Controlled oxide removal for the preparation of damage-free InAs(110) surfaces

Controlled oxide removal from InAs(110) surfaces using atomic hydrogen (H*) has been achieved by monitoring the contaminant vibrational modes with high resolution electron energy loss spectroscopy (HREELS). The contributing oxide vibrational modes of the partially H* cleaned surface have been identified. Following hydrocarbon desorption during preliminary annealing at 360 °C, exposure to atomic hydrogen at 400 °C initially removes the arsenic oxides and indium suboxides; complete indium oxide removal requires significantly higher hydrogen doses. After a total molecular hydrogen dose of 120 kL, a clean, ordered surface, exhibiting a sharp (1×1) pattern, was confirmed by low energy electron diffraction and x-ray photoelectron spectroscopy. Energy dependent HREELS studies of the near-surface electronic structure indicate that no residual electronic damage or dopant passivation results from the cleaning process

Propagation of Ultrasonic Waves in Liquid Mixtures and Intermolecular Forces II

The electronic properties of clean and sulfur-terminated surfaces of InSb(001) and (111) B are investigated using x-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy. The clean surfaces exhibit upward band bending (electron depletion) consistent with the charge neutrality level in InSb lying at the valence band maximum. The surface Fermi level to valence band maximum separation is increased for the S terminated compared with the clean surface, leading to flat bands and downward band bending (electron accumulation) for the (001) and (111) B surfaces, respectively. This is discussed in terms of compensation of native acceptor surface states. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000567

Increased p-type conductivity in GaNxSb1−x, experimental and theoretical aspects

The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaN x Sb 1− x layers grown by molecular beam epitaxy, and the results have been compared with calculated first-principle results corresponding to different defect structures. From the calculated data, binding energies for nitrogen-related defects have also been estimated. Based on the results, the increase in residual hole concentration is explained by an increase in the fraction of negative acceptor-type defects in the material. As the band gap decreases with increasing N concentration, the ionization levels of the defects move closer to the valence band. Ga vacancy-type defects are found to act as positron trapping defects in the material, and the ratio of Ga vacancy-type defects to Ga antisites is found to be higher than that of the p-type bulk GaSb substrate. Beside Ga vacancies, the calculated results imply that complexes of a Ga vacancy and nitrogen could be present in the material

Band gap reduction in GaNSb alloys due to the anion mismatch

The structural and optoelectronic properties in GaNxSb1–x alloys (0<=x<0.02) grown by molecular-beam epitaxy on both GaSb substrates and AlSb buffer layers on GaAs substrates are investigated. High-resolution x-ray diffraction (XRD) and reciprocal space mapping indicate that the GaNxSb1–x epilayers are of high crystalline quality and the alloy composition is found to be independent of substrate, for identical growth conditions. The band gap of the GaNSb alloys is found to decrease with increasing nitrogen content from absorption spectroscopy. Strain-induced band-gap shifts, Moss-Burstein effects, and band renormalization were ruled out by XRD and Hall measurements. The band-gap reduction is solely due to the substitution of dilute amounts of highly electronegative nitrogen for antimony, and is greater than observed in GaNAs with the same N content

Growth and properties of GaSbBi alloys

Molecular-beam epitaxy has been used to grow GaSb 1− x Bi x alloys with x up to 0.05. The Bi content, lattice expansion, and film thickness were determined by Rutherford backscattering and x-ray diffraction, which also indicate high crystallinity and that >98% of the Bi atoms are substitutional. The observed Bi-induced lattice dilation is consistent with density functional theory calculations. Optical absorption measurements and valence band anticrossing modeling indicate that the room temperature band gap varies from 720 meV for GaSb to 540 meV for GaSb 0.95Bi0.05, corresponding to a reduction of 36 meV/%Bi or 210 meV per 0.01 Å change in lattice constant

Valence band offset of the ZnO/AlN heterojunction determined by X-ray photoemission spectroscopy

The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values

High Bi content GaSbBi alloys

The epitaxial growth, structural, and optical properties of GaSb 1– x Bi x alloys have been investigated. The Bi incorporation into GaSb is varied in the range 0 < x ≤ 9.6% by varying the growth rate (0.31–1.33 μm h−1) at two growth temperatures (250 and 275 °C). The Bi content is inversely proportional to the growth rate, but with higher Bi contents achieved at 250 than at 275 °C. A maximum Bi content of x = 9.6% is achieved with the Bi greater than 99% substitutional. Extrapolating the linear variation of lattice parameter with Bi content in the GaSbBi films enabled a zinc blende GaBi lattice parameter to be estimated of 6.272 Å. The band gap at 300 K of the GaSbBi epitaxial layers decreases linearly with increasing Bi content down to 410 ± 40 meV (3 μm) for x = 9.6%, corresponding to a reduction of ∼35 meV/%Bi. Photoluminescence indicates a band gap of 490 ± 5 meV at 15 K for x = 9.6%

Evidence of a second-order Peierls-driven metal-insulator transition in crystalline NbO2

The metal-insulator transition of NbO2 is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed NbOx that may be inherently different to crystalline NbO2. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial NbO2 thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at ~810⁰C. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline NbO2 as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap