Raman scattering reveals strong LO-phonon-hole-plasmon coupling in nominally undoped GaAsBi: optical determination of carrier concentration

We report room-temperature Raman scattering studies of nominally undoped (100) GaAs1−xBix epitaxial layers exhibiting Biinduced (p-type) longitudinal-optical-plasmon coupled (LOPC) modes for 0.018≤x≤0.048. Redshifts in the GaAs-like optical modes due to alloying are evaluated and are paralleled by strong damping of the LOPC. The relative integrated Raman intensities of LO(Γ) and LOPC ALO/ALOPC are characteristic of heavily doped p-GaAs, with a remarkable near total screening of the LO(Γ) phonon (ALO/ALOPC →0) for larger Bi concentrations. A method of spectral analysis is set out which yields estimates of hole concentrations in excess of 5 × 1017 cm−3 and correlates with the Bi molar fraction. These findings are in general agreement with recent electrical transport measurements performed on the alloy, and while the absolute size of the hole concentrations differ, likely origins for the discrepancy are discussed. We conclude that the damped LO-phonon-hole-plasmon coupling phenomena plays a dominant role in Raman scattering from unpassivated nominally undoped GaAsBi

Deep-level defects in n-type GaAsBi alloys grown by molecular beam epitaxy at low temperature and their influence on optical properties

Deep-level defects in n-type GaAs1-x Bi x having 0 ≤ x ≤ 0.023 grown on GaAs by molecular beam epitaxy at substrate temperature of 378 °C have been injvestigated by deep level transient spectroscopy. The optical properties of the layers have been studied by contactless electroreflectance and photoluminescence. We find that incorporating Bi suppresses the formation of GaAs-like electron traps, thus reducing the total trap concentration in dilute GaAsBi layers by over two orders of magnitude compared to GaAs grown under the same conditions. In order to distinguish between Bi- and host-related traps and to identify their possible origin, we used the GaAsBi band gap diagram to correlate their activation energies in samples with different Bi contents. This approach was recently successfully applied for the identification of electron traps in n-type GaAs1-x N x and assumes that the activation energy of electron traps decreases with the Bi (or N)-related downward shift of the conduction band. On the basis of this diagram and under the support of recent theoretical calculations, at least two Bi-related traps were revealed and associated with Bi pair defects, i.e. (VGa+BiGa)(-/2-) and (AsGa+BiGa)(0/1-). In the present work it is shown that these defects also influence the photoluminescence properties of GaAsBi alloys

ДИНАМИКА СВОБОДНЫХ НОСИТЕЛЕЙ В ТЯЖЕЛЫХ СКИНТИЛЯЦИОННЫХ МАТЕРИАЛАХ СО СТРУКТУРОЙ ГРАНАТА

Ce-doped Lu3 Al5 O12 (LuAG), and Gd3 Al2 Ga3 O12 (GAGG) crystals with and without codoping by Mg2+ ions have been studied by the nonlinear absorption spectroscopy method. A faster rise time of transient optical absorption has been observed in all crystals codoped with Mg in comparison to Mg-free samples. A significant difference in the time evolution of the differential optical density in GAGG in comparison to LuAG crystals is revealed. in gadolinium garnets an absorption band peaked in the blue-green range and decaying with characteristic time of ~2 ps is observed. This band is considered to be due to absorption of free electrons before their trapping by Ce3+doping ions. A broad transient absorption band in the yellowred region is attributed to absorption from the Ce3+ excited states. Легированные ионами Се кристаллы Lu3 Al5 O12 (LuAG) и Gd3 Al2 Ga3 O12 (GAGG) с и без кодопирования ионами Mg2+ исследованы методом нелинейной абсорбционной спектроскопии. Ускорение времени нарастания переходного оптического поглощения наблюдалось в кристаллах, кодопированных ионами Mg, по сравнению с образцами без Mg. Выявлено существенное различие во временной эволюции дифференциальной оптической плотности в GAGG в сравнении с кристаллом LuAG. В гадолиниевых гранатах наблюдается полоса поглощения в сине- зеленом диапазоне с затуханием с характерным временем ~2 пс. Эта полоса соотнесена поглощению свободными электронами до их захвата примесными ионами Ce3+. Широкая полоса переходного поглощения в желто-красной области обусловлена поглощением из возбужденных состояний ионов Ce3+.

