Local structure of Mn-doped ferromagnetic semiconductors

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 erromagnetic 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 = 300C, 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 sublattice 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 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

Local Structure and Magnetism of (Ga,Mn)As

Throughout the years, dilute magnetic semiconductors (DMS) have emerged as promising materials for semiconductor-based spintronics. In particular, (Ga,Mn)As has become the model system in which to explore the physics of carrier-mediated ferromagnetism in semiconductors and the associated spintronic phenomena, with a number of interesting functionalities and demonstrated proof-of-concept devices. It constitutes the perfect example of how the magnetic behavior of DMS materials is strongly influenced by local structure. In this thesis, we address key aspects of the interplay between local structure and ferromagnetism of (Ga,Mn)As. We unambiguously identify the lattice site occupied by interstitial Mn as the tetrahedral interstitial site with As nearest neighbors T(As). We show, furthermore, that the T(As) is the most energetically favorable site regardless of the interstitial atom forming or not complexes with substitutional Mn. We also evaluate the thermal stability of both interstitial and substitutional Mn sites occupied by Mn for two representative Mn concentrations (1% and 5%), and its influence on the material’s structure and magnetism. We show that compared to the substitutional Mn, interstitial Mn becomes mobile at lower temperatures, for both low (1%) and high (5%) Mn concentration. Moreover, the diffusion temperatures are lower for the high concentration than for the low concentration case. These diffusion temperatures are concentration dependent, with aggregation of impurities occurring for the high concentration at lower temperatures than for the low concentration. These findings translate into two key conclusions: at typical growth temperatures (200-300°C) the interstitial Mn is mobile for high concentration (5%) but not for low concentration (1%); and substitutional Mn impurities become mobile in a temperature regime that is well below what has been previously reported. We also observe a decrease in the activation energy for the diffusion of both substitutional and interstitial impurities with increasing Mn concentration. This decrease has a different origin for each case: for substitutional diffusion, with activation energies of 2.3-2.6 eV at 1% Mn to 1.9-2.0 eV at 5% Mn, a vacancy-assisted mechanism occurs that is favored with increasing impurity concentration; for interstitial diffusion, with activation energies of 1.5-2.1 eV at 1% Mn to 1.3-1.8 eV at 5% Mn, as charge screening effects become stronger with increasing Mn (and consequently carrier) concentration, interstitial Mn defects are effectively “neutralized”, and therefore experience lower migration barriers. Additionally, we conducted a comprehensive study of the local structure and magnetism in the different diffusion regimes. We show that annealing at 200°C promotes the passivation of the interstitial Mn for 5% Mn (Ga,Mn)As, as an increase in TC and magnetization is observed. No improvement is observed in the 1% Mn case, which can be understood from the absence of interstitial impurities incorporated during growth for this concentration regime. Annealing at 300°C induces the precipitation of Mn into Mn-rich regions for both concentrations studied, with no signs of ferromagnetism. Finally, annealing at 600°C led to the formation of well-defined secondary-phases in both concentrations, consistent essentially of superparamagnetic MnAs nanoclusters of two types: zincblende and hexagonal NiAs-type. The results presented in this thesis are direct evidence for the complex interplay between the local structure and the carrier-mediated ferromagnetism present in this DMS system. It constitutes an important step in the understanding of the fundamental physics behind (Ga,Mn)As, motivating a wider investigation of other dilute magnetic semiconductors within the III-Mn-V family

Drawing the geometry of 3d transition metal-boron pairs in silicon from electron emission channeling experiments

Although the formation of transition metal-boron pairs is currently well established in silicon processing, the geometry of these complexes is still not completely understood. We investigated the lattice location of the transition metals manganese, iron, cobalt and nickel in n- and p+-type silicon by means of electron emission channeling. For manganese, iron and cobalt, we observed an increase of sites near the ideal tetrahedral interstitial position by changing the doping from n- to p+-type Si. Such increase was not observed for Ni. We ascribe this increase to the formation of pairs with boron, driven by Coulomb interactions, since the majority of iron, manganese and cobalt is positively charged in p+-type silicon while Ni is neutral. We propose that breathing mode relaxation around the boron ion within the pair causes the observed displacement from the ideal tetrahedral interstitial site. We discuss the application of the emission channeling technique in this system and, in particular, how it provides insight on the geometry of such pairs

Lattice location of Mg in GaN: a fresh look at doping limitations

Radioactive 27Mg (t1/2=9.5 min) was implanted into GaN of different doping types at CERN’s ISOLDE facility and its lattice site determined via beta− emission channeling. Following implantations between room temperature and 800°C, the majority of 27Mg occupies the substitutional Ga sites, however, below 350°C significant fractions were also found on interstitial positions ~0.6 Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p-type with 2E19 cm−3 stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350°C converts interstitial 27Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV

Identification of the interstitial Mn site in ferromagnetic (Ga,Mn)As

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (TAs) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (TGa) is negligible. TAs is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding TAs versus TGa occupancy of interstitial Mn in (Ga,Mn)As