Copper phthalocyanine organic thin-film transistors with calcium fluoride gate insulator

We report the fabrication of organic thin-film transistors (OTFTs) with copper phthalocyanine (CuPc) as the semiconductor and calcium fluoride (CaF2) as the gate dielectric on the glass substrate. The fabricated transistors show a gate voltage dependent carrier field effect mobility that ranges from 0.001 to 0.5 cm(2) V-1 s(-1). In the devices, the CaF2 dielectric is formed by thermal evaporation; thus OTFTs with a top-gate structure can be fabricated. This provides a convenient way to produce high-performance OTFTs on a large scale and should be useful for the integration of organic displays.

Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F16CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F16CuPc films are confirmed respectively.

A meso elastoplastic constitutive model for polycrystalline metals based on equivalent slip systems with latent hardening

A meso material model for polycrystalline metals is proposed, in which the tiny slip systems distributing randomly between crystal slices in micro-grains or on grain boundaries are replaced by macro equivalent slip systems determined by the work-conjugate principle. The elastoplastic constitutive equation of this model is formulated for the active hardening, latent hardening and Bauschinger effect to predict macro elastoplastic stress-strain responses of polycrystalline metals under complex loading conditions. The influence of the material property parameters on size and shape of the subsequent yield surfaces is numerically investigated to demonstrate the fundamental features of the proposed material model. The derived constitutive equation is proved accurate and efficient in numerical analysis. Compared with the self-consistent theories with crystal grains as their basic components, the present theory is much simpler in mathematical treatment.

Simulations of Mechanical Behavior of Polycrystalline Copper with Nano-Twins

Mechanical behavior and microstructure evolution of polycrystalline copper with nano-twins were investigated in the present work by finite element simulations. The fracture of grain boundaries are described by a cohesive interface constitutive model based on the strain gradient plasticity theory. A systematic study of the strength and ductility for different grain sizes and twin lamellae distributions is performed. The results show that the material strength and ductility strongly depend on the grain size and the distribution of twin lamellae microstructures in the polycrystalline copper.

An anisotropic elastic-plastic constitutive model for single and polycrystalline metals. II—Experiments and predictions concerning thin-walled tubular OFHC copper

Predictions based on an anisotropic elastic-plastic constitutive model proposed in the first part of this paper are compared with the experimental stress and strain data on OHFC copper under first torsion to about 13% and partial unloading, and then tension-torsion to about 10% along eight different loading paths. This paper also describes the deformation and stress of the thin-walled tubular specimen under finite deformation, the numerical implementation of the model, and the detailed procedure for determining the material parameters in the model. Finally, the model is extended to a general representation of the multiple directors, and the elastic-viscoplastic extension of the constitutive model is considered.

An anisotropic elastic-plastic constitutive model for single and polycrystalline metals .1. Theoretical developments

An anisotropic elastic-plastic constitutive model for single and polycrystalline metals is proposed. The anisotropic hardening of single crystals, at first, is discussed with the viewpoint of yield surface and a new formulation of it is proposed. Then, a model for the anisotropic hardening of polycrystals is suggested by increasing the number of slip systems and incorporating the interaction of all slip systems. The interaction of grains through grain boundaries is shown to be similar to, and incorporated into, the interaction of slip systems in grains. The numerical predictions and their comparisons with experiments will follow in Part II of this paper.

Existence of the transverse relaxation time in optically excited bulk semiconductors

Two basic types of depolarization mechanisms, carrier-carrier (CC) and carrier-phonon (CP) scattering, are investigated in optically excited bulk semiconductors (3D), in which the existence of the transverse relaxation time is proven based on the vector property of the interband transition matrix elements. The dephasing rates for both CC and CP scattering are determined to be equal to one half of the total scattering-rate-integrals weighted by the factors (1 - cos chi), where chi are the scattering angles. Analytical expressions of the polarization dephasing due to CC scattering are established by using an uncertainty broadening approach, and analytical ones due to both the polar optical-phonon and non-polar deformation potential scattering (including inter-valley scattering) are also presented by using the sharp spectral functions in the dephasing rate calculations. These formulas, which reveal the trivial role of the Coulomb screening effect in the depolarization processes, are used to explain the experimental results at hand and provide a clear physical picture that is difficult to extract from numerical treatments.

