Chemical studies of the passivation of GaAs surface recombination using sulfides and thiols

Steady-state photoluminescence, time-resolved photoluminescence, and x-ray photoelectron spectroscopy have been used to study the electrical and chemical properties of GaAs surfaces exposed to inorganic and organic sulfur donors. Despite a wide variation in S2–(aq) concentration, variation of the pH of aqueous HS–solutions had a small effect on the steady-state n-type GaAs photoluminescence intensity, with surfaces exposed to pH=8, 0.1-M HS–(aq) solutions displaying comparable luminescence intensity relative to those treated with pH=14, 1.0-M Na2S·9H2O(aq). Organic thiols (R-SH, where R=–CH2CH2SH or –C6H4Cl) dissolved in nonaqueous solvents were found to effect increases in steady-state luminescence yields and in time-resolved luminescence decay lifetimes of (100)-oriented GaAs. X-ray photoelectron spectroscopy showed that exposure of GaAs surfaces to these organic systems yielded thiols bound to the GaAs surface, but such exposure did not remove excess elemental As and did not form a detectable As2S3 overlayer on the GaAs. These results imply that complete removal of As0 or formation of monolayers of As2S3 is not necessary to effect a reduction in the recombination rate at etched GaAs surfaces. Other compounds that do not contain sulfur but that are strong Lewis bases, such as methoxide ion, also improved the GaAs steady-state photoluminescence intensity. These results demonstrate that a general class of electron-donating reagents can be used to reduce nonradiative recombination at GaAs surfaces, and also imply that prior models focusing on the formation of monolayer coverages of As2S3 and Ga2S3 are not adequate to describe the passivating behavior of this class of reagents. The time-resolved, high level injection experiments clearly demonstrate that a shift in the equilibrium surface Fermi-level energy is not sufficient to explain the luminescence intensity changes, and confirm that HS– and thiol-based reagents induce substantial reductions in the surface recombination velocity through a change in the GaAs surface state recombination rate

Pulse phase and precession phase resolved spectroscopy of Her X-1: studying a representative Main-On with RXTE

We performed a detailed pulse phase resolved spectroscopy of the accreting binary X-ray pulsar Her X-1 in the energy range 3.5-75 keV and have established pulse phase profiles for all spectral parameters. For the centroid of the cyclotron line, the photon index and the flux of the 6.4 keV iron line, we have studied the variation as a function of 35 d phase. We analyzed RXTE observations of the Main-On of November 2002. Four different time intervals of about 1 d duration were selected to provide a good coverage of a complete Main-On. The intervals are centered at 35 d phase 0.03, 0.10, 0.15, and 0.20, respectively. All spectral parameters show a strong modulation with pulse phase. While the centroid energy of the cyclotron line follows roughly the shape of the pulse profile, both the photon index and the iron line intensity exhibit distinct minima around the peak of the X-ray pulse. With respect to variations of the observed profiles with 35 d phase, we find that there is a clear evolution of the shape of the pulse profiles (flux versus pulse phase), a moderate increase of the maximum cyclotron line energy (found around pulse phase 0.7), but no significant evolution of the shape of the pulse phase profiles of the cyclotron line energy, the spectral power law index or the iron line intensity. The variation of spectral parameters as a function of the pulse phase provides important information about the system: 1. the disappearance of the Fe line flux near the highest continuum flux may be an indication of a hollow cone geometry of the accretion structure; ii. the apparent non-dependence of the cyclotron line energy profiles on 35 d phase provides a new possibility to test the model of free precession of the neutron star, proposed to be responsible for the systematic variations in the pulse profiles.Comment: 10 pages, 11 figures, Accepted by A&A on the 22/12/201

Dinamiche della fecondit\ue0 contemporanea: intenzioni individuali e tendenze aggregate in Europa

