Calogero–Sutherland system with two types interacting spins

The dynamics of low-temperature (T = 5 K) photoluminescence spectra of Si/Si_1-x Ge _x /Si heterostructures (x = 0.045) under the influence of a stream of nonequilibrium phonons (heat pulses) propagating in the structure is investigated. The rapid evaporation of the electron–hole liquid in the quantum well of the structure is observed as the liquid is heated by nonequilibrium phonons. It is established that an increase in the exciton-gas density in the quantum well is caused by the evaporation of the electron–hole liquid and by an increase in the rate of exciton capture by the quantum well. It is shown that the interaction with nonequilibrium phonons results in the dissociation of bound-exciton complexes in the Si layers, which is accompanied by an increase in the exciton concentration and lifetime.     

Thorough small‐angle X‐ray scattering analysis of the instability of liquid micro‐jets in air

Liquid jets are of interest, both for their industrial relevance and for scientific applications (more important, in particular for X‐rays, after the advent of free‐electron lasers that require liquid jets as sample carrier). Instability mechanisms have been described theoretically and by numerical simulation, but confirmed by few experimental techniques. In fact, these are mainly based on cameras, which is limited by the imaging resolution, and on light scattering, which is hindered by absorption, reflection, Mie scattering and multiple scattering due to complex air/liquid interfaces during jet break‐up. In this communication it is demonstrated that synchrotron small‐angle X‐ray scattering (SAXS) can give quantitative information on liquid jet dynamics at the nanoscale, by detecting time‐dependent morphology and break‐up length. Jets ejected from circular tubes of different diameters (100–450 µm) and speeds (0.7–21 m s^?1) have been explored to cover the Rayleigh and first wind‐induced regimes. Various solvents (water, ethanol, 2‐propanol) and their mixtures have been examined. The determination of the liquid jet behaviour becomes essential, as it provides background data in subsequent studies of chemical and biological reactions using SAXS or X‐ray diffraction based on synchrotron radiation and free‐electron lasers.     

Assessing Carrier Recombination Processes in Type‐II SiGe/Si(001) Quantum Dots

In this work, it is shown how different carrier recombination paths significantly broaden the photoluminescence (PL) emission bandwidth observed in type‐II self‐assembled SiGe/Si(001) quantum dots (QDs). QDs grown by molecular beam epitaxy with very homogeneous size distribution, onion‐shaped composition profile, and Si capping layer thicknesses varying from 0 to 1100 nm are utilized to assess the optical carrier‐recombination paths. By using high‐energy photons for PL excitation, electron‐hole pairs can be selectively generated either above or below the QD layer and, thus, clearly access two radiative carrier recombination channels. Fitting the charge carrier capture‐, loss‐ and recombination‐dynamics to PL time‐decay curves measured for different experimental configurations allows to obtain quantitative information of carrier capture‐, excitonic‐emission‐, and Auger‐recombination rates in this type‐II nano‐system.     

All‐optical modulation based on silicon quantum dot doped SiOx:Si‐QD waveguide

All‐optical modulation based on silicon quantum dot doped SiO_x:Si‐QD waveguide is demonstrated. By shrinking the Si‐QD size from 4.3 nm to 1.7 nm in SiO_x matrix (SiO_x:Si‐QD) waveguide, the free‐carrier absorption (FCA) cross section of the Si‐QD is decreased to 8 × 10⁻¹⁸ cm² by enlarging the electron/hole effective masses, which shortens the PL and Auger lifetime to 83 ns and 16.5 ps, respectively. The FCA loss is conversely increased from 0.03 cm⁻¹ to 1.5 cm⁻¹ with the Si‐QD size enlarged from 1.7 nm to 4.3 nm due to the enhanced FCA cross section and the increased free‐carrier density in large Si‐QDs. Both the FCA and free‐carrier relaxation processes of Si‐QDs are shortened as the radiative recombination rate is enlarged by electron–hole momentum overlapping under strong quantum confinement effect. The all‐optical return‐to‐zero on‐off keying (RZ‐OOK) modulation is performed by using the SiO_x:Si‐QD waveguides, providing the transmission bit rate of the inversed RZ‐OOK data stream conversion from 0.2 to 2 Mbit/s by shrinking the Si‐QD size from 4.3 to 1.7 nm.     

Nongeminate recombination in organic bulk heterojunction solar cells: Contribution of exciton–polaron interaction

A new model to explain nongeminate recombination in organic bulk heterojunction solar cells is presented. We suggest that the annihilation of excitons on charge carriers at the interface between donor and acceptor phases competes with the bimolecular recombination of Coulombically bound electron–hole pairs. The exciton–polaron interaction gives visible contribution to the reduction of Langevin recombination. An analytical formula, which describes the reduction prefactor, has been derived. We demonstrate that exciton–charge carrier interactions cause an increase of the recombination order. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct measurement of carrier spin relaxation times in opaque solids using the specular inverse Faraday effect

Using a spectroscopic technique based on the transient specular inverse Faraday effect (SIFE) we found the electron spin relaxation time in bulk GaAs at room temperature to be 11 ± 1 ps. The hole spin dynamics gives a SIFE signal of the opposite sign and may be resolved in the time-domain.     

