L. Rino

Annealing study of the formation of nickel-related paramagnetic defects in diamond

Abstract We report an annealing study of paramagnetic defects found in synthetic diamond grown from nickel solvent/catalysts. Substitutional N0 and Ni− defects in diamond show a similar behavior in the course of annealing at temperatures in the range 1550–2000 °C. New paramagnetic centers labeled AB6 and AB7 become detectable upon annealing. The AB5 center is found to appear in as-grown HPHT diamond rich in nitrogen, and its concentration decreases with increasing annealing temperature. The AB1, AB3, AB6 and AB7 defects are produced at early stages of the annealing process and are suppressed by heating diamond to temperatures in excess of 1900 °C. Possible origins of the AB paramagnetic centers are discussed in the light of the recently proposed mechanisms of nitrogen aggregation and nickel–nitrogen complexes formation in HPHT diamond.

Photoluminescence studies of a perceived white light emission from a monolithic InGaN/GaN quantum well structure

In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 °C, 1100 °C and 1200 °C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 °C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 °C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260–4000 K.

Multiple temperature effects on up-conversion fluorescences of Er3+-Y b3+-Mo6+ codoped TiO2 and high thermal sensitivity

We report multiple temperature effects on green and red up-conversion emissions in Er3+-Y b3+-Mo6+ codoped TiO2 phosphors. With increasing temperature, the decrease of the red emission from 4F9/2→4I15/2, the increase of green emission from 2H11/2→4I15/2 and another unchanged green emission from 4S3/2→4I15/2 were simultaneously observed, which are explained by steady-state rate equations analysis. Due to different evolution with temperature of the two green emissions, higher thermal sensitivity of optical thermal sensor was obtained based on the transitions with the largest fluorescence intensity ratio. Two parameters, maximum theoretical sensitivity (Smax) and optimum operating temperature (Tmax) are given to describe thermal sensing properties of the produced sensors. The intensity ratio and energy difference ΔE of a pair of energy levels are two main factors for the sensitivity and accuracy of sensors, which should be referred to design sensors with optimized sensing properties.

Spectroscopic analysis of LYSO:Ce crystals

Rare earth orthosilicates are among the most widely used scintillator materials in the last decades. Particularly, lutetium-yttrium oxyorthosilicate (LYSO) is known to exhibit great potentialities in the field of radiation detectors for medical imaging. Consequently, an in-depth knowledge of the material properties is of utmost interest for the mentioned applications. In this work the spectroscopic properties of commercial cerium doped lutetium-yttrium oxyorthosilicate crystals (LYSO:Ce) were investigated by Raman spectroscopy, steady state photoluminescence, photoluminescence excitation and time resolved photoluminescence. Site selective excitation was used under steady state (325nm) and pulsed (266nm) conditions to separately investigate the temperature dependence of the 5d→4f Ce1 and Ce2 luminescence, allowing to establish the thermal quenching dependence of the Ce2 optical center. In the case of the Ce1 optical center, a luminescence quantum efficiency of 78% was obtained from 14K to room temperature with 266nm photon excitation.

Doping of Ga2O3 bulk crystals and NWs by ion implantation

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.

The effect of high-pressure–high-temperature annealing on paramagnetic defects in diamond

The formation and annealing out of paramagnetic defects in high-pressure and high-temperature diamond are investigated through a comprehensive heat treatment study of as-grown crystals. Diamond crystals are subjected to a sequence of isochronal anneals in the temperature range between 1550 and 2000 °C and the concentrations of paramagnetic defects are determined by electron paramagnetic resonance measurements after each annealing step. The quantitative data are analysed in the light of recently published results on the behaviour of nickel and/or nitrogen defects upon annealing. Like the P1 and W8 centres, the NIRIM1 nickel-related defects show a strong decrease in their concentration upon annealing at 1900 °C, whereas the NIRIM2 centres seem to be rather stable. The evolution of the nickel-related centre concentration through the annealing sequence shows that not all W8 defects transform into paramagnetic nickel–nitrogen complexes. Indeed, most of the W8 defects are transformed in other non-paramagnetic centres, with the AB1, AB3, and AB6 defects being probably by-products of this process. The relatively low concentration of nickel–nitrogen paramagnetic complexes formed upon annealing shows that their formation does not play any fundamental role in the kinetics of the nitrogen aggregation during heat treatments.

High pressure synthetic diamond: optical studies of nickel related defects

Abstract The optical spectrum of synthetic diamond, grown by the high pressure method using a nickel-containing alloy as solvent catalyst is quite different not only from the one of natural diamond and that of diamond films, but also from the one of other high pressure diamond synthesised using a catalyst not containing Ni. We report high resolution optical studies of the 1.885 eV system present in the spectrum of synthetic diamonds containing Ni and a high concentration of N (> 50 ppm). Temperature dependence measurements show that this system consists of by 3 zero-phonon lines, which, using uniaxial stress, we show occurance at a rhombic I symmetry defect in transitions between a common ground state A 1 into three excited states transforming as the B 1 representation of the C 2v point group. A model for the defect is suggested.

Prospects on laser processed wide band gap oxides optical materials

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).

Spectroscopic analysis of the NIR emission in Tm implanted AlxGa1-xN layers

AlxGa1-xN samples, with different AlN molar fractions, x = 0, 0.15, 0.77, and 1, grown by halide vapor phase epitaxy were implanted with Tm ions. Photoluminescence (PL) measurements revealed that after thermal annealing all the samples exhibit intraionic Tm3+ luminescence. In samples with x > 0, the low temperature emission is dominated by the lines that appear in the near infrared (NIR) spectral region, corresponding to the overlapped 1G4 → 3H5 and 3H4 → 3H6 multiplet transitions. A detailed spectroscopic analysis of NIR emission of the thulium implanted and annealed AlxGa1-xN layers is presented by using temperature dependent steady-state PL, room temperature PL excitation, and time resolved PL. The results indicate that the excitonic features sensitive to the alloy disorder are involved in the excitation population processes of the Tm3+ luminescence and the highest thermal stability for the NIR emission occurs for the AlN:Tm sample.

Optical studies of nickel complexes in high pressure synthetic diamond

Abstract We report uniaxial stress measurements on the 2.242 eV photoinduced zero phonon line, observed on nickel- and nitrogen-containing synthetic diamonds after annealing at 1600 °C. We show that the line is an electric dipole transition, which occurs at a defect of rhombic I symmetry. The shape of the vibronic sideband and the temperature dependence of the line position and width can be understood assuming electron–phonon coupling to totally symmetric vibrational modes centred at ℏ ω=55 meV. Electron–lattice interaction cannot explain the large decrease of ZPL intensity with temperature, and it is suggested that some thermal activated de-excitation path is responsible for the observed behaviour.