pubs.acs.org/crystal Published on Web 09/02/2009 r 2009 American Chemical Society DOI: 10.1021/cg900018p 2009, Vol. 9 4308–4314 Selective Crystalline Seed Layer Assisted Growth of Vertically Aligned MgZnO Nanowires and Their High-Brightness Field-Emission Behavior Dong Chan Kim, Bo Hyun Kong, Sanjay Kumar Mohanta, Hyung Koun Cho,* Jae Hong Park, and Ji Beom Yoo School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440-746, Korea Received January 8, 2009; Revised Manuscript Received July 9, 2009 ABSTRACT: This article reports a new approach for fabricating vertically aligned MgZnO nanowire arrays using selective crystalline seed layers deposited at relatively high temperatures by metalorganic chemical vapor deposition. In the phase- separated MgZnO seed layers, Zn-rich regions with a wurtzite structure allowed the growth of slim MgZnO nanowires, while the Mg-rich seeds with a cubic or amorphous-like structure hindered the formation of nanowires. The photoluminescence of these nanowires showed a remarkable increase in the activation energy of the excitons, ∼100 meV. The field-emission performance showed excellent emission behavior with extremely improved brightness, ∼80 times higher current density than that of the ZnO nanowires grown under the same conditions. Models for the growth evolution of slim nanowires on Si substrates were also proposed, based on microstructural characterization. 1. Introduction The synthesis of low-dimensional nanostructures in various forms has attracted considerable technological and scientific interest due to the physical and optical properties and the discovery of new physical meaning induced by the quantum- confinement effect. 1,2 One-dimensional (1D) ZnO nanostruc- tures (i.e., nanowires, nanorods, nanobelts, nanorings, etc.) have been the subject of intense research for applications in short-wavelength light-emitting devices and ultraviolet field emitters on account of their wide bandgap and high exciton binding energy, which can allow the production of devices with their performance unaffected by temperature. 3,4 Verti- cally aligned ZnO nanowire arrays are more suitable struc- tures for the fabrication of electronic devices, considering the interconnection of a metal layer and the feasibility of integra- tion processes. 5,6 The ZnO nanowires-based field emitters are expected to have a longer lifetime than carbon-based field emitters due to the chemical and environmental stability of oxide materials. Although many techniques have been used to synthesize vertically well-aligned ZnO nanowire arrays, 5-9 metalorganic chemical vapor deposition (MOCVD) has been demonstrated to be a promising tool with particular advan- tages of accurate doping, low-temperature, catalyst-free epi- taxial growth, etc. 9-11 However, this method produces dense and broad ZnO nanowires (>50 nm). Moreover, although the nanowires are highly crystalline with excellent optical quality, it is difficult to control the density and tip shape of the nanowires, which has a detrimental effect on the perfor- mance of field emitters due to the blunt tip shape and electric arcing. Therefore, the low emission intensity of ZnO-based field emitters and the difficulty in obtaining vertically aligned ZnO nanowire arrays with a narrower shape without the use of metal catalysts has limited field-emission applications to carbon nanotubes (CNTs). 12,13 In this context, the incorpora- tion of a small quantity of Mg atoms in the ZnO matrix can enhance the emission efficiency, 14,15 and an increase in synth- esis temperature can improve the crystalline quality and luminescence intensity. 16 Therefore, it is believed that both the addition of a small quantity Mg and the synthesis of vertically aligned nanowire arrays with the appropriate den- sity at high temperatures will lead to significant improvement in the field-emission behavior. This paper reports a method for synthesizing very slim, vertically aligned single crystalline MgZnO nanowire arrays by growing small wurtzite crystal- line Zn-rich phases selectively at high temperatures by MOCVD. The nanowires showed tremendous improvement in emission behavior due to the high crystalline quality and vertical alignment of nanowire arrays with the appropriate density at high growth temperatures. The detailed growth mechanism for these vertically aligned MgZnO nanowire arrays based on the microstructural characterizations is also presented. 2. Experimental Section 1D MgZnO nanowires were grown on Si substrates using a low- pressure MOCVD method, with diethylzinc (DEZn, purity 99.9995%), bis-cyclopentadienyl-magnesium (Cp 2 Mg, purity 99.9995%), and pure oxygen gas (O 2 , purity 99.9999%) as the reactive gases. Argon was used as the carrier gas. All Si substrates were degreased in an ultrasonic bath with acetone, ethanol, and deionized water for 5 min each. The Cp 2 Mg flow rate during the growth of MgZnO nanowires was 1.4 μmol/min, while the DEZn flow rates was kept constant at 6.1 μmol/min. The nanowires were grown at temperatures ranging from 450-500 °C with a reactor pressure of 1 Torr for 30 min. No buffer layers or metal catalysts were required to produce the vertically aligned nanowires in this MOCVD method. The maximum level of Mg incorporation in the ZnO that allows the synthesis of vertically aligned nanowires under these growth conditions was approximately ∼8 at %. The ZnO nanowires were also grown for comparison. The morphology of the fabricated nanowires was observed by field-emission scanning electron microscopy (FESEM, JSM6700F). The microstructure was examined by transmission electron micro- scopy (TEM, JEM 3010) and high-resolution TEM operated at 300 kV. The TEM specimens were prepared by mechanical polishing and ion- milling at a low current Ar þ ion dose to prevent deformation during *To whom correspondence should be addressed. E-mail: chohk@ skku.edu.