Impact of structural modifications on dielectric properties of epitaxial rare-earth oxides on silicon

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A very promising way to realize advanced future devices is using single-crystalline, closely lattice matched oxides, which will be deposited on the substrate of choice. Binary crystalline rare earth oxides have proved to be a very promising group of dielectrics for epitaxial growth. Under certain conditions, thin rare earth oxide layers epitaxially grown on Si exhibit dielectric constants that are much larger than the known bulk values. The reason for that enhancement effect is not fully understood yet. In the on-going project, we investigate the effect of lattice-mismatch induced strain on dielectric properties. We could show that tetragonal distortion of the cubic lattice is not sufficient to explain the enhancement in K. Therefore, we propose more severe strain induced structural phase deformations into a monoclinic phase. In the prosecution of that project, we will investigate this phase transition in more detail. First results show that Gd2O3 (under tensile stress) and Nd2O3 (under compressive stress) on Si exhibit nearly the same enhancement in the dielectric constant. By growing Nd2O3 on Ge and Si substrates, we will have a model system with identical amount of stress (the lattice mismatch is equal in both cases) but opposite sign (tensile on Ge but compressive on Si). Investigation of the K enhancement for layers with varying thicknesses for both cases should deliver a detailed picture of the stress dependence of this phase transition. Preliminary investigations reveal that Gd2O3 layer growth on Si at low temperatures leads to tetragonal distorted cubic lattices. The temperature was probably not sufficient for a structural phase transition. We expect to understand the dynamics of such a phase transition by growing and evaluating thin Gd2O3 at different temperatures. Complimentary, post-growth anneals of tetragonal distorted layers will be investigated. Our investigations revealed that N doping in epitaxial Gd2O3 thin films can be instrumental in improving their dielectric properties. Substantial reduction of the leakage current density and disappearance of hysteresis in capacitance-voltage characteristics observed in the Gd2O3:N layers indicate that nitrogen incorporation in Gd2O3 effectively eliminates the adverse effects of the oxygen vacancy induced defects in the oxide layer. However, one needs to consider that N incorporation can also dramatically influence the electrical properties of high-K oxides by changing their electronic structures. In order to optimize the electrical properties of the epi-rare earth oxides by means of N doping, it is therefore imperative to investigate the evolution of their electronic structure as a function of N concentration. As an extension to our work on N doped epitaxial Gd2O3 layers we need to investigate the effect of N incorporation on the electronic structure of Gd2O3:N layers which is essential to optimize their electrical properties.