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The introduction of atomic layer deposition (ALD), to the microelectronics industry has introduced a large number of new possible materials able to be deposited in layers with atomic thickness control. One such material is the high-κ oxide HfO2; thermally stable and ultrathin HfO2 films deposited by ALD are a significant contender to replace SiO2 as the gate oxide in capacitor applications. We present a mechanistic study of the first deposition cycle of HfO2 on the Si(111) surface using tetrakis(dimethylamido) hafnium (TDMAHf) and water as precursors using operando ambient pressure x-ray photoelectron spectroscopy. Here, we show that the hydroxylation of the clean Si(111) surface by residual water vapor, resulting in a 0.3 monolayer coverage of hydroxyls, leads to instantaneous full surface coverage of TDMAHf. The change in the atomic ratio of Hf to C/N found during the first deposition half-cycle, however, does not match the assumed immediate ligand loss through reaction with surface hydroxyls. One would expect an immediate loss of ligands, indicated by a Hf:N ratio of approximately 1:3 as TDMAHf deposits onto the surface; however, a Hf:N ratio of 1:3.6 is observed. The partial hydroxylation on the Si(111) surface leads to binding through the TDMAHf ligand N atoms resulting in both N and CH3 being found remaining on the surface post water half-cycle. Although there is evidence of ligand exchange reactions occurring at Si–OH sites, it also seems that N binding can occur on bare Si, highlighting the complexity of the substrate/precursor reaction even when hydroxyls are present. Moreover, the initial low coverage of Si–OH/Si–H appears to severely limit the amount of Hf deposited, which we hypothesize is due to the specific geometry of the initial arrangement of Si–OH/Si–H on the rest- and adatoms. [ABSTRACT FROM AUTHOR]

The correlation between the structural phase transition (SPT) and oxygen vacancy in SrRuO3 (SRO) thin films was investigated by in situ X-ray diffraction (XRD) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In situ XRD shows that the SPT occurs from a monoclinic SRO phase to a tetragonal SRO phase near ∼200 °C, regardless of the pressure environment. On the other hand, significant core level shifts in both the Ru and Sr photoemission spectra are found under ultrahigh vacuum, but not under the oxygen pressure environment. The directions and behavior of the core level shift of Ru and Sr are attributed to the formation of oxygen vacancy across the SPT temperature of SRO. The analysis of in situ XRD and AP-XPS results provides an evidence for the formation of metastable surface oxide possibly due to the migration of internal oxygen atoms across the SPT temperature, indicating the close relationship between oxygen vacancy and SPT in SRO thin films. [ABSTRACT FROM AUTHOR]

Utilizing ambient pressure x-ray photoelectron spectroscopy, the surface segregation and the oxidation dynamics of platinum (Pt)-3d transition metal (M) alloys, Pt3M (M = Ti, V), are investigated. Under oxygen-rich conditions, i.e., P(CO)/P(O2) = 0.1, Pt3V surface forms Pt skin layer while Pt3Ti shows the presence of both Ti and Pt atoms. As temperature increases to 450 K, V atoms make surface segregation to form oxide while Ti atoms start to form various Ti oxides (Ti2O3 and TiOx) on the surface. When CO oxidation occurs at 600 K, the oxidation continues on both surfaces of Pt3M (M = Ti, V), showing the enhancement of TiOx for Pt3Ti and V2O5 and VOx for Pt3V. Also, during CO oxidation, a sign of pure metallic Pt state is found at Pt 4f spectra in both surfaces, suggesting the redistribution of electrons from the transition metals during the oxide formation. [ABSTRACT FROM AUTHOR]

