First-principles study of stability of point defects and their effects on electronic properties of GaAs/AlGaAs superlattice

When the GaAs/AlGaAs superlattice-based devices are used under irradiation environments, point defects may be created and ultimately deteriorate their electronic and transport properties. Thus, understanding the properties of point defects like vacancies and interstitials is essential for the successful application of semiconductor materials. In the present study, first-principles calculations are carried out to explore the stability of point defects in GaAs/Al_0.5Ga_0.5As superlattice and their effects on electronic properties. The results show that the interstitial defects and Frenkel pair defects are relatively difficult to form, while the antisite defects are favorably created generally. Besides, the existence of point defects generally modifies the electronic structure of GaAs/Al_0.5Ga_0.5As superlattice significantly, and most of the defective SL structures possess metallic characteristics. Considering the stability of point defects and carrier mobility of defective states, we propose an effective strategy that Al_As, Ga_As, and Al_Ga antisite defects are introduced to improve the hole or electron mobility of GaAs/Al_0.5Ga_0.5As superlattice. The obtained results will contribute to the understanding of the radiation damage effects of the GaAs/AlGaAs superlattice, and provide a guidance for designing highly stable and durable semiconductor superlattice-based electronics and optoelectronics for extreme environment applications.     

First principles study of Ca in BaTiO3 and Bi0.5Na0.5TiO3

BaTiO₃–Bi_0.5Na_0.5TiO₃ is one of the promising candidates as a high-temperature relaxor with a high Curie temperature and several preferred dielectric characteristics. It has been found experimentally for a long time that adding calcium to BaTiO₃–Bi_0.5Na_0.5TiO₃ improves its temperature characteristic of the capacitance [J. Electron. Mater. 39, 2471]. In this study, Calcium (Ca) defects in perovskite BaTiO₃ and Bi_0.5Na_0.5TiO₃ have been studied based on first-principles calculations. In both BaTiO₃ and Bi_0.5Na_0.5TiO₃, our calculations showed that Ca atom energetically prefers to substitute for the cations, that is Ba, Bi, Na and Ti, depending on the growth conditions. In most cases, Ca predominantly substitutes on the A-site without providing additional electrical carriers (serve as either neutral defects or self-compensating defects). The growth conditions where Ca can be forced to substitute for B-site (with limited amount) and the conditions where Ca can be forced to serve as an acceptor are identified. Details of the local structures, formation energies and electronic properties of these Ca defects are reported.     

Determination of defects in 6H-SiC single crystals irradiated with 20 MeV Au ions

In this paper, the determination of the defects induced by 20 MeV Au irradiations in hexagonal silicon carbide single crystals is discussed. The evolution of the irradiation-induced defects as a function of the ion fluence has been studied as a function of depth below the surface using 0.5-25 keV positron beam based Doppler annihilation-ray broadening spectrometry. Results show the detection of two different kinds of defects, depending on the ion fluence.     

Dynamics of magnetic defects in heavy fermion LiV2O4 from stretched exponential 7Li NMR relaxation

7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small concentration of magnetic defects within the structure drastically changes the nuclear magnetization relaxation versus time from a pure exponential as in pure LiV2O4 to a stretched exponential, indicating glassy behavior of the magnetic defects. The stretched exponential function is described as arising from a distribution of 7Li nuclear spin-lattice relaxation rates and we present a model for the distribution in terms of the dynamics of the magnetic defects. Our results explain the origin of recent puzzling 7Li NMR literature data on LiV2O4 and our model is likely applicable to other glassy systems.     

轴类零件内部缺陷超声检测与重构方法研究

为了探究轴类零件内部缺陷的无损检测方法,本文以曲轴为研究对象,基于超声ALOK技术对其内部多个小孔进行了无损检测实验,并建立了相应的声学仿真有限元模型,提出利用声程—角度散点图中的趋势线,可快速判别内部缺陷特征和位置的方法。并且改进了ALOK算法,即采用邻近探头构建的方程或曲线重构缺陷,模拟和实验结果表明使用改进的算法提高了缺陷位置和尺寸的检测精度,通过实验数据重构所得的缺陷位置误差和半径误差分别在2 mm和0.5 mm左右。研究结论对于轴类零件内部缺陷精确定量化无损检测提供了理论和技术指导。     

Electrical characterization of argon-ion sputtered n-GaAs

Epitaxially grown n-type GaAs was sputtered with 0.5 keV Ar ions at doses of 10¹² and 10¹⁵ ions/cm². The sputter-induced defects in the GaAs were characterized using deep-level transient spectroscopy (DLTS) and the effect of these defects on the characteristics of Au Schottky barrier diodes (SBD's) fabricated on the sputtered GaAs was evaluated by current-voltage measurements. It was found that the barrier height of the SBD's decreased with ion dose from 0.97 eV for unsputtered diodes to 0.48 eV after sputtering at a dose of 10¹⁵/cm². DLTS showed the presence of a multitude of sputter-induced defects at and near the GaAs surface. The defects formed during the initial stages of sputtering had the same properties as some of the primary defects introduced during electron and proton irradiation of GaAs. Isochronal annealing at temperatures of up to 350°C showed that although some defects were removed by annealing, others appeared.     

