Tapering Control of Si Nanowires Grown from SiCl₄ at Reduced Pressure

Abstract

Device applications of tapered Si nanowire (SiNW) arrays require reliable technological approaches for fabricating nanowires with controlled shape and orientation. In this study, we systematically explore effects of growth conditions on tapering of Si nanowires grown by chemical vapor deposition (CVD) at reduced pressure from SiCl(4) precursor. Tapering of SiNWs is governed by the interplay between the catalyzed vapor-liquid-solid (VLS) and uncatalyzed vapor-solid (VS) growth mechanisms. We found that the uncatalyzed Si deposition on NW sidewalls, defined by a radial growth rate, can be enhanced by lowering SiCl(4)/H(2) molar ratio, applying higher gas flow rate, or reducing growth pressure. Distinct dependences of the axial and radial growth rates on the process conditions were employed to produce SiNWs with a tapering degree (i.e., a ratio of the radial/axial growth rates) varying by almost 2 orders of magnitude. The results are explained by an interplay between the thermodynamic and kinetic effects on the axial (VLS) and radial (VS) growth rates. Established correlation between the SiCl(4)/H(2) molar ratio and vertical alignment of nanowires was used to develop a two-stage growth procedure for producing tapered SiNW arrays with a predominantly vertical orientation.

Monolithically integrated thermoelectric energy harvester based on silicon nanowire arrays for powering micro/nanodevices

“…In addition to the supporting silicon beams, these high and low temperature areas are physically connected by the Si NW arrays. The temperature difference attainable across the thermoelectric legs depends on the actual length of the connecting nanowires, which can be technologically limited due to tapering effects during their growth [20][21][22][23]. Microstructures composed by multiple ordered arrays consecutively bridged by transversal microspacers have been implemented to overcome this problem, leading to larger effective Si NW lengths.…”

Thermal Test of an Improved Platform for Silicon Nanowire-Based Thermoelectric Micro-generators

“…The temperature difference attainable across such devices when operated in harvesting mode (placed on top of a heat source) will essentially depend on the length of the thermoelectric material connecting the high-and low-temperature areas, which is technologically limited by the tapering effect during NWs growth. [12][13][14][15] Trenches for the successive linkage of multiple Si NW arrays have been developed in order to overcome this problem, providing larger effective Si NW lengths. A detailed schematic of the intended final structure is shown in Fig.…”

Overview and status of bottom-up silicon nanowire electronics

“…Additional material deposited on the sidewalls is often regarded as a growth byproduct which can severely alter NW properties, especially in situations when dopants are introduced during growth (see Section 3). Yet, tapered NWs are best suited for specific applications including field-emission [9] and photovoltaics [10] and methods have been developed to control these overgrown layers [11]. As regards the axial growth, temperatures higher than the eutectic temperature (T E ) characteristic of the Si-metal system under consideration allow the formation of a liquid alloy.…”