DFT-based study of the structural, optoelectronic, mechanical and magnetic properties of Ti3AC2 (A = P, As, Cd) for coating applications.

The first-principles approach has been used while employing the Perdew-Burke-Ernzerhof exchange-correlation functional of generalized gradient approximation (PBE-GGA) along with the Hubbard parameter to study the structural, optoelectronic, mechanical and magnetic properties of titanium-based MAX materials Ti3AC2 (A = P, As, Cd) for the first time. As there is no band gap found between the valence and conduction bands in the considered materials, these compounds belong to the conductor family of materials. A mechanical analysis carried out at pressures of 0 GPa to 20 GPa and the calculated elastic constants C ij reveal the stability of these materials. Elastic parameters, i.e., Young's, shear and bulk moduli, anisotropy factor and Poisson's ratio, have been investigated in the framework of the Voigt-Reuss-Hill approximation. The calculated values of relative stiffness are found to be greater than ½ for Ti3PC2 and Ti3AsC2, which indicates that these compounds are closer to typical ceramics, which possess low damage tolerance and fracture toughness. Optical parameters, i.e., dielectric complex function, refractive index, extinction coefficient, absorption coefficient, loss function, conductivity and reflectivity, have also been investigated. These dynamically stable antiferromagnetic materials might have potential applications in advanced electronic and magnetic devices. Their high strength and significant hardness make these materials potential candidates as hard coatings.

ab initio study of oxygen vacancy effects on structural, electronic and thermoelectric behavior of AZr1-xMxO3 (A = Ba, Ca, Sr; M= Al, Cu, x = 0.25) for application of memory devices.

The structural, electronic and thermoelectric properties of AZr1-xMxO3 (A = Ba, Ca, Sr; M = Al, Cu, x = 0.25) without and with an oxygen vacancy (Vo) have been unveiled using the Perdew-Burke-Ernzerhof Generalized Gradient Approximation (PBE-GGA) functional along with Tran-Blaha modified Becke-Jonhson (TB-mBJ)approximation based on Density Functional Theory (DFT) in the framework of WIEN2k code for memristors applications. Moreover, isosurface charge density plots have been calculated by using Vienna ab initio Simulation Package (VASP) simulation code. The analysis of structural parameters reveals that substituting Zr4+ with Al3+ and Cu2+ causes the lattice distortion which tends to increase in the presence of Vo along with dopant. The study of band structure, density of states (DOS) and isosurface charge density plots predict the enhanced charge conduction and formation of conducting filaments (CFs) for all composites with dopant and/or Vo. Moreover, spin polarized density of states for Cu doped composites has also been calculated to confirm the large exchange splitting of Cu-3d states. The thermoelectric characteristics of considered composites have also been explored using the Boltztrap code to better explain the semi-classical Boltzmann transport theory. Thermoelectric parameters confirm the semiconductor nature of all composites, ensuring the compatibility for memristors and thermoelectric devices applications. In addition to this spin polarized thermoelectric behavior of Cu doped composites that ensure the contribution of spin down (↓) states of Cu for charge transport mechanism. The SrZrCuO3+Vo composite is found most promising candidate followed by BaZrCuO3 for memristors applications while, CaZrCuO3 is found most suitable amongst studied composites for thermoelectric devices.

Eradicating negative-Set behavior of TiO2-based devices by inserting an oxygen vacancy rich zirconium oxide layer for data storage applications.

Memristors, with low energy consumption, long data storage and fast switching speed, are considered to be promising for applications such as terabit data storage memory and hardware based neurocomputation applications. However, unexpected negative-Set behavior is a serious issue that causes deterioration of reliability and uniformity of switching parameters. In this work, negative-Set behavior of TiO2-based RRAM is successfully eradicated by inserting a thin oxygen vacancy rich ZrO2-x layer. In addition, oxygen vacancy rich ZrO2-x layer is also responsible for the enhancement of resistive switching characteristics in terms of excellent endurance performance (2000 DC cycles), good data retention upto 104 s and uniformity in Set/Reset voltages. Experimental results and density functional theory (DFT) analysis confirm that an interface layer TiOx has formed between highly reactive electrode (Ti) and ZrO2 interlayer. This interface layer is serving as a low series resistance layer and oxygen ion reservoir in Set-process and oxygen ions supplier in Reset-process to generate/refill the oxygen vacancies in the formation and rupture of conductive filaments. Comparing with the single layer Ti/TiO2/Pt device, it is noteworthy that the switching process in the bilayer (BL) Ti/ZrO2-x/TiO2/Pt memristor device is not affected even at high Reset-voltages, but the negative-Set behavior has been eradicated effectively. This work demonstrates that the insertion of a thin oxygen vacancy rich ZrO2-x interlayer into TiO2-based devices is a feasible approach to solve unpredicted negative-Set behavior of RRAM devices.

Ab-initio prediction of the mechanical, magnetic and thermoelectric behaviour of perovskite oxides XGaO3 (X = Sc, Ti, Ag) using LDA+U functional: For optoelectronic devices.

