R. Winn Hardin
OTTAWA -- With an eye toward more powerful and smaller diode lasers, researchers at the National Research Council of Canada in Ottawa recently turned something semiconductor manufacturers consider undesirable into quantum dot lasers capable of emitting visible light.
Normally, wafer fabricators frown on what they call "beaded" semiconductor layers due to their poor quality and varying nature. For the laser researchers, making uniform defect-free islands with a variety of III-V semiconductors proved the path to self-assembled quantum dot lasers.
Beaded semiconductor layer surfaces are punctuated by clusters of atoms, or quantum dots, which trap electrons in small spaces. The spaces -- measuring just a few nanometers each -- produce powerful, tunable and efficient beams by limiting the movement of excited electrons. The energy that would normally be wasted in movement within a quantum well goes into photon emission, resulting in lasers with one to two orders of magnitude more power than diode lasers of the same size.
Because quantum dot lasers offer more power per unit volume and more control, head researcher Simon Fafard says they may offer integrated optical circuit manufacturers -- and eventually optical computer designers -- a better alternative than present-day diode lasers.
"But there's still a lot more research to do," he said. Fafard's team achieved the necessary uniformity by fine-tuning the chemical composition of the composite layers and using a self-assembling method to grow quantum dots.
Seeking room temperature
Scientists used molecular beam epitaxy in the Stranski-Krastanow growth mode at 530 °C to produce defect-free, uniform islands of InAlAs atoms on a GaAs substrate. The Canadian council managed to create a 707-nm beam with a peak power of 200 mW.
"Our goal is to get the laser operating at room temperature with lower thresholds," Fafard said. "To do that we need to stack more quantum dot layers and use wider-bandgap contact layers." Since announcing visible emission by quantum dots, Fafard's team managed to stack five layers of quantum dots, but the layers' thick mass would not allow room-temperature lasing. The team is confident, however, that a proper-width layer with sufficient bandgaps can be grown within the next 6 to 12 months.
While infrared-emitting quantum dot lasers popped on the scene in 1994, the Canadian council is the first to urge quantum dot lasers into the visible spectrum.