The role of metal contacts in the stability of n-type organic field effect transistors

We are presenting a long-time bias stress stability of C₆₀-based n-type organic field effect transistors (OFETs), in bottom gate, top contacts configuration, with aluminium (Al), silver (Ag) and gold (Au) source–drain contacts. The results clearly shows that the bias stress effects in C₆₀-based n-type OFETs is similar to p-type OFETs and it can be reduced by using an appropriate metal for the source–drain contacts. During the bias stress time, the threshold voltage shift and an increase in the contacts resistance have also been measured. On the basis of the stability of the device parameters, it is proposed that the Al source–drain contact-based devices gives better stability as compared to the devices with Ag and Au source–drain contacts. Our results show that the bias stress-induced threshold voltage shift is due to the trapping of charges in the channel region and in the vicinity of the source–drain contacts.     

Anomalous negative-differential-resistance (NDR) characteristics of n+-GaAs/n−-GaAs/n-In0.2Ga0.8As/i-GaAs structure

An n⁺-GaAs/n⁻-GaAs/n-In_0.2Ga_0.8As/i-GaAs field-effect transistor (FET) structure has been fabricated and studied. An anomalous three-terminal-controlled negative-differential-resistance (NDR) phenomenon resulting from the real-space transfer effect is observed. This N-shaped NDR behavior is found in the higher drain-to-source voltage (V_DS) regime. Furthermore, the NDR is obtained at positive and negative gate-to-source bias (V_GS). The influence ofV_GSbias on the NDR characteristics is investigated.     

Fluorescence of the nematic liquid crystal 5CB in nanoporous glasses

The fluorescence spectra of the nematic liquid crystal n-pentyl-n′-cyanobiphenyl (5CB) in porous glasses with pores from 1 to 44 nm in diameter are investigated. A decrease in the pore diameter leads to suppression of some long-wavelength spectral components corresponding to H-type predimer and dimer pairs (the molecular sieve effect). The spectrum of 5CB in small pores (smaller than 4 nm in diameter) can be explained by the superposition of the monomer fluorescence and the fluorescence of J-type dimer pairs of 5CB molecules, as well as associates of 5CB molecules and surface groups on pore walls. Exposure of samples to UV light enhances the molecular interaction in associates, possibly, due to the formation of strong chemical bonds.     

Spin-polarized transport properties of a pyridinium-based molecular spintronics device

By applying a first-principles approach based on non-equilibrium Green's functions combined with density functional theory, the transport properties of a pyridinium-based “radical-π-radical” molecular spintronics device are investigated. The obvious negative differential resistance (NDR) and spin current polarization (SCP) effect, and abnormal magnetoresistance (MR) are obtained. Orbital reconstruction is responsible for novel transport properties such as that the MR increases with bias and then decreases and that the NDR being present for both parallel and antiparallel magnetization configurations, which may have future applications in the field of molecular spintronics.     

First-principle study of the electronic transport properties of a new dumbbell-like carbon nanocomposite

Using a first-principle density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of a new dumbbell-like carbon nanocomposite, in which one carbon nanotube segment is capped with two C₆₀ fullerenes. Our results show that the current–voltage curve reveals a highly nonlinear feature. A negative differential resistance (NDR) behavior is obtained at a very low bias, which is expected to be helpful for the development of low bias NDR-based molecular devices. Moreover, the carbon nanotube length and fullerene type can affect the NDR behavior strongly. The electronic transport is analyzed from the transmission spectra and the molecular projected self-consistent Hamiltonian states under different applied biases.     

Negative differential resistance and rectification effects in zigzag graphene nanoribbon heterojunctions: Induced by edge oxidation and symmetry concept

By applying non-equilibrium Green's functions (NEGF) in combination with tight-binding (TB) model, we investigate and compare the electronic transport properties of H-terminated zigzag graphene nanoribbon (H/ZGNR) and O-terminated ZGNR/H-terminated ZGNR (O/ZGNR–H/ZGNR) heterostructure under finite bias. Moreover, the effect of width and symmetry on the electronic transport properties of both models is also considered. The results reveal that asymmetric H/ZGNRs have linear I–V characteristics in whole bias range, but symmetric H-ZGNRs show negative differential resistance (NDR) behavior which is inversely proportional to the width of the H/ZGNR. It is also shown that the I–V characteristic of O/ZGNR–H/ZGNR heterostructure shows a rectification effect, whether the geometrical structure is symmetric or asymmetric. The fewer the number of zigzag chains, the bigger the rectification ratio. It should be mentioned that, the rectification ratios of symmetric heterostructures are much bigger than asymmetric one. Transmission spectrum, density of states (DOS), molecular projected self-consistent Hamiltonian (MPSH) and molecular eigenstates are analyzed subsequently to understand the electronic transport properties of these ZGNR devices. Our findings could be used in developing nanoscale rectifiers and NDR devices.     

