Lattice Strain Relaxation Research Articles

Strain Relaxation in Surface Nano-Structures Studied by X-Ray Diffraction Methods

We study the lattice strain relaxation in pseudomorphic surface gratings using high resolution X-ray diffraction (XRD), grazing incidence diffraction and elasticity theory. By means of grazing incidence diffraction we determine the grating shape and detect a depth dependent lattice strain relaxation in the grating. Symmetrical and asymmetrical XRD gives evidence of a non-uniform strain relaxation in the etched structures and the creation of a periodic strain field deep in the substrate. The experimental findings are confirmed by an elasticity model which describes the interaction of the different crystalline media. Comparing the measured diffraction maps with calculated ones, we determine the actual strain distribution in the trapezoidal grating and in the substrate.

Effects of stress relaxation of epitaxial SrRuO3 thin film on microstructures

We report the effect of lattice stress relaxation on the microstructures of epitaxial thin films by domain structure studies of epitaxial SrRuO3 thin films grown on vicinal (001) SrTiO3 substrates. X-ray diffraction analysis revealed that the as-grown films are single domain and have a strained lattice due to the lattice mismatch with the substrate. In contrast, plan-view transmission electron microscopy (TEM) images obtained from the same films showed the coexistence of domains with three different crystallographic orientations. The discrepancy is attributed to the lattice stress relaxation occurring on the TEM specimens as the substrate material is eliminated by ion milling or etching, resulting in the formation of elastic domains with different crystallographic orientations. These studies directly reveal a crucial effect of the lattice strain relaxation on the microstructures and properties of epitaxial thin films when the substrate material is removed.

Relaxation of the Si lattice strain in the Si(001)–SiO2 interface by annealing in N2O

It was clearly observed with medium energy ion scattering spectroscopy that the strain in the Si(001)–SiO2 interface of thermal oxides is relaxed by annealing in N2O. The strain relaxation could be correlated with the improved hot-electron hardness of the nitrided oxides compared with the thermal oxides. Based on the direct observation of the strain relaxation, it is suggested that the incorporated N atoms at the interface release the strain and increase the immunity of trap generation under the current stress.

Study of the lattice strain relaxation in the Ga1-XAl X Sb/GaSb system by x-ray topography and high resolution diffraction

High resolution x-ray diffraction and topographic methods have been used to study lattice strain relaxation in the Ga1-XAl X Sb/GaSb system. Samples with layer thickness ranging between 0.1 and 6 μm and with Al concentration x=0.402±0.005 have been grown by molecular beam epitaxy at 550 °C on (0 0 1) oriented undoped GaSb LEC substrates. A first critical thickness (tC1), related to the misfit dislocation generation, has been found to be between 0.16 < tC1 < 0.20 μm. Due to the weak sensitivity of the rocking curve to the onset of relaxation, this result has been obtained by means of a double crystal topographic technique. A “plateau” region in the curve of the residual strain versus thickness has been observed for t ranging between 0.2 and 0.5 μm. The residual strain ɛres shows a dependence close to t–0.5 above a second critical thickness value tC2 slightly larger than 0.5 μm. Finally, in the last region above a layer thickness of 3 μm, strong dislocation interaction effects seem to affect the relaxation. A comparison with theoretical models has been made.

Strain relaxation in InAs/GaSb heterostructures

Lattice strain relaxation in InAs/GaSb heterostructures was investigated by x-ray diffraction. Two types of structures, grown by molecular beam epitaxy, are compared. In the first, GaSb buffer layers were grown on GaAs substrates, followed by 0.05–1.0 μm thick InAs layers. In the second, InAs layers were grown directly on GaSb substrates. For a given thickness, the InAs layers retain significantly more strain when grown on GaSb substrates, reflecting the lower threading dislocation density in the GaSb substrates relative to the GaSb buffer layers grown on GaAs.

Ion bombardment induced relaxation of strained [formula omitted] heterostructures studied by the complementary use of RBS-channeling and X-ray synchrotron radiation

Al x Ga 1− x As epitaxial layers grown on semi-insulating GaAs (SI-GaAs) substrates with x = 0.25 and on in-doped GaAs with x = 0.45 were implanted at room temperature with 1.5 MeV Se ions to fluences ranging from 0.6 × 10 14 to 4 × 10 14at/cm 2. Implanted crystals were analyzed with 1.8 MeV 4He ions using the RBS/channeling technique. In order to determine the defect structure and their depth profiles measurements were carried out at two different temperatures: 295 and 105 K. XRD with synchrotron beams was used for structural analysis. White beam topography was used to detect lattice deformation and strain relaxation. The lattice parameters were determined by rocking curve measurements with a monochromatic beam. For all samples a significant tetragonalization of the implanted region was observed. For the layers grown on SI-GaAs the lattice distortion increases with increasing ion dose until a critical lattice strain is attained. At that point the complete relaxation of the epitaxial layer occurs. In contrast, no relaxation was observed for layers grown on practically defect free in-doped substrates. The mechanism of this transformation is attributed to the much higher dislocation density in the SI-GaAs substrate. Nucleation of new dislocations at defects produced by ion implantation and their interaction with threading dislocations leads to the strain relaxation by plastic deformation of epitaxial layers.

