Negative Differential Behavior Research Articles

Vertical Current Transport in Monolayer MoS<sub>2</sub> Heterojunctions with 4H-SiC Fabricated by Sulfurization of Ultra-Thin MoO<sub>x</sub> Films

In this paper, we report on the growth of highly uniform MoS2 films, mostly consisting of monolayers, on SiC surfaces with different doping levels (n- SiC epitaxy, ~1016 cm-3, and n+ SiC substrate, ~1019 cm-3) by sulfurization of a pre-deposited ultra-thin MoOx films. MoS2 layers are lowly strained (~0.12% tensile strain) and highly p-type doped (<Nh>≈4×1019 cm−3), due to MoO3 residues still present after the sulfurization process. Nanoscale resolution I-V analyses by conductive atomic force microscopy (C-AFM) show a strongly rectifying behavior for MoS2 junction with n- SiC, whereas the p+ MoS2/n+ SiC junction exhibits an enhanced reverse current and a negative differential behavior under forward bias. This latter observation, indicating the occurrence of band-to-band-tunneling from the occupied states of n+ SiC conduction band to the empty states of p+ MoS2 valence band, is a confirmation of the very sharp hetero-interface between the two materials. These results pave the way to the fabrication of ultra-fast switching Esaki diodes on 4H-SiC.

Ab Initio Investigations of Gallium Nitride Nanoribbons for Spin Filter and Negative Differential Behavior

This work demonstrates the influence of alternative edge passivation via H/F on the electronic and magnetic characteristics of gallium nitride nanoribbons (GaNNRs). For this study, the first-principles density functional theory (DFT) and non-equilibrium green function (NEGF) frameworks are deployed. Our study demonstrates that the selective edge passivation with F/H zigzag GaN nanoribbon (NR) is a strong contender for spintronics due to its half-metallic property under specific conditions. Various nano-configurations for the H/F ZGaNNRs passivation are investigated here. Thermodynamically, most stable configuration is alternate fluorinated F-GaN-F NR. Further, the negative differential resistance (NDR) behavior is also reported along with the perfect spin-filtering properties. This study paves the way to employ these 2-D nanostructures-based devices for spin filters, spin logic switches, and steep switching nanoelectronic devices.

Computational study of spin caloritronics in a pristine and defective antimonene nanoribbon

In this paper, by using first-principle density functional theory (DFT) combined with non-equilibrium Green's function (NEGF), thermally induced spin current in zigzag and armchair Antimonene Nanoribbon (SbNR) is investigated. Also, we obtain higher spin current in Armchair nanoribbon (ANR) than zigzag nanoribbon (ZNR), because the start energy of transmission for ANR is closer to the Fermi level than ZNR. The results show that the device has a perfect spin Seebeck effect under temperature difference without gate voltage or bias voltage. For the ANR configuration, the competition between spin up holes and spin down electrons leads to negative differential behavior of charge current, which is essential for the design of thermal transistors. We also investigate the effect of a single vacancy and double vacancy on the structural and electronic properties of ZNR and ANR. The results show that vacancy in ZNR leads to a decrease in spin currents, whereas in ANR, spin currents increase. Double vacancy in ANR can significantly enhance spin up current up to about 2.5 times the pristine one. The results verify the Armchair Nanoribbon as a promising candidate for spin caloritronics devices, which can be used in future low power consumption technology.

The spin filtering effect and negative differential behavior of the graphene-pentalene-graphene molecular junction: a theoretical analysis.

Density functional theory (DFT) combined with nonequilibrium Green's function (NEGF) formalism are used to investigate the effects of substitutional doping by nitrogen and sulfur on transport properties of AGNR-pentalene-AGNR nanojunction. A considerable spin filtering capability in a wide bias range is observed for all systems, which may have potential application in spintronics devices. Moreover, all model devices exhibit a negative differential effect with considerable peak-to-valley ratio. Thus, our findings provide a way to produce multifunctional spintronic devices based on nitrogen and sulfur doped pentalene-AGNR nanojunctions. The underlying mechanism for this interesting behavior was exposed by analyzing the transmission spectrum as well as the electrostatic potential distribution. In addition, a system doped with an odd number of dopant shows a rectifying efficiency comparable to other systems. The above findings strongly imply that such a multifunctional molecular device would be a useful candidate for molecular electronics. Graphical abstract The graphene-pentalene-graphene molecular junction.

