Effect Of Strain Relaxation Research Articles

The effect of surface strain relaxation on HAADF imaging

In this work the effects of strain on high-angle annular dark field (HAADF) images taken in zone axis conditions have been quantitatively studied. In particular, the presence of dark contrast zones in experimental HAADF images of InGaAs–GaAs interfaces is here interpreted in terms of strain relaxation at the surface. The consistence of this assumption is demonstrated by means of experiments and simulations performed for different In compositions, specimen tilt and thickness conditions. It is shown how the HAADF contrast mechanism is related to the bending of the lattice planes in the first surface region. Finally, a generalization of the 1s approximation that is able to qualitatively describe the effect of strain on HAADF images is presented.

Effect of strain relaxation on active-region formation in InGaN/(Al)GaN heterostructures for green LEDs

Processes of active-region formation for green LEDs on the basis of multilayer strained InGaN/GaN nanoheterostructures have been studied. It is shown that the formation of structures of this kind is highly affected by elastic stress relaxation leading to a larger amount of indium incorporated into InGaN layers. For structures emitting in the blue spectral range, an increase in the number of quantum wells (QWs) from 1 to 10 does not lead to stress relaxation or to a shift of the emission wavelength, whereas for structures emitting in the green spectral range, raising the number of QWs from one to five causes a monotonic increase in the emission wavelength.

Characterisation of differential shrinkage of bonded mortar overlays subjected to drying

This work presents an experimental technique developed to assess sensitivity of cracking due to the property gradients in the layers of cement-based materials used as a bonded overlay. The non-uniform moisture distribution in materials causes differential length changes as a result of the combination of shrinkage, creep and strain relaxation effects. In this study, several tests are used for the purpose of predicting the differential behaviour (hydric, hydration, porosity and strain gradients) of material subjected to drying. Two formulations are investigated. Hydric and hydration gradients are measured by using the “slice test” and strain gradients are measured at various positions in the material layer using embedded strain gauges. The material is subjected to drying along a preferential direction into a controlled atmosphere (relative humidity = 65%; temperature = 20°C). Also, in order to study the influence of the overlay–substrate interface, specimens are cast directly on to either a non-adhesive substrate to simulate length changes for the free shrinkage case or an adhesive substrate to simulate length changes for the restrained shrinkage case. For both, the behaviour of the material differs significantly according to depth. So, the strain gradients and consequently the related gradients of mechanical properties are closely linked to hydric heterogeneities generated by non-uniform drying in the layer of material.

Investigation of Composition-dependent Optical Phonon Modes in AlxGa1-xN Epitaxial Layers Grown on Sapphire Substrates

AbstractWe reported the systematical study of optical properties of hexagonal AlxGa1-xN epitaxial films grown on c-sapphire substrate using metal-organic chemical vapor deposition. By performing Fourier transform infrared spectroscopy measurements, the high-frequency dielectric constants and phonon frequencies can be obtained by theoretically fitting the experimental infrared reflectance spectra using a four-phase layered model. The high-frequency dielectric constant of AlxGa1-xN varies between 4.98 and 4.52 for ε∝,│(polarization perpendicular to the optical axis) and between 4.95 and 4.50 for ε∝,//(polarization parallel to the optical axis) respectively when the aluminum composition changes from 0.15 to 0.24. Furthermore, from experimental infrared reflectance spectra of AlxGa1-xN films, a specific absorption dip at 785 cm-1 was observed when the aluminum composition is larger than 0.24. The dip intensity increases and the dip frequency shifts from 785 to 812 cm-1 as aluminum composition increases from 0.24 to 0.58. According to the reciprocal space map of x-ray diffraction measurements, the emergence of this dip could be resulted from the effects of strain relaxation in AlGaN epitaxial layers due to the large lattice mismatch between GaN and AlGaN epitaxial film.

