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Getting Semiconductors Ready to Take a Spin

Photonics Spectra
Mar 2008
Laser-annealing method scribes magnetic areas.

Hank Hogan

For semiconductors, the hope is that everything will not be just about electron charge in the future. Instead, having devices also exploit electron spin — resulting in new kinds of memory and logic chips — is a goal of spintronics, the spin-based analog of modern electronics.

To operate, such devices must generate a spin-polarized electron current in nonferromagnetic semiconductor materials. Spin injection via a ferromagnetic material is seen as the most promising way to achieve this. Now researchers from the University of California, Berkeley, and from Lawrence Berkeley National Laboratory, also in Berkeley, have come up with a laser-based method to write the minuscule ferromagnetically active regions needed to fabricate useful spintronic devices.

TWanneal.jpg

Lasers could help build devices that tap into electron spin. (Left) Shown is a schematic of the optical near-field configuration used for the irradiation of hydrogenated gallium manganese arsenide (GaAs:Mn-H) with a Nd:YAG laser. The light pulses selectively remove the hydrogen, rendering spots in the film ferromagnetically active and therefore able to inject spin-polarized electronsinto nonferromagnetic semiconductors. (Below) This conductance atomic force microscopy image shows a laser-irradiated spot. Annealing with the laser results in the electrical and ferromagnetic activation of the manganese acceptors in select regions of GaMnAs, which remain surrounded by ferromagnetically inactive material. Courtesy of Oscar D. Dubon, University of California, Berkeley.


“A major benefit of this technique is the potential to achieve sub-100-nm resolution,” said Oscar D. Dubon, associate professor of materials science at the university. He noted that graduate student Rouin Farshchi and a group led by mechanical engineering professor Costas Grigoropoulos played key roles in the research.

Nanoscale resolution is particularly important for devices in which spin injection into the semiconductor GaAs from the ferromagnetic material GaMnAs is required. Short diffusion lengths keep the injected spin current from penetrating very far in the GaAs, giving rise to the need for fine resolution.

Investigators add hydrogen to control the electrical and ferromagnetic properties of the GaMnAs. When incorporated into the film, the hydrogen forms a complex with the manganese, rendering the latter inert without introducing any defects. Removing the hydrogen restores the film’s electrical and magnetic characteristics. That removal must be done in a well-controlled manner with high spatial resolution.

In their demonstration of a laser-mediated way to do this, the researchers built upon earlier work they had done showing that the semiconductor surface could be melted with a laser pulse. The surface then solidified into a single-crystalline film. According to Dubon, these results led them to consider whether they could mimic a furnace anneal to remove hydrogen in a controlled manner.

The scientists implanted manganese ions in GaAs wafers and healed the damage with a single pulse from a KrF excimer laser made by GSI Lumonics of Novi, Mich. They added hydrogen to the GaMnAs and irradiated the hydrogenated material with low-power pulses from a 532-nm Nd:YAG laser from New Wave Research of Fremont, Calif., to dissociate the hydrogen in precise regions. They focused the beam to a 4-μm spot size, and tests indicated that a fluence of 60 mJ/cm2 removed the manganese-hydrogen complexes in the film. As a result, they formed ferromagnetically active regions in a nonactive but otherwise structurally identical film.

The investigators then applied the same techniques using a near-field scanning optical microscope. They injected the laser into a probe tip with a diameter of about 800 nm and found that, at a fluence of 2.5 mJ/cm2, the activated feature was only about 150 nm wide. Dubon noted that the injected beam has a Gaussian spread, which explains the narrow activated region.

“We expect appropriate fluences to lead to activation at only the center region,” he said.

The ultimate goal is to fabricate all-semiconductor spin valves, where the defect-free interface between the ferromagnetic and neighboring nonferromagnetic regions could lead to efficient spin injection. Laser-based methods, Dubon said, integrate well with the planar nature of the devices.

Applied Physics Letters, Jan. 7, 2008, Vol. 92, 012517.


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