V. Cambel

Formation of GaAs three-dimensional objects using AlAs “facet-forming” sacrificial layer and H3PO4,H2O2,H2O based solution

We propose here that sacrificial layers used in III–V technology can be utilized also as “facet-forming” sacrificial layers if the lateral etching rate of the embedded layer is close to the vertical etching rate of the basic material. The idea is supported by computer simulations as well as by several experiments in which high and smooth GaAs mesas with controlled shape and tilt were formed. The wet-chemical etching method uses H3PO4, H2O2, and H2O based solution and an AlAs layer embedded into GaAs. The AlAs layer controls the lateral etching rate and influences the cross-sectional profile of GaAs three-dimensional objects. We show that the mesa slope can be precisely tuned via the solution composition, AlAs thickness, and etching depth. The method suppresses the influence of the GaAs crystallography on the final mesa shape, which simplifies its epitaxial overgrowth. The mesa sidewalls obtained are smooth enough for epitaxial growth of nonplanar heterostructures for microelectromechanical systems and for...

Dynamics of vortex nucleation in nanomagnets with broken symmetry

We investigate fundamental processes that govern dynamics of vortex nucleation in sub-100 nm mesoscopic magnets. We focus on a structure with broken symmetry - Pacman-like nanomagnet shape - in which we study micromagnetic behavior both by means of a simple model and numerically. We show that it is possible to establish desired vortex chirality and polarity by applying only quasi-static in-plane magnetic field along specific directions. We identify the modes of vortex nucleation that are very robust against external magnetic field noise. These vortex nucleation modes are common among wide range of sub-100 nm magnets with broken rotational symmetry.

Fabrication of GaAs symmetric pyramidal mesas prepared by wet-chemical etching using AlAs interlayer

We present a wet-chemical-etching method developed for the preparation of GaAs four-sided pyramid-shaped mesas. The method uses a fast lateral etching of AlAs interlayer that influences the cross-sectional profiles of etched structures. We have tested the method using H3PO4:H2O2:H2O etchant for the (100) GaAs patterning. The sidewalls of the prepared pyramidal structures together with the (100) bottom facet formed the cross-sectional angles 25° and 42° for mask edges parallel, resp. perpendicular to {011} cleavage planes. For mask edges turned in 45° according to the cleavage planes, 42° cross-sectional angles were obtained. Using the method, symmetric and more than 10-μm-high GaAs “Egyptian” pyramids with smooth tilted facets were prepared.

Contactless characterization of melt-textured superconducting junctions using micro-Hall sensor arrays

We use spatially resolved Hall-probe magnetometry and magneto-optical imaging to study current-carrying properties of melt-textured high-temperature superconducting welds at high magnetic fields. Magneto-optical images show no deterioration of the current-carrying properties of the sample in the presence of the superconducting weld. The study of the vortex dynamics near the junction using high-resolution Hall-probe magnetometry revealed enhanced relaxation of the magnetization at the junction at high temperatures. We attribute this behavior to an increased local concentration of defects near the weld that, depending on the temperature range, either facilitates enhanced relaxation or strengthens the local pinning properties.

Fabrication of a vector Hall sensor for magnetic microscopy

We have developed a micromachined Hall sensor for scanning the entire magnetic field vector whose active dimensions are an order of magnitude smaller (∼5 μm) than the smallest existing vector field sensor. It is realized by patterning three Hall probes on the tilted faces of epitaxy-overgrown GaAs-based pyramidal-shaped mesa structures. Data from these “tilted” Hall probes are used to reconstruct the full magnetic field vector.

Single-gap superconductivity in β − B i 2 Pd

This work was supported by the Slovak projects APVV-0036-11, APVV-14-0605, VEGA 2/0149/16, VEGA 1/0409/15, by the ERDF EU grant under contract No. ITMS26220120005 and the COST action MP1201 as well as by the U.S. Steel Kosice. Sample growth was partially supported by the projects No. FIS2014-54498-R (MINECO, Spain) and No. S2013/MIT-2850 Nanofrontmag-CM (Region of Madrid, Spain) and by the Engineering and Physical Sciences Research Council, UK. E.H. acknowledges support from the Grant No. 560-2012 programa de doctorados en el exterior (COLCIENCIAS)

High resolution switching magnetization magnetic force microscopy

We introduce switching magnetization magnetic force microscopy based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within this approach the sum of the scanned data with reversed tip magnetization depicts local van der Waals forces, while their differences map the local magnetic forces. Here we implement this method by fabricating low-momentum magnetic probes that exhibit magnetic single domain state, which can be easily reversed in low external field during the scanning. Measurements on high-density parallel and perpendicular magnetic recording media show enhanced spatial resolution of magnetization.

Large-scale high-resolution scanning Hall probe microscope used for MgB2 filament characterization

The scanning Hall probe microscope (SHPM) is an important imaging tool used for detailed studies of superconductors in basic science as well as in the industrial sector. It can be used for the studies of losses, current distribution, and effects at grain boundaries. However, only a few SHPMs for magnetic field imaging at temperatures below 77 K have been proposed up to now, most of them designed for small-area () scanning. We present a large-scale low-temperature SHPM developed for imaging the entire magnetic field in close proximity to magnetic and superconducting samples at 4.2–300 K. The microscope combines a large scanned area and high spatial and magnetic field resolution. The instrument is designed as an insert of standard helium flowing cryostats. The Hall sensor scans an area up to 7 × 25 mm2 in the whole temperature interval with a spatial resolution better than 5 µm. The presented system is used for the study of ex situ prepared MgB2 filament. We show that external magnetic field induces local supercurrents in the MgB2, from which the critical current can be estimated. Moreover, it indicates the microstructure and space homogeneity of the superconductor.

The influence of sample conductivity on local anodic oxidation by the tip of atomic force microscope

We analyze the role of the electric field distribution in the nano-oxidation process realized by the tip of atomic force microscope (AFM) experimentally and theoretically as well. We show the importance of the sample conductivity and the water bridge in the process applied to bulk GaAs and Ga[Al]As heterostructures in both contact and noncontact AFM modes. The experimental results show that the lines written in contact mode are much wider then those written in noncontact mode. Moreover, saddlelike profile lines can appear for high-resistive samples. These effects are explained by the numerical simulations using finite-element method. We show that the electric field distribution in the system tip-sample is controlled by the sample conductivity. In the case of low-conductive samples, maximum field is located apart from the tip apex for both contact and noncontact AFM modes.

OMCVD growth of InP and InGaAs on InP non-planar substrates patterned with {110} quasi facets

Non-planar low-pressure organometallic chemical vapour deposition (LP OMCVD) of InP and InGaAs was performedon patterned(1 0 0) semi-insulating InP substrates. The patterns were 15- mm-high long mesa ridges bounded by (1 1 0) and ð1 % Þ quasi facets and(1 0 0) top surfaces. Growth rates andsurface morphologies on the facets were studied using scanning electron microscopy (SEM), atomic force microscopy (AFM) and optical microscopy. Both InP andInGaAs nucleatedsufficiently on the facets. While InP layers were smooth, InGaAs nucleatedthrough facetted trapezoidal islands. The mechanism of facetting is qualitatively discussed. The InGaAs facetting as well as accompanying roughness were partly suppressedat lower growth temperatures. r 2001 Elsevier Science B.V. All rights reserved.