Magnetoelectric, multiferroic, wide-gap, and polar oxides for advanced applications: first-principles theoretical studies

This Ph.D. thesis reports a theoretical study of electronic and structural properties of several materials relevant for electronic and optical applications. In the last few years, in fact, the renaissance of many physical effects has evolved rapidly, firstly due to new nano-fabrication techniques that allow us to implement advanced materials in numerous innovative structures and devices. The first part of this thesis is related to a new class of multi-functional magnet materials called multiferroics, where magnetism and ferroelectricity are strongly coupled together. Because of that, these materials can be considered as suitable candidates for several technological applications, such as storage devices. Among the class AnBnO3n+2 of layered-perovskite oxides, I have considered the Lanthanum titanate, La2Ti2O7 (LTO), and in order to achieve multiferroicity in this topological ferroelectric I have suggested an isovalent substitution of the Ti cation, non magnetic, by a magnetic one, Mn, obtaining the compound La2Mn2O7 (LMO). Operationally, I have optimized the structures involved in the paraelectric (PE) ferroelectric (FE) transition. Then, I have determined that LMO is a multiferroic materials since ferroelectric (FE) and magnetic order coexist in the same phase. Finally, I have demonstrated that LMO is also a magnetoelectric materials showing a non-zero lattice-mediated magnetoelectric tensor, α. Moreover, magnetic noncollinear spin-orbit calculations reveal that spins point along the c direction but manifests a spin canting in the bc plane generating a weak ferromagnetism interpretable by Dzyaloshinsky-Moriya (DM) interaction. The second part of this thesis is based on the investigation about Gallium oxide, Ga2O3, Indium oxide, In2O3, and their solid solutions. This study is motivated by the recently attracting interest on novel materials systems for high-power transport devices as well as for optical ultraviolet absorbers and emitters. Resorting to an appropriated optimization of physical properties and nanostructuration of Gallium- and Indium-based semiconductor layers of chosen composition, it is possible to tune their key properties (such as band gaps, interface band off-sets, vibrational absorptions, as well as, potentially, the magnetic behavior) leading overall to novel multi-functional nanomaterials, nanostructures and devices. This may enable the design of devices based on interfaces Ga2O3/(Ga1−xInx)2O3 or In2O3/(Ga1−xInx)2O3 such as high-power field effect transistors and far-UV photodetectors or emitters. Operationally, I have studied the electronic and local structural properties of pure Ga2O3 and In2O3. Then, starting from the monoclinic (β) structure of Ga2O3, I have explored alloyed oxides, (Ga1−xInx)2O3, for different In concentrations (x). The structural energetics of In in (Ga1−xInx)2O3 causes most sites to be essentially inaccessible to In substitution, thus limiting the maximum In content to somewhere between 12 and 25% in this phase. In this framework, the gap, the volume and the band offset to the parent compound exhibit also anomalies as function of In concentration. Furthermore, I have explored alloyed oxides based on the bixbyite equilibrium structure of In2O3 in all the In concentration range. The main result is that the alloy shows a phaseseparation in a large composition range, exhibiting a huge and temperature-independent miscibility gap. In addition, in accord with experimental results, intermediate alloying shows an additional crystallographic phase, in competition with the bulk Ga2O3 and In2O3 phases. Finally, I have investigated the orthorhombic (ε) phase of Ga2O3, that results to be the second most stable structure beside β-Ga2O3. Moreover, ε-Ga2O3 exhibits a large spontaneous polarization and a sizable diagonal piezoelectric coefficient, comparable with typical polar semiconductors.