Constriction of an Atmospheric Pressure RF Plasma for Si Nanocrystal Synthesis

Low-temperature plasma reactors have become popular for synthesizing semiconductor nanoparticles. In many cases, these nanoparticles exhibit exciting and novel optical and mechanical behavior, and plasmas offer unique tailoring of these properties. Low-temperature or non-thermal plasmas (LTPs) are considered to be room-temperature synthesis tools, due to the non-equilibrium between the highly energetic electrons and the low-temperature heavy species such as ions and neutrals. This means that LTPs can be used to synthesize materials that require high crystallization temperatures, such as silicon, while maintaining compatibility with a wide variety of substrates. For flow-through synthesis of nanoparticles at lower pressure (1-10 Torr) it’s convenient to use radiofrequency (RF) power sources, and reactor sizes can be as large as 1-2” in diameter. However, this low-pressure regime also introduces difficulty in merging the synthesis process with other fabrication techniques. Thus, using mm-scale or μm-scale reactors at atmospheric pressure is an alternative strategy to generate nanoparticle-producing plasmas using RF power. Additionally, these small-scale standard-pressure reactors can be integrated with direct-write schemes to provide on-demand nanoparticle deposition in controlled patterns.<br/>Prior work by Kramer et al. (Phys. Rev. B, 2015) suggested that LTP synthesis of crystalline silicon nanoparticles at atmospheric pressure requires a much higher ion density and/or atomic hydrogen density as compared to the low-pressure operating regime. This has led to some speculation about how crystalline nanoparticles can be synthesized even in smaller-scale reactor volumes, when using RF power at atmospheric pressure. Here, we present our results on crystalline silicon nanoparticle synthesis using a mm-scale RF LTP at atmospheric pressure. Observation of the plasma indicates that it diffusely fills the reactor volume during nanoparticle synthesis, similar to low-pressure reactors. In that case, predicted ion densities are not high enough to result in crystalline nanoparticles for these weakly-ionized plasmas. However, high-speed imaging of the plasma during synthesis indicates that what is perceived at steady-state to be a diffuse plasma is in fact a fluctuating filamentary discharge, indicating that species densities in regions of the reaction zone may be much higher than predicted based on a diffuse full-volume discharge.<br/>Here we present our results using a constricted RF plasma for generation of crystalline silicon nanoparticles with sizes ranging from 5 nm – 20 nm using an atmospheric pressure RF-driven millimeter discharge. The reactor was a quartz tube with a 1-mm inner diameter and external ring electrodes supplied RF power (13.56 MHz) ranging from 40 W – 115 W (nominal). Argon was flown at 100-300 sccm and a 1% SiH₄/99% Ar mixture was flown at 0.5-25 sccm during the reaction, and nanoparticles were collected at the exit of the tube onto glass substrates. We performed XRD and TEM to confirm the crystallinity and size of the Si nanoparticles. We imaged the plasma using a Photron high-speed camera at a frame rate of 3600 fps. Argon-only plasmas appeared diffuse even when imaged at this high speed. However, the addition of silane into the reactor led to a filamentary discharge under all synthesis conditions. We hypothesize that the larger sized crystalline particles arise via high rates of reactions in the constricted portion of the plasma, allowing these nanoparticles to reach the higher temperatures required for crystallization even if their diameters are too large for size-related melting temperature depression.