We theoretically investigate germanium-tin alloy as a semiconductor for the design of near infrared optical modulators in which the Ge1-xSnx alloy is the active region. We have calculated the electronic band parameters for heterointerfaces between strained Ge1-xSnx and relaxed Si1-yGey Then, a type-I strain-compensated Si0.10Ge0.90/Si0.16Ge0.84/Ge0.94Sn0.06 quantum well heterostructure optimized in terms of compositions and thicknesses is studied by solving Schrödinger equation without and under applied bias voltage. The strong absorption coefficient (>1.5 x 104cm-1) and the shift of the direct transition under large Stark effect at 3 V are useful characteristics for the design of optoelectronic devices based on compressively strained IV-IV heterostructures at near infrared wavelengths. [ABSTRACT FROM AUTHOR]

We investigate the transport characteristics of monolayer graphene p-n junctions by means of the nonequilibrium Green's function technique. It is shown that due to the high interband tunneling of chiral fermions and a finite bandgap opening when the inversion symmetry of the graphene plane is broken, a strong negative-differential-conductance behavior with a peak-to-valley ratio as large as a few tens can be achieved even at room temperature. The dependence of this behavior on the device parameters such as the Fermi energy, the barrier height, and the transition length is then discussed. [ABSTRACT FROM AUTHOR]

We report a microscopic investigation of the spectrum of voltage fluctuations in nanometric n-Si layers. Theory makes use of a Monte Carlo simulator self-consistently coupled with a two-dimensional Poisson solver. We consider layers of variable thickness W in the range of 2–100 nm and variable length L in the range of 10–1000 nm embedded in an external dielectric medium. Calculations are performed at T=300 K for different doping levels and in the presence of an applied voltage of increasing strength. The spectra are found to exhibit peaks centered on the terahertz region. For W≥100 nm and carrier densities of 5×1017 and 5×1018 cm-3, the frequency peaks agree with the value of the three dimensional plasma frequency. For W≤100 nm, the results exhibit a plasma frequency that depends on L, thus implying that the oscillation mode is dispersive. The corresponding frequency covers a wide range of values of 0.2–10 THz and is in agreement with the values of the two-dimensional plasma frequency predicted by existing analytical models. At sufficiently high voltages, the two-dimensional plasma peak is washed out and we observe the onset of a peak in the subterahertz region which is associated with transit time instabilities induced by current saturation conditions. [ABSTRACT FROM AUTHOR]

A vapor-deposited NH3 ice film irradiated at 20 K with 150 eV photons has been studied with near-edge x-ray absorption fine structure (NEXAFS) spectroscopy at the nitrogen K-edge. Irradiation leads to the formation of high amounts (12%) of molecular nitrogen N2, whose concentration as a function of the absorbed energy has been quantified to 0.13 molecule/eV. The stability of N2 in solid NH3 has been also studied, showing that N2 continuously desorbs between 20 and 95 K from the irradiated ammonia ice film. Weak concentrations (<1%) of other photoproducts are also detected. Our NEXAFS simulations show that these features own to NH2, N2H2, and N3-. [ABSTRACT FROM AUTHOR]

Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current. [ABSTRACT FROM AUTHOR]

Motivated by recent studies on the use of graphene for new concepts of electronic/spintronic devices, the authors develop an efficient calculation method based on the nonequilibrium Green’s function to solve the quantum relativisticlike Dirac’s equation that governs the low-energy excited states in graphene. The approach is then applied to investigate the electronic transport and the spin polarization in a single-graphene barrier structure. The obtained results are presented and analyzed in detail aiming to highlight typical properties of the considered graphene structure as well as the efficiency of the developed approach that both may be helpful for further development in electronic devices and in spintronics. [ABSTRACT FROM AUTHOR]

