Metallurgy Division, Material Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
ACS Nano. 2011 Jan 25;5(1):656-64. doi: 10.1021/nn102556s. Epub 2010 Dec 15.
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.
器件应用需要采用可靠的技术来制备具有可控形状和取向的锥形硅纳米线（SiNW）阵列。在这项研究中，我们系统地研究了生长条件对从 SiCl4 前驱体在减压下通过化学气相沉积（CVD）生长的 SiNW 锥形化的影响。SiNW 的锥形化受催化的气-液-固（VLS）和非催化的气-固（VS）生长机制的相互作用控制。我们发现，通过降低 SiCl4/H2 摩尔比、应用更高的气流速率或降低生长压力，可以增强 NW 侧壁上的非催化 Si 沉积，其定义为径向生长速率。轴向和径向生长速率对工艺条件的明显依赖性被用来制备锥形度（即径向/轴向生长速率之比）变化近 2 个数量级的 SiNW。通过对轴向（VLS）和径向（VS）生长速率的热力学和动力学影响的相互作用来解释结果。建立的 SiCl4/H2 摩尔比与纳米线垂直对准之间的相关性被用于开发两阶段生长程序，以制备具有主要垂直取向的锥形 SiNW 阵列。
