A plasma process for the synthesis of cubic-shaped silicon nanocrystals for nanoelectronic devices

Abstract

Low pressure silane plasmas are known for their ability to synthesize silicon nanoparticles via gas phase nucleation. While in the past this particle formation has often been considered from the viewpoint of a contamination problem in semiconductor processing, we here describe a silane low pressure plasma that enables the synthesis of highly oriented, cubic-shaped silicon nanocrystals with a rather monodisperse size distribution. These silicon nanocubes have successfully been used in the manufacture of single nanoparticle vertical transistors. We discuss the advantages of this new paradigm of building nanoelectronic devices. The plasma synthesis process is characterized in more detail than in prior work. The particle nucleation, growth and shape evolution are studied. Results indicate that the process provides two spatially distinct zones: a diffuse plasma for particle growth and a constricted plasma zone for particle annealing. Measurements of the plasma ion density using a capacitive probe suggest that the plasma density in the constricted region of the plasma is about an order of magnitude higher than in the diffuse region, likely aiding the formation of cubic silicon nanocrystals. The process of particle extraction from the plasma reactor is discussed based on the balance of various forces acting on the particles. It is found that the use of a critical orifice for particle extraction enables the detrapping of particles which carry as many as 35 elementary charges.

Plasma‐Assisted Approaches in Inorganic Nanostructure Fabrication

“…However, silicon nanocrystals as large as 100 nm could also be obtained, [112,114] which could not be explained by the above mechanism. Hydrogen [92,98,111,113] or argon [95,114,115] dilution is usually used for crystalline nanoparticle formation. It is well known that hydrogen causes the transition from amorphous silicon to microcrystalline silicon.…”

“…Improved luminescence properties were observed after the silicon nanoparticles were exposed to air or refluxed in organic solvents for a certain period of time, attributed to the surface passivation by oxygen or organic molecules [93][94][95]115,120] Base on this idea, the silicon nanoparticles leaving the plasma zone were introduced into a second plasma zone, which served for surface passivation (Fig. 9).…”

The evolution of pseudo-spherical silicon nanocrystals to tetrahedra, mediated by phosphonic acid surfactants

“…Freestanding silicon nanoparticles have been prepared using gas-phase and solution decomposition of silanes, [5,6] solution-based precursor reduction [7,8] and physical methods involving the use of plasma or pulverization and sonication of porous silicon. [9,10] Size and shape control in the solution synthesis of silicon has not been significantly achieved in comparison to II-VI nanocrystals where an understanding of facet specific surfactant passivation during nucleation and growth allows size monodisperse nanocrystals of a range of geometric shapes to be routinely generated. [11] Alkyl phosphine or alkyl phosphinyl oxide surfactants are particularly effective as they dynamically adsorb on to the surface of the nanocrystal facilitating defined growth.…”

Nonthermal Plasma Synthesis of Faceted Germanium Nanocrystals

“…Figure 1b shows that a large number of the crystallites are of cubic shape, similar to what had been observed for silicon cubic particles previously. [36,38] It is interesting to note, however, that particles appear more agglomerated than the silicon nanocubes found in those studies. The amorphous layer observed around the particles is likely a germanium oxide which quickly grows on exposure of the particles to air.…”

“…Moreover, the difference between the free energies of different shapes was smallest for the largest particles due to the increasing contribution of the bulk free energy compared to the surface, edge, and corner free energies. As we already discussed for the case of silicon cubic particles, we propose that particles are annealed in the low-pressure plasma and reach their equilibrium shape due to the long residence times of several seconds and the particle temperatures which are expected to exceed the gas temperature by several hundred K. [36,38] We believe that the same is true for germanium crystals, which require even lower temperatures for restructuring due to the low melting point of Ge compared to Si (T m,Ge = 1211.4 K vs. T m,Si = 1687 K). However, the fact that hydrogen is present in the discharge, which may etch the Ge crystals, may also contribute to the formation of the faceted particles observed.…”