A. A. Minkevich

Strain field in (Ga,Mn)As/GaAs periodic wires revealed by coherent X-ray diffraction

An experimental and simulation study of the full strain tensor and of strain-induced magnetocrystalline anisotropies in arrays of lithographically patterned (Ga,Mn)As on GaAs(001) is performed using a coherent diffraction lensless microscopy technique. We demonstrate the ability of our technique to get an insight into the strain field propagating in the crystal part belonging to the substrate. The experimentally reconstructed strain fields are in good agreement with those obtained from simulations based on elasticity theory.

Improved success rate and stability for phase retrieval by including randomized overrelaxation in the hybrid input output algorithm

In this paper, we study the success rate of the reconstruction of objects of finite extent given the magnitude of its Fourier transform and its geometrical shape. We demonstrate that the commonly used combination of the hybrid input output and error reduction algorithm is significantly outperformed by an extension of this algorithm based on randomized overrelaxation. In most cases, this extension tremendously enhances the success rate of reconstructions for a fixed number of iterations as compared to reconstructions solely based on the traditional algorithm. The good scaling properties in terms of computational time and memory requirements of the original algorithm are not influenced by this extension.

A portable molecular beam epitaxy system for in situ x-ray investigations at synchrotron beamlines

A portable synchrotron molecular beam epitaxy (MBE) system is designed and applied for in situ investigations. The growth chamber is equipped with all the standard MBE components such as effusion cells with shutters, main shutter, cooling shroud, manipulator, reflection high energy electron diffraction setup, and pressure gauges. The characteristic feature of the system is the beryllium windows which are used for in situ x-ray measurements. An UHV sample transfer case allows in vacuo transfer of samples prepared elsewhere. We describe the system design and demonstrate its performance by investigating the annealing process of buried InGaAs self-organized quantum dots.

Polytypism in GaAs nanowires: determination of the interplanar spacing of wurtzite GaAs by X-ray diffraction

In GaAs nanowires grown along the cubic [111]c direction, zinc blende and wurtzite arrangements have been observed in their stacking sequence, since the energetic barriers for nucleation are typically of similar order of magnitude. It is known that the interplanar spacing of the (111)c Ga (or As) planes in the zinc blende polytype varies slightly from the wurtzite polytype. However, different values have been reported in the literature. Here, the ratio of the interplanar spacing of these polytypes is extracted based on X-ray diffraction measurements for thin GaAs nanowires with a mean diameter of 18-25 nm. The measurements are performed with a nano-focused beam which facilitates the separation of the scattering of nanowires and of parasitic growth. The interplanar spacing of the (111)c Ga (or As) planes in the wurtzite arrangement in GaAs nanowires is observed to be 0.66% ± 0.02% larger than in the zinc blende arrangement.

Lateral ordering, strain, and morphology evolution of InGaAs/GaAs(001) quantum dots due to high temperature postgrowth annealing

The effect of postgrowth annealing on shape and ordering of a single layer of InGaAs/GaAs(001) quantum dots is investigated by three dimensional grazing incidence small angle x-ray scattering. A transition from disordered dots to two-dimensional lateral ordering is found. This transition is accompanying a quantum dot shape transformation. Grazing incidence diffraction measurements relate the observed ordering type to strain driven self organization. The role of different growth conditions leading to lateral correlation is discussed by comparing the results to recent experimental achievements in the field.

Retrieval of the atomic displacements in the crystal from the coherent X-ray diffraction pattern.

The retrieval of spatially resolved atomic displacements is investigated via the phases of the direct(real)-space image reconstructed from the strained crystals coherent X-ray diffraction pattern. It is demonstrated that limiting the spatial variation of the first- and second-order spatial displacement derivatives improves convergence of the iterative phase-retrieval algorithm for displacements reconstructions to the true solution. This approach is exploited to retrieve the displacement in a periodic array of silicon lines isolated by silicon dioxide filled trenches.

Retrieving the displacement of strained nanoobjects: the impact of bounds for the scattering magnitude in direct space

Coherent X-ray diffraction imaging (CXDI) of the displacement field and strain distribution of nanostructures in kinematic far-field conditions requires solving a set of non-linear and non-local equations. One approach to solving these equations, which utilizes only the objects geometry and the intensity distribution in the vicinity of a Bragg peak as a priori knowledge, is the HIO+ER-algorithm. Despite its success for a number of applications, reconstruction in the case of highly strained nanostructures is likely to fail. To overcome the algorithms current limitations, we propose the HIO(O(R))(M)+ER(M)-algorithm which allows taking advantage of additional a priori knowledge of the local scattering magnitude and remedies HIO+ERs stagnation by incorporation of randomized overrelaxation at the same time. This approach achieves significant improvements in CXDI data analysis at high strains and greatly reduces sensitivity to the reconstructions initial guess. These benefits are demonstrated in a systematic numerical study for a periodic array of strained silicon nanowires. Finally, appropriate treatment of reciprocal space points below noise level is investigated.

Asymmetric skew X-ray diffraction at fixed incidence angle: application to semiconductor nano-objects

A procedure for obtaining three-dimensionally resolved reciprocal-space maps in a skew X-ray diffraction geometry is described. The geometry allows tuning of the information depth in the range from tens of micrometres for symmetric skew diffraction down to tens of nanometres for strongly asymmetric skew geometries, where the angle of incidence is below the critical angle of total external reflection. The diffraction data are processed using a rotation matrix formalism. The whole three-dimensional reciprocal-space map can be measured by performing a single azimuthal rotation of the sample and using a two-dimensional detector, while keeping the angle of incidence and the X-ray information depth fixed (FIXD method). Having a high surface sensitivity under grazing-incidence conditions, the FIXD method can be applied to a large variety of Bragg reflections, particularly polar ones, which provide information on strain and chemical composition separately. In contrast with conventional grazing-incidence diffraction, the FIXD approach reveals, in addition to the lateral (in-plane) components, the vertical (out-of-plane) component of the strain field, and therefore allows the separation of the scattering contributions of strained epitaxial nanostructures by their vertical misfit. The potential of FIXD is demonstrated by resolving the diffraction signal from a single layer of InGaN quantum dots grown on a GaN buffer layer. The FIXD approach is suited to the study of free-standing and covered near-surface nano-objects, as well as vertically extended multilayer structures.

Three-dimensional reciprocal space profile of an individual nanocrystallite inside a thin-film solar cell absorber layer

The strain profile of an individual Cu(In,Ga)Se2 nanocrystallite in a solar cell absorber layer is accessed using synchrotron radiation. We find that the investigated crystallite is inhomogeneously strained. The strain is most likely produced by a combination of intergranular strain and composition variations in nanocrystals inside the polycrystalline semiconductor film and carries information about the intercrystalline interaction. The measurements are made nondestructively and without additional sample preparation or x-ray beam nanofocusing. This is the first step towards measurements of strain profiles of individual crystallites inside a working solar cell.