Atomic layer deposition of epitaxial HfO 2 thin films on r -cut sapphire
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extured epitaxial HfO2 thin films of monoclinic structure were grown on r-cut Al2O3 by atomic layer deposition from HfCl4 and H2O at temperatures 450–750 °C. The film-to-substrate out-ofplane orientation was determined to have a single (001)HfO2//(1102)a-Al2O3 relationship. The in-plane orientation showed the existence of two possible relationships: [100]HfO2//[110]a-Al2O3 and [ 100]HfO2//[110]a-Al2O3. In films deposited at 400 °C traces of (010) growth plane were observed in addition to the preferential (001) growth. The lattice of HfO2 was compressed in the surface plane and expanded in the surface normal direction. The strain was highest in the films grown at 450–550 °C. With the increase of deposition temperature to 750 °C, the strain decreased. The strain relaxation in films deposited at 750 °C was in correlation with marked surface roughening in the initial stage of deposition at this temperature. The roughness of the epitaxial films was lower than that of polycrystalline films with comparable thickness deposited on Si(100) and SiO2 substrates.
I. INTRODUCTION
Hafnium dioxide (HfO2) has been extensively studied due to a great number of its applications. For instance, HfO2 has been applied as a high-k dielectric in field-effect transistors1–3 and different kinds of memory devices.4,5 In addition, HfO2 thin films have been used in high-refractiveindex layers of optical coatings6,7 and waveguides.8 In the majority of these applications, amorphous HfO2 films can be used. Crystallization of the films has usually led to the formation of polycrystalline structure with nonuniform material properties and great number of grain interfaces9 that may result in increase of leakage current and/or light scattering. These detrimental effects might be weaker or even negligible in epitaxial films. Advantages of epitaxial dielectrics have been clearly demonstrated e.g., for capacitor structures with RuO2 electrodes and TiO2 dielectrics deposited on single crystal a-Al2O3 substrates.10 Studies on epitaxial growth of HfO2-based dielectrics have been rather scarce, however. Epitaxial and/or quasiepitaxial growth of HfO2 has been reported to take place on Si,11–14 Ge,15,16 GaAs,17,18 MgO,19 and KCl,20 whereas films on Si and Ge substrates have been of more significant practical importance. Main problems with epitaxy of HfO2 on Si, Ge, and GaAs are related to the formation of interfacial oxide, which tends to be amorphous and therefore has to be avoided or removed to realize epitaxial growth. Therefore, in corresponding studies, more attention has to be concentrated on the control of the substrate surface rather than on mechanisms of the film growth and epitaxial quality of films obtained. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.120 1680
J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
http://journals.cambridge.org
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According to literature data, a-Al2O3 seems to be a more convenient substrate material for investigation of epitaxial growth of oxides because the substrate
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