Charge‐transfer‐induced doping at an electron donor (α‐sexithiophene)/acceptor (C60) interface and mobility improvement

We studied various aspects relating to surface charge‐transfer‐induced doping at an organic/organic interface using in situ electrical measurements with a field‐effect transistor (FET) during the formation of the electron donor/acceptor interface. Adsorption of the electron‐accepting molecules (C₆₀) on top of the electron donating molecules (α‐6T) led to an increase in the FET hole mobility in an α‐6T film. Under illumination, the FET hole mobility in the α‐6T film with C₆₀ deposition was significantly increased in comparison with that in the dark due to exciton dissociation at the C₆₀/α‐6T interface, resulting in a large threshold voltage shift. The origin of the mobility increase is explained by the multiple trapping and release (MTR) model in which the mobility is determined by the carrier density. Various phenomena relevant to charge transfer and charge transport at the organic/organic interface are reported and their origins are discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of InGaAs and InGaAsN semiconductor saturable absorber mirrors for high-power mode-locked thin-disk lasers

We report a comparative study of carrier dynamics in semiconductor saturable absorber mirrors (SESAMs) containing InGaAs quantum wells and InGaAsN quantum wells (QWs). The static and dynamic reflectivity spectra were measured with a Fourier-transform-infrared-spectrometer and a pump-probe setup, respectively. The influence of rapid thermal annealing (RTA) on carrier dynamics was studied. Due to the reduction of defect states by RTA we observed an increase of the static reflectivity and an increase of the electron–hole recombination time.     

Exciton relaxation in PbS quantum dots

Exciton relaxation in PbS quantum dots (QDs) in glass and tetrachloroethylene have been investigated and the radiative and non‐radiative relaxation rates of the lowest 1S–1S state have been measured. An estimate of the carrier intra‐band transition rates and their transfer efficiency is calculated. The dependence of the exciton dynamics on the QD surface properties is presented and discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transient saturation absorption spectroscopy excited near the band gap at high excitation carrier density in GaAs

Transient saturation absorption spectroscopy in GaAs thin films was investigated using femtosecond pump and supercontinuum probe technique at excitation densities higher than 1×10^{19}cm^{-3}. The Coulomb enhancement factor of the electron－hole plasma results in a spectrum hole at the pump wavelength. Two distinct transmission peaks at two sides of the pump wavelength are observed, arising from the bleaching of transitions from the heavy- and light-hole bands to the conduction band. The dynamic process of the transient saturation absorption is fitted using a bi-exponential function. The fast decay process is dominated by the carrier-phonon scattering and the slow process may be attributed to the electron－hole recombination.     

Effect of localized states on internal quantum efficiency of III‐nitride LEDs

A model is suggested accounting for effects of localized electron and hole states formed by composition fluctuations in the InGaN active region of a III‐nitride LED on non‐radiative carrier recombination at threading dislocations. The model enables explanation of the abnormal temperature dependence of internal quantum efficiency (IQE) of a green LED structure recently observed at low current densities. The theoretical predictions are in quantitative agreement with experiment in the temperature range between 200 K and 453 K. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of mobilities and carrier concentrations in semi-insulating GaAs

The temperature behavior of electron and hole mobilities and carrier concentrations was measured in p-type Cr doped semi-insulating GaAs. The above quantities were determined by measuring at a given temperature the photomagnetoresistance and photo-Hall effect vs magnetic field intensity. A minimum of hole concentration is observed at 210–225 K. The electron concentration either remains constant below 190–200 K or it rises again at lower temperatures. The mobilities of both carriers show a maximum at almost the same temperatures with those where the behavior of n and p changes. We try to explain the above on the basis of radiative recombination and impurity conduction.     

Observation of ferromagnetic ordering in Mn‐doped 8‐hydroxy‐quinoline aluminum

The ferromagnetic property of Mn‐doped 8‐hydroxy‐quinoline aluminum (Alq₃), synthesized by thermal co‐evaporation of pure Mn metals and Alq₃ powders, was investigated. The weak ferromagnetic property was observed in 5%‐doped Alq₃, with saturation magnetization of around 0.05μ_B/Mn. The doped Mn chemically interacted with O atoms, producing a new gap state at 0.34 eV above the highest occupied molecular orbital and reducing the effective electron concentration. This led to the decrease of the electron affinity and increase of the optical bandgap, resulting in the reduction of the hole‐injection barrier in comparison with the electron‐injection barrier to the Alq₃ layer. From these, the origin of the observed ferromagnetism is suggested. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman spectroscopic studies of ZnSe/GaAs interfaces

