New Possibilities for Ferromagnetic Semiconductors
- PDF / 370,386 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 39 Downloads / 225 Views
New Possibilities for Ferromagnetic Semiconductors
Scott A. Chambers and Robin F.C. Farrow Abstract New candidate ferromagnetic semiconductors have recently been grown by doping semiconducting transition-metal oxides with magnetic impurities. Some of these exhibit ferromagnetism at and above room temperature. The critical question is why? In many cases, complex solid-state chemistry is involved in the synthesis and drives the resulting properties. The observation of room-temperature ferromagnetism in these materials must be accompanied by a careful identification of the phases and structures present in order to accurately identify the origin of the magnetism. Keywords: magnetic materials, molecular-beam epitaxy (MBE), semiconductors, spinpolarized materials, spintronics.
Introduction Diluted magnetic semiconductors (DMSs) are essential ingredients in the spintronics toolbox. A DMS is a nonmagnetic semiconductor doped with a few percent of some magnetic impurity. The most useful kind of DMS is one in which the net electronic (spin plus orbital) angular momenta of the individual magnetic dopants are coupled ferromagnetically by free carriers. Such materials are expected to have spin-polarized states in either the valence or conduction bands. The formation of interfaces with nonmagnetic semiconductors allows for the possibility of spin-polarized carrier injection into semiconductor heterostructures and the fabrication of a variety of device structures that utilize spin. For instance, spin light-emitting diodes (spin-LEDs), spin field-effect transistors (spin-FETs), and spin resonant tunneling diodes (spin-RTDs) are best fabricated using a DMS as the spininjecting material. The conductivity match at the interface can be optimized through selective doping, thereby enabling optimal electrical spin injection. Spin injection is also facilitated when there is little or no electrostatic barrier (band discontinuity or band offset), and when there is a good momentum match at the interface. Momentum match refers to a matching of the crystal momentum for the allowed states of the carrier (electron or hole) at the inter-
MRS BULLETIN/OCTOBER 2003
face. When the momentum match is good, the carrier can cross the interface without having to undergo a scattering event. Such scattering events reduce the probability of transport across the interface. In their article elsewhere in this issue, Jonker et al. discuss the fabrication and highly successful operation of a spin-LED that employs ZnMnSe as the spin injector for a AlGaAs/GaAs quantum-well structure. As is well known, most traditional III–V and II–VI DMS materials are either low-temperature paramagnets or ferromagnets. Thus, practical device structures are precluded because of the need for cryogenic cooling (4.5 K in the case of the spin-LED illustrated in Figure 1 of the article by Jonker et al.). However, some new semiconductors have been discovered in the last few years that exhibit ferromagnetic behavior at or above room temperature. The most robust of these is an oxide– C
Data Loading...
