e has set his sights on a larger goal. SMALLER, LIGHTER POWER ADAPTER "Ultimately, we'd like to develop materials that will let us integrate many dif- ferent technologies on a single platform," Ringel says. The key to Ringel's strategy is the idea of a "virtual substrate" generic chip-like surface that would be compatible with many different kinds of tech- nologies and could easily be tailored to suit different applications. Ringel's current design consists of a substrate of silicon topped with III-V materials such as gallium and arsenide, with hybrid silicon-germanium layers sand- wiched in between. The substrate is 0.7 mm thick, whereas the gallium arsenide layer is only 3 mm-millionths of 1 meter-thick. Other labs have experimented with III-V materials grown on silicon, but none has been able to reduce defect levels below a critical level that would enable devices like light-emitting diodes and solar cells to be achieved, Ringel says. Defects occur when the thin layers of atoms in a film aren't lined up prop- erly. Small mismatches between layers rob the material of its ability to trans- mit electrical charge efficiently. Ringel and his colleagues grew films of III-V semiconductors with a technique known as molecular beam epitaxy, in which evaporated molecules of a substance settle in thin layers on the surface of the silicon-germanium alloy. To search for defects, they used techniques such as transmission electron microscopy. Defects are missing or misplaced atoms that trap electrons within the mate- rial, Ringel explains. That's why engineers typically measure the quality of a solar cell material in terms of carrier lifetime-the length of time an electron can travel freely through a material without falling into a defect. Other exper- imental III-V materials grown on silicon have achieved carrier lifetimes of about two nanoseconds, or two billionths of a second. Ringel's materials have achieved carrier lifetimes in excess of 10 nanoseconds. The engineers have crafted the III-V material into one-square-inch v