Our work deals with dynamic response of semitransparent organic photodetectors over a large frequency range for high and low light illumination intensities through performing a detailed transient response of the device at pulsed photoexcitation of green emission. We present systematic simulation based on drift diffusion approach to analyzing the effect of unbalance charge carrier mobility and non-uniform absorption of thin film organic photodetector on dynamic response of the device. Our result suggests engineering of the interface layers is critical to achieve high performance device for high-speed applications.
High temperature operation of terahertz (THz) sources based on quantum cascade lasers (QCLs) is discussed. THz QCLs are compact, powerful sources but can only operate at cryogenic temperatures. State-of-the art THz QCLs are made with GaAs/AlGaAs heterostructures and use a single composition of AlGaAs for the barrier material. It was recently shown that multi-composition barriers in the band structure can result in gain > loss at temperature as high as ~240K. We demonstrate early experimental results that yield QCLs that operate up to 184K – similar to QCLs based on single composition barrier designs. An alternative method of producing room-temperature THz is based on intra-cavity difference-frequency generation (DFG) in mid-infrared (mid-IR) QCLs. Here we report devices with record conversion efficiency. THz DFG QCLs reported previously are highly inefficient since THz radiation produced more than ~100 μm away from the exit facet is fully absorbed due to high THz losses in the QCL waveguide. Our lasers use a non-collinear Cherenkov DFG scheme to extract THz radiation from the active region. Dual-color mid-IR quantum cascade lasers with integrated giant optical nonlinearity are grown on semi-insulating (S.I.) InP substrates. THz radiation is emitted at an angle into the substrate with respect to the mid-infrared pumps. Since S.I. InP is virtually lossless to THz radiation, this scheme allows for efficient extraction of THz radiation along the whole waveguide length. As a result, our sources demonstrate large mid-infrared-to-THz conversion efficiency. Devices tested at room-temperature produced 18μW peakpower and 75μW/W2 conversion efficiency.
We have performed studies on the correlation between mechanical deformations and electrical conductance on a new interesting hybrid material, a Single Wall Carbon Nanotubes (SWCNTs)/Poly(3,4-ethylenedioxythiophene) (PEDOT) composite. Two are the synthesis techniques utilized to prepare the composite material in form of few hundreds of nm thick films: a spin coating deposition starting from an aqueous dispersion of SWCNTs and PEDOT, and an electrochemical de*position starting from a dispersion of SWCNTs and EDOT monomer. The composite conductance changes induced by a modulated periodic elongation via a coherent technique have been monitored by measuring the voltage variations of a Wheatstone bridge connected with the films. The measurements were performed on SWCNTs/PEDOT composites layered on a rigid substrate. The piezoresistivity gauge factor (GF) of the various samples was evaluated by comparing their responses to mechanical deformations to those of a commercial strain gauge, sticked on a substrate of the same kind. We found no significant piezoresistive effect in the hybrid material films deposited by means of spin coating while the effect is remarkable for the composites prepared by means of the electrochemical technique. In this case the gauge factor is found to be up to 3-4 times higher than that of the commercial strain gauge.
In the present research, purified commercially SWCNTs are used as gas sensing material in an interdigitated electrode platform for NH3, NOx and H2O detection. The SWCNT response to gas absorption is known to be dependent from different parameters and operational conditions, such as the relative orientation of the nanotubes and their organization between the electrodes, the temperature of the sensor, and moreover the voltage applied to a back gate contact. We show the sensor response for the various gas species considered and we analyze the sensor behavior with respect to the sensibility and to the detection velocity. Moreover we studied the effect on absorption/desorption gas processes by applying a gate voltage to the Si substrate beneath the interdigitated electrodes. The results indicate that the acceleration of the time response of the sensor for the detection of NH3 is proportional to the gate voltage in the range 0 V - 40 V.
We reported the design and realization of a carbon nanotube-based integrated multielectrode device. Patterned Si/SiO2/Nb/Nb2O5 multilayer was successfully realized by means of a few, common photolithographic processes with the minimum number of mask alignment steps. Such structure constitutes the patterned substrate of successive Hot Filament Chemical Vapour Deposition (HFCVD) process. Selective growth of highly oriented SWCNT arrays was obtained in the predefined locations while survival of the entire structure was achieved. Field emission measurements of such materials were carried out. Good and reproducible field emission behaviour has been observed in several realized structures.
Our paper is an overview of different methods, which were recently developed or adopted for the simulation of organic electronic devices. In the first part of this work we will briefly review state of the art approaches for simulating current flow through single molecules, while in the second and longer part we will focus on the design of architectures for molecular-scale computing. We will put special emphasis on field-coupling, which is a promising unconventional way for integrating a large number of molecules into a computing device.
A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.
