Angelo Malachias

Three-Dimensional Composition Profiles of Single Quantum Dots Determined by Scanning-Probe-Microscopy-Based Nanotomography

Scanning probe microscopy combined with selective wet chemical etching is employed to quantitatively determine the full three-dimensional (3D) composition profiles of single strained SiGe/Si(001) islands. The technique allows us to simultaneously obtain 3D profiles for both coherent and dislocated islands and to collect data with large statistics. Lateral and vertical composition gradients are observed, and their origin is discussed. X-ray scattering measurements performed on a large sample area are used to validate the results.

Wrinkled-up Nanochannel Networks: Long-Range Ordering, Scalability, and X-ray Investigation

Highly ordered two-dimensional self-organized nanochannel networks as well as free-standing nanomembranes are produced on rigid substrates by means of III-V semiconductor compressively strained layers grown on top of an etchant-sensitive material. The releasing process is controlled by regularly spaced pits obtained from photolithography and a subsequent wet chemical etching. By tuning basic film parameters such as strain and thickness, one obtains periodic arrays of two-dimensional nanochannel networks with symmetries defined by the shape and periodicity of the photolithographic starting pits. Such nanochannel networks with a submicroscale lateral feature size exhibit a surprising flexibility with respect to the crystal lattice symmetry, retaining the original film crystalline quality as confirmed by X-ray grazing-incidence diffraction (GID) measurements. Finite element modeling helps in understanding the particular process of the cross-nanochannel formation.

Direct strain and elastic energy evaluation in rolled-up semiconductor tubes by x-ray microdiffraction

We depict the use of x-ray diffraction as a tool to directly probe the strain status in rolled-up semiconductor tubes. By employing continuum elasticity theory and a simple model, we are able to simulate quantitatively the strain relaxation in perfect crystalline III-V semiconductor bilayers and multilayers as well as in rolled-up layers with dislocations. The reduction in the local elastic energy is evaluated for each case. Limitations of the technique and theoretical model are discussed in detail.

Composition and strain in SiGe/Si(001) nanorings revealed by combined x-ray and selective wet chemical etching methods

The authors used x-ray diffraction to investigate strain and composition in SiGe nanorings formed during partial Si capping of self-assembled SiGe/Si(001) islands. The obtained results are corroborated with selective wet chemical etching experiments. Clear evidence is provided that rings are composed of a Ge rich core surrounded by Si richer ridges indicating that a substantial material redistribution occurs during the shape transformation from SiGe islands to rings. The results suggest that SiGe ring formation is driven by strain relief.

Direct observation of the coexistence of coherent and incoherent InAs self-assembled dots by x-ray scattering

In this letter, grazing incidence x-ray scattering is employed as a method to identify relaxed islands in an ensemble of partially coherent self-assembled InAs quantum dots. A simple model of strained pyramidal islands enables the association of the local lattice parameter of an island to its lateral size. A comparison between the island side length and its strain state allows the identification of coherent and incoherent nanostructures, revealing the size–strain interplay during growth.

Straining nanomembranes via highly mismatched heteroepitaxial growth: InAs islands on compliant Si substrates.

Freestanding, edge-supported silicon nanomembranes are defined by selective underetching of patterned silicon-on-insulator substrates. The membranes are afterward introduced into a molecular beam epitaxy chamber and overgrown with InAs, resulting in the formation of InAs islands on flat areas and at the top of the Si nanomembranes. A detailed analysis of sample morphology, island structure, and strain is carried out. Scanning electron microscopy shows that the membrane stays intact during overgrowth. Atomic force microscopy reveals a lower island density on top of the freestanding membranes, denoting a modified wetting or diffusivity in these areas. An observed bending of the membrane indicates a strain transfer from the InAs islands to the compliant substrate. X-ray diffraction and finite-element modeling indicate a nonuniform strain state of the island ensemble grown on the freestanding membrane. A simulation of the bending of the nanomembranes indicates that the islands at the center of the freestanding area are highly strained, whereas islands on the border tend to be fully relaxed. Finally, continuum elasticity calculations suggest that for a sufficiently thin membrane InAs could transfer enough strain to the membrane to allow coherent epitaxial growth, something not possible on bulk substrates.

Temperature-Induced Coexistence of a Conducting Bilayer and the Bulk-Terminated Surface of the Topological Insulator Bi2Te3

Topological insulators such as Bi2Se3 and Bi2Te3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi2Se3, one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi2Te3. X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior.

Tuning resistive switching on single-pulse doped multilayer memristors

Short-period multilayers containing ultrathin atomic layers of Al embedded in titanium dioxide (TiO(2)) film-here called single-pulse doped multilayers-are fabricated by atomic layer deposition (ALD) growth methods. The approach explored here is to use Al atoms through single-pulsed deposition to locally modify the chemical environment of TiO(2) films, establishing a chemical control over the resistive switching properties of metal/oxide/metal devices. We show that this simple methodology can be employed to produce well-defined and controlled electrical characteristics on oxide thin films without compound segregation. The increase in volume of the embedded Al(2)O(3) plays a crucial role in tuning the conductance of devices, as well as the switching bias. The stacking of these oxide compounds and their use in electrical devices is investigated with respect to possible crystalline phases and local compound formation via chemical recombination. It is shown that our method can be used to produce compounds that cannot be synthesized a priori by direct ALD growth procedures but are of interest due to specific properties such as thermal or chemical stability, electrical resistivity or electric field polarization possibilities. The monolayer doping discussed here impacts considerably on the broadening of the spectrum of performance and technological applications of ALD-based memristors, allowing for additional degrees of freedom in the engineering of oxide devices.

X-ray determination of vertical ordering of InAs quantum dots in InAs/GaAs multilayers

The degree of vertical alignment of InAs quantum dots in InAs/GaAs(001) multilayers was studied using grazing incidence x-ray scattering. We show that it is necessary to access one of the weak (200) x-ray reflections to observe the modulation of the GaAs lattice periodicity produced by the stacking of the InAs dots. The degree of alignment of the dots was assessed by fitting the x-ray diffuse scattering profiles near a GaAs (200) reciprocal lattice point. By using a model of gaussian lateral displacement of the dots, we show that we can determine the average value of the mistake in stacking positions of the islands from one bilayer to the next.

Observation of strain-free rolled-up CVD graphene single layers: toward unstrained heterostructures.

Single layer graphene foils produced by chemical vapor deposition (CVD) are rolled with self-positioned layers of InGaAs/Cr forming compact multi-turn tubular structures that consist on successive graphene/metal/semiconductor heterojunctions on a radial superlattice. Using elasticity theory and Raman spectroscopy, we show that it is possible to produce homogeneously curved graphene with a curvature radius on the 600-1200 nm range. Additionally, the study of tubular structures also allows the extraction of values for the elastic constants of graphene that are in excellent agreement with elastic constants found in the literature. However, our process has the advantage of leading to a well-defined and nonlocal curvature. Since our curvature radius lies in a range between the large radius studied using mechanical bending and the reduced radius induced by atomic force microscopy experiments, we can figure out whether bending effects can be a majoritary driving force for modifications in graphene electronic status. From the results described in this work, one can assume that curvature effects solely do not modify the Raman signature of graphene and that strain phenomena observed previously may be ascribed to possible stretching due to the formation of local atomic bonds. This implies that the interactions of graphene with additional materials on heterostructures must be investigated in detail prior to the development of applications and devices.