J. M. Rebled

Long range epitaxial growth of prismatic heterostructures on the facets of catalyst-free GaAs nanowires

Molecular beam epitaxy is used for the synthesis of catalyst-free GaAs nanowires and related quantum heterostructures. After growth of the nanowire GaAs core, the conditions are changed in situ towards standard MBE planar growth in order to obtain quantum heterostructures on the facets of the nanowires. Depending on the nanowire orientation, different geometries of the quantum heterostructures are obtained. This growth method is fully characterized by high resolution and scanning transmission electron microscopy and Z-contrast electron tomography. The growth conditions are also tuned for the optimization and homogeneity of the optical properties. The feedback of these analyses allows the tuning of the growth conditions according to the required optical properties. This work is the basis for obtaining a new generation of devices based on the heterostructures existing on the nanowire facets.

A new approach for 3D reconstruction from bright field TEM imaging: Beam precession assisted electron tomography

The successful combination of electron beam precession and bright field electron tomography for 3D reconstruction is reported. Beam precession is demonstrated to be a powerful technique to reduce the contrast artifacts due to diffraction and curvature in thin foils. Taking advantage of these benefits, Precession assisted electron tomography has been applied to reconstruct the morphology of Sn precipitates embedded in an Al matrix, from a tilt series acquired in a range from +49° to -61° at intervals of 2° and with a precession angle of 0.6° in bright field mode. The combination of electron tomography and beam precession in conventional TEM mode is proposed as an alternative procedure to obtain 3D reconstructions of nano-objects without a scanning system or a high angle annular dark field detector.

The direct magnetoelectric effect in ferroelectric–ferromagnetic epitaxial heterostructures

Ferroelectric (FE) and ferromagnetic (FM) materials engineered in horizontal heterostructures allow interface-mediated magnetoelectric coupling. The so-called converse magnetoelectric effect (CME) has been already demonstrated by electric-field poling of the ferroelectric layers and subsequent modification of the magnetic state of adjacent ferromagnetic layers by strain effects and/or free-carrier density tuning. Here we focus on the direct magnetoelectric effect (DME) where the dielectric state of a ferroelectric thin film is modified by a magnetic field. Ferroelectric BaTiO3 (BTO) and ferromagnetic CoFe2O4 (CFO) oxide thin films have been used to create epitaxial FE/FM and FM/FE heterostructures on SrTiO3(001) substrates buffered with metallic SrRuO3. It will be shown that large ferroelectric polarization and DME can be obtained by appropriate selection of the stacking order of the FE and FM films and their relative thicknesses. The dielectric permittivity, at the structural transitions of BTO, is strongly modified (up to 36%) when measurements are performed under a magnetic field. Due to the insulating nature of the ferromagnetic layer and the concomitant absence of the electric-field effect, the observed DME effect solely results from the magnetostrictive response of CFO elastically coupled to the BTO layer. These findings show that appropriate architecture and materials selection allow overcoming substrate-induced clamping in multiferroic multi-layered films.

High quality InAlN single layers lattice-matched to GaN grown by molecular beam epitaxy

We report on properties of high quality ∼60 nm thick InAlN layers nearly in-plane lattice-matched to GaN, grown on c-plane GaN-on-sapphire templates by plasma-assisted molecular beam epitaxy. Excellent crystalline quality and low surface roughness are confirmed by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. High annular dark field observations reveal a periodic in-plane indium content variation (8 nm period), whereas optical measurements evidence certain residual absorption below the band-gap. The indium fluctuation is estimated to be ± 1.2% around the nominal 17% indium content via plasmon energy oscillations assessed by electron energy loss spectroscopy with sub-nanometric spatial resolution.

Carrier transport and electroluminescence efficiency of erbium-doped silicon nanocrystal superlattices

A detailed study of transport phenomena and electroluminescence of erbium-doped silicon-rich oxide/silicon oxide superlattices is presented. Extended states conduction is thermally activated from Poole-Frenkel traps located at silicon nanocrystals or its interface. These traps provide bulk limited conduction at low and medium electric fields. In contrast, under high electric fields, conduction is governed by trap-assisted tunneling of electrons from the electrode to the active layer conduction band. Superlattice electroluminescence efficiency at 1.5 μm and injected electron energy distribution in the conduction band are evaluated and compared to a silicon dioxide and a silicon-rich oxide single layer. This work sheds light on the implementation of alternative electroluminescent device architectures with strong emphasis in the hot electron engineering.

