Magnetic tunnel junctions have attracted much attention both for their interest in physics and their potential application in random access memories. These tunnel junctions consist of two ferromagnetic metallic electrodes separated by a nanometer-scaled oxide barrier. Alumina is the usual dielectric layer, which is formed by oxygen plasma on a pre-deposited thin Al film. In such process, the risk of under- and over- oxidation of the aluminium film is high that directly affects the tunnel magnetoresistance. To overcome this problem, we have developed an alternative layer-by-layer deposition method which consists in successive cycles of Al deposition and oxidation under an O2 flux. The growth kinetics are studied by in-situ Auger Electron Spectroscopy. Numerous heterojunctions have been fabricated on Si(001) and SiO2/Si substrates. A wide range of ex-situ characterization techniques including atomic force microscopy, x-ray reflectivity, transmission electron microscopy, Auger profilometry and energy dispersive x-ray spectrometry have been used to assess the layer thickness, composition, stoichiometry, crystalline structure and thermal stability, as well as the surface and interface roughness and abruptness. We demonstrate that it is possible to realize a uniform and homogeneous nanometer-thick AlOx layer with smooth and sharp interfaces. We finally present current-voltage and Kerr effect measurements to investigate the electric and magnetic properties of these junctions.