Complex oxides as novel transparent conductors

Ab-initio density functional approach is employed to investigate the structural, optical, and electronic properties of twelve undoped (non)stoichiometric multicomponent oxides with layered structure RAMO₄ [R³⁺ =In or Sc, A³⁺ =Al or Ga, and M²⁺ - Ca, Cd, Mg, or Zn], as candidates for novel transparent conducting oxides. The compositional complexity of RAMO₄ leads to a wide range of band gaps varying from 2.45eV for InGaCdO₄ to 6.29 eV for ScAlMgO₄. We find that despite the different band gaps in the constituent binary oxides, namely, 2-4 eV in CdO, In₂O₃, or AnO; 5-6 eV for Ga₂O₃ or Sc₂O₃; and 7-9 eV in CaO, MgO, or Al₂O₃, the states of all cations contribute to the bottom of the conduction band of RAMO₄. We show that this hybrid nature of the conduction band originates from the unusual fivefold atomic coordination of A³⁺ and M²⁺ cations and suggests that both structurally and chemically distinct layers of RAMO₄ are expected to participate in carrier transport. This is consistent with the obtained isotropic electron effective mass of 0.3-0.5 me. Next, in order to understand the carrier generation mechanism in RAMO₄, we have systematically investigated the formation of native point defects in three representative InAMO₄ oxides. We find that the donor antisite defect in InGaZnO₄ and InAlZnO₄ occur in higher concentrations than oxygen vacancies which are major donors in binary oxides. Also in contrast to the binary TCOs, the formation energy of cation vacancies is significantly lower in InAMO₄ owing to a large structural relaxation around the defect. As a result, the equilibrium Fermi level is pushed away from the conduction band and deeper into the band gap. The results agree well with the observed dependence of the conductivity on the oxygen partial pressure in InGaZnO₄. These systematic investigations provide a significant insight into the role of chemical composition and structural complexity of RAMO₄ materials on the carrier generation mechanisms and the resulting properties --Abstract, page iv

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

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

EVALUATION OF THE RATIOS OF THE MAIN INDICATORS OF THE DRY SEALING OF THE CYLINDER-PISTON GROUP OF INTERNAL COMBUSTION ENGINES USING A SOLID LUBRICANT

This article presents the results of a study of an alternative method for reducing friction losses in the cylinder-piston group of internal combustion engines based on an experimental installation, taking into account changes in the real state of the working surfaces of the mating parts of the piston ring-cylinder pair depending on the operating time. The factors with a progressive effect on the operation of engine friction units as they wear out are studied, and the degree of their influence on wear is estimated. A model of the friction unit of a cylinder-piston group (piston-piston ring pair) of an internal combustion engine based on a solid antifriction material operating without the use of a lubricating fluid is developed and investigated. Comparative results of determining the wear indicators of sealing rings by various methods of wear control are presented. A method for predicting the resource and the real state of the engine is proposed

Native Point Defects in Multicomponent Transparent Conducting Oxides

The formation of native point defects in layered multicomponent InAMO4 oxides with A 3+=Al or Ga, and M 2+=Ca, Mg, or Zn, is investigated using first-principles density functional calculations. We calculated the formation energy of acceptor (cation vacancies, acceptor antisites) and donor (oxygen vacancy, donor antisites) defects within the structurally and chemically distinct layers of InAMO4 oxides. We find that the antisite donor defect, in particular, the A atom substituted on the M atom site (A M) in InAMO4 oxides, have lower formation energies, hence, higher concentrations, as compared to those of the oxygen vacancy which is know to be the major donor defect in binary constituent oxides. The major acceptor (electron killer) defects are cation vacancies except for InAlCaO4 where the antisite CaAl is the most abundant acceptor defect. The results of the defect formation analysis help explain the changes in the observed carrier concentrations as a function of chemical composition in InAMO4, and also why the InAlZnO4 samples are unstable under a wide range of growing conditions

Carrier Generation in Multicomponent Wide-Bandgap Oxides: InGaZnO₄

To exploit the full potential of multicomponent wide-bandgap oxides, an in-depth understanding of the complex defect chemistry and of the role played by the constituent oxides is required. In this work, thorough theoretical and experimental investigations are combined in order to explain the carrier generation and transport in crystalline InGaZnO4. Using first-principles density functional approach, we calculate the formation energies and transition levels of possible acceptor and donor point defects as well as the implied defect complexes in InGaZnO4 and determine the equilibrium defect and electron densities as a function of growth temperature and oxygen partial pressure. An excellent agreement of the theoretical results with our Brouwer analysis of the bulk electrical measurements for InGaZnO 4 establishes the Ga antisite defect, GaZn, as the major electron donor in InGaZnO4. Moreover, we show that the oxygen vacancies, long believed to be the carrier source in this oxide, are scarce. The proposed carrier generation mechanism also explains the observed intriguing behavior of the conductivity in In-rich vs Ga-rich InGaZnO4