GENEVA Technologists have long sought a silicon light-emitting diode that could be integrated into standard CMOS fabrication lines. Now STMicroelectronics claims to have achieved this critical goal with a record-breaking silicon LED that has the same brightness and efficiency as the gallium arsenide variety. Using a silicon dioxide film containing silicon nanocrystals and erbium ions as the active layer, the company said, its new LED has 100 times the quantum efficiency of any previous silicon light emitter.
A team of researchers at ST's Corporate Technology R&D Organization in Catania, Sicily, developed the diode on standard CMOS fabrication equipment, rather than with specialized lab equipment. As a result, the company will release engineering samples of a circuit using the new device by the end of the year.
"We knew the concept was correct from the beginning, but the implementation was not trivial," said Salvo Coffa, who headed the project. "We combined skills in optics with skills in fabrication."
The first circuit to use the device will be a power chip for motor control, power supplies and solid-state relays. The light-emitting silicon dioxide layer will form an electrically insulating barrier on the chip that will isolate control circuitry from power components. The control and power circuits will communicate via photons generated in the optically active oxide layer.
The breakthrough was based on past work in optical silicon research. One line of research uses erbium ions to boost silicon's optical capabilities. For example, silica fibers doped with erbium created the erbium-doped fiber amplifier, which became a highly successful component of optical communications system.
The other tack is to use quantum confinement in nanoscale particles of silicon to push the material into light-emitting mode. This approach achieved some success in labs using porous silicon, an etched form of pure silicon that produces nanometer-diameter silicon pillars. The porous layers proved to be mechanically fragile, leading to unreliable operation, and the structures were difficult to include in a standard fab process.
ST's LED concept was to use both techniques in a robust silicon dioxide film. "You can get some light-emitting capability from crystalline silicon by doping it with erbium, but that only works at low temperature," Coffa said. "Once you get up to room temperature, the silicon acts as an antenna and draws off the carriers from the erbium ions."
Insulation pros and cons
It is also difficult to get a high concentration of erbium ions into pure silicon. On the other hand, silicon dioxide does not have these problems; it allows high concentrations of erbium and is insulating at room temperature, allowing the erbium ions to remain charged with photon-generating carriers.
But then silicon dioxide's insulating property becomes a problem, blocking the injection and movement of carriers in the film. Adding silicon nanoparticles let the researchers tune the insulating parameter of silicon dioxide, reducing operating voltage from 10 volts to between 2 V and 3 V while increasing the carrier mobility in the film.
"Carriers hop from one silicon particle to the next, and the particles also perform quantum confinement, pushing up the bandgap of silicon so that the erbium ions do not give their charge back," Coffa explained. Another trick made possible by silicon dioxide is to use high-velocity carriers hot electrons to pump the erbium ions.
"You can create silicon nano-particles in silicon dioxide by adjusting the gas concentrations in a standard CVD [chemical vapor deposition] reaction," Coffa said. Excess silicon precipitates from the reaction, and with a careful adjustment of the proportions, pure crystalline nanoparticles of about 1 to 2 nanometers in diameter are created. Thus the optically active layer can be implemented with a specially tuned silicon dioxide deposition step followed by an erbium ion implant, both performed with standard gear.
Other rare earth ions have also been tried with the process, each ion type generating a different color. Erbium produces green light in the center of the visible spectrum, cerium will produce blue light and the researchers have even been able to push the wavelength into the near infrared region used in optical communications, Coffa said.
The silicon LED also opens the door to fully integrated silicon optoelectronic circuits. CMOS silicon photodetectors are well-advanced and currently in the commercial realm.