Matteo Bosi

Review on Atomic Layer Deposition and Applications of Oxide Thin Films

Atomic layer deposition technique is able to grow conformal thin films over high aspect ratio structures. This article reviews the various aspects of oxides grown by this method including applications in photovoltaics and memristors. The main focus of this review is to concentrate on the oxides grown by atomic layer deposition and their growth mechanisms. The oxides deposited using atomic layer deposition are also likely to find application in memristor, an emerging field in the non volatile memories design with the ability to retain data and memory states even in power-off condition. The use of this technique to obtain oxides in surface modification of nanostructures gives the significance of these materials.

Activation and control of visible single defects in 4H-, 6H-, and 3C-SiC by oxidation

In this work, we present the creation and characterisation of single photon emitters at the surface of 4H- and 6H-SiC, and of 3C-SiC epitaxially grown on silicon. These emitters can be created by annealing in an oxygen atmosphere at temperatures above 550 °C. By using standard confocal microscopy techniques, we find characteristic spectral signatures in the visible region. The excited state lifetimes are found to be in the nanosecond regime in all three polytypes, and the emission dipoles are aligned with the lattice. HF-etching is shown to effectively annihilate the defects and to restore an optically clean surface. The defects described in this work have ideal characteristics for broadband single photon generation in the visible spectral region at room temperature and for integration into nanophotonic devices.

Tuning the radial structure of core–shell silicon carbide nanowires

The influence of growth conditions on structural properties is reported for core–shell SiC/SiO2 nanowires grown on silicon substrates by a chemical vapour deposition (CVD) technique. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) studies show a correlation between the growth temperature and the nanowire structure and highlight the possibility to control the inner core diameter by varying the precursor concentration. The nanowire covering of the substrate was considerably enhanced and homogenized using drop casting surfactant-aided deposition of catalysts on an H-terminated silicon 100 surface.

Compositional and optical uniformity of InGaN layers deposited on (0001) sapphire by metal–organic vapour phase epitaxy

InGaN/GaN heterostructures grown by metal–organic vapour phase epitaxy were investigated by micro-Raman spectroscopy, photoluminescence and spectrally-resolved cathodo-luminescence (CL). These methods allowed the precise determination of the indium distribution at the microscale and macroscale. Owing to the axial symmetry of the used vertical reactor, the In molar fraction in the films normally tends to increase from the centre to the edge of the 2-inch wafers. It is also observed that for increasing In content, some additional modes appear in the Raman spectra. They are tentatively associated with In clustering phenomena, most probably occurring around bunches of threading dislocations. This hypothesis is further justified by CL spectral maps.

Characterization of HVPE GaN layers by atomic force microscopy and Raman spectroscopy

GaN layers were deposited on (0001) sapphire by hydride vapour phase epitaxy (HVPE) and then extensively characterized by Raman spectroscopy and atomic force microscopy. It was observed that the GaN epilayers directly deposited on sapphire generally have rough surfaces, mostly made of pyramids. The Raman scattering technique was found to be a powerful tool for quick and non-destructive assessment of the crystallographic quality of HVPE GaN, although care must be taken in the spectra analysis as the surface morphology can affect the spectra structure. The absence of forbidden modes indicate that the surface is planar and smooth and the material is of good crystalline quality. A discussion on the observed red-shift of the A1(LO) mode frequency is presented.

The real structure of ε-Ga2O3 and its relation to κ-phase

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga2O3 epilayers grown at low temperature (650 °C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called e phase; i.e. they exhibited hexagonal P63mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga2O3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5–10 nm size. Each domain has orthorhombic structure with Pna21 space group symmetry, referred to as κ-Ga2O3. Further XRD analysis carried out on thicker samples (9–10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains together – if the domain size falls below the actual resolution of the probing techniques – can be misinterpreted as the disordered structure with its P63mc space group symmetry usually referred to as e-Ga2O3 in the current literature. The crystal structure of these Ga2O3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zig-zag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna21 space group and previous observations.

A Comparative Study of the Morphology of 3C-SiC Grown at Different C/Si ratios

A comparative study of the morphology of 3C-SiC films prepared with different C:Si ratios is presented. The silane precursor controls the growth rate at all values of C:Si ratio but combined of observations using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) indicates that the C:Si ratio is critical in determining the grain size and at values of C:Si close to 1 texturing and faceting become evident. Makyoh Topography reveals various surface defects, a slight mesoscale roughness and bending of the epiwafers.

Studies of nanoindentation and residual stress analysis of Ge/GaAs epilayers

The nanomechanical properties of germanium (Ge) epilayers grown at different temperatures by horizontal home-made metal organic vapor phase epitaxy were studied with nanoindentation using Berkovich and Vickers indenters. The surface morphology of the grown samples was studied by means of atomic force microscopy. High resolution x-ray diffraction (HRXRD) measurements were performed for structural analysis. The present investigation is mainly aimed at the understanding of the relation of hardness with the residual stress. The residual stress values obtained from HRXRD studies are compared with the hardness and elastic modulus values determined from nanoindentation analysis. It was found that the nanomechanical properties are correlated with the observed residual stress. The defects induced mechanism due to the change in load on Ge/GaAs epilayers has been elucidated. This kind of study will lead to improvement of Ge/GaAs films with respect to the deposition conditions understood from the variations in the nanomechanical studies.

3C–SiC nanowires luminescence enhancement by coating with a conformal oxides layer

In this work, we report the light emission properties of different classes of 3C–SiC nanowires. We compare the luminescence of core/shell nanowires where the 3C–SiC core is covered by a SiOx shell of different thickness, or coated with different conformal oxide layers (aluminium oxide and gallium oxide), grown by atomic layer deposition. The main result is the enhancement of the 3C–SiC near-band-edge emission at room temperature with the nanowires surface coating, due to carriers diffusion from the shell to the core, promoted by the alignment between the oxides and SiC bands in a type I quantum-well.

Effects of Growth Parameters on SiC/SiO2 Core/Shell Nanowires Radial Structures

Cubic silicon carbide - silicon dioxide core-shell nanowires have been synthesized in a thermal CVD system from carbon monoxide on silicon substrate. Using a non-ionic surfactant during the coating process of the substrate by the catalyst, the uniformity of the catalytic layer was improved, resulting in a more uniform nanowires growth. It is demonstrated that the core diameter is strongly correlated with the precursor concentration.