Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

First-principles calculations for defects and impurities: Applications to III-nitrides

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

The development of a compact, solid-state light-emitting diode (LED) that emits at 210 nanometres — the shortest wavelength yet achieved for any type of LED — represents an important step towards achieving exciton-related light-emitting devices and replacing inefficient gas light sources with solid-state light sources. Compact high-efficiency ultraviolet solid-state light sources1—such as light-emitting diodes (LEDs) and laser diodes—are of considerable technological interest as alternatives to large, toxic, low-efficiency gas lasers and mercury lamps. Microelectronic fabrication technologies and the environmental sciences both require light sources with shorter emission wavelengths: the former for improved resolution in photolithography and the latter for sensors that can detect minute hazardous particles. In addition, ultraviolet solid-state light sources are also attracting attention for potential applications in high-density optical data storage, biomedical research, water and air purification, and sterilization. Wide-bandgap materials, such as diamond2 and III–V nitride semiconductors (GaN, AlGaN and AlN; refs 3–10), are potential materials for ultraviolet LEDs and laser diodes, but suffer from difficulties in controlling electrical conduction. Here we report the successful control of both n-type and p-type doping in aluminium nitride (AlN), which has a very wide direct bandgap11 of 6 eV. This doping strategy allows us to develop an AlN PIN (p-type/intrinsic/n-type) homojunction LED with an emission wavelength of 210 nm, which is the shortest reported to date for any kind of LED. The emission is attributed to an exciton transition, and represents an important step towards achieving exciton-related light-emitting devices as well as replacing gas light sources with solid-state light sources.

An aluminium nitride light-emitting diode with a wavelength of 210 nanometres

We review the electronic properties of bilayer graphene, beginning with a description of the tight-binding model of bilayer graphene and the derivation of the effective Hamiltonian describing massive chiral quasiparticles in two parabolic bands at low energies. We take into account five tight-binding parameters of the Slonczewski–Weiss–McClure model of bulk graphite plus intra- and interlayer asymmetry between atomic sites which induce band gaps in the low-energy spectrum. The Hartree model of screening and band-gap opening due to interlayer asymmetry in the presence of external gates is presented. The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and we also discuss orbital magnetism, phonons and the influence of strain on electronic properties. We conclude with an overview of electronic interaction effects.

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The electronic properties of bilayer graphene.

We present numerical calculations of the ballistic spin-transport properties of quasi-one-dimensional wires in the presence of the spin-orbit (Rashba) interaction. A tight-binding analog of the Rashba Hamiltonian that models the Rashba effect is used. By varying the robustness of the Rashba coupling and the width of the wire, weak- and strong-coupling regimes are identified. Perfect electron-spin modulation is found for the former regime, regardless of the incident Fermi energy and mode number. In the latter however, the spin conductance has a strong energy dependence due to a nontrivial subband intermixing induced by the strong Rashba coupling. This would imply a strong suppression of the spin modulation at higher temperatures and source-drain voltages. The results may be of relevance for the implementation of quasi-one-dimensional spin-transistor devices.

Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires

We employ effective-mass theory for degenerate hole bands to calculate the acceptor binding energies for Be, Mg, Zn, Ca, C, and Si substitutional acceptors in GaN and AlN. The calculations are performed through the 6$\ifmmode\times\else\texttimes\fi{}$6 Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ) and zinc-blende (ZB) crystal phases, respectively. An analytic representation for the acceptor pseudopotential is used to introduce the specific nature of the impurity atoms. The energy shift due to polaron effects is also considered in this approach. The ionization energy estimates are in very good agreement with those reported experimentally in WZ GaN. The binding energies for ZB GaN acceptors are all predicted to be shallower than the corresponding impurities in the WZ phase. The binding-energy dependence upon the crystal-field splitting in WZ GaN is analyzed. Ionization levels in AlN are found to have similar ``shallow'' values to those in GaN, but with some important differences which depend on the band structure parametrizations, especially the value of the crystal-field splitting used.

Acceptor binding energies in gan and aln

The coupling of geometrical and electronic properties is a promising venue to engineer conduction properties in graphene. Confinement added to strain allows for interplay of different transport mechanisms with potential device applications. To investigate strain signatures on transport in confined geometries, we focus on graphene nanoribbons (GNRs) with circularly symmetric deformations. In particular, we study GNRs with an inhomogeneous out-of-plane Gaussian deformation, connected to reservoirs. We observe an enhancement of the density of states in the deformed region, accompanied with a decrease in the conductance, signaling the presence of confined states. The local density of states exhibits a sixfold symmetric structure with an oscillating sublattice occupation asymmetry that persists for a wide range of energy and model parameters.

Gaussian deformations in graphene ribbons: Flowers and confinement

Coherent quantum transport in ferromagnetic/semiconductor/ferromagnetic junctions is studied theoretically within the Landauer framework of ballistic transport. We show that quantum coherence can have unexpected implications for spin injection and that some intuitive spintronic concepts which are founded in semiclassical physics no longer apply: A quantum spin-valve (QSV) effect occurs even in the absence of a net spin polarized current flowing through the device, unlike in the classical regime. The converse effect also arises, i.e., a zero spin-valve signal for a nonvanishing spin current. We introduce criteria useful for analyzing quantum and classical spin-transport phenomena and the relationships between them. The effects on QSV behavior of spin-dependent electron transmission at the interfaces, interface Schottky barriers, Rashba spin-orbit coupling, and temperature, are systematically investigated. While the signature of the QSV is found to be sensitive to temperature, interestingly, that of its converse is not. We argue that the QSV phenomenon can have important implications for the interpretation of spin injection in quantum spintronic experiments with spin-valve geometries.

Coherent spin-valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions

We present a theory of coherent quantum transport in ferromagnetic/non-magnetic/ferro magnetic heterojunctions. We predict quantum coherence to give rise to a quantum spin valve effect that, unlike its familiar classical analog, occurs even in the absence of a net spin current through the heterostructure. Thus the relationship between spin and charge transport is qualitatively different in the presence of quantum interference than in the (semi-)classical regime. This has important implications for the design of quantum coherent spintronic devices and the interpretation of experiments.

https://iopscience.iop.org/article/10.1209/epl/i2002-00165-4/pdf

Francisco Mireles

Papers

Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires

Acceptor binding energies in gan and aln

Gaussian deformations in graphene ribbons: Flowers and confinement

Coherent spin-valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions

From classical to quantum spintronics: Theory of coherent spin injection and spin valve phenomena