This paper investigates the mobility extraction from channel magnetoresistance, which is widespreading as a powerful experimental method to study transport in short gate devices. A fully self-consistent Monte Carlo device simulator is used to simulate the influence of a transverse magnetic field on electron transport in nanometer scale devices. After validation on a simple silicon magnetic sensor, the method is applied to the simulation of the channel magnetoresistance of nanoscale double gate metal oxide semiconductor field effect transistors. Apparent magnetoresistance mobilities μMR are extracted from channel resistance variation with the applied magnetic field, using a measurement-inspired extraction method. The simulated temperature trends obtained by simulation are consistent with experimental data. As experimentally observed elsewhere, the extracted apparent mobility decreases with the shrinking of the channel length. No additional scattering mechanism specific to short channel devices was needed to observe this effect. This apparent mobility reduction observed in the simulated results is shown to originate from nonstationary transport, which is discussed and interpreted using simple numerical calculations. We propose a Mathiessen-like formalism in order to quantify this effect. Finally, ballistic transport is shown to have a significant impact on the apparent mobility extraction and must be taken into account if the apparent magnetoresistance mobility is to be used as a figure of merit to assess short device performance. [ABSTRACT FROM AUTHOR]

We present a ringlike design for spin field effect transistor electrodes. This configuration solves the local Hall effect drawback as these electrodes do not generate any stray magnetic fields. The shape and size of the electrodes are adjusted in order to match the physical constraints. The gate configuration and channel length are discussed for the [110] growth direction; the GaInAs channel length for the spins to fully switch is calculated to be of the order of 0.15 μm for a gate electric field of 150–300 kV/cm. [ABSTRACT FROM AUTHOR]

The changes in the structure and composition of vapor-deposited ice films irradiated at 20 K with soft x-ray photons (3–900 eV) and their subsequent evolution with temperatures between 20 and 150 K have been investigated by near-edge x-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K edge. We observe the hydroxyl OH, the atomic oxygen O, and the hydroperoxyl HO2 radicals, as well as the oxygen O2 and hydrogen peroxide H2O2 molecules in irradiated porous amorphous solid water (p-ASW) and crystalline (Icryst) ice films. The evolution of their concentrations with the temperature indicates that HO2, O2, and H2O2 result from a simple step reaction fuelled by OH, where O2 is a product of HO2 and HO2 a product of H2O2. The local order of ice is also modified, whatever the initial structure is. The crystalline ice Icryst becomes amorphous. The high-density amorphous phase (Iah) of ice is observed after irradiation of the p-ASW film, whose initial structure is the normal low-density form of the amorphous ice (Ial). The phase Iah is thus peculiar to irradiated ice and does not exist in the as-deposited ice films. A new “very high density” amorphous phase—we call Iavh—is obtained after warming at 50 K the irradiated p-ASW ice. This phase is stable up to 90 K and partially transforms into crystalline ice at 150 K. [ABSTRACT FROM AUTHOR]

Vapor-deposited H[sub 2]O ice films grown between 38 and 150 K under ultrahigh vacuum conditions have been investigated using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy at the oxygen K-edge, in conventional mode—which is bulk sensitive-, and using the photon-stimulated desorption mode (PSD-NEXAFS), which is surface sensitive. By recording simultaneously those two signals, we have evidenced the differences between the surface and bulk electronic and atomic structures, for both amorphous porous ice condensed at 40 K and crystalline ice condensed at 150 K. We have also followed the bulk and surface evolutions of an amorphous ice film annealed from 38 to 147 K. A steep change in the local atomic structure of the bulk is observed, likely related to the high-density amorphous ice→low-density amorphous ice phase transition between 38 and 55 K. We have shown that the surface of crystalline ice is well ordered, but this order is different from that of the bulk. We have evidenced that the H[sub 2]O-H[sub 2]O intermolecular distance at the surface of ice is always longer than in the bulk, and that this difference increases with temperature, as the thermally induced reordering of the surface proceeds. SCF-Xα multiple scattering calculations allow us to figure out those structural differences, both in the bulk and at the surface of amorphous ice, but further calculations are necessary for crystalline ice. We have shown that the PSD-NEXAFS signal is sensitive to the surface morphology that changes with temperature because of the micropores collapse. We have used a model [E. Vichnevetski, A. D. Bass, and L. Sanche, J. Chem. Phys. 113, 3874 (2000)] that quantitatively describes the effect of the surface porosity on the ion yield. The surface of ice at 38 K is well described by a network of vertical cylindrical pores of 20 Å of diameter, separated by 6 Å, collapsing when annealing the film. This model also properly accounts of... [ABSTRACT FROM AUTHOR]

