Оптичні властивості CdTe, легованого Ca

The problems of developing light-emitting structures based on CdTe with an extended range of operating temperatures and radiation-resistant parameters are studied. A technique for obtaining heterostructures has been mastered, technological modes of isovalent substitution have been determined, and radiation sources with a high quantum efficiency η = 7–20% at 300 K in a wide spectral region have been obtained. The design of devices has been developed and light emitters based on CdTe, whose radiation is determined by the interband recombination of free charge carriers and the dominant annihilation of bound excitons, have been fabricated by doping with isovalent impurities Mg, Ca.

The reflectance difference (RD) spectra of nine GaAs(001) surface reconstructions, (2×4)β2, (2×4)α, (2×4)γ2, (2×4)β1, (2×4)γ1 and c(4×4) reconstructions on As-rich surface and (4×2)β2, (4×2)α and (4×2)β1 reconstructions on Ga-rich surface, were studied by using the nearest-neighbor sp3s* tight-binding method. The surface atomic positions and the tight-binding interaction parameters were obtained by the ab inito pseudopotential calculations. We found that the RD spectra have considerably different features between As- and Ga-rich surface reconstructions. The RD spectra of As-rich surfaces are mainly understood by transitions between top As-dimer states, while the RD spectra of Ga-rich surfaces are explained by the surface electronic states resulting from the sinkage of surface Ga atoms into bulk layers. These calculations are compared with the results of recent experiments.

The accurate tight-binding-calculation method of reflectance difference (RD) spectra is developed for surfaces and interfaces with various orientations, taking into consideration the crystalline nature and the finite size of the system adopted in the calculation. It is shown that the bond-polarization picture, i.e., that every bond becomes a unit of polarization, is applicable to the optical response of surfaces and interfaces, and the RD spectra are calculated by averaging the optical responses of even and odd finite-layer systems. The physical origins of the RD spectra are classified into two groups; the RD spectra have either the peak shape or the energy-derivative-of-peak shape depending on whether the electronic states are localized or extended around the surface/interface.

Reflectance difference (RD) spectra for the a–b plane of the single crystals of Gd5Si2Ge2 and b–c planes of Gd5Si2Ge2 and Tb5Si2.2Ge1.8 were obtained in the photon energy range of 1.5–5.5 eV. Several peaks were observed for these crystals in the measured spectrum range. Similar features were observed in the RD spectra for the b–c planes of Gd5Si2Ge2 and Tb5Si2.2Ge1.8, while different features were observed for the a–b plane and b–c plane of Gd5Si2Ge2. The RD spectra for the crystals arise not only from the surface anisotropy but also from the bulk anisotropy due to the monoclinic structure of the bulk crystal.

We report measurements of reflectance difference (RD) spectra for $c(4\ifmmode\times\else\texttimes\fi{}4)$ and $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructed (001) surfaces of GaAs. Surface dimers induce an inhomogeneous orthorhombic strain field that penetrates several monolayers into the crystal. The thickness $d$ of this strained region is smaller than the penetration depth $L$ of the probing light, and an inhomogeneous anisotropy formalism can be used. The dielectric function of GaAs at room temperature can be described by a discrete (Lorentzian) excitonic line shape. For the contribution of the region near the surface to the optical response, the excitonic response should be at least partly quenched because of the electric field present near the surface. Thus, a one-electron contribution (logarithmic line shape) may have to be used to fit the RD spectra. By using an inhomogeneous perturbation formalism and a logarithmic line shape, we have been able to isolate the strain component induced by the dimers around ${E}_{1}$ and ${E}_{1}+{\ensuremath{\Delta}}_{1}$ critical points. We also inferred that the RD spectra include a component related to surface roughness.

Photo‐generation and recombination of free charge carriers in poly‐3 (hexylthiophene) (P3HT) and [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) blend films has been studied at different PCBM concentrations by means of fluorescence spectroscopy and transient photocurrent methods. We show that more than 80% of excitons form charge transfer (CT) states at PCBM concentrations above 4%. Efficiency of the CT state dissociation into free charge carries strongly depends on the PCBM concentration; the dissociation efficiency increases more than 30 times when PCBM concentration increases from 1 to 32%. We attribute the strong concentration dependence to formation of PCBM clusters facilitating electron migration and/or delocalization. Reduced charge carrier recombination coefficient has also been observed at high PCBM concentrations. We suggest that this may be partly caused by the reduced stability of reformed Coulombicaly bound charge pairs.

The influence of the generation and recombination of free charge carriers in the p-i-n structure in strong electric fields on the stationary volt-ampere characteristics is considered. The issues of the stability of stationary solutions are investigated. It is shown that a form of the stationary volt-ampere characteristics and their stability depend on ratios between free electrons and holes in the current injected into the i-layer and in the current outflowing from the layer. When the structure is forward biased, the oscillations may occur.

Photorefractive and photoconductivity steady-state as well as time-resolved measurements have been performed in strongly reduced KNbO3. The recorded photorefractive gratings show a nonexponential dark decay and an intensity-dependent two-wave mixing gain. The measured photoconductivity depends sublinearly on light intensity. The observed behavior is interpreted in terms of a band transport model that uses a single donor level with only electrons as free charge carriers, taking the nonlinear recombination of free charge carriers (owing to variation of the trap density) and the time dependence of the free-charge-carrier density into account. An analytical solution for the nonexponential dark decay of the photorefractive grating is derived. As an application of this model the results of the photorefractive and the stationary and transient photoconductivity experiments are used to determine the model parameters (thermal excitation rate, photoexcitation cross section, recombination coefficient, acceptor and donor densities, and electron mobility).

