Влияние морфологии буферного слоя AlN на структурное качество полуполярного слоя GaN, выращенного на подложке Si(001), по данным просвечивающей электронной микроскопии

Structural features of the interface between semipolar gallium nitride layer and buffer layer of aluminum nitride grown on a SiC/Si(001) template misoriented by an angle of 7° were studied by high-resolution transmission electron microscopy. The effect of interface morphology on the structural quality of the gallium nitride layer is revealed: faceted structure the surface of the buffer layer reduces the threading dislocations density.

Equal channel angular pressing of wire-formed Al6063 by PU rubber-assisted procedure

Expanding suitable severe plastic deformation processes seems essential to design lightweight wire-formed materials for emerging demands. In this regard, 6063 aluminum alloy in the form of wire was processed successfully by polyurethane rubber assisted-equal channel angular pressing up to 16 passes by route BC. It was found that significant improvement of hardness and strength is achieved at the initial passes due to the increment of material’s dislocations density which leads to the crystallite size decrease and lattice microstrain increase. Also, subsequent passes improve the mechanical properties with a gentle rate due to the saturation of dislocation strengthening. The fractography analysis indicated that the ductile fracture mode of the annealed aluminum decreases by imposing the ECAP process. It is related to the formation of cleavage and rive patterns and the reduction in the number and size of the dimples compared to the initial condition. Eventually, X-ray diffraction findings showed that by adding pass numbers, the isotropy degree of the aluminum sample enhances because of the lowest diffraction scattering.

Impact of Long-Term Annealing on Photoluminescence from Ge1−xSnx Alloys

We report on the connection between strain, composition, defect density and the photoluminescence observed before and after annealing at 300 °C for GeSn samples with Sn content of 8% to 10%. Results show how the composition and level of strain influenced the separation between the indirect and direct optical transitions, while changes in the level of strain also influenced the density of misfit dislocations and surface roughness. The effect of annealing is observed to lower the level of strain, decreasing the energy separation between the indirect and direct optical transitions, while also simultaneously increasing the density of misfit/threading dislocations and surface roughness. The reduction in energy separation leads to an increase of photoluminescence (PL) emission, while the increase of misfit/threading dislocations density and surface roughness results in a decrease of PL. Consequently, the competition between these factors is observed to determine the impact of annealing on the PL. As a result, annealing increases the collected PL for small (≤40 meV) separation between the indirect to direct optical transitions in the as-grown sample while decreases the PL for larger (≥60 meV) separations. More generally, these numbers have a small dependence on the level of strain in the as-grown samples.

Effect of nucleation layer thickness on reducing dislocation density in AlN layer for AlGaN-based UVC LED

Purpose The purpose of this study is to investigate the influence of AlN nucleation thickness in reducing the threading dislocations density in AlN layer grown on sapphire substrate. Design/methodology/approach In this work, the effect of the nucleation thickness at 5 nm, 10 nm and 20 nm on reducing the dislocation density in the overgrown AlN layer by metal organic chemical vapor deposition was discussed. The AlN layer without the nucleation layer was also included in this study for comparison. Findings By inserting the 10 nm thick nucleation layer, the density of the dislocation in the AlN layer can be as low as 9.0 × 108 cm−2. The surface of the AlN layer with that nucleation layer was smoother than its counterparts. Originality/value This manuscript discussed the influence of nucleation thickness and its possible mechanism in reducing dislocations density in the AlN layer on sapphire. The authors believe that the finding will be of interest to the readers of this journal, in particular those who are working on the area of AlN.

GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications

III-V semiconductors present interesting properties and are already used in electronics, lightening and photonic devices. Integration of III-V devices onto a Si CMOS platform is already in production using III-V devices transfer. A promising way consists in using hetero-epitaxy processes to grow the III-V materials directly on Si and at the right place. To reach this objective, some challenges still needed to be overcome. In this contribution, we will show how to overcome the different challenges associated to the heteroepitaxy and integration of III-As onto a silicon platform. We present solutions to get rid of antiphase domains for GaAs grown on exact Si(100). To reduce the threading dislocations density, efficient ways based on either insertion of InGaAs/GaAs multilayers defect filter layers or selective epitaxy in cavities are implemented. All these solutions allows fabricating electrically pumped laser structures based on InAs quantum dots active region, required for photonic and sensing applications.

Strain-gradient effect on the crack tip dislocations density

In this study, the influence of a material’s plastic properties on the crack tip fields and dislocation density behavior is analytically and numerically analyzed using the conventional mechanism-based strain-gradient plasticity (CMSGP) theory established using the Taylor model. The material constitutive equation is implemented in a commercial finite element code by a user subroutine, and the crack tip fields are evaluated with novel parameters in the form of the intrinsic material length, characterizing the scale over which gradient effects become significant. As a consequence of the strain-gradient contribution, FE results show a significant increase in the magnitude of the stress fields of CMSGP when the material length parameter is considered. It is found that the density of geometrically necessary dislocations (GND) is large around the crack tip, but it rapidly decreases away from the crack tip. On the contrary, the density of statistically stored dislocations (SSD) is not as large as geometrically necessary dislocations around the crack tip, but it decreases much slower than GND away from the crack tip. A couple effect of material work hardening and the crack tip distance is identified.

Comparison of Buffer Layer Grading Approaches in InGaAs/GaAs (001)

Metamorphic semiconductor devices often utilize compositionally-graded buffer layers for the accommodation of the lattice mismatch with controlled threading dislocation density and residual strain. Linear or step-graded buffers have been used extensively in these applications, but there are indications that sublinear, superlinear, S-graded, or overshoot graded structures could offer advantages in the control of defect densities. In this work we compare linear, step-graded, and nonlinear grading approaches in terms of the resulting strain and dislocations density profiles using a state-of-the-art model for strain relaxation and dislocation dynamics. We find that sublinear grading results in lower surface dislocation densities than either linear or superlinear grading approaches.

Повышение эффективности фильтрации прорастающих дислокаций в темплейтах AlN/c-Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- с фасетированной морфологией поверхности во время их роста методом молекулярно-пучковой эпитаксии

The results of transmission electron microscopy study and X-ray diffraction analysis of AlN/c Al2O3 templates with GaN ultrathin insertions grown by plasma-assisted molecular beam epitaxy are presented. It is shown that AlN buffer layers with faceted surface morphology provide a much higher threading dislocations density reduction then with smooth layers. The filtering action of ultrathin GaN insertions is confirmed.