The surface chemical states of Pd(100) during CO oxidation were investigated using ambient pressure x-ray photoelectron spectroscopy and mass spectroscopy. Under the reactant ratio of CO/O2 = 0.1, i.e. an oxygen-rich reaction condition, the formation of surface oxides was observed with the onset of CO oxidation reaction at T = 525 K. As the reactant ratio (CO/O2) increased from 0.1 to 1.0, ∼ 90 % surface oxides remains on surface during the reaction. Upon the formation of surface oxides, the core level shift of oxygen gas phase peak was observed, indicating that change of surface work function. As CO oxidation takes places, i.e. making a transition from CO covered surface to the oxidic surface, the work functions of surface oxide on Pd(100) and Pt(110) display opposite behavior. [ABSTRACT FROM AUTHOR]

Silicene is an emerging 2D material with advantages of high carrier mobility, compatibility with the silicon-based semiconductor industry, and the tunable gap by a vertical electrical field due to the buckling structure. In this work, we report a first-principles investigation on the spin injection system, which consists of a Fe(111)/silicene stack as the spin injector and pure silicene as the spin channel. An extremely high spin injection efficiency (SIE) close to 100% is achieved. The partial density of states of Fe layers in the Fe(111)/silicene stack shows that spin-down states dominate above the Fermi level, resulting in a negligible spin-up current and high SIE. The transmission spectra have been investigated to analyze the spin-resolved properties. The spin injection system based on silicene is promising for the efficient silicon-based spintronics devices such as switching transistors. [ABSTRACT FROM AUTHOR]

In this work, we present a graphene-based all-spin logic gate (G-ASLG) that integrates the functionalities of perpendicular anisotropy magnetic tunnel junctions (p-MTJs) with spin transport in graphene-channel. It provides an ideal integration of logic and memory. The input and output states are defined as the relative magnetization between free layer and fixed layer of p-MTJs. They can be probed by the tunnel magnetoresistance and controlled by spin transfer torque effect. Using lateral non-local spin valve, the spin information is transmitted by the spin-current interaction through graphene channels. By using a physics-based spin current compact model, the operation of G-ASLG is demonstrated and its performance is analyzed. It allows us to evaluate the influence of parameters, such as spin injection efficiency, spin diffusion length, contact area, the device length, and their interdependence, and to optimize the energy and dynamic performance. Compared to other beyond-CMOS solutions, longer spin information transport length (~μm), higher data throughput, faster computing speed (~ns), and lower power consumption (~μA) can be expected from the G-ASLG. [ABSTRACT FROM AUTHOR]

We investigate the device operation and performance of transistors based on a graphene nanomesh lattice. By means of numerical simulation, we show that this device architecture allows suppressing the chiral tunneling, which reduces drastically the off current and enhances the on/off ratio compared to the pristine graphene counterpart. Additionally, a good saturation of current can be reached in the thermionic regime of transport. Though reduced compared to the case of pristine transistors, the transconductance and the cutoff frequency are still high. Above all, the nanomesh transistors outperform their pristine graphene counterpart in terms of voltage gain and maximum oscillation frequency. [ABSTRACT FROM AUTHOR]

The I-V characteristics of graphene nanoribbon (GNR) p-n junctions have been investigated using atomistic quantum simulation. On the basis of results obtained for simple armchair GNR structures with large bandgap, it is suggested to improve significantly the device operation by inserting a small-bandgap section in the transition region between n and p zones. A giant peak-to-valley ratio (PVR) of negative differential conductance (higher than 103) can be achieved in such hetero-junctions. Additionally, the PVR is proved to be weakly sensitive to the transition length and not strongly degraded by the edge disorder, which is an important feature regarding applications. [ABSTRACT FROM AUTHOR]