Misfit dislocation generation in SiGe epitaxial layers supersaturated with intrinsic point defects

Misfit dislocation generation in SiGe/Si(001) heterostructures supersaturated with the vacancies (LT epitaxial growth) or self-interstitials (ion implantation) was studied by transmission electron microscopy. A model of “optimal” intrinsic point defects (IPDs) for effective strain relaxation is suggested and verified. Supersaturation of compressed SiGe layers with the vacancies (“optimal” IPDs) promotes high strain relaxation, whereas supersaturation with the self-interstitials (“inverse” IPDs) promotes a generation of V-shaped TDs which cannot extend to form MDs.

Homogeneous strain-relaxation effects in La0.67Ca0.33MnO3 films grown on NdGaO3

X-ray diffraction and transport measurements on a series of La0.67Ca0.33MnO3 films grown on (110)-cut NdGaO3 substrates are presented. Contrary to widespread belief assuming strain-free growth, this work shows the presence of strain in a 42nm film. On increasing thickness structural relaxation occurs, reaching a bulklike state for 500nm. No evidence of coexistence of strained and relaxed regions is found. The evolution of lattice parameters toward bulk values is accompanied by an increase of the metal-to-insulator transition temperature and a decrease of the polaron activation energy. Therefore, strain effects cannot always be neglected in La0.67Ca0.33MnO3 films grown on small-mismatch NdGaO3.

The effect of strain relaxation on electron transport in undoped Al 0.25Ga 0.75N/GaN heterostructures

The two-dimensional electron gas (2DEG) transport properties of two-step growth undoped Al 0.25Ga 0.75N/GaN heterostructures with semi-insulating buffer, grown by MOCVD, were investigated in a temperature range of 20–350 K. Using the quantitative mobility spectrum analysis (QMSA) method, it was shown that significant parallel conduction does not occur in worked structures. In-plain growth axis strains are calculated using the total polarization-induced charge density taken as the sheet carrier density measured from the Hall effect. It was found that the calculated strain values are in good agreement with those reported. Influences of the two-step growth parameters such as growth ramp time, the annealing temperature of the GaN nucleation layer on the mobility, and density of the 2DEG are also discussed.

Mechanism for persistent hexagonal island formation in AlN buffer layer during growth on Si (111) by plasma-assisted molecular beam epitaxy

The characteristics of structure and morphology of AlN grown by a growth interruption method on Si (111) with plasma-assisted molecular beam epitaxy are investigated. It is found that the growth interruption method would improve the surface flatness of the AlN layer without the formation of Al droplets. However, AlN hexagonal islands were present and persistent throughout the entire growth owing to effective strain relaxation and Eherlich-Schowebel barrier effect of preexistent surface islands grown on higher terraces of the Si substrate. The density of threading dislocations underneath the hexagonal islands is much less than elsewhere in the film, which is presumably due to dislocation annihilation during the island growth process.

