Elastic Strain Relaxation Research Articles

Structure and electronic properties of epitaxial fluorite-typeIrSi2on Si(001)

An epitaxially stabilized Ir-silicide phase was grown in ultrathin two-phase films on Si(001). Using transmission electron microscopy it was found to have the fluorite structure. Due to the misfit between this epitaxially stabilized phase and the silicon substrate, elastic and plastic strain relaxation can be observed. Optoelectronic measurements of transmission, resistivity, and Schottky barrier height show a transition from infrared absorbing to infrared transparent films depending on thickness and reaction temperature. First-principles calculations confirm the experimental data on the structure and electronic properties of fluorite-type Ir disilicide.

Epitaxial growth of Ge thick layers on nominal and 6° off Si(0 0 1); Ge surface passivation by Si

We have grown various thickness Ge layers on nominal and 6° off Si(0 0 1) substrates using a low-temperature/high-temperature strategy followed by thermal cycling. A combination of ‘mounds’ and a perpendicular cross-hatch were obtained on nominal surfaces. On 6° off surfaces, three sets of lines were obtained on top of the ‘mounds’: one along the ⟨1 1 0⟩ direction perpendicular to the misorientation direction and the other two at ∼4.5° on each side of the ⟨1 1 0⟩ direction parallel to the misorientation direction. The surface root mean square roughness was less than 1 nm for 2.5 µm thick nominal and 6° off Ge layers. Those slightly tensily strained Ge layers (R ∼ 104%) were characterized by 5 × 107 cm−2 (as-grown layers) −107 cm−2 (annealed layers) threading dislocation densities, independently of the substrate orientation. We have then described the 550 °C/650 °C process used to passivate nominal Ge(0 0 1) surfaces with Si prior to gate stack deposition. An ∼5 Å thick SiGe interfacial layer is self-limitedly grown at 550 °C and then thickened at 650 °C (5 Å min−1) thanks to SiH2Cl2 at 20 Torr. Such a Ge surface passivation yields state-of-the-art p-type metal oxide semiconductor field effect transistors provided that 15 Å Si layer thickness is not exceeded. For higher thickness, elastic strain relaxation (through the formation of numerous 2D islands) occurs, followed by plastic relaxation (for a 35 Å thick Si layer).

AlGaN/GaN heterostructures for hot electron and quantum effects

This paper reports on the transport and material characterization of AlGaN/GaN heterostructures. We compared the overheating temperatures in the transmission line model patterned devices during normal operation and analyze their dependence on the buffer thickness in order to optimize the thermal budget of the structures. It is demonstrated that noise spectra can be used to monitor the transport mechanisms and determine the activation energy of the traps. Small-dose gamma irradiation is used as an effective treatment for improving the structural properties. Based on cathodoluminescence and X-ray diffraction spectra shape, we explain the improvement as a result of the relaxation of elastic strains and structural-impurity ordering in the AlGaN barrier layer under irradiation. An irreversible improvement in mobility of the electrons in the channel, controlled by a proper dose of gamma radiation treatment, at considerably reduced self-heating is promising to achieve novel quantum transport regimes on the nanoscale.

In-place bonding of GaAs/InGaAs/GaAs heterostructures to GaAs (0 0 1)

Elastically strain-relaxed GaAs/In0.08Ga0.92As/GaAs heterostructures on GaAs (0 0 1) substrates were fabricated by the in-place bonding method. Pseudomorphic heterostructures were patterned and an underlying sacrificial AlAs layer was removed by selective etching. As the GaAs/InGaAs/GaAs structure is released from the substrate, elastic strain relaxation occurs in agreement with a force-balance model and the strain-relaxed structures are weakly bonded in place to the substrate. The bond between the strain-relaxed structure and the substrate was subsequently strengthened by annealing at 400 °C. The increase in the in-plane lattice parameter of these bonded GaAs/In0.08Ga0.92As/GaAs structures is 0.25–0.37%.

