Investigations into the impact of various substrates and ZnO ultra thin seed layers prepared by atomic layer deposition on growth of ZnO nanowire array
© Ding et al.; licensee Springer. 2012
Received: 13 October 2011
Accepted: 19 June 2012
Published: 3 July 2012
The impact of various substrates and zinc oxide (ZnO) ultra thin seed layers prepared by atomic layer deposition on the geometric morphology of subsequent ZnO nanowire arrays (NWs) fabricated by the hydrothermal method was investigated. The investigated substrates included B-doped ZnO films, indium tin oxide films, single crystal silicon (111), and glass sheets. Scanning electron microscopy and X-ray diffraction measurements revealed that the geometry and aligment of the NWs were controlled by surface topography of the substrates and thickness of the ZnO seed layers, respectively. According to atomic force microscopy data, we suggest that the substrate, fluctuate amplitude and fluctuate frequency of roughness on ZnO seed layers have a great impact on the alignment of the resulting NWs, whereas the influence of the seed layers' texture was negligible.
Zinc oxide (ZnO) is a semiconductor with wide band-gap (3.37 eV) and possesses high excited binding energy of 60 meV.[1, 2] It has been widely studied and applied to field effect transistors, field emitters, photodetectors, gas sensors, dye-sensitized solar cells, and other optoelectronic devices[8, 9] because of its special properties, such as long-term stability, relatively low material costs, simple processing due to its compatibility with wet chemical etching, biocompatibility, environmental friendliness, excellent radiation resistance, and so on. In these applications, one-dimensional (1D) and nanoscale ZnO materials (e.g., nanorods, nanowires, and nanotubes) have attracted considerable attention due to their significantly different electronic and photoelectron-chemical properties and have potential applications in electronic and photonic devices[10–14].
To obtain 1D ZnO materials, there are several methods including physical vapor phase growth that required high temperature and chemical approaches working at low temperature, in which hydrothermal synthesis is a good chemical approach for the synthesis of ZnO nanowire arrays (NWs) by fabricating ZnO seeds with the morphology of thin films or nanoparticles on substrates firstly[15, 16]. Atomic layer deposition (ALD) is a good method for growing high-quality ZnO seed layers[17–20] because it requires low growth temperature and can offer excellent conformality, easy and accurate thickness control, good reproducibility, and high uniformity over a large area. However, the reported thickness of the seed layers prepared by ALD was greater than 10 nm[19, 20]. Moreover, although the effect of roughness and texture of seed layers on the alignment of NWs has been reported[19, 21, 22], few research focused on the mechanism on how the roughness of seed layers affected the orientation of NWs.
Presentation of the hypothesis
In this paper, we first deposited ZnO seed layers with different thickness (2 to 50 nm) on different substrates by ALD method. The effect of the substrates and the seed layers' thickness on morphology and alignment of subsequent ZnO nanorods prepared by hydrothemal method was studied. It was found that roughness rather than the texture of the ZnO seed layers had a great impact on the alignment of the resulting NWs.
Testing the hypothesis
The reaction chamber was pumped down to 1 to 2 Torr before deposition. The operating environment of ZnO deposition was maintained at 3 Torr and 200°C. Each deposition cycle consisted of four steps, which included DEZ reactant, N2 purge, H2O reactant, and N2 purge. The typical pulse time for introducing DEZ and H2O precursors was 0.5 s, and the N2 purge time was 10 s. The deposition cycles of 11, 22, 33, 44, 55, 110, and 275 were chosen to produce ZnO seed layers with the various thickness of 2, 4, 6, 8, 10, 20, and 50 nm. The deposition rate at the above conditions approaches 0.182 nm/cycle.
Finally, the samples were washed with deionized water and dried in air before characterization. The morphology of the NWs was characterized by scanning electron microscopy (SEM, Philips FEIXL30 SFEG, Amsterdam, Netherlands) and transmission electron microscopy (TEM, Hitachi HF-2000, Chiyoda, Tokyo, Japan). TEM samples were prepared by gently dragging the holey (400 mesh Cu, SPI supplies, West Chester, PA, USA) carbon grids along the surface of the samples. X-ray diffraction (XRD) analysis was performed with a Rigaku Dmax-2000 diffractometer using CuKa radiation (Rigaku Corporation, Tokyo, Japan). The morphology of the seed layers and roughness was characterized by an atomic force microscope (AFM, Park Systems XE-100, Santa Clara, California, USA). The photoluminescence (PL) spectroscopy is performed on an Olympus BX51 microscope with Hg illumination and UV filter cube (U-MWU2, excitation, Olympus Shinjuku, Tokyo, Japan).
Implications of the hypothesis
We demonstrate that the growth of the ZnO NWs on ultra-thin seed layers is strongly influenced by the substrates and thickness of the seed films. NWs could be obtained on the smooth substrates covered with seed layers whose thickness is larger than 4 nm and have good alignment when roughness of the seed layers is also suitable. Besides, it is found that the thickness of the seed layers affects fluctuation amplitude and frequency of the roughness, which affects the alignment of the resulting NWs in succession. However, the crystal defects were influenced greatly by substrates instead of seed layers. The research provides prospect for preparation of the ZnO NWs on thin seed layers.
JND is a professor at the Center for Low-Dimensional Materials, Micro-Nano Devices and System, Changzhou University, Changzhou 213164, China and at Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou, 213164, China. YBL and CBT are both post-graduates at the Center for Low-Dimensional Materials, Micro-Nano Devices and System, Changzhou University, Changzhou 213164, China. NYY is a professor at the Center for Low-Dimensional Materials, Micro-Nano Devices and System, Changzhou University, Changzhou 213164, China.
This work was supported by the National High Technology Research and Development Program 863 (2011AA050511), Qing Lan Project (2008–04), Jiangsu ‘333’ Project (201041), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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