자외선 광여기 전자현미경을 이용한 Si 표면 위에 Ge 나노구조의 성장 동역학에 관한 실시간 연구

자외선 광여기 전자현미경 (Ultraviolet - Photoelectron Emission Microscopy: UV-PEEM)을 이용하여 Si (001)과 (113) 표면에 Ge을 증착하면서 실시간으로 나노구조의 형성과 크기 및 형태 변화과정을 조사하였다. Ge은 PBEM에 부착된 e-beam 증착기를 이용하여 <TEX>$450-550^{\circ}C$</TEX> 온도에서 in situ로 증착하면서 표면의 변화를 PEEM으로 관찰하였다. Ge을 <TEX>${\sim}0.4\;ML/min$</TEX>의 증착율로 <TEX>${\sim}4\;ML$</TEX> 이상 두께로 증착했을 때, 두 Si 표면에서 Ge의 균일한 변형층(strained layer) 위에 island 구조가 형성되었다. 초기에 형성된 원형 모양의 island는 연속적인 Ge 증착에 따라, ripening 과정에 의해 크기가 점차 성장되었고 밀도는 감소하였으나, 형태는 원형 모양을 유지하였다. 시료 성장 후 공기 중 AFM 측정 결과, Si(001) 표면에는 dome 형태의 Ge island가 Si(113) 표면에는 윗면이 평판하고 다면의 옆면을 지닌 island 구조가 형성됨이 확인되었다. 반면에 <TEX>${\sim}0.15\;ML/min$</TEX>의 낮은 증착율로 Ge을 증착했을 때, Si(113) 표면에서 원형의 Ge island가 길죽한(elongated) 형태의 나노선 구조로 변형됨이 관찰되었다. 또한, 계속적인 Ge 증착 두께를 증가시킴에 따라 표면에는 새로운 island가 형성되지 않고, 기존의 island들이 점차 길이 방향으로 크기가 증가하면서 [<TEX>$33\bar{2}$</TEX>] 방향으로 배열하였다. 이와 같은 Ge 나노구조의 형성과 형태 변화는 나노구조 형성과정에서 변형이완(strain relaxation)과 가원자(adatom)의 표면 동역학적 효과와 깊은 관련이 있는 것으로 분석된다. The evolution dynamics of nanoscale Ge islands on both Si (001) and (113) surfaces is explored using ultraviolet photoelectron emission microscopy (UV-PEEM). Real-time monitoring of the in-situ growth of the Ge island structures can allow us to study the variation of the size, the shape and the density of the nanostructures. For Ge depositions greater than <TEX>${\sim}4$</TEX> monolayer (ML) with a growth rate of <TEX>${\sim}0.4\;ML/min$</TEX> at temperatures of <TEX>$450-550^{\circ}C$</TEX>, we observed island nucleation on both surfaces indicating the transition from strained layer to island structure. During continuous deposition the circular islands grew larger via ripening processes. AFM measurements showed that the islands grown on Si (001) were dome-shaped while the islands on Si (113) were multiple-side faceted with flat tops of (113)-orientation. In contrast, for Ge deposition with a lower growth rate of <TEX>${\sim}0.15\;ML/min$</TEX> on Si(113), we observed the shape transition from circular into elongated island structures. The elongated islands grew longer along the [<TEX>$33\bar{2}$</TEX>] during continuous Ge deposition. The shape evolution of the islands is discussed in terms of strain relaxation and kinetic effects.

Structural and optical properties of InP quantum dots grown on GaAs (001)

AbstractWe present structural and optical properties of type‐II self‐assembled InP/GaAs quantum dots using different techniques. The results reveal that the uncapped InP dots present an efficient optical emission and are partially relaxed: strain relaxation increases with the amount of InP deposited. The photoluminescence spectra show two optical emission bands associated to the quantum dots, in agreement with the bi‐modal dot‐height distribution observed by atomic force microscopy. We observed distinct photoluminescence results for uncapped and capped samples, which are mainly attributed to surface state and strain relaxation effects. A remarkable result is the large blue shift of the optical emission band from uncapped sample as compared to capped one for increasing excitation intensities. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-resolution XRD analysis of swift heavy ion irradiated InGaAs/GaAs heterostructures

Strain relaxation and swift heavy ion irradiation effects on molecular beam epitaxy grown InGaAs/GaAs heterostructures have been studied by high-resolution X-ray diffraction. Symmetric and asymmetric scans were analyzed to understand the irradiation effects on strain and defects. The broad layer peak of the experimental scans in the samples was observed. The excess width of the layer peak than the expected one has been attributed to the strain relaxation induced defects. Irradiated samples show an additional peak close to the substrate peak of compressive nature, which has been attributed to radiation damage in the substrate due to the heavy ions. Dynamical scattering theory based simulations have been compared with the experimental scans. It has also been used to deconvolute the additional peak. The simulation was comparable with the experimental scan and a satisfactory fit was obtained. The dislocation density has been reduced upon irradiation. The observed decrease in the defect density is attributed to the high electronic energy loss of the ions near the interface region. Also, the damage created by the heavy ions near the interface would enhance the diffusion of indium across the interface and hence reduce the relative mismatch at the interface.

