Femtosecond infrared spectroscopy of semiconductors and semiconductor nanostructures

Infrared spectroscopy on ultrafast time scales represents a powerful technique to investigate the nonequilibrium dynamics of elementary excitations in bulk and nanostructured semiconductors. In this article, recent progress in this field is reviewed. After a brief introduction into electronic excitations below the fundamental bandgap and ultrafast processes in semiconductors, infrared pulse generation and the methodology of time-resolved infrared spectroscopy are reviewed. The main part of this paper is devoted to coherent optical polarizations and nonequilibrium excitations of the electronic system in the spectral range below the fundamental band gap. The focus is on the physics of single component plasmas, i.e. electrons or holes. In particular, intraband, inter-valence and intersubband transitions are considered. Processes of phase relaxation, carrier and energy redistribution are analyzed. The potential of ultrafast infrared technology and spectroscopy for future applications is discussed in the final part.     

Exact quantum critical points and phase separation instabilities in Betts Hubbard nanoclusters

Spontaneous phase separation instabilities with the formation of various types of charge and spin pairing (pseudo)gaps in U>0 Hubbard model including the next nearest neighbor coupling are calculated with the emphasis on the two-dimensional (square) lattices generated by 8- and 10-site Betts unit cells. The exact theory yields insights into the nature of quantum critical points, continuous transitions, dramatic phase separation instabilities and electron condensation in spatially inhomogeneous systems. The picture of coupled antiparallel (singlet) spins and paired charged holes suggests full Bose condensation and coherent pairing in real space at zero temperature of electrons complied with the Bose-Einstein statistics. Separate pairing of charge and spin degrees at distinct condensation temperatures offers a new route to superconductivity different from the BCS scenario. The conditions for spin liquid behavior coexisting with unsaturated and saturated Nagaoka ferromagnetism due to spin-charge separation are established. The phase separation critical points and classical criticalities found at zero and finite temperatures resemble a number of inhomogeneous, coherent and incoherent nanoscale phases seen near optimally doped high-T_c cuprates, pnictides and CMR nanomaterials.     

动态应力下超薄栅氧化层经时击穿的可靠性评价

实验发现动态电压应力条件下,由于栅氧化层很薄,高电平应力时间内隧穿入氧化层的电子与陷落在氧化层中的空穴复合产生中性电子陷阱,中性电子陷阱辅助电子隧穿.由于每个周期的高电平时间较短(远远低于电荷的复合时间),隧穿到氧化层的电子很少,同时低电平应力时间内一部分电荷退陷,形成的中性电子陷阱更少.随着应力时间的累积,中性电子陷阱达到某个临界值,栅氧化层突然击穿.高电平时形成的陷阱较少和低电平时一部分电荷退陷,使得器件的寿命提高. 关键词： 超薄栅氧化层 斜坡电压 经时击穿     

Synthesis and characterization of LSMO manganite-based biocomposite

In this paper we study the structural, morphological and magnetic properties of La_0.67Sr_0.33MnO₃ (LSMO) manganite nanoparticles (NPs) and its biocomposite, obtained by mixing NPs of hydroxyapatite (HA). From the studies of X-ray diffraction and Fourier transmission of infrared spectroscopy it is evident that in the biocomposite sample both the individual phases are distinguishable from each other. The measurements of direct current (DC) magnetization and hysteresis loops reveal that the basic magnetic behaviour of LSMO–HA is similar to that of LSMO; however, the admixture of HA makes the sample magnetically softer. From the investigation of transmission electron microscopy it is observed that such a biocomposite is composed of the NPs of LSMO surrounded by HA particles, which can be found suitable for biomedical applications.     

