Mixed Semiconductor Research Articles

Hydrotalcites of zinc and titanium as precursors of finely dispersed mixed oxide semiconductors for dye-sensitized solar cells

Hydrotalcites are layered materials whose sheets are constituted by octahedra occupied by two different metals (one divalent and the other tri- or tetravalent) and having oxide or hydroxide at the corners. The excess of positive charge of the sheets is compensated by anions located at the interlamellar space. In the present work we have synthesized two hydrotalcites containing zinc and titanium (Zn/Ti atomic ratio in the gel synthesis 6.25) that differ on the absence or presence of sodium dodecyl sulfate in the interlamellar spaces. Calcination of these hydrotalcites leads to a film of intimately dispersed mixed oxide semiconductor that was used as semiconductor for dye sensitised solar cells. Using a ruthenium polypyridyl complex as dye, two photovoltaic cells constructed by films of mixed oxides derived from hydrotalcite calcination were prepared. The highest efficiency parameters were VOC = 0.63 V, JSC = 2.18 mA cm−2, FF = 0.465, η = 0.64%. These efficiency values are not far from those obtained for an analogous photovoltaic cell prepared using P25 titania.

Impurity vibration modes in II–VI and III–V mixed semiconductors

AbstractThe analysis of vibration frequencies in dilute semiconducting alloys is addressed, as part of a more vast paradigm for the description of lattice dynamics in semiconductor alloys within the “1 bond – 2 mode” concept. Specific cases considered are those of Be impurities in ZnSe (system with high elastic contrast; aspects of impurity clustering) and of (Ga,In)As alloys, exhibiting much smaller elastic contrast and almost vanishing value of the mode splitting parameter, Δ. First‐principles calculations of frequencies are done by Siesta method, using 64‐atom supercells. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Elucidation of Charge Transport and Optical Parameters in the Newly 1CR-dppm Organic Crystalline Semiconductors

As a part of a wide plan concerning the novel organic materials, a set of newly organic semiconductors has been obtained in our laboratories. In this paper, a clear overview was reported for elucidating the preparation processes of the chromium−copper mixed crystals. In line with the novel pyrazole diphenylphosphino-methane (M-dppm) derivatives, two mixed organic semiconductors were obtained: the one cupper apyrazole ring compound 5a, C78H74CrCuN4O3P4 (1CR-dppm) and the two cupper apyrazole ring compound 6a, C78H74CrCu2N4O4P4 (2CR-dppm). The structure of the obtained crystals were examined by the X−ray diffraction, and the structures of the two mentioned compounds were discussed based on elemental analysis (EA), IR, 1H NMR, 31P NMR spectroscopic data, and fast atom bombardment mass spectra. The compounds under investigation show typical semiconductor behavior due to of delocalization of the π-electrons in the structure. The temperature dependence of charge transports and optical parameters for the 5a crys...

Lattice dynamics of II–VI mixed semiconductor ZnS 1− xSe x

Lattice dynamical properties of II–VI compounds having zinc-blende structure have been calculated by three-body shell model. This model incorporates the effect of the short-range repulsive interactions up to and including the second nearest neighbours, in addition to the long-range Coulombic interactions in the frame work of the rigid-shell model with both the ions are polarizable. The model involves in total eleven disposable parameters. Using the above proposed model the phonon dispersion relations for mixed II–VI semiconductor ZnS 1− x Se x are plotted. We find an overall good agreement with the experimental results. The application of the present model has been made to calculate the phonon dispersion relations of ZnS, ZnSe and mixed semiconductor ZnS 1− x Se x . The comparison of the theoretical results with the available experimental has been made along high symmetry directions. A reasonably good agreement is observed between theory and experiments.

Power optimization and performance improvement for embedded Ethernet SOC

Information appliance is the combination of traditional home appliances and the internet technology. In this article, an Ethernet controller system-on-chip (SOC) solution for information appliances is presented. To achieve high performance, the embedded 8 bits 8051 micro control unit (MCU) is optimized by an independent instruction bus and a data bus. Besides, a two-stage pipeline feature is added. Compared with the existing 8051 core, the enhanced one-cycle MCU offers ten times improvement in instruction execution efficiency. Mean- while, the performance of media access control (MAC) circuit is greatly improved by adopting various techniques such as direct memory access (DMA) control, paging strategy, etc. To reduce the power consumption, clock gating, low power supply, and multi-working-clock are adopted. Moreover, to achieve rapid data communication in different clock frequency circuits, a simple ping-pong first in first out (FIFO) circuit is realized. The chip is implemented using TSMC 0.25 μm two-poly four-metal mixed signal complementary metal oxide semiconductor (CMOS) technology. Its die area is 4.8mm × 4.6mm. The test results show that the maximum throughput of Ethernet packets can reach 7 Mb/s while the power consumption is rather low—the working current is just about 200 mA.

