Selective MOCVD growth of Fe/sub 3/O/sub 4/ epitaxial thin films for nanostructure

Abstract

Coductive ferromagnetic oxides, manganese perovskites and magnetite (Fes04) have been interested as leading materials for spin electronics because of half-metallic nature in which electrons near Fermi level are 100 % spin-polarized. These specific electrical properties are expected to evolve advanced hnctions in a low dimensional mesoscopic system using these oxides such as quantum dots and wires in which spin-polarized electrons are confined in a mesoscopic space. Metalorganic chemical vapor deposition (MOCVD) is a promising technique making these mesoscopic structures with excellent quality because it allows us to deposit the films only onto a selected small area. However, there is no research on selective epitaxial growth of the magnetic oxides because stable metalorganic sources of transition metal elements suitable for deposition are so few that the MOCVD growth itself of the magnetic thin films is not familiar. We have previously reported that the thin films of magnetic oxides including Fe304 are successfully grown by the MOCVD using metal-acetylacetonate complex [I]. The growth process was dominated by the Langmuir-Hinshelwood mechanism in which an oxidation reaction occurs between the molecules of the complex and oxygen adsorbed onto the surface. Atomic layer epitaxy of the Fe304 thin films supporting this mechanism has been actually achieved by altemately adsorbing the complex and oxygen [2]. In this paper, we demonstrate the selective growth of Fe304 thin films using MOCVD for the first time and discuss the growth mechanism from the view point of catalytic activity of the substrate surface for the oxidation of the organic molecules. ExperimentalThe Fe304 thin films were grown by a low pressure MOCVD using a cold wall reactor. The reactor was evacuated to a base pressure of the order of Torr prior to the film deposition.Ferric acetylacetonate Fe(C5H702)3 precursor was used as the iron source. This precursor was carried to the reactor by a flow of argon after vaporized at 393 K and supplied onto substrate surface along with oxygen flow. The flow rates of oxygen and argon were varied in the range of 0 to 3 ccm and 0 to 10 ccm respectively by mass flow controllers. Single crystals of(100) MgO and basal c-plane sapphires were used as the substrates, in addition to quartz glass. In order to demonstrate the selective film growth, some of the substrates were partially covered with the patterned thin layers of NiO or Fe304 prepared by sputtering or pulsed laser deposition techniques. The substrates were heated in the range of 523 K to 773 K.Results and discussion Temperature dependence of partial oxygen pressure required for the film growth for a constant supply o f the metalorganic source showed that the threshold amount of oxygen I oa ul I pressure required for the film growth increases exponentially with temperature increasing. Such a presence of threshold oxygen amount is characteristic of tangmuir-Hinshelwood type silrface reactinn mechanism; the film growth ~lrrvrs when the amount of oxygen adsorbed onto t...