[en] Gallium oxide microrods have been grown by an evaporation-deposition method by using a precursor containing lithium in order to check the influence of such dopant on the morphology and physical properties of the obtained β -Ga₂O₃ structures. SEM studies show that the morphology is modified with respect to undoped gallium oxide, promoting the growth of micropyramids transversal to the microwire axis. Raman analysis reveals good crystal quality and an additional Raman peak centred at around 270 cm⁻¹, characteristic of these samples and not present in undoped monoclinic gallium oxide. The presence of the Li⁺ ions also influences the luminescence emission by inducing a red-shift of the characteristic UV-blue defect band of gallium oxide. In addition, an intense sharp peak centred around 717 nm observed both by cathodoluminescence (CL) and photoluminescence (PL) is also attributed to the presence of these ions. The Li related luminescence features have also been investigated by PL excitation (PLE) spectra and by the temperature dependence of the luminescence. (paper)

[en] Erbium doped β-Ga₂O₃ nanowires and microwires have been obtained by a vapour-solid process from an initial mixture of Ga₂O₃ and Er₂O₃ powders. X-ray diffraction (XRD) analysis reveals the presence of erbium gallium garnet as well as β-Ga₂O₃ phases in the microwires. Scanning electron microscopy (SEM) images show that the larger microwires have a nearly rectangular cross-section. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis show good crystal quality of the β-Ga₂O₃ nanowires. The nanostructures have been studied by means of the cathodoluminescence technique in the scanning electron microscope. Er intraionic blue, green and red emission lines are observed in luminescence spectra even at room temperature, which confirms the optical activity of the rare earth ions in the grown structures. Mapping of the main 555 nm emission intensity shows a non-homogeneous distribution of Er ions in the microstructures

[en] 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-Ga₂O₃ and GeO₂ structures implanted with Eu show the characteristic red luminescence peak centered at around 610 nm, due to the ⁵D₀-⁷F₂ Eu³⁺ intraionic transition. Sharpening of the luminescence peaks after thermal annealing is observed in Eu implanted β-Ga₂O₃, which is assigned to the lattice recovery. Gd³⁺ as-implanted samples do not show rare earth related luminescence. After annealing, optical activation of Gd³⁺ is obtained in both matrices and a sharp ultraviolet peak centered at around 315 nm, associated with the Gd³⁺⁶P_7/2-⁸S_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.

[en] Visible luminescence from erbium oxide layers grown on crystalline and amorphous silicon (c-Si and a-Si) has been investigated. The results show strong red and green cathodoluminescence bands due to intraionic Er³⁺ radiative transitions at room temperature. The use of c-Si or a-Si as substrate led to a red or green dominant emission, respectively, which has been explained in terms of the oxygen content in the substrate. The results obtained from samples grown in different atmospheres also support this assumption. (author)

No abstract available

[en] Branched, hierarchically grown ZnGa_2O_4 and Zn_1_ _−_ _xMn _xGa_2O_4 (0.1 < x < 0.21) micro- and nanostructures have been fabricated by a thermal evaporation method. Comparison of both materials shows that the presence of Mn favours the formation of the branched morphology, with oriented nanowires of high crystalline quality. The origin of the growth of these nanostructures is discussed. Raman peaks are observed to broaden and shift as a function of Mn content in  the alloy. Cathodoluminescence analysis shows that ZnGa_2O_4 structures emit  the  characteristic defect-related UV-blue band and Zn_1_ _−_ _xMn _xGa_2O_4 nanowires show efficient green emission due to intraionic Mn"2"+ transitions. Zn_1_ _−_ _xMn _xGa_2O_4 structures have been doped with Cr"3"+ during thermal treatments in presence of Cr. This leads to structures with additional red emission related to Cr"3"+. The excitation conditions allow selection of the dominant luminescence band. Photocurrent measurements have also been carried out in ZMGO structures, showing the characteristic intraionic Mn"2"+ transitions. (papers)

[en] Cathodoluminescence and photoluminescence techniques have been used to investigate room temperature light emission from β-Ga₂O₃:Eu nanostructures, which were obtained by two methods. In one of them, a mixture of Ga₂O₃/Eu₂O₃ powders was used as precursor material and annealed under an argon flow. In the other one, undoped β-Ga₂O₃ 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 Eu³⁺ intraionic transitions from β-Ga₂O₃:Eu has been observed. Waveguiding of this red emitted light through the structures was shown.

[en] The luminescence properties of Er doped β-Ga₂O₃ and of the erbium gallium garnet Er₃Ga₅O₁₂ (ErGG) have been investigated both in the visible and in the infrared (IR) ranges by means of photoluminescence (PL). Doping of the β-Ga₂O₃ was obtained in two different ways: erbium ion implantation into β-Ga₂O₃ and high temperature annealing of a mixture of Er₂O₃ and Ga₂O₃ powders. X-ray diffraction shows that the latter samples present both β-Ga₂O₃ and ErGG phases. The PL studies demonstrate that the β-Ga₂O₃ in these samples is doped with erbium. The differences in the luminescence emission and excitation peaks of the Er³⁺ ions in these two hosts are studied through selective PL measurements. Strong near IR emission and weak green emission from Er³⁺ in the β-Ga₂O₃ matrix is obtained. The opposite is obtained for Er³⁺ in ErGG when excited under the same conditions. Room temperature luminescence is observed from erbium in the two hosts

[en] 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. (paper)

[en] Core–shell gallium nanoparticles (Ga NPs) have recently been proposed as an ultraviolet plasmonic material for different applications but only at room temperature. Here, the thermal stability as a function of the size of the NPs is reported over a wide range of temperatures. We analyze the chemical and structural properties of the oxide shell by x-ray photoelectron spectroscopy and atomic force microscopy. We demonstrate the inverse dependence of the shell breaking temperature with the size of the NPs. Spectroscopic ellipsometry is used for tracking the rupture and its mechanism is systematically investigated by scanning electron microscopy, grazing incidence x-ray diffraction and cathodoluminescence. Taking advantage of the thermal stability of the NPs, we perform complete oxidations that lead to homogenous gallium oxide NPs. Thus, this study set the physical limits of Ga NPs to last at high temperatures, and opens up the possibility to achieve totally oxidized NPs while keeping their sphericity. (paper)