RU1790762C - Method of determining concentration of point flaws with a predetermined charge in ferroelectric crystals - Google Patents

Abstract

Usage: physicochemical analysis of solids. Summary of the invention: measure the dependence of the angle tangent of low-frequency dielectric losses on the amplitude of the measuring electric field, determine the threshold electric field on the curve and determine the desired value from the value of the threshold field and the thermodynamic parameters of the ferroelectric 2 silt0

Description

"

Yo

: The invention relates to physicochemical analysis of solids and can be used to determine the number of point defects in the crystal structure in real ferroelectric crystals (SE) o

Defects in the crystal lattice make a significant, and in many cases a determining contribution to the most important properties of solar cells, therefore, the value of their concentration in the crystal is very important for predicting and evaluating the possibilities of applying solar cells. A known method of x-ray analysis, with which you can determine the concentration of point defects in crystalline solids. However, this method is quite complicated, it is very labor intensive,

requires the use of expensive equipment: and highly qualified staff. In addition, during the experiment, even at low doses of radiation, Structural- appear in the crystal; damage and the appearance of radiation defects, which greatly reduces the accuracy of determining the initial concentration of defects in the crystal.

Closest to the invention technical solution. is a method for determining the concentration of point defects q, which is to measure. of the dielectric loss tangent tgS due to the through conductivity of the sample, as well as the dielectric constant in, the mobility of charge carriers (point charge

3 a

to

s

P

(1)

defect q and determination by calculation of the desired value by the formula

 t5Jvcp.Ј

However, this method of determining the concentration of point defects is very complicated and time-consuming, since in addition to measuring tgSoH it requires the measurement of a large number of other parameters of the sample and, in particular, the mobility of charge carriers, which in itself is a difficult task. The disadvantage of this method is also the low accuracy of determination defect concentrations, since the errors of all values included in the calculation formula are summarized and, in addition, the mobility of charge carriers is usually determined only by the order of magnitude; for example, when using the Hall effect, it is possible to determine the Hall mobility, which differs by a factor of 1, and depends on: the mechanism of charge dissipation by imperfections of the crystal lattice, different at different temperatures. The range of applicability of the method is limited to those cases when the dielectric losses are due to through conductivity samples

The aim of the invention is to simplify the method for determining the concentration of point defects in ferroelectrics and to increase the accuracy of determining the desired value.

The goal is achieved by applying metal electrodes perpendicular to the ferroelectric axis of the crystal on the ferroelectric sample, keeping the sample at a temperature exceeding the Curie temperature (T) by at least 5 K for 0.5-1 hours, then cooling to temperature 2–3 K below Tk with a speed of no more than 1 K per minute, measure 6 and tg 8 depending on the amplitude of the measuring electric field (E), determine the threshold field (E „) from the obtained dependence and calculate the concentration of point defects according to the formula

P

 En vri. and;

where n is the charge of the defect; about - first,

Q g O 5 0

d

0 5

0

5

A - the second coefficients in the expansion of the thermodynamic potential in even degrees of polarization; % - correlation constant The proposed method is based on the use of different types of motion of ferroelectric domain walls interacting with point defects of the crystal lattice in electric fields of various amplitudes. In weak fields smaller than the threshold field Ep, a small elastic displacement of the domain the boundaries of relatively fixed point defects, m, e0, their slight deflection between the nearest fixing points. In this case, the dependence tgЈ on the amplitude of the electric field is observed. The field is faster, quieter than HF, individual curved sections of the domain walls are detached from the pinpoint defects, then an avalanche-like increase in their length begins until the domain wall is completely separated from of point defects ,, As a result, an irreversible displacement of the domain wall over large distances follows, This leads to a sharp exponential increase in tgЈ with increasing field amplitude. The value of Eft strongly depends on the concentration of point defects, what is the basis of the present invention ",

Fig. 01 shows the dependences of tpS on the amplitude Et at a frequency of I kHz at a temperature of 319 K for samples of triglycine sulfate ferroelectrics: nominally pure (curve 1), irradiated with an x-ray dose of 0.08 crd (curve 2) and an x-ray dose of 1.8 krd ( curve 3), as well as with the addition of chromium ions introduced into the solution during crystal growth (curve rO with a concentration of 2 x 10 in Fig. 2, the dependences tgo (E) are approximated by an exponential function: a nominally pure crystal (curve 1) irradiated dose of 0.08 krd (curve 2), irradiated first dose 1.8 MRC (curve 3), a Cr-doped crystal (curve 4) "

Example. On a plane-parallel sample of a TGS ferroelectric crystal with an admixture of chromium ions perpendicular to ferroelectric51

silver axis, silver electrodes are deposited with the Y axis. After annealing the sample for 1 h at a temperature of 373 K and cooling it at a speed of 1 K per minute to a temperature of 319 K (not 3 K below Tk), at this temperature at a frequency 1 kHz using the P571 bridge (the dependences of B and tpo on the amplitude of the measuring field were measured

The value of the threshold field Ep was found as the point of intersection of the straight lines in the coordinates In tg & (His2) see 2.

After substituting in the formula for determining the concentration of defects Ep and q values known from the literature, q 4.8-KG ° units, 0С05Е, оЈ 3.9-1П- grad.-, | L В-10- ° unit0 CGSE, Ј 5.7 , OS 10-15 cm2, the value of n was obtained, which coincides well with the true value of the concentration of the Cr impurity in the crystal, since the concentration of the impurity in the crystal is always lower than in the solution () from which the crystal was grown,

The method is simpler and less time consuming in comparison with the prototype, since it determines only one value from the experiment - the threshold field

The method improves the accuracy of determining the concentration of point defects, since the error in determining all the values included in the calculation formula does not exceed 20%, and the error in determining only the mobility of the charge carriers used in the prototype is 150-200%.

907626

Claims (1)

The claims A method for determining the concentration of point defects with a known charge in ferroelectric crystals, which consists in applying metal electrodes to the sample and simultaneously measuring the dielectric tangent tgЈ and dielectric constant 6 of the sample, the values of which indicate the concentration of defects, characterized in that, in order to increase accuracy, metal electrodes are applied perpendicular to the ferroelectric axis of the crystal, the sample is kept at a temperature exceeding the temperature Curie is not less than 5 K, for 0.5-1 h, then cooled to a temperature of 2-3 K below the Curie temperature at a speed of not more than 1 K / min, a variable measuring 25 voltage and measure the dielectric loss tangent and permittivity as a function of the amplitude of the measuring electric field Et, from the obtained dependence 30 determine the threshold field Eft and calculate the concentration of point defects according to the formula 35  Get where q is the charge of the defect, OS is the first, A is the second coefficient in the expansion of the thermodynamic potential in even degrees of polarization, and X is correlation constant.