Synthesis and characterization of Ag-doped p-type ZnO nanowires

P-type ZnO nanowires with silver (Ag) doping were synthesized via a chemical vapor deposition process. The incorporation of Ag was confirmed by selected-area energy-dispersive x-ray spectroscopy. The formation of acceptor states was demonstrated by temperature and excitation power-dependent photoluminescence measurements. Characterization of field-effect transistors using Ag-doped ZnO nanowires as channels showed p-type conductivity of the nanowires with a hole concentration of 4.9× 1017 cm−3 and a carrier mobility of approximately 0.18 cm2 V−1 s−1. ZnO, a wide-band-gap semiconductor with large exciton binding energy of 60 meV at room temperature [1, 2], is a promising candidate for optoelectronic devices such as blue-light emitting diodes [3, 4], ultraviolet laser diodes [5, 6], and photodiodes [7, 8]. It is well known that one of the biggest challenges toward good ZnO-based optoelectronic devices is the difficulty of reliably fabricating p-type ZnO due to the self-compensating effect from native defects (for example, oxygen vacancy Vo and zinc interstitial Zni) and/or H incorporation [9]. There has already been a great deal of efforts on the fabrication of p-type ZnO films by doping group I (Na, Ag) [10, 11] and group V elements (N, P, As, Sb) [12–15] as p-type dopants. In contrast, there have been only a few reports on p-type ZnO nanowires (doped with N, P, and Na) [16–18]. Recently, researchers are interested in developing optoelectronic devices based on ZnO nanowires such as biosensors, ultraG. Wang · S. Chu · N. Zhan · H. Zhou · J. Liu ( ) Quantum Structures Laboratory, Department of Electrical Engineering, University of California at Riverside, Riverside, CA 92521, USA e-mail: [email protected] violet detectors, ultraviolet light emitting diodes, and electrically driven nanowire lasers. The growth of p-type ZnO nanowires with good stability will be an essential step for the applications of nanowires in nanoelectronics and optoelectronics. Ag, a group Ib element, was predicted to be an acceptor in ZnO when incorporated into substitutional Zn sites [19]. Indeed, researchers experimentally demonstrated reliable fabrication of p-type ZnO thin films doped with Ag on sapphire substrate [11] with hole concentrations up to 6 × 1017 cm−3 and also demonstrated the possibility of achieving Ag-doped p-type ZnO nanowires [20, 21]. However, the electrical and optical properties of singlecrystalline Ag-doped ZnO nanowires have not been comprehensively studied yet. In this paper, we report the synthesis and characterization of single-crystalline Ag-doped p-type ZnO nanowires. P-type ZnO nanowires were grown from a mixture of zinc powder and silver (I) oxide using chemical vapor deposition (CVD). The incorporation of Ag was confirmed by selected-area energy-dispersive xray spectroscopy (SAEDX) and photoluminescence (PL) studies. Single-nanowire field effect transistors were subsequently fabricated, which exhibited hole conduction channels, confirming the p-type conductivity of the Ag-doped ZnO nanowires. The growth of nanowires was performed at 730°C for 15 minutes with a gas mixture, containing 600 sccm nitrogen and 300 sccm argon/oxygen (99.5:0.5). The mixture source of zinc powder and silver (I) oxide (1:2 in mass) in a quartz boat was placed in the center of the quartz tube. The silicon substrate was closely attached to the quartz boat on the downstream side. Figures 1(a) and (b) show the scanning electron microscopy (SEM) image of the nanowires from the top and regular transmission electron microscopy (TEM) image of an individual nanowire, respectively. The length