Narrow Results

A geometrical approach is constructed to explain the dependence of lattice thermal conductivity (LTC) in InAs/GaAs multi-layered superlattice structures (SLs). The Morelli–Callaway model is applied to calculate the LTC of InAs/GaAs SLs with the assumption of InAs+GaAs thickness as a grain boundary and the total layer thickness as the sample size. Calculations gave a systematically conventional temperature dependence of LTC for nine different samples depending on their number of layers and the InAs and GaAs layer thicknesses having their ratio from 0.13 to 0.295. The dependence of LTC for both peak values at the low and room temperature were examined according to sample size, number of layers and layer thickness ratio. A new relation is examined for the assessment of the grain boundary effects. The best dependence of LTC was obtained by introducing the effects of the layer thickness ratio (InAs/GaAs) and the sample size D in the form [D/(InAs/GaAs)] which gave a systematic mathematical fitting equation.

A comprehensive investigation was conducted to analyze the physical properties, including electronic structure, optical characteristics, and thermoelectric properties, of four zinc blend structures: BAs, AlAs, BBi, and BSb. This analysis utilized first-principles calculations based on Density Functional Theory (DFT) and Boltzmann transport theories, implemented in the WIEN2K simulator program. The compounds examined displayed intriguing electronic and optical properties, such as low indirect band gaps of 0.726, 1.888, 0.867, and 1.51 eV for AlAs, BAs, BBi, and BSb, respectively. Moreover, these compounds exhibited high absorption in the UV–Visible region. Among the four compounds studied, BAs demonstrated exceptional structural stability due to its high bulk modulus and negative formation energy. The thermoelectric study revealed that the Seebeck coefficient decreased with increasing temperature, while the figure of merit was proportional to temperature enhancement. This behavior suggests that the investigated materials hold promise for applications in visible-light solar cell devices.

1,4,7,10-Tetrathia (12-crown-4)-bridged new polymeric phthalocyanines 9 and 10 have been synthesized by the reaction of tetracyanodibenzo[1,4,7,10-tetrathia (12-crown-4)] in the presence of a strong organic base or Zn(OAc)₂ respectively. Furthermore, 1,4,7,10-tetrathia (12-crown-4)-linked peripherally octa-substituted dimeric phthalocyanine 12, which contains a combination of hexakis(alkylthia) side chains was synthesized by the reaction of subphthalocyanine 11 with the iminoisoindoline derivative 8. The new compounds have been characterized by elemental analyses, IR, UV/vis, mass, ¹H NMR and ¹³C NMR spectroscopy. The thermal stabilities of the compounds were determined by thermogravimetric analysis. The electrical conductivity of the polymeric, 9 and 10, and dimeric phthalocyanines, 12 and 13, are in the semiconductor range; chemical doping with NOBF₄ increases the d.c. conductivity of 10 by a factor of four.

Here we discuss the use of the Cellular Monte Carlo (CMC) method for full band simulation of semiconductor transport and device modeling. The electronic band structure and phonon spectra are used as direct inputs to the program for both cubic, hexagonal, and strained crystal structures using both empirical and ab initio methods. As a particular example, this method is applied to study high field transport in GaN and GaN/AlGaN heterostructures, where good agreement is obtained between the simulated results, and experimental pulse I-V measurements of transport. For device simulation, the CMC algorithm is coupled to an efficient 2D/3D multi-grid Poisson solver. We discuss the application of this algorithm to several technological problems of interest, including ultra-short channel Si/Ge MOSFETs, III-V compound HEMTs, and AlGaN/GaN HEMTs.

Magnetotactic bacteria (MTB), discovered in early 1970s contain single-domain crystals of magnetite (Fe₃O₄) called magnetosomes that tend to form a chain like structure from the proximal to the distal pole along the long axis of the cell. The ability of these bacteria to sense the magnetic field for displacement, also called magnetotaxis, arises from the magnetic dipole moment of this chain of magnetosomes. In aquatic habitats, these organisms sense the geomagnetic field and traverse the oxic-anoxic interface for optimal oxygen concentration along the field lines. Here we report an elegant use of MTB where magnetotaxis of Magnetospirillum magneticum (classified as AMB-1) could be utilized for controlled navigation over a semiconductor substrate for selective deposition. We examined 50mm long coils made out of 18AWG and 20AWG copper conductors having diameters of 5mm, 10mm and 20mm for magnetic field intensity and heat generation. Based on the COMSOL simulations and experimental data, it is recognized that a compound semiconductor manufacturing technology involving bacterial carriers and carbon-based materials such as graphene and carbon nanotubes would be a desirable choice in the future.

A density functional study for structural and electronic properties of Zinc Oxide (ZnO), in wurtzite, rock salt and zinc-blende phases has been performed using full potential-linearized augmented plane wave/linearized augmented plane wave plus local ideal orbital (FP-LAPW/L(APW+lo) approach as realized in WIEN2k code. To approximate exchange correlation energy and corresponding potential, a special GGA parameterized by Wu–Cohen has been implemented. Our results of lattice constants, bulk moduli as well as for internal parameter with GGA-WC are found to be more reliable. This study reveals that value of internal parameter decreases with increasing volume whereas computed electronic band structure confirms the direct band gap behavior of ZnO in B4 and B3 phases while indirect band gap behavior in B1 phase. Moreover, two fold degeneracy at the maxima of valence band for B4 and B1 phases whereas three fold for B3 is observed. A detailed comparison with experimental and other first principles studies is also made.

