Chem. Rev. 2010, 110, 361–388 361 Growth, Thermodynamics, and Electrical Properties of Silicon Nanowires† V. Schmidt,*,‡ J. V. Wittemann,‡ and U. Go¨sele‡,§,| Max Planck Institute of Microstructure Physics, Halle, Germany, and School of Engineering, Duke University, Durham, North Carolina Received April 7, 2009 Contents 1. Introduction 361 2. Silicon Nanowire Synthesis Techniques 363 2.1. High Temperature Chemical Vapor Deposition 363 2.2. Low Temperature Chemical Vapor Deposition 364 2.3. Supercritical-Fluid-Based and Solution-Based 365 Growth Techniques 2.4. Molecular Beam Epitaxy 366 2.5. Laser Ablation 367 2.6. Silicon Monoxide Evaporation 367 3. Catalyst Materials 368 3.1. Gold as Catalyst 368 3.2. Alternative Catalyst Materials 369 3.2.1. Type-A, Au-like Catalysts 370 Volker Schmidt studied Physics at the Bayerische Julius-Maximilians- 3.2.2. Type-B, Low Si Solubility Catalysts 371 Universita¨t Wu¨rzburg, Germany, and at the State University of New York at Buffalo. He received his Ph.D. from the Max Planck Institute of 3.2.3. Type-C, Silicide Forming Catalysts 371 Microstructure Physics in Halle, Germany, working on growth and 4. Crystallography 372 properties of silicon nanowires. Volker Schmidt also worked as a guest 5. Heterostructures 373 scientist at the IBM Zurich research laboratories in Ru¨schlikon, Switzerland, 6. Surface Induced Lowering of the Eutectic 375 and at the Materials Science Department of Stanford University, CA. Temperature 7. Diameter Expansion of the Nanowire Base 376 8. Surface Tension Criterion 378 9. Growth Velocity and Gibbs-Thomson Effect 379 10. Doping 380 11. Dopant Ionization 381 12. Surface States and Charge Carriers 382 13. Summary and Open Questions 385 14. Acknowledgments 386 15. References 386 1. Introduction Research on silicon nanowires has developed rapidly in recent years. This can best be inferred from the sharply Joerg V. Wittemann studied Physics at the Bayerische Julius-Maximilians- increasing number of publications in this field. In 2008, more Universita¨t Wu¨rzburg, Germany, and at the University at AlbanysState than 700 articles on silicon nanowires were published, which University of New York, where he received a M.Sc. in 2007. Afterward, is twice the number published in 2005. Because of this strong he joined the Max Planck Institute of Microstructure Physics as a Ph.D. increase in research activities and output, the vast majority student under the supervision of Prof. U. Go¨sele. He is currently working of publications on silicon nanowires are found to be younger on fabrication and characterization of silicon nanowires. than ten years. At first glance, one could therefore be tempted fashionable subject, driven by potential applications in to assume that Si nanowire research is a very young research nanoelectronics and sensors. field. This, however, is not the case. Si nanowire research The review, which to our knowledge is the first on silicon had a rather long incubation period before it became a wires, dates back to the late 1950s.1 Therein, Treuting and Arnold reported the successful synthesis of 〈111〉 oriented * To whom correspondence should be addressed. E-mail: vschmidt@ Si whiskers. The term whisker was at that time the commonly mpi-halle.de. used expression when reference was made to filamentary † This article is dedicated to the memory of Professor Ulrich Go¨sele. crystals. Nowadays, the term whisker has almost disappeared. ‡ MPI of Microstructure Physics. § Duke University. Instead, the terms “wire” and “nanowire” have found | Deceased. widespread use. In this article, we will adopt this newer 10.1021/cr900141g 2010 American Chemical Society Published on Web 01/13/2010 362 Chemical Reviews, 2010, Vol. 110, No. 1 Schmidt et al. Figure 1. (a) Schematics of the vapor-liquid-solid growth mechanism. (b) Scanning electron micrograph of epitaxially grown Si nanowires on Si 〈111〉. Transmission electron micrograph of the interface region between Si nanowire and substrate. Note the epitaxy Ulrich Go¨sele was Director of the Experimental Department II at the Max and the curved shape of the nanowire flank. Parts b and c are Planck Institute of Microsctructure Physics, Halle, Germany, Honorary reprinted from ref 3 with permission from Zeitschrift fu¨r Met- Professor of Experimental Physics at Martin Luther University Halle- allkunde, Carl Hanser Verlag, Mu¨nchen. Wittenberg, Germany, as well as Adjunct Professor of Materials Science at Duke University’s School of Engineering, Durham, North Carolina. While dissolved in the Au-Si droplets. The additional supply of staying with the Max Planck Institute of Metal Physics, Stuttgart, Germany, he received his Ph.D. from the University of Stuttgart in 1975. Afterward, Si from the gas phase therefore forces the droplets to find a he worked as a researcher for Siemens (Munich, Germany), IBM (Yorktown