Colossal injection of catalyst atoms into silicon nanowires

• Published in: Nature (London) Vol. 496; no. 7443; pp. 78 - 82
• Main Authors: Moutanabbir, Oussama, Isheim, Dieter, Blumtritt, Horst, Senz, Stephan, Pippel, Eckhard, Seidman, David N.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 04.04.2013; Nature Publishing Group
• Subjects: 639/301/357/1016; 639/301/930/2735; Aluminum; Binomial distribution; Catalysts; Catalytic methods; Chemical properties; Cross-disciplinary physics: materials science; rheology; Dissolution (Chemistry); Electrical properties; Exact sciences and technology; Humanities and Social Sciences; Injection; Lasers; letter; Materials science; Methods of nanofabrication; multidisciplinary; Nanotechnology; Nanowires; Observations; Physics; Quantitative research; Science; Silicon; Studies; Tomography; United States; Impurity distribution; Catalysts; Impurity density; Doping; VLS growth; Precipitates; Atom probe; Aluminium additions; Growth mechanism; Tomography; Silicon; Kinetics; Nanowires; Microstructure; Nanomaterial synthesis
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11999

Aluminium catalyst is trapped during growth of a silicon nanowire from vapour phase at concentrations vastly beyond equilibrium solid solubility, but is homogeneously distributed as atoms and not found as clusters or precipitates; this is a potential route to tailoring the composition and properties of nanowires. Aluminum self-dopant in silicon nanowires Semiconductor nanowires are building blocks for electronics, quantum information processing and other nanoscale technologies. Doping — the controlled incorporation of impurities — is used to control nanowire properties. Aluminium has emerged as an effective catalyst for making functional silicon nanowires, replacing gold, and also acts as a dopant. Now, using highly focused ultraviolet laser-assisted atom probe tomography, Oussama Moutanabbir et al . have generated three-dimensional atom-by-atom maps of a single aluminium-catalysed silicon nanowire, revealing a homogenous distribution of the aluminium impurities. Despite the fact that aluminium is present at concentrations well beyond equilibrium solid solubility, there is no clustering or precipitation. The authors explain this in terms of a kinetic model of step-flow nanowire growth with aluminium solutes trapped at step edges. This work suggests potential methods of controlling catalyst injection and tailoring the composition and properties of nanowires for specific properties. The incorporation of impurities during the growth of nanowires from the vapour phase alters their basic properties substantially, and this process is critical in an extended range of emerging nanometre-scale technologies 1 , 2 , 3 , 4 . In particular, achieving precise control of the behaviour of group III and group V dopants has been a crucial step in the development of silicon (Si) nanowire-based devices 5 , 6 , 7 . Recently 8 , 9 , 10 , 11 it has been demonstrated that the use of aluminium (Al) as a growth catalyst, instead of the usual gold, also yields an effective p-type doping, thereby enabling a novel and efficient route to functionalizing Si nanowires. Besides the technological implications, this self-doping implies the detachment of Al from the catalyst and its injection into the growing nanowire, involving atomic-scale processes that are crucial for the fundamental understanding of the catalytic assembly of nanowires. Here we present an atomic-level, quantitative study of this phenomenon of catalyst dissolution by three-dimensional atom-by-atom mapping of individual Al-catalysed Si nanowires using highly focused ultraviolet-laser-assisted atom-probe tomography. Although the observed incorporation of the catalyst atoms into nanowires exceeds by orders of magnitude the equilibrium solid solubility 12 and solid-solution concentrations in known non-equilibrium processes 13 , 14 , the Al impurities are found to be homogeneously distributed in the nanowire and do not form precipitates or clusters. As well as the anticipated effect on the electrical properties, this kinetics-driven colossal injection also has direct implications for nanowire morphology. We discuss the observed strong deviation from equilibrium using a model of solute trapping at step edges, and identify the key growth parameters behind this phenomenon on the basis of a kinetic model of step-flow growth of nanowires. The control of this phenomenon provides opportunities to create a new class of nanoscale devices by precisely tailoring the shape and composition of metal-catalysed nanowires.