Perfect spin filtering effect and negative differential behavior in phosphorus-doped zigzag graphene nanoribbons.

On the basis of the density functional theory combined with the Keldysh nonequilibrium Green’s function method, we investigate the spin-dependent transport properties of single-edge phosphorus-doped ZGNR systems with different widths. The results show a perfect spin filtering effect reaching 100% at a wide bias range in both parallel (P) and antiparallel (AP) spin configurations for all systems, especially for 6-ZGNR-P system. Instructively, for the AP spin configuration, the spin down current of the 4-ZGNR-P system exhibits a negative differential effect. By analyzing the transmission spectrum and the spin-resolved band structures of the electrodes, we elucidate the mechanism for these peculiar properties. Our findings provide a new way to produce multifunctional spintronic devices based on phosphorus-doped zigzag graphene nanoribbons.

Rectifying performance and negative differential behavior in graphite—chain—carbon nanotube junctions

In this paper, the (5, 5) capped carbon nanotubes (CNTs) in contact with different lengths of sp monoatomic chains grown on the surface of graphite substrate are fabricated and its electronic transport properties sandwiched between CNT and graphite electrodes are investigated. The first-principles calculations based on nonequilibrium Green's function in combination with density-functional theory show that their rectifying performance and negative differential resistance behavior are observed under very low biases and obviously are enhanced when the length increases. From our analysis, the charge transfer, transmission spectra, projected density of states and evolutions of molecular orbitals are responsible for these phenomena.

Field-enhanced trapping in deep levels by multiple phonon emission in semi-insulating GaAs

We have carried out the time, temperature, and illumination dependencies of the current density in a semi-insulating GaAs sample grown at 300 °C under strong electric field. Standard ohmic behavior was observed at room temperature. A negative differential behavior as a function of the applied electric field was observed by lowering the temperature and increasing the photon flux, and this phenomenon was associated to the field-enhanced trapping effect. We have fit our data with a model for enhanced capture by a multiple-phonon emission capture process assisted by the applied electrical field.

Magnetoresistance and temperature effects in dotlike lateral surface superlattices.

The magnetotransport in dotlike lateral surface superlattices is investigated as a function of Fermi energy and temperature. At low magnetic fields, the magnetoresistance shows a well-defined negative differential behavior, which is followed by one or two large peaks. In this regime, Hall measurements indicate that the number of electrons that contribute to the current depends strongly on the applied magnetic field. Moreover, a clear negative temperature coefficient of the magnetoresistance is observed. Both effects are explained in terms of a semiclassical picture

The physics of space charge instabilities and temporal chaos in extrinsic photoconductors

This paper reviews experimental and theoretical work carried out on space charge instabilities and temporal chaotic behavior in cooled extrinsic p-type Germanium photoconductors. Measured dc current-voltage (I–V) characteristics of these devices are strongly nonlinear for moderate electric fields ≧0.1 V/cm due to field dependence of the rates of free hole capture and impurity impact ionization. Below the threshold field for impurity breakdown, Ge samples behave like damped nonlinear oscillators, exhibiting characteristic chaotic response when driven by a time-periodic voltage. Above impurity breakdown, we observe voltage-controlled negative differential resistance (NDR) in the I–V curves accompanied by spontaneous current oscillations due to moving space charge domains with velocities 103 to 104 cm/s. Measurements are well explained by a simple rate equation model in which negative differential behavior in the impact ionization rate plays a crucial role. Related work on semiconductor chaos and possible future directions for research are also mentioned.