Narrow-band Semiconductor Research Articles

Spin-wave dispersion in ferromagnetic semiconductor superlattices and thin films in the narrow-band limit

Bulk and surface magnetic excitations of the semi-infinite ferromagnetic semiconductor (FMS) superlattices and thin films described by Heisenberg and s – d model are analyzed using the transfer matrix method, developed in our previous work. Results are discussed in the narrow-band limit. The spin-wave frequencies for the semi-infinite narrow-band semiconductors are analyzed in both low- and high-frequency regions. Energies of localized excitations are compared to the bulk and the results of Green function formalism. Depending on the parameters of the system, the surface spin waves appear as “acoustical” and “optical”, and there are only some quantitative difference in the high-frequency region, comparing our method and the Green function method. In the framework of the same methodology, bulk and surface magnetic excitations of more complicated superlattices and thin films made of the FMS superlattices are analyzed in terms of dependence of the system parameters. It is shown that the s – d interaction governs the behavior of the systems. Dependence on bulk and surface parameters is discussed.

Effect of chemical doping on the thermoelectric properties of FeGa3

Thermoelectric properties of the chemically-doped intermetallic narrow-band semiconductor FeGa3 are reported. The parent compound shows semiconductor-like behavior with a small bandgap (Eg = 0.2 eV), a carrier density of ∼1018 cm−3, and a large n-type Seebeck coefficient (S ∼ − 400 μV/K) at room temperature. Hall effect measurements indicate that chemical doping significantly increases the carrier density, resulting in a metallic state, while the Seebeck coefficient still remains fairly large (∼− 150 μV/K). The largest power factor (S2/ρ = 62 μW/m K2) was observed for Fe0.99Co0.01(Ga0.997Ge0.003)3, and its corresponding figure of merit (ZT = 1.3 × 10−2) at 390 K improved by over a factor of 5 from the pure material.

Terahertz radiations from narrow band gap of semiconductor irradiated by femtosecond pulses with different pump intensities

The characteristics of terahertz (THz) radiations from the surfaces of two kinds of narrow-band semiconductors InN and InAs excited by femtosecond laser pulses with different pump powers (from 10 to 320mW) are investigated experimentally. The results show that InAs can irradiate a stronger THz signal than that of InN under the same pump power so its radiation efficiency is higher. However, the spectral widths of THz radiations from these semiconductor surfaces increase with the increase of pump power. When the intensity of pump laser is high enough, the spectral Half-Maximum-Full-Width (HMFW) of THz radiation tends to be a constant. Compared with InAs, InN can reach this constant HMFW THz spectrum at a lower pump power. This research is significant for investigating the THz radiation mechanism from semiconductor surfaces, and it is also a good reference for exploring a THz radiation source with low cost and high efficiency.

Oriented Bi 2Se 3 nanoribbons film: Structure, growth, and photoelectric properties

Large-area oriented Bi 2Se 3 nanoribbons films were first synthesized via a simple chemical route. The aligned Bi 2Se 3 nanoribbons with a typical width of 120–300 nm, thickness of 40–70 nm and length of tens microns were grown on a metal Ti foil. Optical absorption experiments revealed that the Bi 2Se 3 nanoribbons are narrow-band semiconductors with a band gap E g ≈1.57 eV. The photoelectric properties of the oriented Bi 2Se 3 nanoribbons film was also investigated with a liquid junction photoelectrochemical solar cells system, and the highest short circuit photocurrent may be estimated at 1 mA cm −2.