Bonding of Elastically Strain-Relaxed GaAs/InGaAs/GaAs Heterostructures to GaAs(001)

Bonding of elastically strain-relaxed GaAs/InGaAs/GaAs heterostructures has been achieved on GaAs(001) substrates by the method of in-place bonding. Pseudomorphic heterostructures were patterned and a sacrificial AlAs layer was removed by selective etching. As etching proceeds and the GaAs/InGaAs/GaAs structure is released from the substrate, elastic strain relaxation occurs and the strain-relaxed structures are weakly bonded in-place to the substrate. The bond between the strain-relaxed structure and the substrate was then strengthened by annealing under conditions similar to those used for whole wafer bonding of GaAs. The degree of strain relaxation of the InGaAs layer is determined by the relative thickness of the GaAs and InGaAs layers. The increase in the in-plane lattice parameter of these bonded GaAs/InGaAs/GaAs structures is 0.3-0.4%.

Stress analysis of Si 1−xGe x embedded source/drain junctions

The purpose of this paper is to evaluate the impact of the geometry of embedded Si 1− x Ge x source/drain junctions on the stress field. Stress simulations were performed using TSUPREM4 2D software to further investigate the elastic strain relaxation as a function of Si 1− x Ge x alloy active size, in the regime where no plastic relaxation is present. Moreover, the role of the epilayer thickness and the Ge content on the stress levels is also discussed. The work is complemented with experimental Raman spectroscopy.

Anisotropic strain state of the [11¯00] GaN quantum dots and quantum wires

The strain state of the [11¯00] GaN quantum dots and quantum wires has been studied by a combination of multiwavelength anomalous diffraction and diffraction anomalous fine structure under grazing incidence. The three components of the anisotropic deformation have been independently determined. On one hand, a weak residual strain state along the c-axis, similar for quantum dots and quantum wires, is found. On the other hand, along the [112¯0] direction, quantum wires appear to be more relaxed than quantum dots. Based on high resolution transmission electron microscopy, this is assigned to elastic strain relaxation of GaN governed by either misfit dislocation formation or three-dimensional islanding, depending on the anisotropic strain state of the [11¯00] AlN buffer layer.

Fabrication of uniaxially strained silicon nanowires

In this letter we propose a method for the fabrication of suspended strained silicon nanowires. Tensile uniaxially strained silicon is obtained by elastic strain relaxation of patterned tensile biaxially strained silicon on insulator layer. Electron beam lithography, reactive ion etching and oxidation are employed to write and transfer the nanowires down to a dimension of 15 nm in diameter. Surface smoothing and wire diameter reduction are controlled by an oxidation process.

Short-range mechanism of elastic strain relaxation during heteroepitaxial deposition

Kinetics of coherent strained epitaxy are discussed in the framework of a simple atomistic model. It is argued that large monolayer-high islands exceeding some critical size may dispose of their excessive elastic strain energy by ejecting interior atoms. In some cases, this may result in the formation of missing-row defects. The morphologies observed in $\mathrm{N}∕\mathrm{Cu}(100)$ systems qualitatively agree with our predictions. In addition, the growth kinetics simulated within a simple $1+1$ dimensional model produced bimodal island size distributions and N-shaped temperature dependence of the total number of islands similar to those observed in metallic heteroepitaxy.

Mechanism of mobility increase of the two-dimensional electron gas in AlGaN∕GaN heterostructures under small dose gamma irradiation

The effect of a small dose of gamma irradiation on transport characteristics of the two-dimensional electron gas (2DEG) in AlGaN∕GaN heterostructures was investigated. It is shown that the carrier concentration remains practically unchanged after an irradiation dose of 106rad, while the 2DEG mobility exhibits a considerable increase. The results are explained within a model that takes into account the relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

Metastability in 2D Self-Assembling Systems

We show that 2D self-assembled domains can remain trapped in a large variety of long-lived and metastable shapes that arise from an interplay of crystalline anisotropy and relaxation of elastic strain. On commonly used cubic (111) substrates, these shapes include extended or stacked structures made up of triangular domains connected at their corners, compact shapes with both convex and concave curvatures and others with narrow and elongated arms. We show that all of these distinct experimentally observed shapes can be explained within a unified framework and present a phase diagram that systematically classifies the metastable shapes as a function of their size.

