Dielectric Single Crystals Research Articles

In English

In review, deals with the theory of exciton quasimolecules (formed of spatially separated electrons and holes) in a nanosystems that consists of semiconductor and dielectric colloidal quantum dots (QDs) synthesized in a dielectric and semiconductor matrixs. It has been shown that the exciton quasimolecule formation is of the threshold character and possible in a nanosystem, where the distance D between the surfaces of QD is given by the condition (where and are some critical distance). We have shown that in such a nanoheterostructures acting as “exciton molecules” are the QDs with excitons localizing over their surfaces. The position of the quasimolecule state energy band depends both on the mean radius of the QDs, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure. It was found that the binding energy of singlet ground state of exciton quasimolecules, consisting of two semiconductor and dielectric QDs is a significant large values, larger than the binding energy of the biexciton in a semiconductor and dielectric single crystals almost two orders of magnitude. It is shown that the major contribution to tue binding energy of singlet ground state of exciton quasimolecule is made by the energy of the exchange interaction of electrons with holes and this contribution is much more substantial than the contribution of the energy of the Coulomb interaction between the electrons and holes. It is established that the position of the exciton quasimolecule energy band depends both on the mean radius of the QDs and the distance between their surfaces. It is shown that with increase in temperature above the threshold (), a transition can occur from the exciton quasimolecule to exciton state. It has been found that at a constant concentration of excitons (i.e. constant concentration of QD) and temperatures Т below , one can expect a new luminescence band shifted from the exciton band by the value of the exciton quasimolecule binding energy. This new band disappears at higher temperatures (). At a constant temperature below , an increase in exciton concentration (i.e. in QD concentration) brings about weakening of the exciton luminescence band and strengthening of the exciton quasimolecule. These exciton quasimolecules are of fundamental interest as new quasi-atomic colloidal nanostructures; they may also have practical value as new nanomaterials for nanooptoelectronics. The fact that the energy of the ground state singlet exciton quasimolecule is in the infrared range of the spectrum, presumably, allow the use of a quasimolecule to create new infrared sensors in biomedical research.

Nonlinear Refraction of Dielectric Single Crystals under Excitation with Ultrashort Near-Infrared Laser Pulses

Nonlinear refraction in a number of crystals (KGW, KGW:Eu3+, Ba(NO3)2, BaWO4, PbMoO4, and CdWO4) that are promising for excitation of multifrequency stimulated Raman scattering is investigated by a single-beam Z-scan method. Picosecond laser pulses with a wavelength of 1064 nm generated by a YAG:Nd3+ laser were used for excitation of nonlinear optical processes. The values of n2 are compared depending on the type of the crystal, optical-axis orientation, and impurity concentration.

The formation of an oscillating system during the sputtering of sapphire single crystal with pulsed glow discharge

Sapphire (α-Al2O3, transparent corundum) single crystals were analyzed with pulsed direct current glow discharge mass spectrometry. Combined hollow cathode was used as a discharge cell. To obtain stable sputtering of dielectric material, a formation of initial surface conductivity via preliminary vacuum deposition of thin metallic layer was proposed. Al and Ta film of different thickness (30–200 nm) were considered for this purpose. The approach was found to provide the effective sputtering of dielectrics. The formation of an oscillating system was shown during the sputtering of sapphire samples in a tantalum combined hollow cathode cell. For oriented sapphire single crystals, periodic oscillations of 27Al+ intensity were acquired. This phenomenon was observed only for dielectric single crystals and not for other dielectric samples, e.g. alumina ceramic or fused quartz. The linear dependence of oscillation period on the duration of discharge pulse was found. The origin of these oscillations seems to be attributed to periodic fluctuations of surface conductivity. Oscillation periods calculated for two different orientations of sapphire single crystals (001 and 012) were found to be proportional to the main period of sapphire lattice. Therefore, an assumption that the crystal internal structure of the sample might be the cause of the oscillations is discussed.

