Silicon nanocrystals are particularly useful for applications in nanoelectronic, optoelectronic and biological disciplines, and their production has become increasingly important. These nanocrystals are usually generated by electrochemical etching of bulk silicon into porous silicon. This particular method is however not adapted for the fabrication of nanocrystals in well defined locations or for the creation of arranged microscopic lattices.
Researchers from the institute of Physics in Prague, Czech Republic, have introduced a solid state-process taking place at room temperature and resulting in the formation of microscopic crystalline rings and dots at controlled positions in an amorphous silicon thin film [Rezek, et al., Nanotech. (2009) 20, 045302]. This method is now known as field-enhanced metal-induced solid phase crystallisation (FE-MISPC).
Hydrogenated amorphous silicon (a-Si:H) has been a material of extraordinary interest in the industrial environment over the past decades. Its amorphous structure, with no ordered lattice and a low amount of defects, gives rise to specific features such as continuous density of electronic states and enhanced light absorption, making it a prime material for use in photovoltaics, sensors and other devices.
The FE-MISPC process consists in the deposition by plasma enhanced chemical vapor deposition (PE-CVD) of a 200 nm thick a-Si:H thin film on a glass substrate coated with a nickel film, 40 nm thin. A constant electric field generated by an external source unit is then applied across the a-Si:H film between a conductive sharp atomic force microscopy (AFM) tip and the bottom nickel electrode. In order to protect the sample and shunt a potential excess charge, a MOSFET transistor (metal-oxide-semiconductor field-effect transistor) is implemented in parallel with the sample. The miniaturization of the current-induced crystallization process below 100 nm is achieved by limiting the passing current and electrical discharge between the conductive tip and the bottom electrode.
The growth of silicon nanocrystals thus achieved is corroborated by micro-Raman spectroscopy. This technique indeed resolves a sharp crystalline silicon peak on a broader amorphous spectrum.
Rezek and co-workers demonstrated that the nanoscale FE-MISPC process with MOSFET protection can be employed reproducibly for the fabrication of arrays. These extremely encouraging results led Rezek et al. to envisage that “the presented technology may be useful in creating/positioning silicon nanocrystals in predefined locations with nanoscale accuracy, creating crystalline pathways in the amorphous matrix, or creating nanowells for microscale chemistry or data storage.”