The effects of thermal annealing in self-assembled Ge/Si quantum dots

The effects of thermal annealing in Si base self-assembled Ge dots have been investigated by Raman spectra and PL spectra. An obvious Raman frequency shift under different annealing temperature can be observed. There are two main effects during the annealing procession: one is the inter-diffusion of the Si and Ge quantum dots; the other is the relaxation of the elastic strain. With the calculated results, PL blue shift can be related to strain relaxation effects, and/or a general decrease of Ge content due to the Ge–Si intermixing.

Coercive fields in ultrathin BaTiO3 capacitors

Thickness-dependence of coercive field (EC) was investigated in ultrathin BaTiO3 capacitors with thicknesses (d) between 30 and 5.0nm. The EC appears nearly independent of d below 15nm, and decreases slowly as d increases above 15nm. This behavior can be explained not by effects of interfacial passive layers or strain relaxation, but by domain nuclei formation models. Based on domain nuclei formation models, the observed EC behavior is explainable via a quantitative level. A crossover of domain shape from a half-prolate spheroid to a cylinder is also suggested at d∼15nm, exhibiting good agreement with experimental results.

Influence of strain relaxation on magnetotransport properties of epitaxialLa0.7Ca0.3MnO3 films

In this paper, we study the effect of strain relaxation on the magnetotransport properties ofLa0.7Ca0.3MnO3 (LCMO) epitaxial films (200 nm thick), which were deposited by pulsed laser depositiontechnique under identical conditions. All the films are epitaxial and have a cubic unit cell. Theamount of strain relaxation has been varied by taking three different single crystal substrates ofSrTiO3,LaAlO3 and MgO. It has been found that for thicker films the strain becomes relaxed and producesa variable amount of disorder depending on the strength of strain relaxation. The magnitude oflattice relaxation has been found to be 0.384%, 3.057% and 6.411% for film deposited onSrTiO3,LaAlO3 and MgO respectively.The films on LaAlO3 and SrTiO3 showhigher TIM (insulator–metal transition temperature) of∼243 and 217 K respectivelyas compared to TIM of ∼191 K for the film on MgO. Similarly the Curie temperatureTC of the filmson SrTiO3 andLaAlO3 is sharper and hasvalues of ∼245 and 220 Krespectively, whereas the TC of the film on MgO is ∼186 K. A higher degree of relaxation creates more defects and hence theTIM (TC) of the film on MgO is significantly lower than those ofSrTiO3 and LaAlO3. We have adopted a different approach to correlate the effect of strain relaxation on themagnetotransport properties of LCMO films by evaluating the resistivity variation throughMott’s variable range hopping (VRH) model. The variable presence of disorder in thesethick films due to lattice relaxation, which have been analysed through Mott’s VRH model,provides strong additional evidence that the strength of lattice relaxation producesdisorder dominantly by an increase in density of defects such as stacking faultsand dislocations, which affect the magnetotransport properties of thick epitaxialLa0.7Ca0.3MnO3 films.

Magnetic linear dichroism studies of in situ grown NiO thin films

We report thickness dependence of magnetic linear dichroism (MLD) of in situ grown NiO(0 0 1) films on Ag(0 0 1) substrate at the Ni L 2 absorption edge. Antiferromagnetic domains at the surface of NiO(0 0 1) films are found to be preferentially aligned in-plane. For films thinner than a critical thickness t c (20–40ML), we observe a softening of the in-plane magnetic domain alignments with increasing film thickness, arising from the strain-relaxation effects. Films thicker than t c exhibits a residual in-plane anisotropy, possibly related to the finite-thickness effects.