Graphene layers have been grown by molecular beam epitaxy (MBE) on the [formula] C-face of SiC and have been characterized by atomic force microscopy, low energy electron diffraction (LEED), and UV photoelectron spectroscopy. Contrary to the graphitization process, the step-terrace structure of SiC is fully preserved during the MBE growth. LEED patterns show multiple orientation domains which are characteristic of graphene on SiC [formula], indicating non-Bernal rotated graphene planes. Well-defined Dirac cones, typical of single-layer graphene, have been observed in the valence band for few graphene layers by synchrotron spectroscopy, confirming the electronic decoupling of graphene layers. [ABSTRACT FROM AUTHOR]

Using the nonequilibrium Green’s function method, the electronic transport in a gate-induced barrier bilayer graphene structure is investigated. Strong resonant effects are shown to result in high amplitude oscillation of conductance as a function of Fermi energy and barrier height. Beyond a small effect of negative differential conductance (with peak to valley ratio less than 2), strong oscillations of transconductance are achieved. The amplitude of such oscillations between positive and negative values may exceed 5 mS/μm. This effect might be helpful for further development of graphene-based nanoelectronics. [ABSTRACT FROM AUTHOR]

This letter shows that a moderate degree of edge disorder can explain the fact that the experimentally measured bandgaps of graphene nanoribbons (GNRs) do not depend on orientation. We argue that GNRs actually behave similarly to Anderson insulators and the measured bandgaps should thus be interpreted as quasi-mobility edges. Calculations in the tight binding approach reveal that in the presence of edge disorder, quasi-mobility edge and electronic structures become independent of orientation and that quasi-mobility edge follows a quasi-universal law similar to experimental data, although with different parameters. [ABSTRACT FROM AUTHOR]

We study spin transport in bilayer graphene structures where gate electrodes are attached to ferromagnetic graphene. Due to the exchange field in the gated regions, the current becomes spin dependent and can be controlled by tuning the gate voltages. It is shown that thanks to strong resonant chiral tunneling inherent in bilayer graphene, very high spin polarization and tunneling magnetoresistance can be achieved in the considered structures. Different possibilities for controlling the spin current are discussed. The study demonstrates the potential of bilayer graphene structures for spintronic applications with significant improvement over previously predicted results in monolayer graphene structures. [ABSTRACT FROM AUTHOR]

Photodissociation of acetonitrile (CH[sub 3]CN) condensed, physisorbed on Au(111), and chemisorbed on Pt(111), has been studied at the C(1s) and N(1s) edges in H[sup +] yield and/or in total positive ion yield. The H[sup +] yield C(1s) absorption experiment on the condensed acetonitrile shows a selective fragmentation of the C-H bond at the C 1s→π[sup *]CH[sub 3] excitation, that is still observed when acetonitrile is physisorbed on Au(111) and chemisorbed on Pt(111). Transitions to the π[sup *]C=N state give rise to unselective fragmentations, which are strongly quenched by charge transfer from the substrate when acetonitrile is chemisorbed on Pt(111). The π[sub ⊥][sup *]C=N level that plunges into the Pt(111) is the most affected and vanishes in the total ion yield measurement. The quenching is weaker on the π[sub ||][sup *]C=N which is parallel to the surface, and thus is less hybridized with the metal than the π[sub ⊥][sup *]C=N. For acetonitrile physisorbed on Au(111), the influence of the surface is limited to the quenching of the desorption of the heaviest outgoing fragments. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An original approach is developed to investigate the electron spin dynamics in III-V quantum wells using a particle Monte Carlo transport model. We study the spin precession related to the D'yakonov-Perel' mechanism, which is believed to be the predominant spin relaxation phenomenon in AlGaAs/GaAs quantum wells at room temperature. Using a Monte Carlo approach, the effect of electron/crystal scatterings on the D'yakonov-Perel' mechanism can be both simply and accurately taken into account. Finally, including interface roughness in the calculation leads to spin relaxation times in good agreement with the experimental data. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]