Marcello Salis

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.

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.

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 ...

On the formation of aggregates in silica – rhodamine 6G type II hybrids

The spectroscopic features of dye aggregates in organic–inorganic Rhodamine 6G–silica hybrids are investigated in sol–gel prepared type II bulk and thick film samples in the 10−4–10−3 mol l−1 concentration range. No aggregates are observed in film samples irrespective of the dye concentration and the spectroscopic features are ascribed to fluorescent monomers. On the contrary, by means of excitation and emission fluorescence measurements and the analysis of fluorescence decay kinetics the formation of fluorescent aggregates is reported in bulk samples and the geometry of the formed aggregates at the silica surface is discussed on the single exciton theory framework. Beside the distribution of a single emitting specie observed in bulk samples and individuated as oblique J dimers, the analysis of the excitation spectra indicate also the presence of “dark” monomer units. An energy transfer mechanism is hypothesized to explain the interaction of the monomer–dimer pair.

Investigation of energy transfer in terbium doped Y 2SiO5 phosphor particles

The kinetics of luminescence of sol-gel synthesized terbium doped Y (2)SiO(5) (YSO) phosphor particles is investigated in detail with reference to Tb concentration in the 0.001%-10% range. By increasing the dopant concentration, the luminescence profile changes from a blue to a green peaked emission spectrum because of the energy transfer among centers. The inter-center energy transfer mechanism is well accounted for by the Inokuti-Hirayama (IH) kinetic model which is based on a statistical average of inter-center distance dependent decay modes of the donor luminescence. The distribution of the decay modes is implemented from the Förster-Dexter resonance theory of energy transfer by assuming a rate constant for the energy transfer by multipolar interactions between donors and acceptors. However, the experimental results recorded in the low concentration limit show the presence of green emission contributions in the luminescence spectrum which cannot be related to the Tb concentration; for this reason an additional internal energy transfer mechanism, occurring among levels of the same center, is proposed to account for the recorded emission properties. Thus, a new and more exhaustive model which includes both the internal and external energy transfer processes is considered; the proposed model allows a better explanation of the spectroscopic features of Tb related centers in YSO crystals and discloses the critical concentration and the quantum yields of the different energy transfer mechanisms.

Changing the environment of mesoporous silica to investigate the origin of UV and visible photoluminescence of surface centers

The surface-related emission properties of mesoporous silica are investigated by changing the environment surrounding the silica surface, that is by filling the pores of the transparent matrix with different media such as air and nitrogen, or under high vacuum conditions. The interaction of the emitting surface centers with the filling media induces large modifications on the surface of silica matrices and modifies the optical features of the emitting centers. The aim of the paper is to assign the origin of the gathered UV and visible emissions: the modifications induced by the different treatments support the involvement of OH species in the formation of the surface centers responsible of both the emissions and can be rationalized within the framework of two different dehydration reaction schemes previously proposed.

On the photo-stimulated luminescence of BaFBr:Eu+2 phosphors

Abstract The distinctive features of photo-stimulated luminescence (PSL) of the X-ray storage phosphor BaFBr:Eu+2 are interpreted on the ground of a monomolecular electron–hole recombination mechanism. The thermal enhancement of PSL intensity is explained by taking into account a three level scheme for F(Br−) centres. A comparison with data on PSL intensity versus temperature reported in literature is presented.

Emission properties of Tb3 + ions in LYSO: evidence of a cross relaxation mechanism explained by a kinetic model

The optical properties of Tb(3+) ions in oxyorthosilicates of lutetium and yttrium (LYSO) are reported. The introduction of a small number of terbium ions (nominal content 10 ppm) generates, in the otherwise transparent absorption spectrum of the matrix, an ultraviolet absorption band peaked at about 240 nm. By exciting within the reported UV band, line shaped emissions in the 350-600 nm range are detected. These transitions are related to the (5)D(3) and (5)D(4) levels of the Tb(3+) ions and are characterized by decay times in the millisecond time domain. Analysis of the decay time measurements allows us to individuate a cross relaxation mechanism among terbium ions even at the low dopant concentration investigated. We propose a three-level kinetic model which is able to successfully reproduce the experimental data, allowing us to discriminate among the radiative and non-radiative contributions to the observed emissions.

Thermodynamic basis of the concept of recombination resistance

M. Salis(*), P. C. Ricci and F. RagaDipartimento di Fisica - Universita` di CagliariINFM- UdR CagliariCittadella Universitaria , 09042 Monserrato-Cagliari ,ItalyThe concept of ”recombination resistance” introduced by Shockley and Read(Phys. Rev. 87, 835 (1952)) is discussed within the framework of the thermo-dynamics of irreversible processes ruled by the principle of the minimum rate ofentropy production. It is shown that the affinities of recombination processesrepresent ”voltages” in a thermodynamic Ohm-like law where the net rates ofrecombinations represent the ”currents”. The quantities thus found allow forthe definition of the ”dissipated power” which is to be related to the rate ofentropy production of the recombination processes dealt with.PACS numbers: 05.70.Ln; 72.20.Jv; 82.20 M

A model for pore growth in anodically etched gallium phosphide

The electrochemical etching process of porous gallium phosphide was studied by means of the characteristic current–potential (I–V) curves. Measurements were performed in H2SO4 0.5‐M aqueous solution both in the dark and by illuminating the samples with the 351‐nm line of an argon laser. Raman spectroscopy was applied to investigate the surface morphology of the samples prepared under different anodizing conditions within the potentiostatic regime. Based on a few reasonable assumptions, a simple model of pore growth is proposed. The enhancing effect in current intensity due to the branching of pores and the opposite effect due to a concomitant decrease in the effective cross area available for carrier transport are accounted for to explain the main features of the recorded I–V curves.