Session H33: Functional Oxides

Enhancement of Thermal Conductivity Across the Metal-Insulator Transition in Vanadium Dioxide

The metal-to-insulator transition (MIT) in vanadium dioxide (VO₂) is investigated by electrical and thermal transport measurements. We report an order-of-magnitude enhancement of thermal conductivity across the MIT region in the VO₂ single crystal. Magnetic field dependent measurements reveal that the thermal conductivity peak doesn't show an obvious dependence on the magnetic field, which indicates that the enhancement of thermal conductivity could come from neutral heat carriers such as phonons. Our experiment explores a potential direction for achieving thermal management in phase-change materials.
  

Structural and Electronic properties of Copper Doped amorphous Alumina

To understand the atomic properties of Conducting Bridge Random Access Memory(CBRAM) devices based on Alumina with Copper as one of the active electrodes, we offer new models of amorphous Alumina with varying concentrations of Cu having general form (a-Al₂O₃)_1-nCu_n with n = 0, 0.10, 0.20 and 0.30. We study the structural properties of these models. It turns out that Cu atoms cluster because of its less proclivity to diffuse in the highly ionic host a-Al₂O₃. The clustering produces a narrow conducting path for electrical current. To elucidate the electric conduction path, we employ the Kubo-Greenwood formula in a novel space-projected form[1]. We show that the conduction-active component of the network is an interconnected Cu cluster.

[1] K. Prasai, KN. Subedi, K. Ferris, P. Biswas and DA. Drabold, physica status solidi(RRL) 12, 1800238(2018)   

Phase transitions and optical properties of Sr2SnO4

Layered perovskite Ruddlesden Popper (RP) transition metal oxides are interesting because of the effects of their 2 dimensional crystal structure on their lattice response, as well as electronic and optical properties. The Stannate Sr₂SnO₄ has the K₂NiF₄ type RP structure at high temperatures, but undergo multiple structural phase transitions as the temperature is lowered. In this talk, we present our first principles Density Functional Theory results that elucidate the mechanism behind these phase transitions. We discuss the details of the octahedral tilting and examine its effects on the electronic and optical properties of Sr₂SnO₄.   

Formulation and characterization of sinterless Barium Strontium Titanate (BST) nanocomposite dielectric inks for flexible RF and Microwave electronics applications

The demand for printable electronics over conventional electronics is rising exponentially due to various reasons. Tunable radio frequency (RF) and microwave (MW) devices make a high impact on the size, cost and the overall performance of flexible electronics. The dielectric constant of BST can be tailored by an applied electric field, which allows BST to be used in varactors for tunable RF and MW applications such as tunable filters and conformal antennas.

The Curie temperature (T_C) of (Ba_XSr_1-XTiO₃) can be tailored by adjusting the molar fraction of barium. BST has a paraelectric phase above the Curie temperature and below the Curie temperature, BST has a ferroelectric phase. BST shows highest dielectric properties such as tunability, high dielectric constant and low loss tangent in the paraelectric phase.

These inks can be processed below 150 ⁰C and a high performance can be achieved without sintering the BST nanoparticles. Also, these inks are compatible with different types of printing technologies, such as dispensing and ink jet and aerosol jet. These BST-polymer inks showed 44.29 ± 1.85 as the highest dielectric constant and 0.02 ± 0.01 as the lowest loss tangent at 10 GHz. The maximum tunability achieved was 15% at 10 GHz with an applied electric field of 10 V/µm.   

Chromium Pairs in Combustion Synthesized α-Alumina

High surface area alumina has gained much attention as a support structure for a number of possible biological and biomedical applications. Combustion synthesis provides a simple route to produce large quantities of alumina with optically active impurities after liquid phase mixing. In this report, we present results on chromium ion separation in combustion-synthesized highly-doped ruby. Aluminium and chromium nitrates are dissolved in water/urea solutions. The solutions are then heated above 500 degrees, at which point they undergo a self-propagating combustion reaction transforming them into high-surface area solids. X-ray diffraction confirms the single phase a-alumina structure. Optically excited fluorescence (excited at 473 nm) spectra are used to measure the ratio of the fourth nearest neighbour emission peak to that of the single ion peak to quantify impurity pair formation as a function of chromium concentration. The ratio increases linearly over the concentration range up to four atomic percent. Time-resolved decay dynamics of the pairs will be presented.   