ZnSe/semi‐insulating GaAs interfaces were studied by observing photogenerated plasmon–LO (PPL) coupled modes by nonresonant micro‐Raman spectroscopy. The effect of the carriers generated by the focused laser beam was investigated for a series of different thicknesses of ZnSe epitaxial layers. The PPL mode in GaAs was observed in the micro‐Raman spectra for all samples, but with different magnitude. The plasma is believed to be an electron gas as a result of the negative nature of the interfacial region that contains predominantly hole traps. The free carrier concentration is estimated to be > 10¹⁸ cm⁻³ and their lifetime ∼0.1 ns. This relatively long lifetime suggests that the ZnSe/GaAs interface has to be of high structural quality leading to a low recombination velocity. ZnSe/GaAs heterostructures of less crystalline quality (as determined by resonant Raman measurements) shows the effect of photogenerated carriers only to lesser extent. Copyright © 2007 John Wiley & Sons, Ltd.     

On the observation of electron–hole liquid luminescence under low excitation in Al2O3‐passivated c‐Si wafers

We report on low‐temperature photoluminescence (PL) from aluminum oxide (Al₂O₃)‐passivated c‐Si wafers, which surprisingly exhibits clear signature of the formation of the so‐called electron–hole liquid (EHL), despite the use of excitation powers for which the condensed phase is not usually observed in bulk Si. The elevated incident photon densities achieved with our micro‐PL setup together with the relatively long exciton lifetimes associated with a good quality, indirect band‐gap semiconductor such as our float‐zone c‐Si, are considered the key aspects promoting photogenerated carrier densities above threshold. Interestingly, we observe a good correlation between the intensity of the EHL feature in PL spectra and the passivation performance of the Al₂O₃ layer annealed at different temperatures. The change in the extension of the sub‐surface space‐charge region that results from the balance between the induced fixed charge in the Al₂O₃ and the defect states at the alumina/Si interface is at the origin of the observed correlation. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Excellent passivation of thin silicon wafers by HF‐free hydrogen plasma etching using an industrial ICPECVD tool

In this work, hydrogen plasma etching of surface oxides was successfully accomplished on thin (～100 µm) planar n‐type Czochralski silicon wafers prior to intrinsic hydrogenated amorphous silicon [a‐Si:H(i)] deposition for heterojunction solar cells, using an industrial inductively coupled plasma‐enhanced chemical vapour deposition (ICPECVD) platform. The plasma etching process is intended as a dry alternative to the conventional wet‐chemical hydrofluoric acid (HF) dip for solar cell processing. After symmetrical deposition of an a‐Si:H(i) passivation layer, high effective carrier lifetimes of up to 3.7 ms are obtained, which are equivalent to effective surface recombination velocities of 1.3 cm s^–1 and an implied open‐circuit voltage (V_oc) of 741 mV. The passivation quality is excellent and comparable to other high quality a‐Si:H(i) passivation. High‐resolution transmission electron microscopy shows evidence of plasma‐silicon interactions and a sub‐nanometre interfacial layer. Using electron energy‐loss spectroscopy, this layer is further investigated and confirmed to be hydrogenated suboxide layers. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Transient grating study of the intermolecular dynamics of liquid nitrobenzene

Femtosecond time-resolved transient grating(TG) technique is used to study the intermolecular dynamics in liquid phase. Non-resonant excitation of the sample by two crossing laser pulses results in a transient Kerr grating, and the molecular motion of liquid can be detected by monitoring the diffraction of a third time-delayed probe pulse. In liquid nitrobenzene(NB), three intermolecular processes are observed with lifetimes of 37.9±1.4 ps, 3.28±0.11 ps, and 0.44±0.03 ps, respectively. These relaxations are assigned to molecular orientational diffusion, dipole/induced dipole interaction, and libration in liquid cage, respectively. Such a result is slightly different from that obtained from OKE experiment in which the lifetime of the intermediate process is measured to be 1.9 ps. The effects of electric field on matter are different in TG and optical Kerr effect(OKE) experiments, which should be responsible for the difference between the results of these two types of experiments. The present work demonstrates that TG technique is a useful alternative in the study of intermolecular dynamics.     

Direct detection of carrier traps in Si solar cells after light‐induced degradation

In solar cells fabricated from boron‐doped Cz‐Si wafers minority and majority carrier traps were detected by deep level transient spectroscopy (DLTS) after so‐called “light‐induced degradation” (LID). The DLTS signals were detected from mesa‐diodes with the full structure of the solar cells preserved. Preliminary results indicate metastable traps with energy levels positioned at E_V + 0.37 eV and E_C – 0.41 eV and apparent carrier capture cross‐sections in the 10^–17–10^–18 cm² range. The concentration of the traps was in the range of 10¹²–10¹³ cm^–3. The traps were eliminated by annealing of the mesa‐diodes at 200 °C. No traps were detected in Ga‐doped solar cells after the LID procedure or below the light protected bus bar locations in B‐doped cells. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)