Silicon Nanowires: Growth Studies Using Pulsed PECVD

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Silicon Nanowires: Growth Studies Using Pulsed PECVD David Parlevliet and John C.L. Cornish Physics, Murdoch University, South Street, Murdoch, WA 6150, Australia ABSTRACT Silicon nanowires with high aspect ratios have been grown at high density using a variation of Plasma Enhanced Chemical Deposition (PECVD) known as Pulsed PECVD (PPECVD). Growth rate and morphology were investigated for a range of catalysts: gold, silver, aluminum, copper, indium and tin. The thickness of the catalyst layer was 100nm. Deposition was carried out in a parallel plate PECVD chamber at substrate temperatures up to 350°C, from undiluted semiconductor grade Silane. A 1 kHz square wave was used to modulate the 13.56 MHz RF power. Samples were analyzed using either a Phillips XL20 SEM or a ZEISS 1555 VP FESEM. The average diameter for nanowires grown using a gold catalyst layer was 150nm and the average length was 4µm although some nanowires were observed with lengths up to 20µm. Back-scattered-electron images clearly show gold present at the tips of the silicon nanowires grown using gold as a catalyst, confirming their growth by the vapor liquid solid (VLS) mechanism. Sporadic growth of nanowires was detected when using copper as a catalyst. Although gold performed best as catalyst for nanowire growth it was, however, closely followed by tin. The other catalysts produced nanowires with properties between these extremes.

INTRODUCTION Silicon nanowires have potential uses in the semiconductor industry including possibly the next generation of photovoltaic solar cells. The most common method of fabrication involves the Chemical Vapor Deposition (CVD) of a gas containing silicon and the subsequent growth of silicon nanowires by the Vapor Liquid Solid (VLS) mechanism first proposed by Wagner and Ellis [1]. The presence of a metal catalyst droplet on one end of a nanowire is indicative of the VLS mechanism. Plasma Enhanced Chemical Vapor Deposition (PECVD) is a technique widely used in the production of amorphous and nanocrystalline silicon thin films. When used with substrates covered with a metal catalyst, PECVD has been used to produce silicon nanowires and is known to improve their deposition rate. [2]. A modification of PECVD is pulsed PECVD which uses a modulated plasma to aid the deposition process. We have previously shown [3] that PPECVD can be used to produce silicon nanowires with a greater area density than conventional PECVD. In the VLS mechanism the role of the catalyst is to encourage the growth of single crystal silicon nanowires. For silicon to be absorbed by the catalyst it needs to be highly soluble in the chosen metal. The choice of the metal catalyst is also known to affect the electrical properties of resulting nanowires [4].

A number of catalysts are commonly used in the growth of silicon nanowires. The most common is gold which does not form a silicide and has a bulk Au/Si eutectic temperature that is fairly low (363°C), this results in low temperature growth [2]. Gallium is another catalyst used mainly i