Characteristics of the elastic strain of a Ge(111) surface under a mechanical load

Estimates are obtained for various components of the elastic strain in a Ge(111) surface layer under isotropic lateral tension. Agreement with the reversible components of surface thinning, derived from energy shifts in the volume and surface plasma peaks, suggests that the latter have an elastic character. It is conjectured that the anomalously high strain in a thin surface layer is due to the partially quasielastic variation of the relief of strained semiconductor surfaces. Fiz. Tverd. Tela (St. Petersburg) 41, 641–644 (April 1999)     

Strained layer quantum well semiconductor optical amplifiers: Polarization insensitive amplification

Abstract

Polarization insensitive optical amplification was demonstrated in newly developed semiconductor optical amplifiers that have strained GalnAsP quantum well structures. We tailored the active region of the quaternary strained layer quantum well structure with a small biaxially tensile strain of 0.2% in the well layers for polarization insensitive operation.     

Thermal annealing effects on a compositionally graded SiGe layer fabricated by oxidizing a strained SiGe layer

Thermal annealing effects on a thin compositionally graded SiGe buffer layer on silicon substrate fabricated by oxidizing a strained SiGe layer are investigated with X-ray diffraction, ultraviolet Raman spectra and atomic force microscopy. Interestingly, we found that the surface roughness and the threading dislocation densities are kept low during the whole annealing processes, while the Ge concentration at the oxidizing interface decreases exponentially with annealing time and the strain in the layer is only relaxed about 66% even at 1000 °C for 180 min. We realized that the strain relaxation of such a compositionally graded SiGe buffer layer is dominated by Si-Ge intermixing, rather than generation and propagation of misfit dislocations or surface undulation.     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Morphological breakdown during growth of a high nitrogen content GaInNAs thin film

The breakdown in surface morphology during the growth of an 8 nm thick Ga_0.7In_0.3N_0.05As_0.95 layer has been investigated by scanning tunnelling microscopy. During initial growth (<0.5 nm) the alloy layer is planar but strained. Lateral composition modulation due to spinodal decomposition leads to the co-existence of tensile strained N-rich regions and compressively strained N-poor regions, creating an oscillatory strain field (OSF) across the surface. The overall strain increases with layer thickness up to ∼0.5 nm, after which it is relieved by a transition from two-(2D) to three-dimensional (3D) growth, which manifests itself as an undulating, pitted layer. We propose that the region at the bottom of each pit is N-rich and that overgrowth of such regions is inhibited, thereby avoiding the strain caused by lattice mismatch. The results offer insight into the mechanisms involved in the breakdown of the 2D growth of thin dilute nitride layers at relatively high N concentrations.     

Internal strain in elastically strained germanium and silicon

X-ray diffraction of “forbidden” reflections in elastically strained germanium and silicon shows that the four nearest neighbor bonds are not strained according to the strain tensor but rather keep their length almost unchanged because of the occurrence of an internal strain. Consequently the bond angles are changed. From the structure factor of the (600) reflection, this internal strain has been found to be in good quantitative agreement with theoretical calculations.     

The influence of strain rate on the visibility of dislocations in transmission electron microscopy images of deformed Ti–6 wt% Al–4 wt% V and in Timet 550

Samples of Ti–6?wt%?Al–4?wt%?V and Timet 550 (Ti–4?wt%?Al–4?wt%?Mo–2?wt%?Sn–0.5?wt%?Si) have been subjected to strain rates between 10^?1 and 10³?s^?1and detailed examination of the dislocation structure in the α grains has been carried out using transmission electron microscopy (TEM). For samples deformed to a strain of 0.1 at 10^?1?s^?1, detailed analysis of the defects can be carried out using all diffracting vectors and the presence of (c +?a) dislocations and a dislocations thus confirmed. In contrast, for samples strained to the same strain of 0.1 but at 5?s^?1, it is not possible to obtain images of dislocations when using any diffracting vectors other than 0002. Thus the presence of dislocations which have a Burgers vector containing a c component can be confirmed in the samples strained at 5?s^?1 but the presence of a-component dislocations can only be inferred from TEM of these samples because of the difficulty of obtaining images with diffracting vectors other than 0002. Limited observations on samples strained at 10³?s^?1 show that similar difficulties are found in imaging dislocations as are found in samples deformed at 5?s^?1 but at this strain rate, the highest used, the difficulties are reduced since images can be obtained in some grains using diffracting vectors other than 0002. These results are discussed in terms of the nature of damage as a function of strain rate and the factors that influence contrast from dislocations in crystals.     