Investigation of Optical Nonlinearities and Carrier Dynamics in In-Rich InGaN Alloys

We present experimental studies of nonequilibrium carrier dynamics in InGaN alloys with 70-90% content of In by using picosecond transient grating technique. The observed faster recombination rate in alloys with higher Ga content and formation of a thermal grating via a lattice heating, being more pronounced for layers with larger band gap, indicated that the main reason of the heating is not the excess energy of photons, but the defect density which increases with Ga content. A gradual decrease in carrier lifetime with excitation or with increasing temperature in 50-300 K range point out the role of potential barriers in carrier recombination

Free carrier absorption in self-activated PbWO_4 and Ce-doped Y_3(Al_0.25Ga_0.75)_3O_12 and Gd_3Al_2Ga_3O_12 garnet scintillators

Nonequilibrium carrier dynamics in the scintillators prospective for fast timing in high energy physics and medical imaging applications was studied. The time-resolved free carrier absorption investigation was carried out to study the dynamics of nonequilibrium carriers in wide-band-gap scintillation materials: self-activated led tungstate (PbWO_4, PWO) ant two garnet crystals, GAGG:Ce and YAGG:Ce. It was shown that free electrons appear in the conduction band of PWO and YAGG:Ce crystals within a sub-picosecond time scale, while the free holes in GAGG:Ce appear due to delocalization from Gd^3+ ground states to the valence band within a few picoseconds after short-pulse excitation. The influence of Gd ions on the nonequilibrium carrier dynamics is discussed on the base of comparison the results of the free carrier absorption in GAGG:Ce containing gadolinium and in YAGG without Gd in the host lattice

Timing properties of Ce-doped YAP and LuYAP scintillation crystals

The timing performance of radiation detectors based on Ce-doped perovskites YAlO3 (YAP) and (Lu1−x-Yx) AlO3 (LuYAP) coupled with near-UV sensitive silicon photomultipliers (NUV-HD SiPMs) have been compared in coincidence time resolution (CTR) experiments. The single-detector time resolution full width at half maximum (FWHM) of YAP was found to be 169 ps, i.e., by a factor of two better than that observed earlier with photomultiplier readout. Introduction of lutetium into YAP structure deteriorates the time resolution to 286 ps and 309 ps for a Lu/Y atomic ratio of 1:1 and 0.7:0.3, respectively. The study of the dynamics of Ce3+ excited state population after selective optical excitation with short (200 fs) pulses by using the differential absorption technique in pump and probe configuration evidenced the importance of electron trapping, which is enhanced by antisite LuAl defects having favorable conditions to occur in LuAP

Improvement of response time in GAGG:Ce scintillation crystals by magnesium codoping

Dynamics of the population of the excited Ce states responsible for the luminescence response time in Gd3Al2Ga3O12:Ce scintillating crystals is studied by revealing the dynamics of nonequilibrium carriers in the picosecond domain. Optical pump and probe technique exploiting selective excitation of structural units of the crystal and probing the induced absorption as a function of time and spectral position is exploited. A fast response within a few picoseconds due to the absorption by holes at Gd ions and by electrons occupying the first excited state of Ce ions with the intracenter relaxation time of 500 fs are identified. Trapping of nonequilibrium electrons during their migration through the matrix to the emitting Ce ions are shown to be responsible for the slow component in the population of the excited Ce state. Elimination of the slow component is evidenced even at Mg codoping as low as 10 ppm. The elimination correlates with the acceleration of the response in coincidence time resolution experiments showing potential of GAGG:Ce, Mg in medical and high-energy physics applications

Application of two-photon absorption in PWO scintillator for fast timing of interaction with ionizing radiation

This work was aimed at searching for fast phenomena in scintillators in sub-10-ps range, a benchmark timing for the time response of radiation detectors in particle colliders. The pump-and-probe optical absorption technique with a tunable-wavelength parametric oscillator as the pump and a continuous-spectrum source as the probe beam was used to study lead tungstate PbWO4 (PWO) single crystals. It is shown that the rise time of the probe pulse absorption induced by the pump pulse is shorter than the pump pulse width of 200 fs. The approximately linear dependence of the probe absorption on the pump pulse energy density evidences that the induced absorption is caused by two-photon absorption involving one probe and one pump photon. We demonstrate that the intensity of the induced absorption at certain wavelengths is influenced by gamma irradiation, provided that an appropriate light polarization is selected. The application of the irradiation-sensitive nonlinearity for fast timing in radiation detectors is discussed

Improvement of the Time Resolution of Radiation Detectors Based on Gd3Al2Ga3O12 Scintillators With SiPM Readout

Coincidence time resolution (CTR) of scintillation detectors based on Ce- and Mg-codoped Gd3Al2Ga3O12 (GAGG) scintillation crystals and high-density silicon photomultipliers (SiPMs) is shown to be 165 ps (full width at half maximum) for 511-keV γ -quanta, approaching that achieved by using LSO scintillators. To study the prospective for further improvement of the time resolution, the population of the emitting Ce centers was investigated by optical pump and probe technique using selective photoexcitation and probing by a white light continuum with subpicosecond time resolution. The importance of free electron trapping for excitation transfer to emitting Ce ions was revealed. The influence of transfer delay on the scintillation response time is described, and the dynamics of electron relaxation to the lowest excited level of Ce ion is studied experimentally and analyzed by taking into account intracenter relaxation and relaxation via conduction band. The influence of electron diffusivity on the rise time of the population of the emitting level is described. It is shown that codoping of GAGG:Ce by magnesium even at the level as low as 10 ppm efficiently decreases the scintillator response time by enhancing the electron diffusivity