La rapida diminuzione dei tassi di fecondit\ue0 verificatasi in tutti i paesi industrializzati a partire dagli anni Settanta del secolo scorso si trova sempre pi\uf9 spesso al centro dell\u2019attenzione di studiosi e policymaker a causa della molteplicit\ue0 di implicazioni che comporta sia per gli individui sia per la societ\ue0 nel suo complesso. A partire da tale quadro, ci si propone di approfondire il ruolo dei principali fattori che presiedono all\u2019andamento dei tassi di fecondit\ue0 \u2013 livello macro - e alle decisioni individuali \u2013 livello micro \u2013 attraverso la definizione di un framework concettuale che tenga conto della duplicit\ue0 di piani che caratterizza i comportamenti riproduttivi. L\u2019analisi e la valutazione dei modelli utilizzati in ricerche sullo stesso tema hanno portato a scegliere il framework RWA (Ready \u2013 Willing \u2013 Able), proposto da Coale nel 1973 per spiegare la riduzione della fecondit\ue0, riletto alla luce del mutato contesto in cui vengono prese oggi le decisioni di fecondit\ue0. Nel passaggio dalla Prima alla Seconda Transizione Demografica, il \u201cnuovo comportamento\u201d sembra rappresentato in modo pi\uf9 adeguato dalla scelta di avere un(altro) figlio piuttosto che da quella di non averlo. Nel momento in cui la possibilit\ue0 di avere un figlio viene ad essere il frutto di una scelta deliberata, il \u201cdesiderarlo\u201d non \ue8 pi\uf9 sufficiente e gli individui, a partire dal contesto in cui si trovano a vivere, devono, contemporaneamente, essere pronti (ready), cio\ue8 ritenere la decisione di avere un figlio positiva per loro stessi e per la loro famiglia; volerlo (willing), cio\ue8 considerare la genitorialit\ue0 una scelta legittima, condivisa e supportata dalla societ\ue0; poterlo fare (able), cio\ue8 avere a disposizione tutti i mezzi, tecnici e istituzionali, che possono facilitare la realizzazione del loro desiderio. L\u2019ipotesi che si intende valutare \ue8 se la decisione di avere o meno un(altro) figlio, per quanto indubbiamente frutto di considerazioni del tutto personali, risenta in modo non irrilevante anche dell\u2019ambiente sociale in cui gli individui stessi si trovano a vivere, ovvero del panorama di vincoli ed opportunit\ue0 a partire dal quale essi possono prefigurarsi ed elaborare le proprie strategie di vita familiare e personale. Si ritiene, infatti, che contesti istituzionali e socio-culturali diversi contribuiscano a creare milieu pi\uf9 o meno family friendly ovvero pi\uf9 o meno adatti a sostenere le decisioni riproduttive rispondendo ad una sorta di domanda latente di supporto familiare riscontrabile, in particolare, nei paesi a bassa fecondit\ue0. Dal punto di vista operativo, si ipotizza che la formulazione dell\u2019intenzione di avere un(altro) figlio nei successivi tre anni e il valore assunto dalla componente ready del framework RWA \u2013 la valutazione dei vantaggi/svantaggi legati alla nascita di un figlio \u2013 siano influenzati, oltre che dalle variabili individuali, anche dal contesto in cui i soggetti vivono e, in particolare, dalla componente able che esprime la presenza in un determinato paese di condizioni tecniche e istituzionali in grado di sostenere la genitorialit\ue0. I dati utilizzati per l\u2019analisi empirica provengono dai database messi a punto dall\u2019Unece nell\u2019ambito del Generations and Gender Programme (http://www.ggp-i.org/), integrati dai database Eurostat, del Multilink Project e dell\u2019European Values Study. La verifica condotta a livello contestuale attraverso il confronto dei cluster di paesi definiti in base alla relazione tra gli indicatori macro-sociali classici, le componenti RWA e i tassi di fecondit\ue0 conferma la capacit\ue0 del framework RWA di esprimere la \u201cposizione\u201d dei paesi rispetto alla fecondit\ue0 in modo pi\uf9 preciso rispetto alle analisi condotte separatamente su singoli indicatori. A livello individuale, risulta confermata l\u2019ipotesi, formulata a partire dal framework RWA originale, dell\u2019esistenza di una relazione tra le tre precondizioni ready-willing-able e le intenzioni di fecondit\ue0, mentre la stima dei modelli multilivello per la verifica degli effetti contestuali consente una verifica solo parziale dell\u2019ipotesi di un\u2019influenza esercitata dalle politiche familiari \u2013 espresse tramite la precondizione able-macro \u2013 sulle valutazioni di livello individuale. Tale risultato rinvia da un lato