Emilio Nogales

Cathodoluminescence from β‐Ga2O3 nanowires

β‐Ga2O3 nano- and microwires with diameters ranging from tens of nanometers to about one micron and lengths of up to tens of microns, have been obtained by sintering Ga2O3 powder under argon flow. The structures have been investigated by cathodoluminescence in the scanning electron microscope. The samples showed the violet-blue emission characteristic of Ga2O3 and a red emission at 1.73 eV dominant in the nanowires and other nano- and microstructures formed during the sintering treatment. At temperatures below 210 K, this band exhibits sharp peaks separated by 20 meV. This observation suggests the exchange of phonons in the recombination process.

Red luminescence of Cr in β‐Ga2O3 nanowires

Red luminescence emission from chromium doped β‐Ga2O3 nanowires has been studied by means of photoluminescence (PL) techniques. PL excitation shows several bands in the ultraviolet-visible region. Time decay values, obtained by time resolved PL, at different temperatures fit a three level model with thermal population of the upper level from the middle one. From the results, the origin of the emission is assigned to Cr3+ ions in the β‐Ga2O3 host, and values for the energy level separation and Huang-Rhys factor of the broad T24‐A24 emission are estimated.

Europium doped gallium oxide nanostructures for room temperature luminescent photonic devices

Cathodoluminescence and photoluminescence techniques have been used to investigate room temperature light emission from beta-Ga(2)O(3):Eu nanostructures, which were obtained by two methods. In one of them, a mixture of Ga(2)O(3)/Eu(2)O(3) powders was used as precursor material and annealed under an argon flow. In the other one, undoped beta-Ga(2)O(3) nanostructures were first obtained by thermal oxidation of metallic gallium and europium was subsequently incorporated by a diffusion process. Room temperature luminescence at 610 nm due to Eu(3+) intraionic transitions from beta-Ga(2)O(3):Eu has been observed. Waveguiding of this red emitted light through the structures was shown.

Doped gallium oxide nanowires with waveguiding behavior

Er and Cr doped β-Ga2O3 nano- and microwires have been grown by thermal treatments of Ga2O3 powder containing a fraction of Er2O3 powder or in the presence of Cr2O3 powder, respectively. Doping gives rise to the characteristic red photoluminescence (PL) of Cr3+ ions at about 700nm and to green emission of Er3+ at about 555nm. Waveguiding of the ion related PL light excited in the wires has been studied for wires with different sizes. Waveguide behavior for incident visible light of different wavelengths was demonstrated.

Cathodoluminescence of rare earth implanted Ga2O3 and GeO2 nanostructures

Rare earth (RE) doped gallium oxide and germanium oxide micro- and nanostructures, mostly nanowires, have been obtained and their morphological and optical properties have been characterized. Undoped oxide micro- and nanostructures were grown by a thermal evaporation method and were subsequently doped with gadolinium or europium ions by ion implantation. No significant changes in the morphologies of the nanostructures were observed after ion implantation and thermal annealing. The luminescence emission properties have been studied with cathodoluminescence (CL) in a scanning electron microscope (SEM). Both β-Ga(2)O(3) and GeO(2) structures implanted with Eu show the characteristic red luminescence peak centered at around 610 nm, due to the (5)D(0)-(7)F(2) Eu(3+) intraionic transition. Sharpening of the luminescence peaks after thermal annealing is observed in Eu implanted β-Ga(2)O(3), which is assigned to the lattice recovery. Gd(3+) as-implanted samples do not show rare earth related luminescence. After annealing, optical activation of Gd(3+) is obtained in both matrices and a sharp ultraviolet peak centered at around 315 nm, associated with the Gd(3+) (6)P(7/2)-(8)S(7/2) intraionic transition, is observed. The influence of the Gd ion implantation and the annealing temperature on the gallium oxide broad intrinsic defect band has been analyzed.

Crossed Ga2O3/SnO2 multiwire architecture: a local structure study with nanometer resolution.