Nonsubstitutional single-atom defects in theGe1−xSnxalloy

${\text{Ge}}_{1\ensuremath{-}x}{\text{Sn}}_{x}$ alloys have proved difficult to form at large $x$, contrary to what happens with other group IV semiconductor combinations. However, at low $x$ they are typical examples of well-behaved substitutional compounds, which is desirable for harnessing the electronic properties of narrow-band semiconductors. In this paper, we propose the appearance of another kind of single-site defect $(\ensuremath{\beta}\text{-Sn})$, consisting of a single Sn atom in the center of a Ge divacancy, that may account for these facts. Accordingly, we examine the electronic and structural properties of these alloys by performing extensive numerical ab initio calculations around local defects. The results show that the environment of the $\ensuremath{\beta}$ defect relaxes toward a cubic octahedral configuration, facilitating the nucleation of metallic white tin and its segregation, as found in amorphous samples. Using the information stemming from these local defect calculations, we built a simple statistical model to investigate at which concentration these $\ensuremath{\beta}$ defects can be formed in thermal equilibrium. These results agree remarkably well with experimental findings, concerning the critical concentration above which the homogeneous alloys cannot be formed at room temperature. Our model also predicts the observed fact that at lower temperature the critical concentration increases. We also performed single-site effective-field calculations of the electronic structure.

Band Structure Calculations and Magnetic Relaxation in Correlated Semiconductors FeSb<sub>2</sub> and RuSb<sub>2</sub>

A comparative study of electronic properties of the two isostructural narrow-band semiconductors FeSb2 and RuSb2 by means of experimental NQR spectroscopy as well as theoretical ab-initio band structure calculations is presented. The temperature dependence of 1/T1 consists of two distinct intervals: above 40 K (HT) with activated behavior for FeSb2 with D/kB @ 450 K and below 40 K (LT) with smooth maximum at 10 K. Here the relaxation is governed by in-gap states. We propose the model of inherent Sb-deficiency (as prepared non-stoichiometry) of both FeSb2 and RuSb2 as a possible reason of the in-gap states. This results in creation of a small portion of Fe (Ru) ions possessing formal oxidation number +3 with d5 configuration and forming narrow energy level of localized S = ½ spins near the bottom of the conduction band. Due to much higher gap value in RuSb2, the activation mechanism for RuSb2 in the HT range is inefficient and the 1/T1 dependence in the HT range is more close to T2 behavior characteristic for phonon relaxation mechanism by two-phonon (Raman) scattering.

Current-voltage characteristic of a narrow-band semiconductor taking into account ionization of impurities

The current-voltage characteristic of a material with a narrow conduction band in a strong electric field is studied taking into account ionization of deeply lying impurities in the model, in which collisions of charge carriers are described in the Fokker-Planck approximation. The results are compared with analogous results obtained in the v approximation. It is shown that the results obtained earlier in the v approximation coincide with our results to within ∼ 5%. It is noted that the Fokker-Planck model additionally provides the temperature dependence of the current-voltage characteristic.

Bafe3.39(5)Ru2.61(5)O11 And BaCo1.85(6)Ru4.15(6)O11. Preparation, Crystal Structures, And Magnetic And Transport Properties Of Quaternary Transition Metal Oxoruthenates

Black plate-like single crystals of BaFe3.39(5)Ru2.61(5)O11 and BaCo1.85(6)Ru4.15(6)O11 were grown from a BaCl2 flux [hexagonal, space group P63/mmc (No. 194), Fe: a = 5.856(1), c = 13.587(1) Å , R1 = 0.029, wR2 = 0.084; Co: a = 5.842(1), c = 13.573(3) Å, R1 = 0.033, wR2 = 0.075]. The crystal structures contain two crystallographic sites with mixed Ru and Fe/Co occupation of different level in octahedral coordination and one site purely occupied by the respective 3d metal. The latter position is in trigonal bipyramidal coordination, with some indication for a displacement of the metal atom towards tetrahedral coordination. According to the charge balance, the ruthenium atoms are incorporated with different electronic situations in the two Ru containing sites. The Co compound may be described as containing Co2+ and Ru5+ next to Ru3+. Magnetic susceptibility data support this assignment. According to magnetization measurements on oriented crystals, BaCo1.85(6)Ru4.15(6)O11 is a soft ferromagnetic material with low coercive field and a spontaneous magnetization below Tc = 105 K. It behaves as an electric conductor. However, BaFe3.39(5)Ru2.61(5)O11 is a narrow band semiconductor material with ferrimagnetic ordering at Tc = 440 K

Optical properties of quantum dots produced from inverted-gap semiconductors

We investigate the electron spectrum and optical response of quantum dots (QDs) produced from narrow-band semiconductors with the inverted band structure (the negative sign of the energy gap). It is shown that in addition to usual size-quantized states which are located above the gap edge in normal band materials, there exist states residing inside the gap (analogous of the Tamm states) in inverted band structures. Such states manifest themselves in the far infrared part of emission and absorption spectra, measurement of which allows us to extract information about the QD surface.

