Gallium oxide is being widely studied, mainly for high-power electronics applications. It is a very promising material for photonic/optoelectronic applications, such as solar-blind UV detectors and light emitters. In this work, we study the temperature-dependent behavior of the optical properties of microcavities based on luminescent β-Ga2O3:Cr nanowires that emit an intense red-infrared band. Two distributed Bragg reflectors (DBR), when milled with a focused ion beam (FIB) and separated some microns, result in an optical microcavity that confines the light longitudinally. Both chromium R lines and Fabry-Perot spectral resonances (FPSR) are observed to shift as temperature varies, making these DBRs a valuable thermometer in a wide temperature range, due to both luminescent and interferometric transducing mechanisms. The underlying origin of this shift, in the case of the FPSR, is mainly the variation of the refractive index with temperature and the thermal expansion of the cavity. Ellipsometry studies carried out at different temperatures in bulk β-Ga2O3 yielded the dispersion relations for the three main crystal axes, i.e. its temperature-dependent anisotropic refractive index. These results were implemented in finite-difference time-domain (FDTD) simulations to compare the expected spectral shift of the FPSR in the modelled system with the experimental shift in the DBR cavities, as obtained experimentally by micro-photoluminescence. The results from these two approximations, and a third one based on solving the relevant analytical equations, are compared.
In this work, we present our recent results on the applicability of optical microcavities based on Cr doped Ga2O3 wires to operate as a nanothermometer in a wide temperature range (at least from 150 up to 550 K) and achieving a temperature precision of around 1 K. To this purpose, DBR (distributed Bragg reflectors) have been used to enhance the reflectivity at the lateral ends of the wires. The transduction mechanism encompasses both the luminescence features of the characteristic R-lines of Cr3+ ions in this host as well as the interferometric effects of the Fabry-Perot resonances within the cavity.
Ga2O3 micro- and nanowires-based optical microcavities have been obtained by patterning pairs of distributed Bragg reflectors (DBRs) with a focused ion beam (FIB) microscope. DBRs result in widely tunable high reflectivity bands. The microcavities have been designed and optimized with the aid of simulations and optically characterized by micro-photoluminescence. Tunable strong modulations are confirmed in the NUV-blue as well as in the red-NIR ranges for unintentionally doped and chromium doped wires, respectively. Experimental, analytical and simulations results will be compared and some possible applications of these cavities will be assessed.
Nano and microstructures of ternary oxide compounds, such as nickel gallate, indium-zinc-oxide compounds, and lithium stagnates, have been successfully synthetized by a vapor-solid method. Following this synthesis process, a significant amount of material is produced in an economic and scalable way. NiGa2O4 nano- and microneedles are grown using mixtures of Ga2O3 powders and Ni and Ga metallic powders as main precursors. In the case of In2ZnkO3+k compounds, the precursor blend contains ZnO and InN or In2S3 as a source for indium atoms, producing 1-dimensional or 2-dimentional preferential growth, respectively. The growth of complex branched structures of Li2SnO3 has been achieved with the use of SnO2 and Li2CO3. The temperature and precursor selection allow us to engineer the size, grade of complexity and final morphology of the structures. The growth mechanism of the obtained nano and microstructures is discussed and the driven force behind it is identified as anisotropic growth, autocatalytic process and dislocation driven mechanisms, depending on the specific materials and experimental conditions. Ternary compounds will be presented together with their properties characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), transmission electron microscopy (TEM) and Raman spectroscopy techniques. The possibility of synthetizing nanocomposites will be also briefly discussed.
On one hand, interest on the tunability of the optical microcavities has increased in the last few years due to the need for selective nano- and microscale light sources to be used as photonic building blocks in several applications. On the other, transparent conductive oxide (TCO) β-Ga2O3 is attracting attention in the optoelectronics area due to its ultra wide band gap and high breakdown field. However, at the micro- and nanoscale there are still some challenges to face up, namely the control and tuning of the optical and electrical properties of this oxide. In this work, Cr doped Ga2O3 elongated microwires are grown using the vapor-solid (VS) mechanism. Focused Ion Beam (FIB) etching forms Distributed Bragg Reflector (DBR)-based resonant microcavities. Room temperature microphotoluminescence (μ-PL) spectra show strong modulations in the red-NIR range on five cavities with different lengths. Selectivity of the peak wavelengths is obtained, proving the tunability of this kind of optical systems. The confined modes are analyzed experimentally, analytically and via finite difference time domain (FDTD) simulations. Experimental reflectivities up to 78% are observed.
