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.
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.
Laser assisted flow deposition (LAFD) is a very high yield method based on a vapor-solid mechanism, allowing
the production of ZnO crystals in a very short time. The LAFD was used in the growth of different morphologies
(nanoparticles, tetrapods and microrods) of ZnO micro/nanocrystals and their microstructural characterization confirms
the excellent crystallinity of the wurtzite structure. The optical properties of the as-grown ZnO crystals investigated by
low temperature photoluminescence (PL) evidence a well-structured near band edge emission (NBE) due to the
recombination of free (FX), surface (SX) and donor bound (D0X) excitons. Among the most representative emission
lines, the 3.31 eV transition was found to occur in the stacking faults-free microrods. The luminescence behavior
observed in H passivated samples suggests a closer relationship between this optical center and the presence of surface
Besides the unintentionally doped micro/nanocrystals, ZnO/Ag and ZnO/carbon nanotubes (CNT) hybrid structures were
processed by LAFD. The former aims at the incorporation of silver as a p-type dopant and the latter envisaging
photovoltaic applications. Silver-related spherical particles were found to be inhomogeneously distributed at the
microrods surface, accumulating at the rods tips and promoting the ZnO nanorods re-nucleation. Despite the fact that
energy dispersive X-ray measurements suggest that a fraction of the silver could be incorporated in the ZnO rods, no new
related luminescence lines or bands were observed when compared with the as-grown samples. For the case of the
ZnO/CNT composites two main approaches were adopted: i) a direct deposition of ZnO particles on the surface of
vertically aligned multi-walled carbon nanotubes (VACNTs) forests without employing any additional catalyst and ii)
new ZnO/CNT hybrids were developed as buckypaper nanocomposites. The use of the LAFD technique in the first
approach preserves the CNTs structure and alignment and avoids the collapse of the VACNTs array, which is a major
advantage of this method. On the other hand, LAFD grown ZnO nanoparticles and tetrapods were used to produce
ZnO/CNT buckypaper nanocomposites. When compared with the as-grown samples the PL spectra of the composites
structures behave differently. For the case of the ZnO/VACNTs no changes on the peak position and spectral shape were
observed. Only an enhancement of the overall luminescence was found to occur. On contrary, for the buckypaper
nanocomposites notable changes on the spectral shape and peak position were observed, likely due to distinct surface
band bending effects for the ZnO nanoparticles and tetrapods embedded in the CNTs.
Wide band gap oxide media including 4fn or 3dn ions attracts a considerable attention in the context of photonics and bio-photonics applications due to the electromagnetic widespread spectral range covered by the intraionic radiative relaxation of the charged lanthanide and transition metal ions. Converting ultraviolet commercial light into visible luminescence continues to raise interest for the solid state light market, justifying the demand for new and efficient phosphors with wide spectrum coverage and improved thermal quenching behavior. New materials and methods have been thoroughly investigated for the desired purposes. In this work, we report on laser processing for the growth of oxides media such as ZrO2, ZnO among other oxide hosts. The transparent crystalline materials in-situ doped with different amounts of lanthanide or transition metal ions are explored in order to enhance the room temperature ions luminescence by pumping the samples with ultraviolet photons. Spectroscopic studies of the undoped and doped oxide hosts were performed using Raman spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE).
The origin of a double peak, detected by X-ray diffraction (XRD), in a wurtzite CdxZn1-xO (x=0.17) epilayer, is investigated using Rutherford backscattering spectrometry in channeling geometry (RBS/C). In-depth compositional characterization by RBS/C demonstrates that strain relaxation does not take place via compositional phase separation and does not cause any compositional pulling effects. On the contrary, RBS/C angular scans demonstrate that relaxation is a consequence of progressive structural changes during the heteroepitaxial growth of the film on MgZnO, likely due to the large distortion of the lattice induced by the high Cd content.
The radiation damage formation upon low temperature ion implantation and neutron irradiation has been compared for
GaN and ZnO. Both materials exhibit strong dynamic annealing effects during implantation, even at 15 K, leading to
high amorphisation thresholds. The damage build-up with fluence was found to proceed in a similar way for GaN and
ZnO, both showing two saturation regimes below the amorphisation level where, over wide fluence regions, the damage
level increases only very slowly. For low fluences the damage accumulation rate is similar for both materials. For higher
fluences, on the other hand, GaN shows considerably higher damage levels and finally collapses into an amorphous
structure while ZnO remains single crystalline up to the highest fluence of 7×1016 Ar/cm2.
Neutron irradiation produces similar defects as ion implantation but within the entire sample while the defect density is
much lower. The main effect of irradiation on the structural properties of GaN is an expansion of the c-lattice parameter.
Optical properties are significantly deteriorated after irradiation and only recover partially after annealing. ZnO does not
suffer such a pronounced change of the lattice parameters but reveals a strong deterioration of the surface, possibly due
to blistering and exfoliation. At the same time the optical properties are less affected than for GaN. The near band edge
emission is partly quenched but recovers to a large extend after annealing while broad defect bands are observed below
the bandgap for irradiated samples, before and after annealing.
In this work we report for the first time the electroluminescence of two different kinds of rare earth complexes based
Organic Light Emitting Diodes, the Tb(ACAC)3bipy [Tris (acetylacetonate) - 2,2' - bipyridinyl - terbium(III)] and the
Eu(TTA)3bipy [tris(4,4,4 -trifluoro -1 - (2 - thienyl) -1,3 - butanediono - 2,2' - bipyridinyl - europium(III)]. In both
devices the corresponding electroluminescence spectrum is obtained (red for europium with (x,y) CIE coordinates near
(0.64, 0.34) and green for terbium near (0.28, 0.55) coordinates) at a driving voltage near 16 - 17 V with a maximum
electrical current of 1 mA. The Wall Plug Efficiency is about 10-3% in both cases.
In this work a detailed study of the electroluminescence of an Organic Light Emitting Diode based on Europium
Complex is studied as a function of the emissive layer thickness and growth rate evaporation. The device structure is
glass:ITO / TPD [N,N' - bis (3 - methylphenyl) - N,N' - diphenylbenzidine] / Eu(DBM)3phen [tris (dibenzoylmethane)
- mono (4,7-dimethylphenantroline) europium (III)] / Alq3 [aluminum - tris (8 - hydroxyquinoline)] / Aluminum. The
minimum driving voltage is about 15 Volts although the electrical current is about only 2 mA (wall plug efficiency up to
0.002 %). The electroluminescence spectra and external efficiency are clearly dependent on the Eu (DBM)3phen layer
thickness and growth rate evaporation. This results in a strong color change (CIE coordinates). With these results, a
model for the device opto-electrical behavior is presented, allowing the device final optimization.