Polymer single crystal dielectrics for organic field-effect transistors

Single crystal dielectrics are ideal candidates for high-performance organic field-effect transistors (OFETs) due to the ordered molecular packing and smooth surface. However, it doesn't obtain much attention due to the lack of facile preparation methods. Here, solution-grown smooth poly(ε-caprolactone) (PCL) single crystals with width in tens of microns are obtained and used as the dielectric for the OFETs. The interface between the semiconductor and the PCL single crystal dielectrics are more favorable to the charge carrier transport than spin-coated PCL thin-film dielectrics, exhibiting much higher charge mobility. With 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) single crystal as semiconductor layer, the average OFET mobility reaches up to 1.70 ± 0.97 cm2V−1s−1 with the highest value of 4.95 cm2V−1s−1. This result demonstrated that structural ordering of the polymer dielectrics is critical to further advance the performance of OFET devices.

Spectroscopy of Excitons in Heterostructures with Quantum Dots

We consider the appearance of a new type of electronic excitation in heterostructures with semiconductor and dielectric quantum dots: excitons formed by spatially separated electrons and holes. We have observed the effect of a substantial increase in the binding energy of such an exciton (the hole moves within the quantum dot, while the electron is localized above the spherical surface of the quantum dot–matrix interface) in nanosystems containing semiconductor and dielectric quantum dots, compared with the binding energy of an exciton in semiconductor and dielectric single crystals.

Thermal conductivity of single crystals with a fluorite structure: Cadmium fluoride

The thermal conductivity of Ca, Sr, Ba, and Cd difluoride single crystals and the CdF2 samples doped by 3 mol % NdF3, 15 mol % HoF3, and 10 mol % ErF3 has been studied using the method of steady longitudinal heat flow in the temperature range 50–300 K. The thermal conductivity of the matrices of these compounds decreases in the order CaF2-SrF2-BaF2-CdF2. The temperature dependences of the phonon mean free path for these crystals have been calculated from experimental data and exhibit different behaviors. It has been assumed that the intense phonon scattering observed in the undoped CdF2 sample is caused by the specific features of the processes of phonon-phonon scattering. The formation of heterovalent solid solutions of cadmium difluoride and rare-earth trifluorides is accompanied by a drastic decrease in the thermal conductivity and a change in its character from that typical of dielectric single crystals to that typical of glassy materials.

Measuring equipment and a comparator for measuring high values of thermal conductivity

The problems involved in ensuring unity of measurements of the thermal conductivity of solids when it has high values are considered. The results of the development of instruments for reproducing and transferring values of the thermal conductivity in the 100‐500 W/(m·K) range are presented. The need to increase the level of the metrological backup of thermal quantities, which include the thermal conductivity of substances and materials, is urgent in achieving the energy-saving politics of the state, governed by the Federal Law On Energy Conservation and the present critical technologies at the Federal level “Technologies for achieving energy saving in transportation systems and heat and electrical energy distribution and consumption,” which are a fundamental part of one of the priority areas of the development of science and technology “Power engineering and energy conservation.” Moreover, the increasing development of metrology in the area of ensuring unity of measurements of the properties of substances and materials, including thermal properties, has been recognized by the International Committee on Weights and Measures as a priority. One of the most important thermal quantities, which determine the ability of bodies to transmit heat, is the thermal conductivity. At the present time, the main provision of unity of measurements of thermal conductivity in Russia is provided by the State Primary Standard, which enables the unit to be reproduced and its dimensions to be transferred in the 0.02‐20 W/(m·K) range [1]. The range of values in which metrological backup is not yet provided is in the range λ > 20 W/(m·K), in which the thermal conductivity of large number of conductors, a considerable number of semiconductors, and also some dielectrics lie. Silicon, aluminum, copper, carbon and also many noble metals, which are important for practical applications, are also highly heat-conducting elements. Many alloys based on noble metals, copper, aluminum, magnesium, compounds of semiconductors, and certain dielectric single crystals, for example, beryllium oxide, have thermal conductivities greater than 100 W/(m·K). Hence, it is an urgent problem in modern metrology to extend the range in which unity of measurements of thermal conductivity is provided into the range of higher values of the thermal conductivity. To solve this problem it has been necessary to develop methods of reproducing the unit of thermal conductivity and of transferring its dimensions in 20‐500 W/(m·K) range, and also to develop and manufacture corresponding measuring instruments, which make use of these methods. The technological basis of the reproduction of the unit of thermal conductivity has traditionally been standard measuring instruments, in which the absolute method of reproduction is employed. The dimensions of the unit are transferred using standard measures or thermal conductivity comparators [2, 3]. Samples made of a material, verified as a standard sample, are usually used as standard measures of thermal conductivity. Of the metal materials used for such samples at the present, only VT-6 titanium alloy ( λ≈ 7 W/(m·K)) and 12KhN10T stainless steel ( λ≈ 15 W/(m·K)), having high values of the thermal conductivity, are available.

