Preparation of c-Axis Oriented La2CuO4Thin Films on the Si Substrate by Pulsed Laser Deposition

Abstract

The parent materials of the first high-TC cuprate, La2CuO4 (LCO) are known to be correlated with Mott insulators that become metallic and superconducting upon doping with charge carriers. 1 A large number of researches have been performed on bulk samples in the LCO-based materials due to their simple chemical composition and crystal structures as well as the rich physical properties. 2 The transition from the Mott-Hubbard insulating state to the metallic state is called the metal insulator transition (MIT), 3 and is a topic of great current interest. As an important application of the MIT, a high-speed Mott switching device using an abrupt current jump was predicted for a transistor. 4 LCO is known to have three different structures, 5 T-phase (K2NiF4-type) containing edge-shared CuO6 octahedra arranged in a planar array, T’-phase (Nd2CuO4-type) in which the apical oxygen in the T-phase is shifted away from the Cu atoms, and S-phase (Sr2CuO3) which has isolated CuO3 chains running along the a-axis of the orthorhombic structure. Transitions among T, T’ and S phases were found in the annealing process of bulk LCO. These are quasi-two-dimensional materials which consist of layers of copper-oxide planes separated. It seems that most of the properties are determined by electrons moving within CuO2 planes. The preparation of the thin film 6 is a key point to develop the devices such as thermal switching, thermochromatic coatings, non-volatile memory etc. applying MIT properties of LCO. High quality thin film of LCO is required for a variety of fundamental studies. The electrical and transport properties of LCO-based thin films are highly sensitive to the degree of c-axis orientation in the film growth and induced strain due to lattice mismatch with the substrate. A large number of substrates have been used to obtain highly oriented LCO thin films with good electrical properties such as NdGaO3, LaALO3, SrTiO3, and SrLaAlO3 substrates with a small lattice mismatch. 7 However, these cannot be used in the development of the MIT devices as common substrates because it is too difficult and expensive for the preparation of their single crystalline structure. LCO thin film is required to be grown with c-axis orientation on the surface of common substrates such as SiO2 acting as a gate oxide for fabrication of MIT devices. In the present work, we report the first preparation of caxis oriented LCO thin films on Si substrate, which is largely lattice-mismatched with LCO, by pulsed laser deposition (PLD) following in-situ post-annealing in oxygen ambient. PLD technique is a well-known deposition technique providing stoichiometric transfer of target materials via congruent evaporation for a wide range of materials including metals, alloys and oxide films.