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The physical mechanisms responsible for electrically-induced parametric degradation in GaN-based high electron mobility transistors are examined using a combination of experiments, device simulation, and first-principles defect analysis. A relatively simple formulation is developed under the assumption that the hot-electron scattering cross-section is independent of the electron energy. In this case, one can relate the change in defect concentration to the operational characteristics of a device, such as the spatial and energy distribution of electrons (electron temperature), electric field distribution, and electron energy loss to the lattice.
It is widely believed that switching to the conductive state in memristive materials is triggered by the external field that drives defect dynamics. In polycrystalline materials, grain boundaries are further believed to cause switching by enabling faster defect motion. Here, we report a first-principle study of oxygen vacancy dynamics at a grain boundary (GB) in polycrystalline ZnO and show that switching to the conductive state is triggered by a recombination-enhanced motion of vacancies perpendicular to the GB. We call this mechanism the “breathing” trigger of memristive switching.
Colossal ionic conductivity was recently discovered in YSZ/SrTiO3 multilayers and was explained in terms of strain- and interface-enhanced disorder of the O sublattice. In the present paper we use a combination of scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) and theoretical EELS simulations to confirm the presence of a disordered YSZ O sublattice in coherent YSZ/SrTiO3 multilayers. O K-edge fine structure simulated for the strained disordered O sublattice phase of YSZ possesses blurred-out features compared to that of ordered cubic bulk YSZ, and experimental EELS fine structure taken from the strained YSZ of coherent YSZ/SrTiO3 thin films is similarly blurred out. Elemental mapping is shown to be capable of resolving ordered YSZ O sublattices. Elemental mapping of O in the coherent YSZ/STO multilayers is presented in which the O sublattice is seen to be clearly resolved in the STO but blurred out in the YSZ, indicating it to be disordered. In addition, we present imaging and EELS results which show that strained regions exist at the incoherent interfaces of YSZ islands in STO with blurred out fine structure, suggesting these incoherent regions may also support high ionic conductivities. Recently, Cavallaro et al. reported electronic conductivities in samples of incoherent disconnected islands embedded in STO that are similar to the islands described herein. The presence of a region of O depleted STO at the interface with incoherent YSZ islands is revealed by EELS elemental mapping, implying the n-type doping of STO/YSZ nanocomposites with disconnected incoherent YSZ islands.
The elucidation of physical properties of organic materials is important
for further optimization of related electronic and optoelectronic
devices. Here we review briefly various first-principles computational tools
for the modeling of these materials by investigating key structural,
electronic, and chemical properties of prototype organic semiconductors.
In particular, we discuss the site-selectivity for band formation in pentacene
and rubrene, hydrogenation and transformations of metal-free phthalocyanines,
and the bonding topology in a hybrid organic-inorganic system.