Epitaxial core – shell and core – multishell nanowire heterostructures

Semiconductor heterostructures with modulated composition and/or doping enable passivation of interfaces and the generation of devices with diverse functions. In this regard, the control of interfaces in nanoscale building blocks with high surface area will be increasingly important in the assembly of electronic and photonic devices. Core–shell heterostructures formed by the growth of crystalline overlayers on nanocrystals offer enhanced emission efficiency, important for various applications. Axial heterostructures have also been formed by a one-dimensional modulation of nanowire composition and doping. However, modulation of the radial composition and doping in nanowire structures has received much less attention than planar and nanocrystal systems. Here we synthesize silicon and germanium core–shell and multishell nanowire heterostructures using a chemical vapour deposition method applicable to a variety of nanoscale materials. Our investigations of the growth of borondoped silicon shells on intrinsic silicon and silicon–silicon oxide core–shell nanowires indicate that homoepitaxy can be achieved at relatively low temperatures on clean silicon. We also demonstrate the possibility of heteroepitaxial growth of crystalline germanium–silicon and silicon–germanium core–shell structures, in which band-offsets drive hole injection into either germanium core or shell regions. Our synthesis of core–multishell structures, including a high-performance coaxially gated field-effect transistor, indicates the general potential of radial heterostructure growth for the development of nanowire-based devices. Our approach to the synthesis of core–shell nanowire structures is based upon control of radial versus axial growth (Fig. 1). We have previously developed a general method for synthesis of semiconductor nanowires using nanocluster catalysts to direct axial growth by a vapour–liquid–solid growth process (Fig. 1a). Axial growth is achieved when reactant activation and addition occurs at the catalyst site and not on the nanowire surface (Fig. 1b). Correspondingly, it is possible to drive conformal shell growth by altering conditions to favour homogeneous vapour-phase deposition on the nanowire surface (Fig. 1c). Subsequent introduction of different reactants and/or dopants produces multiple shell structures of nearly arbitrary composition, although epitaxial growth of these shells requires consideration of lattice structures. This approach to core–shell nanowire heterostructures is elaborated below for the technologically important silicon (Si) and germanium (Ge) systems. Homoepitaxial Si–Si core–shell nanowires were grown by chemical vapour deposition (CVD) using silane as the silicon reactant (Fig. 2). Intrinsic silicon (i-Si) nanowire cores were prepared by gold-nanocluster directed axial growth, which yields single-crystal structures with diameters controlled by the nanocluster diameter, and then boron-doped (p-type) silicon (p-Si) shells were grown by homogeneous CVD, where the shell thickness was directly proportional to the growth time. Radial shell growth can be ‘turned-on’ by the addition of diborane, which serves both to lower the decomposition temperature of silane and acts as a p-type dopant. Transmission electron microscopy (TEM) images of the i-Si/p-Si product obtained from constant temperature growth shows a uniform core–shell structure consisting of a crystalline Si core and amorphous Si shell (Fig. 2a), where the core diameter, 19 nm, is consistent with the 20-nm nanocluster used in the initial axial growth step. TEM images show reproducible crystalline faceting at the core–shell interface (Fig. 2b). This faceting suggests that the nanowire surfaces are sufficiently clean following axial growth to nucleate epitaxial shell growth. To understand and control Si on Si homoepitaxy in core–shell nanowire structures we carried out several distinct experiments. First, i-Si/p-Si core–shell nanowires prepared as above were annealed in situ at 600 8C. TEM images recorded on the annealed samples exhibited no diffraction contrast between the core and shell (Fig. 2c), and lattice-resolved images and electron diffraction data further show that the shell crystallizes to yield a single-crystal structure (Fig. 2d). Second, the importance of the initial nucleation for achieving epitaxy in the shell was probed using in situ oxidation of the silicon core (see Methods), which produces a thin amorphous silicon oxide layer at the surface of the core, before silicon-shell growth. Significantly, TEM images of i-Si/SiOx/p-Si core–shell–shell structures show a smooth and abrupt interface between the crystalline core and amorphous shell (Fig. 2e, f). The low roughness of the interface is comparable to that observed in nanowires after only axial growth, and contrasts sharply with the faceted interface of the low-temperature homoepitaxy (Fig. 2b). These results show that the thin oxide layer completely disrupts homoepitaxy; further annealing and TEM studies show that oxidation inhibits crystallization of