Theory of Zn-enhanced disordering in GaAs/AlAs superlattices

First-Principles Study of Point Defects in GaAs/AlAs Superlattice: the Phase Stability and the Effects on the Band Structure and Carrier Mobility

Advanced semiconductor superlattices play important roles in critical future high-tech applications such as aerospace, high-energy physics, gravitational wave detection, astronomy, and nuclear related areas. Under such extreme conditions like high irradiative environments, these semiconductor superlattices tend to generate various defects that ultimately may result in the failure of the devices. However, in the superlattice like GaAs/AlAs, the phase stability and impact on the device performance of point defects are still not clear up to date. The present calculations show that in GaAs/AlAs superlattice, the antisite defects are energetically more favorable than vacancy and interstitial defects. The AsX (X = Al or Ga) and XAs defects always induce metallicity of GaAs/AlAs superlattice, and GaAl and AlGa antisite defects have slight effects on the electronic structure. For GaAs/AlAs superlattice with the interstitial or vacancy defects, significant reduction of band gap or induced metallicity is found. Further calculations show that the interstitial and vacancy defects reduce the electron mobility significantly, while the antisite defects have relatively smaller influences. The results advance the understanding of the radiation damage effects of the GaAs/AlAs superlattice, which thus provide guidance for designing highly stable and durable semiconductor superlattice based electronic and optoelectronics for extreme environment applications.

Native Defects in Wurtzite GaN And AlN

The results of an extensive theoretical study of native defects in GaN and of vacancies in AlN are presented. We have considered cation and anion vacancies, antisites, and intersti-tials. The computations were carried out using quantum molecular dynamics, in supercells containing 72 atoms. Due to the wide gap of nitrides, the formation energies of defects depend strongly on the position of the Fermi level. The N vacancy in GaN introduces a shallow donor level that may be responsible for the n-type character of as-grown GaN.Other defects introduce deep states in the gap, with strongly localized wave functions.

Simulations of SI(100) Growth: Step Flow and Low Temperature Growth

We have investigated Si (100) homoepitaxy in variety of different temperature regimes. In the high temperature step flow regime, the growth properties of the different steps play an important role. The binding sites and diffusion barriers for a Si adatom moving over the buckled Si(100) surface and single-height steps were investigated with the ab initio Car-Parrinello scheme. The SA step edge was found to be a relatively poor sink for adatoms. By contrast, growth takes place much more rapidly at the SB step edges, so that one can understand the relatively fast growth observed at the ends of dimer rows. We have also investigated Si(100) homoepitaxy with classical molecular dynamics simulations at very low temperatures, where typically an amorphous deposit forms. By tuning the energy of the incoming atomic beams, one can lower the temperature of the amorphous to crystalline transition considerably and thereby enhance epitaxial growth.

Properties of one-dimensional molybdenum nanowires in a confined environment

The atomistic mechanism for the self-assembly of molybdenum into one-dimensional metallic nanowires in a confined environment such as a carbon nanotube is investigated using quantum mechanical calculations. We find that Mo does not organize into linear chains but rather prefers to form four atom per unit cell nanowires that consist of a subunit of a Mo body-centered cubic crystal. Our model explains the 0.3 nm separation between features measured by high-resolution transmission electron microscopy and why the nanotube diameter must be in the 0.70-1.0 nm range to accommodate the smallest stable one-dimensional wire. We also computed the electronic band structure of the Mo wires inside a nanotube and found significant hybridization with the nanotube states, thereby explaining the experimentally observed quenching of fluorescence and the damping of the radial breathing modes as well as an increased resistance to oxidation.

Solid-State Composite Electrolyte LiI/3-Hydroxypropionitrile/SiO2 for Dye-Sensitized Solar Cells

A new compound, LiI(3-hydroxypropionitrile)(2), is reported here. According to its single-crystal structure (C2/c), this compound has 3-D transporting paths for iodine. Further ab initio calculation shows that the activation energy for diffusion of iodine (0.73 eV) is much lower than that of lithium ion (8.39 eV) within the lattice. Such a mono-ion transport feature is favorable as solid electrolyte to replace conventional volatile organic liquid electrolytes used in dye-sensitized solar cells (DSSC). LiI and 3-hydroxypropionitrile (HPN) can form a series of solid electrolytes. The highest ambient conductivity is 1.4 x 10(-)(3) S/cm achieved for LiI(HPN)(4). However, it tends to form large crystallites and leads to poor filling and contact within porous TiO(2) electrodes in DSSC. Such a drawback can be greatly improved by introducing micrometer-sized and nanosized SiO(2) particles into the solid electrolyte. It is helpful not only in enhancing the conductivity but also in improving the interfacial contact greatly. Consequently, the light-to-electricity conversion efficiency of 5.4% of a DSSC using LiI(HPN)(4)/15 wt % nano-SiO(2) was achieved under AM 1.5 simulated solar light illumination. Due to the low cost, easy fabrication, and relatively high conversion efficiency, the DSSC based on this new solid-state composite electrolyte is promising for practical applications.

Surface segregation of Ge at SiGe(001) by concerted exchange pathways

The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.

Ab initio investigations of lithium diffusion in carbon nanotube systems

Li-nanotube systems can substantially improve the capacity of Li-ion batteries by utilizing both nanotube exteriors and interiors. Our ab initio simulations show that while Li motion through the sidewalls is forbidden, Li ions can enter tubes through topological defects containing at least nine-sided rings, or through the ends of open-ended nanotubes. Once inside, their motion is not diffusion limited. These results suggest that "damaging" nanotube ropes by either chemical or mechanical means will yield superior material for electrochemical storage.