Relaxation Of Compressive Strain Research Articles

Self-Assembled Nanofold Network Formation on Layered Crystal Surfaces during Metal Intercalation

We study the formation of planar network nanostructures, which develop during metal deposition on initially smooth surfaces of layered compounds. Using in situ low-energy electron microscopy for dynamic observation and high-resolution transmission electron microscopy for structure analysis, we have observed the rapid formation of hexagonal networks of linear "nanofolds" with prismatic cavities on top of layered VSe2 crystals. Their formation results from relaxation of compressive strains which build up during Cu intercalation into a thin surface layer.

Laser Assisted Catalytic Growth of ZnS/CdSe Core-Shell and Wire-Coil Nanowire Heterostructures

ZnS/CdSe core-shell and wire-coil nanowire heterostructures have been synthesized by chemical vapor deposition assisted with pulsed laser ablation. Measurements from high-resolution transmission electron microscopy and selected area electron diffraction have revealed that both ZnS/CdSe core-shell and wire-coil nanowires are of single-crystalline hexagonal wurtzite structures and grow along the [0001] direction. While the lattice parameters of ZnS and CdSe in the core-shell nanowires are nearly equal to those of bulk ZnS and CdSe, change of the lattice parameters in the CdSe-coil is attributed to the doping of Zn into CdSe, resulting in the relaxation of compressive strain at the interface between CdSe-coil and ZnS-wire. Composition variation across the interfacial regions in the ZnS/CdSe nanowire heterostructures ranges only 10–15 nm despite the pronounced lattice mismatch between ZnS and CdSe by ˜11%. Growth mechanisms of the ZnS/CdSe nanowire heterostructures are discussed.

Effect of compressive strain relaxation in GaN blue light-emitting diodes with variation of n+-GaN thickness on its device performance

The effects of compressive strain relaxation with increasing n-GaN thickness on device performances of GaN blue light-emitting diodes (LEDs) were investigated. It was found that the compressive strain relaxation in LEDs with thicker n-GaN occurred more considerably, following by the growth of active layer and p-GaN, and generated many stacking faults right beneath the InGaN active layer, which might be related to a decrease of the LED output power. On the contrary, the LED photoluminescence intensity increased surprisingly with n-GaN thickness. It was understood that the compressive strain relaxation enhanced localized states in InGaN wells.

Optical properties of InAs quantum dots with InAlAs∕InGaAs composite matrix

InAs quantum dots (QDs) in various matrices, such as InAlAs, InGaAs, InAlAs∕InGaAs, and InGaAs∕InAlAs, are prepared to clarify how the overgrown layers affect their optical properties. It is shown that strain reduction mechanism dominates the emission wavelength extension when the thickness of the overgrown layer is thin. The relaxation of compressive strain at the QD bottom by InAlAs overgrown layer cannot only extend the QD emission wavelength but also narrow the spectral linewidth and increase the state separation between the ground state and the first excited state.

Microscopic bonding and macroscopic strain relaxations at Si-SiO 2 interfaces

Optical second harmonic generation studies of the temperature dependence of the phase angle differences from contributions of terrace and edge bonding at silicon–silicon dioxide interfaces prepared on vicinal Si(111) wafers have revealed an interface relaxation process at an annealing temperature between 850 and 900 °C. Complementary studies by synchrotron soft X-ray photoelectron spectroscopy have established that this relaxation is associated with changes in the concentration and composition of local suboxide bonding environments in an ultrathin, interfacial transition region that is approximately one molecular layer thick. This relaxation occurs at a significantly lower temperature than an approximately 990±10 °C relaxation of macroscopic compressive strain in the bulk of the SiO2 film. This paper establishes an analogy between i) the Si–SiO2 interface in which there is a transition from a ‘rigid’ substrate, Si, to an ‘ideal continuous random covalent network’, SiO2, in which the average number of bonding constraints/atom matches the network dimensionality, and ii) a concentration dependent transition between ‘under-’ and ‘over-constrained’ local bonding in non-crystalline glass alloys such as GexSe1-x. The interfacial suboxide transition region is demonstrated to have properties in common with a regime of alloy compositions in which ‘self-organization’ reduces bond constraint induced strain, thereby stabilizing these compositions against ‘aging’ as for example in time-dependent changes in the glass transition temperature. These comparisons provide important new insights into defect formation at Si–SiO2 interfaces, as well as interfaces between Si and alternative high-k dielectrics being considered for advanced Si devices including Al2O3 and transition metal silicate alloys, e.g., (ZrO2)x(SiO2)1-x. This new perspective is also extended to interfaces between GaN and SiO2, where the interfacial transition region is a suboxide of Ga, GaOx, with x<1.5.

Characterisation of III–V optoelectronic devices by Internal Second-Harmonic Generation technique

A surface characterization technique based on the Internal Second-Harmonic Generation (SHG) in optoelectronic devices has been used to study the mirror degradation of 1.55 μm semiconductor optical amplifiers (SOAs) exposed to an antireflection coating deposition technique and 980 nm pump lasers suffering from mirror oxidation during life-tests. In both cases, an overall increase in SH emission was observed, as a consequence of mechanical stress enhancement and defects formation due to oxidation phenomena occurring at the interface between semiconductor and coating film. In the case of pump lasers, an oscillating, periodic trend superimposed to the general increasing one was also observed and interpreted as the periodic relaxation of compressive strain in the active layer due to vacancies formation in the near-mirror semiconductor layer.

