Statistics of electrons in PbS with <Emphasis Type="Italic">U</Emphasis> centers

The dependence of the concentration of two-electron tin centers in the intermediate charge state Sn³⁺ in PbS on the correlation energy is obtained using the Gibbs distribution. It is shown that this state cannot be observed by Mössbauer spectroscopy on an ¹¹⁹Sn isotope (due to insufficient sensitivity), but it can manifest itself in the temperature dependence of the hole density in Pb_1?x?ySn_xNa_yS solid solutions.     

The Polarization and Efficiency of the Terahertz Pulses Generated by Tilted-Pulse-Front Pumping Scheme

Optical rectification of laser pulses in LiNbO₃ by tilted-pulse-front pumping(TPFP) is a powerful way to generate terahertz(THz) pulses. However, comprehensive theoretical analysis is still lack. In this work, we first established and presented a detailed theoretical model for TPFP scheme, which then was used to analyze the pump beam polarization dependent terahertz pulses generated by this scheme. The terahertz pulses polarization and generation efficiency for various pump beam polarization angle were investigated by using nonlinear susceptibility tensor of LiNbO₃ crystal. The results indicate that one can change the polarization state of the terahertz pulse by changing the pump beam polarization.     

Self-ordering of Mg and O isoelectronic impurities in ZnSe

Self-ordering of isoelectronic magnesium and oxygen impurities in ZnSe is described. The formation of 1O4 Mg tetrahedral cells in ZnSe-enriched Mg_xZn_1?x O_ySe_1?y alloy (x ≥ 4y) in the ultradilute limit (1 × 10^?8 ≤ y ≤ 1 × 10^?3) of oxygen concentrations is predicted. Under certain conditions that are established for the temperatures of growth (300°C) and annealing (500°C), all of the oxygen impurity atoms should be surrounded by Mg atoms only. The origin of this phenomenon is the thermodynamic preference for Mg-O and Zn-Se bond formation compared to Mg-S and Zn-O bonds and a decrease in the elastic-stress energy after self-ordering of the alloy.     

Magnetotransport in a semimetal channel in p-Ga1−x InxAsySb1−y /p-InAs heterostructures with various compositions of the solid solution

Magnetotransport properties of an electron channel at the heteroboundary in type II separated p-Ga_1?x In_xAs_ySb_1?y /p-InAs heterostructures grown by LPE (x=0.09–0.22) were studied in the temperature range of 77–300 K. It is shown that an electron channel, which is formed at the heteroboundary and has high mobility μ=(3–5)×10⁴ cm² V^?1 s^?1, exists throughout the whole composition range. The band diagram of the heterostructures under study is discussed, and some parameters of the electron channel are evaluated. It is found that the electron channel with high mobility persists up to room temperature. Type II GaInAsSb/p-InAs heterostructures can find application in new Hall sensor devices with an electron channel at the heteroboundary.     

High Energy THz Pulse Generation by Tilted Pulse Front Excitation and Its Nonlinear Optical Applications

Generation of high energy THz pulses by tilted pulse front excitation is reviewed. The basic idea and the realized setups are described. Properties of THz pulses generated by using different pump lasers are summarized. Nonlinear optical effects induced by the high energy THz pulses were observed, such as nonlinear refraction, self-phase modulation, saturation of THz absorption, and free carrier generation. The main results on these phenomena are described. The possibility of using THz pulses as a quasi-dc field for increasing the cut-off frequency in high harmonic generation is analyzed. The THz generation setup used in recent experiments is analyzed and arrangements for achieving better THz beam quality and higher THz pulse energy are considered. Finally, some results of calculations indicating significantly increased generation efficiency of 0.2-THz pulses compared to generation efficiency of 1-THz pulses are presented.     

Hole concentration and thermoelectric figure of merit for Pb1−x SnxTe:Te solid solutions

The maximum densities of holes generated by cation vacancies, as well as thermoelectric parameters of (Pb_1?x Sn_x)_1?y Te_y solid solutions with tin content x in the range from 0.4 to 0.6, were investigated. It is shown that each vacancy produces four holes in the valence band and that only for small x can the concept of doubly charged vacancy be used. The maximum thermoelectric figure of merit Z is (1.0?1.1)×10^?3 K^?1 at T=800–850 K. The relatively high value of Z achieved without doping is due to the high electrical conductivity provided, first, by the small effective mass of holes and, second, by the high electrical activity of the vacancies.     