Control of Magnetic Anisotropy in(Ga,Mn)Asby Lithography-Induced Strain Relaxation

We report control of magnetic anisotropy in epitaxial (Ga,Mn)As by anisotropic strain relaxation in patterned structures. The strain in the structures is characterized using reciprocal space mapping by x-ray techniques. The magnetic anisotropy before patterning of the layer, which shows biaxial easy axes along [100] and [010], is replaced by a hard axis in the direction of large elastic strain relaxation and a uniaxial easy axis in the direction where pseudomorphic conditions are retained.

VARIATION OF IN-PLANE LATTICE CONSTANT OF Si/Ge/Si HETEROSTRUCTURES WITH Ge QUANTUM DOTS

Experimental data are presented on variations of the in-plane lattice constant of Ge and Si films in the course of the MBE film growth on the silicon (100) surface. The in-plane lattice constant of the silicon film is shown to alter as the film grows; the changes reflect the process of relaxation of elastic strains that result from the misfit of the germanium and silicon lattice constants. Due to the presence of germanium islands, a considerably thicker silicon film is required to provide the strain relaxation. The dependence of distortion penetration depth to the silicon film on the effective germanium film thickness is obtained. TEM studies indicate the vertical ordering of the germanium island layers when the thickness of the Si layer in between Ge layers is not sufficient to provide the full strain relaxation.

Low density of threading dislocations in AlN grown on sapphire

We report on high resolution x-ray diffraction studies of the crystalline perfection and the relaxation of elastic strain in AlN grown by MBE on sapphire(0001). Thin (200–300 nm thick) AlN layers were grown with a very low density of threading screw dislocations. A density of 1.75−8.5×105 cm−2, the lowest value ever reported for III-Nitride epitaxial layers, was observed in a surface layer formed over a defective nucleation layer. Residual elastic strain was found in investigated AlN layers. Stress was found to be close to that expected from thermal expansion mismatch between the AlN and sapphire(0001). A model for the structural transformation of crystalline defects accounts for these observations.

Effect of elastic strain redistribution on electronic band structures of compressively strained GaInAsP/InP membrane quantum wires

The effect of redistribution of elastic strain relaxation on the energy band structures of GaInAsP/InP compressively strained membrane quantum wires fabricated by electron-beam lithography, reactive-ion etching and two-step epitaxial growth is theoretically studied using an 8-band k⋅p method. Anisotropic strain analysis by the finite element method shows that due to etching away the top and the bottom InP clad layers in membrane structures, redistribution of strain occurs. It is found that strain redistribution increases the effective bandgap of membrane quantum wire structures causing a blueshift of the emission frequency. Comparison with effective bandgap calculations neglecting confinement and band mixing demonstrates that neglect of these effects leads to an overestimation of the change in the bandgap. We have also investigated the effect of variation of wire width, barrier strain compensation, number of stacked quantum wire layers, and thickness of the top and the bottom residual InP layers in membrane structures on the change in the effective bandgap of membrane structures.

Crystalline perfection of GaN and AlN epitaxial layers and the main features of structural transformation of crystalline defects

We report on high-resolution X-ray diffraction studies of crystalline perfection and relaxation of elastic strain in GaN and AlN layers grown on SiC and sapphire substrates. Thin (300–500 nm) GaN layers are grown with a threading screw dislocation density of about 1–2×10 7 cm −2. A density of 1.75–8.5×10 5 cm −2, the lowest value ever reported for III-nitride epitaxial layers, is observed in thin (200–300 nm) AlN layers. In both cases, low density is observed in a surface layer formed over the defective nucleation layer. A model of spatial distribution of crystalline defects, based on generation of point defects on the growth surface, diffusion and further structural transformation under the action of volumetric elastic strain accounts for these observations. Perfect thin layers may be used as templates for epitaxial growth of perfect layers of other compositions, for example, GaN or InGaN layers for photovoltaic applications.

