Spin-polarization effects in homogeneous and non-homogeneous diluted magnetic semiconductor heterostructures

Spin polarization is a key characteristic in developing spintronic devices. Diluted magnetic heterostructures (DMH), where subsequent layers of conventional and diluted magnetic semiconductors (DMS) are alternate, are one of the possible ways to obtain it. Si being the basis of modern electronics, Si or other group-IV DMH can be used to build spintronic devices directly integrated with conventional ones. In this work we study the physical properties and the spin-polarization effects of p-type DMH based in group-IV semiconductors (Si, Ge, SiGe, and SiC), by performing self-consistent (k) over right arrow . (p) over right arrow calculations in the local spin density approximation. We show that high spin polarization can be maintained in these structures below certain values of the carrier concentrations. Full spin polarization is attained in the low carrier concentration regime for carrier concentrations in the DMS layer up to similar to 2.0 x 10(19) cm(-3) for Si and up to similar to 6.0 x 10(19) cm(-3) for SiC. Partial, but still important spin polarization can be achieved for all studied group-IV DMH, with the exception of Ge for carrier concentrations up to 6.0 x 10(19) cm(-3). The role played by the effective masses and the energy splitting of the spin-orbit split-off hole bands is also discussed throughout the paper.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq[550.126/05-8/CTPETRO]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq[303.817/05-4/PQ]CNPq[304936/2009-0/PQ]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq[303578/2007-6/PQ]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq[577.219/2008-1/JP]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CAPESCoordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FACEPE[1077-1.05/08/APQ]Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPES

Optical absorption and electronic band structure first-principles calculations of alpha-glycine crystals

Light absorption of alpha-glycine crystals grown by slow evaporation at room temperature was measured, indicating a 5.11 +/- 0.02 eV energy band gap. Structural, electronic, and optical absorption properties of alpha-glycine crystals were obtained by first-principles quantum mechanical calculations using density functional theory within the generalized gradient approximation in order to understand this result. To take into account the contribution of core electrons, ultrasoft and norm-conserving pseudopotentials, as well as an all electron approach were considered to compute the electronic density of states and band structure of alpha-glycine crystals. They exhibit three indirect energy band gaps and one direct Gamma-Gamma energy gap around 4.95 eV. The optical absorption related to transitions between the top of the valence band and the bottom of the conduction band involves O 2p valence states and C, O 2p conduction states, with the carboxyl group contributing significantly to the origin of the energy band gap. The calculated optical absorption is highly dependent on the polarization of the incident radiation due to the spatial arrangement of the dipolar glycine molecules; in the case of a polycrystalline sample, the first-principles calculated optical absorption is in good agreement with the measurement when a rigid energy shift is applied