Applied Bias Increases Research Articles

Modulation of Negative Differential Resistance in Black Phosphorus Transistors.

Negative differential resistance (NDR), which describes the current decrease as the applied bias increases, holds great potential for varieties of electronic applications including radio-frequency oscillators, multipliers, and multivalue logics. Here, the modulation of a unique NDR effect in ambipolar black phosphorus (BP) transistors is reported, which is activated by specific electrical field dependence of lateral carrier distribution and is distinct from conventional NDR devices that rely on quantum tunneling. The NDR device exhibits a high peak current density (34 µA µm-1 ) and a high operating temperature. More importantly, due to the strong coupling between the channel and the gate electrode, both the NDR peak current and peak/valley voltages can be effectively modulated by the electrostatic gate. Furthermore, it is demonstrated that light can serve as an additional terminal for NDR modulation. The findings could provide an important insight into the transport behavior of BP transistors and contribute to the design of ambipolar-semiconductor-based electrical circuits.

Energy levels of bilayer graphene quantum dots

Within a tight binding approach we investigate the energy levels of hexagonal and triangular bilayer graphene (BLG) quantum dots (QDs) with zigzag and armchair edges. We study AA- and AB- (Bernal) stacked BLG QDs and obtain the energy levels in both the absence and the presence of a perpendicular electric field (i.e., biased BLG QDs). Our results show that the size dependence of the energy levels is different from that of monolayer graphene QDs. The energy spectrum of AB-stacked BLG QDs with zigzag edges exhibits edge states which spread out into the opened energy gap in the presence of a perpendicular electric field. We found that the behavior of these edges states is different for the hexagonal and triangular geometries. In the case of AA-stacked BLG QDs, the electron and hole energy levels cross each other in both cases of armchair and zigzag edges as the dot size or the applied bias increases.

Magnetotransport in an impurity-doped few-layer graphene spin valve

Using Keldysh nonequilibrium Green's-function method we study the spin-dependent transport through impurities-doped few-layer graphene sandwiched between two magnetic leads with an arbitrary mutual orientations of the magnetizations. We find for parallel electrodes magnetizations that the differential conductance possesses two resonant peaks as the applied bias increases. These peaks are traced back to a buildup of a magnetic moment on the impurity due to the electrodes spin polarization. For a large mutual angle of the electrodes magnetization directions, the two resonant peaks approach each others and merge into a single peak for antiparallel orientation of the electrodes magnetizations. We point out that the tunneling magnetoresistance (TMR) may change sign for relatively small changes in the values of the polarization parameters. Furthermore, we inspect the behavior of the differential conductance and TMR upon varying the temperature.

Effect of length of anodized TiO 2 tubes on photoreactivity: Photocurrent, Cr(VI) reduction and H 2 evolution

Anodized tubular TiO 2 electrodes (ATTEs) are prepared using an organic additive consisting of either (i) ethylene glycol (EG) or (ii) glycerol (Gly) to make various photoanodes with different length of TiO 2 tubes and thereby to investigate the effect of their length on the photo-driven activity for hydrogen evolution and Cr(VI) reduction, as well as on the photocurrent. The ATTEs with EG have longer TiO 2 tubes (3.42–15.6 μm) than those with Gly (0.26–1.95, 6.82 μm). The former samples exhibit higher photocurrent densities (22.8–32.8 mA cm −2) than the latter (8.0–19.4, 20.3 mA cm −2). The latter samples (tube length of less than 7 μm) clearly exhibit a change of the rate-determining step from electron migration to photohole capture as the scanned applied bias increases, since the photocurrent shows a plateau for tube lengths above 2 μm. Meanwhile, the samples with EG remain in the electron migration step up to a tube length of 16 μm and is due to the difference of the morphology, crystal phase and crystallinity. This favourable characteristic is also applied to and well matched with the results from the reactions of Cr(VI) reduction and hydrogen evolution (up to ca. 250 μmol h −1).