This paper investigates the impact of the 1986 US-Japan Semiconductor Trade Agreement (STA) and antidumping actions by the US on Japanese firms. We conduct an event study employing WLS and OLS estimations on the daily returns of eight large electronics firms over roughly a two year period. We find that the STA had a positive effect on the daily returns while the antidumping rulings were found to be insignificant. These results are consistent with some authors' views that the STA policy may have facilitated collusive behavior to the benefit of not only US, but Japanese firms as well. These results shed some light on the ambiguous results found in the Voluntary Import Expansion (VIE) literature.

We discuss three recent magnetooptical experiments using the explosive driven flux-compression technique that have been performed in cooperation of the Berlinian with the Russian group of VNIIEF, Sarov. We present an overview on the experimental techniques touching the frontiers of physics as well as a detailed discussion of the results obtained on the semiconducting materials GaN, GaAs and HgSe. Special emphasis will be laid on the interpretation in context with theoretical predictions and analysis that go beyond the ordinary k•p-formalism but are also valid in the limit of the HOFSTADTER butterfly.

Carrier mobility in a narrow GaAs semiconductor quantum well wire embedded in the GaAlAs matrix is investigated using a simple model developed by Lee and Spector. Five different screening functions with three different impurity distributions are used in the calculations. The results show that (i) the choice of the screening function is important as the mobility values vary by two orders of magnitude, and (ii) the mobility values not only depend on the impurity distribution but also vary differently with the wire radius. While hydrostatic pressure reduces the mobility values, temperature increases the values. The polaronic effect decreases the mobility values irrespective of temperature and pressure, the maximum contribution being 9%.

Quantum 1/f noise is the manifestation of the coherent and conventional quantum 1/f effects (Q1/fE). The conventional Q1/fE is a fundamental quantum fluctuation of physical cross sections σ and process rates Γ, caused by the bremsstrahlung (recoil) energy and momentum losses of charged particles, when they are scattered, or accelerated in any way. The closely related coherent Q1/fE is present in any current carried by many particles. It is caused by the energy spread characterizing any coherent state of the electromagnetic field oscillators. According to the Heisenberg's uncertainty principle, because an approximation of the phase or position variable is known, exact knowledge of the energy is precluded. This energy spread results in nonstationary energy values, or fluctuations in the energy of the oscillators. To find the spectral density of these inescapable basic fluctuations, which are known to characterize any quantum state, which is not an energy eigenstate, we use an elementary physical derivation based on Schrödinger's definition of coherent states, which can be supplemented by a rigorous derivation from a well-known quantum-electrodynamical branch-point propagator. The example of a simple harmonic oscillator is also useful for illustrating the uncertainty that arises due to Q 1/f Noise. Clearly illustrating the relation between the uncertainty principle and Q 1/f noise.

Trapping and de-trapping of the acceptor-donor impurity pairs in semiconductors have been studied in crystalline silicon. The existence of such pairs in semiconductors are reported to have influence on the optical and electrical properties of materials. The perturbed γ - γ angular correlation (PAC) method is employed here to study such thermally induced dynamics of impurity complexes in the host lattice. The various types of pairs which are identified via the measured quadrupole interaction frequencies (QIF) showed distinct population of the complexes at different annealing temperatures. Efforts made to re-trap formed complexes after their dissociation by high sample temperatures produce positive results for some impurities.

The aptitude of magneto-spectroscopic methods for studying the electronic and spin properties of semiconductor structures is demonstrated with a few examples of our recent work on two-dimensional electron gases and semiconductor quantum dots, on bulk GaAs and GaN, as well as on thin graphitic layers.

The aim of this work is to analyze the band-structure parameters of Mn_xCd_yHg_1-x-yTe (MMCT) alloys by means of Magnetophonon Resonance (MPR) and to compare that obtained for Zn_xCd_yHg_1-x-yTe (ZMCT). MPR was observed for two samples of Mn_xCd_yHg_1-x-yTe (MMCT) (I- x=0.095 and y=0.09, II-0.04 and y=0.19). The measurements of MPR were performed in pulsed magnetic fields at different temperatures ranging from 77 K to 200 K. The several wide maxima are clearly visible on the experimental curves corresponding to four series of peaks. These series are related to three kinds of phonons analogous to ZMCT.

The system of MPR peaks in MMCT is shifted significantly to the higher magnetic fields in comparison with the ZMCT resonance curves of approximately the same content of Zn. The value of effective mass obtained by MPR experiment is of 66% greater than those calculated in the framework of three-band Kane model, in which the value of E_g, was extracted from the photoconductivity data.

