Corresponding author: Viktor M. Timokhin ( t.v.m@inbox.ru ) © 2020 Viktor M. Timokhin, Vladimir M. Garmash, Valentin A. Tedzhetov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Timokhin VM, Garmash VM, Tedzhetov VA (2020) Technology of thermally stimulated diagnostics of anisotropy and optical axes in crystals. Modern Electronic Materials 6(4): 125-132. https://doi.org/10.3897/j.moem.6.4.65351
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For implementing the technology of thermally stimulated diagnostics of anisotropy and optical axes in crystals, the sample is thermostated at a temperature not exceeding the melting point, an electric field not exceeding the breakdown field is applied to the sample and polarization is produced for a time greater than the relaxation time at this temperature. After that, without switching off the electric field, the sample is cooled to the liquid nitrogen temperature, following which the field is switched off, the sample is linearly heated to a temperature above the polarization temperature and the resultant thermally stimulated depolarization (TSD) spectra taken along and perpendicular to the optical axis of the crystal are examined. When comparing the spectra the presence of anisotropy is detected and the direction of the optical axes is determined from the magnitude and presence of the TSD maxima.
diagnostics, crystalline materials, anisotropy, optical axis, thermally stimulated currents
Optically uniaxial α-LiIO3 single crystals possess unique optical, electrical, pyroelectric and piezoelectric properties and are used as short wave frequency doublers in various semiconductor lasers for laser-beam ship guidance applications. There are several methods of optical axis diagnostics [
The method of determining the direction of the optical axis for a phase-anisotropic crystalline λ/4 wafer was described earlier [
The closest counterpart of the technology suggested herein by the technical principle and the achieved result is the method of determining the onset temperature of the tunneling effect in dielectrics and electrically insulating materials [
The aim of this work is to provide a technology of thermally stimulated diagnostics of anisotropy and optical axes in crystals on the basis of TSD spectrum examination. The main objectives of this study are: 1) obtaining TSD spectra in different directions in the 77–473 K temperature range; 2) obtaining thermally stimulated luminescence (TSL) spectra and comparing them with the TSD spectra; 3) obtaining electrical conductivity spectra and comparing them with the TSD spectra. The importance of this study is dictated by the necessity of providing a technology of the diagnostics of anisotropy and optical axes in crystalline laser materials used on board marine ships and at onshore facilities, especially those operating at low temperatures.
The principle of the thermally stimulated depolarization method (TSD) is the non-isothermal electrical relaxation of charged particles due to the fact that when the sample temperature varies following a certain law the material changes from nonequilibrium to equilibrium state. The process is accompanied by energy release which can be detected by measuring the conductivity, thermally stimulated depolarization currents, thermally stimulated luminescence etc. The current vs temperature dependences allow one to determine the energy parameters of the electrophysical processes in the sample. The first prototypes of the method were developed in the late 1960s by C. Bicci, R. Fieschi et al. [
Theoretical models of thermally stimulated depolarization currents (TSDC) were developed earlier [
The measurement of thermally stimulated depolarization currents is carried out as follows. The prepared sample is placed between the electrodes, isothermally aged at the polarization temperature Тp, the electric field Еp is applied to the sample and polarization is carried out for the time tp; this produces an inhomogeneous distribution of carries and an anisotropic orientation of polar molecules in the sample. The electric field also causes migration of free carriers. The pinning of charges shifted through a macro-distance may occur in the vicinity of the electrodes (near-electrode polarization) or inside the crystal. The polarized object is rapidly cooled without switching off the electric field Еp to the temperature Т0 (in the case considered, the liquid nitrogen temperature, i.e., 77 K), and as a result the polarized state is retained for a long time and the relaxation time increases substantially. Then the electric field is switched off, the sample is connected to a V7-30 measuring instrument and heated at a constant rate. As a result a depolarization current is produced in the outer circuit and the maximum corresponding to the specific type of defects appears in the TSD spectrum.
For determining the total depolarization charge one integrates the area under the I = f (T) curve [
(1)
where β = dT/dt is the sample heating rate, d is the sample thickness and j is the current density.
The initial rise method is based on the fact that at low temperatures (about the liquid nitrogen temperature) the initial sections of TSD current curves have the following pattern regardless of the relaxation process kinetics:
. (2)
It can be seen from Eq. (2) that the initial section of the TSD current curve in the Arrhenius coordinates (lnI ~ 1/kT) is a straight line (see Fig.
