arXiv subjects

Xiao Shen

Publications and source records attributed to Xiao Shen.

59 records · Page 4Linked to original sources

arXiv2015

Two-dimensional nanovaristors at grain boundaries account for memristive switching in polycrystalline BiFeO3

Memristive switching in polycrystalline materials is widely attributed to the formation and rupture of conducting filaments, believed to be mediated by oxygen-vacancy redistribution. The underlying atomic-scale processes are still unknown, however, which limits device modeling and design. Here we combine experimental data with multi-scale calculations to elucidate the entire atomic-scale cycle in undoped polycrystalline BiFeO3. Conductive atomic force microscopy reveals that the grain boundaries behave like two-dimensional nanovaristors while, on the return part of the cycle, the decreasing current is through the grains. Using density-functional-theory and Monte-Carlo calculations we deduce the atomic-scale mechanism of the observed phenomena. Oxygen vacancies in non-equilibrium concentrations are initially distributed relatively uniformly, but they are swept into the grain boundaries by an increasing voltage. A critical voltage, the SET voltage, then eliminates the barrier for hopping conduction through vacancy energy levels in grain boundaries. On the return part of the cycle, the grain boundaries are again non-conductive, but the grains show nonzero conductivity by virtue of remote doping by oxygen vacancies. The RESET voltage amounts to a heat pulse that redistributes the vacancies. The realization that nanovaristors are at the heart of memristive switching in polycrystalline materials may open possibilities for novel devices and circuits.

cond-mat.mtrl-sci↗

arXiv2015

Atomic origin of high temperature electron trapping in MOS devices

MOSFETs based on wide band-gap semiconductors are suitable for operations at high temperature, at which additional atomic-scale processes that are benign at lower temperatures can get activated which results in device degradation. Recently significant enhancement of electron trapping was observed under positive bias in SiC MOSFETs at temperatures higher than 150°C. Here we report first-principle calculations showing that the enhanced electron trapping is associated with thermally activated capturing of a second electron by an oxygen vacancy in SiO2, by which the vacancy transforms into a new structure that comprises one Si dangling bond and a bond between a five-fold and a four-fold Si atoms. The results suggest a key role of oxygen vacancies and their structural reconfigurations in the reliability of high-temperature MOS devices.

cond-mat.mtrl-sci↗

arXiv2012

Competing Atomic and Molecular Mechanisms of Thermal Oxidation

The oxidation of SiC and Si provide a unique opportunity for studying oxidation mechanisms because the product is the same, SiO2. Silicon oxidation follows a linear-parabolic law, with molecular oxygen identified as the oxidant. SiC oxidation obeys the same linear-parabolic law but has different rates and activation energies and exhibits much stronger face-dependence. Using results from first-principles calculations, we show that atomic and molecular oxygen are the oxidant for Si- and C-face SiC respectively. Comparing SiC with Si, we elucidate how the interface controls the competition between atomic and molecular mechanisms.

cond-mat.mtrl-sci↗

arXiv2010

Identification of the major cause of endemically poor mobilities in SiC/SiO2 structures

Materials with good carrier mobilities are desired for device applications, but in real devices the mobilities are usually limited by the presence of interfaces and contacts. Mobility degradation at semiconductor-dielectric interfaces is generally attributed to defects at the interface or inside the dielectric, as is the case in Si/SiO2 structures, where processing does not introduce detrimental defects in the semiconductor. In the case of SiC/SiO2 structures, a decade of research focused on reducing or passivating interface and oxide defects, but the low mobilities have persisted. By invoking theoretical results and available experimental evidence, we show that thermal oxidation generates carbon di-interstitial defects inside the semiconductor substrate and that they are a major cause of the poor mobility in SiC/SiO2 structures.

cond-mat.mtrl-sci↗

arXiv2006

Electronic Structure of ZnO nanowire

This paper presents two first-principles calculations on the lattice- and electronic- structures of a small-diameter infinite and truncated ZnO nanowire. The two calculations show excellent agreement with each other. For the small diameter nanowire, the lattice and electronic properties are determined by the relaxed surface. We find a similarity between the nanowire surface and the surface of the bulk material, and discuss the assignment of the rotational quantum number m to the bands.

cond-mat.mtrl-sci↗