Electrostriction in dielectrics and metals

We examine electrostriction in ellipsoidal samples of dielectrics and metals. We show that electrostriction in metals is independent of the external shape of the sample, whereas in dielectrics electrostriction is a quadratic function of the shape parameter. State Teachers University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 53–58, November, 1998.     

Nanoscopic measurements of the electrostriction responses in P(VDF/TrFE) ultra-thin-film copolymer using atomic force microscopy

Atomic force microscopy (AFM) has been used as a new method to perform nanoscale measurements of the electrostriction coefficients in the lamellae structure of the ferroelectric P(VDF/TrFE) 73/27 copolymers. The result found shows that the electrostriction coefficient inside (in the middle of) the lamella crystals is 6×10^-19 (m²V^-2), which is three times larger than that at the boundary, 2×10^-19 (m²V^-2). To explain the dependence of the electrostriction coefficients with those two regions, some suggestions are proposed. By heat treatment at 140 °C during 2 h, the sample changed its morphology as well as its crystallinity; the amorphous phase is much reduced and the degree of the crystallinity inside the lamellae is higher than that in the border. Also, it is suggested that in the lamellae’s boundary the macromolecular chains come to an end, or one monolayer folds over the other layer. In this case, the electrostriction was suppressed due to the loss of surface energy in the lamellae’s boundary. The achievements will supply a guideline to develop new and better devices for electromechanical and actuator applications. Received: 23 June 2000 / Accepted: 23 August 2000 / Published online: 5 October 2000     

Untersuchungen am Kubischen Bariumtitanat Nach der Methode des elektrostriktiven Resonators

The temperature dependence of the constant of elasticity could be measured in the cubic phase on [100] rods in the absence of a d.c. field using the electrostriction resonator method. The temperature range reached from the transition point up to 300°C. A steep decrease in mechanical quality could be observed several degrees above the transition point.The influence of the dielectric non-linearity on the observed electrostriction resonance is discussed. The piezoelectrical resonances found additionally are explained as flexural vibrations excited by a tetragonal boundary layer.     

Relaxational behavior of the piezoelectric constant and the electrostriction constant in polymers

Piezoelectricity in polymer films is classified by its mechanism into three groups: A-1, intrinsic piezoelectricity due to internal strain in nonpolar crystals; A-2, intrinsic piezoelectricity due to the strain-dependence of the spontaneous polarization in polar crystals; and B, piezoelectricity due to the heterogeneity of macroscopic strain.

The relaxational behavior of piezoelectricity in Type A-1 is discussed for two cases: (a) the piezoelectric phase is relaxing, and (b) the piezoelectric phase is nonrelaxing but the nonpiezoelectric phase is relaxing. For Case a, a thermody-namic theory is developed, yielding relations among relaxation strengths of piezoelectric constant, dielectric constant, and elastic modulus. For Case b, on the other hand, the inequalities e″/e′ < 0 and d″/d′ > 0 are verified for a generalized composite model of two phases, where e′ — ie” and d′ — id″ are complex piezoelectric stress and strain constants, respectively.

Relaxational behavior of the piezoelectric constant for Type A-2 and Type B is expected to reflect that of the electrostriction constant as suggested by theories. The complex electrostriction constant in a range from 30 Hz to 500 kHz is presented for unrolled and rolled poly(vinylidene fluoride) films. The frequency dependence of the complex electrostriction constant can explain the relaxational behavior of the piezoelectric constant of Types A-2 and B, and at the same time gives a new aspect of the relaxational character of polymers, the strain-dependence of the relaxation time, and its anisotropy.     

Ultrasonic amplification semiconductors subjected to deformation

It is shown that in semiconductors with inherent piezo-effect or high relative dielectric permittivity (?_rel ? 10³) which have undergone uniaxial deformation an additional electron-phonon interaction is possible, this being conditioned by the non-linear deformation piezo-effect or electrostriction. The non-linear piezo-module in CdS and the electrostriction constant of the second order in SrTiO₃ have been experimentally investigated.Additional ultrasound amplification conditioned by deformation can exceed the amplification in a non-deformed semiconductor by one order.     

