Dielectric Effects in Proton-Irradiated KBr Crystals
Abstract Generated abstract
This study examines radiation-induced dielectric absorption in KBr crystals irradiated with 4.5 MeV protons, using temperature and frequency measurements of dielectric loss and electrical conductivity between 20 and 200°C. Irradiation produces a dielectric-loss maximum near 140 to 170°C, while conductivity measurements and thermal bleaching of color centers indicate that the effect is not due to ionic conductivity changes or destruction of F and V2 centers. The frequency dependence of the loss peak is interpreted qualitatively as resonance absorption by low-frequency, electrically active complex ionic formations created by irradiation. Experiments with strontium-doped KBr suggest that localized holes participate in these formations, probably as complex vacancy aggregates capable of trapping several holes.
Full Text
PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV
Vol. 139 1965
DIELECTRIC EFFECTS IN PROTON-IRRADIATED KBr CRYSTALS
Yu. M. ANNENKOV
(Presented by the scientific seminar of the Laboratory of Dielectrics and Semiconductors)
The present level of development of the radiation physics of alkali-halide crystals has been achieved mainly thanks to methods of high-frequency spectroscopy and luminescence analysis. At the same time, experimenters judge a wide range of ionic phenomena indirectly.
The method of studying the electrical properties of crystals gives direct information on the development of ionic processes. However, in the field of studying radiation-induced changes in the electrical properties of crystals there are only isolated works [1, 2, 3].
The energy of radiation is stored in the crystal not only in the formation of color centers, but also in the generation of more complex disturbances, which may manifest themselves in dielectric absorption.
In studying the influence of ionizing types of radiation on dielectric losses in alkali-halide crystals, we recorded one intense maximum in the temperature dependences of $\operatorname{tg}\delta$ for KBr and KJ. The purpose of the present work is, using KBr crystals as an example, to study in greater detail the nature of the radiation defects responsible for the observed dielectric absorption.
The temperature-frequency dependences of $\operatorname{tg}\delta$ in the audio-frequency range and the electrical conductivity in the temperature interval $(20—200)^\circ\mathrm{C}$ were investigated.
Irradiation was carried out with protons of energy 4.5 MeV from two sides of the specimen, the thickness of the crystal being chosen so as to ensure uniform coloration throughout the volume. The integral dose was varied from $1\cdot10^{14}$ protons/cm$^2$ to $7\cdot10^{15}$ protons/cm$^2$.
Typical results for the influence of proton bombardment on the electrical properties of KBr crystals are shown in Fig. 1. Irradiation lowers the dielectric losses below 120°C and causes the appearance of a maximum of $\operatorname{tg}\delta$ in the temperature interval $(140—170)^\circ\mathrm{C}$. Increasing the irradiation dose leads to an increase in $\operatorname{tg}\delta$ and to some shift of its position into the high-temperature region.
The most interesting information about the type of dielectric absorption can be obtained by considering the temperature-frequency dependences of $\operatorname{tg}\delta$.
Figure 1 presents the dependences of the effect of radiation-induced change in $\operatorname{tg}\delta$ on temperature at several frequencies. With increasing
as the frequency of the electric field is increased, the position of the maximum shifts somewhat toward higher temperatures, while its amplitude decreases.
Fig. 1. Variation of \(\operatorname{tg}\delta\) with temperature at a frequency of 50 Hz for unirradiated KBr crystals (2) and crystals irradiated with protons (1). Dependence of the radiation effect of the change in \(\operatorname{tg}\delta\) on temperature at different frequencies: 3—50 Hz, 4—100 Hz, 5—1000 Hz, 6—5000 Hz. The irradiation dose was \(1 \cdot 10^{15}\) protons/cm\(^2\).
Fig. 2. Dependence of the logarithm of the electrical conductivity on temperature for KBr.
1 — unirradiated crystals, 2 — crystals irradiated with a dose of \(1 \cdot 10^{15}\) protons/cm\(^2\).
The electrical conductivity of the crystals studied after irradiation decreases in the temperature interval \((20—180)^\circ\mathrm{C}\). At higher temperatures, the values of the electrical resistance of irradiated and radiation-undamaged specimens coincide (Fig. 2).
Consequently, it may be assumed that the radiation-induced change in the ionic conductivity of the crystals is not responsible for the appearance of the maximum of \(\operatorname{tg}\delta\).
On the other hand, in the region of thermal destruction of color centers, an electronic component of the current may appear. In order to clarify the role of the electronic conductivity of the crystal in the occurrence of dielectric absorption, the thermal-bleaching curves of the \(F\)- and \(V_2\)-bands were studied. The heating rate of the KBr specimens in studying thermal bleaching and electrical properties was kept constant and was \((0.8—1.0)^\circ\mathrm{C}\) per min. The experimental data indicate that the maximum rate of destruction of the \(F\)- and \(V_2\)-bands in KBr crystals falls in the temperature interval \((60—100)^\circ\mathrm{C}\) (Fig. 3), whereas the maximum of dielectric losses occurs at higher—
Fig. 3. Thermal-bleaching curves of the \(F\)- and \(V_2\)-absorption bands for KBr crystals.
1 — \(F\)-band \((620\ m\mu)\), 2 — \(V_2\)-band \((230\ m\mu)\).
—temperatures. Consequently, the increase of \(\operatorname{tg}\delta\) in pre-irradiated KBr crystals is connected only with polarization phenomena. The observed dielectric absorption cannot be explained by relaxation polarization caused by orientational transitions [4].
