Spintronics device concepts

Spin-dependent phenomena in semiconductors may lead to devices with new or enhanced functionality, such as polarised solid-state light sources (spin light-emitting diodes), novel microprocessors and sensitive biological and chemical sensors. The realisation of robust semiconductor spin-device technology requires the ability to control the injection, transport and detection of polarised carriers, and to manipulate their density by a field gating. The absence of Si-based or room-temperature dilute magnetic semiconductors has subdued the initial excitement over semiconductor spintronics, but recent reports demonstrate that progress is far from dormant. The authors give examples of a number of different spin-device concepts for polarised light emission, spin field-effect transistors and nanowire sensors. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field which has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analogue. There have been demonstrations of device-type operation in structures based on GaMnAs and InMnAs at low temperatures. The most promising materials for room-temperature polarised light emission are thought to be GaN and ZnO, but results to date on realising such devices have been disappointing. The short spin-relaxation time observed in GaN/InGaN heterostructures probably results from the Rashba effect. Possible solutions involve either cubic phase nitrides or the use of additional stressor layers to create a larger spin-splitting, to get polarised light emission from these structures, or to look at alternative semiconductors and fresh device approaches.