Crossed nanowire structures are the basis for high-density integration of a variety of nanodevices. Owing to the critical role of nanowires intersections in creating hybrid architectures, it has become a challenge to investigate the local structure in crossing points in metal oxide nanowires. Thus, if intentionally grown crossed nanowires are well-patterned, an ideal model to study the junction is formed. By combining electron and synchrotron beam nanoprobes, we show here experimental evidence of the role of impurities in the coupling formation, structural modifications, and atomic site configuration based on crossed Ga2O3/SnO2 nanowires. Our experiment opens new avenues for further local structure studies with both nanometer resolution and elemental sensitivity.

Resonant cavity modes in gallium oxide microwires

Fabry Perot resonant modes in the optical range 660-770 nm have been detected from single and coupled Cr doped gallium oxide microwires at room temperature. The luminescence is due to chromium ions and dominated by the broad band involving the T-4(2)-(4)A(2) transition, strongly coupled to phonons, which could be of interest in tunable lasers. The confinement of the emitted photons leads to resonant modes detected at both ends of the wires. The separation wavelength between maxima follows the Fabry-Perot dependence on the wire length and the group refractive index for the Ga2O3 microwires.

β-Ga 2 O 3 nanowires for an ultraviolet light selective frequency photodetector

The behaviour of β-Ga2O3 nanowires as photoconductive material in deep ultraviolet photodetectors to operate in the energy range 3.0–6.2 eV has been investigated. The nanowires were grown by a catalyst-free thermal evaporation method on gallium oxide substrates. Photocurrent measurements have been carried out on both undoped and Sn-doped Ga2O3 nanowires to evidence the influence of the dopant on the photodetector performances. The responsivity spectrum of single nanowires show maxima in the energy range 4.8–5.4 eV and a strong dependence on the pulse frequency of the excitation light has been observed for undoped nanowires. Our results show that the responsivity of β-Ga2O3 nanowires can be controlled by tuning the chopper frequency of the excitation light and/or by doping of the nanowires. Non-linear behaviour in characteristic current–voltage curves has been observed for Ga2O3 : Sn nanowires. The mechanism leading to this behaviour has been discussed and related to space-charged-limited current effects. In addition, the responsivity achieved by doped nanowires at lower bias is higher than for undoped ones.

Visible and infrared luminescence study of Er doped β-Ga2O3 and Er3 Ga5O12

The luminescence properties of Er doped β-Ga_2O_3 and of the erbium gallium garnet Er_3 Ga_5O_12(ErGG) have been investigated both in the visible and in the infrared (IR) ranges by means of photoluminescence (PL). Doping of the β-Ga_2O_3 was obtained in two different ways: erbium ion implantation into β-Ga_2O_3 and high temperature annealing of a mixture of Er_2O_3 and Ga_2_O3 powders. X-ray diffraction shows that the latter samples present both β-Ga_2O_3 and ErGG phases. The PL studies demonstrate that the beta-Ga2O3 in these samples is doped with erbium. The differences in the luminescence emission and excitation peaks of the Er^3+ ions in these two hosts are studied through selective PL measurements. Strong near IR emission and weak green emission from Er^3+ in the β-Ga_2O_3 matrix is obtained. The opposite is obtained for Er^3+ in ErGG when excited under the same conditions. Room temperature luminescence is observed from erbium in the two hosts.

Near-field cathodoluminescence studies on n-doped gallium nitride films

Near-field cathodoluminescence (NFCL) has been used to characterize hydride vapor phase epitaxy grown n-GaN films. This technique can obtain high resolution luminescence images and perform local measurements of the diffusion length for minority carriers in different parts of the sample. NFCL contrast observed in round growth hillocks at the sample surface, with a diameter of less than 10 μm, is compared with that observed by conventional cathodoluminescence in scanning electron microscope (CLSEM) techniques. In particular NFCL images reveal features not detected by CLSEM which is explained by the fact that under near field conditions the signal arises from a depth of only several tens of nanometers and is then directly related to the surface hillocks. Diffusion lengths of about 0.4 and 4 μm have been found for the holes in different regions of the samples at room temperature. The order of magnitude of these minority carriers diffusion lengths is in good agreement with previous measurements performed at di...