DOI: 10.1007/s00339-004-2667-1 Appl. Phys. A 80, 263–266 (2005) Materials Science & Processing Applied Physics A y.w. heo 1 m. kaufman 1 k. pruessner 1 k.n. siebein 1 d.p. norton 1, ✉ f. ren 2 ZnO/cubic (Mg,Zn)O radial nanowire heterostructures 1 Dept. of Materials Science and Engineering, University of Florida, 106 Rhines Hall, P.O. Box 116400, Gainesville, FL 32611, USA 2 Dept. of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA Received: 11 December 2003/Accepted: 4 February 2004 Published online: 4 May 2004 • © Springer-Verlag 2004 ABSTRACT The formation and properties of radial heteroepi- taxial ZnO/(Mg,Zn)O nanowires is reported in which the (Mg,Zn)O is cubic. Synthesis is achieved via a catalyst-driven molecular beam epitaxy technique. The nanowires were grown on Ag-coated Si substrates at growth temperatures ranging from T g = 300 to 500 ◦ C, using Zn, Mg, and O 3 /O 2 as the reac- tive flux. Structural and compositional analyses indicate that the core of the nanowire is ZnO possessing the hexagonal wurtzite structure, with the (Mg,Zn)O sheath assuming the cubic rock salt structure. Since (Mg,Zn)O has a larger band-gap energy (up to 7.8 eV) than that of ZnO (3.37 eV), these radial heterostruc- ture nanorods provide an interesting system for quantum con- finement and one-dimensional nanoscale device studies. PACS 81.05.Dz; 81.07.Vb The synthesis and properties of semiconducting nanorods/ nanowires have attracted significant attention in recent years. Potential applications for these structures include electron field emitters, bio/chemical nanosensors, field-effect tran- sistors, single-electron transistors, and nano-LEDs [1–6]. One of the more interesting semiconducting materials for nanowire fabrication and exploitation is ZnO [7–11]. This binary oxide is an n-type semiconductor with a direct band gap of 3.37 eV. ZnO is attractive for optical applications, given its large band gap and large exciton binding energy (60 meV) [12]. The band gap for ZnO can be increased to nearly 4.0 eV, while still maintaining the wurtzite struc- ture, by doping with Mg in epitaxial (Zn,Mg)O thin-film alloys. As with other semiconductors, heterostructures of (Zn,Mg)O can be fabricated to generate band offsets and carrier confinement [13]. ZnO/(Zn,Mg)O heterostructures have been realized that exhibit quantum confinement in quantum-well structures. While the formation of planar semiconductor heterostructures is common for thin films, the synthesis of one-dimensional heterostructures is diffi- cult. Axial heterostructures, in which the chemical mod- ulation is imposed along the length of the nanowire axis, have been reported for a few systems, such as InAs/InP and Si/SiGe nanowires [14, 15]. ZnO/Zn 1−x Mg x O quantum- ✉ Fax: +1-352/846-1182, E-mail: dnort@mse.ufl.edu well nanorods have been grown as axial heterostructures as well [16]. Little work has addressed the synthesis of nanowire heterostructures in which the chemical modulation extends radially from the wire center [17]. In this paper, we report the formation of ZnO-based one- dimensional radial heterostructure nanowires possessing a ra- dial modulation in composition and structure. In particular, the nanowires consist of wurtzite ZnO cores surrounded by a rock salt-structured (Mg,Zn)O sheath. The crystal structure, microstructure, and composition of the cored ZnO nanowire were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and photolumines- cence measurements. Despite the mismatch in lattice symme- try, the cubic (Mg,Zn)O sheath is found to be epitaxial on the wurtzite ZnO core. The growth experiments were performed using a conven- tional molecular beam epitaxy (MBE) system. The background base pressure of the growth chamber was ∼ 5 × 10 −8 mbar. An ozone/oxygen mixture was used as the oxidizing source. The nitrogen-free plasma discharge ozone generator yielded an O 3 /O 2 ratio on the order of 1%–3%. No effort was made to separate the molecular oxygen from the ozone. The fluxes of Zn and Mg were provided by Knud- sen effusion cells using high-purity Zn metal (99.9999%) and Mg metal (99.995%) as the sources. Cation and O 2 /O 3 partial pressures were determined via a nude ionization gauge that was placed at the substrate position prior to growth. The beam pressure of the O 3 /O 2 mixture was varied between 5 × 10 −6 and 5 × 10 −4 mbar, controlled by a leak valve between the ozone generator and the chamber. The Zn pressure was varied between 5 × 10 −7 and 5 × 10 −6 mbar and, for Mg, the pres- sure ranged between 1 × 10 −7 and 1 × 10 −6 mbar. Si wafers with a native SiO 2 layer terminating the surface were used as substrates. Ag served as the catalyst. The ZnO/(Mg,Zn)O nanowires were nucleated and grown on Si substrates coated with Ag for catalytic growth. Typ- ical growth times for nanowires on the Ag-coated silicon were 2 h with growth temperatures ranging from T g = 300 to 500 ◦ C. After growth, the samples were examined by field-emission scanning electron microscopy (JEOL 6335F), transmission electron microscopy (Philips 420EM), and high- resolution TEM (JEOL 2010F). Energy-dispersive spec- trometry (EDS) spectra from single nanowires were col- lected by TEM (Philips 420EM). Compositional line scans, profiled across the nanowire, were measured by scanning