F. Bogani

Thermally activated carrier transfer and luminescence line shape in self-organized InAs quantum dots

We investigated the temperature dependence (10–180 K) of the photoluminescence (PL) emission spectrum of self‐organized InAs/GaAs quantum dots grown under different conditions. The temperature dependence of the PL intensity is determined by two thermally activated processes: (i) quenching due to the escape of carriers from the quantum dots and (ii) carrier transfer between dots via wetting layer states. The existence of different dot families is confirmed by the deconvolution of the spectra in gaussian components with full width half maxima of 20–30 meV. The transfer of excitation is responsible for the sigmoidal temperature dependence of the peak energies of undeconvoluted PL bands.

Controlled tuning of the radiative lifetime in InAs self-assembled quantum dots through vertical ordering

Multilayer structures of InAs quantum dots have been studied by means of photoluminescence techniques. A strong increase of the radiative lifetime with increasing number of stacked dot layers has been observed at low temperatures. Moreover, a strong temperature dependence of the radiative lifetime, which is not present in the single layer samples, has been found in the multistacked structures. The observed effects are nicely explained as a consequence of the electronic coupling between electrons and holes induced by vertical ordering.

Review of the development of diamond radiation sensors

Abstract Diamond radiation sensors produced by chemical vapour deposition are studied for the application as tracking detectors in high luminosity experiments. Sensors with a charge collection distance up to 250 μm have been manufactured. Their radiation hardness has been studied with pions, proton and neutrons up to fluences of 1.9×10 15 π cm −2 , 5×10 15 p cm −2 and 1.35×10 15 n cm −2 , respectively. Diamond micro-strip detectors with 50 μm pitch have been exposed in a high-energy test beam in order to investigate their charge collection properties. The measured spatial resolution using a centre-of-gravity position finding algorithm corresponds to the digital resolution for this strip pitch. First results from a strip tracker with a 2×4 cm 2 surface area are reported as well as the performance of a diamond tracker read out by radiation-hard electronics with 25 ns shaping time. Diamond pixel sensors have been prepared to match the geometries of the recently available read-out chip prototypes for ATLAS and CMS. Beam test results are shown from a diamond detector bump-bonded to an ATLAS prototype read-out. They demonstrate a 98% bump-bonding efficiency and a digital resolution in both dimensions.

Growth patterns of self-assembled InAs quantum dots near the two-dimensional to three-dimensional transition

Self-assembled InAs quantum dots have been grown by molecular beam epitaxy in such a way as to obtain a continuous variation of InAs coverages across the wafer. Structured photoluminescence spectra are observed after excitation of a large number of dots; deconvolution into Gaussian components yields narrow emission bands (full width at half-maximum 20–30 meV) separated in energy by an average spacing of 30–40 meV. We ascribe the individual bands of the photoluminescence spectra after low excitation to families of dots with similar shapes and with heights differing by one monolayer, as strongly supported by numerical calculations of the fundamental electronic transitions in quantum dot structures.

Fermi resonance in solid CO2

A theory of Fermi resonance in molecular crystals is described and applied to the ω1; 2ω2 Fermi resonance region of the CO2 crystal. It is shown by means of the Green functions method that bound states can originate out of the two‐phonon manifold of ω2+ω2 vibrational states for sufficiently high values of the anharmonicity constant. Starting from the crystal harmonic Hamiltonian, Green functions for the excitations of interest are built. Introducing as anharmonic contribution to the Hamiltonian the intramolecular coupling between ω1 and 2ω2, renormalization of the ω2 density of states results. The profiles of the renormalized density of states are seen to change both with mechanical anharmonicity and the unperturbed ω1 frequency. Two cases of practical interest are discussed: in the first the ω1 state is immersed into the ω2+ω2 manifold while in the second ω1 is well separated from that. The intensity of the Raman spectrum is discussed in terms of a direct mechanism, i.e. scattering of light by ω2(k) and ...

Volume versus surface-mediated recombination in anatase TiO2 nanoparticles

We present an experimental study of the radiative recombination dynamics in size-controlled anatase TiO2 nanoparticles in the range 20–130 nm. From time-integrated photoluminescence spectra and picosecond time-resolved experiments as a function of the nanoparticle size, excitation density, and temperature, we show that photoluminescence comes out from a bulk and a surface radiative recombination. The spectral shift and the different time dynamics provide a clear distinction between them. Moreover, the intrinsic nature of the emission is also proven, providing a quantitative evaluation of volume and surface contributions.

Pulse height distribution and radiation tolerance of CVD diamond detectors

The paper reviews measurements of the radiation tolerance of CVD diamond for irradiation with 24 GeV/c protons, 300 MeV/c pions and 1 MeV neutrons. For proton and neutron irradiation, the measured charge signal spectrum is compared with the spectrum calculated by a model. Irradiation by particles causes radiation damage leading to a decrease of the charge signal. However, both the measurements and the outcome from the model shows that for tracker applications this drawback is at least partly counterbalanced by a narrowing of the distribution curve of the charge signal. In addition, we observed after proton irradiation at the charge signal spectrum a decrease of the number of small signals. As a result, the efficiency of a CVD diamond tracker is less affected by irradiation than the mean charge signal. (11 refs).

A comparative study of the thermoluminescent response to beta irradiation of CVD diamond and LiF dosimeters

Abstract The thermoluminescent (TL) response of Chemical Vapour Deposited (CVD) diamond films to beta irradiation has been investigated. A numerical curve-fitting procedure, calibrated by means of a set of LiF TLD100 experimental spectra, has been developed to deconvolute the complex structured TL glow curves. The values of the activation energy and of the frequency factor related to each of the TL peaks involved have been determined. The TL response of the CVD diamond films to beta irradiation has been compared with the TL response of a set of LiF TLD100 and TLD700 dosimeters. The results have been discussed and compared in view of an assessment of the efficiency of CVD diamond films in future applications as in vivo dosimeters.

Status of the R&D activity on diamond particle detectors

Chemical Vapor Deposited (CVD) polycrystalline diamond has been proposed as a radiation-hard alternative to silicon in the extreme radiation levels occurring close to the interaction region of the Large Hadron Collider. Due to an intense research effort, reliable high-quality polycrystalline CVD diamond detectors, with up to 270μm charge collection distance and good spatial uniformity, are now available. The most recent progress on the diamond quality, on the development of diamond trackers and on radiation hardness studies are presented and discussed.

Tracking with CVD diamond radiation sensors at high luminosity colliders

Recent progress on developing diamond-based sensors for vertex detection at high luminosity hadron colliders is described. Measurements of the performance of diamond sensors after irradiation to fluences of up to 5/spl times/10/sup 15/ hadrons/cm/sup 2/ are shown. These indicate that diamond sensors will operate at distances as close as 5 cm from the interaction point at the Large Hadron Collider (LHC) for many years at full luminosity without significant degradation in performance. Measurements of the quality of the signals from diamond sensors as well as spatial uniformity are presented. Test beam results on measurements of diamond-based microstrip and pixels devices are described.