Effect of Temperature and Stress on the Structure and Creep Parameters of Pb—2 at% Sb Alloy

The transient and steady state creep parameters of Pb–2at% Sb alloy are determined in the temperature range from 443 to 503 K. The transient creep parameters β and n increase with working temperature and applied stress and have values changing from 43×10—4 to 355×10—4 and from 0.46 to 0.79, respectively. The strain rate sensitivity parameter m changes from 0.12 to 0.2 which points to a cross slipping dislocation mechanism. The activation energies of both transient and steady state creep are calculated. In the low temperature range (443 to 473 K), the steady state activation energy is found to be stress dependent. The X-ray analysis supports that relaxation of the internal lattice strain takes place at the transformation temperature (473 K).

Effect of Temperature and Stress on the Structure and Creep Parameters of Pb—2 at% Sb Alloy

The transient and steady state creep parameters of Pb–2at% Sb alloy are determined in the temperature range from 443 to 503 K. The transient creep parameters β and n increase with working temperature and applied stress and have values changing from 43×10—4 to 355×10—4 and from 0.46 to 0.79, respectively. The strain rate sensitivity parameter m changes from 0.12 to 0.2 which points to a cross slipping dislocation mechanism. The activation energies of both transient and steady state creep are calculated. In the low temperature range (443 to 473 K), the steady state activation energy is found to be stress dependent. The X-ray analysis supports that relaxation of the internal lattice strain takes place at the transformation temperature (473 K).

Lattice strain relaxation study in the Ga1−xAlxSb/GaSb system by high resolution x-ray diffraction

Lattice strain relaxation has been studied in the Ga1−xAlxSb/GaSb system by analyzing layers with thicknesses ranging between 0.1 and 6 μm and with Al concentration x=0.4. The samples have been grown by molecular beam epitaxy at 550 °C on (001) oriented undoped GaSb substrates. The heterostructures were investigated by a high resolution x-ray diffraction method. The experimental critical thickness for the strain relaxation has been found to be tc≈ 310 nm. The measured residual strain εres shows a t−0.47 dependence on the layer thickness t. This result is in agreement with the theoretical predictions based on the energy balance model, and cannot be described in the frame of the equilibrium theories which yield εres∝t−1. The values of εres have been corrected by the so-called thermal misfit (ΔT) for epitaxial systems with different thermal expansion coefficients. For the Ga0.6Al0.4Sb/GaSb epitaxial system grown at TG=550 °C, the value ΔT=−3.28×10−4 has been calculated from the literature data.

Generation of vacancy-type point defects in single collision cascades during swift-ion bombardment of silicon

Silicon samples of n-type have been implanted with low doses (${10}^{7}$-${10}^{10}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$) of $^{11}\mathrm{B}$, $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{28}\mathrm{Si}$, $^{74,76}\mathrm{Ge}$, and $^{120}\mathrm{Sn}$ ions using energies between 0.4 and 8.0 MeV. Because of the low doses employed, single-collision cascades prevail, and an analysis of the implanted samples by deep-level transient spectroscopy (DLTS) reveals two dominant point defects of vacancy type, the divacancy (${\mathrm{V}}_{2}$) and the vacancy-oxygen (VO) center. The generation of ${\mathrm{V}}_{2}$ and VO is studied in detail as a function of ion mass, dose, dose rate, sample depth, and sample temperature. Surface-enhanced annihilation of migrating defects is found to play a significant role in depleting the concentration of ${\mathrm{V}}_{2}$ and VO centers in proximity to the surface. The concentration of ${\mathrm{V}}_{2}$ and VO increases linearly with ion dose, but the generation efficiency per incoming ion decreases at high dose rates after displaying a constant value below \ensuremath{\sim}${10}^{8}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. This dose rate effect exhibits a dependence on ion mass, and is qualitatively predicted by computer simulations of the defect reaction kinetics, applying a model where the interaction between individual collision cascades is primarily due to fast-diffusing Si self-interstitials. At dose rates \ensuremath{\leqslant}${10}^{8}$${\mathrm{ncm}}^{\mathrm{\ensuremath{-}}2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, the generation of ${\mathrm{V}}_{2}$ centers per ion-induced vacancy in the damage peak region is identical, within \ifmmode\pm\else\textpm\fi{}10%, for all the different types of ions studied. However, the ${\mathrm{V}}_{2}$ centers formed by heavy ions are strongly perturbed, as shown by large deviations from a one-to-one relation between the two DLTS peaks associated with the singly and doubly negative charge state of ${\mathrm{V}}_{2}$, and also by a broadening of the two peaks. In contrast to that for ${\mathrm{V}}_{2}$, the generation of VO centers per ion-induced vacancy decreases with increasing ion mass, consistent with the picture that light ions are more effective in generating point defects than heavy ions. At elevated implantation temperatures, gradual relaxation of lattice strain occurs together with recrystallization of disordered zones, promoting the formation of unperturbed ${\mathrm{V}}_{2}$ centers and increasing the generation efficiency of VO centers.

