Depth Distribution Of Strain Research Articles

Understanding the origin of mobility enhancement in wedge-shaped c-GaN nanowall networks utilizing spectroscopic techniques

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically.

X-ray Reciprocal Space Mapping of Graded Al x Ga1 - x N Films and Nanowires.

The depth distribution of strain and composition in graded AlxGa1 − xN films and nanowires (NWs) are studied theoretically using the kinematical theory of X-ray diffraction. By calculating left(20overline{2}5right) reciprocal space maps (RSMs), we demonstrate significant differences in the intensity distributions from graded AlxGa1 − xN films and NWs. We attribute these differences to relaxation of the substrate-induced strain on the NWs free side walls. Finally, we demonstrate that the developed X-ray reciprocal space map model allows for reliable depth profiles of strain and Al composition determination in both AlxGa1 − xN films and NWs.

Nanoscale organization by elastic interactions between H and He platelets in Si

We used ion implantation of H and He in Si and thermal treatments to produce two systems allowing to study the effects of global and local mechanical stress fields on the formation energy of H-precipitates called H-platelets. In the first part of the work, the depth-distribution of different crystallographic orientations of the precipitates formed along the implanted layer was characterized by transmission electron microscopy. The global strain in the region was measured by X-ray diffraction, and the depth distribution of strain was reconstructed using a dynamical-theory-based code. Elasticity theory was used to develop a model based on mechanical interactions, explaining the preferential presence of (001)-oriented precipitates in the more stressed region of the implanted layer. In a second part, local sources of stress of nanometer size and cylindrical symmetry were introduced in a deeper region of the matrix, before the nucleation of H-platelets. The local stresses were embodied by (001) He-plate precipitates. Upon annealing, a specific arrangement of crystallographic variants of {111}-oriented H-platelets in a four-fold configuration was observed. To explain these experimental observations, and to calculate the variations of the formation energy of the precipitates under the presence of local stress tensors components, analytical and numerical (finite element method) approaches were used to develop 2D and 3D models based on elasticity theory. The concepts and modeling strategy developed here paves the way for determining the required conditions to create controlled architecture of precipitates at the nanoscale using local stress engineering.

Influence of freely diffusing excitons on the photoluminescence spectrum of Si thick films with depth distribution of strain

We present an interpretation of photoluminescence (PL) for relatively thick Si films (200–300 nm) with depth distribution of strain, in which freely diffusing excitons influence the PL signal that originates from layers with different strain conditions. Micro-PL was excited by a 325 nm laser at 8.5 K for the Si films. The PL spectra clearly depended on the thickness of the strained part (ts) in the strained Si film. Under the condition of ts greater than the penetration depth (dp) of 325 nm line for Si, only the strained-part-related PL (PLs) could be observed but not the unstrained-part-related PL (PLus). With a decrease in ts, PLus gradually appeared and became strong, and simultaneously PLs became weak. For positions with very small ts, PLs could never be observed. The strain completion for the Si films was also investigated based on exciton behaviors in strained semiconductor. These characteristics of PL are useful references for understanding exciton behavior in semiconductor with depth distribution of band gap.

325 nm-laser-excited micro-photoluminescence for strained Si films

Low-temperature micro-photoluminescence (PL) was performed for strained Si (sSi) films by 325 nm-laser excitation at 8.5 K. All of the sSi films were thicker than the penetration depth ( d p) of the 325-nm line for Si. The dependence of the PL spectra on the strain condition was studied by comparing d p to the thickness of the strained part ( t s), which varied in the sSi film plane. Under the condition t s > d p, the strained-part-related PL (PL-S) was observed, but not the unstrained-part-related PL (PL-US). Under the condition t s < d p, PL-US appeared and its intensity negatively depended on t s, while the intensity of PL-S positively depended on t s. Under the condition of a very small t s, PL-S was never observed. These phenomena were explained by exciton behaviors in sSi film with a band-gap distribution, and enable a deeper understanding of PL characteristics in a relatively large-scale sample with a depth distribution of strain.

Depth distribution of strain in GaN/AlN/SiC heterostructures by DUV micro‐Raman spectroscopy

We measured the depth distribution of strain in GaN/AlN/SiC heterostructures by using deep ultraviolet (DUV) micro-Raman and photoluminescence spectroscopies with bevel shaped specimens. Raman signal of AlN were detectable for the layers thicker than 15 nm. AlN and GaN layers had compressive strain in the whole region and the strain relaxation along the growth direction was observed. In this study, the variation of the Raman shift was well explained by a strain relaxation model that the misfit dislocations are distributed not only at the interface, but also throughout the film. The relaxation rate related to the generation rate of misfit dislocations was different for the AlN and GaN layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of implanted semiconductors by means of white-beam and plane-wave synchrotron topography

