C. Schwab

The OPERA experiment Target Tracker

The main task of the Target Tracker detector of the long baseline neutrino oscillation OPERA experiment is to locate in which of the target elementary constituents, the lead/emulsion bricks, the neutrino interactions have occurred and also to give calorimetric information about each event. The technology used consists in walls of two planes of plastic scintillator strips, one per transverse direction. Wavelength shifting fibres collect the light signal emitted by the scintillator strips and guide it to both ends where it is read by multi-anode photomultiplier tubes. All the elements used in the construction of this detector and its main characteristics are described.

Electron paramagnetic resonance spectroscopy of fast neutron‐generated defects in GaAs

A series of fast neutron‐irradiated GaAs samples (neutron fluence range of 2×1015–2.5×1017 cm−2) has been investigated by electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra at 9 GHz exhibit a broad (∼1 kG) Lorentzian singlet at g≊2.09 superimposed on the AsGa quadruplet. The singlet intensity scales linearly with neutron fluence as does that of the quadruplet. The presence of this new defect has not been reported in as‐grown GaAs known to have large concentrations of AsGa defects. EPR measurements at 35, 159, and 337 GHz indicate that the singlet linewidth increases with the microwave frequency.

Raman scattering evaluation of lattice damage and electrical activity in Be-implanted GaAs

Systematic Raman scattering experiments were carried out on [001] GaAs implanted with 70‐keV Be+ ions both after implantation for samples subjected to fluences spanning the 1013–1015 cm−2 range and after successive isochronal thermal annealing steps performed between 200 and 900 °C for a selected series of Be doses. The implantation‐induced damages were observed via the modifications with increasing fluence, of the first‐order Raman line parameters (mainly for the LO peak) classified in terms of mode energy softening and linewidth increase. These modifications were quantitatively accounted for within the spatial coherence‐length reduction formalism, which makes it possible to evaluate the mean size of unperturbed regions in as‐implanted GaAs. Upon successive thermal treatments, the implanted layers were found to recover their crystalline perfection at a temperature as low as 500 °C. Above this temperature the Be electrical activation is evidenced by the sudden and drastic modification of only the ‘‘longit...

Electron paramagnetic resonance monitoring of recovery of fast neutron irradiated GaAs

Electron paramagnetic resonance spectra of fast neutron irradiated semi‐insulating GaAs, recorded at 9 GHz and 4.2 K, have been studied as a function of isochronal thermal anneals. Their decomposition into quadruplet and singlet allows one to determine the main annealing temperatures of the corresponding defects, previously identified as As4+Ga and V2−Ga, which occur respectively at 400 and 600 °C. Comparison with the behavior of implantation damage shows that electrical activation of Be+ implants is correlated with the annealing of the main defects on the cation sublattice. Finally, the linewidth variation of the quadruplet during its decay indicates a concomitant change of the local environment of the As4+Ga center.

Electron paramagnetic resonance determination of the generation rate of As antisites in fast neutron irradiated GaAs

The generation rate of paramagnetic defects has been measured by means of Electron Paramagnetic Resonance over the 1015–1017‐10n  cm−2 fluence range, using a low flux (7×1010 10n  cm−2 s−1) of fast neutrons; much higher yields are achievable than reported up to now. A comparison of 4.2‐  and 295‐K spectra evidences a narrowing of the hyperfine structure separation of the components of the AsGa quadruplet with increasing temperature.The generation rate of paramagnetic defects has been measured by means of Electron Paramagnetic Resonance over the 1015–1017‐10n  cm−2 fluence range, using a low flux (7×1010 10n  cm−2 s−1) of fast neutrons; much higher yields are achievable than reported up to now. A comparison of 4.2‐  and 295‐K spectra evidences a narrowing of the hyperfine structure separation of the components of the AsGa quadruplet with increasing temperature.

Multiple-Phonon Resonant Raman Scattering in CuGaS2.

Resonant enhancements of multiple-phonon Raman scattering in CuGaS2 excited by a wide range of laser energy are studied. At 8.6 K, the intensities of the multiple-phonon modes show a sharp resonance maximum at the scattered-photon energy just below that of the free exciton. Polarization characteristics of these modes have the same selection rule as the free exciton. The behavior is attributed to the intraband Frohlich interaction for longitudinal E modes of polar phonons. The resonant scattering by this mechanism exhibits a temperature-dependent enhancement correlated with the thermal shift and broadening of the free exciton energy. On the other hand, the temperature dependence of the resonance is greatly decreased for the nonpolar A1 mode which has a large deformation potential.

Fast neutron-induced defects in undoped and iron-doped indium phosphide

Undoped and iron‐doped InP samples have been investigated by conventional electron paramagnetic resonance measurements before and after fast neutron irradiations. Besides the expected anion antisite PIn defect, they reveal the presence of a broad spectrum that could be ascribed to an anion vacancy Vp on the basis of detailed linewidth and g‐shift scalings of the resonance parameters of already identified intrinsic defects in GaP and InP. The irradiated InP:Fe sample shows both the nearest and next‐nearest FeIn‐Ini pairs.

Identification of paramagnetic AsGa and optical EL2 centers in semi‐insulating gallium arsenide

After a brief recall of our main results obtained during a recent theoretical cluster calculation on anion antisite‐related defects in GaAs, we discuss the consequences of their resonance parameters. Experimentally, we perform a detailed analysis of the electron paramagnetic resonance data in plastically deformed undoped and Cr‐doped semi‐insulating materials in conjunction with simultaneous EL2° optical absorption measurements. Combining theoretical calculations and experimental results, we are able to identify the ‘‘As+Ga’’ spectrum as a superposition of spectra ascribed to the isolated As+Ga and to its binary and ternary vacancy complexes, whereas the ternary complex AsGaVGaVAs only is believed to be the probable configuration for EL2.

Spectroscopic ellipsometry and Raman scattering study of the annealing behavior of Be‐implanted GaAs

The annealing of the lattice damage induced by Be‐ion implantation in GaAs has been studied by spectroscopic ellipsometry and Raman scattering after each step of an isochronal thermal treatment. These two optical (i.e., nondestructive) techniques are shown to be very sensitive both to the lattice recovery and to the electrical activation of the implants. It has been observed that the latter process occurs after the lattice perfection recovery at T=550 °C, while for the ultimate annealing temperatures (T>750 °C) a significant Be redistribution takes place resulting in a decrease of the electrical activity which is confirmed by differential Hall effect measurements.

Persistent photoquenching and anion antisite defects in neutron‐irradiated GaAs

A potential origin for the reported discrepancies on the low‐temperature photosensitivity of particle‐induced anion antisites in GaAs is revealed by a systematic study of the relative fraction of photoquenchable paramagnetic As4+Ga centers as a function of neutron fluence and annealing temperature. The electron paramagnetic resonance data show that the As4+Ga centers can be split into two subsets.