Formation of a hydroxide thin film by laser ablation and conversion to an oxide film with topotaxy

<jats:p>It is important to control the crystallographic orientation of oxide films for application in electronic devices. In the case of oxide superconductors, the orientation of epitaxial films has been controlled by varying the substrate temperature and the partial pressure of oxygen. To fix the orientation irrespective of the substrate temperature and the pressure of oxygen, the metal layer of the deposited film should be parallel to the substrate, and the formation of the oxide film by heat treatment should preserve this orientation. Hydroxides are one of the promising compounds that satisfy the above condition. By heating the hydroxide to achieve dehydration, oxides will be formed with the atomic arrangement preserved, that is, topotaxially. The purpose of this study is to form a completely oriented [001] Ca(OH)2 thin film, to form a perfectly [111] oriented CaO film after topotaxial dehydration, and to demonstrate formation of Ca(OH)2 and CaO films in vacuum. The [001] oriented Ca(OH)2 film was formed on a MgO substrate by laser ablation, indicating that the Ca layer of the film was parallel to the substrate. Pole figure measurements showed that the planar orientation of the Ca(OH)2 was random. This is due to the mismatch between the film and the substrate. By heating the Ca(OH)2 film at 600–800 °C to achieve dehydration, a [111] oriented CaO film was formed. Pole figure measurements showed that the CaO film is randomly oriented in the plane. This is because the mismatch between the CaO and the MgO is as large as 23%. It is to be noted that a [100] oriented film tends to be formed when the CaO film is deposited directly from a CaO target. The change of the orientation from [001] to [111] is discussed in connection with the topotaxy mechanism. These results show a possibility of “hydrogen bond epitaxy,” that is, formation of an atomically controlled oxide film through a hydroxide.</jats:p>