Typical Semiconductor Material Research Articles

Stability of spintronic devices based on quantum ring networks

Transport properties in mesoscopic networks are investigated, where the strength of the (Rashba-type) spin-orbit coupling is assumed to be tuned with external gate voltages. We analyze in detail to what extent the ideal behavior and functionality of some promising network-based devices are modified by random (spin-dependent) scattering events and by thermal fluctuations. It is found that although the functionality of these devices is obviously based on the quantum coherence of the transmitted electrons, there is a certain stability: moderate level of errors can be tolerated. For mesoscopic networks made of typical semiconductor materials, even cryogenic temperatures can smear out the desired transport properties. When the energy distribution of the input carriers is narrow enough, it turns out that the devices can operate close to their ideal limits even at relative high temperature. As an example, we present results for two different networks: one that realizes a Stern-Gerlach device and another that simulates a spin quantum walker. Finally we propose a simple network that can act as a narrow band energy filter even in the presence of random scatterers.

Optical properties of new photovoltaic materials: AgCuO 2 and Ag 2Cu 2O 3

AgCuO 2 and Ag 2Cu 2O 3 are new types of semiconductor materials. A theoretical study is presented for both the electronic and optical properties of these new photovoltaic materials in the framework of density functional theory (DFT). The calculated cohesive energy is −3.606 eV/atom and −3.723 eV/atom for Ag 2Cu 2O 3 and AgCuO 2, respectively. Electronic calculations indicate that AgCuO 2 is a small band gap semiconductor and Ag 2Cu 2O 3 is metallic in nature. The valency state of Cu is divalent in Ag 2Cu 2O 3 and trivalent in AgCuO 2. The largest absorption coefficient of CuO 2 is 332 244, which is significantly greater than that of CuInSe 2, CdTe, GaAs, etc.

Synthesis and field effect characteristics of Na 2Ti 3O 7 nanowires

Na 2Ti 3O 7 nanowires with diameters of about 80-130 nm and lengths up to several tens of micrometers are synthesized via a simple hydrothermal method and characterized by the field-emission scanning electron microscopy and X-ray diffraction. Back-gate field-effect transistors based on these nanowires are fabricated on indium tin oxide glass substrates with polymethyl-methacrylate-co-glyciclyl-methacrylate as the gate insulator layers. Typical p-type semiconductor material properties are observed in our investigations. The field-effect mobility is about 0.1 cm 2/Vs. The capacitance per unit area of the dielectric is 3.43 nF/cm 2 (dielectric constant, k = 3.9). The on/off ratio is around 10 3 at the conduction of 10 V.

Inducing growth of highly ordered molybdenum oxide nanoplates under ambient conditions

Large area uniform nanoplates of molybdenum oxide (MoO3), a typical semiconductor material, have been synthesized under soft conditions by using carboxymethyl cellulose (CMC) as template. Under ambient condition, hydrolysis of ammonium molybdate into layered molybdenum oxide, and its subsequent inclusion of CMC polymers results in formation of lamellar CMC/molybdenum oxide hybrid. Calcinations of this lamellar hybrid at 500 °C lead to formation of large area uniform nanoplates of orthorhombic phase of MoO3. Scanning electron microscopy and transmission electron microscopy images show that these MoO3 nanoplates are regularly packed, about 100 nm in thickness and 10–100 μm in length. The mechanism of the hybrid reaction and the templating ability of CMC polymers have been extensively discussed. The oriented growth of short molybdenum oxide flakes into long-range ordered plates has been induced by CMC polymers because of the shrinking of CMC during the hybrid reaction. This is the first report that large area highly ordered molybdenum oxide nanometer materials have been obtained under soft reaction conditions.

Prediction of the intrinsic thermal conductivity of phonons in dielectric and semiconductor materials based on the density of the lattice vibration energy

A novel method is proposed to simulate the intrinsic thermal conductivity of the phonons in dielectric and semiconductor materials by introducing the concept of the density of the lattice vibration energy, which is a function of frequency and temperature. A quantitative relationship between the density of the lattice vibration energy and the mean free path of the phonons is established. The heat capacity and sound velocity can also be calculated by using the phonon density of states, the theoretical densities, and the elastic modulus. The thermal conductivities of some typical dielectric and semiconductor materials are then calculated, and it is found that the agreement with experimental data is good for some materials. In contrast to traditional semi-empirical methods, there is no need to input any experimental data.

Stability of Solitary Waves in a Semiconductor Drift-Diffusion Model

We consider a macroscopic (drift-diffusion) model describing a simple microwave generator, consisting of a special type of semiconductor material that, when biased above a certain threshold voltage, generates charge waves. These waves correspond to travelling wave solutions of the model equation which, however, turn out to be unstable in a standard formulation of the travelling wave problem. Here a different formulation of this problem is considered, where an external voltage condition is applied in the form of an integral constraint. Global existence of this novel Cauchy problem is proven and the results of numerical experiments are presented, which suggest the stability of solitary waves. In addition, a small amplitude limit is considered, for which linearized orbital stability of solitary waves can be proven.

