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
