(Invited) Tuning Dielectric Properties of Epitaxial Lanthanide Oxides on Silicon

A very promising way to realize advanced future devices is using single-crystalline, closely lattice matched oxides, which will be grown on the substrate of choice. The dielectric properties of such oxides are sensitive to small variations in structure and symmetry. It is known that thin layers of crystalline binary rare earth oxides can exhibit significant larger dielectric constants compared to bulk materials. For example, thin crystalline Gd₂O₃ films epitaxially grown on silicon exhibit dielectric constants up to 20 although the known bulk value is only around 13. The reason for that “enhancement effect” is not fully understood yet. Here, we will report about different investigations on strain-induced effects on dielectric properties. As model systems, we chose Gd₂O₃ and Nd₂O₃ having very similar bulk dielectric constants and band gaps. The crystalline structures are also identical. On the other hand, the lattice spacing in Nd₂O₃ is larger while that of Gd₂O₃ smaller than the lattice spacing in silicon; i.e. one layer should be under compressive strain and the other under tensile strain. First, we report on the dependence of the dielectric constant on layer thickness for epitaxial Gd₂O₃ on Si(111). The K-value strongly decreases with increasing layer thickness and reaches the bulk value at around 8 nm. Controlling the oxide composition in ternary (Gd_1-xNd_x)₂O₃ thin films enables us to tune the lattice mismatch to silicon, and thus the strain-induced variation in the dielectric constants of the layer from 13 (close to the bulk value) up to 20. Finally, we will show that solely tetragonal distortion of the cubic lattice is not sufficient to explain the huge lattice-mismatch induced enhancement in K-values. Thus, we will explain these effects by more severe strain induced structural phase deformations. Further, dielectric properties of epitaxial oxide thin films grown on Si have been found to improve significantly by incorporation of suitable dopants. We observe substantial reduction of the leakage current density in nitrogen-doped Gd₂O₃ layers. To achieve optimum electrical properties from such doped oxides it is important to understand the correlation between doping and the electronic structure of the material. X-ray photoelectron spectroscopy investigations revealed band gap narrowing in epitaxial Gd₂O₃ due to nitrogen doping, which leads to reduction in the valence band offset to Si. The observed reduction of the leakage current densities in the these layers with increasing nitrogen content suggests that nitrogen doping can be an effective route to eliminate the adverse effects of the oxygen vacancy induced defects in the oxide layers.