Dynamic characteristics of double-barrier nanostructures with asymmetric barriers of finite height and widths in a strong ac electric field

The theory of the interaction of a monoenergetic flow of injected electrons with a strong high-frequency ac electric field in resonant-tunneling diode (RTD) structures with asymmetric barriers of finite height and width is generalized. In the quasi-classical approximation, electron wavefunctions and tunneling functions in the quantum well and barriers are found. Analytical expressions for polarization currents in RTDs are derived in both the general case and in a number of limiting cases. It is shown that the polarization currents and radiation power in RTDs with asymmetric barriers strongly depend on the ratio of the probabilities of electron tunneling through the emitter and collector barriers. In the quantum mode, when δ = ? ? ? _r = ?ω ? Γ (? is the energy of electrons injected in the RTD, ? is Planck’s constant, ω is the ac field frequency, ? _r and Γ are the energy and width of the resonance level, respectively), the active polarization current in a field of E ≈ 2.8?ω/ea (e is the electron charge and a is the quantum-well width) reaches a maximum equal in magnitude to 84% of the direct resonant current, if the probability of electron tunneling through the emitter barrier is much higher than that through the collector barrier. The radiation-generation power at frequencies of ω = 10¹²–10¹³ s^?1 can reach 10⁵–10⁶ W/cm² in this case.     

Population inversion between Γ subbands in quantum wells under the conditions of Γ-<Emphasis Type="Italic">L</Emphasis> intervalley transfer

Monte Carlo simulations of electron transport in Al_xGa_1−x As/GaAs/In_yGa_1−y As double-quantum-well heterostructures in high lateral electric fields are carried out. It is shown that, under the conditions of intervalley Γ-L electron transfer, there exists a population inversion between the first and the second quantum-confinement subbands in the Γ valley. The population inversion appears in the fields exceeding 4 and 5.5 kV/cm at 77 and 300 K, respectively. The gain in a superlattice composed of such quantum wells is estimated to be on the order of 100 cm⁻¹ for radiation with a wavelength of 12.6 μm. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 224–229. Original Russian Text Copyright ? 2003 by Aleshkin, Andronov, Dubinov.     

Study of lifetimes and photoconductivity relaxation in heterostructures with Hg x Cd1 ? x Te/Cd y Hg1 ? y Te quantum wells

Carrier lifetimes in the continuum of the quantum well of a Hg _x Cd_{1 ? x} Te/Cd _y Hg_{1 ? y} Te hetero-structure were studied by terahertz pump-probe spectroscopy. It is found that the relaxation duration of the transmission signal is ??65 ps and is independent of the pump power. Such rapid relaxation in these structures is most likely determined by the interaction of holes with acoustic phonons due to a high density of states in the valence band and a larger effective mass compared with electrons. By the obtained data, the times of the interband nonradiative recombination of holes are determined. In this publication, we report the results of numerical calculation of the energy spectrum of the model structure, in which the possibility of obtaining population inversion at specified concentrations of nonequilibrium carriers is analyzed.     

Influence of Sn<Superscript>4+</Superscript> on Structural and DC Electrical Resistivity of Ni-Zn Ferrite Thick Films

Among the soft ferrites, Ni-Zn ferrite is one of the most versatile ceramic materials because of their important electrical and magnetic properties. These properties were improved by substituting Sn⁴⁺ in Ni-Zn ferrites with chemical composition of Ni _x Zn_1+y?x Fe_2?2y Sn _y O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0; y = 0.1, 0.2). To achieve homogenous ferrite powder at lower sintering temperature and smaller duration in nano-size form, the oxalate co-precipitation method was preferred as compared to other physical and chemical methods. Using this powder, ferrite thick films (FTFs) were prepared by the screen printing technique because of its low cost and easy use. To study structural behavior, the FTFs were characterized by different techniques. The x-ray diffraction and thermo-gravimetric and differential thermal analysis studies show the formation of cubic spinel structure and ferrite phase formation, respectively. There is no remarkable trend observed in lattice constants for the Sn⁴⁺ (y = 0.1)- and Sn⁴⁺ (y = 0.2)-substituted Ni-Zn ferrites. The bond lengths as well as ionic radii on the A-site of Ni-Zn-Sn ferrites were found to decrease with increasing nickel content. The bond length and ionic radii on the B-sites remained almost constant for Sn⁴⁺ (y = 0.1, 0.2)-substituted Ni-Zn ferrites. The energy dispersive x-ray analysis confirms the elemental analysis of FTFs. The Fourier transform infrared spectra show two major absorption bands near 400 cm^?1 and 600 cm^?1 corresponding to octahedral and tetrahedral sites, respectively, which also confirms the formation of the ferrites. The field emission scanning electron microscopy images shows that the particles are highly porous in nature and located in loosely packed agglomerates. The average particle size of the FTFs lies in the range 20–60 nm. Direct current (DC) resistivity of Ni-Zn-Sn FTFs shows the semiconductor nature. The DC resistivity of Ni-Zn-Sn_0.2FTFs is lower than Ni-Zn-Sn_0.1 FTFs. The DC resistivity is found to decrease with the increase in Ni²⁺ content up to x = 0.6. It increases thereafter for a further increase in Ni²⁺ content up to x = 1.0, and a similar trend is observed for the variations of activation energy with Ni²⁺ content.     

