J. P. Renard

Ga kinetics in plasma-assisted molecular-beam epitaxy of GaN(112¯2): Effect on the structural and optical properties

GaN directly deposited on m-sapphire by plasma-assisted molecular-beam epitaxy settles into two main crystalline orientation domains: GaN﴾112¯2﴿ and GaN{101¯3}. The dominant phase is GaN﴾112¯2﴿ with GaN║ sapphire and GaN║ sapphire in-plane epitaxial relationships. Deposition of GaN on top of an AlN﴾112¯2﴿ buffer layer and growth under slightly Ga-rich conditions reduce GaN{101¯3} precipitates below the detection limits. Studies of Ga adsorption demonstrate that it is possible to stabilize up to one Ga monolayer on the GaN﴾112¯2﴿ surface. The presence of this monolayer of Ga excess on the growth front reduces the ﴾112¯2﴿ surface energy and hence minimizes the surface roughness. Photoluminescence from two-dimensional GaN﴾112¯2﴿ layers is dominated by a near-band-edge emission, which is assigned to excitons bound to stacking faults, present with a density around 3x105 cm−1.

Growth of m-plane GaN quantum wires and quantum dots on m-plane 6H-SiC

Growth of m-plane GaN quantum nanostructures on an AlN buffer layer on m-plane SiC is investigated. GaN nanostructures with different shapes are obtained depending on the AlN buffer layer thickness and the amount of GaN deposited. For AlN buffer layer below 300nm, GaN quantum wires, elongated perpendicularly to the c axis, are obtained independently of the amount of GaN deposited. For buffer layer thickness above 300nm, and for an amount of GaN below (above) 5 ML (monolayer), GaN quantum dots (wires) are obtained. The difference in m-plane GaN morphology is related to the buffer layer stress state and to the anisotropic surface diffusion of m-plane GaN. Optical properties suggest an absence of internal quantum confined Stark effect.

Magnetoresistance of very thin cobalt-gold multilayers with perpendicular anisotropy

We report magnetoresistance (MR) measurements on Au/Co/Au sandwiches and Au/Co/Au multilayers, deposited on polished glass substrates in ultrahigh vacuum, with Co thickness ranging between 3 and 30 A. The experiments were performed at room temperature in the field range 0–0.8 T and at low temperature 1.3–4.2 K in the field range 0–5 T for different field orientations with respect to the film and to the current. For Co thickness below 11 A, the experimental data are consistent with an easy axis perpendicular to the film, in agreement with previous ferromagnetic resonance and magnetization measurements. From the hysteretic behavior of the MR, coercive fields of a few 102 Oe at room temperature are observed. In Co bilayers, the MR is enhanced with respect to the MR of simple sandwiches and indicates a square hysteresis loop. The abnormally large MR of the ultrathin Co films is attributed to a Co/Au interface resistance related to walls between Co domains with alternating perpendicular magnetization.