Synthèse et caractérisation structurale d'épicouches hétérogènes semi-conductrices/ferromagnétiques : le cas d'agrégats de MnP encastrés dans une matrice de GaP

Integration of magnetic materials in thin film semiconductors is a promising way to develop new integrated electronic devices. In this context, heterogeneous epitaxial magnetic semiconductors are interesting candidates. This class of materials corresponds to transition metal-based ferromagnetic nanoclusters embedded in III-V semiconductor layers. Those layers are ferromagnetic at room-temperature and exhibit interesting magnetic functionalities (giant magneto-resistance, large magneto-optic and spin polarization effects). The magnetic properties of the layer are strongly dependent on the structural properties of the clusters, in particular the alloy, the dimension, the density, the spatial distribution and the texture (i.e. the statistical distribution of the crystallographic orientations). In order, to exploit the magnetic functionalities at their full potential, we must control the structural properties of the nanoclusters and understand their growth mechanism. The general objective of this work is to bring a better understanding of the growth mechanism and the influence of the growth parameters on the microstructure of the heterogeneous magnetic semiconductors layers. Toward this end, we have undertaken a detailed study on the structural characteristics of the GaP:MnP ferromagnetic semiconductor thin films grown by metal organic vapour phase epitaxy (MOVPE). We have focused our effort on three specific objectives: (1) to demonstrate the growth of epitaxial heterogeneous GaP:MnP layers; (2) to establish the influence of the growth parameters on the microstructure of the matrix and nanoclusters; (3) to obtain a detailed structural characterisation of the texture of the clusters as a function of the growth parameters. We have successfully grown epitaxial heterogeneous GaP:MnP layers without structural defects on GaP substrates at 650 °C. The layers contain a uniform ensemble of 15–50 nm quasi-spherical MnP nanoclusters within a dislocation-free GaP epilayer matrix that is fully coherent with the substrate. The clusters occupy 3 to 8% of the total volume of the layer, controlled by the flow of the Mn precursor in the vapor phase. We showed that the growth temperature strongly affect the microstructure of the GaP matrix. At 700 °C the surface roughness increases and we have observed 100 nm wide cavities in the GaP matrix. The layers grown at 600 °C contain a large density of pile-up defects along GaP{111} facets. To explain these defects we propose the following mechanism: (1) the nucleation of clusters on the GaP growth surface change the