Comparison of electrical characteristic between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance–voltage curves,the electrical characteristics of AlN/GaN Schottky diode,such as Schottky barrier height,turn-on voltage,reverse breakdown voltage,ideal factor,and the current-transport mechanism,are analyzed and then compared with those of an AlGaN/GaN diode by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the dislocation-governed tunneling is dominant for both AlN/GaN and AlGaN/GaN Schottky diodes.However,more dislocation defects and a thinner barrier layer for AlN/GaN heterostructure results in a larger tunneling probability,and causes a larger leakage current and lower reverse breakdown voltage,even though the Schottky barrier height of AlN/GaN Schottky diode is calculated to be higher that of an AlGaN/GaN diode.     

Electrostrictive counterforce on fluid microdroplet in short laser pulse

When a micrometer-sized fluid droplet is illuminated by a laser pulse, there is a fundamental distinction between two cases. If the pulse is short in comparison with the transit time for sound across the droplet, the disruptive optical Abraham-Minkowski radiation force is countered by electrostriction, and the net stress is compressive. In contrast, if the pulse is long on this scale, electrostriction is cancelled by elastic pressure and the surviving term of the electromagnetic force, the Abraham-Minkowski force, is disruptive and deforms the droplet. Ultrashort laser pulses are routinely used in modern experiments, and impressive progress has moreover been made on laser manipulation of liquid surfaces in recent times, making a theory for combining the two pertinent. We analyze the electrostrictive contribution analytically and numerically for a spherical droplet.     

Theoretical resistance of an ionic crystal to destruction by an electric field

We describe an electrostriction mechanism for destruction of an ionic crystal by an electric field. We present a simple theory that transforms into the well-known Rogowskii theory when there is no electrostatic breakdown. We show that electrostatic breakdown of the crystal lattice is due to separation of nearestneighbor ions with different charges to the point where the electric field exceeds the interionic attractive forces. The theoretical tensile strength of a crystal lattice undergoing electrostriction by an electric field is 15 MV/cm.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 47–50, March, 1989.     

Characterization of GaN/AlGaN epitaxial layers grown by metalorganic chemical vapour deposition for high electron mobility transistor applications

GaN and AlGaN epitaxial layers are grown by a metalorganic chemical vapour deposition (MOCVD) system. The crystalline quality of these epitaxially grown layers is studied by different characterization techniques. PL measurements indicate band edge emission peak at 363.8 nm and 312 nm for GaN and AlGaN layers respectively. High resolution XRD (HRXRD) peaks show FWHM of 272 and 296 arcsec for the (0 0 0 2) plane of GaN and GaN in GaN/AlGaN respectively. For GaN buffer layer, the Hall mobility is 346 cm²/V-s and carrier concentration is 4.5 × 10¹⁶/cm³. AFM studies on GaN buffer layer show a dislocation density of 2 × 10⁸/cm² by wet etching in hot phosphoric acid. The refractive indices of GaN buffer layer on sapphire at 633 nm are 2.3544 and 2.1515 for TE and TM modes respectively.     

Untersuchungen Zur Elektrostriktion und Piezoelektrizität der Ferroelektrika

The electromechanical effects of ferroelectric crystals are shown on the basis of the thermodynamical theory. A piezoelectric resonator is most frequently used to determine the piezoelectric constants. In some cases, however, it was found that direct measurement of the deformation as a function of the electric displacement was useful since quadratic effects can be investigated in the same way. The electrostriction constants can also be found by the dynamic method. This method (an electrostriction resonator) also provides the possibility of exciting and investigating mechanical oscillations without the superposition of a linearizing d.c. field. A survey of the experimental results is given for some ferroelectrics. In the neighbourhood of the Curie point, in particular, some considerable deviations from the expected results can be observed.     

