Papers by Author: Nuno P. Barradas

Abstract: The Hotbird is a state of the art X-ray laboratory for advanced materials characterisation, installed at ITN since 1999. Several major improvements in its capabilities have been implemented. On the one hand, new hardware developments have extended the applications that can be studied and on the other hand, new software has enabled both enhanced automated control of the system, and improved data analysis that leads to extraction of further precise information from the data. One improvement was the implementation of the x-ray reflectometry (XRR) technique, which is a major expansion of the Hotbird capabilities. XRR is well-suited to characterise film thickness and roughness with high resolution. Furthermore, several optics improvements, such as a Göbel mirror and monochromators were introduced. The combination of this optics allows one to use either a higher intensity beam (orders of magnitude better) or a higher resolution beam configuration. A new high-temperature chamber was developed, which allows one to perform in-situ experiments with excellent temperature control up to 800 °C, in all possible configurations. Data simulation/fitting analysis software for XRR was developed. Also, to control the diffractometer and perform experiments, a new user-friendly software package was developed. In order to illustrate the Hotbird capabilities improvements, several experimental examples will be described.

Abstract: Quaternary GaInAsSb films alloys were grown by MOVPE technique on GaSb substrates with different growth conditions such as substrate orientation and thickness. The composition of the films determines their bandgap, and also how well they are lattice matched to the substrate. It is thus essential to determine it accurately, which is not a trivial task in this system. The composition of the samples was studied with a combination of Particle Induced Xray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) experiments. The RBS experiments were done with a 2 MeV 4He+ or H+ ion beam, according to the thickness of the films, and were used to determine the thickness of the samples. The PIXE experiments were performed at grazing angle conditions and provided accurate elemental composition information. It was found that for thin layers (300 nm) there is a dependence of In incorporation into the matrix according to the substrate orientation, although this tendency was not found for thicker films (24m).

Abstract: Ga0.81In0.19As0.14Sb0.86 layers were grown on (100)-Te doped GaSb substrates 2º missoriented towards (110), (111)A and (111)B directions by metalorganic vapour deposition (MOVPE) at 540 °C. X-ray reciprocal space maps done in symmetric (224) and asymmetric (115) directions show a super-lattice structure due to the phase separation with a 5 nm period and independent of substrate orientation. The x-ray maps show different stage of relaxation of the films and in same cases an interdiffusion region near the substrate. Despite of the phase separation, channelling experiments with H ions as projectiles showed a good quality of the films. Channelling experiments show that the crystalline quality gets worse with increasing the In and As concentration.

Abstract: Antiferromagnets (AF) such as MnPt and MnNi can be used in spin valves and tunnel junctions due to their high exchange coupling, high blocking temperature, and fair corrosion resistance. They are used as pinning layer in a AF/FM/barrier/FM structure, where FM is a ferromagnet such as CoFe and the barrier is an ultra-thin insulating layer. However, as deposited MnPt and MnNi films with thickness around 20 nm are in the fcc phase, and show no exchange bias in AF/FM bilayers. A transition from fcc to fct is required, and takes place upon annealing. We present an X-ray diffraction study of the phase transition in glass/Ta 7nm/Ru 3 nm/MnPt 20 nm/CoFe 5 nm/Ta 3 nm AF/FM bilayers. We observe the MnPt phase transition from fcc to fct around 250°C. We correlate the phase transition with the change of the bilayers magnetic properties.