Amorphous silicon has long been the king of flat-panel displays. It began its reign in PC monitors and high-definition TV, then conquered netbooks, e-readers, and smartphones. No other substance was as suitable for the thin-film transistors that sit behind a display's hundreds of thousands of pixels, turning each one on or off.
But soon the dominion of amorphous silicon will pass, because it can't provide what coming generations of electronic products will require. For one thing, it isn't fast enough. Next-generation LCD TVs will be refreshed at least 240 times a second, which is two to four times as quick as today's versions; that way, they'll provide sharper fast-action sports and movies. Three-dimensional displays will need refresh rates twice again as high, to provide all that fast-motion goodness to each eye.
Nor are today's thin-film transistors stable enough for displays that use the organic light-emitting diode (OLED), a thin, efficient, high-contrast technology. Stability matters because the "threshold voltage" that an amorphous silicon transistor needs to turn on tends to drift as the transistor works. And both the problems of slow switching and drift get worse when amorphous silicon devices are made on flexible plastic, which is a critical design requirement for tomorrow's roll-up displays. Such displays will enable a laptop-size screen to fold away for storage inside a smartphone.
For these reasons, researchers and display manufacturers need a replacement in hand when the day comes for amorphous silicon to step down from its throne. And they already have their eyes set on a promising heir—in fact, a whole family of materials, known as amorphous oxide semiconductors. They're amorphous because like today's silicon standby, they lack a regular crystalline structure, and they're oxides because they're made of oxygen compounded with two or three metals, most commonly selected from zinc, indium, gallium, and tin.
Amorphous oxides can form thin films that are transparent and electrically conductive, which is why they already serve as the see-through electrode layer in displays and solar cells. It was this quality that led to the surge in research that began in 1996, when Hideo Hosono and his colleagues at the Tokyo Institute of Technology first noted the merits of amorphous transparent conducting oxides.
We believe that amorphous oxides could do more than simply serve as a passive electrode. They could also replace amorphous silicon as the active semiconducting material that does the heavy lifting as the channel in thin-film transistors.
Here's what's great about amorphous oxides: First and foremost, charge can zip through them 20 to 40 times as fast as in amorphous silicon. This speed is defined by a material's charge-carrier mobility, and the higher it gets, the faster transistors can switch.
Second, unlike amorphous silicon, amorphous oxides can be deposited at low temperatures without compromising their electronic properties. That means they can be laid down or even printed on pliable plastic sheets to make paperlike displays. Such printing could make flexible electronics cheap and ubiquitous. Finally, because the materials are transparent, they could be used in electronic-ink screens that could be laminated on windshields, windows, and eyeglasses.