Direct Synthesis of Silicon Wires on a Polymer Substrate without the Substrate Heating using Hot-Wire-Enabled VLS Method

Rapidly growing field of flexible electronics calls for the coupling of modern nanomaterials and polymeric substrates. Silicon nanowires (SiNWs) grown by Vapor-Liquid-solid (VLS) mechanism are a preferred material for advanced electronic devices on account of their tunable properties and growth compatibility with any substrate. However, no report of direct growth of SiNWs on low operating temperature (~150°C) polymeric substrates is available as yet. In the present work, synthesis of Silicon wires (SiWs) having an average diameter in the sub-micrometre range has been successfully attempted directly on a polymer sheet (Kapton) via the reduction of an Indium Tin Oxide (ITO) film to In-Sn metal alloy particles followed by the synthesis of SiWs without the substrate heating by HotWire Chemical Vapor Processing technique. These SiWs were synthesized using a variation of the conventional VLS method wherein the hot wire plays a vital role for the growth of SiWs on low-melting-point ($\leq$156°C) In-Sn metal alloy particles. We therefore call this as the Hot-Wire-Enabled VLS (HW-VLS) method. The surface layer of In-Sn metal alloy particles formed after the reduction exhibit two distinct size distributions with average particle sizes and densities to be$\sim $58nm, 4x1$0^{9}{c}\mathrm{m}^{-2}$and$\sim $310nm, 2.2x1$0^{8}{c}\mathrm{m}^{-2}$. SiWs grown using these particles as a seed layer have a good density with an average diameter of $\sim $550 nm and a high growth rate of$\sim $7.4nm/sec. The direct growth as proposed in the present scheme has an added advantage of facile and cost-effective fabrication with fewer processing steps.

I. Introduction

SiNWs (Silicon Nanowires) based electronic devices have been successfully fabricated on polymeric substrates [1–2]. However, in all these reports SiNWs integration on polymer sheets was achieved by a post-growth transfer method. These methods are often costly, low yield and multistep processes. The only report of direct integration of SiNWs on a polymer substrate employs a substrate temperature as high as restricting the choices among polymeric substrates [3]. Essentially, this limitation is posed by the divergency between the eutectic temperature of the metal-silicon alloy used in the vaporliquid-solid (VLS) method and the operating temperature of a polymer [4]. Alet et al. and Soam et al. addressed this issue and demonstrated SiNWs growth at low substrate temperatures using low melting point (LMP) metals such as In or Sn by Plasma Enhanced CVD and HotWire Chemical Vapor Processing (HWCVP) techniques respectively [5–6]. Following a different route, Yu et al. synthesized SiNWs via the Hydrogen-plasma reduction of widely used transparent conducting oxide (TCO) materials such as Indium Tin Oxide (ITO) and Tin Oxide to their constituent metal particles that subsequently assist in the VLS growth of SiNWs at a temperature as low as [7].