We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Polymer-based organic photovoltaic systems hold the promise for a cost-effective, lightweight solar energy conversion platform, which could benefit from simple solution processing of the active layer. The function of such excitonic solar cells is based on photoinduced electron transfer from a donor to an acceptor. Fullerenes have become the ubiquitous acceptors because of their high electron affinity and ability to transport charge effectively. The most effective solar cells have been made from bicontinuous polymer–fullerene composites, or so-called bulk heterojunctions. The best solar cells currently achieve an efficiency of about 5 %, thus significant advances in the fundamental understanding of the complex interplay between the active layer morphology and electronic properties are required if this technology is to find viable application.

Polymer–Fullerene Composite Solar Cells

About Supramolecular Assemblies of π-Conjugated Systems

In solid-state materials with strong relativistic spin-orbit coupling, charge currents generate transverse spin currents. The associated spin Hall and inverse spin Hall effects distinguish between charge and spin current where electron charge is a conserved quantity but its spin direction is not. This review provides a theoretical and experimental treatment of this subfield of spintronics, beginning with distinct microscopic mechanisms seen in ferromagnets and concluding with a discussion of optical-, transport-, and magnetization-dynamics-based experiments closely linked to the microscopic and phenomenological theories presented.

Spin Hall effects

Modern computing technology is based on writing, storing and retrieving information encoded as magnetic bits. Although the giant magnetoresistance effect has improved the electrical read out of memory elements, magnetic writing remains the object of major research efforts. Despite several reports of methods to reverse the polarity of nanosized magnets by means of local electric fields and currents, the simple reversal of a high-coercivity, single-layer ferromagnet remains a challenge. Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay of data storage media, owing to their ability to retain a stable magnetization state over long periods of time and their amenability to miniaturization. However, the same anisotropy properties that make a material attractive for storage also make it hard to write to. Here we demonstrate switching of a perpendicularly magnetized cobalt dot driven by in-plane current injection at room temperature. Our device is composed of a thin cobalt layer with strong perpendicular anisotropy and Rashba interaction induced by asymmetric platinum and AlOx interface layers. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. The symmetry of the switching field is consistent with the spin accumulation induced by the Rashba interaction and the spin-dependent mobility observed in non-magnetic semiconductors, as well as with the torque induced by the spin Hall effect in the platinum layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the cobalt layer and the oxidation of the aluminium layer, which is uppermost, suggesting that the Rashba interaction has a key role in the reversal mechanism. To prove the potential of in-plane current switching for spintronic applications, we construct a reprogrammable magnetic switch that can be integrated into non-volatile memory and logic architectures. This device is simple, scalable and compatible with present-day magnetic recording technology.

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Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

We present a cross-sectional scanning-tunneling microscopy investigation of the shape, size, and composition of InAs quantum dots in a GaAs matrix, grown by molecular beam epitaxy at low growth rate. From the dimensional analysis we conclude that the investigated quantum dots have an average height of 5 nm, a square base of 18 nm oriented along [010] and [100] and the shape of a truncated pyramid. From outward relaxation and lattice constant profiles we conclude that the dots consist of an InGaAs alloy and that the indium concentration increases linearly in the growth direction. Our results justify the predictions obtained from previous photocurrent measurements on similar structures and the used theoretical model.

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Determination of the shape and indium distribution of low-growth-rate InAs quantum dots by cross-sectional scanning tunneling microscopy

We have experimentally achieved spin polarization by a lateral current in a single nonmagnetic semiconductor heterojunction. The effect does not require an applied magnetic field. The current-induced spin orientation can be seen as the inverse of the circular photogalvanic effect (also often referred to as spin photocurrents): the nonequilibrium spin changes its sign as the current reverses.

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Current-induced spin polarization at a single heterojunction

The wave function of a hole bound to an individual Mn acceptor in GaAs is spatially mapped by scanning tunneling microscopy at room temperature and an anisotropic, crosslike shape is observed. The spatial structure is compared with that from an envelope-function, effective mass model and from a tight-binding model. This demonstrates that anisotropy arising from the cubic symmetry of the GaAs crystal produces the crosslike shape for the hole wave function. Thus the coupling between Mn dopants in GaMnAs mediated by such holes will be highly anisotropic.

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Spatial structure of an individual Mn acceptor in GaAs.

We experimentally observed an oscillating carrier capture time as a function of quantum well thickness. The capture times were obtained in a separate confinement quantum well structure by subpicosecond rise time measurements of the quantum well luminescence as well as by pump-probe correlation measurements of the population decay in the barrier layer. Both experimental techniques yield an oscillating capture time between 3 and 20 ps, in excellent agreement with the theoretical predictions. In a classical picture, our results correspond to a local capture time oscillating between 0.1 and 1.8 ps. Furthermore, the dependence of the capture time on the excitation energy is analyzed and the time-dependent position of the quasi-Fermi-level in the barrier layer is tracked experimentally. We find that the carrier capture time is very sensitive to the detailed structure parameters as well as to the carrier distribution in the barrier. Carrier capture is found to be an ambipolar process in which the oscillations of the observed capture times are due to the quantum-mechanical oscillation of the electron wave-function overlap above the well. Finally, electron capture is demonstrated to be dominated by LO-phonon emission.

Carrier capture into a semiconductor quantum well.

We present an atomic-scale analysis of the indium distribution of self-assembled In(Ga)As quantum rings (QRs) which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20nm which is consistent with the observed electronic radius of QR structures.

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JH Joachim Wolter

Papers

Determination of the shape and indium distribution of low-growth-rate InAs quantum dots by cross-sectional scanning tunneling microscopy

Current-induced spin polarization at a single heterojunction

Spatial structure of an individual Mn acceptor in GaAs.

Carrier capture into a semiconductor quantum well.

Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs