Optical and microstructural properties of ZnO/TiO2 nanolaminates prepared by atomic layer deposition
© Gu et al.; licensee Springer. 2013
Received: 18 November 2012
Accepted: 27 January 2013
Published: 27 February 2013
ZnO/TiO2 nanolaminates were grown on Si (100) and quartz substrates by atomic layer deposition at 200°C using diethylzinc, titanium isopropoxide, and deionized water as precursors. All prepared multilayers are nominally 50 nm thick with a varying number of alternating TiO2 and ZnO layers. Sample thickness and ellipsometric spectra were measured using a spectroscopic ellipsometer, and the parameters determined by computer simulation matched with the experimental results well. The effect of nanolaminate structure on the optical transmittance is investigated using an ultraviolet–visible-near-infrared spectrometer. The data from X-ray diffraction spectra suggest that layer growth appears to be substrate sensitive and film thickness also has an influence on the crystallization of films. High-resolution transmission electron microscopy images show clear lattice spacing of ZnO in nanolaminates, indicating that ZnO layers are polycrystalline with preferred (002) orientation while TiO2 layers are amorphous.
KeywordsZnO/TiO2 nanolaminates ALD Transmittance HRTEM
ZnO is a low-cost and widely used semiconductor material with outstanding physical and chemical characteristics. At room temperature, the band gap and exciton binding energy of ZnO are 3.37 eV and 60 meV, respectively, both contributing to its extraordinary chemical and thermal stability. Thus, ZnO thin films exhibit magnificent applications in the manufacturing process of optoelectronic devices . Also, being a promising semiconductor material that is transparent to visible light and has excellent optical transmittance, TiO2 is widely used in the synthesis of semiconductor photocatalysts, solar cell electrodes, and sophisticated electronic optical devices [2–5].
ZnO and TiO2 thin films, both with a wide band gap, high refractive index, high stability, and good catalysis, are suitable partners for multilayer nanostructures. On the one hand, TiO2 could serve as a buffer layer between ZnO and Si substrates. The lattice and thermal mismatches can be reduced, and the quality of ZnO films will be enhanced because TiO2 can inhibit the surface silicon atoms from plundering oxygen atoms in ZnO films [6, 7]. Moreover, growing very thin ZnO films over a porous TiO2 electrode can improve the surface state and surface atomic mobility, so high-powered solar cells with better utilization efficiency can be produced . There are also researches on ZnO/TiO2 multilayer mirrors at ‘water-window’ wavelengths with high reflectivity around 2.7 nm, indicating its potential in multilayer optics .
ZnO/TiO2 multilayers have been prepared by many techniques, such as chemical vapor deposition, pulsed laser deposition, and co-sputtering [10–12]. However, high-quality nanolaminate films require precisely controlled factors including interfacial roughness, interdiffusion between layers, layer-to-layer consistency, and conformality. Atomic layer deposition (ALD) is more powerful in preparing such multilayers than other techniques, which keeps the precursors separated during the reaction . By sequentially dosing the surface with appropriate chemical precursors and then promoting surface reactions that are inherently self-limiting, the atomic layer control of film growth can be obtained. There has been a variety of publications on ALD-prepared ZnO or TiO2 films [14–17]. Thus, studies on ZnO/TiO2 multilayers prepared by ALD are of increasing importance in this field [18, 19]. In this study, a series of ZnO/TiO2 nanolaminates were prepared by ALD. The optical and microstructural properties of ZnO/TiO2 were measured and compared by spectroscopic ellipsometry (SE), ultraviolet–visible-near-infrared (UV–vis-NIR) spectrometry, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM).
The thicknesses of the multilayer were measured by spectroscopic ellipsometry (Sopra GES5E, SOPRA, Courbevoie, France) where the incident angle was fixed at 75° and the wavelength region from 230 to 900 nm was scanned with 5-nm steps. The optical transmission spectra were obtained using a UV spectrophotometer (UV-3100) in a wavelength range of 200 to 900 nm at room temperature in air. The crystal structures of the films were obtained using an X-ray diffractometer (D8 ADVANCE, Bruker AXS, Inc., Madison, WI, USA) using Cu Kα radiation (40 kV, 40 mA, λ = 1.54056 Å). High-resolution transmission electron microscopy and electron diffraction experiments were performed in a Philips CM200-FEG system operated at 200 kV. The specimens were prepared by mechanical polishing and dimpling, followed by Ar+ ion milling to electron transparency with 4.0-keV beam energy at an angle of 6° using a Gatan precision ion polishing system (Pleasanton, CA, USA).
Results and discussion
The measured layer thickness of films with indexes 1 to 5 grown on Si by SE
Total thickness (nm)
Crystallized ZnO shows clear lattice in the image, while a crystal structure could hardly be observed in TiO2 layers. Thus, TiO2 films are amorphous, that is why no diffraction peaks are observed in XRD. Fast Fourier transformation (FFT) image is shown in the HRTEM image (Figure 6b). The reciprocal lattice spacing can be identified to be 3.795 nm−1. As a result, the interplanar spacing is 2.6 Å, which is consistent with the calculated data for ZnO (002) orientation. Thus, it could be concluded that ZnO films grow on TiO2 along the (002) direction [26, 27]. Besides, the crystallite size of ZnO film shown in TEM images is also very close to the values calculated from XRD peaks, further confirming the structure features of ZnO/TiO2 nanolaminate.
ZnO/TiO2 nanolaminates were grown on Si (100) and quartz substrates by ALD technique at 200°C. The optical and microstructural properties of samples with different numbers of bilayers are investigated. The thickness and growth rate of ZnO and TiO2 films are obtained using a spectroscopic ellipsometer, indicating the high accuracy of the ALD technique in controlling the growth of nanolaminates. The transmittance of multilayers in the visible wavelength increases gradually as the number of sample bilayers increases. The XRD spectra show that ZnO films grown on quartz are polycrystalline with preferred (002) orientation while TiO2 films are amorphous. The high-resolution TEM image for a representative sample shows clear lattice spacing along with the grain size of ZnO, confirming the structural properties of nanolaminated ZnO/TiO2 multilayers.
This work is supported by the Important National Science & Technology Specific Projects (no. 2011ZX02702-002), the National Natural Science Foundation of China (no. 51102048), the SRFDP (no. 20110071120017), and the Independent Innovation Foundation of Fudan University, Shanghai.
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