Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field

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Semiconductor Spintronics

The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30?at% The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD These particles are well oriented with the ZnO crystal structure By increasing the base pressure of the vacuum chamber to pressures above 1?10?5?Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ~008??B/Co Depositions in the presence of oxygen background gas at 002?mTorr decreases the magnetization The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out

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Structure and magnetism of cobalt-doped ZnO thin films

We have investigated experimentally the effects of n-type impurities such as Al and H on the magnetic properties of ZnCoO. The carrier density changes slightly upon the introduction of hydrogen but increases significantly in the case of Al. A measurement of magnetic circular dichroism indicates that, of the two impurities, only H induces ferromagnetism in ZnCoO. Our experimental data suggest that, unlike Al, H plays an important role in the enhancement of ferromagnetic spin-spin interactions that goes much beyond a carrier-mediated effect.

Hydrogen-induced ferromagnetism in ZnCoO

Electronic and magnetic properties of Co-doped ZnO diluted magnetic semiconductor

Semiconductor materials form the basis of modern electronics, communication, data storage and computing technologies. One of today’s challenges for the development of future technologies is the realization of devices that control not only the electron charge, as in present electronics, but also its spin, setting the basis for future spintronics. Spintronics represents the concept of the synergetic and multifunctional use of charge and spin dynamics of electrons, aiming to go beyond the traditional dichotomy of semiconductor electronics and magnetic storage technology. The most direct method to induce spin-polarized electrons into a semiconductor is by introducing appropriate transition-metal or rare-earth dopants producing a dilute magnetic semiconductor (DMS). At the same time the seamless integration of future spintronic devices into nanodevices would require the fabrication of one-dimensional DMS nanostructures in well-defined architectures. In this review we focus on recent advances in the synthesis of DMS nanowires as well discussing the structural, optical and magnetic properties of these materials.

Dilute magnetic semiconductor nanowires

Both ion implantation and epitaxial crystal growth provide convenient methods of introducing transition metals such as Mn,Cr,Fe,Ni and Co into GaN, GaP, SiC and ZnO for creating dilute magnetic semiconductors exhibiting room temperature ferromagnetism. In this paper we review progress in wide band gap ferromagnetic semiconductors and the role of defects and doping on the resulting magnetic properties.

http://www.phys.ufl.edu/~afh/reprints/EffectsofDefects_WideBandSemiconductors_Pearton.pdf

Effects of defects and doping on wide band gap ferromagnetic semiconductors

Single-phase (Ga,Mn)N layers grown by molecular beam epitaxy with ∼3 at. % Mn were exposed to 2H or 1H plasmas under conditions designed to permeate layers with hydrogen. No evidence of the formation of Mn–H complexes could be detected from either infrared spectroscopy or the changes in magnetic properties. However, the residual magnetization increased ∼20% after hydrogenation, consistent with passivation of crystal defects in (Ga,Mn)N. Hydrogenation may be an attractive method for improving the magnetic properties of these heteroepitaxial materials.

http://www.phys.ufl.edu/~afh/reprints/hydrogen_in_GaMnN_apl.pdf

Effects of hydrogen incorporation in GaMnN

Carbon-doped (p ∼ 10 1 9 cm - 3 ) GaMnP with ∼5 atom % Mn grown by gas-source molecular beam epitaxy showed ferromagnetism to <50 K in the as-grown state. After exposure to a 2 H plasma for 3 h at 250°C, the apparent Curietemperature was increased to ∼230 K. A plausible mechanism for this improvement in magnetic properties is hydrogen passivation of some of the carbon acceptors and also defects in the epi layer that trap free carriers or promote antiferromagnetic coupling. The results reinforce the notion that crystal defects degrade the magnetic properties of III-V ferromagnetic semiconductors.

W. Tang

Papers

Effects of defects and doping on wide band gap ferromagnetic semiconductors

Effects of hydrogen incorporation in GaMnN

Hydrogenation effects on magnetic properties of GaMnP