Magnetically Mediated Transparent Conductors: In₂O₃ Doped with Mo

First-principles band structure investigations of the electronic, optical, and magnetic properties of Mo-doped In2O3 reveal the vital role of magnetic interactions in determining both the electrical conductivity and the Burstein-Moss shift which governs optical absorption. We demonstrate the advantages of the transition metal doping which results in smaller effective mass, larger fundamental band gap, and better overall optical transmission in the visible as compared to commercial Sn-doped In2O3. Similar behavior is expected upon doping with other transition metals opening up an avenue for the family of efficient transparent conductors mediated by magnetic interactions

Electronic properties of layered multicomponent wide-bandgap oxides: a combinatorial approach

The structural, electronic, and optical properties of twelve multicomponent oxides with layered structure, RAMO$_4$, where R$^{3+}$=In or Sc; A$^{3+}$=Al or Ga; and M$^{2+}$=Ca, Cd, Mg, or Zn, are investigated using first-principles density functional approach. The compositional complexity of RAMO$_4$ leads to a wide range of band gap values varying from 2.45 eV for InGaCdO$_4$ to 6.29 eV for ScAlMgO$_4$. Strikingly, despite the different band gaps in the oxide constituents, namely, 2-4 eV in CdO, In$_2$O$_3$, or ZnO; 5-6 for Ga$_2$O$_3$ or Sc$_2$O$_3$; and 7-9 eV in CaO, MgO, or Al$_2$O$_3$, the bottom of the conduction band in the multicomponent oxides is formed from the s-states of all cations and their neighboring oxygen p-states. We show that the hybrid nature of the conduction band in multicomponent oxides originates from the unusual five-fold atomic coordination of A$^{3+}$ and M$^{2+}$ cations which enables the interaction between the spatially-spread s-orbitals of adjacent cations via shared oxygen atoms. The effect of the local atomic coordination on the band gap, the electron effective mass, the orbital composition of the conduction band, and the expected (an)isotropic character of the electron transport in layered RAMO$_4$ is thoroughly discussed.Comment: 15 page

Composition-Dependent Oxygen Vacancy Formation in Multicomponent Wide-Band-Gap Oxides

The formation and distribution of oxygen vacancy in layered multicomponent InAMO 4 oxides with A3 +=Al or Ga and M2 +=Ca or Zn and in the corresponding binary oxide constituents is investigated using first-principles density functional calculations. Comparing the calculated formation energies of the oxygen defect at six different site locations within the structurally and chemically distinct layers of InAMO 4 oxides, we find that the vacancy distribution is significantly affected not only by the strength of the metal-oxygen bonding, but also by the cation\u27s ability to adjust to anisotropic oxygen environment created by the vacancy. In particular, the tendency of Zn, Ga, and Al atoms to form stable structures with low-oxygen coordination results in nearly identical vacancy concentrations in the InO 1.5 and GaZnO 2.5 layers in InGaZnO 4, and only an order of magnitude lower concentration in the AlZnO 2.5 layer as compared to the one in the InO 1.5 layer in InAlZnO 4. The presence of two light-metal constituents in the InAlCaO 4 along with Ca failure to form a stable fourfold coordination as revealed by its negligible relaxation near the defect, leads to a strong preference of the oxygen vacancy to be in the InO 1.5 layer. Based on the results obtained, we derive general rules on the role of chemical composition, local coordination, and atomic relaxation in the defect formation and propose an alternative light-metal oxide as a promising constituent of multicomponent functional materials with tunable properties

Hopping versus bulk conductivity in transparent oxides: 12CaO - 7Al₂O₃

First-principles calculations of the mayenite-based oxide, [Ca12Al14O32]2+(2e-), reveal the mechanism responsible for its high conductivity. A detailed comparison of the electronic and optical properties of this material with those of the recently discovered transparent conducting oxide, H-doped UV-activated Ca12Al14O33, allowed us to conclude that the enhanced conductivity in [Ca12Al14O32]2+(2e-) is achieved by elimination of the Coulomb blockade of the charge carriers. This results in a transition from variable range-hopping behavior with a Coulomb gap in H-doped UV-irradiated Ca12Al14O33, to bulk conductivity in [Ca12Al14O32]2+(2e-). Further, the high degree of delocalization of the conduction electrons obtained in [Ca12Al14O32]2+(2e-) indicates that it cannot be classified as an electride, as originally suggested

Metallic Networks and Hydrogen Compensation in Highly Nonstoichiometric Amorphous In2 O3-X

The unique response of amorphous ionic oxides to changes in oxygen stoichiometry is investigated using computationally intensive ab initio molecular dynamics simulations, comprehensive structural analysis, and hybrid density-functional calculations for the oxygen defect formation energy and electronic properties of amorphous In2O3-x with x=0-0.185. In marked contrast to nonstoichiometric crystalline nanocomposites with clusters of metallic inclusions inside an insulating matrix, the lack of oxygen in amorphous indium oxide is distributed between a large fraction of undercoordinated In atoms, leading to an extended shallow state for x0.185. The calculated carrier concentration increases from 3.3x1020cm-3 at x=0.037 to 6.6x1020cm-3 at x=0.074 and decreases only slightly at lower oxygen content. At the same time, the density of deep defects located between 1 and 2.5 eV below the Fermi level increases from 0.4x1021cm-3 at x=0.074 to 2.2x1021cm-3 at x=0.185. The wide range of localized gap states associated with various spatial distributions and individual structural characteristics of undercoordinated In is passivated by hydrogen that helps enhance electron velocity from 7.6x104 to 9.7x104 m/s and restore optical transparency within the visible range; H doping is also expected to improve the material\u27s stability under thermal and bias stress