Producing p-ZnO via rf magnetron sputtering onto a thin evaporated layer of Zn3As2

Zinc oxide (ZnO) is a wide band gap semiconductor with many potential applications, including UV lasers, transparent circuits, and radiation resistant devices. The native defects in ZnO cause it to be n-type, and stable high quality p-type ZnO has proven hard to create. We report on a technique whereby As-doped p-ZnO is created using rf magnetron sputtering onto a thin evaporated layer of Zn₃As₂. The optical characteristics and thickness of the Zn₃As₂ layer have been determined by spectroscopic ellipsometry. ZnO quality and doping has been studied with photoluminescence, X-ray diffraction, Hall effect, and other techniques. Annealing in a vacuum leads to better sample quality, although the p-type characteristics have not always been reproducible.   

Observation of the Charge-Stripe Crystal Phase in an Insulating Cuprate

High-temperature superconductivity in cuprates arises from carrier doping an antiferromagnetic Mott (AFM) insulator. This carrier doping leads to the formation of electronic liquid crystal phases. The insulating charge-stripe crystal phase is predicted to form when a small density of holes is doped into the charge-transfer insulator state, but this phase is yet to be experimentally observed. We realized the lightly-doped charge transfer insulating state of the cuprate Bi2Sr2CaCu2O8+x by surface annealing. In this insulating state with a charge transfer gap at the order of ~1 eV, we discovered a unidirectional charge-stripe order with a commensurate 4a0 period along the Cu-O-Cu bond. Notably, this insulating charge stripe crystal phase develops before the onset of the pseudogap and the formation of the Fermi surface. Our work provides fresh insight into the microscopic origin of electronic inhomogeneity in high-Tc cuprates.
  

Integration of Eu2O3 with GaN (0001) and determination fundamental material properties

As semiconductor technology moves beyond silicon, III-nitrides are strong candidates for high-power, high-temperature and high-frequency MOS applications. Rare-earth (RE) oxides are well suited as gate dielectrics on III-nitrides due to high dielectric constants, thermodynamic stability and large bandgaps. The RE oxide Eu2O3 was grown by MBE on the III-nitride GaN. The Eu2O3 film was characterized by in-situ RHEED, XPS, UPS and ex-situ XRD, STEM and inverse PES. The bandgap was extracted from XPS and inverse PES and the electron affinity determined from UPS, the band alignment with the GaN substrate from XPS. The correlated nature of the Eu 4f core-levels, overlapping the valence band states in binding energy, do not allow an interpretation of photoelectron spectra of the valence band in terms of initial (ground) state properties, hence a direct comparison with ab-initio theory DOS is not valid in this case. The combination of high- and low-energy excitations of XPS and UPS allow to probe matrix element effects in the photoemission process enabling the partial assignment of valence band structures to O 2p or Eu 4f emission hinting towards importance of hybridization between Eu 4f and O 2p.   

Structurally triggered metal-insulator transition in rare-earth nickelates and related eg1 perovskite

Rare-earth nickelates (RNiO3), apart for LaNiO3, exhibit on cooling a metal-insulator transition (MIT), a concurrent structural phase transition and a magnetic phase transition. Here, I first assess the ability of first-principles calculations within the DFT+U formalism to describe the properties of rare-earth nickelates. Then, using such a formalism, I show that the MIT in nickelates can be seen as a structurally triggered phase transition [1] highlighting so a first concrete example of such a kind of phase transition in simple perovskites. The origin of this unusual mechanism is then traced back in the electronic and magnetic properties, revealing a Peierls-type instability, structurally triggered by oxygen rotation motions and eventually assisted by magnetic ordering. This knowledge helps rationalizing the evolution of the MIT in thin films and heterostructures [2] and is also relevant to other e_g¹ perovskites like alkaline-earth ferrites and rare-earth manganites [3]. Work done in collaboration with A. Mercy, Y. Zhang, M. Schmitt, He Xu, J. Bieder & E. Bousquet. Research supported by ARC project AIMED and ERA.NET project SIOX. [1] A. Mercy et al., Nature Commun. 8, 1677 (2017). [2] Z. Liao et al., PNAS in press (2018). [3] Y. Zhang et al., Phys. Rev. B 98, 081108(R) (2018).   

Electronic structure of group IV ternary alloy of CSiSn

CSiSn as the ternary group IV alloy has promising applications in optoelectronic devices due to its possible direct bandgap. The proposed alloy is capable of engineering the bandgap by changing the composition of each element which also has the possibility of lattice matching to Si as the substrate. However, electronic bandstructure of these alloys is heavily dependent on the band bowing which would change the compositions needed to achieve direct bandgap. In this talk, we use the Density Functional Theory to predict the electronic band gap of binary alloys of C-Si-Sn as well as ternary alloy of CSiSn. In addition, we present how the band gap is correlated with the proportion of each element.   

