Quasi-non-uniform gradient-level exchange-correlation approximation for metals and alloys

The flexibility of the common generalized gradient approximation for the exchange-correlation energy is investigated by monitoring the equilibrium volume of transition metals. It is shown that no universal gradient-level approximation yielding consistent errors for all metals exists. Based on an element-specific optimization, the concept of quasi-non-uniform gradient-level approximation is introduced. The strength of the scheme is demonstrated on several transition-metal alloys

Magnetic origin of the chemical balance in alloyed Fe-Cr stainless steels: First-principles and Ising model study

Iron-chromium is the base material for most of the stainless steel grades. Recently, new insights into the origins of fundamental physical and chemical characteristics of Fe-Cr based alloys have been achieved. Some of the new results are quite unexpected and call for further investigations. The present study focuses on the magnetic contribution in the atomic driving forces related to the chemical composition in Fe-Cr when alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab initio exact muffin-tin orbitals method combined with an Ising-type spin model, we demonstrate that the magnetic moment of the solute atoms with the induced changes in the magnetic moments of the host atoms form the main factor in determining the mixing energy and chemical potentials of low-Cr Fe-Cr based alloys. The results obtained in the present work are related to the designing and tuning of the microstructure and corrosion protection of low-Cr steels. (C) 2014 Elsevier B. V. All rights reserved.</p

Crystalline and oxide phases revealed and formed on InSb(111)B

Oxidation treatment creating a well-ordered crystalline structure has been shown to provide a major improvement for III–V semiconductor/oxide interfaces in electronics. We present this treatment’s effects on InSb(111)B surface and its electronic properties with scanning tunneling microscopy and spectroscopy. Possibility to oxidize (111)B surface with parameters similar to the ones used for (100) surface is found, indicating a generality of the crystalline oxidation among different crystal planes, crucial for utilization in nanotechnology. The outcome is strongly dependent on surface conditions and remarkably, the (111) plane can oxidize without changes in surface lattice symmetry, or alternatively, resulting in a complex, semicommensurate quasicrystal-like structure. The findings are of major significance for passivation via oxide termination for nano-structured III–V/oxide devices containing several crystal plane surfaces. As a proof-of-principle, we present a procedure where InSb(111)B surface is cleaned by simple HCl-etching, transferred via air, and post-annealed and oxidized in ultrahigh vacuum

Unusual oxidation-induced core-level shifts at the HfO2/InP interface

X-ray photoelectron spectroscopy (XPS) is one of the most used methods in a diverse field of materials science and engineering. The elemental core-level binding energies (BE) and core-level shifts (CLS) are determined and interpreted in the XPS. Oxidation is commonly considered to increase the BE of the core electrons of metal and semiconductor elements (i.e., positive BE shift due to O bonds), because valence electron charge density moves toward electronegative O atoms in the intuitive charge-transfer model. Here we demonstrate that this BE hypothesis is not generally valid by presenting XPS spectra and a consistent model of atomic processes occurring at HfO2/InP interface including negative In CLSs. It is shown theoretically for abrupt HfO2/InP model structures that there is no correlation between the In CLSs and the number of oxygen neighbors. However, the P CLSs can be estimated using the number of close O neighbors. First native oxide model interfaces for III-V semiconductors are introduced. The results obtained from ab initio calculations and synchrotron XPS measurements emphasize the importance of complementary analyses in various academic and industrial investigations where CLSs are at the heart of advancing knowledge

GaSbBi alloys and heterostructures: fabrication and properties

International audienceDilute bismuth (Bi) III-V alloys have recently attracted great attention, due to their properties of band-gap reduction and spin-orbit splitting. The incorporation of Bi into antimonide based III-V semiconductors is very attractive for the development of new optoelectronic devices working in the mid-infrared range (2-5 µm). However, due to its large size, Bi does not readily incorporate into III-V alloys and the epitaxy of III-V dilute bismides is thus very challenging. This book chapter presents the most recent developments in the epitaxy and characterization of GaSbBi alloys and heterostructures

Molecular Beam Epitaxy Growth and Properties of GaAsBi and AlAsBi

GaAsBi alloys have been extensively studied in recent years, and the highest Bi concentration yet reached has been 22 %. Many photoelectric devices using this material have been produced, such as quantum well lasers, LEDs, solar cells, etc. The Bi incorporated into AlAs is expected to change the bandgap from indirect to direct. There are only a few theoretical reports on AlAsBi, however, experimental research results are seldom reported. In this chapter, we review the molecular beam epitaxy of GaAsBi and analyze the growth mechanism. Besides, we present the synthesis of AlAsBi by molecular beam epitaxy. The growth temperature, As/Ga flux ratio, Bi flux and the growth rate all have great influence on the Bi incorporation. Bismuth atoms play a surfactant role under As-rich conditions and an anti-surfactant role under Ga-rich conditions. Droplets tend to be formed on the surface of GaAsBi alloys due to the atomic size mismatch between Bi atoms and As atoms. The high-angle annular dark-field mode of scanning transmission electron microscopy images confirm Bi atoms cluster exsiting in GaAsBi films. Furthermore, we show the optical properties of GaAsBi and discuss the localized states induced by Bi. The photoluminescence wavelength of GaAsBi redshifts with increasing Bi concentration. The bandgap of GaAsBi is insensitive to temperature, which is important for developing un-cooled lasers. We discuss the influence of Bi incorporation on the electric and transport properties of GaAsBi. The types of dominant point defects induced by Bi incorporation are analyzed. The measurement results of the electron effective mass demonstrate that Bi incorporation not only changes the valence band but also has non-negligible influence on the conduction band in GaAsBi. For AlAsBi, we review the theoretical simulations and present the molecular beam epitaxy growth without substrate rotaion to investigate the influence of\ua0As/Al flux raio and the Bi flux on\ua0Bi incorporation