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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]

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]

An efficient calculation method based on the nonequilibrium Green's function is developed for solving the quantum relativisticlike Dirac's equation that has governed the low-energy excited states in graphene. This method is used for examining the electron transport and the spin polarization in a single-graphene barrier structure.

The article presents a complete analysis of the spin-polarized current and tunneling magnetoresistance exhibited by the different ferromagnetic gate bilayer graphene structures. The use of the structures for different spintronic applications is also discussed.