Wide and ultrawide bandgap oxides, such as β-Ga2O3, ZnGa2O4 and Zn1-xGa2-2xGexO4, are of particular importance for a myriad of technological applications, including electronics, optoelectronics, and medical devices. In the case of the latter, the development of new, affordable and non-invasive methodologies for bioimaging and diagnosis is of crucial importance towards solutions that can improve health and wellbeing of the populations. For these purposes, red/near infrared emitters within the biological transparency window are required. Therefore, the here studied oxides were subjected to a controlled Cr-doping giving rise to intraionic emission in this region. In the case of the here studied oxide systems, we will investigate the intraionic luminescence properties of Cr3+, with particular emphasis on the persistent luminescence recorded in micro/nano particles of Cr-doped ZnGa2O4 and Zn1-xGa2-2xGexO4 synthesised by laser ablation in liquid media.
K. Lorenz, A. Redondo-Cubero, M. Lourenço, M. Sequeira, M. Peres, A. Freitas, L. Alves, E. Alves, M. Leitão, J. Rodrigues, N. Ben Sedrine, M. Correia, T. Monteiro
Compositional grading of InGaN/GaN multi quantum wells (QWs) was proposed to mitigate polarization effects and Auger losses in InGaN-based light emitting diodes [K. P. O'Donnell et al., Phys. Status Solidi RRL 6 (2012) 49]. In this paper we are reviewing our recent attempts on achieving such gradient via quantum well intermixing. Annealing up to 1250 °C resulted in negligible interdiffusion of QWs and barriers revealing a surprising thermal stability well above the typical MOCVD growth temperatures. For annealing at 1400 °C results suggest a decomposition of the QWs in regions with high and low InN content. The defect formation upon nitrogen implantation was studied in detail. Despite strong dynamic annealing effects, which keep structural damage low, the created defects strongly quench the QW luminescence even for low implantation fluences. This degradation could not be reversed during thermal annealing and is hampering the use of implantation induced quantum well intermixing in InGaN/GaN structures.
Wide band gap oxides, such as ZnO, SnO2 and ZrO2, are functional materials with a wide range of applications in several important technological areas such as those including lighting, transparent electronics, sensors, catalysis and biolabeling. Recently, doping and co-doping of oxides with lanthanides have attracted a strong interest for lighting purposes, especially among them nanophosphors for bioassays. Tailoring the crystalline materials physical properties for such applications often requires a well-controlled incorporation of dopants in the material lattice and a comprehensive understanding of their role in the oxides matrices. These undoped or intentionally doped oxides have band gap energies exceeding 3.3 eV at room temperature and are known to exhibit optically active centers that span from the ultraviolet to the near infrared region. Typically, by using photon energy excitation above the materials band gap, high quality undoped materials display narrow emission lines near the band edge due to free and bound-exciton recombination, as well as shallow donor-acceptor recombination pairs. Additionally, broad emission bands are often observed in these wide band gap hosts, hampering some of the desired physical properties for further applications. Recognizing and understanding the role of the dopant-related defects when deliberately introduced in the oxide hosts, as well as their influence on the samples luminescence properties, constitutes a matter of exploitation by the scientific community worldwide. In this work, we investigate the luminescence properties of undoped and lanthanide doped oxide materials grown by laser assisted techniques. Laser assisted flow deposition (LAFD) and pulse laser ablation in liquids (PLAL) were used for the growth of ZnO, SnO2 and yttria stabilized ZrO2 (YSZ) micro and nanocrystals with different morphologies, respectively. Regarding the YSZ host, trivalent lanthanide ions were optically activated by in-situ doping and co-doping. The influence of the defect energy states on the optical properties of the different undoped and doped metal oxide hosts is investigated under ultraviolet and infrared excitation by means of photoluminescence and photoluminescence excitation.
NiO/ZnO heterostructures were fabricated on FTO/glass and bulk hydrothermal ZnO substrates by pulsed laser
deposition. X-Ray diffraction and Room Temperature (RT) Raman studies were consistent with the formation of (0002)
oriented wurtzite ZnO and (111) oriented fcc NiO. RT optical transmission studies revealed bandgap energy values of
~3.70 eV and ~3.30 eV for NiO and ZnO, respectively and more than 80% transmission for the whole
ZnO/NiO/FTO/glass stack over the majority of the visible spectrum. Lateral p-n heterojunction mesas (~6mm x 6mm)
were fabricated using a shadow mask during PLD growth. n-n and p-p measurements showed that Ti/Au contacting
gave an Ohmic reponse for the NiO, ZnO and FTO. Both heterojunctions had rectifying I/V characteristics. The junction
on FTO/glass gave forward bias currents (243mA at +10V) that were over 5 orders of magnitude higher than those for
the junction formed on bulk ZnO. At ~ 10-7 A (for 10V of reverse bias) the heterojunction leakage current was
approximately two orders of magnitude lower on the bulk ZnO substrate than on FTO. Overall, the lateral p-NiO/n-
ZnO/FTO/glass device proved far superior to that formed by growing p-NiO directly on the bulk n-ZnO substrate and
gave a combination of electrical performance and visible wavelength transparency that could predispose it for use in
various third generation transparent electronics applications.
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