Effects of BaTiO_3 and SrTiO_3 as the buffer layers of epitaxial BiFeO_3 thin films

BiFeO_3 (BFO) thin films with BaTiO_3 (BTO) or SrTiO_3 (STO) as buffer layer were epitaxially grown on SrRuO_3-covered SrTiO_3 substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Strain analysis of free-standing strained silicon-on-insulator nanomembrane

Based on the ultra-thin strained silicon-on-insulator(s SOI) technology, by creatively using a hydrofluoric acid(HF)vapor corrosion system to dry etch the Si O2 layer, a large area of suspended strained silicon(s Si) nanomembrane with uniform strain distribution is fabricated. The strain state in the implemented nanomembrane is comprehensively analyzed by using an UV-Raman spectrometer with different laser powers. The results show that the inherent strain is preserved while there are artificial Raman shifts induced by the heat effect, which is proportional to the laser power. The suspended s SOI nanomembrane will be an important material for future novel high-performance devices.     

Large acoustic transients induced by nonthermal melting of InSb

We have observed large-amplitude strain waves following a rapid change in density of InSb due to nonthermal melting. The strain has been measured in real time via time-resolved x-ray diffraction, with a temporal resolution better than 2 ps. The change from the solid to liquid density of the surface layer launches a high-amplitude strain wave into the crystalline material below. This induces an effective plane rotation in the asymmetrically cut crystal leading to deflection of the diffracted beam. The uniform strain in the layer below the molten layer is 2.0(+/-0.2)%. A strain of this magnitude develops within 5 ps of the incident pulse showing that the liquid has reached the equilibrium density within this time frame. Both the strain amplitude and the depth of the strained material in the solid can be explained by assuming a reduction in the speed of sound in the nonequilibrium liquid compared to measured equilibrium values.     

质子交换MgO:LiNbO3波导的双层结构

本文报导MgO:LiN_bO₃质子交换波导X射线双晶衍射和红外吸收的实验结果。这些结果表明质子交换波导由浅表面层和深表面层构成。浅表面层是一个连续应变层,有着大量随机排列的O—H—O形式的缔合OH^-基团。深表面层是一个均匀应变层,其内H⁺以自由OH^-基团形式出现。质子交换波导折射串不稳定性与双层结构中的结构驰豫密切相关。通过对质子交换过程的分析,提出两条避免双层结构的途径,其一为降低交换剂中的H⁺浓度,另一条是采用有限源工艺。     

长波长大应变InGaAs/InGaAsP分布反馈激光器的材料生长与器件制备

采用低压金属有机化合物气相沉积法(LP-MOCVD)生长并制作了1.6—1.7μm大应变InGaAs/InGaAsP分布反馈激光器.采用应变缓冲层技术，得到质量良好的大应变InGaAs/InP体材料.器件采用了4个大应变的量子阱，加入了载流子阻挡层改善器件的温度特性.1.66μm和1.74μm未镀膜的3μm脊型波导器件阈值电流低(小于15mA)，输出功率高(100mA时大于14mW).从10—40℃，1.74μm激光器的特征温度T₀=57K，和1.55μm InGaAsP分布反馈激光器的特征温度相当. 关键词： MOCVD InGaAs/InGaAsP 应变量子阱 分布反馈激光器     

Adiabatic invariance of diffraction by regularly strained crystals

Adiabatic invariants occurring in the dynamic Laue diffraction of X rays, high-energy electrons, and thermal neutrons are calculated for asymmetric scattering by regularly strained crystals. A criterion for adiabatically smooth variations of strain is found using a generalized pendulum analogy. The conceptual equivalence of the adiabatic and ray modes of Bloch wave propagation in strained crystals is established.     

