ST develops high-efficiency Si-based LED
Silicon is a poor light emitter due to its indirect bandgap. Even complex silicon-based LED structures can only achieve efficiencies in the 0.01-0.1% range. Last year, scientists at the University of New South Wales used techniques developed in the photovoltaics industry to achieve an efficiency of 1%.
STMicroelectronics trick is to use a structure in which ions of rare-earth metals such as erbium or cerium are implanted in a layer of silicon-rich oxide (SRO), i.e. silicon dioxide enriched with silicon nanocrystals. These nanocrystals, which are 1-2 nm in diameter, are formed by laser-induced pyrolysis of silane.
The nanocrystals are coupled with the rare-earth ions in the silicon dioxide matrix. Recombination of electron-hole pairs in the nanocrystals causes excitation of the rare-earth ions, which then emit light. The frequency of the emitted light depends on the choice of rare-earth dopant and ST has patented key techniques for implanting the rare-earth ions into the silicon.
"The ability to combine optical and electronic processing on the same chip presents enormous opportunities for ST to be the first to develop many new types of semiconductor products, especially as the technology is compatible with existing production process flows and equipment," said GianGuido Rizzotto, Director of ST s Corporate Technology R&D organization, which is based in Catania, Italy. The process was developed using the same pilot line that ST uses to develop new MOSFET and bipolar devices.
ST says it has already identified a number of promising applications. One is to build power control devices in which the control circuitry is electrically isolated from the power switching transistors, thereby replacing external devices such as relays, transformers or discrete optocouplers. ST has patented a novel structure in which two circuits, built on the same chip but electrically separated from each other by insulating silicon dioxide, communicate via optical signals using integrated silicon light emitters and detectors. The company says these devices will have numerous applications, including motor control, power supplies, solid-state relays and similar applications where the power circuit needs to handle much higher voltages than the control circuit. Engineering samples will be available by the end of 2002.
In the longer term, ST is investigating optical distribution of clock signals in CMOS circuits which control optical data-transmission systems.