Polymer photodetector with voltage-adjustable photocurrent spectrum

Polymer photodetectors with voltage-adjustable photoresponse from visible to near infrared range are demonstrated. Poly(3-hexylthiophene) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) blend is used as the active layer. The photoresponse can be continuously adjusted by the thickness of the active layer as well as the applied voltage bias. The thickness of the active layer is varied from 250 nm to 16.2 μm. The mechanism for the photoresponse adjusted by the thickness can be attributed to the absorption of the photons in the infrared range by thick PCBM layer. The mechanism for the photoresponse adjusted by the applied bias can be attributed to the carrier recombination reduction when the applied bias increases. The adjustable photodetector also has high operating speed up to 10 kHz.

Photocarrier behavior in organic heterojunction photovoltaic cells

We have investigated the bias dependence of photocurrent in several organic heterojunction cells to elucidate the behavior of photogenerated charge carriers. Both the planar and planar-mixed heterojunction devices are shown to always have negative photocurrent even at large forward biases; this phenomena has been attributed to an increased driving force for carrier diffusion away from the heterointerface as the applied bias increases. In contrast, the drift current generally dominates in mixed heterojunction devices due to distributed nature of charge generation throughout the active layer, leading to a photocurrent that is highly dependent on the internal electric field. This dependence gives rise to the reversal of the photocurrent direction at high biases when compared to that at the short-circuit condition. However, the voltage yielding zero photocurrent shows appreciable wavelength dependence, which is strongly correlated to the detailed charge carrier generation profile within the active layer.

Effect of donor-complex-defect-induced dipole field on InAs∕GaAs quantum dot infrared photodetector activation energy

In order to understand dopant incorporation in quantum dot infrared photodetectors, three quantum dot (QD) Schottky diodes (undoped, delta doped, and modulation doped) have been investigated. Donor-complex-defect (DX) centers have been observed by photocapacitance quenching in the doped diodes only. When the applied bias increases, the doped samples show a rapid increase in dark current and a resulting dramatic decrease in QD activation energy. The activation energy reduction could be related to a dipole field between positively charged DX centers and electrons in QDs. A transport mechanism is proposed to explain the observed activation energy bias dependence in the doped samples.

Wave-vector filtering and spin filtering in a magnetic double-barrier and double-well structure

We investigate wave-vector filtering and spin filtering for electrons tunneling through a magnetic double-barrier and double-well quantum structure in the presence of an external electric field, where the double magnetic wells are within or outside the double magnetic barriers. It is shown that electronic resonant tunneling shows strong wave-vector-dependent enhancement and suppression as the applied bias increases. It is also shown that although the current density decreases with the strength of the inhomogeneous magnetic field increasing, the degree of spin polarization is generally enhanced and shows a more wider and nearly 100% spin polarization plateau in small external bias range. The general rules for relations between wave-vector filtering and the magnetic configuration as well as between spin filtering and the magnetic configuration are summarized and explained.

Effects of Coulomb blockade on the photocurrent in quantum dot infrared photodetectors

We present theoretical studies of the effects of Coulomb blockade on the photocurrent of quantum dot infrared photodetectors within the Anderson model with two localized levels coupled with the electromagnetic field. We use the Keldysh Green function method to calculate the photocurrent. The energy levels, on-site Coulomb energy, and coupling parameters between leads and quantum dot states, as functions of the applied field, are evaluated within an effective mass model. It is found that the Coulomb interaction and level mixing in the many-body open system lead to a double-peak spectrum for the intraband transition. The center of gravity of the spectrum is redshifted as the applied bias increases, which competes with the blueshift caused by the Stark effect. Furthermore, the photocurrent is found to be a nonlinear function of the steady-state electron density of the quantum dot, in sharp contrast to quantum well infrared photo-detectors.

Midinfrared photoconductivity of Ge/Si self-assembled quantum dots

We have investigated the midinfrared photoconductivity of Ge/Si self-assembled quantum dots. The self-assembled quantum dots were grown by ultra-high-vacuum chemical vapor deposition on Si(001). The photoresponse of the p-type device exhibits resonances in the midinfrared around 10 μm wavelength. The resonance of the photocurrent shifts to lower energy as the applied bias increases. The photocurrent is weakly dependent on the incoming polarization of the infrared light. The photocurrent is analyzed in terms of bound-to-bound and bound-to-continuum transitions in the valence band. The photocurrent peaks are correlated to the photoluminescence of the device.