STRUCTURAL INVESTIGATION OF THIN FILMS: FROM METAL HETEROSTRUCTURES TO METAL/ORGANIC SYSTEMS

COSSARO, ALBANO

STRUCTURAL INVESTIGATION OF THIN FILMS: FROM METAL HETEROSTRUCTURES TO METAL/ORGANIC SYSTEMS

COSSARO, ALBANO

2005-03-25

http://thesis2.sba.units.it/store/handle/item/13175

http://hdl.handle.net/10077/11489

Doctoral Thesis

Contributor(s)

SENATORE, GAETANO

Abstract

The semiconductor and electronic industry has seen a remarkable miniaturization trend driven by many scientific and technologic innovations in which the device fabrication and control on the nanometric lengthscales have become a fundamental issue. The experimental quest for highly ordered metal-semiconductor, metal-metal and organic-inorganic interfaces in the modem electronics is further motivated by the observed increased electric charge carrier mobility over well ordered and defect free layers, which can be produced by epitaxial film growth under Ultra-High- Vacuum conditions. Furthermore, heterogeneous epitaxy is a widely exploited technique to fabricate artificial materials, since a proper choice of the substrate allows to introduce controlled structural modifications of the growing film and to tune in this way its electronic properties. For these reasons the capability of studying not only the spectroscopic but also the structural properties of these systems has become of fundamental importance in the last years. In this work a multi-technique experimental approach has been applied to reach an exhaustive comprehension of the interface structure and morphology on a few case studies. Thin films have been studied by means of Grazing Incidence X-Ray Diffraction (GIXRD), X-ray Photoelectron Diffraction (PED), Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS), Helium Atom Scattering (HAS) and Low Energy Electron Diffraction (LEED). The thesis work has been performed at the ALOISA beamline of ELETTRA Synchrotron in Trieste. A brief review of the experimental techniques is given. Two experiments on simple systems are presented to describe the application of GIXRD (the structural determination of the l x2 missing row Au(llO) surface reconstruction) and PED and NEXAFS ( determination of the diatomic molecule CN orientation on Pd(llO) substrate) techniques and their implementation in the experimental apparatus of ALOISA. In the case of more complex systems, a complete structural characterization requires the simultaneous use of most of the above introduced techniques. Two case studies are reported in this thesis.l. Ni film growth on Pd(lOO) surface. Thin metallic films, when grown on suitable substrates can display unusual chemical and magnetic properties due to the strain induced by the substrate. To reach a better understanding of these heterosystems a structural characterization is needed. In the case of Ni on Pd(OOl), in the early stages of deposition, Ni atoms forma tetragonally strained fcc phase with the same laterallattice spacing of the substrate (pseudomorphic phase). Previous PED measurements suggested a structural transition occurring when the film thickness exceeds approximately 9-121ayers. At this stage the film was supposed to transform into its bulk-like Ni fcc structure. We have used GIXRD and PED to fully characterize the evolution of the two phases as a function of the thickness. A critical value of l O layers has been determined as the onset for the transition to the bulk phase. The morphology and the distribution of the two species of the system has been determined. GIXRD scans analysis puts in evidence how the transition, accompanied by a strong increase of the intermixing at the Ni/Pd interface, proceeds by the lateral growth of the Ni fcc domains at the expense of the pseudomorphic phase. 2.The structural characterization of a thin Copper-Phthalocyanine (CuPc) fùm grown on Au(llO) surface. Metal-Phthalocyanines films are among the most promising organic semiconductors devices for applications in electronics, solar celis, gas sensors. Their transport properties can be improved by optimizing the film morphology, which depends on the interface structure; for this reason the study of the early stages of deposition of CuPc on metals is relevant for possible applications. W e have studied the structure of a thin Cu-Pc film on the Au(llO) surface by means of HAS, LEED, NEXAFS and GIXRD as a function of thickness in the 0-2 monolayer range. W e have shown how different reconstructions are induced by the interaction between the molecules and the substrate as a function of the film thickness. In particular, two phases have been studied displaying different reconstructions along the [l 00] direction of the surface: a x5 reconstruction is present in the submonolayer range; at the monolayer coverage a x3 phase appears. GIXRD measurements allowed to determine the structure of the x3 cell which is present in the both phases and is accompanied by residua! x2 cell of the clean surface in the x5 phase. The use of two differently surface sensitive techniques · such as HAS and LEED allowed to distinguish between the order of the substrate and the organic layer; while the molecular film is partly disordered at the monolayer stage, the substrate presents well defined long range order. The NEXAFS spectra determine the molecules orientation with respect to the substrate. A tilt angle of about 25 degrees between the molecular plane and the surface in the x3 phase has been determined, consistent with an asymmetric distortion of the structure of the x3 cell.