Alberto Anedda

A 1.9 eV photoluminescence induced by 4 eV photons in high‐purity wet synthetic silica

Photoluminescence (PL) of type III (high‐purity wet synthetic) silica excited by a XeCl excimer laser (hν=4 eV) is reported. Time‐resolved spectra show that a PL band peaked at 1.9 eV can be induced by the XeCl laser irradiation. This band exhibits a fast rise time (<20 ns) but a decay time of several microseconds. The possible mechanisms of photogeneration and photoexcitation of the defects related to this emission are discussed.

Persistent photoconductivity and optical quenching of photocurrent in GaN layers under dual excitation

Persistent photoconductivity (PPC) and optical quenching (OQ) of photoconductivity (PC) were investigated in a variety of n-GaN layers characterized by different carrier concentrations, luminescence characteristics, and strains. The relation between PPC and OQ of PC was studied by exciting the samples with two beams of monochromatic radiation of various wavelengths and intensities. The PPC was found to be excited by the first beam with a threshold at 2.0 eV, while the second beam induces OQ of PC in a wide range of photon energies with a threshold at 1.0 eV. The obtained results are explained on the basis of a model combining two previously put forward schemes with electron traps playing the main role in PPC and hole traps inducing OQ of PC. The possible nature of the defects responsible for optical metastability of GaN is discussed.

Photocurrent in epitaxial GaN

A simple kinetic model concerning photocurrent in epitaxially grown GaN is presented. Utilizing a minimal set of rate equations and kinetic parameters, it is shown that in the presence of hole centers with small probabilities of electron-hole recombinations, the time dependence of photocurrent is ruled by competition between capture of conduction bandelectrons by deep electron traps and electron-hole recombinations. If the probability of electron capture exceeds that of recombination, the decay of current after excitation is turned off shows the usual persistent photocurrent trend. If, on the contrary, the probability of recombination is larger than that of electron capture, a slow photocurrent quenching, past a maximum, can be observed. In some circumstances, after excitation is turned off, the current drops below the steady dark current, at which point the negative persistent photoconductivity effect comes into play.

Optical properties of α-ZnAl2S4:Cr single crystals

The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state 4A2g and the excited levels 2Eg, 2T1g, 2T2g, 4T2g, and 4T1g of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the 2Eg, 2T1g, and 4T2g levels and the energy gaps between the 2Eg–2T1g and 2Eg–4T2g states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.

RECOMBINATION IN HgGaInS4 SINGLE CRYSTALS

Abstract Photoelectronic processes in HgGaInS4 have been studied by photoconductivity, thermally stimulated conductivity and photoluminescence. An exponential distribution of traps with a slope of 23 meV/decade as well as a further electron trap system with an activation energy of 70 meV have been localized below the bottom of the conduction band. Radiative electron transitions are shown to occur mainly from exponentially distributed traps to an acceptor level characterized by an activation energy of 220 meV.

Light assisted dimer to monomer transformation in heavily doped rhodamine 6G-Porous silica hybrids.

The spectral properties of postdoping prepared type I Rhodamine 6G-silica hybrids were investigated in the case of intentionally large doping (5 x 10(-2) mol/L). Beside expected concentration quenching effects, steady state and time resolved optical spectroscopy measurements displayed the presence of different kind of aggregates, both fluorescent and nonfluorescent. As the irradiation dose on the sample increases, the emission features change: the overall emission increases, the peak of the emission is blue-shifted, and the emission decay time also increases. The modifications of the spectral properties under light excitation are interpreted in terms of dimer to monomer light assisted transformation, and a thermodynamic model based on a monomolecular kinetics is presented.

Photoconductive properties of HgGa2S4

Abstract Mercury thiogallate, HgGa 2 S 4 is a defect chalcopyrite semiconductor with the space group S 4 2 which offers a combination of attractive properties for applications. In order to obtain information about the electron states in the energy gap, photoconductivity measurements are performed in the 80–300 K range. Photoconductivity spectra show two peaks related to intrinsic and extrinsic excitation at about 410 and 500 nm, respectively; these maxima show a temperature dependence similar to the linear coefficient of the energy gap. Thermally stimulated currents have been studied by exciting the samples with intrinsic light at different temperatures. For all excitation temperatures a single TSC peaks were obtained. The analysis of TSC curves allowed one to estimate the kinetics of the trap emptying, trap energy distribution and thermal activation energy. A model for the level distribution in the semiconductor energy gap is suggested which in good agreement with the results of a previous photoluminescence study.

A kinetics model for Tb3+ recombinations in low doped Tb:Lu1.8Y0.2SiO5 crystals

The detailed characterization of the terbium related optical properties of low doped oxyorthosilicates of lutetium and yttrium is reported. The introduction of terbium ions generates an ultraviolet absorption band peaked at about 242 nm and line shaped emissions in the 350–600 nm range. The transitions are related to the D53 and D54 levels and the analysis of the decay time measurements allows to individuate a cross relaxation mechanism among Terbium ions. We propose a three level kinetic model which is able to reproduce the experimental data allowing to discriminate among the radiative and nonradiative contributions to the transitions in the case of low content of Tb ions (nominal content 10 ppm). The reported study addresses two important goals, providing, from one side, a detailed characterization of possible inorganic phosphors, and from the other side, since traces of unwanted elements were identified in numerous commercial samples of cerium doped oxyorthosilicates, it can contribute to increase the ...

Fundamental optical constants of CdIn2S4

Abstract The reflectivity of CdIn2S4 has been measured at 300 K and 77 K in the 2–10 eV energy range. The analysis of the low temperature data by means of the Kramers Kronig relations led to the determination of the optical constants. The results have been compared with recent band structure calculations and a substantial agreement between theory and experiment has been found.

OH-dependence of ultraviolet emission in porous silica

Abstract Photoluminescence and Raman measurements have been performed on sol–gel synthesized porous silica monoliths. Porous silica and polyethilenglycole/silica hybrids thermally treated at different temperatures have been analyzed. By exciting at 5.6 eV a structured photoluminescence emission centered at 3.7 eV is found in the whole set of samples. Hydration treatment of the pore surfaces increases the H-bonding interaction between silanols, inhibits the isolated silanols vibrations and changes the photoluminescence spectrum increasing the emission peaked at 3.7 eV.