Al(x)Ga(1-x)As epitaxic layers, with x = 0.43 and GaAs single crystals implanted with various doses of high-energy Se and Si ions, and silicon single crystals implanted with high-energy light ions were studied by means of different X-ray diffraction methods employing either a strongly limited white beam or a highly collimated monochromatic beam. The methods provided complementary characterization of lattice parameter changes and lattice deformation in the implanted layers. The synchrotron rocking curves recorded with a small-diameter beam provided a very good separation of interference maxima and enabled determination of the strain profile. A characteristic difference in strain depth distributions between the implanted A(III)B(V) compounds and silicon was noticed. Ion implantation in A(III)B(V) compounds produced a relatively thick layer with an almost constant and distinctly increased lattice parameter in regions close to the surface, whereas, in the case of silicon, shot-through layers with almost unchanged lattice spacing were observed. Other important information obtained from the synchrotron micro-Laue pattern was that the interference fringes caused by crystal curvature or strain gradient are located in the plane of diffraction, while parts of the Laue spot corresponding to the deformed regions are usually displaced.

Structural strain in a-axis oriented thin films studied by electron backscatter diffraction

The electron backscatter diffraction technique has been used to study the depth distribution of the structural strain in a-axis oriented films made by a self-template method. It has been found that the structural strain was mainly formed in the upper layer near the interface between the template and upper layer. In contrast, there is little strain in the template layer. The strain can be relaxed when the thickness of the upper layer exceeds a certain value. The results agree with the observation of a dramatic improvement in the transition temperature and metallic behaviour when the upper layer was thicker than about 200 nm. Such a depth distribution of strain has been discussed in correlation with the degradation of in a-axis films. The microstructures of these films have been investigated by transmission electron microscopy.

Strain and defects depth distributions in undoped and boron-doped Si1−xGex layers grown by solid phase epitaxy

A detailed characterization of undoped and heavily boron-doped Si1−xGex layers with x=0.21, 0.26, and 0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The starting material for solid phase epitaxial growth was prepared by amorphization of epitaxial SiGe-Si heterostructures by ion implantation. In order to obtain doped layers, boron was also implanted into some of the amorphous samples. After regrowth, the strain depth distributions of the SiGe layers were measured using axial channeling angular scans and the defect distributions were observed by high-resolution electron microscopy. A defect-free region ranging from 0 nm (undoped layer of x=0.34) to 30 nm (doped layer of x=0.21) in thickness was observed next to the layer-substrate interface. In the upper region of the layers, strain-relieving defects, identified as planar faults, were observed. Some isolated defects were also present at the layer-substrate interfaces of most of the samples. The measured strain depth profiles show that (i) the defect-free regions are not always fully strained; the defects located at the interfaces being responsible for this partial relaxation; (ii) the strain is almost constant throughout the defect-free layers because it cannot be relieved due to the absence of defects; and (iii) the strain progressively decreases towards the sample surfaces in the region of the layer where the strain-relieving defects are located. Comparison between the undoped and boron-doped layers show the consequences of the strain compensation effect due to the incorporation of boron atoms into the lattice. The defect-free regions of the doped layers are thicker and closer to coherency than those in the undoped layers of the same composition and the defect density in the upper region of the layers is significantly reduced. As a result of the strain compensation effect, a 30-nm-thick heavily doped layer with x=0.21 is found to be defect free and fully strained throughout its whole thickness although the corresponding undoped layer was partially relaxed and showed strain-relieving defects.

Characterization of beryllium-implanted GaAsP/GaP strained-layer superlattice structure

A dynamic model is presented based on Takagi's theory of dynamic diffraction of a distorted crystal, which incorporates depth-dependent strain and spherically symmetric gaussian distribution of random displaced atoms. The analysis of X-ray rocking curves using this model provides damage and strain depth distributions for ion-implanted strained-layer superlattices. This model has been used to interpret an experimental logarithmic (400) X-ray diffraction intensity vs. 2θ angle profile of a 75 keV beryllium-implanted GaAsP/GaP strained-layer superlattice structure reported in the literature.

Parallel stress and perpendicular strain depth distributions in [001] silicon amorphized by ion implantation

Measurements performed by x-ray diffraction techniques on silicon wafers implanted with silicon ions show that surface amorphous film and underlying interstitial defects are able to induce wafer convexity. The simulation of the rocking curves obtained by double-crystal diffraction before and after annealing, the measurement of the total vertical expansion of the implanted layer, the determination of the curvature radius, and the use of a theoretical model, directly compatible with the output of rocking curve simulation, recently developed for multilayered crystalline structures, allow us to determine the tangential stress depth distribution of the implanted wafer and the average relaxed volume expansion of the amorphous layer. The results obtained show that both stress and volume expansion increase with dose for a given ion-beam energy. A comparison is made with similar data reported recently in the literature for high-dose arsenic-implanted silicon.

Parallel Stress and Perpendicular Strain Depth-Distributions in [001] Silicon Amorphized by Ion Implantation

Parallel Stress and Perpendicular Strain Depth-Distributions in [001] Silicon Amorphized by Ion Implantation