Leakage current analysis of La0.67Sr0.33MnO3/Nb:SrTiO3 p–n junctions

La_0.67Sr_0.33MnO₃ (LSMO) films were grown on 0.7 wt% Nb-doped SrTiO₃ (NSTO) single-crystal substrates by pulsed laser deposition. The crystal phase structure and surface morphology of the LSMO films were investigated by means of X-ray diffraction method and atomic force microscopy, respectively. The diode-like behavior was observed in the leakage currents of the LSMO/NSTO heterojunction, by which I–V curves were measured at room temperature. The leakage current of LSMO/NSTO heterojunction follows the space-charge-limited conduction (SCLC) model under lower forward bias. As the forward bias increases, the barrier at the LSMO/NSTO interface becomes narrower and lower, which allows electrons to go over/through the interface barrier by the conduction mechanisms of Schottky emission and interface-limited Fowler–Nordheim tunneling, respectively. Under the same backward bias, the leakage current still undergoes the Ohmic law region according to the SCLC model, which is due to the drift currents of holes in the LSMO films and electrons in the NSTO substrates.     

The effects of Gd3+ doping on the physical structure and photocatalytic performance of Bi2MoO6 nanoplate crystals

Gd³⁺ doped Bi₂MoO₆ nanoplate crystals were fabricated by solvothermal combined calcination method. The effects of Gd³⁺ doping with different concentrations on the texture, crystal and optical properties of Bi₂MoO₆ were investigated by N₂ physical adsorption, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR) and ultraviolet–visible diffuse reflection spectrum (UV–vis DRS), photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Under simulated solar light irradiation, the influences of Gd³⁺doping on photocatalytic activity of Bi₂MoO₆ were evaluated by photocatalytic degradation of Rhodamine B. The characterization results showed that with Gd³⁺ doping, a contraction of lattice and a decrease in crystallite size occurred. Meanwhile, an increase in surface area over Gd³⁺ doped Bi₂MoO₆ was observed. Moreover, Gd³⁺ doping could obviously enhance the visible light harvesting of Bi₂MoO₆ and promoted the separation of photogenerated electrons and holes. With optimum Gd³⁺(6 wt%) doping, Gd/Bi₂MoO₆ exhibited the best activity and stability in degradation of Rhodamine B.     

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.     

Short-range order,large-scale potential fluctuations,and photoluminescence in amorphous SiN<Subscript>x</Subscript>

The short-range order and electron structure of amorphous silicon nitride SiN_x (x<4/3) have been studied by a combination of methods including high-resolution X-ray photoelectron spectroscopy. Neither random bonding nor random mixture models can adequately describe the structure of this compound. An intermediate model is proposed, which assumes giant potential fluctuations for electrons and holes, caused by inhomogeneities in the local chemical composition. The characteristic scale of these fluctuations for both electrons and holes is about 1.5 eV. The photoluminescence in SiN_x is interpreted in terms of the optical transitions between quantum states of amorphous silicon clusters.     

Ultrahigh charging of dust particles in a nonequilibrium plasma

Charging of dust particles in a plasma with the two-temperature energy distribution of electrons has been studied. It has been shown that the dust-particle potential divided by the electron temperature decreases with increasing electron temperature in the plasma with cold ions. Owing to this behavior, the potential of the dustparticle surface increases with the electron temperature more slowly than the linear function and is lower than the electron temperature (divided by the elementary charge) for T _e > 5.5 eV in hydrogen and for T _e > 240 eV in argon. The fraction of fast electrons at which these electrons begin to contribute to the charge of dust particles has been determined. It has been shown that the charge of micron particles can reach 10⁶ elementary charges. The effect of the cold and thermal field emission on the charge of dust particles has been analyzed. The possibility of obtaining ultrahigh charges (to 10⁷ elementary charges on dust particles with a radius of 50–100 μm irradiated by a 25-keV 1-mA electron beam has been demonstrated.     

Microwave properties of Nd0.5Sr0.5MnO3: a key role of the (x2−y2)-orbital effects

Transmittance of the colossal magnetoresistive compound Nd_0.5Sr_0.5MnO₃ showing metal–insulator phase transition has been studied by means of the submm- and mm-wavelength band spectroscopy. An unusually high transparency of the material provided direct evidence for the significant suppression of the coherent Drude-weight in the ferromagnetic metallic state. Melting of the A-type antiferromagnetic states has been found to be responsible for a considerable increase in the microwave transmission, which was observed at the transition from the insulating to the metallic phase induced by magnetic field or temperature. This investigation confirmed a dominant role of the (x²−y²)-orbital degree of freedom in the low-energy optical properties of Nd_0.5Sr_0.5MnO₃ and other doped manganites with planar (x²−y²)-orbital order, as predicted theoretically. The results are discussed in terms of the orbital-liquid concept.     

