Phase diagram and polarization of stable phases of (Ga1−x_{1-x}Inx_x)2_2O3_3

Using density-functional ab initio calculations, we provide a revised phase diagram of (Ga$_{1-x}$In$_{x})_2$O$_3$. Three phases --monoclinic, hexagonal, cubic bixbyite-- compete for the ground state. In particular, in the $x$$\sim$0.5 region we expect coexistence of hexagonal, $\beta$, and bixbyite (the latter separating into binary components). Over the whole $x$ range, mixing occurs in three disconnected regions, and non-mixing in two additional distinct regions. We then explore the permanent polarization of the various phases, finding that none of them is polar at any concentration, despite the possible symmetry reductions induced by alloying. On the other hand, we find that the $\varepsilon$ phase of Ga$_2$O$_3$ stabilized in recent growth experiments is pyroelectric --i.e. locked in a non-switchable polarized structure-- with ferroelectric-grade polarization and respectable piezoelectric coupling. We suggest that this phase could be used profitably to produce high-density electron gases in transistor structures.Comment: 5 pages, 3 figure

Low In solubility and band offsets in the small-xx β\beta-Ga2_2O3_3/(Ga1−x_{1-x}Inx_x)2_2O3_3 system

Based on first-principles calculations, we show that the maximum reachable concentration $x$ in the (Ga$_{1-x}$In$_x$)$_2$O$_3$ alloy in the low-$x$ regime (i.e. In solubility in $\beta$-Ga$_2$O$_3$) is around 10%. We then calculate the band alignment at the (100) interface between $\beta$-Ga$_2$O$_3$ and (Ga$_{1-x}$In$_x$)$_2$O$_3$ at 12%, the nearest computationally treatable concentration. The alignment is strongly strain-dependent: it is of type-B staggered when the alloy is epitaxial on Ga$_2$O$_3$, and type-A straddling in a free-standing superlattice. Our results suggest a limited range of applicability of low-In-content GaInO alloys.Comment: 3 pages, 3 figure

High thermoelectric figure of merit and thermopower in layered perovskite oxides

We predict high thermoelectric efficiency in the layered perovskite La$_2$Ti$_2$O$_7$, based on calculations (mostly ab-initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit $ZT$ computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around 10$^{20}$ cm$^{-3}$. The Seebeck thermopower coefficient is between 200 and 300 $\mu$V/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the potential of this material is due to its lattice thermal conductivity of order 1 W/(K m); using a model based on ab initio anharmonic calculations, we interpret this low value as due to effective phonon confinement within the layered-structure blocks.Comment: 18 preprint pages, 9 figures, accepted on PR Material

Theory of thermoelectricity in Mg3_3Sb2_2 with an energy- and temperature-dependent relaxation time

We study the electronic transport coefficients and the thermoelectric figure of merit ZT in $n$-doped Mg$_3$Sb$_2$ based on density-functional electronic structure and Bloch-Boltzmann transport theory with an energy- and temperature-dependent relaxation time. Both the lattice and electronic thermal conductivities affect the final ZT significantly, hence we include the lattice thermal conductivity calculated ab initio. Where applicable, our results are in good agreement with existing experiments, thanks to the treatment of lattice thermal conductivity and the improved description of electronic scattering. ZT increases monotonically in our T range (300 to 700 K), reaching a value of 1.6 at 700 K; it peaks as a function of doping at about 3$\times$10$^{19}$ cm$^{-3}$. At this doping, ZT$>$1 for T$>$500 K.Comment: 8 pages, 6 figures, further expanded, now accepte

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

Towards Ge-based electronic devices: Increased longevity of alkanethiol-passivated Ge(100) in low humidity environments

