Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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Fundamentals of zinc oxide as a semiconductor

We have performed a comprehensive first-principles investigation of native point defects in ZnO based on density functional theory within the local density approximation (LDA) as well as the $\mathrm{LDA}+U$ approach for overcoming the band-gap problem. Oxygen deficiency, manifested in the form of oxygen vacancies and zinc interstitials, has long been invoked as the source of the commonly observed unintentional $n$-type conductivity in ZnO. However, contrary to the conventional wisdom, we find that native point defects are very unlikely to be the cause of unintentional $n$-type conductivity. Oxygen vacancies, which have most often been cited as the cause of unintentional doping, are deep rather than shallow donors and have high formation energies in $n$-type ZnO (and are therefore unlikely to form). Zinc interstitials are shallow donors, but they also have high formation energies in $n$-type ZnO and are fast diffusers with migration barriers as low as $0.57\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$; they are therefore unlikely to be stable. Zinc antisites are also shallow donors but their high formation energies (even in Zn-rich conditions) render them unlikely to be stable under equilibrium conditions. We have, however, identified a different low-energy atomic configuration for zinc antisites that may play a role under nonequilibrium conditions such as irradiation. Zinc vacancies are deep acceptors and probably related to the frequently observed green luminescence; they act as compensating centers in $n$-type ZnO. Oxygen interstitials have high formation energies; they can occur as electrically neutral split interstitials in semi-insulating and $p$-type materials or as deep acceptors at octahedral interstitial sites in $n$-type ZnO. Oxygen antisites have very high formation energies and are unlikely to exist in measurable concentrations under equilibrium conditions. Based on our results for migration energy barriers, we calculate activation energies for self-diffusion and estimate defect-annealing temperatures. Our results provide a guide to more refined experimental studies of point defects in ZnO and their influence on the control of $p$-type doping.

Native point defects in ZnO

Nanomaterials, such as metal or semiconductor nanoparticles and nanorods, exhibit similar dimensions to those of biomolecules, such as proteins (enzymes, antigens, antibodies) or DNA. The integration of nanoparticles, which exhibit unique electronic, photonic, and catalytic properties, with biomaterials, which display unique recognition, catalytic, and inhibition properties, yields novel hybrid nanobiomaterials of synergetic properties and functions. This review describes recent advances in the synthesis of biomolecule-nanoparticle/nanorod hybrid systems and the application of such assemblies in the generation of 2D and 3D ordered structures in solutions and on surfaces. Particular emphasis is directed to the use of biomolecule-nanoparticle (metallic or semiconductive) assemblies for bioanalytical applications and for the fabrication of bioelectronic devices.

Integrated Nanoparticle–Biomolecule Hybrid Systems: Synthesis, Properties, and Applications

The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Summary form only given. Hafnium silicate (HfSi/sub x/O/sub y/) is being studied as a potential candidate material to replace SiO/sub 2/ as the gate dielectric in scaled silicon transistors due to its relatively high dielectric constant and high thermal stability. The dielectric constant of HfSi/sub x/O/sub y/ is /spl sim/8-15, depending on composition, which is 2-4 times higher than SiO/sub 2/. In this paper we describe the formation and characterization of hafnium silicate by UV/O/sub 3/ oxidation of hafnium silicide as a potential gate dielectric candidate for high performance thin-film transistors on plastic substrates.

Low temperature deposition of hafnium silicate gate dielectrics for TFTs on plastic substrates

A permeation barrier system that could self-heal upon the influx of moisture through defects and subsequently seal the cracks would find potential applications in multilayer composite materials. Nanoporous polylactic acid (PLA) spheres and fibers were prepared by electrospraying and electrospinning respectively. The porous spheres and fibers were exposed to titanium (IV) chloride to introduce the metal oxide precursor into the pores and subsequently seal by annealing and solvent treatment.

Self-healing of metal oxide composite materials

Energy loss of channeled ions and ion-induced electron emission (IIEE) are principally governed by electronic stopping. The authors report experimental observations of the angular dependence of the energy loss and the corresponding IIEE yields of channeled protons near the 〈100〉 direction in a thin silicon crystal. The Rutherford backscattering spectrometry (RBS) yield drops to a minimum in the channeled condition as also do the transmitted-ion energy loss and the IIEE yields (forward, backward and total). For 1 MeV protons, the effective IIEE yields in the channeled condition are reduced in approximately the same proportion as the electronic stopping power. The present data support a modification of Schou's theory of IIEE.

Correlation between energy loss and ion-induced electron emission of 1 MeV protons channeled along the 〈100〉 axis of a thin silicon crystal

Embodiments of vicinity sensor systems are described herein. Other embodiments and related methods are also disclosed herein.

Vicinity sensor systems and related methods

A thin film radiation detection device includes a photosensitive p-n diode (102), a polysilicon thin film transistor (TFT) (104), a radiation detection layer (106), and a substrate (108). The photosensitive p-n diode and the TFT are formed on the substrate. The radiation detection layer is formed above the substrate and receives multiple radiations. The photosensitive p-n diode receives a conversion output signal from the radiation detection layer and generates a detector signal. The TFT generates an amplified signal based on the detector signal. A first portion (110a) of a polysilicon layer is the cathode of the photosensitive diode and a second portion (110b) of said polysilicon layer is the gate of the TFT.

Bruce E. Gnade

Papers

Low temperature deposition of hafnium silicate gate dielectrics for TFTs on plastic substrates

Self-healing of metal oxide composite materials

Correlation between energy loss and ion-induced electron emission of 1 MeV protons channeled along the 〈100〉 axis of a thin silicon crystal

Vicinity sensor systems and related methods

Method for the integration of monlithic thin film radiation detector systems