Crystalline Oxides on Silicon

Abstract

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 gate dielectric replacements in future Metal Oxide Semiconductor (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. On Si (100) oriented surfaces, crystalline Gd₂O₃ grows as (110)-oriented domains, with two orthogonal in-plane orientations. Layers grown under best vacuum conditions often exhibit poor dielectric properties due to the formation of crystalline interfacial silicide inclusions. Additional oxygen supply during growth improves the dielectric properties significantly. Experimental results for Gd₂O₃-based MOS capacitors grown under optimized conditions show that these layers are excellent candidates for application as very thin high-k materials replacing SiO₂ in future MOS devices. Epitaxial growth of lanthanide oxides on silicon without any interfacial layer has the advantage of enabling defined interfaces engineering. We will show that the electrical properties of epitaxial Gd₂O₃ thin films on Si substrates can further be improved significantly by an atomic control of interfacial structures. The incorporation of few monolayers of Ge chemisorbed on the Si surface has been found to have significant impact on the electrical properties of crystalline Gd₂O₃ grown epitaxially on Si substrates. Efficient manipulation of Si(100) 4° miscut substrate surfaces can lead to single domain epitaxial Gd₂O₃ layer. Such epi-Gd₂O₃ layers exhibited significant lower leakage currents compared to the commonly obtained epitaxial layers with two orthogonal domains. For capacitance equivalent thicknesses below 1 nm, this difference disappears, indicating that for ultrathin layers, direct tunneling becomes dominant. Further, we investigate the effect of post-growth annealing on layer properties. We show that a standard forming gas anneal can eliminate flat-band voltage instabilities and hysteresis as well as reduce leakage currents by saturating the dangling bonds caused by the bonding mismatch. In addition, we investigated the impact of rapid thermal anneals on structural and electrical properties of crystalline Gd₂O₃ layers grown on Si. Finally, we will present a new approach for nanostructure formation which is based on solid-phase epitaxy of the Si quantum-well combined with simultaneous vapor-phase epitaxy of the insulator on top of the quantum-well. Ultra-thin single-crystalline Si buried in a single-crystalline insulator matrix with sharp interfaces was obtained by this approach on Si(111). In addition, structures consisting of a single-crystalline oxide layer with embedded Si nano-clusters for memory applications will also be demonstrated.