Low-temperature crystallization of semiconductor solid solutions that are promising for the realization of the extrinsic thermophotovoltaic effect

Versions are proposed of the formation of the molecules of the multi-component compounds that alloy the elementary semiconductors in the low-temperature crystallization of the solid solutions with the subnano- and nano-defects. The possibilities for the realization of the extrinsic thermophotovoltaic effect in the photocells that are based on GaSb and Ge and are alloyed with the narrow-band semiconductor compounds are discussed.

Anomalous Hall effect in Rashba two-dimensional electron systems based on narrow-band semiconductors: Side-jump and skew scattering mechanisms

We employ a helicity-basis kinetic equation approach to investigate the anomalous Hall effect in two-dimensional narrow-band semiconductors considering both Rashba and extrinsic spin-orbit (SO) couplings, as well as a SO coupling directly induced by an external driving electric field. Taking account of long-range electron-impurity scattering up to the second Born approximation, we find that the various components of the anomalous Hall current fit into two classes: (a) side-jump and (b) skew scattering anomalous Hall currents. The side-jump anomalous Hall current involves contributions not only from the extrinsic SO coupling but also from the SO coupling due to the driving electric field. It also contains a component which arises from the Rashba SO coupling and relates to the off-diagonal elements of the helicity-basis distribution function. The skew scattering anomalous Hall effect arises from the anisotropy of the diagonal elements of the distribution function and it is a result of both the Rashba and extrinsic SO interactions. Further, we perform a numerical calculation to study the anomalous Hall effect in a typical $\mathrm{InSb}∕\mathrm{AlInSb}$ quantum well. The dependencies of the side-jump and skew scattering anomalous Hall conductivities on magnetization and on the Rashba SO coupling constant are examined.

High‐Quality Ultralong Bi2S3 Nanowires: Structure, Growth, and Properties.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Integrated twin-photon sources for the silicon absorption band: a numerical study

We numerically study a narrowband semiconductor integrated source of counterpropagating twin photons for line-of-sight experiments in quantum key distribution. Considering the air transparency and the availability of silicon avalanche photodiodes to be around 770 nm, we focus on GaN- and ZnSe-based quadratically nonlinear multilayer waveguides. The design of a quasi-phase-matched χ(2) profile in the growth direction allows us to generate 3×105 guided photon pairs per second for a pump power around 300 mW.

High-Quality Ultralong Bi2S3 Nanowires: Structure, Growth, and Properties

A simple one-step hydrothermal method for large-scale synthesis of ultralong single-crystalline Bi2S3 nanowires was reported, and the nanowires were comprehensively characterized. The diameters of the nanowires are about 60 nm, and their lengths range from tens of microns to several millimeters. The structure of the nanowires was determined to be of the orthorhombic phase, the growth direction was along [001], and the growth mechanism was investigated based on extensive high-resolution transmission electron microscopy observations. Optical absorption experiments revealed that the Bi2S3 nanowires are narrow-band semiconductors with a band gap E(g) approximately 1.33 eV. Electrical transport measurements on individual nanowires gave a resistivity of about 1.2 ohms cm and an emission current of 3.5 microA at a bias field of 35 V/microm. This current corresponds to a current density of about 10(5) A/cm2, which makes the Bi2S3 nanowire a potential candidate for applications in field-emission electronic devices.

Modeling of Terahertz Radiation from InSb and InAs

In this paper the THz radiation dynamics of InSb and InAs, two typical narrow band semiconductors, was investigated using the ensemble Monte Carlo method. Our simulations indicated that the single mechanism of current surge only can result in small difference of THz emission efficiency for InSb and InAs. The great advantage of InAs over InSb in THz emission efficiency that was found in a published work is possibly due to the mechanism of optical rectification. In addition, under low excitation level, we found the emission efficiency of InSb is advantage over that of InAs, but under high excitation level, the result is reversed. On the other hand, through the Fourier transforms of temporal THz waveforms we found that the main frequency of THz pulses from InAs is always higher than that of InSb.

