B. Méndez

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.

Nature of compensating luminescence centers in Te-diffused and -doped GaSb

Diffusion of tellurium in undoped p‐GaSb has been carried out. Using the cathodoluminescence and photoluminescence techniques, the luminescence centers in Te‐diffused samples have been identified and compared with the Te‐doped bulk GaSb. Fundamental differences in the radiative levels are observed between the diffused and the as‐grown doped samples. Evidence for self‐compensating acceptor complexes are seen in diffused samples. With short and moderate diffusion times, a compensating acceptor complex VGaGaSbTeSb is observed. For long diffusion times, the dominant acceptor center has been attributed to the antisite defect GaSb or related complex. The reasons for the formation of various acceptor centers have been discussed.

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.

High aspect ratio GeO2 nano-and microwires with waveguiding behaviour

High aspect ratio GeO2 nano- and microwires have been grown by thermal treatment at 600 °C of compacted Ge powder under argon flow. The wires have cross-sectional dimensions from less than 100 nm to about 1 µm, depending on the duration of the treatment, and lengths of up to about 2000 µm. Waveguide behaviour of the wires was demonstrated for visible light, which shows the potential applications of these structures for optical nanodevices.

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.

Cathodoluminescence studies of growth and process-induced defects in bulk gallium antimonide

The homogeneity and luminescence properties of undoped bulk GaSb have been studied by the cathodoluminescence (CL) technique in the scanning electron microscope. CL images have revealed a nonuniform distribution of native defects in GaSb wafers prepared from as-grown single crystals. Postgrowth annealing in vacuum, gallium, and antimony atmospheres has been performed to obtain more accurate information about the defect structure in this material. In general, on annealing, homogeneous distribution of impurities is observed throughout the wafers. CL spectra show that a luminescence band (centered at 756 meV) is enhanced by annealing in a gallium atmosphere, suggesting that Ga atoms play an important role in the formation of this acceptor center. The 756 meV peak has been attributed to a transition from conduction band to an acceptor center comprised of Ga-Sb or a related complex. Interestingly, localized crystallization at the subgrain boundaries seems to occur by annealing in Ga atmosphere.

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.