Microstructure of epitaxial VO 2 thin films deposited on (1120) sapphire by MOCVD
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Epitaxial VO 2 thin films grown on (1120) sapphire ( a - A l 2 O 3 ) substrates by MOCVD at 600 °C have been characterized by conventional electron microscopy and high resolution electron microscopy (HREM). Three different epitaxial relationships between the monoclinic VO 2 films and sapphire substrates have been found at room temperature: I. (200) [010] monoclinic VO 2 || (1120) [0001] sapphire, II. (002) [010] monoclinic VO 2 || (1120) [0001] sapphire, and III. (020) [102] monoclinic VO 2 || (1120) [0001] sapphire. Expitaxial relationships II and III are equivalent to each other when the film possesses tetragonal structure at the deposition temperature; i.e., they can be described as (010) [100] tetragonal VO 2 || (1120) [0001] sapphire and (100) [010] tetragonal VO 2 II (1120) [0001] sapphire. HREM image shows that the initial nucleation of the film was dominated by the first orientation relationship, but the film then grew into the grains of the second and the third (equivalent to each other at the deposition temperature) epitaxial relationships. Successive 90° transformation rotational twins around the a-axis are commonly observed in the monoclinic films. I. INTRODUCTION Oxides, due to the great variety and complexity of their structures, form a class of materials that exhibit a wealth of interesting and useful properties that can be exploited for a variety of technological applications. 12 Many of the oxide materials have been studied in the form of thin films because of the requirements imposed for specific applications. Until quite recently, almost all of these thin films were in the polycrystalline or amorphous forms; little work has been carried out to study epitaxial oxide thin films3 as compared to semiconductor or metal. In the past few years, however, this situation has changed rather rapidly, and more and more researchers have shown interest in this area. The simple oxide VO 2 , due to its simple crystal structure4 and interesting property associated with the metal-semiconductor transition,5"10 was selected as a model material in our thin film growth effort by MOCVD on a variety of single-crystal oxide substrates such as sapphire ( a - A l 2 O 3 ) , MgO, SrTiO3, etc. Our goal was to gain a fundamental understanding of the processingstructure-property relationship for oxide heteroepitaxy in general.11"14 Our results showed that sapphire was the most suitable substrate for growing epitaxial VO 2 films due to the structural similarity (the atomic arrangement) between the two materials. X-ray diffraction study showed there were two possible film-substrate epitaxial relationships for films grown on sapphire (1120) at different temperatures, i.e., (100) [010] VO 2 || (1120) [0001] sapphire and (010) [102] VO2 11(1120) [0001] sapphire. The x-ray results also showed that these films had good crystallinity judging from the rocking curves of
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