X-ray reflectivity study of solution-deposited ZrO 2 thin films on self-assembled monolayers: Growth, interface properti
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.P. Niesen and F. Aldinger Max-Planck-Institut fu¨r Metallforschung and Institut fu¨r Nichtmetallische Anorganische Materialien, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, D-70569 Stuttgart, Germany (Received 7 February 2000; accepted 31 August 2000)
Thin films of ZrO2 were deposited from aqueous solution on Si(100) substrates precovered by functionalized alkyltrichlorosilane self-assembled monolayers (SAMs). The interface structure, thermal stability, and densification of these films in the temperature range from room temperature to 750 °C in vacuum were measured using in situ x-ray reflectivity. The growth rate is a nonlinear function of time in solution, with a pronounced nonuniformity during the first 30 min. The as-deposited films exhibit about 3-nm roughness and a density below that of bulk ZrO2. Measurements in vacuum reveal decreasing film thickness, increasing film density, and decreasing roughness upon annealing up to 750 °C. The densification saturates at the highest measured temperatures, presumably following evaporation of residual contaminants from the aqueous synthesis procedure. Above 200 °C the SAM/ZrO2 interface began to deteriorate, possibly due to interdiffusion. The ZrO2 film structure obtained at the highest annealing temperatures persisted upon cooling to room temperature, and there was no visible evidence of stress-induced microstructural changes, such as peeling or cracking.
I. INTRODUCTION
Recent technological demand for thin films of metal oxides like ZrO2 in applications such as oxygen sensors, high refractive-index optical coatings, and laser mirrors has spurred the development of synthesis techniques based on chemical processes, such as precipitation from aqueous solution and sol-gel,1,2 rather than physical ones, such as sintering and molecular beam deposition.3,4 Advantages of the chemical synthesis routes include low cost, suitability at low temperatures, uniform coverage, and the possibility of coating nonflat surfaces. Of the chemical synthesis techniques, a promising new method is precipitation from an aqueous precursor solution at temperatures below 100 °C onto a surface covered by an ordered organic array of functional groups, a technique which has been dubbed “biomimetic” inasmuch as it replicates the biological mechanisms for ceramic synthesis at low temperature. 5,6 Self-assembled monolayers (SAMs) provide such an organic surface onto which ada)
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J. Mater. Res., Vol. 15, No. 12, Dec 2000 Downloaded: 05 Apr 2015
ditional adsorbates can be anchored.7,8 Thin films of ZrO2 and Y2O3-doped ZrO2,9 Y2O3,10 SnO2,11 ZnO,12 TiO2,13 and V2O514,15 have thus far been prepared using this technique, and they have been characterized at room temperature via transmission electron microscopy (TEM), x-ray diffraction, and atomic force microscopy.9,16 Studies of the temperature dependence of the thin film and interface properties have been less commonly unde
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