Optical energies of AllnN epilayers

Optical energy gaps are measured for high-quality Al1−xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of 6 eV and differences with earlier reports are discussed. Very large Stokes' shifts of 0.4-0.8 eV are observed in the composition range 0.13<x<0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric fiel

Thin film growth of semiconducting Mg2Si by codeposition

Includes bibliographical references (page 1088).Ultrahigh vacuum evaporation of magnesium onto a hot silicon substrate (⩾200 °C), with the intention of forming a Mg2Si thin film by reaction, does not result in any accumulation of magnesium or its silicide. On the other hand, codeposition of magnesium with silicon at 200 °C, using a magnesium-rich flux ratio, gives a stoichiometric Mg2Si film which can be grown several hundreds of nm thick. The number of magnesium atoms which condense is equal to twice the number of silicon atoms which were deposited; all the silicon condenses while the excess magnesium in the flux desorbs. The Mg2Si layers thus obtained are polycrystalline with a (111) texture. From the surface roughness analysis, a self-affine growth mode with a roughness exponent equal to 1 is deduced

On the growth kinetics of Ni(Pt) silicide thin films

We report on the effect of Pt on the growth kinetics of δ-Ni2Si and Ni 1−xPtxSi thin films formed by solid phase reaction of a Ni(Pt) alloyed thin film on Si(100). The study was performed by real-time Rutherford backscattering spectrometry examining the silicide growth rates for initial Pt concentrations of 0, 1, 3, 7, and 10 at. % relative to the Ni content. Pt was found to exert a drastic effect on the growth kinetics of both phases. δ-Ni2Si growth is slowed down tremendously, which results in the simultaneous growth of this phase with Ni 1−xPtxSi. Activation energies extracted for the Ni 1−xPtxSi growth process exhibit an increase from Ea = 1.35 ± 0.06 eV for binary NiSi to Ea = 2.7 ± 0.2 eV for Ni 1−xPtxSi with an initial Pt concentration of 3 at. %. Further increasing the Pt content to 10 at. % merely increases the activation energy for Ni 1−xPtxSi growth to Ea = 3.1 ± 0.5 eV

Surface quality studies of high Tc_{c} superconductors of the Hg , Tl and Hgx_{x}Tl1−x_{1-x}-families: RBS and resonant C and O backscattering studies

The composition, crystallinity, uniformity, purity, and thermal stability of cuprate superconductors have been studied by Rutherford backscattering and channeling spectrometry, and 3.045 MeV He$^{+}$ oxygen non-Rutherford resonant scattering. Further experiments have been performed with 1.75 MeV H$^{+}$carbon non-Rutherford resonant scattering. Three sets of samples were studied : HgBa$_{2}$CuO$_{(4+\delta)}$ (Hg1201), Hg$_{x}$T1$_{1-x}$Ba$_{2}$Ca$_{2}$Cu$_{3}$O$_{(2n+\delta)}$ (Hg,T1-1223) and T1$_{1.85}$Ba$_{2}$CuO$_{6}$/LaA10$_{3}$ (T1-2201), either in bulk or as an epitaxial thin film. It was observed that the superconductors exhibit a metal deficiency near the surface, which is largely compensated by excess oxygen. Moreover, the samples are significantly contaminated with carbon within the probing region of the H$^+$ beam. The thermal stability and surface degradation were studied in both oxidizing ambient and vacuum. As a general trend, the heavy metal deficiency — and consequently the compensating oxygen excess — is enhanced as the temperature increases

Interface induced out-of-plane magnetic anisotropy in magnetoelectric BiFeO3-BaTiO3 superlattices

Room temperature magnetoelectric BiFeO3-BaTiO3 superlattices with strong out-of-plane magnetic anisotropy have been prepared by pulsed laser deposition. We show that the out-ofplane magnetization component increases with the increasing number of double layers. Moreover, the magnetoelectric voltage coefficient can be tuned by varying the number of interfaces, reaching a maximum value of 29 V/cmOe for the20×BiFeO3-BaTiO3 superlattice. This enhancement is accompanied by a high degree of perpendicular magnetic anisotropy, making the latter an ideal candidate for the next generation of data storage devices

Stability studies of Hg implanted YBa2_{2}Cu3_{3}O6+x_{6+x}

High quality YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO) superconducting thin films were implanted with the radioactive $^{197m}$Hg (T$_{1/2}$ = 24 h) isotope to low fluences of 10$^{13}$ atoms/cm$^{2}$ and 60 keV energy. The lattice location and stability of the implanted Hg were studied combining the Perturbed Angular Correlation (PAC) and Emission Channeling (EC) techniques. We show that Hg can be introduced into the YBCO lattice by ion implantation into unique regular sites. The EC data show that Hg is located on a highly symmetric site on the YBCO lattice, while the PAC data suggests that Hg occupies the Cu(1) site. Annealing studies were performed under vacuum and O$_{2}$ atmosphere and show that Hg starts to diffuse only above 653 K

Studies of colossal magnetoresistive oxides with radioactive isotopes

We propose to study Colossal Magnetoresistive (CMR) oxides with several nuclear techniques, which use radioactive elements at ISOLDE. Our aim is to provide local and element selective information on some of the doping mechanisms that rule electronic interactions and magnetoresistance, in a complementary way to the use of conventional characterisation techniques. Three main topics are proposed: \\ \\ a) Studies of local [charge and] structural modifications in antiferromagnetic LaMnO$_{3+ \delta}$ and La$_{1-x}$R$_{x}$MnO$_{3}$ with R=Ca and Cd, doped ferromagnetic systems with competing interactions: - research on the lattice site and electronic characterisation of the doping element. \\ \\ b) Studies of self doped La$_{x}$R$_{1-x}$MnO$_{3+\delta}$ systems, with oxygen and cation non-stoichiometry: -learning the role of defects in the optimisation of magnetoresistive properties. \\ \\ c) Probing the disorder and quenched random field effects in the vicinity of the charge or orbital Ordered/Ferromagnetic phase instability: - Investigating the local environment of ions at the Mn site, which trigger the ferromagnetic phase. Our approach to study these problems, combines complementary techniques such as Perturbed Angular Correlation, Emission Channeling and Electrical/Magnetic Measurements in pellets, single crystals and high quality thin films of CMR oxides doped with radioactive isotopes. Preliminary results obtained in La Cd MnO$_{3+x}$ pellets and thin films implanted with $^{111m}$Cd are also presented