A. Barbier

INDRA, a 4π charged product detection array at GANIL

Abstract INDRA, a new and innovative highly segmented detector for light charged particles and fragments is described. It covers geometrically 90% of the 4π solid angle and has very low detection thresholds. The detector, operated under vacuum, is axially symmetric and segmented in 336 independent cells allowing efficient detection of high multiplicity events. Nucleus identification down to very low energy threshold (≈ 1 A MeV) is achieved by using ionization chambers operated with low pressure C 3 F 8 gas. Residual energies are measured by a combination of silicon (300 μm thick) and cesium iodide (5 to 14 cm in length) detectors. Very forward angles are covered by fast counting phoswich scintillators (NE102/NE115). Charge resolution up to Z = 50 is achieved on a large energy dynamic range (5000 to 1 for silicon detectors). Isotopic separation is obtained up to Z = 3. The treatment of the signals is performed through specifically designed and highly integrated modules, most of which are in the new VXIbus standard. Full remote control of parameter settings, including visualization of signals, is thus allowed. The detector is continuously monitored with a laser source and electronic pulsers and is found stable over several days. Energy calibration procedures, making use of specific detectors and the ability of the GANIL accelerator to deliver secondary beams, have been developed. First experiments were performed in the spring of 1993.

Growth of aluminum nitride on (111) silicon: Microstructure and interface structure

The growth of hexagonal aluminum nitride directly on (111) silicon has been studied by grazing incidence x-ray diffraction and high resolution electron microscopy as a function of film thickness. Two epitaxial relationships were observed: (1) AlN (0001) [211¯0]//Si(111) [022], which prevails at deposition temperatures larger than 650 °C, and (2) AlN (0001) [1010]//Si(111) [022]. For a 40 A thick layer, the average in-plane crystallite size is 162 A, the in-plane rotation is ∼2° and the dislocations induce an average strain distribution of 0.8%. The Si/AlN interface is very sharp and complete relaxation (down to ∼0.2%) occurs within one bilayer. No long range order was observed at the interface. This implies a low mobility of the AlN species on Si, inhibiting any structural rearrangement. In particular the in-plane rotations originate from the early stage of the layer growth and decrease with the layer thickness, especially for thicknesses larger than 250 A.

Very-high-quality MgO(001) surfaces : Roughness, rumpling and relaxation

Abstract The structure of MgO(001) single crystal surfaces that are of very high crystalline quality, extremely flat, clean and stoichiometric, was investigated by grazing incidence X-ray scattering. A new procedure for preparing these surfaces is described, which includes high-temperature annealing in air followed by high-temperature ion bombardment, and annealing under partial oxygen pressure. The relaxation, −0.56±0.35%, and rumpling, 1.07±0.5%, were determined with a much better accuracy than in any previous experimental study. These values are compared with those given in previous experimental and theoretical studies. The atomic structure of the Ca-segregated MgO(001) surface was also investigated. The most likely structure is the theoretically predicted ( 2 × 2 ) R45° reconstruction. Atomic displacements within this unit cell were determined.

A NEW UHV DIFFRACTOMETER FOR SURFACE STRUCTURE AND REAL TIME MOLECULAR BEAM DEPOSITION STUDIES WITH SYNCHROTRON RADIATIONS AT ESRF

Abstract We describe a new surface X-ray diffractometer, which is optimized to combine surface X-ray diffraction (SXRD), absorption spectroscopy (SEXAFS) and grazing incidence small angle scattering (GISAXS). This instrument is particularly well suited for real time studies of material elaborated in-situ with molecular beam deposition (MBD) techniques. The goniometer allows for large in-plane and out-of-plane momentum transfer with high accuracy. Owing to the flipping mechanism of the sample holder, it can perform absorption experiments with polarisation directions normal and parallel to the sample surface, while keeping a grazing incidence. Several MBD sources as well as complementary surface sensitive electron techniques (RHEED and Auger spectroscopy) can be used simultaneously with X-rays. Finally, the samples can easily be inserted in the X-ray chamber via an UHV transport system and a fast entry load-lock module.

