In the context of spintronics development using semiconductors, new phenomena have been discovered and begin to be explored at the nanoscale. Spin injection in semiconductors from a ferromagnetic metal and semiconductors doping with magnetic atoms opened the way to new devices conception using both the electron charge and spin. But these topics raised also new questions concerning the physics of impurities and interfaces that play a crucial role when devices shrink to nanoscale. The goal of the SSAS project is to address these questions of magnetism in semiconductors heterostructures using Scanning Tunneling Microscopy (STM) techniques. Innovating techniques will be developed and applied to semiconductors samples to probe magnetic properties down to the atomic scale, namely the Scanning Tunneling Spectroscopy (STS), the Spin-Polarized STM (SP-STM) and the Inelastic Electrons Tunneling Spectroscopy (IETS). A first part of the project will be devoted to nanomagnetism of heterostructures. The local magnetization behavior and the magnetic anisotropy will be probed near semiconductor interfaces (for example on Fe/GaAs hybrid structures used in spin injection experiments), and in magnetically doped semiconductors (in the ferromagnetic material Ga1-xMnxAs). SP-STM will be used in these samples for the first time. Using the ability of tunneling spectroscopy to probe the local electronic density of states, the band structure modification of semiconductors will be investigated when one introduce magnetic impurities in a semiconductors matrix. Magnetic doping will be explored with two points of view: the band structure of ferromagnetic samples strongly doped, and the local electronic modification brought by single magnetic atoms on the surrounded semiconductors material. In this context, the effect of magnetic doping will also be studied in semiconductors nanostructures presenting discrete electronic structure as quantum dots. Finally, a new type of approach is suggested. Instead of studing samples used for actual studies in spintronics or nanoeletronics, the next step of future devices will be explored using structures containing only few magnetic atoms. STM offers the possibility to manipulate individual atoms on a surface and to construct artificial structures of the whished shape and size. The STM atomic manipulation will be developed in this project on semiconductors surfaces with magnetic atoms. The magnetic properties of structure with two or three atoms will then be investigated by spin dependent spectroscopy, in particular the magnetic coupling existing between them and the spin orientation of each as compare to the others. The specificity of the magnetic interaction in case of semiconductor environment will be extract as compare to metallic samples.