Negative differential resistance in thin-film electroluminescent emitters based on zinc sulfide

S-and N-type negative differential resistance (NDR) has been observed in thin-film electroluminescent emitters based on zinc sulfide doped with manganese, and conditions for its emergence have been identified. It has been found that when a negative half-wave of voltage is applied to the nontransparent top electrode, an S-type NDR with a region of decreasing current is observed, and when it is applied to the transparent bottom electrode, the NDR will be N-type. The emergence of NDR is due to space charges which form in the near-cathode and near-anode layers of the phosphor.     

Band-to-band Auger processes at high carrier concentration

The strong decrease of lifetime in semiconductors under high doping conditions is discussed. The Auger effect should be responsible for it in both p-type and n-type material. Formulas for the begin of decrease are discussed and the appropriate carrier concentrations are calculated for several III–V compounds.     

Theoretical study on transport properties of a BN co-doped SiC nanotube

We investigate the electronic transport properties of silicon carbide nanotubes (SiCNT) in presence of both boron (B) and nitrogen (N) impurities. The results show that co-doping BN impurities suppresses the important negative differential resistance (NDR) property. NDR suppression is attributed to the introduction of new electronic states near the Fermi level followed by weak orbital localization. BN co-doping results in exponential current-voltage (I-V) characteristics which is in contrast to linear I-V characteristics for individual boron and nitrogen doped SiCNTs. HOMO has no contribution from B impurity, whereas, LUMO has contribution from N impurity at low and high bias.     

Quaternary n-Al0.08In0.08Ga0.84N/p-Si-based solar cell

Quaternary n-type Al_0.08In_0.08Ga_0.84N grown on p-Si using molecular beam epitaxy technique was fabricated as a pn-junction and an anti-reflection coating (ARC) of solar cells. The structural properties and surface morphology of the solar cells were investigated using scanning electron and atomic force microscopy. Optical reflectance was obtained using an optical reflectometery system (Filmetric F20-VIS). Current–voltage characteristics were examined under 100 mW cm^?2 illumination conditions. Quaternary n-type Al_0.08In_0.08Ga_0.84N coating was found to be an excellent ARC against incident light compared with other ARCs. This material also exhibited good light trapping over a wide wavelength spectrum, which produced highly efficient solar cells. The unique and strong polarization, as well as the piezoelectric effect, of the quaternary-nitrides was employed to reduce surface recombination velocities and enhance the solar cell performance. A solar cell with reasonable conversion efficiency of 9.74% was obtained when the n-Al_0.08In_0.08Ga_0.84N/p-Si was employed.     

Electrical transport properties of hydrogen-exposed p-type PbTe films

Hall coefficient and d.c. conductivity measurements were made on p-type PbTe epitaxial films exposed to molecular hydrogen gas at high pressures (100–500 psi) in the temperature range (100–300K). It is found that hydrogen converts p-type films to n-type at a pressure of about 300 psi. The results are explained by assuming that the action of hydrogen is to provide donor electrons.     

Exchange bias and training effect in Ni/Ag-doped NiO bilayers

A series of polycrystalline Ag-doped Ni_1−xAg_xO/Ni bilayers with x up to 0.2 were prepared by magnetron sputtering. X-ray diffraction, atomic force microscopy and transmission electron microscopy analyses reveal that Ag doping significantly reduces the mean NiO grain size and leads to the appearance of Ag nanoparticles on the surface of the Ag-doped NiO films. As x increases, the exchange bias field and coercivity at room temperature decrease as a consequence of the reduced thermal stability of smaller NiO grains and the screening effect resulting from the interfacial Ag nanoparticles. At lower temperatures, a slight enhancement of the exchange bias field is observed in the Ag-doped sample, indicating that the Ag doping increases the uncompensated NiO spin density. In addition, our studies find that the training effect of the Ag-doped sample can be well described by a spin configurational relaxation model, regardless of the presence of Ag nanopartiles at the interface.     

Investigation of the electrical and optical properties of InAs/InGaAs dot in a well solar cell

The electroreflectance (ER) and current–voltage (J–V) of InAs/InGaAs dots in a well (DWELL) solar cell (SC) were measured to examine the optical and electrical properties. To investigate the carrier capturing and escaping effects in the quantum dot (QD) states the above and below optical biases of the GaAs band gap were used. In the reverse bias region of the J–V curve, the tunneling effect in the QD states was observed at low temperature. The ideality factors (n) were calculated from the J–V curves taken from various optical bias intensities (I_ex). The changes in the ideality factor (n) and short circuit current (J_SC) were attributed mainly to carrier capture at low temperature, whereas the carrier escaping effect was dominant at room temperature. ER measurements revealed a decrease in the junction electric field (F_J) due to the photovoltaic effect, which was independent of the optical bias source at the same temperature. At low temperature, the reduction of photovoltaic effect could be explained by the enhancement carrier capturing effect due to the strong carrier confinement in QDs.     