The mechanical, magnetic and thermoelectric properties of spin polarized XGaO3 (X = Sc, Ti, Ag) perovskite oxides in cubic phase have been investigated using LDA + U functional through ab-initio study based on density functional theory (DFT) in the framework of WIEN2K simulation code. The Full Potential Linearized Augmented Plane Wave (FP-LAPW) technique along with PBE-GGA functional have been used to optimize the systems and determining exchange-correlation potential. However, in order to address on-site self-interactions error and overcome limitations of PBE-GGA functional, LDA + U has been employed because Hubbard parameter 'U' is found an appropriate remedy to consider on-site self-interactions, and to calculate improved electronic energy band gap. All spin polarized band structures reveal indirect band gap with different energies Eg (eV) such as ↑↓ 0.98 eV for ScGaO3, ↑1.05 eV and ↓1.70 eV for TiGaO3, ↑1.13 eV and ↓2.19 eV for AgGaO3. Thus, all compounds are semiconductor in nature. The analysis of spin polarized total and partial density of states unveil that ScGaO3 is non-magnetic material, whereas, TiGaO3 and AgGaO3 are characterized by strong exchange splitting of 3d (Ti) and 4d (Ag) states with significant spin magnetic moments, i.e., 1.0002 µB and -2.0002 µB, respectively. The elastic constants, i.e., Bulk, Young and Shear moduli, Poisson's coefficient, Anisotropy factor, Pugh's ratio, Cauchy pressure and melting temperature are calculated through Viogt-Reuss-Hill approximation. The thermoelectric response of the considered perovskites has been determined through semi-classical Boltzmann transport theory in the framework of BoltzTraP simulation code. Basic understandings of the mechanical, magnetic and thermoelectric properties of these compounds are studied for the first time in this manuscript.

Effect of Bilayer CeO2-x/ZnO and ZnO/CeO2-x Heterostructures and Electroforming Polarity on Switching Properties of Non-volatile Memory.

Memory devices with bilayer CeO2-x/ZnO and ZnO/CeO2-x heterostructures sandwiched between Ti top and Pt bottom electrodes were fabricated by RF-magnetron sputtering at room temperature. N-type semiconductor materials were used in both device heterostructures, but interestingly, change in heterostructure and electroforming polarity caused significant variations in resistive switching (RS) properties. Results have revealed that the electroforming polarity has great influence on both CeO2-x/ZnO and ZnO/CeO2-x heterostructure performance such as electroforming voltage, good switching cycle-to-cycle endurance (~ 102), and ON/OFF ratio. A device with CeO2-x/ZnO heterostructure reveals good RS performance due to the formation of Schottky barrier at top and bottom interfaces. Dominant conduction mechanism of high resistance state (HRS) was Schottky emission in high field region. Nature of the temperature dependence of low resistance state and HRS confirmed that RS is caused by the formation and rupture of conductive filaments composed of oxygen vacancies.

Endurance and Cycle-to-cycle Uniformity Improvement in Tri-Layered CeO2/Ti/CeO2 Resistive Switching Devices by Changing Top Electrode Material.

Resistance switching characteristics of CeO2/Ti/CeO2 tri-layered films sandwiched between Pt bottom electrode and two different top electrodes (Ti and TaN) with different work functions have been investigated. RRAM memory cells composed of TaN/CeO2/Ti/CeO2/Pt reveal better resistive switching performance instead of Ti/CeO2/Ti/CeO2/Pt memory stacks. As compared to the Ti/CeO2 interface, much better ability of TaN/CeO2 interface to store and exchange plays a key role in the RS performance improvement, including lower forming/SET voltages, large memory window (~102) and no significant data degradation during endurance test of >104 switching cycles. The formation of TaON thinner interfacial layer between TaN TE and CeO2 film is found to be accountable for improved resistance switching behavior. Partial charge density of states is analyzed using density functional theory. It is found that the conductive filaments formed in CeO2 based devices is assisted by interstitial Ti dopant. Better stability and reproducibility in cycle-to-cycle (C2C) resistance distribution and Vset/Vreset uniformity were achieved due to the modulation of current conduction mechanism from Ohmic in low field region to Schottky emission in high field region.

Forming-free bipolar resistive switching in nonstoichiometric ceria films.

The mechanism of forming-free bipolar resistive switching in a Zr/CeOx/Pt device was investigated. High-resolution transmission electron microscopy and energy-dispersive spectroscopy analysis indicated the formation of a ZrOy layer at the Zr/CeOx interface. X-ray diffraction studies of CeOx films revealed that they consist of nano-polycrystals embedded in a disordered lattice. The observed resistive switching was suggested to be linked with the formation and rupture of conductive filaments constituted by oxygen vacancies in the CeOx film and in the nonstoichiometric ZrOy interfacial layer. X-ray photoelectron spectroscopy study confirmed the presence of oxygen vacancies in both of the said regions. In the low-resistance ON state, the electrical conduction was found to be of ohmic nature, while the high-resistance OFF state was governed by trap-controlled space charge-limited mechanism. The stable resistive switching behavior and long retention times with an acceptable resistance ratio enable the device for its application in future nonvolatile resistive random access memory (RRAM).