Anisotropy of Laser-Induced Bulk Damage of Single Crystals

The regularities of laser-induced damage of anisotropic materials, such as LiNbO3 and KDP dielectric single crystals, are experimentally studied. It is revealed that the shape of laser-induced damage in the dielectric crystals depends on the elastic symmetry of crystal and the propagation direction of the laser beam. When the beam propagates along the optic axis of crystals, the figures of the laser damage are six-path stars for LiNbO3 and four-path ones for KDP crystals. For the direction parallel to X and Y axes in KDP crystal, the damage has initially cross-like configuration, with further splitting of Z-oriented crack into two cracks in the process of damage evolution, leading to transformation of orthogonal-type damage to a hexagonal-type one.

Single crystal microwave dielectrics at low temperature: losses and non-linearities

Single crystal dielectrics (SCDs)at cryogenic temperatures ( < 100 K)provide some of the highest Q microwave resonators which may be realised and, as a result, may have important applications across the microwave communications industry. We summarise measurement techniques used for characterising the complex permittivity of single crystal dielectrics at cryogenic temperatures. Various measurement geometries are described, including Courtney resonators, composite dielectric pucks, an indirectly heated high Q resonator perturbation technique (using both microwave and bolometric readouts)and coupled dielectric resonators. A potential technique for making spatially resolved measurements on a scale small compared with the free space wavelength is also outlined. The methods are illustrated by measurements made on a range of single crystal materials including sapphire, rutile and strontium titanate. Numerical modelling is an important element in accurate evaluation of the measurement data and this will also be considered. Until now the thermal properties of the dielectrics have been ignored in evaluating their microwave properties. We suggest that this is an issue which should not be neglected, particularly when considering the non-linear behaviour of the permittivity as a function of microwave power. # 2003 Elsevier Ltd. All rights reserved.

Development of horizontally oriented crystallization of high-melting dielectric single crystals

Special features of mathematical heat- and mass-transfer models are considered; systems of equations that describe the height change for growing single crystals are determined, and recommendations are given according to which the single-crystal height is practically constant. Under conditions of ideal mixing of a melt, the dopant distribution in the crystal is given.

Low Loss Sintered Dielectric Resonators with HTS Thick Films

The combination of high-Q dielectric resonators and high-temperature superconducting (HTS) films offer many advantages in the area of cellular and satellite communications. The high cost of single crystal dielectrics and HTS thin films may be unattractive in certain applications. Superconducting thick films and polycrystalline ceramic dielectrics offer a high performance, low-cost alternative to high-Q thin film/single crystal dielectric resonators. The loss of polycrystalline ceramics of Al 2 O 3 , Ba(Mg 1/3 Ta 2/3 )O 3 (BMT), and Zr 0.875 Sn 0.25 Ti 0.875 O 4 (ZTS) has been studied. Alumina, Al 2 O 3 , has been studied as a model material for dielectric loss. Theory predicts that the loss in single crystal sapphire should follow a T 5 dependence. However, at low temperatures the loss is dominated by extrinsic losses due to crystal imperfection, residual dopant atoms, dislocations, and other lattice defects and the T 5 dependence does not hold. In polycrystalline alumina the intrinsic loss is immediately masked by these extrinsic losses, even at room temperature, and a simple T dependence is observed. Results on polycrystalline alumina show that Q's well in excess of 10 5 at 10 GHz and 77 K can be achieved in a design made compact by the use of a HTS thick film shield.

High Q dielectric resonators using YBa/sub 2/Cu/sub 3/O/sub x/ thick films and polycrystalline dielectrics

The combination of high Q dielectric resonators and high temperature superconducting (HTS) films offer many advantages in the area of cellular and satellite communications. The high cost of single crystal dielectrics and HTS thin films may be unattractive in certain applications. Superconducting thick films and polycrystalline ceramic dielectrics offer a high performance, low cost alternative to high Q thin film/single crystal dielectric resonators. Results on polycrystalline dielectrics show that Q's well in excess of 10/sup 5/ at 10 GHz and 77 K can be achieved in a design made compact by the use of a HTS thick film shield.