Passive Q-switching of short-length Tm3+-doped silica fiber lasers by polycrystalline Cr2+:ZnSe microchips

We demonstrate passive Q-switching of short-length double-clad Tm3+-doped silica fiber lasers near 2 mu m pumped by a laser diode array (LDA) at 790 nm. Polycrystalline Cr2+:ZnSe microchips with thickness from 0.3 to 1 mm are adopted as the Q-switching elements. Pulse duration of 120 ns, pulse energy over 14 mu] and repetition rate of 53 kHz are obtained from a 5-cm long fiber laser. As high as 530 kHz repetition rate is achieved from a 50-cm long fiber laser at similar to 10-W pump power. The performance of the Q-switched fiber lasers as a function of fiber length is also analyzed. (c) 2008 Elsevier B.V. All rights reserved.

Synthesis and magnetic properties of Ni-doped zinc oxide powders

Polycrystalline Zn1-xNixO diluted magnetic semiconductors have been successfully synthesized by an auto-combustion method. X-ray diffraction measurements indicated that the 5 at% Ni-cloped ZnO had the pure wurtzite structure. Refinements of cell parameters from powder diffraction data revealed that the cell parameters of Zn0.95Ni0.05O were a little bit larger than ZnO. Transmission electron microscopy observation showed that the as-synthesized powders were of the size similar to 60 nm. Magnetic investigations showed that the nanocystalline Zn0.95Ni0.05O possessed room temperature ferromagnetisin with the saturation magnetic moment of 0.1 emu/g (0.29 mu(B)/Ni2+). (c) 2005 Elsevier B.V. All rights reserved.

Effect of Copassivation of Cl and Cu on CdTe Grain Boundaries

Using a first-principles method, we investigate the structural and electronic properties of grain boundaries (GBs) in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this Letter, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs.

Proposal for the direct optical detection of pure spin currents in semiconductors

We suggest a different practical scheme for the direct detection of pure spin current by using the two-color Faraday rotation of optical quantum interference process (QUIP) in a semiconductor system. We demonstrate theoretically that the Faraday rotation of QUIP depends sensitively on the spin orientation and wave vector of the carriers, and can be tuned by the relative phase and the polarization direction of the omega and 2 omega laser beams. By adjusting these parameters, the magnitude and direction of the spin current can be detected.

Effects of coupling lens on optical refrigeration of semiconductors

Optical refrigeration of semiconductors is encountering efficiency difficulties caused by nonradiative recombination and luminescence trapping. A commonly used approach for enhancing luminescence efficiency of a semiconductor device is coupling a lens with the device. We quantitatively study the effects of a coupling lens on optical refrigeration based on rate equations and photon recycling, and calculated cooling efficiencies of different coupling mechanisms and of different lens materials. A GaAs/GaInP heterostructure coupled with a homo-epitaxial GaInP hemispherical lens is recommended.

Doped polycrystalline 3C-SiC films deposited by LPCVD for radio-frequency MEMS applications

Polycrystalline 3C-SiC films are deposited on SiO2 coated Si substrates by low pressure chemical vapour deposition (LPCVD) with C3H8 and SiH4 as precursors. Controlled nitrogen doping is performed by adding NH3 during SiC growth to obtain the low resistivity 3C-SiC films. X-ray diffraction (XRD) patterns indicate that the deposited films are highly textured (111) orientation. The surface morphology and roughness are determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface features are spherulitic texture with average grain size of 100 nm, and the rms roughness is 20nm (AFM 5 x 5 mu m images). Polycrystalline 3C-SiC films with highly orientational texture and good surface morphology deposited on SiO2 coated Si substrates could be used to fabricate rf microelectromechanical systems (MEMS) devices such as SiC based filters.

Revised ab initio natural band offsets of all group IV, II-VI, and III-V semiconductors

Using an all-electron band structure approach, we have systematically calculated the natural band offsets between all group IV, III-V, and II-VI semiconductor compounds, taking into account the deformation potential of the core states. This revised approach removes assumptions regarding the reference level volume deformation and offers a more reliable prediction of the "natural" unstrained offsets. Comparison is made to experimental work, where a noticeable improvement is found compared to previous methodologies.

Origin and Enhancement of Hole-Induced Ferromagnetism in First-Row d(0) Semiconductors

The origin of ferromagnetism in d(0) semiconductors is studied using first-principles methods with ZnO as a prototype material. We show that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row d(0) semiconductors and can be attributed to the localized nature of the 2p states of O and N. We find that acceptor doping, especially doping at the anion site, can enhance the ferromagnetism with much smaller threshold hole concentrations. The quantum confinement effect also reduces the critical hole concentration to induce ferromagnetism in ZnO nanowires. The characteristic nonmonotonic spin couplings in these systems are explained in terms of the band coupling model.