Fabrication of well-defined individual dislocations in SiGe as a novel one-dimensional system

We show a new way to fabricate well-defined individual dislocations in SiGe. We started with a fully pseudomorphic but metastable SiGe layer grown on Si(0 0 1) by molecular beam epitaxy. Next, elongated (1 mm) mesa stripes with various widths (0.5–3 μm) were fabricated by a combination of isotropic and anisotropic etching. For smaller stripes, elastic relaxation of the strained SiGe layer can occur, transforming the originally biaxial strained layer into uniaxial strained subsystems. Subsequent strain relaxation caused by high temperature treatments leads to the formation of individual dislocation along the mesa stripes. The number of parallel dislocation can be adjusted by the original strain (Si:Ge ratio and layer thickness) and the mesa widths. We were able to fabricate structures with exactly one dislocation. Finally, contact pads were added to the stripes enabling the electrical characterization of individual dislocation.     

Strain induced changes in the magnetic phase diagram of metamagnetic heteroepitaxial EuSe/PbSe(1-x)Tex multilayers

Multilayers of strained metamagnetic EuSe intercalated with nonmagnetic PbSe1-xTex were grown by molecular beam epitaxy under conditions optimized by electron diffraction. From detailed structural and magnetic characterization using anomalous synchrotron x-ray diffraction and magnetization measurements, the phase transition temperatures and the magnetic phase diagrams of strained EuSe as a function of the in-plane lattice constant are determined. In this way, it is demonstrated that the magnetic properties of the samples can be significantly changed by applying biaxial strain on EuSe in superlattice structures.     

Theories of surface elasticity for nanoscale objects

The emergence of nanotechnology has driven recent interest in systems having surface atoms as a significant fraction of all atoms present, in particular nano-sheets (ultra-thin slabs), nano-wires, and nano-particles. In these systems, the bulk (i.e. non-surface region or interior) is typically strained in response to the stress of the surface. This elastic strain of the bulk in turn changes the surface lattice constants. Since the bulk and the surface are coupled, the problem must be solved self-consistently. Solving this problem requires a quantitative model of the surface elastic properties which are different from the bulk. In this paper we consider various models that have been proposed for surface elasticity. Our goal is to elucidate the relationship between two contrasting approaches: (1) the Shuttleworth equation which defines a surface stress based on the strain derivative of the surface energy and (2) the Gurtin-Murdoch (GM) theory which considers the surface layer as a membrane with residual strain and with elastic constants different from the bulk. The GM theory is analogous to the 2-D Frenkel-Kontorova (FK) model and can be used to obtain quantitative parameters for the FK model. We present an embedded atom method calculation of the surface elastic constants of Cu(1 1 1) using the GM theory with the surface represented by a membrane one atomic layer thick. This quantitative approach describes the elastic properties of surfaces in a physically appealing way. Just as the bulk elastic constants provide direct information regarding the stress/strain relationship in a bulk material, the surface elastic constants provide similar information for a surface monolayer. This theory will allow elasticity analysis and atomistic calculations of properties of nano-scale objects.     