Electronic and optical properties of van der Waals vertical heterostructures based on two-dimensional transition metal dichalcogenides: First-principles calculations

Four vertical heterostructures based on two-dimensional transition-metal dichalcogenides (TMDs) – MoS₂/GeC, MoSe₂/GeC, WS₂/GeC, and WSe₂/GeC, were studied by density functional theory calculations to investigate their structure, electronic characteristics, principle of photogenerated electron–hole separation, and optical-absorption capability. The optimized heterostructures were formed by van der Waals (vdW) forces and without covalent bonding. Their most stable geometric configurations and band structures display type-II band alignment, which allows them to spontaneously separate photogenerated electrons and holes. The charge difference and built-in electric field across the interface of these vdW heterostructures also contribute to preventing the photogenerated electron–hole recombination. Finally, the high optical absorption of the four TMD-based vdW heterostructures in the visible and near-infrared regions indicates their suitability for photocatalytic, photovoltaic, and optical devices.     

Properties of carbon and iron modified TiO2 photocatalyst synthesized at low temperature and photodegradation of acid orange 7 under visible light

The nanoparticles of TiO₂ modified with carbon and iron were synthesized by sol-gel followed solvothermal method at low temperature. Its chemical composition and optical absorption were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence emission spectroscopy (PL), UV-vis absorption spectroscopy, and electron paramagnetic resonance (EPR). It was found that carbon and iron modification causes the absorption edge of TiO₂ to shift the visible light region. Fe(III) cation could be doped into the matrix of TiO₂, by which could hinder the recombination rate of excited electrons/holes. Superior photocatalytic activity of TiO₂ modified with carbon and iron was observed for the decomposition of acid orange 7 (AO7) under visible light irradiation. The synergistic effects of carbon and iron in modified TiO₂ nanoparticles were responsible for improving visible light photocatalytic activity.     

Magneto-optics of strongly correlated two-dimensional electrons in single heterojunctions

Investigations of two-dimensional (2D) electron systems in semiconductors subjected to a strong perpendicular magnetic field with the use of photoluminescence are reviewed. The foundation of the optical spectroscopy method using the radiative recombination of 2D electrons with photoexcited holes bound to acceptors in a δ-doped monolayer in GaAs/Al _x Ga_1-x As single heterojunctions is presented. Optical spectroscopy studies of the energy spectra of 2D electrons imposed on transverse magnetic fields in the regimes of the integer and fractional quantum Hall effects are discussed. The relationship between the mean energy of the 2D electron gas and the first moment of their radiative recombination is analysed. It is shown that the magnetic field dependence of the first moment provides a method to measure the cyclotron, enhanced spin and quasiparticle energy gaps at the same time. Therefore it is shown how magneto-optics ‘see’ the ground state of interacting 2D electrons in the extreme quantum limit and how an optical ‘tool’ is efficient for the determination of Coulomb gaps of incompressible Fermi fluids in the fractional quantum Hall effect. Finally optical observations and studies of the Wigner crystallization of 2D electrons are presented. The corresponding liquid-solid phase diagram is discussed.     

Synthesis,structural, optical,and magnetic properties of Co doped,Sm doped and Co+Sm co-doped ZnS nanoparticles

The compositional, structural, optical and magnetic properties of ZnS, Zn_0.98Co_0.02S, Zn_0.98Sm_0.02S and Zn_0.96Co_0.02Sm_0.02S nanoparticles synthesized by a hydrothermal method are presented and discussed. X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies revealed that all the samples exhibited cubic structure without any impurity phases. X-ray photoelectron spectroscopy (XPS) results revealed that the Co and Sm ions existed in +2 and +3 states in these samples. The photoluminescence (PL) spectra of all the samples exhibited a broad emission in the visible region. The room temperature magnetization versus applied magnetic field (M–H) curves demonstrated that the Sm+Co doped nanoparticles exhibited enhanced ferromagnetic behavior compare to Co and Sm individually doped ZnS nanoparticles, which is probably due to the exchange interaction between conductive electrons with local spin polarized electrons on the Co²⁺ or Sm³⁺ ions. This study intensifies the understanding of the novel performances of co-doped ZnS nanoparticles and also provides possibilities to fabricate future spintronic devices.     

