Short-wavelength visible light emission from silicon nanocrystals

Si is the material of choice for modern microelectronics but, as an indirect-bandgap semiconductor, it is not an efficient light emitter. An electrically pumped Si laser would present a breakthrough for optoelectronic integration that may enable optical interconnect to make computers faster. Si light emitting diodes may revolutionize solid-state lighting and displays because of the low cost and environmental friendliness of Si. One of the most challenging problems of Si-based lighting and displays is the lack of a reliable and efficient full visible spectrum emission. Si nanocrystals (Si-NCs) have so far been the most promising form of Si to emit light. Most of the synthesis approaches of Si-NCs, however, only lead to red light emission. Our recent work on Si-NCs synthesized by non-thermal plasmas has focused on extending their light emission into the short-wavelength range. Firstly, the process of oxidation-etching-oxidation of Si-NCs is investigated. This process causes the size of Si-NCs to decrease, leading to shorter wavelength light emission from Si-NCs. Yellow or green photoluminescence (PL) has been observed from initially oxidized red light emitting Si-NCs after HF vapour etching and atmospheric oxidation. The intensity of PL from Si-NCs, however, decreases by a factor up to 100. It is found that HF etching restructures the surface of Si-NCs. This leads to a decrease in the incorporation of O during subsequent oxidation, which finally results in silicon suboxide SiO$_{1.9}$. Such an understoichiometry indicates a high density of defects such as Si dangling bonds at the Si-NC/oxide interface. Therefore, the PL efficiency is extremely low for short-wavelength light emitting Si-NCs obtained by the process of oxidation-etching-oxidation. Secondly, an integrated two-stage plasma system is employed to achieve the light emission from Si-NCs in the full visible spectrum range. Red-light-emitting Si-NCs are produced in the first stage by the plasma decomposition of SiH$_{4}$. In the second stage a tetrafluoromethane (CF$_{4})$-based plasma etches Si-NCs and at the same time passivates them with carbon and fluorine. After the two-stage process Si-NCs emit light in the short-wavelength region from yellow to blue. We find that a self-limited oxidation process blueshifts the light emission until saturation is reached. Significantly, relatively high quantum yields of short-wavelength light emission from Si-NCs are obtained in spite of oxidation. It is interesting to note that Si-NCs treated by CF$_{4}$-based plasma are hydrophilic while those without CF$_{4}$-based plasma treatment are hydrophobic.