SETUP FOR STUDYING THE PHOTOCONDUCTIVITY AND DIELECTRIC PROPERTIES OF SOLIDS AT A FREQUENCY OF \(10^{10}\) Hz

V. I. MAMATYUK, S. I. RUKOLEEV

(Presented by the scientific seminar of the Department of Radiation Chemistry, Faculty of Chemical Technology)

Studies of the photoelectric properties of solids by microwave methods have become widespread comparatively recently, mainly in the last decade. In some cases they have a number of advantages over methods of measurement at direct current and at low frequencies. At ultrahigh frequencies, bulk properties have a number of special features, since the period of microwave oscillations is of the same order as the lifetime of current carriers and the relaxation times of certain types of polarization. These features make it possible to obtain additional information on the properties of solids [1]. In addition, microwave methods have the advantage that the samples do not require contacts, which sometimes alter the properties of the samples.

This article describes a setup for studying solids at a frequency of 10000 MHz. The use of automatic frequency tuning to the working resonator makes it possible to eliminate the influence of the photodielectric effect in measurements of photoconductivity, while synchronous detection of the signal makes it possible to increase the sensitivity of the setup in comparison with other setups of similar type [2, 3].

In work [4], for similar measurements, an electron paramagnetic resonance radiospectrograph with a bolometric detector was used. Its high cost and design features make its use in such studies inadvisable. In addition, when recording a modulated photoconductivity signal, the sensitivity of bolometric detectors decreases because of their inertia.

The block diagram of the setup is shown in Fig. 1. Microwave oscillations from the klystron generator GS-624M [1], through a ferrite isolator [2] and transformer [3], enter the cylindrical measuring resonator [4], excited through a coupling aperture in the \(E_{010}\) wave. The GS-624M generator makes it possible to cover the frequency range from 7700 to 10300 MHz and to switch on an automatic frequency-tuning system. The ferrite isolator, with an attenuation of 0.5 dB in the forward direction and 20 dB in the reverse direction, as well as bidirectional attenuators at the generator output, eliminate the influence of the load on the microwave generator.

Matching of the resonator with the waveguide tract to VSWR values of 1.1–1.3 is achieved by means of a full-resistance transformer. The use of a resonator excited in the wave

Fig. 1. Block diagram of the setup. 1—GS-624M microwave generator; 2—ferrite valve; 3—impedance transformer; 4—working resonator; 5—sample; 6—microwave detector; 7—switch; 8—28 IM amplifier; 9—synchronous detector; 10—FSK-1 photoresistor; 11—EPP-09 self-recording potentiometer; 12—obturator; 13—motor; 14—illuminator; 15—DRSh-500 lamp; 16—automatic frequency-control system; 17—S1-19A oscilloscope; 18—mixer

of the \(E_{010}\) type makes it possible to interpret the measurement results rather easily, since the field distribution for such oscillations does not depend on the length of the resonator or on the azimuth. The resonator has a diameter of 24.5 mm and a length of 35 mm. Coupling holes 4.5 mm in diameter were located in the side wall. A semiconductor diode DKS-4 was used as the microwave detector [6]. When measuring photoconductivity, the sample, in the form of a thin disk, is placed at the maximum of the resonator’s electric field. When the sample is illuminated and a photocurrent arises, the \(Q\)-factor of the resonator changes, which leads to a change in the power passing through the resonator. Accordingly, at the output of the microwave detector a signal appears with a frequency equal to the frequency of the light modulation. After the 28 IM amplifier [8], the signal is fed to the input of the synchronous detector [9], whose reference voltage is formed by the FSK-1 photoresistor [10]. The synchronous detector is made according to the Schuster circuit. From the output of the synchronous detector, the signal, proportional to the photoconductivity of the sample, is recorded on an EPP-09 self-recording potentiometer. To match the output resistance of the synchronous detector to the input of the EPP-09, a balanced stage with a cathode load was used. The automatic frequency-control system is made according to a modified Pound circuit.

When switch [7] is set to position II and under continuous illumination, the setup makes it possible to study the photodielectric effect in the same sample. To measure the resonant frequency of the resonator, the wavemeter resonator of the GS-624 generator was used. Signals from the microwave detector and from the resonance indicator were fed through the mixer [18]

are fed to the input of the vertical amplifier of the S1-19 oscilloscope [17]. The microwave oscillations were frequency-modulated by a sawtooth voltage from the generator’s internal modulation. This same sawtooth voltage produced the horizontal sweep of the beam. As a result, the resonance curve of the working resonator and the marker from the wavemeter resonator were visible on the oscilloscope screen, which made it possible to measure the resonance frequency and the quality factor of the resonator. For measuring dielectric permittivity and dielectric losses, the use of this same resonator is possible in principle; however, such measurements can only be relative.

Therefore, for measuring dielectric permittivity and dielectric losses, a resonator excited in the \(H_{011}\) mode was used; it made it possible to investigate materials with dielectric permittivity \(\varepsilon\) from 1 to 200 and dielectric losses \(\operatorname{tg}\delta\) from \(10^{-4}\) to \(10^{-1}\). The resonator was tuned by means of a noncontact piston, which made it possible to avoid the excitation in the resonator of other oscillations that interfere with the measurement, especially oscillations of type \(E\). To measure the piston displacement with an accuracy of \(10^{-3}\), a micrometer from a standard waveguide load was used. When measuring large values of dielectric permittivity and losses, the sample was placed on the resonator piston. For small values of \(\varepsilon\) and \(\operatorname{tg}\delta\), it is advisable to place the samples at the maximum of the electric field of the resonator. The quality factor and resonant wavelength of the resonator were measured by the oscillographic method.

REFERENCES

1. Bhar. Proc. IEEE. 51, No. 11, p. 1623, (1963).
2. E. S. Meylikhov. FTT, 7, 5, 1531, (1965).
3. V. G. Erofen'ev, L. N. Kurbatov. FTT, 31, 2, 595, (1961).
4. E. V. Baranov, I. A. Akimov. DAN SSSR, 154, 1, 184, (1964).