The synthesis of complex nanostructures that combine materials and dimensionality, promises the ability to identify novel designs and architectures with enhanced properties that could be used in new devices. One of the building blocks in nanomaterials are nanowires, which offer several possibilities to get complex nanostructures. We present two kinds of morphologies based on oxide nanowires obtained by a thermal evaporation method. The common feature of both morphologies is a central oxide nanowire and, depending on the growth parameters, nanowires with either nanocrystallites or nano/microrods attached to the central wire are obtained. We have previously reported the fabrication of several single oxide nanowires and in particular, gallium oxide (β-Ga2O3) and zinc germanate oxide (Zn2GeO4) nanowires. Here we report the shape evolution of these nanowires by the suitable modification of the growth parameters. The addition of tin oxide (SnO2) to the precursors and variation of the thermal treatments duration result in the formation of the above-mentioned complex nanostructures. Structural and chemical characterizations were performed by electron microscopy techniques and Raman spectroscopy. The results shed light on the understanding of the driving mechanisms that lead to the formation of complex oxide nanostructures.
K. Lorenz, M. Peres, M. Felizardo, J. Correia, L. Alves, E. Alves, I. López, E. Nogales, B. Méndez, J. Piqueras, M. Barbosa, J. Araújo, J. Gonçalves, J. Rodrigues, L. Rino, T. Monteiro, E. Villora, K. Shimamura
KEYWORDS: Crystals, Europium, Annealing, Ions, Picture Archiving and Communication System, Backscatter, Ion implantation, Doping, Sensors, Chemical species
Ga2O3 bulk single crystals have been implanted with 300 keV Europium ions to fluences ranging from 1×1013 to 4×1015 at/cm2. The damage build-up and Eu-incorporation was assessed by Rutherford Backscattering Spectrometry in the channeling mode (RBS/C). RBS/C results suggest that implantation causes a mixture of defect clusters and extended defects such as dislocations. Amorphisation starts at the surface for fluences around 1×1015 at/cm2 and then proceeds to deeper regions of the sample with increasing fluence. Amorphous regions and defect clusters are efficiently removed during rapid thermal annealing at ~1100 °C; however, Eu diffuses towards the surface. Nevertheless, Eu ions are optically activated and show cathodoluminescence at room temperature. Results in bulk samples are compared to those in Eu-implanted Ga2O3 nanowires and despite strong similarities in the structural properties differences were found in the optical activation. Furthermore, damage and dopant incorporation studies were performed using the Perturbed Angular Correlation technique, which allows probing the immediate lattice surroundings of an implanted radioactive probe at the atomic level.
Interest on the control of light at the nano- and microscale has increased in the last years because of the incorporation of nanostructures into optical devices. In particular, semiconductor oxides microstructures emerge as important active materials for waveguiding and confinement of light from UV to NIR wavelengths. The fabrication of high quality and quantity of nano- and microstructures of semiconductor oxides with controllable morphology and tunable optical properties is an attractive challenge in this field. In this work, waveguiding and optical confinement applications of different micro- and nanostructures of gallium oxide and antimony oxide have been investigated. Structures with morphologies such as nanowires, nanorods or branched nanowires as elongated structures, but also triangles, microplates or pyramids have been obtained by a thermal evaporation method. Light waveguide experiments were performed with both oxides, which have wide band gap and a rather high refractive index. The synthesized microstructures have been found to act as optical cavities and resonant modes were observed. In particular, photoluminescence results showed the presence of resonant peaks in the PL spectra of Ga2O3 microwires and Sb2O3 micro-triangles and rods, which suggest their applications as optical resonators in the visible range.
Monoclinic gallium oxide, β-Ga2O3, is a transparent conducting oxide (TCO) that presents one of the widest band gaps
among this family of materials. Its characteristics make it highly interesting for applications in UV - visible - IR
optoelectronic and photonic devices. On the other hand, the morphology of nanowires made of this oxide presents
specific advantages for light emitting nanodevices, waveguides and gas sensors. Control of doping of the nanostructures
is of the utmost importance in order to tailor the behavior of these devices.
In this work, the growth of the nanowires is based on the vapor-solid (VS) mechanism during thermal annealing
treatment while the doping process was carried out in three different ways. In one of the cases, doping was obtained
during the growth of the wires. A second method was based on thermal diffusion of the dopants after the growth of
undoped nanowires, while the third method used ion implantation to introduce optically active ions into previously
grown nanowires. The study of the influence of the different dopants on the luminescence properties of gallium oxide
nanowires is presented. In particular, transition metals and rare earths such as Cr, Gd, Er or Eu were used as optically
active dopants that allowed selection of the luminescence wavelength, spanning from the UV to the IR ranges. The
benefits and drawbacks of the three different doping methods are analyzed. The waveguiding behavior of the doped
nanowires has been studied by room temperature micro-photoluminescence.
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