Elastic and plastic deformation in low mismatched

We have measured, by X-ray diffraction, the deformation of uniform cadmium mercury telluride layers epitaxially grown on cadmium zinc telluride substrates with varying zinc concentration. It is then possible, within the same sample, to clearly identify elastic deformation in the regions of small lattice mismatch and plastic deformation in the regions with larger mismatch. The deformation has been analysed in a framework based on a stress relaxation model published by Fontaine et al. This analysis permits determination of the energies of formation of misfit dislocations. They have been compared with values obtained from ab initio models. From this comparison possible relaxation mechanisms are proposed. A clear dissymmetry is also observed between relaxation of tensile and compressive strain. Pre-relaxation dislocation movements are also detected by diffraction peak broadening. Possible reasons for these behaviours are discussed.

Strain Relaxation in Hexagonally Close-Packed Metal-Metal Interfaces.

Scanning tunneling microscopy observations on epitaxial Cu/Ru(0001) show the relaxation of (tensile) lattice strain to occur in four different stages with increasing film thickness, from pseudomorphic to unilaterally contracted to (quasi-)isotropically contracted film structures, with decreasing lattice strain and increasing interface energy. The underlying film layers rearrange during growth. Comparison with other data suggests this scheme to be more generally valid for tensile and, with slight modifications, also compressive strain relaxation in hexagonally close-packed metal surfaces and metal-metal interfaces.

The relaxation of compressive biaxial strains in SOS via microtwins

For heteroepitaxial systems, such as silicon on sapphire (SOS), microtwins occur in significant numbers and are thought to contribute to strain relief in the silicon thin film. The size of this contribution can be assessed from TEM measurements, of the differential volume fraction of microtwins, dV/dν (the derivative of the microtwin volume V with respect to the film volume ν), for SOS grown by both chemical vapor deposition (CVD) and molecular beam epitaxy (MBE).In a (001) silicon thin film subjected to compressive stress along the [100] axis , this stress can be relieved by four twinning systems: a/6[211]/( lll), a/6(21l]/(l1l), a/6[21l] /( l1l), and a/6(2ll)/(1ll).3 For the a/6[211]/(1ll) system, the glide of a single a/6[2ll] twinning partial dislocation draws the two halves of the crystal, separated by the microtwin, closer together by a/3.

Dislocation mechanisms of epitaxial strain relaxation in Ta/V multilayer films

The relaxation of epitaxial elastic strains ε hkl generated at the nucleation stage of tantalum growth on the (110) plane of a vanadium substrate was studied as a function of tantalum film thickness h Ta by the X-ray and electron microscopy methods. The pseudomorphic state of the tantalum film was found to be broken down along the [1 1 0] and [002] crystallographic directions of the Ta(110)/V interface at different values of h Ta: 0.45 nm and 1.8 nm respectively. The relaxation of ε hkl results from the introduction of vacancy loops in the tantalum film. This process has a multistage character owing to the large value of the lattice misfit (8.7%) and the low symmetry of the (110) interface. As in the other b.c.c. metals, the relaxation of compressive strains ε hkl in tantalum films starts with the nucleation of imperfect vacancy loops with the Burgers vector b= 1 2 〈110〉 in the Ta{110} crystallographic planes. By shear reaction at some critical size R cr 1 the imperfect loops are transformed into perfect loops with b= 1 2 〈111〉 or b=〈001〉 which subsequently change their loop planes at R cr 2> R cr 1. The type of the perfect loop depends on the value and the sign of the residual epitaxial strain of the tantalum film.

Thermal strain in lead thin films II: strain relaxation mechanisms

A deformation mechanism map was constructed to study the mechanisms of strain relaxation in lead thin films which were deposited on oxidized silicon wafers at room temperature and which were then thermally cycled between room temperature and liquid helium temperature. The stress level, which was calculated from the strain measured by an X-ray diffraction technique, was plotted on the map. By comparing the calculated and experimental stress levels the following observations were obtained. In the cooling process the strain was relaxed rapidly in a field of dislocation glide mechanism for films of greater than 0.2 μm thickness. In the heating process most of the strain was again believed to be relaxed by the glide mechanism. For a film 0.5 μm thick the stress (after the primary relaxation was completed) was found to be (1–1.5) × 10 9 dyn cm -2 for the cooling process and (0.17–0.24) × 10 9 dyn cm -2 for the heating process at temperatures around 200–280 K. Slow secondary relaxations were observed after the primary relaxations were completed. The measured compressive strain relaxation rate at room temperature was very close to the rate calculated on the assumption of grain boundary diffusion creep. This suggested that the secondary relaxation mechanism of compressive strain was grain boundary diffusion creep at temperatures near room temperature. These suggestions were supported by scanning electron microscopy observations in which dislocation slip lines were observed inside grains and hillocks were observed on grain boundaries.