Germanium is a critically important material for future complementary metal-oxide-semiconductor devices, however, to maximise its potential it is necessary to develop a robust passivation process that prevents Ge re-oxidation for a queue time of 24 h. Self-assembled monolayers (SAMs) of alkanethiols on Ge have previously been shown to inhibit oxidation; however, re-oxidation eventually occurs when exposed to ambient conditions. Herein, it is shown that humidity plays a key role in the degradation of the SAM, ultimately resulting in re-oxidation. To demonstrate this, thiol-passivated Ge(100) surfaces are exposed to controlled humidity environments with different levels of relative humidity (RH). The rate of re-oxidation of the Ge surfaces are tracked using X-ray photoelectron spectroscopy and water contact angle analysis to discern what role RH plays in the re-oxidation of the Ge and the degradation of the SAM passivation. Atomic force microscopy data is presented to show that humidity-mediated re-oxidation of the Ge has little or no impact on the route mean square roughness of those surfaces. Finally, atomistic modelling of thiol-SAM passivated Ge in the presence of water molecules has been studied using first principles density functional theory in order to simulate experimental conditions and to understand the atomic level processes that determine stability in hydrophilic and hydrophobic configurations

Electrical switching of magnetization in ferromagnetic V-doped La2Ti2O7

We showed recently that V-doped La2Ti2O7 is properly multiferroic [1]. Ferromagnetism is driven by the ordering of V into dimerized chains along the a crystal axis, whereas polarization is due to composite, mainly rotational modes (around the same axis) whose net dipoles along the c crystal axis fail to compensate due to the layered structure. In [1] we estimated that the vibrational magnetoelectric response around the equilibrium structure due to the subset of modes inducing ferroelectricity should be marginal. Here instead we directly explore magnetoelectric coupling upon complete polarization reversal between the two equivalent equilibrium states, a reversal which is easily obtained by a DC field. Using ab-initio magnetic-anisotropy calculations, we find that V spins point approximately along the b crystal axis in one polarization state, whereas they reverse to -b in the other state (with a substantial ~1 meV b/-b anisotropy energy). The spins seem to reorient by rotating through the a direction, which is the hard axis at equilibrium. In summary, polarization switching is accompanied by magnetization switching, and therefore the magnetization M can indeed be switched electrically. We note in passing that while LTO has P=(0,0,Pc), upon V doping polarization acquires a component along a, i.e. P=(PV,0,Pc). As the polarization Pc c is reversed (e.g. by an electric field E0c, and M=MVb is reversed with it, as we showed), the component Pv a remains unchanged, i.e. the polarization is not inverted but rather reflected through the a-b plane. We suspect, and are investigating, a relation of this finding with magnetization inversion

Ab initio thermal conductivity of thermoelectric Mg3Sb2: Evidence for dominant extrinsic effects

The lattice thermal conductivity of the candidate thermoelectric material Mg3Sb2 is studied from first principles, with the inclusion of anharmonic, isotope, and boundary scattering processes, and via an accurate solution of the Boltzmann equation. We find that the anomalously low observed conductivity is due to grain- boundary scattering of phonons, whereas the purely anharmonic conductivity is an order of magnitude larger. Mass disorder due to alloying and off-stoichiometry is also found to contribute significantly to its decrease. Combining ab initio values vs sample size with measured grain-size distributions, we obtain an estimate of κ vs T in nanopolycrystalline material in good agreement with typical experiments, and compute the ZT figure of merit in the various cases

Properties of (Ga1−xInx)2O3 over the whole x range

Using density-functional ab initio theoretical techniques, we study (Ga1-xInx)2O3 in both its equilibrium structures (monoclinic and bixbyite) and over the whole range of composition. We establish that the alloy exhibits a large and temperature-independent miscibility gap. On the low-x side, the favored phase is isostructural with -Ga2O3; on the high-x side, it is isostructural with bixbyite In2O3. The miscibility gap opens between approximately 15% and 55% In content for the bixbyite alloy grown epitaxially on In2O3, and 15% and 85% In content for the free-standing bixbyite alloy. The gap, volume and band offsets to the parent compound also exhibit anomalies as function of x. Specifically, the offsets in epitaxial conditions are predominantly type-B staggered, but have opposite signs in the two end-of-range phases

Structure, electronics and vibrations in Ga2O3, (Ga1-xInx)2O3, and interfaces

We report ongoing work to assess electronic, structural, and vibrational properties of pure and In-substituted Ga2O3 using the density functional theory (DFT) in various versions. We obtain estimates of the gap as function of pressure and concentration of In, besides providing structural hints on the reasons of the difficult In miscibility in Ga2O3