Doping of semiconductor nanowires
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A cornerstone in the successful application of semiconductor nanowire devices is controlled impurity doping. In this review article, we discuss the key results in the field of semiconductor nanowire doping. Considerable development has recently taken place in this field, and half of the references in this review are less than 3 years old. We present a simple model for dopant incorporation during in situ doping of particle-assisted growth of nanowires. The effects of doping on nanowire growth are thoroughly discussed since many investigators have seen much stronger and more complex effects than those observed in thin-film growth. We also give an overview of methods of characterizing doping in nanowires since these in many ways define the boundaries of our current understanding.
I. INTRODUCTION
Semiconductor nanowires (NWs) have emerged as one of the most powerful and versatile building blocks for integration of nanoscale devices with current technology. The geometry of NWs permits efficient strain relaxation, which in turn allows the combination of materials not normally compatible, as well as the growth of III–V NWs on Si.1,2 Numerous technically and scientifically interesting devices have been demonstrated using NWs, such as field effect transistors (FETs),3 single-electron transistors,4 and spin-orbit qubits.5 Doping is necessary in most device applications to control the conductivity of the NWs. Doping of semiconductor NWs was first investigated almost two decades ago by the pioneering group of Hiruma, who demonstrated GaAs p–n junctions.6 Interest in NWs developed rapidly after Lieber’s group demonstrated controlled doping in Si NWs,7 and successful doping has since then enabled fabrication of photovoltaic devices,8 single-quantum-dot light-emitting diodes,9 and tunneling FETs.10 The rapid development of the field of NW doping is demonstrated by the fact that about half of the references in this review were published during the past 3 years. Despite these advances, many challenges remain in both understanding and controlling the doping of NWs. The size of NWs prevents the use of standard Hall measurements to determine doping levels, and the evaluation of carrier concentrations and mobilities remains difficult. The dynamics of NW growth is complex, even without considering doping, and, as discussed in a special section, the effects of doping are often much stronger
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.214 2142
J. Mater. Res., Vol. 26, No. 17, Sep 14, 2011
http://journals.cambridge.org
Downloaded: 12 Mar 2015
than in thin-film growth. Furthermore, NWs are often grown at significantly lower temperatures than thin films, for which an understanding of doping has been developed.11 While the non-nitride III–V semiconductors grow in the cubic zincblende (ZB) form in bulk, III–V NWs commonly exhibit a mixture of ZB and the lower symmetry, hexagonal wurtzite (WZ) crystal structure. This polytypism has been the focus of much research during recent years,12 and t
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