G. M. Sipahi

Towards high-frequency operation of spin-lasers

Injecting spin-polarized carriers into semiconductor lasers provides important opportunities to extend what is known about spintronic devices, as well as to overcome many limitations of conventional (spin-unpolarized) lasers. By developing a microscopic model of spin-dependent optical gain derived from an accurate electronic structure in a quantum well-based laser, we study how its operation properties can be modified by spin-polarized carriers, carrier density, and resonant cavity design. We reveal that by applying a uniaxial strain, it is possible to attain a large birefringence. While such birefringence is viewed as detrimental in conventional lasers, it could enable fast polarization oscillations of the emitted light in spin lasers which can be exploited for optical communication and high-performance interconnects. The resulting oscillation frequency (

Valence band structure of cubic AlGaN/GaN superlattices

>200

Hole charge localization and band structures of p-doped GaN/InGaN and GaAs/InGaAs semiconductor heterostructures

GHz) would significantly exceed the frequency range possible in conventional lasers.

Exchange-correlation effects on the hole miniband structure and confinement potential in zinc-blende AlxGa1-xN/GaN superlattices

The influence of different material parameters, spin-orbit interaction, and strain effects on the valence band structure of cubic AlGaN/GaN superlattices is investigated. One-particle hole state calculations are carried out within the k⋅p theory by means of a full six-band Luttinger-Kohn Hamiltonian in a plane-wave representation. It is shown that the use of distinct values for the Luttinger parameters for the barrier and well regions leads to significant changes in the hole levels, particularly for large Al content. Spin-orbit interaction effects are responsible for strong nonparabolicities due to the light- and split-off-hole bands mixing. Besides, the hole levels are very sensitive to strain effects. Thus, it is demonstrated that these effects cannot be neglected in a realistic description of the valence band structure of these materials.

Inter- and Intraband Transitions in Cubic Nitride Quantum Wells

Hole band structures of p-doped semiconductor heterostructures are presented. The full six-band Luttinger-Kohn Hamiltonian generalized to treat different materials is solved in conjunction with the Poisson equation in a plane-wave representation. Self-consistent solutions of the multiband effective-mass-Poisson equations are obtained for unstrained and biaxially strained zinc-blende GaN/InxGa1-xN and GaAs/InxGa1-xAs quantum wells and superlattices (SLs), in which the acceptor doping concentration and its profile, the SL period, and the alloy content x are varied. The particular features observed in the valence subband structure of GaN/InGaN systems are stressed in a comparison with other selected In-derived III-V heterostructures, such as GaAs/InGaAs SLs.

Calculations of electronic and optical properties in p-doped AlGaN/GaN superlattices and quantum wells

We present valence band-structure calculations for undoped and p-doped cubic AlxGa1-xN/GaN superlattices (SLs), in which the coupling between the heavy-hole, light-hole, and spin-orbit-split-hole bands and strain effects due to lattice mismatch are taken into account. The calculations are performed within a self-consistent approach to the kp theory by means of a full six-band Luttinger-Kohn Hamiltonian. Exchange-correlation effects within the two-dimensional hole gas are included in the calculations in the local density approximation. Results for hole minibands and potential profiles are shown as functions of the SL period. It is shown that exchange and correlation play an important role in the correct description of the systems.

Optical and electronic properties of AlInGaN/InGaN superlattices

In this work we analyze the luminescence emissions from selected isolated GaN/InGaN quantum wells comparing measured and theoretical photoluminescence (PL) spectra. The calculations are performed within the k.p method by means of an 8 x 8 Kane Hamiltonian, generalized to treat different materials. Strain effects due to the large lattice mismatch between InN and GaN are taken into account. From the direct comparison with experimental results, we found evidence for transitions involving confined levels which, besides those related to quantum dots, may be ascribed to the first electron-heavy-hole transition in the quantum wells. Since the studies of optical properties of quantum wells based on cubic nitrides are at an early stage, the results reported here will provide guidelines for the interpretation of forthcoming experiments.

Charge and spin distributions in Ga 1 − x Mn x As ∕ GaAs ferromagnetic multilayers

We present the band structure and photoluminescence spectra calculations of h-AlGaN/GaN multiple quantum wells and superlattices. The calculations are performed within a self-consistent approach to the k . p theory by means of the resolution of a full eight-band Kane Hamiltonian together with Poisson equation for the carriers charge density. Exchange-correlation effects are included within the local density approximation. These are found to play a important role in the correct determination of band structures and potential profiles. In our calculations, electric fields, strain effects and split-off hole band are taken into account. Theoretical PL spectra of the systems are shown. Similar calculations to c-AlGaN/GaN MQWs and SLs were done for comparison.

Systematic study of magnetic linear dichroism and birefringence in (Ga,Mn)As

In this work we analyze the theoretical photoluminescence (PL) spectra from strained AlGaInN/InGaN and InGaN/AlGaInN superlattices (SLs). The calculations are performed within the k.p framework by means of the solution of the 8×8 effective mass Kane Hamiltonian generalized to treat layers of different materials. Strain effects due to lattice mismatch and the split-off-hole band were also taken into account. The results indicate that PL emissions observed in these systems are due to recombination from confined states inside the quantum well. In both cases, considering the quaternary nitride alloys in the barrier and in the well, we have feasibility of light emissions sweeping the visible region spectra from blue through red. This is the first attempt to show theoretical luminescence spectra for cubic AlInGaN/InGaN SLs and can be used as a guide for the design of white light emission diodes (LEDs) and other devices.

Spin polarization of Co(0001)/graphene junctions from first principles

A self-consistent electronic structure calculation based on the Luttinger-Kohn model is performed on GaMnAs/GaAs multilayers. The Diluted Magnetic Semiconductor layers are assumed to be metallic and ferromagnetic. The high Mn concentration (considered as 5% in our calculation) makes it possible to assume the density of magnetic moments as a continuous distribution, when treating the magnetic interaction between holes and the localized moment on the Mn(++) sites. Our calculation shows the distribution of heavy holes and light holes in the structure. A strong spin-polarization is observed, and the charge is concentrated mostly on the GaMnAs layers, due to heavy and light holes with their total angular momentum aligned anti-parallel to the average magnetization. The charge and spin distributions are analyzed in terms of their dependence on the number of multilayers, the widths of the GaMnAs and GaAs layers, and the width of lateral GaAs layers at the borders of the structure.