Impact of strain relaxation of AlGaN barrier layer on the performance of high Al-content AlGaN/GaN HEMT

The effects of strain relaxation of AlGaN barrier layer on the conduction band profile, electron concentration and two-dimensional gas (2DEG) sheet charge density in a high Al-content AlGaN/GaN high electron mobility transistor (HEMT) are calculated by self-consistently solving Poisson’s and Schrodinger’s equations. The effect of strain re-laxation on dc I-V characteristics of Al x Ga 1- x N/GaN HEMT is obtained by developing a nonlinear charge-control model that describes the accurate relation of 2DEG sheet charge density and gate voltage. The model predicts a highest 2DEG sheet charge density of 2.42×10 13 cm -2 and maximum saturation current of 2482.8 mA/mm at a gate bias of 2 V for 0.7 μm Al 0.50 Ga 0.50 N/GaN HEMT with strain relaxation r =0 and 1.49×10 13 cm -2 and 1149.7 mA/mm with strain relaxation r =1. The comparison between simulations and physical measurements shows a good agreement. Results show that the effect of strain relaxation must be considered when analyzing the characteristics of high Al-content AlGaN/GaN HEMT theoretically, and the performance of the devices is improved by decreasing the strain relaxation of AlGaN barrier layer.

Strain relaxation and surface migration effects in InGaAlAs and InGaAsP selective-area-grown ridge waveguides

Surface migration of the group-III precursors and strain relaxation at the ridge sidewalls are compared for 2.5μm wide waveguides based on InGaAsP and InGaAlAs multiple-quantum-well (MQW) structures. The cross-sectional thickness and strain variations have been measured using synchrotron radiation-based x-ray diffraction with an angular resolution of 2arcs and a beam size of (0.24×0.35)μm2. Indium-rich overgrowth has been observed for the InGaAsP-based waveguides, while InGaAlAs-based waveguides demonstrate thickness uniformity of the MQW active region with a strain relief of 0.4%∕μm at the sidewalls.

Strain relaxation and quantum confinement in InGaN/GaN nanoposts

Nanoposts of 10–40 nm top diameter on an InGaN/GaN quantum well structure werefabricated using electron-beam lithography and inductively coupled plasma reactive ionetching. Significant blue shifts up to 130 meV in the photoluminescence (PL) spectrum wereobserved. The blue-shift range increases with decreasing post diameter. For nanopostswith significant strain relaxation, the PL spectral peak position becomes lesssensitive to carrier screening. On the basis of the temperature-dependent PL andtime-resolved PL measurements and a numerical calculation of the effect of quantumconfinement, we conclude that the optical behaviours of the nanoposts are mainlycontrolled by the combined effect of 3D quantum confinement and strain relaxation.

Investigation of the strain relaxation and ageing effect ofYBa2Cu3O7−δ thin films grown on silicon-on-insulator substrates with yttria-stabilized zirconia bufferlayers

The integration of high-temperature superconductors into advanced semiconductortechnology is rather intriguing in the microelectronics field. In this paper, we report onYBa2Cu3O7−δ (YBCO) thin films deposited on silicon-on-insulator (SOI) substrates buffered byyttria-stabilized zirconia (YSZ) layers using the in situ pulsed laser deposition (PLD)technique. The strain in the YBCO films and the ageing effect caused by the thermallyinduced cracks are carefully studied. It is believed that because of the adoption of the SOIsubstrate, the strain in the YBCO films is greatly relaxed and the quality and stability ofYBCO films are rather good, which is advantageous to the superconductive devices.

Temperature and polarization dependent polynomial based non-linear analytical model for gate capacitance of Al mGa 1− mN/GaN MODFET

A compact and accurate analytical charge control model for finding gate capacitance of Al m Ga 1− m N/GaN MODFET is presented. This temperature dependent model incorporates the effect of strain relaxation, donor neutralization and free carrier generation in the Al m Ga 1− m N layer. Non-linear Fermi potential ( E F)-variation with the sheet carrier concentration ( n s), and the charge induced in the two-dimensional electron gas (2-DEG) due to highly dominant effects of spontaneous and piezoelectric polarization at the heterojunction of MODFET are also considered. The model is based on closed form expression and does not involve any fitting parameter or elaborate computation. Agreement of calculated data with published simulated/experimental data proves the validity and applicability of the model.