We have investigated the electrical properties of organic thin-film transistor by using two-dimensional drift-diffusion simulations. The dependence of electrical haracteristics on the mobility model and on the barrier height of the contacts is carried out. We found that the field dependence of the carrier mobility is responsible for non-linearity of the drain current. This non-linear behavior is mainly related to the field-dependence of the mobility and to the barrier height of the contacts. The simulation allow us to clear understand the differences in the mobility derived by the analysis of I-V curve (as done experimentally by using standard MOSFET theory) and the intrinsic mobility of the organic layer. The effects of the interface traps has also be considered. The dependence of the threshold voltage on the density, energy level and model of the traps has been outlined. Results of the simulations have been compared with experimental data. The comparison between experimental data and
simulation allow us to clearly identify the physical mechanism responsible for the measured characteristics. Finally we also consider the effect of the device bending on the electrical characteristic of all-plastic OTFT.
Density Functional theory calculations combined with non-equilibrium Green's function technique have been used to compute electronic transport in organic molecules. In our approach the system Hamiltonian is obtained by means of a self-consistent density-functional tight-binding (DFTB) method. This approach allows a first-
principle treatment of systems comprising a large number of atoms. The implementation of the non-equilibrium Green's function technique on the DFTB code allows us to perform computations of the electronic transport properties of organic and inorganic molecular-scale devices. The non-equilibrium Green's functions are used to compute the electronic density self-consistently with the the open-boundary conditions naturally encountered in transport problems and the boundary conditions imposed by the potentials at the contacts. The Hartree potential of the density-functional Hamiltonian is obtained by solving the three-dimensional Poisson's equation involving the non-equilibrium charge density.
In the present work we investigate the influence of molecular vibrations on the tunneling of electrons through a molecule sandwiched between two metal contacts. The study is confined to the elastic scattering only, but beyond the harmonic approximation. The problem is tackled both from a classical and a quantum-mechanical point of view. The classical approach consists in the computation of the time-dependent current uctuations calculated at each step of a molecular dynamics (MD) simulation. On the other hand, the vibrational modes are treated quantum-mechanically and the tunneling current is computed as an ensemble average over the distribution of
the atomic configurations obtained by a suitable approximation of the density matrix for the normal mode oscillators. We show that the lattice fluctuations modify the electron transmission. At low temperatures the quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. However, for temperatures higher than few hundreds Kelvin the simple harmonic approximation which leads to the phonon modes breaks because the oscillation amplitudes of the lowest energy modes become large.
Single Wall Carbon Nanotubes (SWCNTs) based nanotechnology appears to be promising for future nanoelectronics. The SWCNT may be either metallic or semiconducting and both metallic and semiconducting types of SWCNTs have been observed experimentally. This gives rise to intriguing possibilities to put together semiconductor-semiconductor and semiconductor-metal junctions for diodes and transistors. The potential for nanotubes in nanoelectronics devices, displays and nanosensors is enormous. However, in order to realize the potential of SWCNTs, it is critical to understand the properties of charge transport and to control phase purity, elicity and arrangement according to specific architectures. We have investigated the electrical properties of various SWCNTs samples whit different organization: bundles of SWCNTs, SWCNT fibres and different membranes and tablets obtained using SWCNTs purified and characterized.
Electrical characterizations were carried out by a 4155B Agilent Semiconductor Parameter Analyser. In order to give a mechanical stability to SWCNTs fibres and bundles we have used a nafion matrix coating, so an electrical characterization has been performed on samples with and without this layer. I-V measurements were performed in vacuum and in air using aluminium interdigitated coplanar-electrodes (width=20mm or 40mm) on glass substrates. The behaviour observed is generally supralinear with currents of the order of mA in vacuum and lower values in air with the exception of the tablet samples where the behaviour is ohmic, the currents are higher and similar values of current are detected in air and vacuum.
We have performed studies on the correlation between mechanical deformation and conductivity on a set of carbon samples constituted by 70% of single-walled carbon nanotubes. The samples, in form of slabs (6 × 5 mm, thickness: 400 mm), were obtained by compacting the nanotube material at 200 and 600 bar. The changes of conductivity have been monitored by measuring the current variations induced by a modulated periodic elongation of the slabs via a coherent technique. The mechanical deformations were produced by forces applied vertically at the center of each slab, horizontally placed on a sample holder. A piezoelectric actuator controlled by a lock-in amplifier was fixed to the sample holder. The modulation of the current induced by the mechanical deformation of the nanotube slabs is huge, and the amplitude of the modulation is almost linearly proportional to the elongation induced by the piezoelectric actuator. Such change of conductivity is more than an order of magnitude higher than the change obtained by piezoelectrical deformation of Si. The behaviour of the nanotube samples has been compared to that of a reference sample made of graphite compressed at 200 bar to form a slab with similar dimensions. In this case the change of conductivity was below the sensitivity of the lock-in amplifier, which was unable to lock to the periodicity of the mechanical deformation. We are currently addressing the problem to interpret the strong response of the nanotube slabs, which could be attributed either to a piezoresistive response of the sample or to the direct effect of the deformation on the hopping transport processes.
