Spin dynamics in semiconductors

This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.

The ability of spintronics to re-energize itself in directions that germinate new subfields has made it one of the most fertile grounds for basic research aimed at future applications. A brief overview of the connections between five emerging subfields suggests exciting things to come.

New moves of the spintronics tango

In a classical light source, such as a laser, the photon number follows a Poissonian distribution. For quantum information processing and metrology applications, a non-classical emitter of single photons is required. A single quantum dot is an ideal source of single photons and such single-photon sources in the visible spectral range have been demonstrated with III-nitride and II-VI-based single quantum dots. It has been suggested that short-wavelength blue single-photon emitters would be useful for free-space quantum cryptography, with the availability of high-speed single-photon detectors in this spectral region. Here we demonstrate blue single-photon emission with electrical injection from an In0.25Ga0.75N quantum dot in a single nanowire. The emitted single photons are linearly polarized along the c axis of the nanowire with a degree of linear polarization of ~70%.

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Electrically driven polarized single-photon emission from an InGaN quantum dot in a GaN nanowire

The Technical Committee of the IEEE Magnetics Society has selected seven research topics to develop their roadmaps, where major developments should be listed alongside expected timelines: 1) hard disk drives; 2) magnetic random access memories; 3) domain-wall devices; 4) permanent magnets; 5) sensors and actuators; 6) magnetic materials; and 7) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used, for example, as a cell for a magnetic random access memory and a spin-torque oscillator in their vertical form as well as a spin transistor and a spin Hall device in their lateral form. In these devices, the critical role of magnetic materials is to inject spin-polarized electrons efficiently into a nonmagnet. We have accordingly identified two key properties to be achieved by developing new magnetic materials for future spintronic devices: 1) half-metallicity at room temperature (RT) and 2) perpendicular anisotropy in nanoscale devices at RT. For the first property, five major magnetic materials are selected for their evaluation for future magnetic/spintronic device applications: 1) Heusler alloys; 2) ferrites; 3) rutiles; 4) perovskites; and 5) dilute magnetic semiconductors. These alloys have been reported or predicted to be half-metallic ferromagnets at RT. They possess a bandgap at the Fermi level $E_{F}$ only for its minority spins, achieving 100% spin polarization at $E_{F}$ . We have also evaluated $\text{L1}_{0}$ alloys and $D0_{22}$ –Mn alloys for the development of a perpendicularly anisotropic ferromagnet with large spin polarization. We have listed several key milestones for each material on their functionality improvements, property achievements, device implementations, and interdisciplinary applications within 35 years time scale. The individual analyses and the projections are discussed in the following sections.

Roadmap for Emerging Materials for Spintronic Device Applications

The high-frequency dynamics of spin-polarized carriers and photons in a spin laser have been studied. The transient response of the device obtained from the rate equations is characterized by two sets of relaxation oscillations in the carrier and photon distributions corresponding to the two polarization modes. Consequently two distinct resonant peaks are observed in the small-signal modulation response. The calculated transient characteristics indicate that the best results are obtained from a spin laser when only the favored polarization mode, with lower threshold, is operational. Under this condition the small-signal modulation bandwidth is higher than that in a conventional laser, the threshold current is lower and the output polarization can be 100% with appropriate bias conditions, independent of the spin polarization of carriers in the active region. Measurements were made at 230 K on a InAs/GaAs quantum dot spin vertical cavity surface emitting laser. A time-averaged output polarization of 55% is measured with an active region spin polarization of $5--6\text{ }\mathrm{%}$. The experimental results are in good agreement with calculated data.

High-frequency dynamics of spin-polarized carriers and photons in a laser

The effects of spin-induced gain anisotropy on output polarization and threshold current reduction of electrically pumped spin-polarized lasers have been studied. Analytical forms of these parameters are derived by considering diffusive transport from the spin injector to the active region. The calculated values of the parameter are in excellent agreement with values obtained from measurements made at 200 K on an InAs/GaAs quantum dot spin-polarized vertical cavity surface-emitting laser. Electrical modulation of the output polarization of the laser is demonstrated with a peak modulation index of 0.6.

Optical polarization modulation and gain anisotropy in an electrically injected spin laser.

Quantum dot lasers: From promise to high-performance devices

A high-speed multiwavelength quantum dot comb laser, grown by molecular beam epitaxy, is demonstrated. The device is characterized with a 75.9 nm (full width at half maximum) and a 91.4 nm (Δ−15 dB) wide lasing spectrum. There are 105 and 185 simultaneously emitted longitudinal modes with a maximum channel intensity nonuniformity of less than 3 dB in the spectral range of 1231–1252 nm and 1274–1311 nm, respectively, for a laser with 1040 μm cavity length. The channel spacing can be tuned with cavity length and remains invariant in the temperature range of 300–323 K. The small signal modulation bandwidth is 7.5 GHz.

High performance tunnel injection quantum dot comb laser

The electrical properties of the AlxGa1-xAs alloys for x>or=0.24 are dominated by deep donors which appear in undoped and doped crystals. The corresponding defects, which are now more commonly known as D-X centres, also behave as electron traps. The properties of these defects in undoped and doped alloys are reviewed. Results of Hall, transient capacitance, deep-level transient spectroscopy and thermally stimulated capacitance measurements on crystals grown by liquid phase epitaxy and molecular beam epitaxy are presented and discussed. The deep levels have a constant thermal activation energy of 0.21 eV in the direct band-gap region, while in the same composition range the defect binding energy, as determined from Hall measurements, increases monotonically. Beyond the crossover region, the thermal activation energy and binding energy assume similar values and both decrease monotonically with increase of x. These energy values and the electron capture properties of the defects can be consistently explained by involving the interaction of the defects with indirect L and X minima. Defects with similar properties are also identified in other alloy systems in which the L and X conduction minima are lowest in energy.

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P. Bhattacharya

Papers

High-frequency dynamics of spin-polarized carriers and photons in a laser

Optical polarization modulation and gain anisotropy in an electrically injected spin laser.

Quantum dot lasers: From promise to high-performance devices

High performance tunnel injection quantum dot comb laser

The relationship of the D-X centre in AlxGa1-xAs and other III-V alloys with the conduction band structure