Phonon Knudsen flow in nanostructured semiconductor systems

We determine the size effect on the lattice thermal conductivity of nanoscale wire and multilayer structures formed in and by some typical semiconductor materials, using the Boltzmann transport equation and focusing on the Knudsen-flow effect. For both types of nanostructured systems we find that the phonon transport is reduced significantly below the bulk value by boundary scattering off interface defects and/or interface modes. The Knudsen-flow effects are important for almost all types of semiconductor nanostructures but we find them most pronounced in Si and SiC systems due to the very large phonon mean free paths. We apply and test our wire thermal-transport results to recent measurements on Si nanowires. We further investigate and predict size effects in typical multilayered SiC nanostructures, for example, a doped-SiC∕SiC∕SiO2 layered structure that could define the transport channel in a nanosize transistor. Here the phonon-interface scattering produces a heterostructure thermal conductivity smaller than what is predicted in a traditional heat-transport calculation, suggesting a breakdown of the traditional Fourier analysis even at room temperatures. Finally, we show that the effective thermal transport in a SiC∕SiO2 heterostructure is sensitive to the oxide depth and could thus be used as an in situ probe of the SiC oxidation progress.

Optical breakdown for silica and silicon with double femtosecond laser pulses

The optical breakdown thresholds (OBTs) of typical dielectric and semiconductor materials are measured using double 40-fs laser pulses. By measuring the OBTs with different laser energy and different time delays between the two pulses, we found that the total energy of breakdown decrease for silica and increase for silicon with the increase of the first pulse energy.

Spectroscopic analysis of external stresses in semiconductor quantum-well materials

ABSTRACTWe present an approach for spectroscopic strain analysis in semiconductor quantum-well devices. This approach is applicable to all types of semiconductor materials, and to spectroscopic techniques which employ the electronic band-structure of the material, such as photoluminescence, photoreflection, photocurrent, and transmittance. The approach is based on two components, namely the theoretical calculation of the strain-sensitivity of the spectral positions of the relevant quantum-confined optical transitions within a particular quantum-well, and the spatially resolved measurement of a substantial part of the optical transition sequence within the quantum-well. The primary experimental technique applied in our approach is photocurrent spectroscopy. InAlGaAs/GaAlAs/GaAs, high-power lasers serve as the model species.

Monitoring the ultrafast electric field change at a mid-infrared plasma Bragg mirror

The electric field change of a femtosecond mid-infrared (MIR) pulse reflected by a new type of Bragg mirror is directly measured by time-resolved cross-correlation spectroscopy. The refractive-index contrast of the plasma Bragg mirror is achieved by use of different doping levels of only one type of semiconductor material (n(+) -doped GaAs and undoped GaAs). The direct measurement of the time dependence of the electric field of a reflected MIR pulse permits the observation of a noninstantaneous response of a Bragg mirror compared with a metallic surface, which is due to the penetration of the pulse into the multilayer structure.

Effects of Constant and Variable Accelerations on Crystals Grown onboard Spacecraft by the Method of Directional Crystallization

The effect of constant and time-dependent accelerations (vibrations) on the melt flow and heat and mass transfer in the process of crystal growth by the method of directional crystallization (Bridgman method) onboard spacecraft is numerically investigated. The mathematical formulation of the problem and the technique to solve it numerically are given. The time-averaged flow arising under the action of vibrations in a nonisothermal fluid is investigated. With the help of a rational choice of dimensionless similitude parameters, a generalized dependence on the intensity of melt flow is obtained for the radial segregation of dopants. This dependence is invariant with respect to the type of motive power and thermal boundary conditions in the region of very small velocities of melt flow (“creeping” flow), which are characteristic for microgravity conditions. The allowable levels of constant accelerations, as well as the frequency dependences of tolerable vibrations, are obtained for five typical semiconductor materials: Ge(Ga), GaAs(Te), InSb(Te), Si(P), and Si(B). It is shown that the radial segregation of dopant is much more sensitive to microaccelerations than the axial one. In the region of small velocities, the latter is determined by the duration of the transition regime, which depends on certain physical properties of the melt. New problems that resulted from the investigations performed are discussed.

Spatially resolved femtosecond time correlation measurements on a GaAsP photodiode

Femtosecond photoconductive near-field microscopy on a GaAsP diffusion type photodiode is used to investigate the response of the pure semiconductor material and impurity defect sites. At an excitation energy of 780 nm, below the bulk band gap, impurity sites ≈1 μm in diameter are observed. These sites exhibit a linear dependence of photoconductivity on light intensity instead of the usual two-photon dependence. The linear response on the defect sites gives rise to a first-order interferometric autocorrelation, while the typical GaAsP semiconductor material in other regions yields a second-order interferometric autocorrelation, which can be used for determinations of temporal shape, phase, and linear chirp of pulses. The results indicate that high concentrations of impurity sites can limit the application of a GaAsP photodiode as a femtosecond auto- or cross-correlator.

