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

A large amount of work world-wide has been directed towards obtaining an understanding of the fundamental characteristics of porous Si. Much progress has been made following the demonstration in 1990 that highly porous material could emit very efficient visible photoluminescence at room temperature. Since that time, all features of the structural, optical and electronic properties of the material have been subjected to in-depth scrutiny. It is the purpose of the present review to survey the work which has been carried out and to detail the level of understanding which has been attained. The key importance of crystalline Si nanostructures in determining the behaviour of porous Si is highlighted. The fabrication of solid-state electroluminescent devices is a prominent goal of many studies and the impressive progress in this area is described.

The structural and luminescence properties of porous silicon

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

We developed two-step solution-phase reactions to form hybrid materials of Mn3O4 nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Mn3O4 nanoparticles grown selectively on RGO sheets over free particle growth in solution allowed for the electrically insulating Mn3O4 nanoparticles wired up to a current collector through the underlying conducting graphene network. The Mn3O4 nanoparticles formed on RGO show a high specific capacity up to ~900mAh/g near its theoretical capacity with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn3O4 nanoparticles grown atop. The Mn3O4/RGO hybrid could be a promising candidate material for high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for design and synthesis of battery electrodes based on highly insulating materials.

Mn3O4-Graphene Hybrid as a High Capacity Anode Material for Lithium Ion Batteries

Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

Wide-bandgap semiconductor ultraviolet photodetectors

This research investigates the impacts of the AlGaN barrier layer thickness and the Al mole fraction on p-GaN gate AlGaN/GaN HEMTs and presents an optimized structure. Based on 2-D drift-diffusion simulation, the effects of the trap level in the UID GaN buffer layer on the transfer and output characteristics of the optimized device are described. The energies of the trap levels are set at 0.28, 0.33, 0.4, 0.58, and 0.9 eV below the conduction band minimum, respectively. The depth of the trap level is found to influence the off-state leakage current and on-state ID,max. The Shockley–Read–Hall recombination model for a single trap level is used to analyze the impact of different trap levels in the UID GaN buffer on p-GaN gate AlGaN/GaN HEMTs.

Effects of the Trap Level in the Unintentionally Doped GaN Buffer Layer on Optimized p-GaN Gate AlGaN/GaN HEMTs

We investigate the electrical and optical properties of graphene–Ag nanodot compounds as transparent conductive layers (TCLs) for gallium nitride (GaN)-based ultraviolet light-emitting diodes (UV-LEDs) with different Ag thicknesses As the thickness of Ag increases from 2 to 5 nm on the 365-nm UV-LEDs, the transmittance decreases from 825% to 652% and ohmic contact resistance decreases from 02 to ${13}{\times} {10}^{-5} \Omega \text {cm}^{2}$  At a wavelength of about 320 nm, the TCLs exhibit high transparency, which is also confirmed by the simulation data of the finite-difference time-domain solution The simulation and experimental results suggest that Ag with a thickness of 5 nm can potentially obtain both low ohmic contact resistance and high transmittance at the wavelength of about 320 nm, which is important for medical applications We also study the nanodot migration and coalescence mechanism of 2- and 5-nm Ag samples by scanning electron microscopy, as well as the influence of the existence of graphene The 5-nm Ag sample has smaller and denser Ag nanodots and thus exhibits better electrical properties In addition, the Ag nanodots after the second annealing procedure with graphene covering are larger than those without graphene covering

The Effect of the Original Thickness of Ag in the Graphene–Ag Nanodots Transparent Conductive Layer on the Electrical and Optical Properties of GaN-Based UV-LEDs

Multi-aperture anode based AlGaN/GaN Schottky barrier diodes with low turn-on voltage and high uniformity

In this letter, we demonstrated a normally-off AlGaN/GaN HEMT using p-NiO as a gate stack combined with a recess structure. The fabricated HEMT exhibits a positive threshold voltage of 1.73 V, a saturation output current of 524 mA/mm, a small subthreshold swing of 79.7 mV/dec and a maximum transconductance as high as 143 mS/mm. This is the first time to demonstrate the breakdown characteristics of a p-NiO gate HEMT with a high breakdown voltage of 1205 V and a low specific ON-resistance of 2.22 <inline-formula> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula>cm<sup>2</sup>, yielding a competitive Baliga’s figure-of-merit of 0.65 GW/cm<sup>2</sup>. The instability evaluation of <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> by step stress and pulse transfer curves shows that the p-NiO gate HEMT has a negligible <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> shift in the entire measured gate bias range, which can be attributed to the counteraction between the electron trapping-induced positive <inline-formula> <tex-math notation="LaTeX">$\text{V}_{\text {TH}}$ </tex-math></inline-formula> shift and hole accumulation induced-negative <inline-formula> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> shift. It is well understood in terms of the carrier transport model based on the large band discontinuity at the interface of the p-NiO/AlGaN type-II heterojunction, which is further verified by transient gate current spectra.

Over 1200 V Normally-OFF p-NiO Gated AlGaN/GaN HEMTs on Si With a Small Threshold Voltage Shift

The III–V ternary semiconductors In x Ga1−x As (0 ≤ x ≤ 1) have attracted a great deal of interests in electronics and optoelectronics due to their superior transport properties, high-efficiency thermoelectric applications and so on. In this work, the optoelectronic properties of bulk In x Ga1−x As materials and the phase transition of In x Ga1−x As nanowires (NWs) at different Indium (In) component have been investigated by first-principles calculation using density functional theory. Different calculation models have been chosen to simulate the (2 × 2 × 1) supercells of pure bulk In x Ga1−x As structures and the most stable one has been adopted for further calculation. NWs models of In0.25Ga0.75As and In0.75Ga0.25As have been employed to represent the In-rich and Ga-rich structures, respectively. Our calculation results have clearly shown that the In-rich nanostructure show more zinc-blende characteristic while the wurtzite phase exhibit dominating role in the Ga-rich NWs. The band gap of bulk models decreases with increasing the In component, which is well consistent with the theoretical formula. With the In component increasing, the lower valence band shifts toward high energy region and the peak value increases, while the absorption edge and peak value move to lower energy side and the static dielectric constant increases. The metal reflective properties emerge in certain photon energy range. Our results offer the guidance to make use of the different properties of In x Ga1−x As materials at different In component.

https://iopscience.iop.org/article/10.1088/2053-1591/aaa7a8/pdf

Youdou Zheng

Papers

Effects of the Trap Level in the Unintentionally Doped GaN Buffer Layer on Optimized p-GaN Gate AlGaN/GaN HEMTs

The Effect of the Original Thickness of Ag in the Graphene–Ag Nanodots Transparent Conductive Layer on the Electrical and Optical Properties of GaN-Based UV-LEDs

Multi-aperture anode based AlGaN/GaN Schottky barrier diodes with low turn-on voltage and high uniformity

Over 1200 V Normally-OFF p-NiO Gated AlGaN/GaN HEMTs on Si With a Small Threshold Voltage Shift

First principles study on the structural stability and optoelectronic properties of In x Ga 1-x As materials with different Indium component