2 Tb BY u.v. LIGHT
the different rate.)other During the thermal decay of the heating VK centers, hole-centers,
1293
2 found by EPR methods for linear and 0.30 eV motions of the V~centers in doped SrF random 2.
such as V~and VH centers are formed. These centers were found to be stable up to higher temperatures. 1,2 In the present work it has been shown that TSC can be excited in SrF2 :Tb crystals with photons of about 6 eV, compared to a band gap of about 10 eV. The u.v. excited TSC peaks appeared about the temperatures TL peaks atobserved in same X-rayed doped SrF as the 2 by Tzalmona and Pershan’ It seems therefore that our electrical glow peaks at 157°and 169°K, like Tzalmona and Pershan’s 155° and 160°K luminescence glow peaks, are associated with the two stages of thermal decay of the V~ centers. Their 250°K TL peak has been attributed to the decay of ~ and VH centers. It appears that our 245°K TSC peak is associated with the same process. The thermal activation energies of 0.19 and 0.30 eV of our 157° and 169°KTSC peaks, fit the activation energies of 0.21 and
These findings indicate that 80°Kthe same type of hole centers was generated in doped SrF2 crystals by the non-ionizing u.v. lights, as by the ionizing X-radiation. The creation of VK centers by the non-ionizing radiation is probably connected with a transition of an electron from a F 3~),which ion to a nearby trivalent rare-earth ionof an corresponds to an excitation (Tb electron to an impurity level within the forbidden gap. In additional experiments presently carried out at our laboratory, photoconductivity induced in SrF2: Tb by non-ionizing u.v. light is also studied. In preliminary measurements, no photoconductivity could be detected at 80°Kbut a remarkable photoconductivity was obtained during the illumination of the crystals at temperatures above 200°K. Maximal efficiency was achieved by excitation with wavelengths of about 2lOnm.
REFERENCES 1.
TZALMONA A. and PERSHAN P.S., Phys. Rev. 182, 906 (1969).
2.
BEAUMONT J.H., HAYES W., KIRK D.L. and SUMMERS G.P., Proc. R. Soc., Lond.A.315, 69(1970).
3.
EWANIZKY T., Phys. Rev. 135, A221 (1964).
4.
KRISTIANPOLLER N. and GERSHENSON M. (to be published).
5.
HALPERIN A. and BRANER A.A., Phys. Rev. 117, 408 (1960).
6.
CHEN R., J. appi. Phys. 40, 570 (1969).
Thermisch stimulierte Leitfàhigkeit konnte in SrF2 :Tb Kristallen mit nicht-ionisierender u.v. Strahlung erregt werden. Die elektrischen Glowkurven wurden im Temperaturbereich 80°—300°Kuntersucht und die Aktivierungsenergien wurden nach verschiedenen Methoden berechnet. Em doppelter Glowpeak bei 157°Kund 169°Kwird den zwei Stufen des thermischen Verfalls von V~Zentern zugeschrieben.