Formation of DX-centers in indium doped CdTe

In CdTe, the achievable n-type doping is limited by the formation of DX-centers. A characteristic feature of DX-centers is the ‘persistent photoconductivity (PPC)’ which is created by illumination at low temperatures and caused by a metastable state of the DX-center. The DX-center and the PPC effect in n-type CdTe are theoretically explained by the ‘large lattice relaxation model’. PAC measurements on In doped CdTe using ¹¹¹In/¹¹¹Cd and, in addition, resistivity measurements on the same samples have been performed. Below 150 K, the samples showed a PPC effect that was accompanied by an increase of about 20% of the carrier concentration. This effect is not accompanied by any changes of the observed EFG. Possible explanations of the EFG observed, originally assigned to the DX-center, will be discussed. Finally, first reports on the investigation of DX-centers in CdTe using the radioactive isotope ¹¹⁷Cd decaying to ¹¹⁷In are presented.     

Gd-doping of HfO2

An increase in the density of states between the oxygen 2p bands and the Fermi level is seen with increasing Gd concentrations. In addition, for the Gd-doped HfO₂ films, the Gd 4f photoexcitation peak at 5.5 eV below the valence band maximum was identified using resonant photoemission. Electrical measurements show pronounced rectification properties for lightly-doped Gd:HfO₂ films on p-Si and for heavily-doped Gd:HfO₂ films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level. In addition, there is an increase in the reverse bias current with neutron irradiation.     

Evidences of p-type conductivity in iodine grown gallium selenide

Photoconductivity, photovoltage and photoelectromagnetic effect have been studied at different excitation intensities between 80 and 300°K in GaSe grown by the iodine transport method. The experimental results give evidence that the conductivity of these samples, which are n-type at room temperature, changes to p-type at low temperature.     

Molecular rectification of thiol-linked Au|PTCDI-[CH2]n|Au junctions

The electronic transport properties of the PTCDI-[CH₂]_n(0≤n≤6) molecular junctions with different molecular lengths are theoretically investigated via the first-principles density functional theory (DFT) and non-equilibrium Green's function (NEGF) method. Our results show that the transport properties depend on molecular lengths. The equilibrium conductance of the probed systems decreases exponentially with the increasing number n of the CH₂ unit. With n≥1, the rectifying effect has been found. In the n=6 case, a significant rectification ratio of 72.6 is achieved at the bias of ±2.1 V in our probed voltage range. The rectification effect arises from asymmetric molecular structures. Our results suggest these molecules have great potential application in the molecular-scale device.     

Negative differential resistance in a one-dimensional molecular wire with odd number of atoms

We have investigated the effects of electron-phonon coupling on the current-voltage characteristics of a one-dimensional molecular wire with odd number of atoms. The wire has been modelled using the Su-Schreiffer-Heeger (SSH) Hamiltonian and the current-voltage characteristics have been obtained using the Landauer’s formalism. In the presence of strong electron-lattice coupling, we find that there are regions of negative differential resistance (NDR) at some critical bias, due to the degeneracy in the energies of the frontier molecular orbitals. The presence of the applied bias and the electron-lattice coupling results in the delocalization of these low-lying molecular states leading to the NDR behaviour.     

Auger emission in degenerate GaSb crystals

The dependence of the peak energy of the Auger band on the doping in n-type and p-type degenerate GaSb crystals is investigated, in good agreement with experiments.It is suggested that the continuum for n-type GaSb is only by a factor of 20 stronger than for p-type material, and the maximum value of the stimulated Auger emission in the n-type or p-type GaSb photoluminescence is 4 times weaker than the corresponding maximum value of the near-band-edge emission.     

Current switching in bistable structures: Heavily doped n +-polysilicon-tunnel-oxide layer-n-silicon

Switching from a state with a steady-state nonequilibrium depletion and low current into the “on” state with a high current and low voltage drop on the structure is observed in highly doped n ⁺-polysilicon-tunnel-oxide-n-silicon structures. The structures were prepared on n-silicon substrates with a resistivity of 25 Ω·cm. A structure with an oxide thickness of 23 Å can be switched on both by a radiation pulse with small reverse bias on the structure (50 V) and under dark conditions by increasing the reverse bias to 250–300 V. In the “on” state Auger carrier production is the internal source of minority carriers that is required for compensating tunnel leakage of holes into the n ⁺-polysilicon and for maintaining a quasiequilibrium inversion layer of holes at the n-Si-SiO₂ boundary.