N. E. Zhulovsky State Aerospace University, Kharkov
共Submitted February 3, 1999兲
Pis’ma Zh. Tekh. Fiz. 25, 24–29共December 26, 1999兲
It is shown that acoustic treatment of piezoelectric zinc selenide crystals by exciting strong natural elastic vibrations can specifically alter their photoelectric and other properties. Irreversible changes in the stressed-strained state of the crystals under this treatment are responsible for characteristic features in the spectral dependences of the permittivity
⬘
() and the dielectric loss coefficient⬙
(). These dependences plotted in the form of *() diagrams on thecomplex plane and also the spectrum of natural elastic vibrations can be used to monitor the treatment process. © 1999 American Institute of Physics.关S1063-7850共99兲02012-1兴
Bulk piezoelectric crystals of zinc selenide grown from a flux are characterized by a wide range of structural defects, i.e., strain carriers and growth inhomogeneities, which form a complex stressed-strained state in these crystals. This state strongly influences their optical, photoelectric, and other properties. In particular, it is responsible for the individual nature of the low-frequency spectrum of elastic natural vibrations1,2and also for the difference between the spectral and temperature dependences of the permittivity
⬘
and the dielectric loss coefficient⬙
. Thus, a representation of these dependences in the form of *() and *(T) diagrams on the complex plane reflects the self-consistent changes in the electric and elastic fields of the crystal under external influences.3,4Note that the irreversible changes in these spa- tially nonuniform fields in zinc selenide are difficult to re- duce substantially by heat treatment. In the present paper we show that an effective method of changing the complex stressed-strained state of piezoelectric crystals involves treat- ing these with a strong alternating electric field at frequen- cies of specific natural elastic vibrations which can specifi- cally alter the photoelectric, dielectric, and other properties. We investigated oriented samples in the form of rectan- gular parallelepipeds 共having sides of 10⫻8⫻8 mm and 12⫻12⫻11 mm兲, fabricated from crystal ingots up to 50 mm in diameter. These were grown from a flux by the Bridgman method under argon pressure. After chemical-mechanical polishing, indium–gallium Ohmic contacts were deposited on opposite faces. The acoustic treatment was carried out by exciting natural elastic vibrations in the samples using an alternating electric field of strength 100–800 V/cm. The treatment involved systematically exciting between one and five low-frequency natural elastic vibrations. The duration of the acoustic treatment was determined from the rate of de- crease in the intensity of the light during the treatment pro- cess after this had been passed through a polarizer–crystal– analyzer system. Torsional vibrations were excited using four electrodes, for which a quadrupole electric field was created in the sample. Topograms of the vibrations were vi- sualized by an optical-polarization method. The presence of optical inhomogeneities in the samples was identified by a direct shadow method共luminous point method兲. The dielec-tric parameters were determined using an ac bridge at 1 kHz. For the investigations we selected two groups of samples. The first group was formed by optically more per- fect crystals whose absorption coefficient at 10.6m is bet- ter than 3.3⫻10⫺3cm⫺1. The permittivity of the samples measured at 1 kHz is in the range 9.1–9.4 and is almost independent of the crystallographic direction. Their spectrum of natural elastic vibrations contains various strong vibra- tions whose number predominates over the secondary vibra- tions. In the second group of samples we identified numerous optical inhomogeneities created by two-dimensional struc- tural defects共rotation twins, glide bands, twinning lamellae, and so on兲. These create bands of alternating birefringence which are randomly distributed in the关111兴 direction. In this direction the permittivity has the lowest values 共10.4–11.4兲 whereas in other directions its values are in the range 12–18. In addition, the method of etch figures revealed layers con- taining a substructure in these samples. The spectrum of natural elastic vibrations has a complex structure of second- ary resonances whose number and distribution over the spec- trum are individual characteristics of the sample.2Character- istic features of the three-dimensional image of the shadow pattern of the samples 共the ‘‘deformation’’ of the image of the sample surfaces, and so on兲 are shown in the insets to Figs. 1 and 2. These indicate the complexity of the processes accompanying the growth of plastic deformation. Note that in the shadow pattern of the more deformed samples in the second group we can seen ‘‘traces’’ of plastic rotations. A blurred image of the sample surfaces indicates macroscopic fluctuations of the refractive index 共see inset to Fig. 2兲.