We studied intersubband relaxation of carries during ultrafast photoexcitation in single and coupled quantum wells using ensemble Monte Carlo simulation, Intra- and intersubband scattering due to polar and nonpolar optical phonons, acoustic phonons, and intercarrier scattering are included in the simulation. The polar optical mode description is given in terms of a two-pole dielectric continuum model for the alloy barriers. In the present work we focus on relaxation when the 2-1 subband spacing is smaller than the optical phonon energy so that suppression of the intersubband polar optical phonon scattering rate occurs. Our results for a single well show that intercarrier intersubband scattering dominates over acoustic phonon scattering during the initial relaxation of carriers from 2-1, with a strong contribution due to polar optical phonon emission from the tails of the heated distributions as well. We have studied optical pumping for a 3 level coupled quantum well system in which (Delta) E12 is less than h(omega) 0, and calculate the change in occupancy of the excited subbands through pumping of the 1-3 transition.
A numerical model of bulk electroabsorption modulators has been developed. It consists of a quasi-2D representation based on a drift-diffusion approach and includes the presence of heterostructures and Fermi statistics for the carriers. The non-linear behavior of this type of device, essentially related to the pile-up and the space-charge effects, has been analyzed. Simulations results have been compared with laboratory measurements on a fabricated device that presents an abrupt heterojunction, obtaining a good agreement. Two other types of structures have been simulated, one obtained with the inclusion of a thin quaternary layer and the other with a graded heterojunction, which eliminate the hole pile-up at the InGaAsP-InP heterointerface. The paper demonstrates that it is possible to approach the optimum behavior of the modulator using both the alternatives considered here. Finally, non linear effects in short modulators has been investigated.
We present a theoretical study of hot-carrier induced light emission in III-V semiconductor devices. Carrier heating under the intense electric fields present under high bias conditions are studied via a selfconsistent Monte Carlo simulation. The carrier distribution functions obtained from the simulation are then incorporated into a pseudo-potential algorithm that describes the direct optical transitions and calculates the corresponding spectra. We show that the light emission due to hot carriers is dominated by direct radiative interband transitions within the conduction and valence bands. Good agreement between theory and experiment is obtained for GaAs MESFET and GaAs/AlGaAs HBTs.
We present a detailed study of phonons in rectangular quantum wires within the dielectric continuum model and within a fully microscopic approach. From the latter, we calculate the phonon dispersions and the potentials associated to the individual modes. The classification of such modes is much more complex than in the corresponding two dimensional case, owing to the intrinsic coupling of confined and interface modes associated to both directions perpendicular to the wire. This indicates a failure of the current implementations of the macroscopic dielectric continuum model.
We present a combined experimental and theoretical study of the ultrafast internal thermalization of high energy carriers created by laser excitation. Luminescence up-conversion is used to monitor the spectral and temporal evolution of the photoexcited carrier distributions with a time resolution of about 100 fs. A Monte Carlo simulation joined with a molecular dynamics approach is then used to interpret the experimental results. We show that the coulomb interaction among carriers is responsible for the initial ultrafast thermalization. The simulation allow us to distinguish between binary carrier-carrier collisions and plasmon losses and reconcile the results obtained with time resolved vs. c.w. hot (e, angstroms) luminescence.
KEYWORDS: Scattering, Monte Carlo methods, Phonons, Computer simulations, Laser scattering, Gallium arsenide, Solids, Quantum wells, Dielectrics, Electron transport
We present a Monte Carlo analysis of a `true' GaAs-based quantum wire, whose dimensions correspond to present state-of-the-art technology. Intrasubband and intersubband scattering rates for the electron-polar optical phonon interaction are included in the simulation as well as electron-electron interaction. We have studied the nonequilibrium transport characteristics of the one-dimensional system in two different situations: the response of the electron gas to an external electric field applied along the wire direction, and the cooling dynamics following laser photoexcitation. With respect to 3-D and 2-D systems, we can show that the electron- phonon interaction is not substantially modified, while a strong reduction in the electron- electron scattering rate of the wire is found.
We use an ensemble Monte Carlo simulation of coupled electrons, holes and polar
optical phonons in multiple quantum well systems to model the intersubband
relaxation of hot carriers measured in ultra-fast optical experiments. Our
simulated results are in good agreement with experimental results in modulation
doped quantum wells and coupled double well structures where we find that the
intersubband relaxation time is controlled by the spatial overlap of the subband
envel ope wavefuncti ons.
We discuss the role of LO-phonons confinement in quantum well systems, by comparing two different
phonon models that have been proposed in the literature. A critical discussion concerning
the use of macroscopic approaches for the description of phonons in two dimensional systems is
presented. We use a Monte Carlo simulation which includes nonequilibrium phonon effects as well
as carrier-carrier scattering to determine the effect of phonon confinent on the relaxation of photoexcited
carriers in A1GaAs-GaAs quantum wells. Good agreement with available experimental
data is found. Even at low excitation densities, intercarrier scattering and phonon reabsorption
are important, and need to be taken into account in the interpretation of experimental data.
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