The electronic properties of ZnS/CdS (001) superlattices (SLs) are investigated using the sp³s* tight-binding method versus substrate composition and valence-band offset (VBO). The results show that the electron is always confined within the CdS slabs; and also show a more striking feature due to the prominent localization of the top hole state near the interface region independently from the substrate composition (strain state). Theoretically, we have varied the VBO and inspected its effect on the interface state, and found that this latter persists to exist for VBO values within the interval [-1.4 eV, 0.6 eV]. This interface state is a manifestation of strong band mixing effects induced by the biaxial strain, and is expected to enhance the radiative efficiency of the SLs. The modelling of some available PL data, obtained for thin CdS quantum well embedded inside ZnS, has shown that the samples are of type-I with high energy bandgaps lying within the blue to ultraviolet spectral region.

A review of megagauss experiments using different kinds of field generator up to 1,000 T is given. The effiency of the single-turn coil, the electromagnetic and the explosive flux-compression, and finally the nearly steady-state long-pulse generators are presented. The basic feature of magnetic fields in semiconductor physics is related to the quantity of the magnetic length λ = (ħ/eB)^1/2 where ħ and e are natural constants with the usual meaning and B is the magnetic flux density. For B = 100 T we obtain λ = 2.56 nm. The parameter λ is a measure for the extension of the corresponding wavefunction of the charged particle and completely independent of the cyclotron mass. It should be noted that megagauss magnetic fields and nano structures have thus a natural correspondence: the magnetic length λ serves as a length measure in space in the nm-regime to be tuned by the external magnetic field. A second feature of megagauss fields is the generally extremely short pulse of field realization accompanied by a very high dB/dt. In this way the fields are "transient" and can be used to study the dynamics of the charge-carrier system. Also "eddy current"-effects can modify the system and manifest in new physical phenomena. Setup, performance and data analysis are demonstrated in different examples.

The resultant electric field gradient (EFG) produced at the nucleus of an atom in a solid can be measured using the perturbed γ – γ angular correlation (PAC) method that employs radioactive probe atoms. Several EFGs, associated with different types of defects trapped by the probe, are reported from the crystal silicon and germanium semiconductors. However, the nature of the field gradients is not fully understood because of the many factors contributing for its properties. The proximity of an impurity atom to the probe in the host matrix particularly played a significant role in the determination of the magnitude of the EFG. We discuss here the temperature and stress dependence of the crystal EFGs caused by impurity trapping as well as by the actions of uniaxial stress on semiconductor substrates.

Flower-type ZnO nanorods were synthesized by two-step low temperature solution growth and dye-sensitized with phenosafranine for the first time. The scanning electron microscope result shows that the diameter and the length of a single rod of the flower-type nanostructures depend on the method of synthesis. Optical absorption analysis shows a visible absorption at 517 nm, which was otherwise absent in nano ZnO. The photoluminescence spectra of ZnO and dye-sensitized ZnO nano flowers were also analyzed.

We have carried out thermally stimulated current (TSC) measurements on as-grown Tl₂Ga₂S₃Se layered single crystals in the temperature range 10–60 K with different heating rates of 0.6–1.5 K s¹. The data were analyzed by curve fitting, initial rise, and peak shape methods. The results were in good agreement with each other. Experimental evidence was obtained for trapping center in Tl₂Ga₂S₃Se crystal with activation energy of 11 meV. The capture cross section and concentration of the traps were found to be 1.5 × 10^-23cm² and 1.44 × 10¹⁰cm^-3, respectively. Analysis of the TSC data at different light excitation temperatures leads to a value of 18meV/decade for the traps distribution.

Single phase M₂Si (M = Mg, Ca, Sr) silicides were grown using Si substrates, by thermal treatment of the substrates in the vapors of the metallic sources, M, and the electronic structures and optical property of the silicides were investigated. The electronic band structures of the silicides were calculated using the first-principles total-energy calculation program in pseudopotential schemes with plane-wave basis functions. The calculated optical reflectance spectra were also deduced from the theoretical band structures, and roughly agreed with the experimental results except for the low reflectance intensity around 2 eV. This suggests that the energy band gap of the silicides roughly agree with the calculated values of 0.15, 0.31 and 0.35 eV for Mg₂Si, Ca₂Si and Sr₂Si, respectively, within the underestimation of the band gap by the density functional calculation. The optical property of the silicides is also discussed in relation to the morphological structures of the silicides.

Based on methods of quantum optics, we discuss the possibility of achieving the negative index of refraction in a semiconductor with donor-like impurities. The quantum states of hydrogen-like donor atom and states of an electron in conduction band constitute a discrete-level atomic medium within the optical range. The coherent coupling of an electric dipole transition with a magnetic dipole transition leads to permeability and permittivity responses and, within some frequency band, ensures the negative refractive index. The magnetic moment between two quasi-atomic states separated by optical frequencies is induced by a low-frequency electromagnetic field. The implementation of this scheme is carried out in tin-doped indium oxide, and the calculations show feasibility of the effect within a broad bandwidth ∼2% with a high figure of merit greater than 10 in the optical regime.