M.E. Mora-Ramos

Optical characterization of polytype fibonacci and Thue-Morse quasiregular dielectric structures made of porous silicon multilayers

To investigate the reflection of light in quasiregular dielectrics, we study here the optical properties of porous-silicon-based Fibonacci and Thue–Morse heterostructures. The multilayered systems are fabricated in such a way that each element in the two-block substitutional sequence has a polytype structure. Both delta-like and traditional configurations are considered. The results for the optical reflectance are analysed. Numerical simulation for the transmittance of delta structures along the lines of the transfer matrix approach is also presented.

Tunable resonance transmission modes in hybrid heterostructures based on porous silicon

In this work, we report the experimental results and theoretical analysis of strong localization of resonance transmission modes generated by hybrid periodic/quasiperiodic heterostructures (HHs) based on porous silicon. The HHs are formed by stacking a quasiperiodic Fibonacci (FN) substructure between two distributed Bragg reflectors (DBRs). FN substructure defines the number of strong localized modes that can be tunable at any given wavelength and be unfolded when a partial periodicity condition is imposed. These structures show interesting properties for biomaterials research, biosensor applications and basic studies of adsorption of organic molecules. We also demonstrate the sensitivity of HHs to material infiltration.

Energy states in GaAs delta-doped field effect transistors under hydrostatic pressure

The study of the electronic structure in GaAs-based delta-doped field effect transistors under applied hydrostatic pressure is presented. A combination of the depletion approximation and the local density Thomas-Fermi theory is used to model the potential energy profile. We present a discussion on the possible effect of the hydrostatic pressure in the formation of high conductivity channels in the system.

Polaron Effective Mass and Binding Energy in Semiconducting InxGa1—xN

Polaron binding energy and effective mass are calculated for bulk hexagonal In x Ga 1-x N with the use of a dielectric continuum Frohlich-like electron-phonon Hamiltonian. The calculation includes the evaluation of polaronic corrections up to second order in the coupling constants. The result obtained for the electron effective mass in hexagonal GaN is in very good agreement with recent experimental findings. The relevance of having reliable experimental values for the InN conduction band effective mass and optical dielectric constant in order to clarify the real contribution of second-order corrections to the binding energy and the anisotropy of the polaron mass in this material is discussed. The case of cubic alloys is briefly considered as well.

Influence of the hydrostatic pressure onto the electronic and transport properties of n-type double δ-doped GaAs quantum wells

We present a Thomas-Fermi-based envelope function calculation of the electronic structure for n-type double δd-doped GaAs quantum wells under the influence of applied hydrostatic pressure. An empirical formula for the electron mobility is used to qualitatively describe some transport properties in the system. The optimal interwell distance and hydrostatic pressure for which the mobility would be a maximum are obtained, particularly in the high density limit (above 1013cm-2). This could be of interest for the design and fabrication of high power, high speed electronics.

Donor impurity states and related terahertz range nonlinear optical response in GaN cylindrical quantum wires: Effects of external electric and magnetic fields

We report a study on the optical absorption coefficient associated to hydrogenic impurity interstate transitions in zinc-blende GaN quantum wires of cylindrical shape taking into account the effects of externally applied static electric and magnetic fields. The electron states emerge within the effective mass approximation, via the exact diagonalization of the donor-impurity Hamiltonian with parabolic confinement and external field effects. The nonlinear optical absorption is calculated using a recently derived expression for the dielectric susceptibility, obtained via a nonperturbative solution of the density-matrix Bloch equation. Our results show that this treatment eliminates not only the intensity-dependent bleaching effect but also the change in sign of the nonlinear contribution due to the combined effect of asymmetric impurity location and the applied electric field.

Optical Properties of Non-Periodic Dielectric Systems Made of Nanostructured Porous Silicon

To investigate the optical properties in non-periodic dielectric systems, we study here the reflection of light from nanostructured porous-silicon-based period doubling heterostructures. The multilayered systems are fabricated in such a way that the optical thickness of each layer is one quarter of 650nm. The results for the optical reflectance are presented and compared with that of Fibonacci, Thue-Morse, and random structures fabricated under the same conditions. Numerical simulation for the reflectance along the lines of the transfer matrix approach is performed. In addition, optical reflection from Gaussian porous silicon multilayers is also briefly discussed. We find that porous silicon Period Doubling dielectric multilayers could demonstrate the optical properties similar to the classical periodic Febry-Perot interference filters with one or multiple resonant peaks, but with an advantage of having total optical thickness much lesser than that of the periodic structures.

Polaron Properties in GaN and AlN

The polaron binding energy and effective mass are obtained for bulk III–V nitride compounds with wurtzite crystalline structure with the use of a recently derived dielectric continuum Frohlich-like electron–phonon interaction Hamiltonian. Corrections are calculated up to second order in the coupling constants. Numerical results are reported for GaN and AlN. Effects of phonon polarization mixing are studied and its relevance to the effective mass anisotropy is discussed. For GaN, the calculated effective mass coincides with reported experimental values.

Effect of the hydrostatic pressure on the electron mobility in delta-doped systems

The influence of hydrostatic pressure on the electron states and low-temperature mobility in n-type GaAs δ-doped single quantum wells is studied. Values of hydrostatic pressure consider are below the so-called Γ-X crossover, keeping all attention in the electronic properties at the Brillouin zone center. The effect of the pressure on the electron mobility is described via a relative quantity that is proportional to the ratio between P ≠ 0 and zero pressure results. Calculation is performed using an analytical description of the potential energy function profile, based on the Thomas-Fermi approach, taking explicitly into account the dependence upon P of the main input parameters: effective masses and dielectric constant. The relative mobility increases for higher values of P. The cases of zero and finite -although small- temperature are studied, showing that the influence of T is mainly to lower the values of the relative mobility in the entire range of P considered. Numerical results are reported for a two-dimensional density of ionized impurities equals to 7.5 × 1012 cm-2.

Carrier states and optical response in core–shell-like semiconductor nanostructures

Abstract The charge carrier states in a GaAs/AlGaAs axially symmetric core–shell quantum wire are calculated in the effective mass approximation via a spectral method. The possible presence of externally applied electric and magnetic fields is taken into account, together with the variation in the characteristic in-plane dimensions of the structure. The obtained energy spectrum is used to evaluate the optical response through the coefficients of intersubband optical absorption and relative refractive index change. The particular geometry of the system also allows to use the same theoretical model in order to determine the photoluminescence peak energies associated to correlated electron-hole states in double GaAs/AlGaAs quantum rings, showing a good agreement when they are compared with recent experimental reports. This agreement may validate the use of both the calculation process and the approximate model of abrupt, circularly shaped cross section geometry for the system.