We develop an empirically based optoelectronic model to accurately simulate the photocurrent in organic photovoltaic (OPV) devices with novel materials including bulk heterojunction OPV devices based on a new low band gap dithienothiophene-DPP donor polymer, P(TBT-DPP), blended with PC70BM at various donor-acceptor weight ratios and solvent compositions. Our devices exhibit power conversion efficiencies ranging from 1.8% to 4.7% at AM 1.5G. Electron and hole mobilities are determined using space-charge limited current measurements. Bimolecular recombination coefficients are both analytically calculated using slowest-carrier limited Langevin recombination and measured using an electro-optical pump-probe technique. Exciton quenching efficiencies in the donor and acceptor domains are determined from photoluminescence spectroscopy. In addition, dielectric and optical constants are experimentally determined. The photocurrent and its bias-dependence that we simulate using the optoelectronic model we develop, which takes into account these physically measured parameters, shows less than 7% error with respect to the experimental photocurrent (when both experimentally and semi-analytically determined recombination coefficient is used). Free carrier generation and recombination rates of the photocurrent are modeled as a function of the position in the active layer at various applied biases. These results show that while free carrier generation is maximized in the center of the device, free carrier recombination is most dominant near the electrodes even in high performance devices. Such knowledge of carrier activity is essential for the optimization of the active layer by enhancing light trapping and minimizing recombination. Our simulation program is intended to be freely distributed for use in laboratories fabricating OPV devices.

We investigate the optical property of monolayer and layered BiI$_3$ and reveal the presence of exciton only in the monolayer crystal. We evaluate the energy spectrum of a dielectric function by using time-dependent density functional theory. Bulk crystal of BiI$_3$ is an atomic layered semiconductor with the band gap corresponding to the frequency of visible light. The numerical result for the bulk crystal is confirmed to be consistent with the previous experimental results and it does not depend on the number of layers except the monolayer. We reveal the excitons appearing below the resonant peak associated with the inter-band excitation in the monolayer crystal. The unique optical property can be directly observed in the optical absorption or differential reflectance spectrum and distinguish the monolayer crystal from the stacked BiI$_3$.

Competitive role of nitrogen functionalities of N doped GO and sensitizing effect of Bi2O3 QDs on TiO2 for water remediation

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong near-infrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C60 layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in <10 ns. Adding C60 increases the yield and extends the lifetime of a significant fraction of free carriers to ≫100 ns via interfacial charge separation. Spectral dependencies indicate that the driving force for interfacial electron transfer vanishes for large-diameter SWCNTs, from which we approximate (9,7) s-SWCNT energetics. We estimate a free carrier generation yield of ∼6% in neat s-SWCNT films and 9 GHz...

Intrinsic and extrinsic bands in optical spectra of linear-chained carbon films on sodium and potassium chloride substrates

Carrier recombination through radiative and nonradiative processes in lattice-matched n-Zn0.5Cd0.5Se∕Zn0.21Cd0.19Mg0.6Se multi-quantum-wells (MQWs) was investigated by temperature-dependent time-resolved photoluminescence (PL) spectroscopy. The n-Zn0.5Cd0.5Se∕Zn0.21Cd0.19Mg0.6Se MQW samples with different well widths were grown on InP substrates by molecular beam epitaxy. The PL decay times and the PL intensities were measured as functions of temperature. For a doping level of 1×1018cm−3, the dominant mechanism of the radiative process was found to be free carrier recombination while excitonic recombination was absent due to the effect of strong carrier screening. The nonradiative mechanism was determined to be hole capture through multiphonon emission (MPE). The expressions of the nonradiative MPE recombination lifetime, the PL decay time, and the PL intensity were deduced as functions of temperature and were used to fit the measured temperature dependence of the PL decay times and the PL intensities. The MPE activation energies and relative defect densities for the samples with different well widths were obtained. A simple method is suggested to investigate the interfacial defects of quantum wells.

An apparatus is described here in detail for the transfer of a sample from a metalorganic chemical vapor deposition (MOCVD) reactor to an ultrahigh-vacuum (UHV) chamber without introducing any contamination. The surface of the sample does not change during transfer as is borne out by the identical reflectance difference (RD) spectrum measured first in the MOCVD reactor, i.e., in situ, and afterwards again in the UHV chamber. Making use of the earlier apparatus a semiconductor can be grown in the MOCVD reactor and can afterwards be investigated with any desired tool of surface science, in particular also those that require UHV. All the data collected in UHV can be identified with the RD spectrum measured already in the MOCVD reactor. Several examples are presented here for data collection in UHV on III–V semiconductors grown in the MOCVD reactor. They illustrate the ease and reliability of the here described apparatus for contamination-free sample transfer. Signals are presented in particular for the genuine MOCVD-grown P-rich seemingly (2×1)/(2×2)InP(100) reconstructed surface that until now can only be investigated in UHV if one makes use of the sample transfer system described in this article.