Coherent optical spectroscopy of promising materials for solid-state optical processors

State of the art of optical coherent spectroscopy of doped solids that are promising as information carriers for optical processors is reviewed. Special attention is paid to optical echo spectroscopy of doped crystals classified as the Van-Vleck paramagnets where the long-lived stimulated echo is observed with the optical-memory time reaching several hours at low temperatures. Modern elaborations of optical echo processors based on this echo phenomenon are discussed. Physical principles of femtosecond echo spectroscopy and coherent four-wave mixing spectroscopy are formulated. The abilities of these methods in the diagnostics of fast processes at room temperature are illustrated using examples of a doped polymer films. The results of elaborations of a new branch of optical spectroscopy (biphoton spectroscopy) are also presented. The advantages and possible applications of this method are demonstrated using an example of two crystals (Er³⁺:YAG and Cr³⁺:Al₂O₃).     

Study of hot electrons by measurement of optical emission from the rear surface of a metallic foil irradiated with ultraintense laser pulse

Hot electrons and optical emission are measured from the rear surface of a metallic foil. The spectra of the optical emission in the near infrared region have a sharp spike around the wavelength of the incident laser pulse. The optical emission is ascribed to coherent transition radiation due to microbunching in the hot electron beam. It is found that the optical emission closely correlates with the hot electrons accelerated in resonance absorption.     

Negative permittivity of ZnO thin films prepared from aluminum and gallium doped ceramics via pulsed-laser deposition

Aluminum and gallium doped zinc oxide thin films with negative dielectric permittivity in the near infrared spectral range are grown by pulsed laser deposition. Composite ceramics comprising ZnO and secondary phase Al₂O₃ or Ga₂O₃ are employed as targets for laser ablation. Films deposited on glass from dense and small-grained ceramic targets show optical transmission larger than 70 % in the visible and reveal an onset of metallic reflectivity in the near infrared at 1100 nm and a crossover to a negative real part of the permittivity at approximately 1500 nm. In comparison to noble metals, doped ZnO shows substantially smaller losses in the near infrared.     

Structural aspects of two-dimensional polymers: Li4C60, Na4C60 and tetragonal C60. Raman spectroscopy and X-ray diffraction

We present an analysis of three different two-dimensional polymers, tetragonal C₆₀, Li₄C₆₀, and Na₄C₆₀. Based on X-ray diffraction and Raman spectroscopy, we conclude that Li₄C₆₀ forms a tetragonal structure with intermolecular bonds formed by 2+2 cycloaddition, in the same way as for tetragonal C₆₀. Na₄C₆₀, on the other hand, forms a monoclinic structure with single C–C bonds between the molecules. Our Raman spectroscopy results can be interpreted in two ways: either the charge transfer to the C₆₀ molecules is the same in both doped compounds with four electrons/molecule or the electron charge transfer is smaller from the Li ions than from the Na ions.     

Effect of vanadium doping on the electrical properties of Bi12GeO20 crystals

A study is reported of the temperature dependences of the dc and ac electrical conductivities, as well as of I–V characteristics of pure and vanadium-doped germanosillenite crystals. It has been established that the charge carriers in Bi₁₂GeO₂₀ are electrons and holes. Doping with vanadium gives rise to a strong dependence of the conductivity and its activation energy on the dopant concentration. Within the model, the results explain the hopping-charge transfer in doped, closely compensated semiconductors.     

Charge and spin dynamics in antiferromagnetic metallic phase of iron-based superconductors

The electronic states, charge dynamics, and spin dynamics in the antiferromagnetic metallic phase of iron-arsenide superconductors are investigated by mean-field calculations for a five-band Hubbard model. Taking into account the difference of observed magnetic moments between LaFeAsO (1111 system) and BaFe₂As₂ (122 system), we investigate the effect of the magnitude of the moments on band dispersion, optical conductivity, and dynamical spin susceptibility. We clarify how the magnitude affects on these quantities and predict different behaviors between the 1111 and 122 systems in the antiferromagnetic metallic phase.