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

Conventional electronics is based on the manipulation of electronic charge. An intriguing alternative is the field of ‘spintronics’, wherein the classical manipulation of electronic spin in semiconductor devices gives rise to the possibility of reading and writing non-volatile information through magnetism1,2. Moreover, the ability to preserve coherent spin states in conventional semiconductors3 and quantum dots4 may eventually enable quantum computing in the solid state5,6. Recent studies have shown that optically excited electron spins can retain their coherence over distances exceeding 100 micrometres (ref. 7). But to inject spin-polarized carriers electrically remains a formidable challenge8,9. Here we report the fabrication of all-semiconductor, light-emitting spintronic devices using III–V heterostructures based on gallium arsenide. Electrical spin injection into a non-magnetic semiconductor is achieved (in zero magnetic field) using a p-type ferromagnetic semiconductor10 as the spin polarizer. Spin polarization of the injected holes is determined directly from the polarization of the emitted electroluminescence following the recombination of the holes with the injected (unpolarized) electrons.

Electrical spin injection in a ferromagnetic semiconductor heterostructure

羟氨苄青霉素引起大疱性类天疱疮样疹

Semiconductor devices have become indispensable for generating electromagnetic radiation in everyday applications. Visible and infrared diode lasers are at the core of information technology, and at the other end of the spectrum, microwave and radio-frequency emitters enable wireless communications. But the terahertz region (1-10 THz; 1 THz = 10(12) Hz) between these ranges has remained largely underdeveloped, despite the identification of various possible applications--for example, chemical detection, astronomy and medical imaging. Progress in this area has been hampered by the lack of compact, low-consumption, solid-state terahertz sources. Here we report a monolithic terahertz injection laser that is based on interminiband transitions in the conduction band of a semiconductor (GaAs/AlGaAs) heterostructure. The prototype demonstrated emits a single mode at 4.4 THz, and already shows high output powers of more than 2 mW with low threshold current densities of about a few hundred A cm(-2) up to 50 K. These results are very promising for extending the present laser concept to continuous-wave and high-temperature operation, which would lead to implementation in practical photonic systems.

Terahertz semiconductor heterostructure laser

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 injection of spin-polarized electrons is presently one of the major challenges in semiconductor spin electronics. We propose and demonstrate a most efficient spin injection using diluted magnetic semiconductors as spin aligners. Time-resolved photoluminescence with a Cd0.98Mn0.02Te/CdTe structure proves the feasibility of the spin-alignment mechanism.

Spin injection into semiconductors

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.

Direct Observation of Optically Injected Spin-Polarized Currents in Semiconductors

We present a spectroscopic method for studying spin transport in semiconductors. Our time-resolved experiments have an important implication for spin electronics as they show that spin-polarized electron drift is possible in semiconductors over typical device lengths in high electric fields. We demonstrate an almost complete conservation of the orientation of the electron spin during transport in GaAs over a distance as long as 4 μm and fields up to 6 kV/cm.

Spin transport in GaAs

The recombination kinetics of the electron‐hole plasma in strongly excited, intrinsic GaAs is investigated at room temperature by time‐resolved photoluminescence using a line‐shape analysis of transient spectra. Special structuring of the samples prevents stimulated emission and diffusion. Population of higher energetic subsidiary conduction‐band valleys must be taken into account for densities ≳1.5×1019 cm−3. A significant influence of Auger recombination is detected for densities ≳2.5×1019 cm−3. The bimolecular recombination coefficient and an effective Auger coefficient are found to be B=(1.7±0.2)×10−10 cm3 s−1 and Ceff=(7±4)×10−30 cm6 s−1, respectively.

Auger recombination in intrinsic GaAs

We observe quantum beats in the time-resolved photoluminescence of GaAs quantum wells in a magnetic field perpendicular to the growth direction. These beats originate from Larmor precession of electron spins and show a coherence time of 500 ps, which is much longer than the optical dephasing time of 7 ps. The long coherence time allows us to determine the electron Land\'e g factors with an accuracy better than 1%.

W. W. Rühle

Papers

Spin injection into semiconductors

Direct Observation of Optically Injected Spin-Polarized Currents in Semiconductors

Spin transport in GaAs

Auger recombination in intrinsic GaAs

Quantum beats of electron Larmor precession in GaAs wells.