Masoud Seifikar

Optical absorption of dilute nitride alloys using self-consistent Green's function method

AbstractWe have calculated the optical absorption for InGaNAs and GaNSb using the band anticrossing (BAC) model and a self-consistent Green’s function (SCGF) method. In the BAC model, we include the interaction of isolated and pair N levels with the host matrix conduction and valence bands. In the SCGF approach, we include a full distribution of N states, with non-parabolic conduction and light-hole bands, and parabolic heavy-hole and spin-split-off bands. The comparison with experiments shows that the first model accounts for many features of the absorption spectrum in InGaNAs; including the full distribution of N states improves this agreement. Our calculated absorption spectra for GaNSb alloys predict the band edges correctly but show more features than are seen experimentally. This suggests the presence of more disorder in GaNSb alloys in comparison with InGaNAs.

Self-consistent Green's function method for dilute nitride conduction band structure.

We present a self-consistent Greens function (SCGF) approach for the Anderson many-impurity model to calculate the band dispersion and density of states near the conduction band edge in GaN(x)As(1-x) dilute nitride alloys. Two different models of the N states have been studied to investigate the band structure of these materials: (1) the two-band model, which assumes all N states have the same energy, EN; (2) a model which includes a full distribution of N states obtained by allowing for direct interaction between N sites. The density of states, projected onto extended and localised states, calculated by the SCGF two-band model, are in excellent agreement with those previously obtained in supercell calculations and reveal a gap in the density of states just above E(N), in contrast with the results of previous non-self-consistent Greens function calculations. However, including the full distribution of N states in a SCGF calculation removes this gap, in agreement with experiment.

Enhanced pressure response in ZnO nanorods due to spontaneous polarization charge

We present measurements of the induced charge flow when a compressive force is applied by contacting to a ZnO nanowire (NW). The measured charge transfer from the NW is over 104 times larger than expected from the strain-induced piezoelectric response, and is comparable in magnitude to the spontaneous polarization charge, associated with an ideal ZnO NW. A model is presented that compares the total energy of an isolated NW and external capacitor with the total energy when the capacitor and NW form a closed circuit. The analysis shows that it is possible, for realistic values of surface defect creation energy, to have spontaneous polarization charge transferred from a NW to an external capacitor when a circuit is completed between them. We propose that it is possible to use spontaneous polarization charge to get a significantly enhanced response in ZnO-based NW pressure sensors.

Inverse Scattering Method Design of Regrowth-free Single-mode Semiconductor Lasers for Monolithic Integration

An inverse scattering method is used to design single moded lasers, using etched depth insensitive pits as perturbations in the laser cavity. We compare 10 pit, 15 pit and 20 pit devices, and report strongly single moded lasers (>40dB). OCIS codes: (250.5300) Photonic integrated circuits; (140.3460) Lasers; (140.3570) Lasers, single-mode

Bifurcation and frequency analysis of mutually delay-coupled semiconductor lasers in photonic integrated circuits

A system of two mutually delay-coupled semiconductor lasers for integration in a photonic integrated circuit is investigated. Multi-stabilities and bifurcation scenarios are presented, followed by a comprehensive frequency analysis of the symmetric and symmetry-broken, 1-colour and 2-colour states.

Theory of Electronic Transport in Nanostructures

As the first of three chapters on transport properties, we begin by explaining some of the key factors relevant to electron transport on a macroscopic scale. We then turn to address a range of novel nanoscale transport effects. These include the quantum Hall effect and quantised conductance, as well as the recent prediction and observation of quantised conduction associated with the spin quantum Hall effect in a topological insulator. We next consider graphene and the consequences of its unusual band structure before concluding with an overview of the potential use of “junctionless” transistors as one of the most promising approaches for future nanoscale electronic devices.