Investigating the local structure of topological crystalline insulators using channeling techniques

For decades, ferromagnetic semiconductors have captured the scientific community's interest, harnessing in a single material the carrier's charge as in a semiconductor and the spin as in a ferromagnet. Dilute magnetic semiconductors (DMS) are among the most well-studied families of ferromagnetic semiconductors, and pioneered multiple spintronic device concepts exploring the interactions between spin, electric current, and light. Mn-doped GaAs, the model DMS, has been regarded as the best candidate for technological application, exhibiting carrier-mediated ferromagnetism. However, despite significant progress, its ferromagnetic Curie temperature remains well below room-temperature. On the other hand, multiferroic Rashba semiconductors (MUFERS) have recently joined the class of ferromagnetic semiconductors, promising additional degrees of tunability through the precise control of spin-currents with applied electric fields, thanks to the coexistence of Rashba and Zeeman effects. In order to tune these properties in MUFERS, one must understand the interplay between the ferroelectric polarization and the magnetization, which is intrinsically related to the rhombohedral distortion of the crystal lattice. The origin of this distortion and how it influences the ferroelectricity, the ferromagnetism and their interplay are still a subject of debate. This thesis focuses on the local structure of Mn dopants in these two families of ferromagnetic semiconductors, through the investigation of their lattice location and thermal stability, providing insight on the mechanisms behind their ferromagnetic and electronic properties.

In (Ga,Mn)As, the DMS with the highest reported Curie temperature (Tc = 185 K), the realization of higher Tc may be enabled by a better understanding of the out-diffusion of interstitial Mn impurities and the segregation of substitutional Mn atoms, induced by thermal annealing. In order to understand to what extent thermal annealing can be used to improve the ferromagnetic properties of (Ga,Mn)As and related compounds, we studied the lattice location and thermal stability of Mn in GaP and GaSb semiconductors, as well as in ferromagnetic (Ga,Mn)As, (Ga,Mn)Sb and quaternary (Ga,Mn)(As,X) layers, X=P or X=Sb. Across the III-Mn-V series, the tetrahedral interstitial site with anion nearest neighbors was identified as the preferred minority site occupied by Mn dopants, following the Ga-substitutional as the majority site. Moreover, the stability of interstitial Mn was found to decrease not only with increasing Mn concentration, but also with decreasing atomic packing of the host semiconductor (from GaP to GaAs to GaSb). This decrease directly affects the substitutional Mn thermal stability and results in a narrow annealing temperature range in which interstitial Mn can be removed to increase Tc. This temperature window is limited by segregation of substitutional Mn at low temperatures, which was investigated in (Ga,Mn)As films annealed at T=300°C, revealing the formation of disordered paramagnetic Mn-rich nanoclusters. These results further illustrate the delicate balance between structure and magnetism in III-Mn-V semiconductors, and may inspire new strategies for achieving more effective substitutional doping and higher Curie temperatures.

In (Ge,Mn)Te, the model MUFERS combining ferromagnetism with robust ferroelectric and giant Rashba effects, the ferroelectricity and its coupling to the ferromagnetism are intrinsically related to the rhombohedral distortion. Associated with this distortion is a displacement between cation and anion sub-lattices, which breaks crystal inversion symmetry. In this work the local structure of the Mn sub-lattice in (Ge,Mn)Te with Mn concentration up to 21% was investigated, and observed a smaller displacement for the Mn sub-lattice than for the host Ge sub-lattice. This is consistent with a more central, charge neutral Mn position in the lattice, which reduces the rhombohedral distortion and the ferroelectric moment at higher Mn contents in (Ge,Mn)Te. A preferential orientation of the Mn sub-lattice displacement in the ferroelectric phase was also observed, corresponding to the out-of-surface $\langle111\rangle$ direction, implying  a possible spontaneous ferroelectric polarization. This new insight on the nature of the sub-lattice displacement helps understanding its role in the multiferroic phase diagram, which is in turn relevant for developing future spintronic applications of (Ge,Mn)Te or materials exhibiting similar MUFERS behavior.