(Invited) Epitaxial Oxides on Silicon for CMOS and Beyond
Authors: Osten H. Joerg 
Published: February 2020

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 ability to integrate crystalline metal oxide dielectric layers into silicon structures can open the way for a variety of novel applications which enhances the functionality and flexibility ranging from high-K replacements in future MOS devices to oxide/silicon/oxide heterostructures for nanoelectronic application in quantum-effect devices. We present results for crystalline gadolinium oxides on silicon in the cubic bixbyite structure grown by solid source molecular beam epitaxy. This material has a large band gap of about 6 eV and nearly symmetrical band offsets as well as a low lattice mismatch of about -0.4 % to Si. Layers grown by an optimized process display a sufficiently high-K value to achieve equivalent oxide thickness values below 1 nm, combined with ultra-low leakage current densities, good reliability, and high electrical breakdown voltage. A variety of MOS devices has been fabricated based on these layers.

The dielectric properties of such oxides are sensitive to small variations in structure and symmetry. It is known that thin layers of crystalline 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 25 although the known bulk value is only around 13. The reason for that “enhancement effect” is not fully understood yet. 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 will 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 (Gd1-xNdx)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.

We will finally demonstrate different approaches to grow Si nanostructures embedded into crystalline rare earth oxides. By efficiently exploiting the growth kinetics one could create nanostructures exhibiting various dimensions, ranging from three dimensionally confined quantum dots to the quantum wells, where the particles are confined in one of the dimensions. Double-barrier structures comprising epitaxial insulator as barriers and Si as quantum-well are attractive candidate for resonant tunneling devices. Embedded Si quantum dots (with average sizes below 5 nm) exhibit excellent charge storage capacity with competent retention and endurance characteristics suitable for nonvolatile memory device applications.