alla difficolt\ue0 di cogliere tramite indicatori sintetici la complessit\ue0 delle misure di sostegno alla genitorialit\ue0, dall\u2019altro alla necessit\ue0 di dare spazio e riconoscimento all\u2019autonomia della soggettivit\ue0 e alla riflessivit\ue0 individuale rispetto alle condizioni di contesto.The fast decrease in fertility rates, occurred in all industrialized countries since the Seventies of the last century, is increasingly the center of attention for scholars and policymakers due to its several implications for individuals and society. From this framework, my aim is to study the role of the main determinants of fertility rates \u2013 on the macro level - and of individual decisions \u2013 on the micro level \u2013 by defining a conceptual framework which takes into account the duality that characterizes the reproductive behaviors. The analysis and the evaluation of models used in research on same subject led to choose the framework RWA (Ready - Willing - Able), proposed by Coale in 1973 to explain the decrease in fertility, reviewed in the light of the different context where people make their fertility decisions today. In moving from the First to the Second Demographic Transition, the "new behavior" seems better represented by the choice of having a (nother) child rather than to not have it. When the ability to have a child is the result of a deliberate choice, the "desire" is no longer enough and the individuals, from the context where they live, must at the same time be ready, that is to believe that the decision to have a child is good for themselves and for their family, be willing, that is to consider parenting a legitimate choice, shared and supported by society, and be able, that is to have all the technical and institutional means that can facilitate the achievement of their desire. The main hypothesis to evaluate is whether the decision to have a (nother) child, even though it is doubtless the fruit of the personal characteristics and individual conditions, is affected even by the social environment where individuals live, namely by the background of opportunities and constraints from which they can develop their personal and family life strategies. It is believed, in fact, that the different institutional and socio-cultural contexts contribute to create the different milieux, more or less family friendly, i.e. more or less suitable to support the reproductive decisions responding to a kind of \u201clatent demand of family support\u201d noticeable, especially, in low fertility countries. By an operational standpoint, it is hypothesized that the intention to have a (nother) child in the next three years and the value assumed by the \u201cready\u201d component of the RWA framework - the evaluation of advantages / disadvantages connected with childbearing - are influenced, in addition to individual variables, also by the context where people live and, above all, by the \u201cable\u201d component which expresses the presence, in a given country, of technical and institutional conditions able to support the parenting. The data used for empirical analyses come from the databases developed under the UNECE Generations and Gender Programme (http://www.ggp-i.org/), supplemented by the databases collected by Eurostat, by Multilink Project and by European Values Study. The study carried out on the contextual level, by comparing clusters of countries defined according to the relationship among the classical macro-social indicators, the RWA components and the fertility rates, confirms the ability of the RWA framework in identifying the "position" of the countries with respect to fertility trend in a more accurate way than through the analyses of each indicators separately. On the individual level, the hypothesis formulated from RWA original framework of a link between the three preconditions ready-willing-able and the fertility intentions is confirmed, whereas the multilevel models estimated in order to verify a potential contextual effect on people\u2019s evaluations allows only the partial confirmation of the hypothesis about the influence exerted by family policies expressed by the precondition able-macro. This result refers to the difficulty in capturing with synthetic indicators the complexity of the measures aimed to support parents, on the one hand, and to the need to give space and recognition to the subjects\u2019 autonomy and reflexivity in relation to contextual conditions, on the other