ORDERED GAN/INGAN NANORODS ARRAYS GROWN BY MOLECULAR BEAM EPITAXY FOR PHOSPHOR-FREE WHITE LIGHT EMISSION

The basics of the self-assembled growth of GaN nanorods on Si(111) are reviewed. Morphology differences and optical properties are compared to those of GaN layers grown directly on Si(111). The effects of the growth temperature on the In incorporation in self-assembled InGaN nanorods grown on Si(111) is described. In addition, the inclusion of InGaN quantum disk structures into self-assembled GaN nanorods show clear confinement effects as a function of the quantum disk thickness. In order to overcome the properties dispersion and the intrinsic inhomogeneous nature of the self-assembled growth, the selective area growth of GaN nanorods on both, c-plane and a-plane GaN on sapphire templates, is addressed, with special emphasis on optical quality and morphology differences. The analysis of the optical emission from a single InGaN quantum disk is shown for both polar and non-polar nanorod orientations.

Ti diffusion in (001) SrTiO3–CoFe2O4 epitaxial heterostructures: blocking role of a MgAl2O4 buffer

Titanium diffusion from (001) SrTiO3 (STO) substrates into CoFe2O4 (CFO) films grown using pulsed laser deposition is reported. To elucidate the reasons for Ti interdiffusion, a comparative study of CFO films grown on MgAl2O4 (MAO) and STO substrates, buffered by thin STO and MAO layers, has been made. It is shown that whereas bottom STO layers always result in Ti migration, a thin MAO layer, only 8 nm thick, is effective in blocking it. We argue that this success relies on the lower mobility of Ti ions in the MAO lattice compared to that of CFO. This result should contribute to the development of high quality epitaxial heterostructures of dissimilar complex oxides.

Nontunnel transport through CoFe2O4 nanometric barriers

Electric transport through ultrathin CoFe2O4 (CFO) films of different thicknesses is studied using current sensing atomic force microscopy. Analysis of current distribution maps and I-V characteristics reveals anomalous thickness dependence. Results indicate the existence of an Ohmic conduction channel in parallel with the tunnel one. The origin of the nontunneling, likely non-spin-preserving, channel is discussed in the context of recent results on spin-filtering CFO-based devices.

On the use of Sb to improve the performance of GaInP subcells of multijunction solar cells

GaInP is a material commonly employed for the top subcells of different multijunction solar cells architectures. In this study, the performance of GaInP top cells has been improved by increasing the energy band gap with the use of Sb as a surfactant during the MOVPE growth of the structures. The optimization of the appropriate Sb molar flow was done by Reflectance Anisotropy Spectroscopy. Different characterization techniques have been employed to assess the effect of Sb on the morphology, microstructure and optoelectronic properties of the resulting GaInP grown with different Sb/P ratios. Finally, the performance of several GaInP subcells with different order parameters has been assessed.

Advanced Electron Microscopy Techniques on Semiconductor Nanowires: from Atomic Density of States Analysis to 3D Reconstruction Models

Technology at the nanoscale has become one of the main scientific world challenges as new quantum physical effects appear and can be modulated at will (Kastner, 1992). Superconductors, materials for spintronics, electronics, optoelectronics, chemical sensing, and new generations of functionalized materials are taking advantage of the low dimensionality, improving their properties and opening a new range of applications (De Franceschi et al., 2003; Samuelson et al., 2004; Fischer et al., 2006; Pettersson et al., 2006; Wang et al., 2006; Hernandez-Ramirez et al., 2007; Qin et al., 2007; Appenzeller et al., 2008; Boukai et al., 2008; Hochbaum et al., 2008; Wang et al., 2008; Colombo et al., 2009; Thunick et al., 2009). These new materials for future applications are being synthesized at the nanoscale (ultrathin layers, nanoparticles, nanowires or nanotubes functionalized). Among all these new materials, one-dimensional (1D) nanostructures such as nanowires, are one of the most used and promising morphologies (Lieber, 2003; Yang, 2005; Thelander et al., 2006; Lieber & Wang, 2007). Groups all around the world in the area of materials science, physics, chemistry and biology work in close collaboration with nanoscopy tools as there is a critical need for the structural, chemical and morphological characterization of the synthesized nanostructures at atomic scale in order to correlate these results with the physical and chemical properties and functionalities they present. In order to obtain an accurate control and understanding of these new materials properties, it is essential to access their structure and chemistry at atomic scale. Electron Microscopy and more precisely (scanning) transmission electron microscopy ((S)TEM) and electron spectroscopy related techniques (also known as electron nanoscopies) have thus a preeminent role in advanced materials science. Recent developments in electron microscopy, such as aberration correctors and monochromators are allowing us to reach sub-angstrom and sub-eV, spatial and energy resolutions, respectively. In addition to these advances, the possibility to obtain 3D models of our nanostructures by means of electron tomography, have shown that Electron Microscopy related techniques are the most promising to fully characterize complex