Bake stability of CdTe and ZnS on HgCdTe: An x-ray photoelectron spectroscopy study

Mercury cadmium telluride (Hg1−xCdxTe or MCT) has been commonly used in devices for infrared (IR) detection. For the optimum performance of the device, a compatible surface-passivation technology that provides long-term stability is required. Using x-ray photoelectron spectroscopy (XPS), the present study examines the effects on Hg0.8Cd0.2Te passivated with CdTe and ZnS undergoing baking in vacuum at temperatures typically used for dewar bakeout. Spectra recorded as a function of depth in both cases clearly show out-diffusion of Hg from the substrate toward the surface, even before the bakeout. On baking in vacuum, dramatic changes are observed in the ZnS/MCT case with complete loss of Hg from the sample up to the tested depth of more than 1,000 A. Compositions of the HgCdTe matrix, formed after Hg out-diffusion, before and after the bakeout are also calculated at selected depths (from 250 A to 700 A), which is vital information from a device point of view, as it affects the bandgap of this narrow-band semiconductor.

Theoretical design for 120 GHz semiconductor junction circulators: effects of different numbers of terms in the Green's function

A theoretical analysis of the predicted performance and field distributions for semiconductor junction circulators is based on Bosma's Green's function approach which involves a summation of infinite series in the mathematical derivation. The effects are considered of taking a different number of terms in the series, looking at three different circulators centred at 120 GHz. The narrowband semiconductor circulators show a similar effect to the ferrite junction circulator because the field distributions inside the semiconductor discs can be approximated by only considering the dominant modes. However, there is no dominant mode in the broadband circulator and the higher order modes play an important role in the operation of this circulator. In order to obtain a precise representation of the circulation effects inside both narrowband and broadband circulators, at least up to the 6th term, or 18 terms in all, are required to be added in the summation of Green's functions.

Frequency stabilization of an extended cavity semiconductor diode laser for chirp cooling

We present a technique to stabilize in an atomic transition the chirping frequency of a narrow-band semiconductor diode laser. The technique is demonstrated to chirp-cool Rb85 atoms used for loading a magneto-optical trap. The stabilization process eliminates long-term fluctuations and drifts in the number of atoms caught in the trap. This is a simple, easy-to-implement, and robust method for wide range of laser cooling experiments employing frequency chirping.

Polarons in crystalline and non-crystalline materials

The current state of polaron theory as applicable to transition metal oxides is reviewed, including problems such as impurity conduction where disorder plays a role. An estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy. The binding energy of a polaron to a donor or acceptor in narrow-band semiconductors is discussed. The experimental evidence about the conductivity of TiO 2 and NiO is reviewed. Impurity conduction in NiO and conduction in glasses containing transition metal ions is discussed and it is emphasized that the activation energy for hopping nearly all vanishes at low temperatures. Pollak's theory of a.c. impurity conductivity is reviewed and applied to the problem of dielectric loss in these materials.

Optically controlled GaAs-GaAlAs photonic band gap structure

We study a mechanism of light-by-light control, based on changing the refractive index of the narrow-band layers, in a semiconductor photonic band gap structure (PBGS). This change is caused by the contribution of the non-equilibrium charge carriers generated by the controlling radiation. We theoretically show that this mechanism can be efficient if an optimal controlling light wavelength is taken near the proper absorption edge of the narrow-band semiconductor layers, since small changes of the layer refractive indices at the band gap edge of a PBGS cause esseatial change of the transmission characteristics. A modulation depth of the controlled light of more than 90% is achieved for a PBGS based on GaAs at the wavelength 1.54 × 10 -6 m under the controlling light power equal to 5 × 10 7 W m -2 . The inertia of the mechanism considered is determined by the lifetime of the non-equilibrium charge carriers in semiconductor layers and is equal to 10 -7 s in our case.