Indeed, in the case of Debye losses, the amplitude of the peak does not depend on the field frequency:
\[ \operatorname{tg}\delta_{\max} = \frac{\varepsilon_0-\varepsilon_\infty} {2\sqrt{\varepsilon_0\cdot\varepsilon_\infty}}, \]
where \(\varepsilon_0\) is the static dielectric permittivity,
\(\varepsilon_\infty\) is the high-frequency dielectric permittivity.
To describe such types of polarization as electronic and ionic displacement, the harmonic-oscillator model is widely used. At frequencies of the electromagnetic field close to the natural frequency of oscillation of the oscillator, absorption of energy is observed. In this case the imaginary part of the dielectric permittivity is expressed as follows [5]:
\[ \varepsilon''=\frac{1}{2}(\varepsilon_0-\varepsilon_\infty)\cdot \left[ \frac{\omega\cdot\tau}{1+(\omega+\omega_0)^2\tau^2} + \frac{\omega\cdot\tau}{1+(\omega-\omega_0)^2\tau^2} \right]. \]
The formula for the maximum value of \(\varepsilon''\) has the form:
\[ \varepsilon''_{\max}=\frac{1}{2}(\varepsilon_0-\varepsilon_\infty)\cdot \sqrt{1+\omega_0^2\tau^2}, \]
where \(\omega_0\) is the natural frequency of the oscillator,
\(\tau\) is the time constant for the establishment of polarization,
\(\omega\) is the frequency of the external field.
Assuming that \(\tau\) decreases with increasing temperature and that \(\omega\tau\) is close to 1, the following regularities are observed with increasing frequency:
a) the temperature maximum of \(\varepsilon''\) shifts somewhat toward higher temperatures;
b) the amplitude of the maximum of \(\varepsilon''\) decreases and its half-width increases.
Thus, the dielectric absorption arising in irradiated KBr crystals is qualitatively described by the formulas of resonance absorption. Our experimental results indicate that the natural frequency of oscillation of the defects responsible for resonance absorption has a very low value—less than 50 Hz. This makes it possible to regard the oscillators as complex ionic formations that become electrically active after irradiation.
From the data on thermal bleaching of KBr crystals it follows that color centers are not a component part of the radiation defects responsible for dielectric absorption.
To clarify the nature of the observed absorption, we used the method of introducing divalent cation-substituting impurities into KBr crystals. It is known that alkaline-earth ions are acceptors for holes generated by radiation. Therefore, if localized holes participate in the formation of oscillators, the introduction of strontium ions into the crystal should change the amplitude of the resonance peak of \(\operatorname{tg}\delta\).
It has been shown that small additions of \(\mathrm{Sr}^{++}\) reduce the height of the maximum of dielectric losses induced by irradiation, and at impurity concentrations greater than \(1.5\cdot 10^{-2}\) mole % the resonance peak of dielectric losses disappears and a new absorption region appears (Fig. 4). At low frequencies, a shift of the maximum was traced, as well as a change in the real part of the dielectric permittivity characteristic of relaxation processes. The activation energy of the relaxation polarization is 1.2 eV.
When the relaxation shifts into the temperature region \((90\text{–}110)^\circ\mathrm{C}\), a sharp decrease of the maximum is observed, caused by destruction of the relaxators. The fact that the \(\operatorname{tg}\delta\) peak increases with increasing concentration of \(\mathrm{Sr}^{++}\) impurity in the crystal and is not observed for the system
KBr + Pb\(^{++}\), permits the dipole to be regarded as consisting of an Sr\(^{++}\) ion and one or two holes localized near the impurity ion. Taking into account the closeness of the destruction temperatures of the relaxators and \(V_2\)-centers, the more probable model of the dipole may be considered to be a \(V_2\)-center localized at the cation vacancy of the excited complex.
Fig. 4. Temperature dependences of \(\tan\delta\) and \(\varepsilon\) for KBr + \(3.8\cdot10^{-2}\) mole % Sr\(^{++}\) crystals. Solid lines correspond to unirradiated samples; dashed lines correspond to crystals irradiated with a dose of \(1\cdot10^{15}\) protons/cm\(^2\). 1 — 50 Hz, 2 — 100 Hz, 3 — 500 Hz, 4 — 1000 Hz, 5 — 5000 Hz.
We arrive at the following conclusions:
-
The dielectric absorption that arises in irradiated KBr crystals is not a consequence of “contamination” of the crystals by impurities, but is typical of the intrinsic structure of the compound.
-
The formations responsible for resonant absorption in pre-irradiated KBr crystals include localized holes. These are probably complex vacancy aggregates that have captured several holes. Confirmation of the existence in crystals of vacancy aggregates capable of localizing holes is provided by the change in the radiation-induced decrease of the electrical conductivity of alkali-halide crystals when hole-acceptor impurities are introduced into them [6].
REFERENCES
- V. V. Krasnopevtsev. FTT, 5, 8, 2261, 1963.
- V. V. Krasnopevtsev. FTT, 3, 214, 1961.
- B. V. Budylin, A. A. Vorob’ev. The Action of Radiation on Ionic Structures. Gosatomizdat, Moscow, 1962.
- Yu. M. Annenkov. Izv. TPI, vol. 140, 1965.
- R. Fröhlich. Theory of Dielectrics. IL, Moscow, 1960.
- M. I. Ignat’eva, E. K. Zavadovskaya, I. Ya. Melik-Gaikazyan. FTT, 5, 2775, 1963.