Oxygen-segregation-controlled epitaxy of Y2O3 films on Nb(110)

We demonstrate a very simple and reliable method of manufacturing clean, single-crystalline Y2O3 films on Nb(110) substrates in situ. The method exploits the oxygen bulk contamination of Nb as a source of clean oxygen. For substrate temperatures above 800 K oxygen segregation to the Nb surface is so efficient, that yttrium becomes oxidized during deposition without any background oxygen pressure required in the ultrahigh vacuum system. The crystallinity and stoichiometry of these films can be tuned by the deposition temperature. For Y deposition at 1300 K the formation of well-ordered (111)-oriented Y2O3 films is achieved. PACS: 81.05.Je; 68.55.Jk; 81.15.Ef; 81.65.Mq; 61.14.Hg; 61.14.Qp Yttria (Y2O3) is a highly refractory material that has many practical applications, for example, it is used as sintering aids in the processing of ceramic materials and as a component for rare-earth (RE) doped lasers and optical windows [1]. In the last few years thin yttria films have attracted increasing attention. Its high dielectric constant (ε= 13−17), its high resistivity, and high breakdown strength make Y2O3 a viable candidate for silicon very-large-scale applications such as highdensity storage capacitors in miniaturized dynamic random access memory (DRAM) [2–4]. Moreover, Y2O3/Nb tunneling barriers were found among the best in interface quality and the highest in tunneling resistance and, consequently, meet important prerequisites for superconducting digital electronics [2]. It is obvious, that the unique properties of yttria depend critically on the presence of defects. Therefore, a simple and reliable method of manufacturing yttria films with controlled defect concentrations is desirable for both applications as well as for fundamental research on the electronic structure and bonding in crystalline yttria at the microscopic level. Present address: Departmentof Physics, Pennsylvania State University, PA 16802. (Fax: US-814-865-0978, E-mail: [email protected]) Usually, thin Y2O3 films are manufactured by evaporation, sputtering, and laser ablation of bulk Y2O3 or by postoxidation of Y films in air or oxygen ambient [3–5]. Here we demonstrate a very simple and reliable method of manufacturing clean, single-crystalline Y2O3 films on (110)-oriented Nb substrates. Their crystallinity and stoichiometry can be tuned by the deposition temperature. The crystalline quality of the films was examined by means of X-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED). X-ray photoelectron spectroscopy (XPS) was used to study the chemical bonding in the yttrium oxide films. Film preparation and characterization were carried out in situ. Angle-resolved ultraviolet photoemission spectroscopy (ARUPS) results on the electronic structure of Y2O3 films will be published elsewhere [6]. XPD was chosen because of its chemical sensitivity and its sensitivity to local real-space order. It is a powerful technique for surface structural investigations [7], and it has been shown that full hemispherical XPD patterns provide very direct information about the near-surface structure [8]. At photoelectron kinetic energies above about 500 eV, the strongly anisotropic scattering of photoelectrons by the ion cores leads to a forward-focusing of the electron flux along the emitterscatterer axis. The photoelectron angular distribution, therefore, is to a first approximation a forward-projected image of the local structure around the photoemitters. 1 Experimental procedures Experiments were performed in a modified VG ESCALAB Mk II spectrometer (base pressure in the low 10−11 mbar region) and equipped with a two-axis sample goniometer enabling sequential computer-controlled sample rotation. Photoelectron spectra and diffraction patterns were measured using Mg Kα (hν = 1253.6 eV) radiation with the sample kept at room temperature (RT). In the XPD experiments we typically measured about 4500 angle settings ((θ, φ), photoelectron emission angles θ(polar) and φ(azimuthal)), each displaying the angular-dependent intensity of the selected Published in Applied Physics A: Materials Science & Processing 71, issue 7, 615-618, 2000 which should be used for any reference to this work 1