Figure The image illustrating various applications of SiNWs and their growth process by HWCVP method and results
Room temperature synthesis of In-catalyzed Silicon nanowires (SiNWs) on a flexible substrate for electronic applications
World is moving fast towards flexible electronics. Almost all existing Si wafer-based electronic devices are reproducible on flexible substrates. The quasi-1D nanostructures such as SiNWs is the material of choice for advanced miniaturized and large area electronic devices owing to their unique physical, chemical and electronic properties and uncomplicated synthesis methods. SiNWs-based single and hybrid devices such as wearable health systems (e-skins etc.), supercapacitors, sensors, FETs, and solar cells etc. have been successfully fabricated on flexible substrates with remarkable performance. Among the bottom-up approaches of SiNWs growth, VLS-grown SiNWs offer tunablity of their properties along with a distinguishable advantage of compatibility with CMOS technology and more choices of substrates.
A wide variety of flexible substrates ranges from metal foils, polymer sheets to fabrics and even paperboard in some cases. The most challenging aspect of material synthesis on a flexible substrate is the processing temperature. Therefore, the choice of substrate is limited by the highest possible processing temperature it can withstand. Although, there are reports of SiNWs integrated on polymer sheets by a post-growth transfer step, the direct growth as proposed in the present scheme has an added advantage of facile and cost-effective fabrication with fewer processing steps. Mostly, the SiNW synthesis methods on polymer substrates employ a processing temperature ranging from ~ 60-350 oC. However, a particular choice of flexible substrate and its application may require a processing temperature as low as room temperature. Therefore, the challenge is to get the desired material properties without the aid of thermal energy supplied during the material synthesis process in form of processing temperature known as the substrate temperature (Ts). Keeping this in mind, efforts have been made to synthesize SiNWs on a polymer sheet at room temperature.
The SiNWs’ synthesis is done by HWCVP method via the VLS mechanism but without switching on the substrate heating. In this method precursor gases are pyrolytically dissociated in Si-containing species and atomic hydrogen reaching the substrate and causing the material growth. Room temperature SiNWs synthesis was achieved by innovatively exploiting two inherent features of HWCVP method –
The presence of conspicuous amount of atomic Hydrogen generated by the hot wire.
The presence of indispensible additional thermal energy due to the radiation heating
from the hot wire at 1800 oC, generally an undesirable entity.
By circumventing the thermal route and adopting the chemical route of material synthesis in which the thermal compensation of the growth is made up chemically by harnessing the potential of conspicuous atomic hydrogen present in the chamber. Atomic hydrogen is known to help in the crystallization of silicon chemically although very high amount of hydrogen reduces the growth rate of SiNWs by diluting the primary precursor gas silane for the growth of SiNWs. The growth process essentially consists of reducing the Indium tin oxide (ITO) coated on Kapton polymer sheet to a surface layer of Indium nanotemplate with the help of atomic hydrogen. The nanotemplate serves as the catalytic seed for the growth of the SiNWs when exposed to a source of Silicon via the dissociation of Silane gas over a heated Tantalum wire. Entire process right from the loading the Kapton sheet to the synthesis of SiNWs is accomplished in a single pump-down in an indigenously built HWCVP cluster tool.
We have already realized and shown an unprecedented on-chip integration of a SiNW supercapacitor on a silicon wafer which showed electrochemical performance suitable for energy storage requirements in implantable bio-devices and wireless sensors networks . All the technological steps involved in the fabrication of the on-chip supercapacitor are easily applicable in replicating the device employing the aforementioned SiNWs on Kapton or on any substrates for that matter widening the horizon of choices of substrates.