G. Renaud

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.

Substrate-enhanced supercooling in AuSi eutectic droplets.

The phenomenon of supercooling in metals—that is, the preservation of a disordered, fluid phase in a metastable state well below the melting point—has led to speculation that local atomic structure configurations of dense, symmetric, but non-periodic packing act as the main barrier for crystal nucleation. For liquids in contact with solids, crystalline surfaces induce layering of the adjacent atoms in the liquid and may prevent or lower supercooling. This seed effect is supposed to depend on the local lateral order adopted in the last atomic layers of the liquid in contact with the crystal. Although it has been suggested that there might be a direct coupling between surface-induced lateral order and supercooling, no experimental observation of such lateral ordering at interfaces is available. Here we report supercooling in gold-silicon (AuSi) eutectic droplets, enhanced by a Au-induced (6 × 6) reconstruction of the Si(111) substrate. In situ X-ray scattering and ab initio molecular dynamics reveal that pentagonal atomic arrangements of Au atoms at this interface favour a lateral-ordering stabilization process of the liquid phase. This interface-enhanced stabilization of the liquid state shows the importance of the solid–liquid interaction for the structure of the adjacent liquid layers. Such processes are important for present and future technologies, as fluidity and crystallization play a key part in soldering and casting, as well as in processing and controlling chemical reactions for microfluidic devices or during the vapour–liquid–solid growth of semiconductor nanowires.

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.

Strains Induced by Point Defects in Graphene on a Metal

Strains strongly affect the properties of low-dimensional materials, such as graphene. By combining in situ, in operando, reflection high-energy electron diffraction experiments with first-principles calculations, we show that large strains, above 2%, are present in graphene during its growth by chemical vapor deposition on Ir(111) and when it is subjected to oxygen etching and ion bombardment. Our results unravel the microscopic relationship between point defects and strains in epitaxial graphene and suggest new avenues for graphene nanostructuring and engineering its properties through introduction of defects and intercalation of atoms and molecules between graphene and its metal substrate.

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.

Apparatus for 3D surface X-ray scattering during in situ molecular beam deposition

Abstract A novel surface diffractometer devoted to structural studies of surfaces and interfaces by X-ray scattering is described. It is composed of a very precise goniometer, onto which different UHV chambers can be rapidly aligned and interchanged without breaking the vacuum, thus allowing a more efficient use of the synchrotron X-ray beam. Two UHV chambers have been designed, one for horizontal sample surface, and one for vertical sample surface. The latter is equipped with all necessary tools to prepare the surface and perform molecular beam epitaxy at the same time as surface X-ray scattering. Both allow one to reach very large out-of-plane momentum transfer values with high accuracy.

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.

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.

Elastic strain relaxation in GaN/AlN nanowire superlattice

The molecular-beam epitaxy growth of AlN/GaN nanowire superlattices has been studied by using a combination of in situ x-ray diffraction experiments, high-resolution electron-microscopy analysis and theoretical calculations performed in a valence force field approach. It is found that the nanowire superlattices are in elastic equilibrium, in contrast with the two-dimensional case but in line with the predicted increase in the critical thickness in the nanowire geometry.