way of how to dispose of the excess Si. This is accomplished Heights, New York), NTT (Japan), and a nuclear research center in South by crystallizing solid Si at the droplet-wire interface. A Africa. In 1985, he became Full Professor of Materials Science and, later continuous supply of Si consequently leads to the growth of on, J. B. Duke Professor of Materials at Duke University, Durham, NC. wires with a Au-Si droplet at their tip, as schematically Since 1993, he was a Scientific Member of the Max Planck Society and Director at its Max Planck Institute of Microstructure Physics in Halle. indicated in Figure 1a. Ulrich Go¨sele was a Fellow of the American Physical Society and Great The name vapor-liquid-solid (VLS) mechanism reflects Britain’s Institute of Physics. He was a member of the German National Academy of Sciences and was on the board of the Materials Research the pathway of Si, which coming from the vapor phase Society in the USA. He passed away in November 2009. diffuses through the liquid droplet and ends up as a solid Si wire. Related is the so-called vapor-solid-solid (VSS) terminology. Rodlike crystals with a diameter of less than mechanism, which describes cases where a solid catalyst 100 nm will be referred to as nanowires. In places where particle instead of a liquid droplet is involved. An example rodlike crystals of larger diameters are considered, the term of Au-catalyzed Si nanowires grown homoepitaxially on a wire will be used. The term wire will also be used in a 〈111〉 substrate via the VLS-mechanism is shown in Figure generalized sense, i.e. when reference is to be made to both 1b. These nanowires were grown at about 450 °C using silane wires and nanowires. as precursor.3 The transmission electron micrograph in Figure Regarding silicon wire growth, it is remarkable to see how 1c proves the epitaxial relation between nanowire and much was already known in the 1960s. The best example of substrate. What should also be noted in Figure 1c is the - - this is the vapor liquid solid mechanism of Si wire growth curved shape of the nanowire flank; an aspect that will be proposed by Wagner and Ellis in their seminal article discussed in detail later on in section 7. The most remarkable 2 - - published in March 1964. Till today, the vapor liquid solid feature of the VLS growth mechanism, however, is its (VLS) growth mechanism was the most prominent method universality. VLS growth works well for a multitude of for silicon wire synthesis. The VLS mechanism really catalyst and wire materials and, regarding Si wire growth, represents the core of silicon wire research, though it does not only work for silicon but also for a much broader range over a size range of at least 5 orders of magnitude; from of wire materials. The VLS mechanism can best be explained wire diameters of just a few nanometers up to several on the basis of Au catalyzed Si wire growth on silicon hundred micrometers. substrates by means of chemical vapor deposition (CVD) The VLS mechanism has numerous direct and indirect using a gaseous silicon precursor such as silane. implications for Si wire growth. Consequently, a large part The Au-Si binary phase diagram possesses a characteristic of this review, which is an extended version of a previous peculiarity, namely that the melting point of the Au-Si alloy article,4 focuses on the limitations and implications of the strongly depends on composition. A mixture of 19 atom % VLS mechanism. This concerns experimental issues such as ° Si and 81 atom % Au already melts at 363 C, which is the choice of growth method (section 2) and catalyst material about 700 K lower than the melting point of pure Au and (section 3), the crystallography of the wires (section 4), and more than 1000 K lower than the melting point of pure Si. the synthesis of heterostructures (section 5), as well as Thus, heating Au in the presence of a sufficient amount of Si, considering e.g. a Au film on a Si substrate, to temper- theoretical issues such as the depression of the eutectic atures above 363 °C will result in the formation of liquid temperature (section 6), the expansion of the wire base Au-Si alloy droplets as schematically depicted in Figure (section 7), the surface tension criterion (section 8), and the 1a. Exposing these Au-Si alloy droplets to a gaseous silicon Gibbs-Thomson effect (section 9). The last part of this precursor such as silane, SiH4, will cause precursor molecules article deals with the electrical properties of silicon nanow- to preferentially crack at the surface of these droplets, thereby ires: from nanowire doping (section 10) and the question of supplying additional Si to the droplet. At equilibrium the dopant ionization (section 11) to the influence of surface phase diagram allows only for a limited amount of Si states on the effective charge carrier density (section 12).