Ferroelectronics Lab

Understanding and utilizing non-volatile properties of materials

New Publication! ” Local structure maturation in high entropy oxide (Mg,Co,Ni,Cu,Zn)1-x(Cr,Mn)xO thin films”

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Abstract: High entropy oxides (HEOs) have garnered much interest due to their available high degree of tunability. Here, we study the local structure of (MgNiCuCoZn)0.167(MnCr)0.083O, a composition based on the parent HEO (MgNiCuCoZn)0.2O. We synthesized a series of thin films via pulsed laser deposition at incremental oxygen partial pressures. X-ray diffraction shows lattice parameters to decrease with increased pO2 pressures until the onset of phase separation. X-ray absorption fine structure shows that specific atomic species in the composition dictate the global structure of the material as Cr, Co, and Mn shift to energetically favorable coordination with increasing pressure. Transmission electron microscopy analysis on a lower-pressure sample exhibits a rock salt structure, but the higher-pressure sample reveals reflections reminiscent of the spinel structure. In all, these findings give a more complete picture of how (MgNiCuCoZn)0.167(MnCr)0.083O forms with varying initial conditions and advances fundamental knowledge of cation behavior in high entropy oxides.

Full text available at The Journal of the American Ceramic Society

New Publication! “Thermodynamic Origins of Nonvolatility in Resistive Memory”

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Abstract: Electronic switches based on the migration of high-density point defects, or memristors, are poised to revolutionize post-digital electronics. Despite significant research, key mechanisms for filament formation and oxygen transport remain unresolved, hindering our ability to predict and design device properties. For example, experiments have achieved 10 orders of magnitude longer retention times than predicted by current models. Here, using electrical measurements, scanning probe microscopy, and first-principles calculations on tantalum oxide memristors, we reveal that the formation and stability of conductive filaments crucially depend on the thermodynamic stability of the amorphous oxygen-rich and oxygen-poor compounds, which undergo composition phase separation. Including the previously neglected effects of this amorphous phase separation reconciles unexplained discrepancies in retention and enables predictive design of key performance indicators such as retention stability. This result emphasizes non-ideal thermodynamic interactions as key design criteria in post-digital devices with defect densities substantially exceeding those of today’s covalent semiconductors.

Full text available from Matter

New Publication! “Perspective: Entropy-Stabilized Oxide Memristors”

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Abstract: A memristor array has emerged as a potential computing hardware for artificial intelligence (AI). It has an inherent memory effect that allows information storage in the form of easily programmable electrical conductance, making it suitable for efficient data processing without shuttling of data between the processor and memory. To realize its full potential for AI applications, fine-tuning of internal device dynamics is required to implement a network system that employs dynamic functions. Here, we provide a perspective on multicationic entropy-stabilized oxides as a widely tunable materials system for memristor applications. We highlight the potential for efficient data processing in machine learning tasks enabled by the implementation of “task specific” neural networks that derive from this material tunability.

Full text available from Applied Physics Letters

New Publication! “Investigation of the Influence of Growth Conditions on the Local Structure in High Entropy Oxides Using S/TEM”

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Abstract: Recently, the chemically disordered multi-cation oxide class of materials called High Entropy Oxides (HEOs) has been widely explored due to their tunable functional properties because of their higher configurational entropy and to understand their fundamental phase formation. These HEOs have shown potential applications as thermoelectrics, ionic conductors, electrocaloric materials, etc. Since these systems have multiple aliovalent cations in a single lattice, it necessitates the understanding of elemental distribution and structure of these novel oxides. This would help us understand how the HEOs navigate the configurational space and enable us to establish a correlation between structure and property. In this study, we are investigating the influence of oxygen partial pressure during pulsed laser deposition on the structure, and chemical environment on the HEO thin films. (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O, commonly known as ‘J14’, is the prototype HEO that has been widely studied. We are currently exploring the seven component HEO system with the composition: (Mg0.167Ni0.167Co0.167Cu0.167Zn0.167(Cr, Mn)0.167)O, referred to as ‘J14MnCr’. The goal of this study is to understand the influence of addition of Cr and Mn to J14 on the structure and how the growth conditions modulate the structure and tune magnetic properties. Due to the presence of seven cations with some cations that can adapt multiple valences, it is necessary to probe the structural nuances and chemical environment in these systems. In this study, Scanning/Transmission Electron Microscope is used to characterize the HEOs as S/TEM allows us to probe the structure, composition, and valence variation at nanoscale regime.

We have probed the J14MnCr thin film grown at 5mTorr and 50mTorr oxygen partial pressure. We have observed from Selected Area Electron Diffraction (SAED) and dark field TEM experiments that there is a change in the crystal structure in the films grown at 5mTorr vs. 50mTorr. At 50mTorr, the SAED reveals the formation of rock salt structure with additional secondary phase. The increase in oxygen partial pressure during synthesis has led to the nucleation of a new phase with a different structure which is speculated to influence the magnetic property. We have further performed STEM experiments and will be implementing unsupervised machine learning to detect the local structural variation. STEM-EELS experiments have also been performed to investigate the local changes in structure and to correlate it to changes in the valences of the cations and chemical environment. These S/TEM experiments enable us to understand the influence of processing conditions on microstructures and chemical environment, informing us about the structural nuances at the nanoscale and allowing us to tune the structure for desired properties.

Full text available from Microscopy and Microanalysis

New publication! “Achieving Semi-Metallic Conduction in Al-Rich AlGaN: Evidence of Mott Transition”

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Abstract: The development of high performance wide-bandgap AlGaN channel transistors with high current densities and reduced Ohmic losses necessitates extremely highly doped, high Al content AlGaN epilayers for regrown source/drain contact regions. In this work, we demonstrate the achievement of semi-metallic conductivity in silicon (Si) doped N-polar Al0.6Ga0.4N grown on C-face 4H-SiC substrates by molecular beam epitaxy. Under optimized conditions, the AlGaN epilayer shows smooth surface morphology and a narrow photoluminescence spectral linewidth, without the presence of any secondary peaks. A favorable growth window is identified wherein the free electron concentration reaches as high as ∼1.8 × 1020 cm−3 as obtained from Hall measurements, with a high mobility of 34 cm2/V·s, leading to a room temperature resistivity of only 1 mΩ·cm. Temperature-dependent Hall measurements show that the electron concentration, mobility, and sheet resistance do not depend on temperature, clearly indicating dopant Mott transition to a semi-metallic state, wherein the activation energy (Ea) falls to 0 meV at this high value of Si doping for the AlGaN films. This achievement of semi-metallic conductivity in Si doped N-polar high Al content AlGaN is instrumental for advancing ultrawide bandgap electronic and optoelectronic devices.

Full text available from Applied Physics Letters

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Our research is at the intersection of multiple disciplines, drawing on principles and methodologies from materials science, chemistry, physics, and electrical engineering. Our mission is to pioneer … Read More

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Ferroelectronics Lab
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