Inclination Of Dislocations Research Articles

Tensile stress generation and dislocation reduction in Si-doped AlxGa1−xN films

The effects of Si doping on the evolution of stress in AlxGa1−xN:Si thin films (x≈0.4–0.6) grown on 6H-SiC by metal organic chemical vapor deposition were investigated using in situ wafer curvature measurements. The results were correlated with changes in film microstructure as observed by transmission electron microscopy. The incorporation of Si into the films resulted in a compressive-to-tensile transition in the biaxial stress at the surface, and the magnitude of the tensile stress was found to increase in proportion to the Si concentration. The stress gradient was attributed to Si-induced dislocation inclination resulting from an effective climb mechanism. Si doping also resulted in a decrease in the threading dislocation density in the AlxGa1−xN layers, which was attributed to increased dislocation interaction and annihilation. The model describing tensile stress generated by dislocation effective climb was modified to account for the dislocation reduction and was found to yield an improved fit to the experimental stress-thickness data.

In situ measurement of stress generation arising from dislocation inclination in AlxGa1−xN:Si thin films

The effect of Si-doping on the stress and microstructure of AlxGa1−xN (x≈0.39–0.45) films grown by metalorganic chemical vapor deposition on SiC substrates was investigated. In situ measurements revealed a compressive-to-tensile transition of the stress state at the film surface upon the addition of SiH4 during growth, which correlated with a change in the angle of inclination of threading dislocations in the film. The magnitude of the in situ measured stress gradient was comparable to that predicted by the dislocation effective climb model, suggesting that dislocation inclination is the dominant mechanism responsible for tensile stress generation in the films.

Influence of AlN thickness on strain evolution of GaN layer grown on high-temperature AlN interlayer

The strain evolution of a GaN layer grown on a high-temperature AlN interlayer with varying AlN thickness by metalorganic chemical vapour deposition is investigated. In the growth process, the growth strain changes from compression to tension in the top GaN layer, and the thickness at which the compressive-to-tensile strain transition takes place is strongly influenced by the thickness of the AlN interlayer. It is confirmed from the x-ray diffraction results that the AlN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer. The strain transition process during the growth of the top GaN layer can be explained by the threading dislocation inclination in the top GaN layer. Adjusting the AlN interlayer thickness could change the density of the threading dislocations in the top GaN layer and then change the stress evolution during the top GaN layer's growth.

Crack formation in surface layers with strain gradients

Abstract The problem of crack formation in surface layers where a linear profile of elastic strain prevails through the thickness of a layer is investigated. Such strain gradients can be generated, for example, in graded alloy semiconductor layers or due to specific stress relaxation mechanisms in lattice mismatched layers or previously unanticipated dislocation-related strain gradients. The operation of the relaxation mechanisms related to threading dislocation inclination and leading to the strain gradients in nominally compressed Al x Ga1 – x N layers grown on buffer layers with smaller lattice constants is discussed. A fracture mechanics model is developed for the calculation of the stress intensity factor of mode I cracks initiated at the surface of the layer exhibiting a linear strain dependence. The critical layer thickness for crack formation in such a gradient elastic field has been found in the framework of this fracture mechanics model. Results of the modeling are compared with experimental observations of crack onset in nominally compressed layers of Al x Ga1−x N semiconductors. Good agreement between the model predictions and the experimental data is found.

Heteroepitaxy of AlGaN on bulk AlN substrates for deep ultraviolet light emitting diodes

The authors report the growth of AlGaN epilayers and deep ultraviolet (UV) light emitting diodes (LEDs) on bulk AlN substrates. Heteroepitaxial nucleation and strain relaxation are studied through controlled growth interruptions. Due to a low density of preexisting dislocations in bulk AlN, the compressive strain during AlGaN heteroepitaxy cannot be relieved effectively. The built-up of strain energy eventually induces either an elastic surface roughening or plastic deformation via generation and inclination of dislocations, depending on the stressor interlayers and growth parameters used. AlGaN LEDs on bulk AlN exhibit noticeable improvements in performance over those on sapphire, pointing to a promising substrate platform for III-nitride UV optoelectronics.