斜切蓝宝石衬底MOCVD生长GaN薄膜的透射电镜研究

利用MOCVD技术在斜切角度为0.3°的c面蓝宝石衬底上生长了非故意掺杂 GaN 薄膜, 并采用透射电子显微镜对材料的质量和材料内部缺陷进行了分析. 研究发现斜切蓝宝石衬底上外延的GaN材料中,位错在距离衬底0.8 μm附近大量湮灭, 同时位错扎堆出现.基于上述现象, 提出了斜切衬底上GaN材料中位错的湮灭机制, 解释了斜切衬底能够提高GaN晶体质量的原因.     

The electrostriction of dielectrics

The deformation of bodies in an electric field was first observed by Fontana [1] and Volpicelli [2], and Outer [3] analyzed the phenomenon quantitatively. Lippman predicted the phenomenon of electrostriction from general thermodynamic principles after the discovery of piezoelectricity by the Curie brothers [4].We may presume that any dielectric will exhibit electrostrictive properties to some extent. In fact, Zheludev [5] has shown that the oscillation of a dielectric pendulum is closely connected to, and dependent on, the polarization of the material. Since the polarization effect is inherent in all dielectrics, electrostriction is also a basic property of any dielectric.Both polarization and electrostriction appear in a dielectric under the influence of an electric field. Clearly, these effects will cause a change in the capacitance of condensers containing the experimental samples under the action of the electric field. The goal of the present work is to verify this reasoning and to determine which of the factors under consideration plays the dominating role.     

AlN/蓝宝石模板上生长的GaN研究

研究了在分子束外延制备的AlN/蓝宝石模板上采用金属有机物化学气相外延生长的非故意掺杂GaN的材料性质.采用X射线衍射(XRD)、透射电镜(TEM)和原子力显微镜研究了AlN模板的晶体质量和表面相貌对GaN的影响.结果表明,当AlN的表面粗糙度较小时,尽管AlN模板的位错密度较高((102)面XRD ω扫描半高全宽900—1500 arcsec),但生长得到的GaN依然具有和在蓝宝石衬底上采用"二步法"生长的GaN可比拟的晶体质量((002)面XRD ω扫描半高全宽200—30 关键词： 氮化镓 氮化铝 金属有机物化学气相外延     

Heteroepitaxial GaN films on silicon substrates with porous buffer layers

New complex buffer layers based on a porous material have been developed for epitaxial growth of GaN films on Si substrates. The characteristics of gallium nitride heteroepitaxial layers grown on silicon substrates with new buffer layers by metal-organic vapor phase epitaxy are investigated. It is shown that the porous buffer layers improve the electric homogeneity and increase the photoluminescence intensity of epitaxial GaN films on Si substrates to the values comparable with those for reference GaN films on Al₂O₃ substrates. It is found that a fianite layer in a complex buffer is a barrier for silicon diffusion from the substrate into a GaN film.     

GaN基异质结缓冲层漏电分析

通过对GaN基异质结材料C-V特性中耗尽电容的比较,得出AlGaN/GaN异质结缓冲层漏电与成核层的关系.实验结果表明,基于蓝宝石衬底低温GaN成核层和SiC衬底高温AlN成核层的异质结材料比基于蓝宝石衬底低温AlN成核层异质结材料漏电小、背景载流子浓度低.深入分析发现,基于薄成核层的异质结材料在近衬底的GaN缓冲层中具有高浓度的n型GaN导电层,而基于厚成核层的异质结材料的GaN缓冲层则呈高阻特性.GaN缓冲层中的n型导电层是导致器件漏电主要因素之一,适当提高成核层的质量和厚度可有效降低GaN缓冲层的背景载流子浓度,提高GaN缓冲层的高阻特性,抑制缓冲层漏电. 关键词： AlGaN/GaN异质结 GaN缓冲层 漏电 成核层     

GROWTH MODE AND SURFACE RECONSTRUCTION OF GaN(0001) THIN FILMS ON 6H-SiC(0001)

