DOI: 10.1021/cg900588c
Vertically Aligned Ultraslim ZnO Nanowires Formed by Homobuffer: Growth Evolution and Emission Properties
2009, Vol. 9 4725–4729
Dong Chan Kim, Sanjay Kumar Mohanta, and Hyung Koun Cho* School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440-746, Korea Received June 1, 2009; Revised Manuscript Received July 16, 2009
ABSTRACT: Vertically aligned, ultraslim ZnO nanowire arrays, e 10 nm in diameter, were fabricated by depositing a homobuffer layer on a Si substrate by metalorganic chemical vapor deposition and their growth mechanism was examined. During the heating process, the side facet planes of the nanosheets were transformed from high index planes, such as (0113) and (0112), to low index planes, such as (0111) and (0110), which is related to the available surface energy during growth. The ultraslim nanowires exhibited extremely intense ultraviolet emission with enhanced thermal activation energy in the photoluminescence measurements as well as excellent field-emission performance with intense brightness and a low turn-on field. This extraordinary emission performance of the ultraslim ZnO nanowires was attributed to the concurrent achievement of high crystalline quality and the vertical alignment of the ultraslim nanowires with the appropriate density at high temperatures.
1. Introduction ZnO nanowires have been studied extensively as nanoscale devices, such as ultraviolet lasers, light-emitting diodes, photodetectors, chemical sensors, field emitters, and solar cells, because of their wide direct bandgap of 3.37 eV, large exciton binding energy of 60 meV, high mechanical stability, piezoelectricity, etc.1-5 However, in order to achieve excellent device performance and easy fabrication, it is essential to produce vertically aligned catalyst-free nanowires of suitable density with high crystallinity and optical quality. Hightemperature growth is most effective way of enhancing the crystallinity and emission properties of nanowires.6,7 Although high temperature growth methods, such as carbothermal reduction, pulsed laser deposition and chemical vapor deposition, have been used to synthesize nanowires, it is difficult to maintain vertical alignment and a uniform distribution on large-scale substrates.8-10 In this context, metalorganic chemical vapor deposition (MOCVD) was reported to produce vertically aligned ZnO nanowire arrays with a uniform distribution. However, the diameter of the synthesized nanowires was quite large (>50 nm), which has a detrimental effect on the performance of field emitters because of the blunt tip shape and electric arcing. According to previous reports, ZnO nanowires become increasingly slender with increasing growth temperature.6,7 Therefore, the high-temperature growth of nanowires by MOCVD is a predictable path for achieving ultraslim nanowires with vertical alignment and uniformity, which are believed to show improved emission properties. Furthermore, MOCVD was demonstrated to be a promising tool with the particular advantages of accurate doping and catalyst-free nanowire growth.11,12 The fabrication of vertically aligned ZnO nanowire arrays with a narrower diameter paves the way toward the realization of the intense UV emission devices operating at low threshold voltages. Therefore, methods for growing slim ZnO nanowires with vertical arrangement at
higher temperatures should be developed in order to enhance the efficiency of nano-optoelectronic/photonic devices and fabricate the building blocks of nanodevices using the bottomup and Si integration process. Vertically aligned nanowire arrays on a large-scale Si substrate are promising structures for Si-based integrated optoelectronic devices. This paper reports the successful growth of vertically aligned ultraslim ZnO nanowires by MOCVD at high temperatures by depositing a low-temperature homobuffer. The evolution from nanosheets to nanowires on the homobuffer layer was explained based on the available surface energy during growth. These ultraslim nanowires showed tremendous improvement in emission behavior because of their high crystalline quality and vertical alignment with the appropriate density. 2. Experimental Section
*To whom correspondence should be addressed. E-mail: chohk@skku. edu.
The ZnO nanowires were grown on n-type Si (100) substrates with and without buffer layers in a vertical MOCVD reactor operating at low pressure.13 Diethylzinc (DEZn, purity 99.9995%) and oxygen gas (O2, purity 99.9999%) were used as the precursors, and argon was used as carrier gas for the DEZn source. The reactor pressure was maintained at 1 Torr, while the temperatures for growing the nanowires on the Si substrates were varied between 460-550 °C. The ZnO buffer layers were grown at 260 °C for 10 min, while the growth time for the nanowires was 30 min. After deposition of the buffer layer, there was no pause in the injection of reactive gases during the heat-up process for the growth of slim nanowires, because no nanowires would form under these conditions if there were such a pause. The morphology of ZnO nanowires was examined by scanning electron microscopy (FESEM, JSM6700F). The structural properties of the ZnO nanowires were investigated by X-ray diffraction (XRD, Bruker AXS D8 Discover) and transmission electron microscopy (TEM, JEOL JEM2100F). The room-temperature and variabletemperature photoluminescence (PL) measurements were carried out using the 325 nm line excitation from a He-Cd laser. For the variable temperature PL measurements, the samples were placed inside a closed cycle He cryostat and the laser power was kept to