Role of inclined threading dislocations in stress relaxation in mismatched layers

(0001)-oriented epitaxial wurtzite III-nitride layers grown on mismatched substrates have no resolved shear stress on the natural basal and prismatic slip planes; however, strained III-nitride layers may gradually relax. We report on the stress relaxation of Al0.49Ga0.51N layers grown on nominally relaxed Al0.62Ga0.38N buffer layers on sapphire. The reduction in elastic strain of the Al0.49Ga0.51N was enhanced by Si doping which caused an increased surface roughness. Despite the Si doping, the films always sustained step-flow growth. The extent of relaxation of the Al0.49Ga0.51N layer was determined by on-axis ω-2θ scans of (000l) peaks and reciprocal space maps of inclined (off-axis) peaks. Cross-section and plan-view transmission electron microscopy studies showed that the threading dislocations in the Al0.49Ga0.51N layer inclined from the [0001] direction towards ⟨11¯00⟩ directions by ∼15–25°, perpendicular to their Burgers vector (13⟨112¯0⟩). These inclined threading dislocations have a misfit dislocation component and thus provide stress relief. The contribution of the dislocation inclination to the degree of relaxation has been formulated and the energy release has been determined for dislocation inclination in mismatched stressed layers.

Modeling of misfit and threading dislocations in epitaxial heterostructures

The processes of mechanical stress relaxation in lattice-mismatched heteroepitaxial films usually proceed via misfit dislocation formation at the film/substrate interface and are typically accompanied by the generation ofa high density of threading dislocations in the bulk of the film. Threading dislocations are deleterious for a wide variety of modem electronic and optoelectronic devices. Relaxation in stressed epitaxial films also gives rise to the characteristic cross-hatch surface morphology of the growing heterostructures. This article presents a short review of recent mesoscopic models describing the evolution of dislocation structures in heteroepitaxial films. Dislocation-assisted cross-hatch formation in (001)-oriented fcc film is explained theoretically and discussed on the basis of experimental results. A specific stress relaxation mechanism via threading dislocation inclination in (0001)-oriented wurtzite crystal structure films is considered and analyzed in detail. Finally, a 'reaction-kinetics' approach to threading dislocation reduction in growing films is developed.

Stress relaxation in mismatched layers due to threading dislocation inclination

A recently observed mechanism of elastic stress relaxation in mismatched layers is discussed. The relaxation is achieved by the inclination of pure edge threading dislocation lines with respect to the layer surface normal. The relaxation is not assisted by dislocation glide but rather is caused by the “effective climb” of edge dislocations. The effective dislocation climb may result from the film growth and it is not necessarily related to bulk diffusion processes. The contribution of the dislocation inclination to strain relaxation has been formulated and the energy release due to the dislocation inclination in mismatched stressed layers has been determined. This mechanism explains recently observed relaxation of compressive stresses in the (0001) growth of AlxGa1−xN layers.

High-resolution electron microscopy of dislocations of MgO

Dislocations in MgO introduced by ion irradiation and by plastic deformation are observed by HREM. Depending on the Burgers vector and the dislocation character, various types of lattice images are obtained. Image simulations are performed for the inclination of dislocations, as well as for dissociated dislocations. A comparison of observed and simulated images shows that inclination of nondissociated dislocations makes them appear as if they were dissociated; in reality a/2(110) dislocations in MgO are not dissociated.

Electron Microscopy Images of Rectilinear Dislocations Crossing the Surface at an Arbitrary Angle

Electron microscopy images of rectilinear inclined dislocations crossing the free surface at arbitrary angles are theoretically investigated. The influence of some factors, such as the Burgers vector orientation, the dislocation inclination, the image type, etc. on the electron microscopy image are studied. Images of close-to-the-surface dislocations surrounded by impurity atmospheres are discussed. [Russian Text Ignored].