Determination Of The α-Al 2 O 3 (0001) Surface Relaxation and Termination by Measurements of Crystal Truncation Rods

We have investigated the unreconstructed (0001) surface structure of sapphire (α-Al 2 O 3 ) by Grazing Incidence X-ray Scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surface. The most likely model yields a single Al layer termination with relaxations of the first four planes of -51%, +16%, -29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.

Structural investigation of the NiO(111) single crystal surface

Abstract We report an in situ grazing incidence X-ray scattering study of the NiO(111) single crystal surface. It evidences a relatively high stability of this surface. After an air anneal at 1300 K it is flat and almost not reconstructed. Annealing at 860 K in ultra-high vacuum leads to two domains: one covered by epitaxial and relaxed metallic Ni clusters associated with a highly ordered interfacial supercell on the coincidence site lattice, presumably a misfit dislocation network and one exhibiting a p(2 × 2) reconstruction. After oxidation the metallic Ni transforms again into NiO(111) and only a p(2 × 2) reconstructed flat surface remains. In these conditions macroscopic (100) facetting was never observed. The general behaviour of the NiO(111) surface is discussed and compared with other polar and non-polar oxide surfaces, with emphasis on the specific properties which could explain its stability.

Enhanced photoanode properties of epitaxial Ti doped α-Fe2O3 (0001) thin films

The growth, crystal and electronic structures, and photo-electrochemical properties of undoped and Ti doped hematite epitaxial films were studied. We evidence that Ti4+ substitutes Fe3+ in the hematite lattice inducing a slight modification of the oxygen octahedron. Ti doping is shown to induce a shift of the valence band toward higher binding energy due to a movement of the Fermi level toward the conduction band. The resulting modification of electrical conductivity appears as a possible origin of the improvement of photo-electrochemical properties in the doped sample.

Growth, annealing and oxidation of the Ni/MgO (001) interface studied by grazing incidence x-ray scattering

We report an in situ grazing incidence x-ray scattering study of the Ni/MgO (001) interface during its formation at room temperature. In-plane measurements reveal that the interface is sharp and that the epitaxial relationship is complex. In the early stages of growth, up to one monolayer of “on site” Ni, in-plane strained to the lattice parameter of MgO, is observed. An oxygen epitaxial site is found. For the on-site fraction the interfacial and Ni interlayer distances were determined. For coverages larger than one monolayer two distinct lattices exist: expanded Ni (001) and Ni (110). The latter exhibits several orientations with respect to the substrate depending on the thickness. The Ni (110) orientations disappear by annealing at high temperature, leaving only the Ni cube/cube orientation. The layer was also almost fully transformed into NiO (001) by high temperature oxidation.

Restoration of bulk magnetic properties by strain engineering in epitaxial CoFe2O4 (001) ultrathin films

We report on the significantly enhanced in-plane magnetic properties of CoFe2O4 (001) ultrathin layers (5 nm) grown on MgAl2O4 (001) in comparison to films deposited on MgO (001). The predicted inverse spinel structure is confirmed by x-ray magnetic circular dichroism measurements and transmission electronic microscopy studies reveal a significant in-plane compressive strain, responsible for the strong film anisotropy. These results show that strain engineering can be used to tailor the magnetic properties of oxide ultrathin films. A large compressive strain restores bulk magnetic properties for CoFe2O4 films at tunnel barrier thicknesses.

Stability and stoichiometry of (polar) oxide surfaces for varying oxygen chemical potential

Polar oxide surfaces constitute a class of surfaces long considered as unstable because of their theoretically predicted diverging electrostatic surface potential, although such crystal orientations form spontaneously in nature. Because of the intrinsic hindrances (charge build-up, electron correlation effects) a reliable description of such surfaces necessitates a combined theory-experiment approach. Experimentally, detailed investigations became possible in recent years because of the availability of charge build-up insensitive techniques such as surface x-ray diffraction, which is thoroughly used in the presented work. We consider here prototypical polar oxide surfaces of different crystalline structures and show that their stability regime strongly depends on the external conditions applied to the surface in terms of oxygen chemical potential. In order to understand the structure and stability of polar oxide surfaces, their chemical environment must be included.