Magnetic properties of Pb1−x GexTe alloys doped with ytterbium

Temperature dependences of magnetic susceptibility and magnetic-field dependences of magnetization in the Pb_1?x?y Ge_xYb_yTe (0≤x≤0.02, y≤0.065) solid solutions were studied. It was found that diamagnetic response was replaced by the Curie-Weiss paramagnetic response as temperature decreased. This indicates that Yb³⁺(4f ¹³) magnetic ions are present in the alloys. The magnetic ion concentration and the occupancy of the Yb-induced impurity band were determined from the experimental data.     

Type II heterojunctions in an InGaAsSb/GaSb system: Magnetotransport properties

Magnetotransport properties of the narrow-gap In_xGa_1?x As_ySb_1?y /GaSb heterojunctions grown by liquid-phase epitaxy with various In content in the solid solution (x=0.85–0.95 and E _g ≤0.4 eV) were studied. It is shown that, depending on the In content in these heterostructures, type II staggered-lineup (x=0.85) or broken-gap heterojunctions (x=0.95) with high mobility in the electron channel at the interface (μ?20000 cm²/(V s)) can be realized. For x=0.92, depending on temperature, both types of heterojunctions were observed. Obtained results are in good agreement with the band energy diagram of the type II InGaAsSb/GaSb heterostructures under study.     

Temporal Behavior of the Pump Pulses,Residual Pump Pulses,and THz Pulses for D<Subscript>2</Subscript>O Gas Pumped by a TEA CO<Subscript>2</Subscript> Laser

Temporal behavior of the pump pulses, residual pump pulses, and THz pulses for optically pumped D₂O gas molecules was investigated by using a tunable TEA CO₂ laser as the pumping source. The pulse profiles of pump laser pulses, residual pump pulses, and the THz output pulses were measured, simultaneously, at several different gas pressures. For THz pulse, the pulse delay between the THz pulse and the pump pulse was observed and the delay time was observed to increase from 40 to 70 ns with an increase in gas pressure from 500 to 1700 Pa. Both THz pulse broadening and compression were observed, and the pulse broadening effect transformed to the compression effect with increasing the gas pressure. For the residual pump pulse, the full width at half maximum (FWHM) of the main pulse decreased with increasing gas pressure, and the main pulse disappeared at high gas pressures. The secondary pulses were observed at high gas pressure, and the time intervals of about 518 and 435 ns were observed between the THz output pulse and the secondary residual pump pulse at the pressure of 1400 Pa and 1700 Pa, from which the vibrational relaxation time constants of about 5.45 and 5.55 μs Torr were obtained.     

Laser Line Competition in Optically Pumped D2O THz Laser

Based on the theory of semi-classical density matrix and a model of double three-energy-level (DTEL) system for laser line competition in optically pumped D₂O super-radiant terahertz laser (THz Laser), the mathematical expressions of gain coefficients for pumping signal and THz laser signal are deduced, and the competition between 66?μm and 116?μm laser lines is calculated numerically by means of iteration method. The effects of the operation conditions (including pump power, working gas pressure and the length of cavity) on the competition are analyzed. It is found that 66?μm emission dominated the competition; with the increase of working gas pressure (or the cavity length), the spectrum of 66?μm emission experiences changes from discrete double-peak structure to overlapped double-peak structure and finally to a wide-band structure, and the optimum gas pressure varied conversely with the optimum length of the TH_Z laser cavity.     

Features of the conduction mechanisms of the n-HfNiSn semiconductor heavily doped with the Co acceptor impurity

The crystalline structure, electron density distribution, energy and kinetic parameters of a HfNi_{1 ? x} Co _x Sn semiconductor heavily doped with a Co acceptor impurity are studied in the ranges T = 80?C1620 K and N _A ^Co from 9.5 × 10²⁰ cm^?3 (at x = 0.05) to 7.6 × 10²¹ cm^?3 (at x = 0.40). It is shown that variations in the activation energy of hopping conduction ? ₃ ^?? (x) and the modulation amplitudes of continuous energy bands ? ₃ ^?? (x) are caused by the appearance of a donor source in the n-type HfNi_{1 ? x} Co _x Sn semiconductor. It is shown that the doping of n-HfNiSn with a Co acceptor impurity is accompanied by a change in the degree of compensation of the semiconductor due to the simultaneous generation of both structural acceptor-type defects during Ni atom substitution with Co atoms and structural donor-type defects during the partial occupation of Ni sites by Sn atoms. The results are discussed within the Shklovskii-Efros model for a heavily doped and compensated semiconductor.     