Electronic Structure of Superconducting MgB₂ and Related Binary and Ternary Borides

First-principles full potential linear muffin-tin orbital-generalized gradient approximation electronic structure calculations of the new medium-Tc superconductor (MTSC) MgB2 and related diborides indicate that superconductivity in these compounds is related to the existence of Px,y-band holes at the γ point. Based on these calculations, we explain the absence of medium-Tc superconductivity for BeB2, AlB2, ScB2, and YB2. The simulation of a number of MgB2-based ternary systems using a supercell approach demonstrates that (i) the electron doping of MgB2 (i.e., MgB2-yXy with X=Be, C, N, O) and the creation of defects in the boron sublattice (nonstoichiometric MgB2-y) are not favorable for superconductivity, and (ii) a possible way of searching for similar or higher MTSC should be via hole doping of MgB2 (CaB2) or isoelectronic substitution of Mg (i.e., Mg1-xMxB2 with M = Be, Ca, Li, Na, Cu, Zn) or creating layered superstructures of the MgB2/CaB2 type

Effect of Phosphorus on Cleavage Fracture in Κ-Carbide

To understand the origin of cleavage fracture which dominates in Fe(Mn)-Al-C alloys at a high phosphorus concentration, we performed first-principles study of the phosphorus effect on ideal cleavage energy and critical stress in κ-carbide, Fe3 AlC, a precipitate in the austenitic alloys. We find that phosphorus has higher solubility in Fe3 AlC than in γ-Fe and sharply reduces the cleavage characteristics of κ-carbide. We show that strong anisotropy of the Fe-P bonds in Fe3 (Al,P) C under tensile stress, leads to the appearance of large structural voids and may facilitate crack nucleation

Hydrogen Behavior at Crystalline/amorphous Interface of Transparent Oxide Semiconductor and its Effects on Carrier Transport and Crystallization

The role of disorder and particularly of the interfacial region between crystalline and amorphous phases of indium oxide in the formation of hydrogen defects with covalent (In-OH) or ionic (In-H-In) bonding are investigated using ab initio molecular dynamics and hybrid density-functional approaches. The results reveal that disorder stabilizes In-H-In defects even in the stoichiometric amorphous oxide and also promotes the formation of deep electron traps adjacent to In-OH defects. Furthermore, below-room-temperature fluctuations help switch interfacial In-H-In into In-OH, creating a new deep state in the process. This H-defect transformation limits not only the number of free carriers but also the grain size, as observed experimentally in heavily H-doped sputtered In2Ox. On the other hand, the presence of In-OH helps break O2 defects, abundant in the disordered indium oxide, and thus contributes to faster crystallization rates. The divergent electronic properties of the ionic vs covalent H defects passivation of undercoordinated In atoms vs creation of new deep electron traps, respectively and the different behavior of the two types of H defects during crystallization suggest that the resulting macroscopic properties of H-doped indium oxide are governed by the relative concentrations of the In-H-In and In-OH defects. The microscopic understanding of the H defect formation and properties developed in this work serves as a foundation for future research efforts to find ways to control H species during deposition

Composition-Dependent Structural and Transport Properties of Amorphous Transparent Conducting Oxides

Structural properties of amorphous In-based oxides, In-X-O with X=Zn, Ga, Sn, or Ge, are investigated using ab initio molecular dynamics liquid-quench simulations. The results reveal that indium retains its average coordination of 5.0 upon 20% X fractional substitution for In, whereas X cations satisfy their natural coordination with oxygen atoms. This finding suggests that the carrier generation is primarily governed by In atoms, in accord with the observed carrier concentration in amorphous In-O and In-X-O. At the same time, the presence of X affects the number of six-coordinated In atoms as well as the oxygen sharing between the InO6 polyhedra. Based on the obtained interconnectivity and spatial distribution of the InO6 and XOx polyhedra in amorphous In-X-O, composition-dependent structural models of the amorphous oxides are derived. The results help explain our Hall mobility measurements in In-X-O thin films grown by pulsed-laser deposition and highlight the importance of long-range structural correlations in the formation of amorphous oxides and their transport properties

Tuning the properties of complex transparent conducting oxides: role of crystal symmetry, chemical composition and carrier generation

The electronic properties of single- and multi-cation transparent conducting oxides (TCOs) are investigated using first-principles density functional approach. A detailed comparison of the electronic band structure of stoichiometric and oxygen deficient In$_2$O$_3$, $\alpha$- and $\beta$-Ga$_2$O$_3$, rock salt and wurtzite ZnO, and layered InGaZnO$_4$ reveals the role of the following factors which govern the transport and optical properties of these TCO materials: (i) the crystal symmetry of the oxides, including both the oxygen coordination and the long-range structural anisotropy; (ii) the electronic configuration of the cation(s), specifically, the type of orbital(s) -- $s$, $p$ or $d$ -- which form the conduction band; and (iii) the strength of the hybridization between the cation's states and the p-states of the neighboring oxygen atoms. The results not only explain the experimentally observed trends in the electrical conductivity in the single-cation TCO, but also demonstrate that multicomponent oxides may offer a way to overcome the electron localization bottleneck which limits the charge transport in wide-bandgap main-group metal oxides. Further, the advantages of aliovalent substitutional doping -- an alternative route to generate carriers in a TCO host -- are outlined based on the electronic band structure calculations of Sn, Ga, Ti and Zr-doped InGaZnO$_4$. We show that the transition metal dopants offer a possibility to improve conductivity without compromising the optical transmittance