Objective Indices of Perceived Vocal Strain

BackgroundA limited number of experiments have investigated the perception of strain compared to the voice qualities of breathiness and roughness despite its widespread occurrence in patients who have hyperfunctional voice disorders, adductor spasmodic dysphonia, and vocal fold paralysis among others.ObjectiveThe purpose of this study is to determine the perceptual basis of strain through identification and exploration of acoustic and psychoacoustic measures.MethodsTwelve listeners evaluated the degree of strain for 28 dysphonic phonation samples on a five-point rating scale task. Computational estimates based on cepstrum, sharpness, and spectral moments (linear and transformed with auditory processing front-end) were correlated to the perceptual ratings.ResultsPerceived strain was strongly correlated with cepstral peak prominence, sharpness, and a subset of the spectral metrics. Spectral energy distribution measures from the output of an auditory processing front-end (ie, excitation pattern and specific loudness pattern) accounted for 77–79% of the model variance for strained voices in combination with the cepstral measure.ConclusionsModeling the perception of strain using an auditory front-end prior to acoustic analysis provides better characterization of the perceptual ratings of strain, similar to our prior work on breathiness and roughness. Results also provide evidence that the sharpness model of Fastl and Zwicker (2007) is one of the strong predictors of strain perception.     

Cation diffusion in InP/In0.53Ga0.47As superlattices: strain build-up and relaxation

InP and In_0.53Ga_0.47As are lattice matched and can form superlattices that are free of crystalline defects. Zn indiffusion enhances the diffusion of cations while leaving the anions unaffected; the resultant In_1–x Ga _x P/In_1–x Ga _x As superlattices are strained. Since the as-grown specimens are pseudomorphic, any defects observed after Zn diffusion must be attributed to strain relaxation. Studies of the post-growth strain build-up and relaxation in this novel system suggest a new strain relief mechanism for buried strained layers of face-centred-cubic (fcc) structures. The signature defect of the proposed mechanism is a microtwin along a {111} plane spanning the buried strained layer and terminating at both interfaces with partial dislocations of 1/6112 type. Energy analysis indicates that this new partial-dislocation strain relief mechanism is more effective than the conventional 60 perfect-dislocation mechanism for relieving the in-plane strain in buried strained layers. Therefore, the proposed mechanism is an energetically favourable relaxation channel and limits the useful thicknesses of strained layers in electronic and optoelectronic devices.     

Intersubband absorption in strained AlxGa1−xN/GaN quantum wells with InyGa1−yN nanogroove layers

The intersubband absorption in a four-energy-level system consisting of a strained Al_xGa_1?xN/GaN quantum well with an In_yGa_1?yN nanogroove layer is calculated by considering the strain modification on the material parameters and polarization effect. It is found that the In_yGa_1?yN nanogroove layer in the middle of quantum well can enhance the confinement of electrons and their energy levels which consequently affect the intersubband absorption. With increasing the In composition and the groove thickness or applying a moderate compressive stress, an inflexion of energy levels appears when the lowest energy potentials of the left well and the groove are equivalent. The intersubband absorption spectrum exhibits multiple peaks contributed by different transitions. The position and height of absorption peaks are sensitive to the structural parameters (i.e., In composition and nanogroove thickness) and the strain induced by the groove layer.     

Strain and defect densities in Si/Si1-xGex heterostructures investigated by ion scattering and X-ray diffraction

MBE-grown Si/Si_1-xGe_x heterostructures on (100) Si have been characterized by Rutherford backscattering spectroscopy (RBS), ion channeling and X-ray diffraction to investigate defect densities and tetragonal lattice distortion. Critical layer thickness and relaxation of strain by formation of misfit dislocations are strongly dependent on the growth temperature. A Si_0.67Ge_0.33 layer with a thickness of 2000 Å is found to be still fully strained at a growth temperature of 450°C, whereas the same layer grown at 550°C shows considerable strain relaxation by dislocations. To obtain better depth resolution than with conventional RBS, medium energy ion scattering (MEIS) experiments have been performed on Si/Ge superlattices with layer thicknesses of 10–40 Å. A position-sensitive toroidal electrostatic analyser was employed to detect the backscattered ions simultaneously over an angular range of 30° with an energy resolution of 1 keV FWHM for 250 keV He ions, corresponding to a depth resolution of about 10 Å.