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We use time-resolved terahertz-pulse spectroscopy to study the ultrafast carrier dynamics in InGaAs/GaAs (311)A quantum wires. Anisotropy in the photoconductive dynamics is observed when aligning the terahertz probe polarization parallel versus perpendicular to the wire direction. The origin of this anisotropy is the carrier capture into localized quantum-wire states from delocalized wetting layer or barrier regions over time scales from 6 to 30 ps. The capture efficiency is found to be strongly temperature dependent, with thermal emission dominating above 125 K, while state-filling effects within the wires influence the capture rate below 125 K. Transient spectroscopy reveals a Drude-like carrier conductivity. [ABSTRACT FROM AUTHOR]

A systematic experimental evaluation of the thermal stability of Ru metal gate electrodes in direct contact with SiO2 and Hf-based dielectric layers was performed and correlated with electrical device measurements. The distinctly different interfacial reactions in the Ru/SiO2, Ru/HfO2, and Ru/HfSiOx film systems were observed through cross-sectional high-resolution transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy with electron-energy-loss spectra, and energy dispersive x-ray spectra analysis. Ru interacted with SiO2, but remained stable on HfO2 at 1000 °C. The onset of Ru/SiO2 interfacial interactions is identified via silicon substrate pitting possibly from Ru diffusion into the dielectric in samples exposed to a 900 °C/10-s anneal. The dependence of capacitor device degradation with decreasing SiO2 thickness suggests Ru diffuses through SiO2, followed by an abrupt, rapid, nonuniform interaction of ruthenium silicide as Ru contacts the Si substrate. Local interdiffusion detected on Ru/HfSiOx samples may be due to phase separation of HfSiOx into HfO2 grains within a SiO2 matrix, suggesting that SiO2 provides a diffusion pathway for Ru. Detailed evidence consistent with a dual reaction mechanism for the Ru/SiO2 system at 1000 °C is presented. [ABSTRACT FROM AUTHOR]

The microchemical and microstructural origins of insulation-resistance degradation in BaTiO3-based capacitors are studied by complementary impedance spectroscopy and analytical transmission electron microscopy. The degradation under dc-field bias involves electromigration and accumulation of oxygen vacancies at interfaces. The nonstoichiometric BaTiO3-δ becomes locally more conducting through increased oxygen vacancy concentration and Ti ion reduction. The symmetry across the dielectric layer and locally across each grain is broken during the degradation process. Locally, the nonstoichiometry becomes so severe that metastable lattice structures are formed. The degradation in insulation resistance at the grain boundaries and electrode interfaces is associated with the double Schottky-barrier potential lowering and narrowing. This may correlate with an effective decrease in net acceptor charge density at the grain boundaries. [ABSTRACT FROM AUTHOR]

It was found that by using the same α-Fe2O3 target in fabrication process, an oxygen-deficient and an oxygen-sufficient atmosphere facilitated the formation of Fe3O4 and α-Fe2O3, respectively. The Au/α-Fe2O3/Pt heterostructure showed a memristive bipolar resistance switching. A clear difference was observed in the transport and magnetic properties between the two resistance states. The enhanced conductivity and magnetization in the Au/α-Fe2O3/Pt heterostructure are believed to result from an enhanced electron hopping between Fe3+-Fe2+ pairs and Fe3+-O-Fe2+ double exchange coupling effect. The enhanced Fe2+ comes from a redox transition between two phases (one behaves like Fe3O4 and the other like α-Fe2O3), due to the electrochemical migration of oxygen vacancies. [ABSTRACT FROM AUTHOR]

Describes a substrate material for depositing high-temperature superconducting YBa[sub2]Cu[sub3]O[sub7-δ] (YBCO) thin films. Preparation of the films; Method used to examine the structure of the deposited YBCO thin films.

We investigate the scaling of Al2O3 dielectric on graphene by atomic layer deposition (ALD) using ultra-thin, oxidized Ti and Al films as nucleation layers. We show that the nucleation layer significantly impacts the dielectric constant (k) and morphology of the ALD Al2O3, yielding k = 5.5 and k = 12.7 for Al and Ti nucleation layers, respectively. Transmission electron microscopy shows that Al2O3 grown using the Ti interface is partially crystalline, while Al2O3 grown on Al is amorphous. Using a spatially uniform 0.6 nm-thick Ti nucleation layer, we demonstrate graphene field-effect transistors with top dielectric stacks as thin as 2.6 nm. [ABSTRACT FROM AUTHOR]

By controlling the temperature and oxygen partial pressure, well polycrystalline Fe3O4 thin film on Pt substrate was fabricated, using a stoichiometric α-Fe2O3 target. A ferrimagnetic hysteresis loop and a Verwey transition at about 130 K were shown in the results of magnetic measurement. A sharp hysteretic resistance switching below Verwey transition temperature (Tv) was observed in I-V tests of the Au/Fe3O4/Pt heterostructure. Enlarged hysteresis by larger pulse delay and suppressed metal-insulator transition by larger source currents indicated that enhanced local Joule heating effect played an important role in the hysteresis of resistance switching. [ABSTRACT FROM AUTHOR]

Bipolar resistive switching (RS) behavior was observed in epitaxially deposited TiO2-δ/La2/3Sr1/3MnO3 stacks. An intermediate initial state of this device demonstrated that the original resistive state would switch to either the higher or the lower state, depending only on the electrical polarity of the initial measurement. As the analogue intermediate states were obtained by device fabrication, the direct observation of these switching states was possible, knowing the microscopic physical origin of the RS effect. Based on x-ray photoelectron spectroscopy analysis, an understanding of this interesting phenomenon was proposed, well supporting the oxygen vacancy formation and recombination mechanism. [ABSTRACT FROM AUTHOR]

