Synthesis of ZnO nanorods by a hot-wall high-temperature metalorganic chemical vapor deposition process
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ZnO nanorods with diameter of 30–200 nm were synthesized by a metalorganic chemical vapor deposition process in a hot-wall type chamber at elevated temperatures above 700 °C. At temperatures between 400 and 500 °C, ZnO thin films and wrinkles were synthesized. Above 500 °C, vertically aligned ZnO nanorods were grown on a Si and sapphire substrate without any catalysts. The range of diameter was 30–200 nm. When Au catalysts were deposited on the substrate prior to the deposition, nanocombs and nanosheets as well as nanorods were synthesized. In particylar, ZnO could be grown selectively along the pattern of the Au catalyst with the aid of a Au–Zn alloy.
I. INTRODUCTION
Bottom-up approaches to nanoelectronics using nanoscale building blocks such as quantum dots, nanowires, and nanotubes have received considerable attention to date.1,2 As for one-dimensional (1D) nanostructures, various semiconducting nanorods and nanotubes have been successfully explored.1–3 In particular, ZnO, a wide-bandgap (3.37 eV) semiconductor has attracted considerable attention due to the potential applications for electrooptics, electronics, and piezoelectrics. Thus, various 1D nanostructures of ZnO including nanorods and nanotubes have been fabricated by several different processes.4–10 Typically, ZnO nanorods have been synthesized by a carbothermal reduction process1,4 on an Au-coated silicon substrate by heating a 1:1 mixture of ZnO and graphite. As the ZnO nanorods grow along the Au–Zn eutectic alloy, one can align the nanorods along the pattern of Au-coated substrate. However, as the exact control of oxygen partial pressure and related thermodynamics is troublesome in this process, it is difficult to produce nanorods in a reproducible manner. For nanorods of more stability and large-area compatibility, the metalorganic chemical vapor deposition (MOCVD) process has been used for the fabrication of ZnO nanorods in the temperature range of 400–500 °C.5 In this MOCVD process, however, the selective growth of nanorods by the aid of catalysts is difficult due to such a low process temperature range.
In the present work, a high-temperature hot-wall MOCVD process were developed for the fabrication of ZnO nanorods with several interesting features. In the typical MOCVD systems, the temperature of substrate is controlled 400∼600 °C in a cold-wall chamber. In our work, however, we designed a hot-wall MOCVD chamber so that the temperature of the target and substrate can be controlled between 400∼1100 °C. II. EXPERIMENTAL
Figure 1 shows the hot-wall MOCVD chamber used in this work. In this system, the temperature of the target and substrate can be controlled between 400–1100 °C by the heaters encapsulating the reaction chamber. The deposition was performed on a substrate in a horizontal hot-wall 1-in. reactor. Diethyl zinc (DEZ) was transported into the reactor through the bubbling by highpurity Ar and was mixed with Ar carrier gas. The ratio of DEZ/Ar was controlled in the range of 0.001–0.1, and the
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