This method was first suggested in an earlier work [
In this work we used the methods of thermally stimulated depolarization currents, electrical conductivity and thermally stimulated luminescence (TSL). The instruments were a V7-30 electronic voltmeter-electrometer, a V6-21 voltmeter, stabilized power sources, VM-538 and VM-507 impedance meters, Dewar flasks with liquid nitrogen, an URS-2.0 X-ray apparatus and the experimental system.
The test samples were lithium iodate α-LiIО3 crystals (hexagonal system, point group С6) grown by open evaporation in H2O and natural muscovite mica KAl2[AlSi3O10](OH)2 plate-like crystals (monoclinic system, point group 2/m, prismatic) that are used for the production of electrically insulating materials, e.g., micanite and micafolium. These crystals were chosen as test samples because of their hydrogen bonds. For the tests we cut out, ground and polished several samples of each crystal, with a 0.05 mm thickness for plate-like muscovite crystals and 0.5–1 mm for lithium iodate.
The experimental system was developed, fabricated, patented [
Multifunctional system for studying physical and technical parameters of semiconductors, dielectrics and electrically insulating materials.
An ultrasonic converter 11 having an output 12 to an ultrasonic generator is attached with springs 10 inside the electrode 4. The test sample 13 with a guard electrode 14 and a measuring electrode 15 is placed onto the hollowed bottom electrode 4 and slightly pressed down with a thin spring plate 16 attached to an isolated pole 17 and a molten quartz plate 18. The vacuum electric input consists of a molten quartz plate 26 with a centered hole for the input of a contact 27. The natural sample heating rate was preferably 0.5–1.0 K/min. The pressure inside the system was maintained at 10-3–10-4 mm Hg with a roughing vacuum pump and a diffusion pump through a union 23. The vacuum screening cap has windows 19 and 19a for sample irradiation and reflected radiation detection with a photoelectron amplifier. The temperature is measured with two differential Chromel-Copel thermocouples 20 at the bottom and top electrodes through a contact 21 with a retaining nut 22. The vacuum electric input is assembled from the molten quartz plate 26 with a centered hole for the input of the contact 27. The measurement error of this system is as follows: ±1 × 10-15 A for current, 5% for tgδ at tgδ ≥ 5 × 10-4, 10–30% for tgδ at 1 × 10-4 ≤ tgδ < 5 and 2% for electrical capacity.
The object of this study is achieved by comparing the TSD spectra of the crystals taken along the optical axis with the TSD spectra taken perpendicular to the optical axis and with the TSL and electrical conductivity spectra. The fundamentals of this technology were put forward by us earlier [
We now consider the suggested diagnostic method for the example of hexagonal lithium iodate crystals α-LiIO3 used in laser engineering. The method is implemented as follows. The sample of the test material is silver coated at two sides by vacuum sputtering in a VUP-5 plant for producing silver electrodes. Alternatively contactode or glue electrodes made from AK-113 lacquer and fine nickel powder can be used to provide for similar results. The sample is thermostated at Тp = 300 K accurate to ±0.5 K. If the material contains polar defects they will be detected as maxima in the TSD spectrum on a recorder or on a computer display. Then the resultant spectra taken along and perpendicular to the sixth order optical axis of the crystal are examined, and their comparison reveals the presence of anisotropy and the direction of the optical axes. The measurements were carried out for different samples cut from one crystal in order to eliminate the effect or residual polarization.
Of the 32 known classes of crystalline dielectrics, pyroelectrics are 10 classes which are non-centrosymmetric and have a polar axis. This group also includes hexagonal crystalline lattice crystals to which lithium iodate crystals pertain. Anisotropy is the dependence of physical properties (in the case considered, electrical and optical ones) on direction in the crystal.
If a crystal is pyroelectric this should be seen from its TSD spectrum. We will now dwell upon this phenomenon. The pyroelectric effect is caused by the appearance and accumulation of electric charge on crystal surface during crystal heating or cooling, resulting in the production of an electric moment [
TSD current density of α-LiIO3 single crystals along [0001] axis as a function of Еp for Тp = 323 K, tp = 10 min, d = 0.7 mm, electrode diameter 25 mm and Еp field magnitudes, V/m: (1) 8.5×105; (2) 4×105; (3) 2×105; (4) 5×104.