Microstructure of α-AgI-frozen composites in the AgI- Ag2O-B2O3 system after heat treatment

The microstructure of the α-AgI-frozen 82AgI.13.5Ag 2 O.4.5B 2 O 3 composites after heat treatment at various temperatures were investigated to clarify the morphology changes associated with the relaxation of lattice strain of frozen α-AgI, which brought about the α→β phase transformation. We discuss the stabilization process of α-AgI during melt-quenching and the transformation behaviour of frozen α-AgI during thermal annealing from the viewpoint of morphologies of the composites.

Reflection High-Energy Electron Diffraction Image Analysis during Molecular Beam Epitaxial Growth of InAs/AlGaAsSb Deep Quantum Well Structures

We have investigated the lattice strain relaxation during the initial stage of Alx Ga1-x AsSb (0<x<0.5) growth on GaAs (100) substrates by molecular beam epitaxy (MBE). Using time-resolved reflection high-energy electron diffraction (RHEED) image analysis, we have found that AlGaAsSb on a GaAs surface quickly relaxes within 3 to 7 monolayers (MLs). We have also observed that two-dimensional (2D) growth occurs after deposition of 20MLs of GaAsSb on GaAs surfaces. Furthermore, we have confirmed the InAs grows on AlGaAsSb two-dimensionally because AlGaAsSb is lattice matched to InAs. With a total thickness of less of than 700 nm in the InAs/AlGaAsSb deep quantum wells (DQWs), we have achieved very high electron mobilities of more than 32,000 cm2/(V·s) at room temperature.

Ultra-low Contact Resistivity by High Concentration Germanium and Boron Ion Implantation Combined with Low Temperature Annealing

AbstractA new formation method for an ultra-low resistivity contact and structural analysis are discussed. In this study, a minimum contact resistivity of 6.9× 10−9 Ω·cm2 has been successfully obtained in an AlSi(lwt%)Cu(0.5wt%) alloy / Si system by using heavy dose ion implantations of Ge and B combined with low temperature annealing as low as 550°C. Structural analysis on the Si surface layer has been carried out. As a result, it was found that the crystal lattice was expanded in-plane by 6% in the Si layer near the surface because of the high concentration of Ge and no relaxation of lattice strain at such a low temperature. It was also found that super-saturation of B was stably formed in this system. An ultra-low resistivity contact is considered to be a result of these combined effects.

Investigations on Fe Lattice Strain Relaxation in the Al/Fe/Al Trilayer

AbstractWe have measured polarization-dependent Fe K-edge XAFS spectra from an epitaxially-grown Al/Fe/Al/ZnSe/GaAs(001) sample using synchrotron radiation. The Fourier-transformed XAFS spectra were fit up to four coordination shells simultaneously using theoretical spectra generated with FEFF 6. Our results reveal a small tetragonal distortion of the bcc α-Fe film lattice towards a contraction in the film growth direction (c=2.863 Å and a=2.874 Å; bulk a=2.867 Å). The anomalous tetragonal distortion is consistent with a model involving substantial interdifussion of Fe and Al, leading to Fe1−xAlx alloy formation localized in the interface regions.

Low temperature growth of (100) HgCdTe layers with DtBTe in metalorganic vapor phase epitaxy

Growth characteristics of (100) HgCdTe (MCT) layers by MOVPE at low temperature of 275°C were studied using ditertiarybutyltelluride as a tellurium precursor. Growths were conducted in a vertical narrow-spacing growth cell at atmospheric pressure. Cd composition of MCT layers were controlled from 0 to 0.98 using dimethylcadmium (DMCd) flow. The growth rate was constant for increase of DMCd flow. During the growth, Cd was incorporated preferentially into the MCT layers. Enhancement of Cd incorporation in the presence of Hg was also observed. Crystal quality and electrical properties were also evaluated, which showed that high quality MCT layers can be grown at 275°C. Strain in CdTe layers grown at 425 and 275°C was also evaluated. Lattice parameter of layers grown at 425°C approached bulk value at thickness of 5 μm, while layers grown at 275°C relaxed at 1 μm. The rapid strain relaxation of layers grown at 275°C was considered due to the layer growth on the strain relaxed buffer layer. The effect of the thermal stress on the relaxation of CdTe lattice strain was also discussed.

