Spin polarized transport in semiconductors – Challenges for Nanolithography

I. Preda, L. Soriano, M. Abbate, S. Palacín, A. Gutiérrez, J.M. Sanz Departamento de Física Aplicada, Universidad Autónoma de Madrid, Cantoblanco 28049 Madrid, Spain [email protected] The main purpose of this work is to make a comparative study of the electronic structure of the early stages of growth of NiO on different oxide substrates or, in other words, to study the NiO/oxide interface formation. The study of oxide-oxide interfaces has become a hot topic nowadays since the early stages of growth of the oxide overlayer will define the characteristics of the final thin film. In this work we have studied the NiO/oxide interfaces by depositing very small amounts of NiO (sub-monolayer) on the different oxide substrates at room temperature. Ni has been evaporated at a very small evaporation rate from a tungsten filament surrounded by a Ni wire, in an oxygen atmosphere (1.5x10 Torr). The substrates had been previously prepared by using different methods. For instance, the SiO2 substrate was a 500 Å thick layer obtained by dry oxidation of a p-doped Si wafer. The Al2O3 substrate was an ultra-thin-layer (about 20 Å thick) formed by thermal oxidation of a clean polycrystalline Al film. In the case of MgO, a layer about 200 Å thick of MgO was deposited on a p-doped Si wafer by reactive evaporation of Mg in an oxygen atmosphere (1.5x10 Torr). It is important to note that in all cases, the oxide substrate is in contact with a conductive underlayer (pdoped Si, polycrystalline Al) in order to avoid charging problems during measurements by electron spectroscopies. Another important feature is that all materials used in these experiments were amorphous except highly oriented pyrolitic graphite (HOPG). In spite of that, the great sensitivity to the local order of the Ni2p XPS spectra makes this technique suitable for the analysis of such nonhighly-ordered materials. The electronic structure of NiO has been controversial in the literature for years. In particular, the Ni 2p photoemission spectrum of NiO shows a complex structure with three well defined peaks for the Ni 2p 3/2. The electronic configuration 3d is responsible of such complex spectra. The origin of those three peaks is also controversial. One of the most recent theories assigns the broad satellite lying at higher binding energy (862868 eV) to a mixture of unscreened photoemission, with the c3d final state, and screened photoemission, with the c3dL final state, where c and L stand for a hole in the core level and the ligand respectively. But the most characteristic feature of the Ni 2p photoemission peaks in NiO is the double peaked structure of the main-line (854-860 eV). According to the above theory, the lowest state corresponds to c3dL final state, i.e. there exists a screening process by electrons from the ligand. However, the origin of the second peak is the screening of the photoemission by electrons which do not belong to the nearest neighbors but the next nearest neighbors, or in other words, non-local screening. So that, we can say that the main line of the Ni 2p XPS core level is formed by the competition of two screening processes, local and non-local. Basically, these processes and the balance between local and non-local screening depend strongly on the local symmetry: position of the Ni atoms where the hole is created (bulk-surface), hybridization and valency of the neighboring atoms. For instance, if the Ni atoms are located mainly at the surface, the non-local screening is enhanced. However, if the second neighbors are Mg atoms (Ni impurity in ionic-MgO), only local screening is produced. Also hole doping (valency) leads to a preference of the local screening. We will use below this theory for the interpretation of our experimental results. N S IT