On the migration of defects and impurities in microelectromechanical systems subject to a large number of light pulses

The migration of defects in light-irradiated microelectromechanical systems (MEMS) is treated theoretically. The effects of temperature gradients on atomic demixing are considered. It is found that, when the migration energy of defects and impurities is less than about 0.5 eV, their migration has to be introduced in the modelling of ageing of MEMS. Received: 17 July 2000 / Accepted: 31 March 2000 / Published online: 27 June 2001     

Effects of radiation damage on the charge density wave transitions in NbSe3

We have measured the electrical resistivity of NbSe₃ samples which have been radiation damaged with 2.5 MeV protons up to a defect concentration of 0.5%. We find that, unlike substitutional impurities, the defects do not destroy the charge density wave (CDW) transitions and the samples do not go superconducting. The defects become more effective scatterers below the CDW transitions so that the defect resistivity is temperature dependent. The defects pin the CDWs randomly so that carriers in the unnested regions can be scattered by the CDW. This leads to an enhancement of the defect resistivity. The resistivity of the highly damaged samples is still increasing with decreasing temperatures to below 1 K.     

Nanostructured multiferroic PbFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> and its physical properties

Polycrystalline multiferroic PbFe_0.5Nb_0.5O3 (PFN) fabricated by a solid-phase method is studied. Before sintering, a synthesized PFN powder is processed in Bridgman anvils via a force action in combination with shear deformation (FASD) at room temperature. The electrophysical properties and structural parameters of processed samples and a reference sample are compared. Point defects are shown to play a key role in the formation of the physical properties beginning from an FASD of 200 MPa.     

Instability of an elastically compressed silicon surface under etching

An unusual relief in the form of linear defects resembling quasicracks or grooves was observed on a compressionally stressed surface of a bent silicon surface subjected to chemical etching. The average distance between the forming defects was determined by the magnitude of the surface mechanical strain. The reason for this is assumed to be an Asaro-Tiller-Srolovitz instability in the compressed-surface-etching-agent system. A simple technique is proposed for producing considerable mechanical strains, up to 0.5%, in a silicon surface at room temperature. Fiz. Tverd. Tela (St. Petersburg) 41, 1416–1418 (August 1999)     

Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi_1.5Zn_0.5)(Zn_0.5−x/3Ti_xNb_1.5−2x/3)O₇ (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I₂ in the Ti-rich sample. At 1 MHz their dielectric constant (?′) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan δ) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant.     

Comparison of vacancy and antisite defects in GaAs and InGaAs through hybrid functionals

The formation energies and charge transition levels of vacancy and antisite defects in GaAs and In(0.5)Ga(0.5)As are calculated through hybrid density functionals. In As-rich conditions, the As antisite is the most stable defect in both GaAs and InGaAs, except for n-type GaAs for which the Ga vacancy is favored. The Ga antisite shows the lowest formation energy in Ga-rich conditions. The As antisite provides a consistent interpretation of the defect densities measured at mid-gap for both GaAs/oxide and InGaAs/oxide interfaces.     

Luminescence and point defects in cerium-doped lutetium scandium orthoborate crystals

Cerium-doped lutetium scandium orthoborate single crystals as potential scintillation materials were grown by the floating zone (FZ) method for the first time and the Czochralski (Cz) grown crystal was used for comparison. In this paper, the representative composition Lu_0.5Sc_0.5BO₃:0.5at%Ce (abbreviated as LSBO:Ce) was selected as the research target. The phase structure of FZ-grown LSBO:Ce crystal was characterized by X-ray diffraction and its optical properties were investigated using photoluminescence emission (PL), excitation (PLE), radioluminescence (RL) spectra. The defect properties of LSBO:Ce crystals were studied by thermoluminescence (TL) as a function of temperature (300–600 K). The glow curve of FZ-grown LSBO:Ce crystal shows three TSL peaks, at 350, 400, 552 K, corresponding to a trap depth of 0.99, 1.14 and 1.50 eV, respectively. Based on UV-irradiation TL measurements and air-annealing experiments, the former two kinds of traps could be ascribed to the radiation-induced defects and the third kind originated from the oxygen vacancies formed during crystal growth. These defects also existed in the Cz-grown LSBO:Ce crystal.     

