Interdependence of strain and shape in self-assembled coherent InAs islands on GaAs

Abstract

Self-assembled coherent InAs islands on GaAs (100) have been investigated by a novel version of grazing-incidence diffraction ("iso-strain scattering"). This method permits the determination of the interdependence of strain and shape, as well as the relaxation gradient within the InAs dots. The relaxation in the islands ranges from fully strained at the bottom to completely relaxed at the top of the islands. The radius of the dots at a given height depends linearly on the local elastic lattice relaxation, with a rapidly increasing relaxation gradient when approaching the top of the islands.

Resonant X-ray diffraction of self-assembled epitaxial systems: From direct to complementary chemical information

“…• in both beamline setups [3]. Although high momentum transfer measurements allow for more pronounced anomalous effect -as explored in ref.…”

“…6 for samples B and D. Such measurements probe the existence of an in-plane lattice parameter gradient inside the islands [3,10,25]. The intense GaAs(220) substrate peak is not shown for clarity.…”

“…4(b) can be employed to draw a twodimensional projection map of in-plane local lattice parameter a = a SnTe h/2 (considering |h| = |k|) -for EuTe islands. In such iso-strain projections the areas with each lattice parameter are drawn following the simple ellipse constraint (x 2 + y 2 /L 2 ) ≤ 1, where x,y and L are the coordinates and iso-lattice parameter lateral size along the [1][2][3][4][5][6][7][8][9][10] and [2,11] directions, here considered the same accordingly to the equal L sizes obtained from measurements in the 220 family of reflections [23]. Figure 4(c) shows the in-plane iso-lattice parameter projections for dots.…”

“…X-ray diffraction is a non-destructive technique and can provide statistical information over large areas of the studied materials. Its use to investigate self-assembled quantum dots spans from the first grazing-incidence diffraction experiments [3] to the recent achievements in coherent diffraction imaging [4]. The description of nanostructures by X-ray methods has, therefore, remarkably evolved towards quantitative tomographic results.…”

“…This tuning, however, is not simple, since several aspects influence the physical properties of QDs, such as chemical composition, island size, strain, capping, among others. The pseudomorphic growth of QDs on a substrate with a different lattice parameter leads to the formation of elastically strained three-dimensional structures [8] in which strain relaxation can take place in the form of lattice parameter gradients [3] or by the creation of dislocations. The latter process renders the dots optically inactive, while elastic relaxation by strain and composition gradients may lead to optically active QDs with very different properties with respect to bulk materials [9].…”