Ambipolar charge carrier transport in organic semiconductor blends of phthalocyanine and fullerene

AbstractAmbipolar charge carrier transport is realised in organic field‐effect transistors using mixtures of p‐conductive copper‐phthalocyanine and n‐conductive buckminster fullerene as active layer. These blends are model systems for ambipolar transport with adjustable field‐effect mobilities for electrons and holes by variation of the mixing ratio. Thereby balanced mobilities for both charge‐carrier types are possible.In this paper we discuss the variation of mobility, threshold voltage and electronic energy levels with the mixing ratio. The charge carrier mobilities are strongly reduced upon dilution of the respective conducting phase by the other species. This shows that transport of each carrier species occurs by percolation through the respective phase in the blend. A strong correlation between contact resistance and mobility indicates that carrier injection is diffusion limited. A charge redistribution in the copper‐phthalocyanine causes a hole accumulation at the organic/organic interface and affects thereby the threshold voltage for holes. By analysing the electronic structure using photoelectron spectroscopy no charge transfer was found in the ground state. The common work function of these blends changes linearly between the work functions of the neat materials. Thus a constant ionisation potential for the highest occupied molecular orbitals of the two materials is obtained.Furthermore we have fabricated ambipolar inverters using mixed organic semiconductor layers and compare their behaviour to complementary inverters consisting of discrete p‐ and n‐channel transistors. The experimental findings and concomitant simulations demonstrate the need for balanced elec‐ tron and hole mobilities in order to achieve symmetric inverter characteristics. However, they also reveal the superior performance of true complementary logic inverters towards their ambipolar pendants. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ethernet controller SoC design and its low-power DFT considerations

In this paper, an Ethernet controller SoC solution and its low power design for testability (DFT) for information appliances are presented. On a single chip, an enhanced one-cycle 8-bit micro controller unit (MCU), media access control (MAC) circuit and embedded memories such as static random access memory (SRAM), read only memory (ROM) and flash are all integrated together. In order to achieve high fault coverage, at the same time with low test power, different DFT techniques are adopted for different circuits: the scan circuit that reduces switching activity is implemented for digital logic circuits; BIST-based method is employed for the on-chip SRAM and ROM. According to the fault-modeling of embedded flash, we resort to a March-like method for flash built in self test (BIST). By all means above, the result shows that the fault coverage may reach 97%, and the SoC chip is implemented successfully by using 0.25 μm two-poly four-metal mixed signal complementary metal oxide semiconductor (CMOS) technology, the die area is 4.8×4.6 mm2. Test results show that the maximum throughput of Ethernet packets may reach 7 Mb · s−1.

Linearized force constants method for lattice dynamics in mixed semiconductors

A simple and accurate method of calculating phonon spectra in mixed semiconductors alloys, on the basis of preliminarily (from first principles) relaxed atomic structure, is proposed and tested for (Zn,Be)Se and (Ga,In)As solid solutions. The method uses an observation that the interatomic force constants, calculated ab initio for a number of microscopic configurations in the systems cited, show a clear linear variation of the main (diagonal) values of the interatomic force constants with the corresponding bond length. We formulate simple rules about how to recover the individual 3×3 subblocks of the force constants matrix in their local (bonds-related) coordinate systems and how to transform them into a global (crystal cell-related) coordinate system. Test calculations done for 64-atom supercells representing different concentrations of (Zn,Be)Se and (Ga,In)As show that the phonon frequencies and compositions of eigenvectors are faithfully reproduced in a linearized force constants calculation, as compared to true ab initio calculations.

First-Principles Langevin Molecular Dynamics Studies of Metallic and Semiconductor Clusters: GGA versus LDA Results

The combination of Langevin molecular dynamics for simulated annealing with realistic quantum-mechanical interactions obtained from first principles supercell calculations within the pseudo potential plane-wave method are applied to examine the structural and electronic properties of the following systems i) pure and mixed GenTem (n, m = 0 - 2), Ge3, Te3 and Ge3Te3 semiconductor clusters, and ii) bimetallic PbNan (n ≤ 7) clusters. Results using the local density approximation (LDA) and the generalized gradient approximation (GGA) for the exchange correlation functional are compared with available experimental data. It is found that the GGA leads to larger bond lengths and smaller binding energies than the LDA, but both approximations lead to the same group symmetry for the ground state of these clusters. Two isomeric configurations for PbNa6, having C3v and Oh symmetries, respectively, are found. The dipole moments of semiconductor clusters are compared with those estimated from recent experiments at room temperature. The dipole moments from GGA calculations are, on average, in better agreement with experiments than those calculated with the LDA.