Heating of electrets also produces currents which are however accounted for by depolarization. For these crystals the magnitude of these currents decreases with time, with the pattern of the TSD spectrum changing significantly. Indeed the experiment showed that the first and second TSD spectrum maxima of lithium iodate at 112 and 145 K are reproduced from experiment to experiment. It is noteworthy that the method of partial thermal cleaning does not hold for the TSD current maxima Nos. 1 and 2 at 112 and 145 K: these maxima are not suppressed after reheating from the liquid nitrogen temperature. The method of partial thermal cleaning however holds well for higher temperature maxima. Thus pyroelectric current (maxima 1 and 2) dominates over weak relaxation current by 3–4 orders of magnitude. The activation energies (Table
Maximum No. | Т m, K | U a ║ Z axis, eV | Concentration ║ Z axis, m-3 | U a ┴ Z axis, eV | Concentration ┴ Z axis, m-3 |
1 | 112 | 0.07 ± 0.02 | – | – | – |
2 | 145 | 0.15 ± 0.02 | 1.2×1018 | 0.16 ± 0.03 | 3.8×108 |
3 | 190 | 0.30 ± 0.03 | – | – | – |
4 | 205 | 0.35 ± 0.03 | – | – | – |
5 | 225 | 0.42 ± 0.04 | – | – | – |
6 | 250 | 0.48 ± 0.05 | 5.4×1020 | 0.50 ± 0.05 | 2.4×1015 |
7 | 352 | 0.45 ± 0.05 | 8×1022 | 0.65 ± 0.06 | 4.7×1017 |
Study of the TSD spectra that were also described earlier [
Thermally stimulated luminescence spectrum of α-LiIO3 crystals: tp = 1 h, U = 15 kV, Тp = 80 K. Irradiation on URS-2.0 X-ray apparatus. tp, U and Тp are polarization time, voltage and temperature, respectively.
Most crystalline materials having defect-free lattices exhibit small if any photoluminescence.
However an impurity ion concentration of a few hundredths of a percent is sufficient to initiate luminescent properties in these materials. Recombination luminescence is excited in solid crystalline materials as a result of the generation of metastable carriers (electrons or holes) due to the action of an ionizing radiation source. This is the so-called defect or impurity center luminescence. Any luminescence process can include carrier trapping by traps or mobile defects followed by carrier release upon heating.
As can be seen from the experimental results (Fig.
Comparison of Figs
Thus study of thermally stimulated depolarization current spectra allows diagnostics [
For muscovite mica crystals (Fig.
The conclusions made on the basis of the experimental TSD spectra are confirmed by analysis of the IR spectra [
It can be seen from Fig.
TSD spectrum of α-LiIO3 crystals taken perpendicular to the sixth order axis at Еp = 4 × 105 V/m and Тp = 376 K.
Muscovite mica TSD current density as a function of Еp for Тp = 323 K and tp = 10 min for Еp field magnitudes, V/m: (1) 5×106; (2) 106; (3) 5×105, (4) 2×105.
The reported TSD examination experiments for muscovite and lithium iodate crystals suggest that the technology suggested in this work allows not only detecting the presence of anisotropy and determining optical axis directions but also evaluating the quality of optical crystalline materials. In spite of the different principles underlying the TSL and TSD studies, the experiment showed exact coincidence of the temperatures of maxima in the TSL and TSD spectra indicating that the same defects participate in both processes. The data obtained using the TSD method for determining the direction of the optical axis in crystals agree well with the temperature and frequency spectra of electrical conductivity. Furthermore the TSD method not only detects the presence of anisotropy but also gives the exact direction of the optical Z axis [0001] along which the spectrum contains seven maxima whereas the spectrum taken perpendicular to the optical axis contains only three maxima.
The use of the thermally stimulated depolarization current method allowed completely solving the tasks of this work and puts forward a technology for the diagnostics of anisotropy and optical axes in crystalline materials. The technology developed in this work is doubtlessly of interest for the fundamental research dealing with the tunneling effect and for various practical applications as a method of examining the quality of optical crystals at a sufficiently high scientific and technical level that was unachievable before, resulting in the consequent development of the method of optical diagnostics of optical axes and types of oscillation centers in crystals with hydrogen bonds [
Low-temperature studies are important for applications in extreme north and Antarctic regions where temperatures below –50 °C occur at which the electrophysical and optical parameters of crystalline materials change.
The work was carried out with support from RBRF Grant No. 18-32-00656 mol_a (Study of relationship between optical-luminescent and mechanical phenomena due to reversible ionization of Ce3+ activator in Lu2SiO5:Ce3+ single crystals).