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Spintronics Holds Promise For More Efficient Devices

Based on manipulating the spin - rather than the charge - of electrons, spintronics could one day provide us with faster, more efficient devices, as Jon Newey discovers.
Commercially available electronic and optoelectronic devices rely on the manipulation of charge to produce the desired functions. However, around the world there is ongoing research aimed at combining charge with another, less tangible property of the electron - its spin. So-called spintronics will usher in a whole new generation of faster and more efficient devices. The spin property of electrons has already been exploited in PC data storage drives, and magnetic RAM chips are now at an advanced stage of development. MRAM devices can be rewritten and switched at a greater speed than is possible in current RAM technology, and they also retain their state when power is switched off.

To usefully extend the applications of spintronics to devices such as LEDs and transistors requires semiconductor materials that are ferromagnetic at room temperature. Dilute magnetic semiconductors (DMSs) are made by adding small amounts of a ferromagnetic material (such as manganese, cobalt or iron) to a semiconductor. DMSs based on GaAs, GaN and ZnO are among the most actively developed. Wide-bandgap semiconductors such as GaN and ZnO doped with transition metals are showing particular promise, thanks to the relatively high temperatures at which they retain their ferromagnetic properties.

Cermet s spin on ZnOCermet of Atlanta, GA, has been active both in the development of bulk ZnO, GaN and AlN and in homo- and heteroepitaxy onto these materials. Figure 1 shows an MOCVD-grown ZnO thin film made by the company. In recent years Cermet has won a number of contracts to develop wide-bandgap materials and devices, and is now embarking on two more projects awarded through the US Air Force focused on ZnO-based spin FETs and spin LEDs.

The ultimate performance of FETs in any material system is dictated by the mobility and dissipation of carriers in the channel. Devices using the spin property of electrons can be switched at much higher speeds and at lower powers than conventional FETs. A spin FET looks very much like a conventional device, having a lateral structure with source, drain and gate regions, but the source and drain regions are made from a spintronic material, which in Cermet s device will be epitaxially grown onto ZnO.

The source injects electrons of a particular spin state into the channel. The drain only accepts electrons of that spin state, and rejects all others. The spin state of the electrons in the channel is controlled by the gate voltage, with a small voltage having a large effect on the spins. It is therefore possible to alter the electrons spin before they reach the drain, effectively turning the drain current off.

There are a number of reports of III-V-based spin FETs, but most of these describe a metallic source and drain regions to inject spin-polarized carriers into the channel, such as the devices proposed by Datta and Das in 1990 (figure 2). Using a ferromagnetic semiconductor would allow epitaxial growth of the source and drain with a high-quality interface.

Cermet is working with Hadis Morkoc of Virginia Commonwealth University on spin FETs. Cermet will supply MOCVD-grown structures to Morkoc s team, who will characterize the material and test the FETs.

Spin LEDsCermet is also collaborating with Ian Ferguson s group at Georgia Tech in Atlanta on spin LEDs. A spin LED looks very much like a conventional LED, but again charges are injected into the device from spintronic materials. If the charges recombining in the LED s active region are spin-polarized, then the emitted light will be circularly polarized.

Polarized light is useful for a number of applications including efficient light-coupling into fibers and optical isolation or selection based on polarization. At present LED emission from LEDs needs to be passed through polarizing filters that waste much of the light. Also, measuring the polarization state of the light provides a good measure of the spin-polarization state of the current in spintronic devices, which is itself dependent upon the ferromagnetism of the semiconductor.

The LEDs under development at Cermet are referred to as hybrid spin LEDs, as they will combine the light-emitting properties of GaN with the spintronic properties of ZnO. The polarization of the emitted light will depend on the ferromagnetism of the ZnO, which in turn depends on the level of transition metal doping. The challenge will be to produce highly polarized light, while maintaining material quality and light transmission at the same time.

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