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. (C) 2003 American Institute of Physics.

An analysis is presented on the effect of the strain field originating from a subsurface stressor (point source of dilatation or a dilatating ellipsoidal inclusion) on the electronic properties of nitride semiconductors. With good accuracy, real quantum dots can be modeled as such stressors. We consider the following material structure design: a uniform semi-infinite GaN matrix with a buried stressor or a GaN matrix with a single (In,Ga)N quantum well, which is grown pseuodomorphically between the stressor and the free surface. We utilize isotropic elasticity to determine the strain field in the structures under investigation. We then apply a k.p perturbation theory approach to examine the shifts of the conduction and valence band edges caused by the stressor. We find lateral confinement for electrons and holes, which can be proposed for the realization of strain-induced quantum dots in the quantum well. (C) 2005 American Institute of Physics.

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 , 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 approximate to 45 degrees 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 similar to 70 degrees from c-axis oriented films. These calculations were performed using the most recent and convincing values for the piezoelectric and elasticity constants, and applying Vegard's law to estimate the constants in the ternary InxGa1-xN and AlyGa1-yN 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 Ill-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 omega-2 theta scans of (0001) 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 Al(0.49)Gao(0.5)N layer inclined from the [0001] direction towards (1 (1) over bar 00) directions by similar to 15-25 degrees, perpendicular to their Burgers vector (1/3 ) 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. (c) 2005 American Institute of Physics.

We present further developments and understanding of the commonly observed crosshatch surface morphology in strain-relaxed heteroepitaxial films. We have previously proposed that the crosshatch morphology is directly related with strain relaxation via threading dislocation glide which results in both surface step and misfit dislocation (MD) formation [see Andrews , J. Appl. Phys. 91, 1933 (2002)-now referred to as Part I]. In this article, we have used solutions for the stress fields and displacement fields for periodic MD arrays which include the effects of the free surface. These solutions avoid truncation errors associated with finite dislocation arrays that were used in Part I. We have calculated the surface height profile for relaxed films where the misfit dislocations were introduced randomly or the misfit dislocations were placed in groups with alternating sign of the normal component of their Burgers vector. We have calculated the surface height profiles where the slip step remains at the surface ["slip step only" (SSO)] and where the slip step is eliminated ["slip step eliminated" (SSE)] due to annihilation of opposite sense steps, such as could happen during growth or lateral mass transport. For relaxed films, we find that the surface height undulations, characteristic of crosshatch, increase with increasing film thickness for the SSO case, whereas the surface becomes flatter for the SSE case. Experiments on relaxed In0.25Ga0.75As films on (001) GaAs show that the surface height undulations in the [110] direction increase with increasing film thickness. Thus, we conclude that with increasing film thickness the crosshatch in the slow diffusion [110] direction is best described by the SSO case. (C) 2004 American Institute of Physics.

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-mum-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. (C) 2003 American Institute of Physics.