Electrical Spin Injection and Transport in Semiconductor Spintronic Devices
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Electrical Spin
Injection and Transport in Semiconductor Spintronic Devices
B.T. Jonker, S.C. Erwin, A. Petrou, and A.G. Petukhov Abstract Semiconductor heterostructures that utilize carrier spin as a new degree of freedom offer entirely new functionality and enhanced performance over conventional devices. We describe the essential requirements for implementing this technology, focusing on the materials and interface issues relevant to electrical spin injection into a semiconductor. These are discussed and illustrated in the context of several prototype semiconductor spintronic devices, including spin-polarized light-emitting diodes and resonant tunneling structures such as the resonant interband tunneling diode. Keywords: ferromagnetic materials, semiconductors, spin injection, spin-polarized materials, spintronics.
Introduction The field of semiconductor electronics is based exclusively on the manipulation of charge. The phenomenal progress in increasing circuit performance by reducing device dimensions at a rate commonly referred to as Moore’s law is likely to be curtailed by practical and fundamental limits by the year 2010.1 Consequently, there is keen interest in exploring new avenues and paradigms for future technologies. Since an electron bears spin as well as charge, combining carrier spin as a new degree of freedom with the established bandgap engineering of modern devices offers exciting opportunities for new functionality and performance, as the other articles in this issue have discussed. This approach is referred to as “semiconductor spintronics.”2 Materials research and the physics of new spin-dependent phenomena play key roles in this rapidly growing field as researchers work to develop new materials, such as ferromagnetic semiconductors, and try to understand the basic 740
issues of spin injection and scattering at heterointerfaces. One may distinguish two broad regimes envisioned for spin-dependent device operation: one in which the net spin polarization is the key parameter (i.e., there are more spins oriented in a given direction than in the opposite direction in either current or number density) and a second in which spin phase coherence is important. This article will focus on the former,3 while the latter is relevant to other avenues of research such as the development of spinbased quantum computation, which relies on the controlled entanglement of wave functions.4 One of the earliest proposals for a semiconductor spintronic device was for a spin-polarized field-effect transistor (spinFET),5 in which the source and drain contacts are ferromagnetic materials intended to inject and detect spin-polarized electrons transported in a high-mobility channel. The conductance of the FET would depend on
electron spin orientation in the channel, which would be controlled by the gate voltage relative to the magnetization of the drain contact, producing a spin-based mode of operation. If the magnetization of the source and drain are independently controlled using techniques developed for magnetic memo
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