Chemical Vapor Deposition Growth of Silicon Nanowires with Diameter Smaller Than 5 nm

Abstract

Quantum confinement effects in silicon nanowires (SiNWs) are expected when their diameter is less than the size of the free exciton (with a Bohr radius ∼5 nm) in bulk silicon. However, their synthesis represents a considerable technological challenge. The vapor-liquid-solid (VLS) mechanism, mediated by metallic nanoclusters brought to the eutectic liquid state, is most widely used for its simplicity and control on the SiNWs size, shape, orientation, density, and surface smoothness. VLS growth is often performed within high-vacuum physical vapor deposition systems, where the eutectic composition and the pressure conditions define the minimum diameter of the final nanowire usually around 100 nm. In this article, we present and discuss the SiNWs' growth by the VLS method in a plasma-based chemical vapor deposition system, working in the mTorr pressure range. The purpose is to demonstrate that it is possible to obtain nanostructures with sizes well beyond the observed limit by modulating the deposition parameters, like chamber pressure and plasma power, to find the proper thermodynamic conditions for nucleation. The formation of SiNWs with sub-5 nm diameter is demonstrated.

Figure 1

SEM images acquired in a planar view of the SiNW samples deposited at the plasma power of 20 W and setting the chamber pressure at (a) 20 mTorr, (b) 35 mTorr, and (c) 50 mTorr.

Figure 2

SiNWs size distribution determined by SEM images analysis for the samples deposited at 20 W of plasma power and 20 mTorr (green), 35 mTorr (red), and 50 mTorr (black) of chamber partial pressure.

Figure 3

SEM images acquired in a planar view of the SiNW samples deposited at the Ar/SiH₄ gas pressure of 50 mTorr by varying the plasma power: (a) 20 W, (b) 30 W, and (c) 40 W.

Figure 4

SiNWs size distribution calculated by SEM images analysis for the samples deposited at 50 mTorr and 20 W (black), 30 W (red), and 40 W (green).

Figure 5

TEM acquired on the SiNWs grown at 20 W and 20 mTorr, imaged in high-resolution TEM mode (a–c) and in energy-filtered TEM mode by selecting the 16 eV plasmon peak (d). The SiNW diameters measured correspond, respectively, to 2.8, 3.5, 4, and 3.5 nm.