Thermal transport properties of single crystal lanthanum aluminate

The thermal diffusivity and thermal conductivity of single crystal (100) lanthanum aluminate (LaAlO3) have been determined in the temperature range 77–353 K. The thermal diffusivity was measured using a transient heat-pulse technique and the thermal conductivity by a steady state dc technique. The specific heat capacity was calculated from the thermal diffusivity and thermal conductivity data obtained. The temperature dependence of the thermal diffusivity was found to be of the form [T exp(TD/bdT)] and that of the thermal conductivity of the form [T3 exp(TD/bcT)]. Both results are consistent with the expected behavior of the thermal transport, limited by phonon-phonon collision (‘‘Umklapp’’) processes, in high purity dielectric single crystals. The Debye temperature (TD) was calculated as 720±22 K and the parameters bd and bc were found to be 1.4±0.1 and 0.7±0.1, respectively. The average sound velocity was calculated to be (5.4±0.6)×105 cm/s. The phonon mean-free-path at 303 K was found to be (2.2±0.2)×10−7 cm.

Inorganic nonlinear materials

As optical technology becomes poised to revolutionise the world of communication the need for novel nonlinear optic (NLO) and electrooptic (EO) materials is increasing rapidly. NLO and EO materials come in many shapes and sizes; there are the large, aesthetically-pleasing, optically perfect and well known inorganic single crystals such as lithium niobate (LiNbo3), and the less familiar potassium titanyl phosphate(KTiOPO4 or KTP) and β-barium borate (β-BaB2O4 or BBO). There are also crystalline thin films, organic polymer or composite thin films and transparent inorganic ceramics such as lead lanthanum zirconium titanate (PLZT) I will concentrate here on the wide range of research on inorganic, dielectric single crystals.

Experimental observation of fundamental microwave absorption in high-quality dielectric crystals

This paper is devoted to the experimental study of small microwave losses (tan δ ≈ 10 −4−10 −9) in high-quality dielectric single crystals. Wide-temperature-range measurements of the loss tangent at frequencies 9–72 GHz have indicated the existence of fundamental (lattice) and extrinsic (due to imperfections) microwave absorption in sapphire and YAG crystals. Strong distinctive temperature dependence of the lattice absorption ( tan δ ∼ T α , α = 4 or α = 5 for different crystal symmetries) allows to identify it with the “photon+phonon→phonon” interaction due to lattice anharmonicity.

Аномально малая диссипация электромагнитных волн в совершенных монокристаллах-диэлектриках

Аномально малая диссипация электромагнитных волн в совершенных монокристаллах-диэлектриках

Attenuation of Longitudinal Ultrasonic Waves in Insulators at Room Temperature

It is possible to distinguish between two intrinsic loss mechanisms for longitudinal ultrasonic waves propagating in dielectric single crystals, viz., the thermoelastic loss mechanism and the Akhiezer mechanism. In the normal derivation of the theoretical expressions of the attenuation coefficients for these mechanisms, the former involves the thermal conductivity and the thermal expansion coefficients, while the latter involves the thermal conductivity and the Grüneisen numbers of the thermal phonon modes. These Grüneisen numbers can only be deduced from a knowledge of the third-order elastic constants. We have found experimentally that, for the 15 materials for which sufficient data are available, the attenuation from the Akhiezer mechanism exceeds that from the thermoelastic mechanism by a factor of about 40. This result is shown to be a consequence of the distribution of the Grüneisen numbers of the thermal phonon modes about the mean value. It is suggested that our conclusions can be used to predict the total ultrasonic attenuation in other materials from a knowledge of a few macroscopic properties, even if the third-order elastic constants are unknown. Such predictions are made for GaAs, InSb, and diamond. The third-order elastic constants of GaAs, InSb, and LiF have also been used to compute the ultrasonic attenuation in these materials, and to compute the volume Grüneisen constants. The latter are found to be in good agreement with experimental values except for InSb.