We have demonstrated that the resistance switching (RS) effect can be controlled by the modification of the electrode configurations and the carrier densities in the Ag/SrTiO3 and Ag/Nb-doped SrTiO3(Nb:STO) structures. The elimination of the Schottky junction in the metal/Nb:STO completely destroys the RS effect, which suggests that the RS effect originates from the modification of Schottky-like barrier formed at the interface of metal/Nb:STO. The rectifying I-V curves revealed that the change in resistance was attributed to the trapping or detrapping carriers at the interface. The carrier density plays an important role in the determination of RS effect. The presence of the RS in SrTiO3 requires an appropriate doping level to provide conditions for trapping carriers at the interface. [ABSTRACT FROM AUTHOR]

A resistive switching device based on the stacked α-Fe2O3/Nb-SrTiO3 is proposed and fabricated that demonstrates excellent bipolar resistive switching behaviors including the uniformity, endurance, and retention performance. The Schottky-like current-voltage characteristics correlated with set process were observed in both high resistive states (HRSs) and low resistive states (LRSs) of the device. Importantly, the anticrosstalk characteristic, possessing higher reversed-biased LRS resistance than the forwarded-biased HRS resistance, indicates the potential applications of the stacked α-Fe2O3/Nb-SrTiO3 for multilevel storage in the cross-bar memory arrays. The carrier injection and trapping mechanism is suggested to explain the observed phenomena. [ABSTRACT FROM AUTHOR]

The structure and chemical composition of ZnSe/Ga[sub 1-x]Mn[sub x]As/ZnSe multilayers grown on (100) GaAs substrates are investigated by high-resolution transmission electron microscopy imaging and spectroscopy techniques. While all layers grow epitaxially and the Ga[sub 1-x]Mn[sub x]As layer is free of planar defects, a high density of stacking faults is observed in the ZnSe layer over Ga[sub 1-x]Mn[sub x]As. The composition of the ferromagnetic layer is measured to be Ga[sub 0.93]Mn[sub 0.07]As, and the Mn valence was determined to be 2[sup +]. Compositional profiles across the interfaces quantified by electron energy-loss spectroscopy show that the ZnSe/Ga[sub 1-x]Mn[sub x]As interfaces are wider than the ZnSe/ GaAs-substrate interface, which is mainly attributed to interfacial roughness. [ABSTRACT FROM AUTHOR]

We present a study of the anisotropic properties of two-dimensional electron gases formed in GaAs/AlGaAs heterostructures in which InAs self-assembled quantum dots have been inserted into the center of a GaAs quantum well. We observe an anisotropic mobility for the orthogonal [1¯10] and [110] directions. The mobility in the [1¯10] direction was found to be up to approximately twice that in the [110] direction. It is suggested that the interface roughness scattering due to the inserted InAs material could be a cause for the large anisotropies in mobility. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pronounced bipolar resistance switching with a good retention property has been observed in CeO2/PrBa2Cu3O7-δ/Pt heterostructure. The low resistance state and high resistance state exhibited distinguished ferromagnetic signals, as compared to the nearly non-magnetic initial state. It is found that the migration of the oxygen vacancies under electric field is mainly responsible for the electric and the magnetic changes. The modified interfacial electronic structure by the oxygen vacancy migration and the trapping/detrapping of the carriers leads to the resistance switching. The exchange interaction of the hydrogen-like orbitals formed around the singly occupied oxygen vacancies in CeO2 is accounting for the emerged and modulated ferromagnetic signals. Temperature dependence of resistance in the low resistance state follows a variable range hopping law, further confirming that the amount of oxygen vacancies in the CeO2 layer directly affects the hydrogen-like orbital radius, which determines the strength of the ferromagnetic coupling. [ABSTRACT FROM AUTHOR]

We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system. [ABSTRACT FROM AUTHOR]

Silicon nanowires will find applications in nanoscale electronics and optoelectronics both as active and passive components. Here, we demonstrate a low-temperature vapor–liquid–solid synthesis method that uses liquid-metal solvents with low solubility for silicon and other elemental semiconductor materials. This method eliminates the usual requirement of quantum-sized droplets in order to obtain quantum-scale one-dimensional structures. Specifically, we synthesized silicon nanowires with uniform diameters distributed around 6 nm using gallium as the molten solvent, at temperatures less than 400 °C in hydrogen plasma. The potential exists for bulk synthesis of silicon nanowires at temperatures significantly lower than 400 °C. Gallium forms a eutectic with silicon near room temperature and offers a wide temperature range for bulk synthesis of nanowires. These properties are important for creating monodispersed one-dimensional structures capable of yielding sharp hetero- or homointerfaces. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a study of the transport properties of two-dimensional electron gases formed in GaAs/AlGaAs heterostructures in which InAs self-assembled quantum dots have been inserted in the center of a GaAs quantum well. We observed that, while maintaining a constant carrier density, the mobility increased as the InAs dot density was reduced. The ratio of the transport to the quantum lifetime was measured to be approximately five with the dominant scattering mechanism attributed to short-range scattering from the inserted InAs dots. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have demonstrated that the resistance switching (RS) effect can be controlled by the modification of the electrode configurations and the carrier densities in the Ag/SrTiO3 and Ag/Nb-doped SrTiO3(Nb:STO) structures. The elimination of the Schottky junction in the metal/Nb:STO completely destroys the RS effect, which suggests that the RS effect originates from the modification of Schottky-like barrier formed at the interface of metal/Nb:STO. The rectifying I-V curves revealed that the change in resistance was attributed to the trapping or detrapping carriers at the interface. The carrier density plays an important role in the determination of RS effect. The presence of the RS in SrTiO3 requires an appropriate doping level to provide conditions for trapping carriers at the interface. [ABSTRACT FROM AUTHOR]