Reflection high-energy electron diffraction study of the heterointerface formation of ZnSe/ZnTe

The purpose of this study is to investigate the interface formation process by atomic layer epitaxy (ALE) and to know whether ALE is still realized during the formation of interface in a highly lattice-mismatched case. Based on detailed reflection high-energy diffraction (RHEED) measurements during ALE growth of ZnTe on ZnSe, we will suggest that (1) at the very beginning of heteroepitaxy, the surface coverage is less than unity even for saturation coverage, hence the incorporation of atoms should be smaller than one monolayer (1 ML) during each open period of the shutter; (2) two-dimensional (2D) laminar growth is preserved until lattice strain relaxation takes place; (3) lattice strain relaxation starts at the deposition of nominally 5 ML thick ZnTe, followed by 3D island growth.

An X-ray diffraction study of the lattice strain relaxation in MOVPE GaAs/Ge heterostructures

GaAs/Ge heterostructures were grown by metal-organic vapour phase epitaxy with different V/III flow ratios; the layer thickness was always much larger than the critical threshold for the elastic strain relaxation. The structural properties of the specimens were investigated by both X-ray topography and high-resolution X-ray diffractometry. It has been found that, at high V/III ratios, the layers are affected by a high density of stacking faults, whereas misfit dislocations are completely absent; the fault density decreases on decreasing the V/III value. A small, but significant strain relaxation has also been observed, thus demonstrating the effectiveness of the partial dislocations bordering the faults in relaxing the strain. At lower V/III ratios the dominant defects are the misfit dislocations to which the strain relaxation can be attributed.

Lattice strain relaxation of ZnS layers grown by vapour-phase epitaxy on (100) GaAs

The structural characterization of ZnS epilayers grown on (100) GaAs by H 2 transport vapour-phase epitaxy is reported. High-resolution X-ray diffraction and ion channelling Rutherford backscattering spectrometric measurements were used to evaluate the overall crystalline perfection of the epilayers and the distortion and reciprocal orientation of the ZnS and GaAs lattices and to measure the epilayer built-in lattice strain. High crystalline quality can be achieved at the surface of relatively thick (above ca. 1 μm) ZnS layers, although a dense distribution of extended defects was observed close to the ZnS/GaAs interface. It turns out that the ZnS unit cell is orthorombically distorted, although its deviation from a purely tetragonal distortion is very small. Systematic measurements of the ZnS lattice strain were performed on the epilayers and the results were compared with the thermal strain value obtained by temperature-dependent X-ray diffraction measurements. The data indicate that the ZnS built-in lattice strain can be entirely ascribed to a thickness-dependent thermal strain contribution, the initial lattice misfit at the growth temperature being almost totally relaxed even for relatively thin epilayers. Finally, no appreciable strain was found for epilayer thicknesses above about 3 μm.

Annealing Effects on Magnetic Properties of Co/Pt Multilayers

Co/Pt multilayer films sputter-deposited with and without a substrate bias of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> =-30 V were annealed in vacuum at 300 and 400°C. On annealing the saturation magnetization M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> and the saturation Kerr rotation angle θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</sub> both increased, while the perpendicular anisotropy energy K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> decreased monotonically. The decrease in K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> was larger for -30 V biased films than for 0 V films because of the larger decrease in the surface anisotropy K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> for -30 V biased films. This larger decrease in K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> is discussed taking the relaxation of lattice strain due to annealing into account.

Magnetic anisotropy and lattice strain in Co/Pt multilayers

The correlation is reported between perpendicular anisotropy and in-plane lattice strain in Co/Pt multilayers. (Cox/Pty)n samples, where x, y are the thickness of the individual Co and Pt layers and n is the number of repeats that were prepared by molecular beam epitaxy and studied by means of polar magneto-optic Kerr effect and transmission electron microscopy. Kerr rotation data and electron diffraction experiments show that the largest perpendicular anisotropy and square hysteresis loop occur when x=3 Å while the Pt layers are subjected to about −2% in-plane strain. As Co thickness increases, Co and Pt layers gradually lose coherency and the magnetic anisotropy goes from perpendicular to planar. This is accompanied by a relaxation of lattice strain in both Co and Pt layers. The close relationship between magnetic anisotropy and lattice strain can be explained as magnetoelastic anisotropy or stress anisotropy effect due to lattice mismatch between the adjacent epitaxial layers.