Morphological, optical and electrical properties of γ CuCl deposited by vacuum evaporation

In this study, we have investigated the microstructural and optical properties of thin films of CuCl grown by vacuum evaporation on amorphous silica glass, polycrystalline indium tin oxide (ITO) and Silicon (100) substrates. Room temperature X-ray diffraction measurements reveal that CuCl grows preferentially with a (111) orientation irrespective of the substrate. Atomic force microscopy measurements showed similar surface topologies and roughness for CuCl/Si, CuCl/ITO and CuCl/Glass structures. Photoluminescence measurements at 20 K revealed the bound bi-exciton N1, impurity bound exciton I1 and the free exciton Z3 occurring at 3.152 ± 0.002 eV, 3.18 eV, and 3.204 eV, respectively, for all three structures. In addition to this, temperature dependent impedance measurements using irreversible electrodes (Au) showed that the deposited CuCl is a mixed ionic and electronic semiconductor with a predominantly extrinsic cationic conduction mechanism at temperatures above ∼280 K and with electronic conduction dominant at temperatures below ∼265 K.

Hydrogen generation at irradiated oxide semiconductor–solution interfaces

This review focuses on the use of inorganic oxide semiconductors for the photoassisted generation of hydrogen from water. Representative studies spanning approximately three decades are included in this review. The topics covered include a discussion of the types of water photosplitting approaches, an ideal photoelectrolysis system, an examination of why oxide semiconductors are attractive for this application, a review of both classical and more recent studies on titanium dioxide, tungsten trioxide, and other binary metal oxides, perovskites and other ternary oxides, tantalates and niobates, miscellaneous multinary oxides, semiconductor alloys and mixed semiconductor composites, and twin-photosystem configurations for water splitting.

Growth and Atomistic Structure Study of Disordered SiGe Mixed Semiconductors

The atomistic structure of Czochralski-grown SixGe1-x binary mixed semiconductor was studied experimentally and theoretically. By extended X-ray absorption fine structure (XAFS) studies it was found that bulk SiGe semiconductor is a random mixture and that the Ge-Ge, Ge-Si and Si-Si bond lengths maintain distinctly different lengths and vary in a linear fashion against the alloy composition across the whole composition range 0 < x < 1, in good agreement with expectations derived from the ab-inito electronic structure calculations. The result indicates that SiGe is a suitable model for a disorder mixed material and that the bond lengths and bond angles are distorted with the composition.

Sol-gel synthesis, characterization and photocatalytic activity of mixed oxide ZnO-Fe2O3

Synthesis by sol-gel method of mixed semiconductor ZnO-Fe2O3 was carried out using inorganic salts as precursors. The powders were synthesized at different pH (neutral and basic) in order to study its influence on the properties of the catalysts. These materials were characterized by thermal analysis TGA-DTA, X-ray Diffraction, FTIR spectroscopy, scanning electron microscopy and X-ray energy dispersive spectroscopy. The specific surface area of the solids was calculated from nitrogen adsorption isotherms by BET method. The band gap energy (E g ) of the catalysts was obtained from it UV-Vis spectra (diffuse reflectance) when the crystalline phase was formed under thermal treatment at 350°C by 3 h. The incorporation of Fe2O3 to ZnO semiconductor gives as a result a lower E g value for zinc oxide. Catalytic activity of the materials was evaluated using as test reaction the photocatalytic degradation of potassium cyanide in aqueous dissolution. The catalyst ZnO-Fe2O3 synthesized at pH 7 turned out to be superior than sol-gel and commercial ZnO under similar conditions.

Long-range pair transport in graded band gap and its applications

The transport of electron–hole pairs in graded band gap of mixed crystal semiconductors is examined. It is estimated that a pair can be transferred for about 30 μm within its lifetime in the band gap gradient of 1 meV/μm in the low-temperature AlGaAs quantum well. Various opto-electronic devices by means of the pair transport are proposed. As one of exciting applications, the author proposes the quantum up-converter, where incoherent low-energy photons are converted to high-energy photons.

