A. E. Romanov

Strain-induced polarization in wurtzite III-nitride semipolar layers

This paper presents growth orientation dependence of the piezoelectric polarization of InxGa1−xN and AlyGa1−yN layers lattice matched to GaN. This topic has become relevant with the advent of growing nitride based devices on semipolar planes [A. Chakraborty et al., Jpn. J. Appl. Phys., Part 2 44, L945 (2005)]. The calculations demonstrate that for strained InxGa1−xN and AlyGa1−yN layers lattice matched to GaN, the piezoelectric polarization becomes zero for nonpolar orientations and also at another point ≈45° tilted from the c plane. The zero crossover has only a very small dependence on the In or Al content of the ternary alloy layer. With the addition of spontaneous polarization, the angle at which the total polarization equals zero increases slightly for InxGa1−xN, but the exact value depends on the In content. For AlyGa1−yN mismatched layers the effect of spontaneous polarization becomes important by increasing the crossover point to ∼70° from c-axis oriented films. These calculations were performed u...

On the structure, stress fields and energy of nonequilibrium grain boundaries

Abstract The changes of grain boundary (GB) structure caused by the absorption of lattice dislocations are analysed. The typical result of the absorption is the disordering of networks of disclination dipoles/grain boundary dislocations (GBDs). It is shown that the variance of GBD spacings is a good quantitative measure of the degree of GB structure nonequilibrium. By the use of Monte Carlo technique the abating law of x − 1 2 for long range stress fields of disordered GBD networks is obtained. Excess energies of nonequilibrium boundaries are also calculated. In strongly excited state of GBs characterized by the maximum value of the variance of GBD spacings the excess energy can exceed the equilibrium GB energy. Such high-nonequilibrium boundaries can exist in ultrafine grained materials.

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.

Pentagonal Symmetry and Disclinations in Small Particles

A comprehensive review is given on experimental studies of small particles with fivefold symmetry accompanied by an in-depth theoretical description of their characteristics and computer modeling. The cases of uniform and nonuniform deformations (disclination model), stability and relaxation of elastic stresses in pentagonal particles and needle-like crystals, models of their formation are discussed.

Si doping effect on strain reduction in compressively strained Al0.49Ga0.51N thin films

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-μm-thick Al0.62Ga0.38N buffer layers on sapphire, revealed that increased Si doping promoted the relaxation of the compressively strained layers. The degree of strain relaxation R of the Al0.49Ga0.51N films, as determined by x-ray diffraction (XRD), increased from R=0.55 to R=0.94 with an increase in disilane injection from 1.25 nmol/min to 8.57 nmol/min. Transmission electron microscopy analysis showed that the edge threading dislocations (TDs) in the Al0.49Ga0.51N layers were inclined, such that the redirected TD lines had a misfit dislocation component. The calculated strain relaxation due to the inclined TDs was in close agreement with the values determined from XRD. We propose that the TD line redirection was promoted by the Si-induced surface roughness.

Basal plane misfit dislocations and stress relaxation in III-nitride semipolar heteroepitaxy

This article presents a theoretical analysis of dislocation behavior and stress relaxation in semipolar III-nitride heteroepitaxy, e.g., for AlxGa1−xN and InyGa1−yN layers grown on {hh2−h−m}- or {h0h−m}-type semipolar planes of GaN substrates. We demonstrate that the shear stresses on the unique inclined basal (0001) plane do not vanish for such growth geometries. This leads to the onset of relaxation processes in semipolar III-nitride heterostructures via dislocation glide in the basal slip systems 〈1−1−20〉(0001) and to the formation of misfit dislocations (MDs) with Burgers vectors of (a/3)〈1−1−20〉-type at the semipolar heterointerface. Next we calculate the Matthews-Blakeslee critical thickness for MD formation in semipolar III-nitride layers together with the MD equilibrium spacings for complete misfit relaxation. The component of the MD Burgers vector normal to the film/substrate interface will cause a crystal lattice tilt in the epilayer with respect to the GaN substrate. The calculated magnitudes o...

Lateral ordering of quantum dots by periodic subsurface stressors

We investigate the possibility of using subsurface dislocation arrays as a tool for controlling the nucleation of self-assembled quantum dots (SAQDs). A quantitative model predicts that periodic nonuniform elastic fields on the surface induced by dislocations may control the lateral ordering SAQDs. The effect of dislocations is shown to be comparable to the interaction between buried and surface dots which leads to vertical dot stacking. The periodic subsurface dislocation arrays necessary for dot ordering can be produced by twist wafer bonding and backside substrate removal.

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 disloca...

THEORY OF MICROSTRUCTURE AND MECHANICS OF THE ...A1/A2/A1/A2... DOMAIN PATTERN IN EPITAXIAL FERROELECTRIC AND FERROELASTIC FILMS

The micromechanics of the lamellar ...a1/a2/a1/a2... domain pattern are developed for the case of epitaxial tetragonal ferroelectric or ferroelastic films grown on (001) substrates. This problem is treated in the framework of the theory of defects. Coherency defects are necessary to maintain epitaxy as a result of the symmetry‐reducing phase transition in the film (ferroelectric or ferroelastic transition). The defects include continuous distributions of edge and screw dislocations. The screw dislocation distributions are equivalent to an alternating Somigliana dislocation chain. Using this approach, fully analytic solutions are derived for the stress and strain in the film and substrate. These calculations include all the effects of the free surface. An integral expression is derived for the elastic energy. Using dipole and quadrupole approximations, analytic expressions are derived for the rotations of the crystal axes for individual domains, the elastic energy, and thickness dependence of the domain pe...

444.9 nm semipolar (112¯2) laser diode grown on an intentionally stress relaxed InGaN waveguiding layer

We demonstrate an electrically injected semipolar (112¯2) laser diode (LD) grown on an intentionally stress relaxed n-In0.09Ga0.91N waveguiding layer. Detrimental effects of misfit dislocations (MDs) in the proximity of the active region were effectively suppressed by utilizing a p/n-Al0.2Ga0.8N electron/hole blocking layer between the dislocated heterointerfaces and the active region. The threshold current density of the LD was ∼20.3 kA/cm2 with a lasing wavelength of 444.9 nm. This LD demonstrates an alternative approach in semipolar AlInGaN LD waveguide design where the thickness and composition of the waveguiding and/or cladding layers are not limited by the critical thickness for MD formation.