Epitaxial GaMnAs layers and nanostructures with anisotropy in structural and magnetic properties

We present results on locally strained GaMnAs structures which were deposited by low-temperature MBE on GaAs substrates, GaInAs buffer layers, or self-assembled InAs/GaAs islands. The strain within the GaMnAs is modified by elastic strain relaxation caused by nanopatterning or islanding. The influence of local strain engineering in such GaMnAs/GaInAs nanostructures on optical and magnetic properties is investigated by high-resolution X-ray diffraction, photoluminescence, and magnetometry measurements.

Strain relaxation of patterned Ge and SiGe layers on Si(0 0 1) substrates

We have investigated dislocation morphology and strain relaxation mechanisms of SiGe and Ge sub-micron wide striped mesa lines patterned on Si(0 0 1) substrates. The patterning of SiGe and Ge layers principally leads to asymmetric elastic strain relaxation. Post-patterning anneal induces 60° dislocation introduction to relax the strain but the narrower the line width the more dominant is the elastic strain relaxation. In the case of 250 nm wide SiGe lines, 60° dislocation introduction along the line is critically suppressed so that asymmetric strain distribution is realized. On the other hand, for the Ge line structure, pre-formed pure edge dislocations elongate along both orthogonal directions at the heterointerface independent of the line geometry even with the line width of 250 nm. Thus strain relaxation occurs symmetrically and rigidly. These results can be explained by deference of the introduction and propagation mechanisms of 60° and pure-edge dislocations.

Analysis of the linewidth enhancement factor (α-factor) in compressively strained InGaAsP quantum wire lasers

The linewidth enhancement factor (α-factor) of compressively strained (CS) InGaAsP/InP quantum wire (QWire) lasers is theoretically studied using an eight-band k⋅p formalism. It is found that tensile strained (TS) barriers lower the value of the α-factor by suppressing elastic strain relaxation. When multiple QWire layers are stacked vertically, the α-factor decreases owing to the increase in the modal gain. However, the decrease is lower than expected, because enhanced strain relaxation in multiple-layer stack tends to increase the α-factor. Comparison of the α-factors of QWires and QWells at the lasing frequency shows that the peak value of the α-factor decreases monotonically with increasing confinement, but an improvement in the α-factor of QWires at threshold condition relative to the α-factor of QWell lasers is not achieved unless the QWire width is reduced below a critical value. These results provide useful guidelines for designing CS QWire lasers with reduced values of the α-factor.

Dissociation of a ⟨100⟩ edge superdislocations in the γ ′-phase of nickel-base superalloys

Dissociation of a⟨100⟩ edge superdislocations in Ni3Al, the hardening γ′-phase of nickel-base superalloys, was investigated using molecular dynamics simulations and theory of elasticity. It was shown that these dislocations dissociate either symmetrically or asymmetrically when they are close to the ⟨011⟩ orientation. The symmetric dissociation, called Hirth lock, has the lowest energy. The reasons for the dissociation are the strong energy reduction due to the core splitting and the relaxation of elastic strains within the dissociation area. The dissociation of a⟨100⟩ edge superdislocations is the reason for their alignment in ⟨011⟩ orientation in the γ′-rafts of superalloys. However, the dissociation does not block the movement of the dislocation because they penetrate the γ′-rafts by climbing. Under loading conditions, typical for creep tests of nickel-base superalloys at high temperatures (≥1000°C), the Hirth lock slightly expands but remains stable. The asymmetric configuration is less stable and can transform into the lower energy Hirth lock.