Two-dimensional growth of GaN thin films on an atomically flat C-face 6H-SiC(0001) surface prepared by ultra-high vacuum Si-etching is observed when using an AlN buffer layer in N plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy and reflection high energy electron diffraction observations reveal a series of Ga-stabilized reconstructions which are consistent with those reported for an N-polar GaN(0001) film. The result, including the effect observed previously for GaN thin film on Si-terminated 6H-SiC(0001), agrees with the polarity assignment of heteroepitaxial wurtzite GaN films on polar 6H-SiC substrates, i.e., GaN film grown on SiC(0001) is <0001> oriented (N-face) while that on SiC(0001) is <0001> oriented (Ga-face).     

Desorption induced GaN quantum dots on (0001) AlN by MOVPE

GaN quantum dots (QDs) are realized on (0001) AlN templates by growing a thin GaN layer on an AlN buffer layer and applying a subsequent desorption step without ammonia present. A growth interruption (GRI), which is commonly applied after the GaN growth allowing for QD formation, is systematically investigated regarding the temperature, duration and initial GaN coverage. Without GRI the initial GaN layer exhibits a two‐dimensional nonuniform growth at the step edges. In this study, the surface morphology only changes significantly if the GRI is performed without ammonia exposure. Thus, an initial two‐dimensional GaN layer can be shaped into three‐dimensional nanostructures. Presented coverage studies by atomic force microscopy (AFM) show desorption as the main driving force for island evolution. By tailoring the growth parameters, GaN QDs can be achieved. Uncapped GaN samples exhibit QDs with 1.2 nm in height and 30 nm in diameter. Additionally, capped GaN QDs exhibit excitonic luminescence lines at about 4.3 eV with FWHM down to 2 meV and an excitonic fine structure splitting of 7 meV. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Self-organized domain formation in low-dislocation-density GaN

The growth of high-quality GaN layers on a wafer size appropriate for device applications is based on heteroepitaxy on foreign substrates. Heteroepitaxial GaN layers with low densities (below 10⁶ cm⁻²) of extended structural defects can be achieved by lateral overgrowth of mask-patterned templates or by the growth of extremely thick GaN layers as a route towards free-standing GaN-pseudosubstrates. We present the microscopic analysis of such low-dislocation-density GaN layers by means of scanning cathodoluminescence microscopy (CL). Several state-of-the-art concepts of lateral overgrowth are compared, including two-step epitaxial lateral overgrowth of stripe masks (ELO), multi-stack ELO comprising several mask layers as well as an alternative approach involving in situ SiN nano-masks. The self-organized formation of typical microscopic growth domains with characteristic optical properties is evidenced by CL for all lateral overgrowth techniques. This behavior directly fingerprints the different growth rates and the specific impurity incorporation on non-equivalent GaN facets, e.g. (0001), or , always present during lateral overgrowth and closely related to the mask geometry. Accordingly, characteristic CL line shapes found in ELO on periodic, micrometer scale mask patterns are also detected for GaN on in situ SiN nano-masks and clearly reveal the individual facet structure during overgrowth. For thick GaN layers, CL is used to detect the spontaneous appearance of inclined facets inside inverted pyramidal defects. Optimized, thick GaN layers exclusively formed by (0001)-growth are proven to be laterally homogeneous despite periodically varying residual stress and dislocation density of the underlying ELO template.     

GaN中Zn的均匀掺杂

在Ar-HCl-Ga-NH3气相外延生长GaN的过程中,分析了Zn在GaN中分布不均匀的原因,认为除生长出晶体完整性好的n-GaN提供均匀掺杂的必要条件外,Zn气流的合理分布是获得Zn均匀掺入的关键。我们通过设计新的Zn喷口,能在φ25mm的GaN:Zn样品上,得到了Zn的均匀分布,PL、CL、EL均为一种颜色,室温下测定了EL光谱,不同点的峰值偏差小于10Å。