Breakthrough in silicon photonics

Silicon, one of the base elements of our planet, is the foundation of the modern information society. Modern electronics would be unthinkable without the development of silicon transistors; such transistors are made possible only by the outstanding characteristics and stability of silicon and its oxides.

However, the increasing miniaturisation of microelectronics, the demands of optoelectronics, and the development of optical data transmission also show the limits of silicon technology: silicon is an indirect semiconductor and, as such, has a very inefficient light emission at room temperature. Thus, the structures most used in optoelectronics are based on the III-V elements such as gallium arsenide or indium phosphide, or corresponding combinations, and are not compatible with silicon.

Structures in nanometre range provide a viable solution due to the fact that silicon manifests different characteristics on the nanoscale. In a range of very few nanometres, the movement of electrons and electron vacancies in silicon is narrowly restricted, and so-called 'quantum-confinement' effects appear which enlarge the band gap of silicon and shift the light emission into visible range. The possible advantages of this effect have spurred over ten years of intensive research worldwide in the field of silicon nanocrystals.

The methods of manufacture are diverse, but controlling the size of the nanocrystals has remained problematic. For technological uses it is required that the density, size and position of the nanocrystals be controlled independently. This has not been possible with the prevailing techniques such as the manufacture of porous silicon, ion implantation, and the manufacture of thick SiOx films.

Researchers at the Max Planck Institute for Microstructure Physics have recently developed a means of controlling the size of silicon nanocrystals and custom manufacturing these crystals on 4" wafers. The technique is based on a combination of multi-layer structures with layer thicknesses of very few nanometres and varying band gaps, so-called 'superlattices', and a phase separation in the ultra-thin layers.

The superlattice structure of amorphous silicon oxide layers (SiOx/SiO2) is manufactured using a currently standard technique.

The researchers employed a novel but simple variation of this technique by evaporating the silicon oxide either in a vacuum or in an oxygen-containing atmosphere. The resulting amorphous SiO/SiO2-superlattice structure was then tempered in a nitrogen-containing atmosphere at 1100°C. Through the thus thermally-activated phase separation, the SiO in the ultra-thin sublayers transformed into pure silicon nanocrystals and into amorphous SiO2, whereby the nanocrystals were automatically surrounded with an appropriate barrier material.

The size of the crystals within the range in question - from two to five nanometres - can be determined by the thickness of the layer. The distance between the crystals can be adjusted by varying the thickness of the SiO2 barrier layers and the oxygen content of the SiOx-layers. A higher oxygen content would automatically lead to a higher proportion of the amorphous SiO2 after the phase separation and thereby to a larger distance between the silicon nanocrystals within the sublayer.

The luminescence of the silicon increases with the number of crystals, and the quantum efficiency of small crystals is higher than that of the larger; therefore, primarily very small crystals in high density are required for the highest possible luminenscence intensity. When the nanocrystal structures are implanted with erbium ions, a very effective energy transfer occurs from the nanocrystals to the Er3+-ions, and the luminescence shifts into a technologically useful range of 1.54 micrometres.

This particular wavelength is of great technological importance due to the fact that the fibre optic cable employed in optical data transmission exhibits a transmission maximum at 1.54 micrometres. Additionally, erbium-doped glass fibre is used as an amplifier for the optical data transmission.

Both Motorola and STMicroelectronics have recently announced breakthroughs based on silicon nanocrystal technology - Motorola with a 4 megabit memory, and STMicroelectronics with light-emitting diodes (LED).