Enhancement of Young's modulus in freestanding SrTiO3 thin films

Recent developments in thin fil growth provide a powerful route to free-standing single-crystal films of perovskite oxides by water etching of a sacrificial underlayer (Di Lu et al., Nature Materials 15, 1255 (2016)). We have used atomic force microscopy to systematically probe the elastic properties of suspended SrTiO₃ films, in a thickness series from 4-98 nm, which has been previously inaccessible and unexplored. We observe the thickness dependence of elastic modulus of SrTiO₃ to be non-monotonic, with a strong enhancement of stiffness for thicknesses below 30 nm. We provide evidence that the nature of this elastic stiffening lies in an effective flexoelectric coupling. This study provides the basis for studying strain manipulation and strain control of oxides at the nanoscale. In particular, oxide membranes can be designed to mimic stiffnesses of 2d materials like graphene and transition metal dichalcogenides.   

Examination of the high-temperature optical properties of HfO2 thin films for metamaterial selective-emitter applications

Metamaterial selective-emitters can enhance the efficiency of thermophotovoltaic (TPV) energy conversion systems by selecting the wavelengths of light emitted towards the TPV photodiodes. As such, the optical properties of any material used as part of a metamaterial selective-emitter must be known and controlled. As one of the highest melting point dielectric materials, hafnium dioxide (HfO₂, melting point of ~ 2760°C) has attracted researchers’ attention for possible use in high-temperature applications; such as metamaterial selective-emitters. In this work, HfO₂ was deposited on a Si substrate by atomic layer deposition (ALD) and then diced into smaller pieces. The frequency-dependent complex permittivity of the HfO₂ films was measured via ellipsometry during annealing at temperatures from room temperature (RT) up to 600°C and then again at RT after annealing. The ellipsometric measurements were conducted over a wavelength range of 200 – 500 nm. The degree of crystallization and film density is correlated with the measured permittivity of HfO₂ by X-ray reflectivity (XRR) and X-ray diffraction (XRD).   

phonoRam: A workflow tool for predicting Raman activity with DFT

The Raman activity tensor is a fundamental property describing the coupling of light with matter. As polarized Raman spectroscopy becomes more routine, detailed knowledge of the tensor components is necessary to interpret spectra, in addition to the powder-averaged value measured in unpolarized experiments. We present phonoRam, a python-based workflow tool based on the phonopy phonon analysis package, that simplifies and manages the calculation of the Raman activity tensor using DFT. We present an overview of the package, convergence of the macroscopic dielectric tensor values, and a set of test cases. We present an investigation of U₃O₈, a material with 68 Raman active modes and polarization dependence, using polarized Raman and phonoRam to analyze the crystallite structure.   

Optical Bandgap and Electric Transportation of Co- and Al- co-doped ZnO with Low Defect Concentration

The defects in Co- and Al- co-doped ZnO, Co_0.05Al_0.02Zn_0.93 (CAZO), films were generated during sputtering growth by mixing low percentage of hydrogen, 0.1~1% denoted as CAZO-H2%, along with argon gas at 450^oC. For comparison, an extremely low defect film, except the native oxygen vacancies, was grown under a mixture of oxygen with argon gas, which is denoted as CAZO-O2. The optical band gaps and electric resistivities were measured as a function of the H₂%. We found the CAZO-O2 is insulating while the resistance drops to a single digit (less than 10 Ω) with the addition of a very small amount of H₂, 0.1%, in the growth atmosphere and reaches the minimum for CAZO-1%. Combining previous data with higher defect concentrations, the lowest resistance was observed for the CAZO-1% sample. The optical bandgap, E_op, however, shows the opposite trend that increases smoothly with the defect concentration from E_op=3.4eV for CAZO-O2 sample, similar to the bandgap of pure ZnO, to 3.61eV for CAZO-1.1%. The role of defects on the bandgap and resistance will be presented.   

Special stability of SrCoO2.5 and HSrCoO2.5

In this work, we demonstrate how the electron-counting model (ECM) can be used in understanding the phases, particularly in explaining the stability of the oxygen-vacancy channels (OVCs), and in examining the Co valance problem in SCO2.5 and HSCO2.5. Using density-functional theoretical (DFT) methods, we analyze the crystalline, electronic, and magnetic structures of BM- and H-SCO. We discovered stable phases with large bandgaps (> 1 eV) for both BM-SCO and H-SCO, with reasonable agreement with experiments. Our calculation also indicates limited charge transfer from H to O that may explain the special stability of the H-SCO phase and the reversibility of H incorporation observed in experiments. In contrary to the initial study, our calculation also suggests intrinsic antiferromagnetism (AFM) of H-SCO, showing how the measured ferromagnetism (FM) has possible roots in hole doping.