ST moves towards all-silicon optocouplers
Last October, the company announced silicon emitters with an efficiency some two orders of magnitude better than the best previously achieved results with silicon.
Now, the team has increased the external quantum efficiency of its light-emitting devices by a factor 1.5, as well as increasing the maximum emitted power by a factor 50. As a result, ST is now able to produce more than 1 mW of emitted light power for each square millimetre of silicon.
ST’s 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) - silicon dioxide enriched with silicon nanocrystals measuring 1-2 nm in diameter.
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 at a frequency which depends on the choice of rare-earth dopant.
"[Since last year] ST has made substantial further progress towards turning this pioneering research into both near-term commercial products and potentially disruptive new technologies," said Salvo Coffa, Research Director of the SST group within ST s Corporate R&D organization.
Already, ST has produced fully working prototypes of a completely silicon-based optocoupler that demonstrate that all-silicon devices can deliver similar performance to that of traditional devices while offering the cost benefits of silicon manufacturing.
Traditional optocouplers use compound semiconductor emitters and silicon detectors, while ST eventually aims to produce the whole circuit on a single silicon chip.
"Comparing the performance of our first prototypes with that of existing devices and considering the improvements we have already made to the initial technology, we expect to reach the commercial crossover point in around six months," said Coffa.
The company also expects to develop entirely new types of product that exploit the ability to integrate all three functions - light emission, waveguiding and detection - on a single silicon chip that can be manufactured at low cost on high-volume production lines.
"We already know from our current research that it will be possible to increase this performance by at least another order of magnitude,” said Coffa.
“We are on the threshold of opening up entirely new industrial applications in the fields of displays and even solid-state illumination. In addition to targeting the large existing market for conventional optocouplers, ST will be aggressively pursuing these novel applications.”