Revealing ionic motion molecular solids

Thin films of mixed valence semiconductor copper-tetracyano-quinodimethane (Cu-TCNQ) and small molecule tris(8-hydroxyquinolinato)aluminum (Alq3) were investigated by current-voltage (I-V) and admittance (C-V,G-V) techniques in single-layer configuration with different combinations of electrodes. The predicted hysteresis of I-V curves and nearly constant loss (NCL) could be observed for both materials. When cycling devices between negative and positive biases, slowly decaying ionic transient currents interfere with steady-state currents and point to unidirectional motions of the ionic species with subsequent redox reaction at one of the electrodes. Plotting the frequency f dependence of the equivalent parallel capacitance at zero bias as log10C(0) versus log10(f), the dielectric behavior of Cu-TCNQ and Alq3 complies with the effective-medium model for NCL in ionic conductors [J. R. Macdonald J. Appl. Phys. 94, 558 (2003)]. It also holds for a similar plot of the equivalent parallel zero-bias conductance G(0). The nature of the revealed mobile ions is discussed with emphasis on their sources.

Comparison of gamma radiation performance of a range of CMOS a/D converters under biased conditions

Radiation effects on analog and mixed signal semiconductor devices are a major concern in space and nuclear applications. Significant degradation has been observed in A/D converters in a radiation environment. In fact, recent advances in A/D converters have further reduced the radiation tolerance of these converters on account of their complex circuitry. This paper studies the effects of gamma radiation on several properties (offset error, gain error, integral non linearity etc.) of a whole range of A/D converters under biased conditions. A comparative study of their performance is presented. Four major types of A/D converters, namely- flash, successive approximation, integrating and sigma-delta type, are selected for a detailed evaluation and comparison. Significant degradation of the power supply currents was observed in the biased samples during radiation. Significant improvement in radiation tolerance after adjustment of the offset and calibration registers in the high resolution sigma delta converters can be achieved. All the results and experimental observations are presented

NO x gas response characteristics of thin film mixed oxide semiconductor

Many metal oxide gas sensors have been developed for improving stability during last several decades. In this paper, we proposed the hot-wire type micro-gas sensor with single electrode for improving sensitivity and stability. This type sensor has only a single electrode by forming a sensing material onto Pt heater so the resistance of the single electrode is very low for parallel values of Pt heater resistor and sensing materials of mixed oxide semiconductor. Mixed oxide (Sn–In)O + Pt with various composition Sn–In = 1:1, 2:1, 5:1 and 10:1 for sensing materials were prepared. The fabricated sensor showed high sensitivity for NO x at low gas concentration and good selectivity for CO and HC gases.

Lattice dynamics of the mixed semiconductors (Be,Zn)Se from first-principles calculations

Vibration properties of Zn(1-x)Be(x)Se, a mixed II-VI semiconductor haracterized by a high contrast in elastic properties of its pure constituents, ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation and frozen phonons. The calculations within the local density approximation has been done with the Siesta method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure endsystems were moreover done also by all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes early detected in Raman spectra in the intermediate region (20 to 80%) of ZnBe concentration. This was early discussed on the basis of a percolation phenomenon, i.e., the result of the formation of wall-to-wall --Be--Se-- chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of Be--Se bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence on the short-range order.

Near Field Spectroscopic Investigation of Fluorescence Quenching by Charge Carriers in Pentacene-Doped Tetracene

The recently introduced technique of near field scanning optical microscopy (NSOM) with an electrically biased NSOM probe is applied in this paper to the study of exciton/polaron interactions in single crystals of the mixed organic semiconductor, tetracene doped with pentacene. As in the case of the previously investigated conjugated polymers, the injection of hole polarons induces extreme fluorescence quenching for tetracene/pentacene. However, the spectral selectivity, dynamics, and other features of the tetracene/pentacene data suggest a different mechanism than conjugated polymer for hole polaron interactions. The tetracene/pentacene mechanism is dominated by hole trapping by pentacene.

Localization of excitons in Zn1−xMgxSe and Cd1−xMgxSe crystals

Photoluminescence characteristics as a function of temperature were investigated for Zn1−xMgxSe (0<x<0.63) and Cd1−xMgxSe (0<x<0.55) crystals grown by the high pressure Bridgman method. In the composition range investigated Cd1−xMgxSe crystallizes in a wurtzite structure while Zn1−xMgxSe crystallizes in sphelerite and wurtzite for Mg content lower and higher than 0.18, respectively. The temperature dependence of luminescence can be explained by taking into account, that statistical fluctuations of local composition in solid solutions cause spatial fluctuations of potential, leading to localization of excitons at low temperatures. This phenomenon strongly influences radiative recombination processes in such mixed semiconductors. All observed luminescence characteristics can be explained by this exciton localization model.