In this work we use the multi-scale software tool TiberCAD to study the transport and optical properties of InGaN
quantum disk (QD) - based GaN nanocolumn p-i-n diode structures. IV characteristics have been calculated for
several values of In concentration in the QD and of nanocolumn width. Strain maps show a clear relaxation effect
close to the column boundaries, which tends to vanish for the larger columns. Effects of strain and polarization
fields on the electron and hole states in the QD are shown, together with the dependence of optical emission
spectra on geometrical and material parameters.
In the first part of the present contribution, we will report on transport calculations of nanoscaled devices based on Carbon Nanotubes obtained via self-consistent density-functional method coupled with non-equilibrium Green's function approaches. In particular, density functional tight-binding techniques are very promising due to their intrinsic efficiency. This scheme allows treatment of systems comprising a large number of atoms and enables the computation of the current flowing between two or more contacts in a fully self-consistent manner with the open boundary conditions that naturally arise in transport problems. We will give a description of this methodology and application to field effect transistor based on Carbon nanotubes.
The advances in manufacturing technology are allowing new opportunities even for vacuum electron devices producing radio-frequency radiation. Modern micro and nano-technologies can overcome the typical severe limitations of vacuum tube devices. As an example, Carbon Nanotubes used as cold emitters in micron-scaled triodes allow for frequency generation up to THz region. The purpose of the second part of this contribution will be a description of the modelling of Carbon Nanotube based vacuum devices such as triodes. We will present the calculation of important figures of merit and possible realizations.
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.
The recently published data on phototransit signals in smectic and discotic liquid crystals, have led us to reconsider the old problem of the weak temperature dependence of the mobility in ordered narrow band systems and inthe liquid crystalline phases. We argue that one has to distinguish between currents which are due to light-generated carriers and currents due to band conduction in equilibrium and which can be described using the Kubo-Greenwood formula. We use a first principle band model in an electric field and show how a T-independent mobility can be derived for a single particle which obeys band transport, includes joule energy relaxation, elastic disorder, and agrees with the few carrrier limit of the Kubo formula. The result is essentially a generalized Drude velocity applicable to describe "single particle currents". We also discuss alternative explanations for the observed temperature independent mobilities which are based on hopping with weak disorder and polaron theories.