Effects of green laser fluence on the characteristics of graphene nanosheets synthesized by laser ablation method in liquid nitrogen medium

We have studied the effects of laser fluence on the characteristics of graphene nanosheets produced by pulsed laser ablation technique. In this work, The second harmonic of a Q-switched Nd:YAG laser at 532 nm wavelength and 5 Hz repetition rate with different laser fluences in the range of 0.5–1.8 J/cm² was used to irradiate the graphite target in liquid nitrogen medium. The products of ablation were characterized using Fourier transform infrared spectroscopy, field emission scanning electron microscopy, X-ray diffraction pattern, UV–Vis absorption spectroscopy, Raman spectrum and transmission electron microscopy. The Raman spectroscopy indicates that the quality of the graphene nanosheets was decreased while their structure defects were increased as the laser fluence was increased from 0.5 to 1.4 J/cm². Our results suggest that the amount of defects and the number of layers in graphene nanosheets can be changed by adjusting the laser fluence. This study could be a useful guidance for producing of high quality of graphene nanosheets by laser ablation method.     

Phase shift analysis of below-gap primary photocurrent in hydrogenated amorphous silicon

Below-gap primary photocurrent associated with dangling bond defects in hydrogenated amorphous silicon has been investigated by measuring the amplitude and phase shift spectra with respect to the chopped excitation light. Theoretical analysis has been made on the temperature and chopping frequency dependence of the phase shift, yielding a conclusion that the doubly occupied dangling bond states are located at 0.5-0.6 eV below the conduction band edge.     

Two-dimensional graphene with structural defects: elastic mean free path, minimum conductivity, and Anderson transition

Quantum transport properties of disordered graphene with structural defects (Stone-Wales and divacancies) are investigated using a realistic π-π* tight-binding model elaborated from ab initio calculations. Mean free paths and semiclassical conductivities are then computed as a function of the nature and density of defects (using an order-N real-space Kubo-Greenwood method). By increasing the defect density, the decay of the semiclassical conductivities is predicted to saturate to a minimum value of 4e2/πh over a large range (plateau) of carrier density (>0.5×10(14) cm(-20). Additionally, strong contributions of quantum interferences suggest that the Anderson localization regime could be experimentally measurable for a defect density as low as 1%.     

用正电子湮没技术研究a-Si:H/a-SiNx:H多层膜中的界面缺陷

本文报道应用正电子湮没技术(PAT)对a-Si:H/a-SiN_x:H(x≈0.5)多层膜系列样品所进行的研究。发现,由于a-Si:H和a-SiN_x:H在结构方面的失配,导致在a-Si:H/a-SiN_x:H多层膜中的界面区,产生大量缺陷。在a-Si:H子层中,紧靠界面的是应变层,厚度约为8?;在应变层之后是过渡层,厚度约为50?。在过渡层中存在大量缺陷,这就是所谓界面缺陷。 关键词：     

Magnetic properties of Fe/Pt films by symmetrical insertion of Ag

Ultrathin Ag (0.5 nm) pinning layers (APLs) were symmetrically inserted into [Fe/Pt] bilayers to introduce controllable defects on the interfaces between Ag and Fe/Pt multilayers. The highest coercivity 7700 Oe and remanent squareness 0.95 were obtained with five APLs. The large enhancement in coercivity (75% increment compared with that without APL) is due to the relative uniform defects that introduced pinning effects on the interfaces between the APLs and Fe/Pt multilayers. According to the distribution of angule- dependent coercivity of Fe/Pt multilayers without and with APLs, a tendency is suggested of weakened domain-wall motion while enhanced rotation of reverse domain mode.     

研究激光激发的声表面波与材料近表面缺陷的振荡效应

根据激光激发声表面波的热弹运动方程及热传导方程, 采取有限元技术对方程进行求解, 得到声表面波传播波形图. 当声表面波经过近表面缺陷时, 声表面波与近表面缺陷之间产生一种振荡效应, 通过近表面缺陷的振荡波形幅值存在一个逐渐增加后又逐渐减小的过程. 当声表面波经过不同深度的近表面缺陷时, 振荡信号中心频率存在一定的变化规律. 数值仿真结果表明: 当近表面缺陷深度从0.1 mm到0.5 mm变化时, 振荡效应产生的振荡信号中心频率从0.4 MHz到0.76 MHz变化, 振荡信号中心频率与近表面缺陷深度呈近似线性关系, 这为近表面缺陷的定量检测提供了一种理论基础.