Nickel impurity excitons and photoinduced lattice distortion in ZnSe1−y Sy:Ni and Zn1−x CdxSe:Ni solid solutions

A heavy dependence of the zero-phonon line of the donor Ni excitons and of a series of its phonon replicas on the composition of ZnSe_1?y S_y:Ni and Zn_1?x Cd_xSe:Ni solid solutions was studied. A model of nonradiative recombination of impurity excitons with allowance made for an intermediate virtual state {d ⁸} is discussed in detail. It is shown that this state depends on the lattice distortion induced by the Ni³⁺ impurity center whose charge is positive in reference to the lattice.     

Multi-Cycle Terahertz Emission from <Emphasis Type="Italic">β</Emphasis>-Barium Borate

We report the observation of strong narrowband terahertz emission at 10.6 THz from the common nonlinear second harmonic generation crystal β-barium borate crystal excited by 40 fs laser pulses at a 800 nm center wavelength. The emitted THz field amplitude is linear with incident pump energy with no saturation observed up to 2 mJ of pump pulse energy, and exhibits a crystal azimuthal angular dependence consistent with an optical rectification process. The narrowband emission can be explained by the large dispersion caused by optical phonons in the vicinity of the emission energy.     

Thermoelectric Properties of Heavy Rare Earth Filled Skutterudites Dy y Fe x Co4?x Sb12

Heavy rare earth element Dy-filled skutterudites (Dy _y Fe _x Co_4?x Sb₁₂) have been synthesized by a melting–quenching–annealing method and sintered by the spark plasma sintering technique. Our results suggest that single-phased Dy _y Fe _x Co_4?x Sb₁₂ compounds could be obtained when the Fe content is less than 1.5. The maximum filling fraction of Dy in skutterudites increases with increasing Fe content. We also found significant lattice expansion induced by Fe substitution at Co sites and Dy filling in the voids. The electrical conductivity, Seebeck coefficient, and thermal conductivity have been measured in the temperature range from 300?K to 800?K. The low-temperature Hall coefficient and carrier mobility are reported in the temperature range from 2.5?K to 300?K. The power factor for Dy _y Fe _x Co_4?x Sb₁₂ increases with increasing Fe content. A significant reduction in lattice thermal conductivity is observed in heavy rare earth element Dy-filled skutterudites due to the low localized vibrational frequency of Dy that effectively scatters low-frequency lattice phonons. The sample with composition Dy_0.41Fe_1.45Co_2.55Sb_12.28 has lattice thermal conductivity as low as 1.05?W?m^?1?K^?1 at room temperature. The thermoelectric figure of merit (ZT) reaches a maximum value of 0.67 at 750?K.     

Magnetotransport properties of type II heterojunctions based on GaInAsSb/InAs and GaInAsSb/GaSb

The results of detailed study of the magnetotransport properties of broken-gap type II heterojunctions in a GaInAsSb/InAs(GaSb) system are reported. An electron channel with a high charge-carrier mobility (as high as 50000–60000 cm²/(V s)) is observed and studied for the first time in an isotype broken-gap p-GaInAsSb/p-InAs heterostructure. The effects of electron-channel depletion and semimetal-semiconductor transition in the case of heavy doping of the quaternary alloy with acceptors are studied. Magnetotransport properties at temperatures of 4.2–200 K are studied in detail. Data on the energy spectrum and parameters of two-dimensional charge carriers at the heteroboundary are obtained. It is ascertained experimentally that, depending on the composition, either staggered (at x = 0.85) or broken-gap (at x = 0.95) heterojunctions can be formed in the Ga_1?x In_xAs_ySb_1?y /GaSb, which is confirmed by theoretical calculations. The anomalous Hall effect and negative magnetoresistance were observed in GaInAsSb/InAs:Mn grown on substrates doped heavily with Mn magnetic acceptor impurity so that the hole concentration was as high as p > 5 × 10¹⁸ cm^?3; these phenomena are caused by exchange interaction of Mn ions in InAs with high-mobility charge carriers in the electron channel at the heterointerface.     

Vanadium deep impurity level in diluted magnetic semiconductors Pb1 ? x ? y Sn x V y Te

The crystal structure, Sn and V distribution over the length of single-crystal ingots, and galvanomagnetic effects in low magnetic fields (4.2 K ?? T ?? 300 K, B ?? 0.07 T) in Pb_1?x?y Sn _x V _y Te alloys (x = 0.05?0.21, y ?? 0.015) are studied. It is shown that all the samples are single-phase, while the Sn and V concentrations exponentially increase from the beginning to the end of the ingots. Upon doping with V, a decrease in the concentration of free holes and a metal-insulator transition are found. They are related to the appearance of a deep impurity level of V in the band gap, electron redistribution between the level and the valence band, and pinning of the Fermi-level to the impurity level. The shift rate of the V level relative to the conduction band bottom is determined and a diagram of the reconstruction of the electronic structure of the Pb_{1 ? x ? y} Sn _x V _y Te alloy upon varying the host composition is suggested.