Off-plane angle dependence of photonic band gap in a two-dimensional photonic crystal

We present an analysis of off-plane angle dependence of the photonic band gap in a two-dimensional triangular lattice with circular air rods embedded in a dielectric background. The plane-wave expansion theory is used to calculate band structures within the first Brillouin zone, and an effective refractive index approach is introduced to derive the off-plane angle dependence. The off-plane angle at which the in-plane band gap decreases to half of its gap width has been obtained, which can be increased significantly by increasing the filling fraction of the air rods. For a typical semiconductor material with a refractive index of 3.6 as the background, this angle reaches 26/spl deg/ at a high filling fraction of 0.84 (where the normalized in-plane band gap width is 24%). The corresponding solid angle of 1.75/spl pi/ could cover about 60 of the spontaneous emission from a linear-polarized dipole along the air rods.

Quantum dot lattice and enhanced excitonic optical nonlinearity

We propose a concept of quantum dot lattice for enhancing the excitonic optical nonlinearity, especially its figures of merit. The most important aspect of the quantum dot lattice for enhancing the optical nonlinearity is the enlarged exciton coherence length due to the cooperation of a large number of quantum dots. The optical nonlinearity of the quantum dot lattice composed of typical semiconductor materials is estimated and an enhancement in the figures of merit of several orders of magnitude over the ever known values is predicted.

Electrical properties of tungsten trioxide films

Selectively doped semiconducting metal oxide (SMO) films have been shown to have applications as the sensing element in gas microsensors. Critical to the design and operation of these sensors is the SMO film. In the present work, the electrical properties of both intrinsic and extrinsic (doped with gold) tungsten trioxide (WO3) films, which selectively sorb hydrogen sulfide (H2S), are investigated. Hall effect measurements are performed as a function of film thickness, temperature, gold-doping concentration, and H2S gas concentration. The conductivity was found to be n type and strongly dependent on temperature, gold doping concentration, and H2S gas concentration and less dependent on film thickness. The mobility was relatively high while the intrinsic carrier concentration was low when compared to typical semiconductor materials. The conductivity was shown to exhibit anomalous behavior at certain temperatures and H2S gas concentrations.

Hg Cd Te mis structure: Modelling and application to charge-coupled device infrared imagery

Applications of CCD to read-out techniques signal processing of an infrared detector mosaic could lead to a new generation of sensors where the photosensor element density could be greatly increased. Due to their well-known detection capabilities in the 3–5μm and 8–15μm range, we have dealt with monolithic devices processed on the narrow bandgap semiconductor: mercury-cadmium-telluride. The project will assign optimization criteria which specify the device behaviour, i.e. spectral window, type of semiconductor material, temperature for definite working conditions, scanning and read-out procedure, signal processing. The first part of this paper develops the modelling of the elementary structure of the array designed for read-out through charge transfer techniques. This MIS structure can detect the incident infrared light and collect the corresponding signal charge. Limits imposed by collection and charge integration will modify the infrared system design: and we therefore deduce the specifications of such devices from the model. In the second part, the architecture and read-out mode of a two-dimensional matrix is presented. Choice between charge injection devices and charge-coupled devices should depend on the technology process and material properties, and is discussed with reference to the previous results. One solution worked out for the design of a 8 × 8 matrix array is described.

Neutron activation analysis of impurities in gallium phosphide

Gallium phosphide is a new type of semi-conductor material. Although this substance becomes highly radioactive after neutron irradiation, because of the nature of its component elements (gallium and phosphorus) the chances were that the great sensitivity of the neutron activation technique would at least partly solve this very difficult trace analysis problem. The possibilities of non-destructive analysis of long lived elements (Co−Sb−Sc−Zn−Fe...) were investigated first. To extend this method to the determination of medium-life elements, different methods of radiochemical separation plan was worked out. Finally the method developed gives quantitative analysis of fourteen elements (Ag−As−Au−Ba−Co−Cr−Cu−Fe−Ir−Na−Rb−Sb−Sc−Zn) amounting very often less than one ppb.

Spreading Resistance Between Constant Potential Surfaces

We determine the spreading resistance for a sheet of homogeneous material of uniform thickness with a disk contact source on one side and with current collected (i) over the entire back plane, and (ii) at a corresponding disk on the back plane. A constant driving potential is assumed over the source resulting in mixed boundary conditions in the plane of the source. For each case, a closed form integral solution is derived and then numerically integrated for a range of geometric ratios resulting in a universal spreading resistance curve. The results are used to evaluate the spreading resistances encountered in a typical (1 Ω cm) semiconductor material.