Electronic data connections are increasingly becoming a bottleneck in the exponential growth of data traffic worldwide. Future optical interconnects are the obvious successors but will require ultrasmall light sources with sub-micrometer sizes to achieve low energy consumption and ultrafast speeds. In both nanoscale light-emitting diodes (LEDs) and lasers radiative and nonradiative recombination rates play a key role in the efficiency. Specifically, radiative recombination (both spontaneous and stimulated) is affected by the small mode volume in nanoLEDs and nanolasers, potentially leading to strong Purcell enhancements and higher speed. On the other hand, nonradiative recombination rates are also very high due to the high surface-to-volume ratios, typically leading to low radiative efficiencies.
In this project, using advanced nanofabrication, characterization and modelling methods, we have investigated novel nanoscale light sources consisting of a waveguide-coupled metal-dielectric cavity nanopillar LED on silicon. These devices work at telecommunications wavelengths featuring more than 20 nW waveguide-coupled powers and GHz-range modulation bandwidths at room-temperature (RT). The efficiency of the reported nanoLEDs currently lies between 0.01 and 1 percent, at RT and at 10 K, respectively, mostly limited by nonradiative recombination effects. We have developed a passivation method using sulfur treatment, followed by silicon oxide capping deposited by plasma-enhanced chemical vapor deposition,that strongly suppresses the surface recombination at the InGaAs surfaces of nanopillars from a few hundred picoseconds to more than 20 nanoseconds. These results will ensure substantial improvements in the efficiency of future nanoLEDs and reduce the threshold current in nanolasers, which are of crucial importance for their application in optical interconnects.
The performance of the experimental nanoLEDs was analyzed using a rate equations model which properly takes into account the nanocavity effects in the spontaneous emission rate and the spatial and spectral overlap between carriers and photons. The model was extended to describe the stimulate emission processes occurring in nanolasers. Using this model, the ultimate limits of scaling down these nanoscale lasers and LEDs leading to Purcell enhancement of the emission and higher speeds was theoretically analyzed.
