ABSTRACT
Spinel oxides with the general formula AB2O4 comprise a large family of compounds covering a very wide range of band-gap values (1 eV < Eg < 8 eV) as a function of the nature of the metallic cations A and B. Owing to this, the physical properties of these materials have been largely exploited both from a fundamental point of view, for their variable electronic properties, and for their possible use in numerous engineering applications. Herein, the modeling of ZnAl2O4, ZnGa2O4, MgAl2O4, and MgGa2O4 cubic spinel oxides has been carried out by using the semiempirical approach based on the difference of electronegativity between oxygen and the average electronegativity of cations present in the oxides. The results of recent theoretical extensions of our semiempirical approach to ternary and quaternary oxides have been tested for spinel oxides with metallic ions occupying both octahedrally and tetrahedrally coordinated sites in different ratios. A detailed analysis of the experimental band-gap values and comparison with the theoretically estimated values has been carried out for ternary ZnAl2O4, ZnGa2O4, MgAl2O4, and MgGa2O4 spinels as well as for double spinels Mg(Al2xGa2-x)O4 and Zn(Al2xGa2-x)O4, and quaternary mixed oxides (ZnxMg(1-x))Al2O4 and (ZnxMg(1-x))Ga2O4. The wide range of band-gap values reported in the literature for simple or double spinels has been related to the different preparation methods affecting the grain dimension of crystalline spinel samples as well as to the presence of crystallographic defects and/or impurities in the spinel matrix. The good agreement between experimental band-gap values and the theoretical ones strongly supports the use of our semiempirical approach in the area of band-gap engineering of new materials.
ABSTRACT
A generalization of the modeling equation of optical band gap values for ternary oxides, as a function of cationic ratio composition, is carried out based on the semiempirical correlation between the differences in the electronegativity of oxygen and the average cationic electronegativity proposed some years ago. In this work, a novel approach is suggested to account for the differences in the band gap values of the different polymorphs of binary oxides as well as for ternary oxides existing in different crystalline structures. A preliminary test on the validity of the proposed modeling equations has been carried out by using the numerous experimental data pertaining to alumina and gallia polymorphs as well as the crystalline ternary Ga(1-x)AlxO3 polymorphs (α-Ga(1-x)AlxO3 and ß-Ga(1-x)AlxO3) covering a large range of optical band gap values (4.50-8.50 eV). To make a more rigorous test of the modeling equation, we extended our investigation to amorphous ternary oxides anodically formed on Al-d-metal alloys (Al-Nb, Al-Ta, and Al-W) covering a large range of d-metal composition (xd-metal ≥ 0.2). In the last case, the novel approach allows one to overcome some difficulties experienced in fitting the optical band gap dependence from the Al-d-metal mixed anodic oxide composition as well as to provide a rationale for the departure, at the lowest d-metal content (xd-metal < 0.2), from the behavior observed for anodic films containing higher d-metal content.
ABSTRACT
Redox-based resistive switching memories (ReRAMs) are strongest candidates for the next-generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta2 O5 -based devices by anodizing. This method allows to grow high-quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical-oxide-based devices demonstrate superior properties, i.e., endurance of at least 106 pulse cycles and/or 103 I-V sweeps maintaining a good memory window with a low dispersion in ROFF and RON values, nanosecond fast switching, and data retention of at least 104 s. Multilevel programing capability is presented with both I-V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.
ABSTRACT
A theoretical treatment of a Schottky barrier dynamic response is developed on the basis of a general model of a semiconductor with thickness comparable in length to the space charge region width. It is shown that, when the space charge region approaches the metal/semiconductor interface, the electric field at this interface, induced by the charge accumulated on the metal, becomes significant with respect to the electric field induced by the charge accumulated on the semiconductor. Under this condition, the total capacitance of the Schottky barrier becomes independent of the polarization potential and tends to the value ε/L, like in a pure dielectric insulator. The term thin film is intended to be with respect to the screening length, which is a function of the volumetric charge density. In amorphous materials the transition potential at which the semiconducting to insulating behaviour is observed is dependent on the frequency. An approximated analytical solution for the capacitance of the junction is calculated. The model for finite thickness semiconductors is successfully applied to the study of anodic Nb(2)O(5), formed in phosphate buffer 0.5 M aqueous solution up to different formation potentials (namely 5, 10 and 20 V vs. Ag/AgCl). The finite thickness semiconductor model permits extrapolation to a general behaviour of the oxide in a wide range of frequencies, potentials and thicknesses, and identification of the electron transfer between adsorbed surface species and the conduction band of Nb(2)O(5) at potentials near to the flat band value.
