Semiconductor devices transfer like decals.

Semiconductor devices transfer like decals

Lots of kids master the childhood technology known by some as Silly Putty transfer. By pressing Silly Putty on a newspaper comic, kids lift off ink images and transfer them to, say, the kitchen table. Now scientists report using a related method for transferring ultrathin semiconductor devices from one place to another. Faster computers, new opto-electronic devices and cheaper solar cells are some of the expected payoffs, the researchers say.

Compared to silicon, the newer semiconductor material gallium arsenide makes for faster transistors. And unlike silicon, its crystals can turn laser light into electronic signals or vice versa. But because the two materials clash crystally, gallium arsenide resists growing on silicon to make hybrid opto-electronic devices.

The new material-saving technique called epitaxial liftoff may solve these problems, say researchers at Bell Communications Research (Bellcore), in Red Bank, N.J. "This will hasten the day when the world of optical fiber communications comes all the way into homes," predicts physicist Eli Yablonovitch, leader of the research team. In the February PHOTONICS TECNOLOGY LETTERS, the researchers report using the technique for transferring preformed thin film lasers onto glass from a gallium arsenide template, or substrate, on which they are grown.

Making gallium arsenide devices involves wafers about 0.5 mm thick, yet only the very top microns (0.001 mm) participate in the devices' actions, he says. "Well over 90 percent of the semiconductor chip is just there to mechanically support the active layer, which is incredibly thin," says Yablonovitch. "By separating the part that does the work from the part that supports it, we overcome a lot of problems." These include the substrate's inability to combine with other materials such as silicon and its poor thermal conductivity, which limits its uses to those that don't produce much heat.

Starting with a thick gallium arsenide growth substrate, the researchers successively deposit thin layers of semi- and less-conductive materials to construct devices such as transistors and lasers. First, directly on the substrate, the researchers lay down a roughly seven-atom-deep dissolvable film of aluminum arsenide. Next comes the thin laser assembly: A layer of semiconductive gallium arsenide underlies four more layers that alternate between less-conductive gallium aluminum arsenide and gallium arsenide. The middle gallium arsenide layer responds to electronic currents between the uppermost and bottommost layers of the material by emitting laser light. A layer of mechanically supportive wax tops it all off.

Treatment with hydrofluoric acid, which eats through the aluminum arsenide separation layer while leaving everything else unscathed, frees the thin-layer laser from the now reusable substrate. The laser is then easily transferred to any smooth surface such as glass or polished silicon, Yablonovitch says. Once it's there, a solvent washes away the wax.

Using epitaxial liftoff to make hybrid devices that combine silicon-based chips with the optical capabilities of gallium arsenide is "the hope of the future," comments Paul S. Peercy, manager of compound semiconductor and device research at Sandia national Laboratories in Albuquerque, N.M.