For the first group of samples, the most effective treat- ment involved systematically exciting between three and five of the strongest low-frequency natural vibrations using a strong alternating electric field. Figure 1 shows typical
*() diagrams for this group of samples before and after acoustic treatment 共solid and dashed curves兲 plotted using the spectral dependences
⬘
() and ⬙
(). We can see that after treatment, the permittivity increment to light decreases, which reduces the area enclosed by the curve *(). Note that similar changes in the diagrams were also identified for other directions of the external field.TECHNICAL PHYSICS LETTERS VOLUME 25, NUMBER 12 DECEMBER 1999
980
The efficiency of the acoustic treatment of samples in the second group depends strongly on the crystallographic orientation of the sample, the type of elastic vibrations ex- cited, and the external electric field strength. The efficiency may decrease or increase the dielectric parameters, the ab- sorption coefficient at 10.6m and other parameters, which confirms its close correlation with the stressed-strained state of the crystal. The most significant irreversible changes in the spectra
⬘
() and ⬙
() were identified after treating the samples using a quadrupole alternating electric field at the frequency of the torsional natural vibrations when the exter- nal electric field is perpendicular to the 关111兴 direction. Acomparison between the *() diagrams obtained before and after acoustic treatment共solid and dashed curves in Fig. 2兲 shows that after treatment the range of variation of
⬘
and
⬙
under photoexcitation from the 0.47–0.62m range was almost doubled and the area enclosed by the curve on the*() diagram was almost quadrupled. The photosensitivity in the infrared also dropped significantly. The change in the diagrams after acoustic treatment indicates that the relaxation processes undergo rearrangement. Under strong torsional vi- brations the internal elastic and electric fields created mainly by two-dimensional structural defects undergo self- consistent changes in the crystals. This is confirmed by: a兲 a reduction in the local birefringence in some birefingence bands, b兲 a reduction in the dielectric loss coefficient, and c兲 the increased symmetry of the optical-polarization topo- grams and equalization of the contrast of the nodal lines on these topograms which indicates that the homogeneity of the samples is improved. Note also that after treatment the num- ber of secondary resonances in the natural vibration spectrum decreased. The high efficiency of the acoustic treatment of cubic zinc selenide crystals in a quadrupole electric field may be explained by the fact that the关111兴 direction perpendicu- lar to the twinning and glide planes is selected.
The strong influence of the acoustic treatment of the crystals on nonequilibrium carrier transfer processes and charge transfer between complex centers confirms the impor- tant role played by the elastic fields of structural defects in the formation of photoelectric, dielectric, and other proper- ties. We also note that unlike the treatment of crystals in a traveling ultrasonic wave field,5 the possibility of exciting various types of natural vibrations in samples means that the stressed-strained state of the crystal can be specifically and selectively altered. The *() diagrams and the optical po- larization topograms as well as the spectrum of natural elas- tic vibrations can be used to effectively monitor the treat- ment process. This method of treatment is particularly promising for materials grown under extreme conditions since other methods of treatment are ineffective for these.
The authors thank the Fund for Basic Research of the
FIG. 1. Diagrams of *() for zinc selenide (E parallel to 关111兴,
T⫽293 K兲: 1 — 0.450, 2 — 0.475, 3 — 0.480, 4 — 0.490, 5 — 0.500, 6 — 0.510, 7 — 0.525, 8 — 0.550, 9 — 0.580, and 10 — 0.600m.
FIG. 2. Diagrams of *() for zinc selenide (E parallel to
关111兴, T⫽293 K兲: 1 — 0.450, 2 — 0.475, 3 — 0.480, 4 —
0.490, 5 — 0.500, 6 — 0.510, 7 — 0.525, 8 — 0.550, 9 — 0.580, 10 — 0.600, 11 — 0.650, 12 — 0.700, 13 — 0.800,
14 — 0.900, 15 — 1.000, and 16 — 1.200m.
981 Tech. Phys. Lett. 25 (12), December 1999 I. A. Klimenko and V. P. Migal’
Ministry of Science of the Ukraine for financially supporting this work.
1V. K. Komar’, V. P. Migal’, V. A. Kornienko et al., Pis’ma Zh. Tekh. Fiz.
20共10兲, 71 共1994兲 关Tech. Phys. Lett. 20, 419 共1994兲兴.
2V. K. Komar’, V. P. Migal’, and O. N. Chuga, Neorg. Mater. 34, 800
共1998兲.
3Yu. A. Zagoruko, V. K. Komar’, V. P. Migal’ et al., Fiz. Tekh. Polupro-
vodn. 29, 1065共1995兲 关Semiconductors 29, 552 共1995兲兴.
4Yu. A. Zagoruko, V. K. Komar’, V. P. Migal’ et al., Fiz. Tekh. Polupro-
vodn. 30, 1046共1996兲 关Semiconductors 30, 556 共1996兲兴.
5I. V. Ostrovski and V. N. Lysenko, Fiz. Tverd. Tela 共Leningrad兲 24, 1206
共1982兲 关Sov. Phys. Solid State 24, 682 共1982兲兴.
Translated by R. M. Durham