Crystalline Oxides on Silicon

Currently, researchers worldwide are primarily focused on investigating amorphous high-k materials. These materials are highly ionic metal oxides with relatively low crystallization temperatures. When these temperatures are reached, they tend to form polycrystalline phases, which are unsuitable for use as gate insulators. This is primarily due to the high leakage currents that occur along the grain boundaries and the resulting increased surface roughness. For well-known high-k materials like HfO₂ or ZrO₂, these crystallization temperatures are typically surpassed during a CMOS process. While it's possible to take steps to raise the crystallization temperature (such as adding Si or Al), this approach is always a trade-off, as it tends to decrease the k value, limiting the achievable layer thickness. An alternative solution is to use epitaxially grown crystalline high-k materials, which eliminate the risk of subsequent crystallization. This approach significantly enhances thermal process stability. Additionally, it offers a well-defined interface with silicon, providing better opportunities for interface engineering. However, it's important to note that growing epitaxial high-k materials on silicon requires a degree of compatibility in terms of atomic spacing and lattice symmetry, limiting the range of suitable materials. Apart from various epitaxial perovskite structures, certain binary metal oxides, especially epitaxially grown binary rare-earth oxides (as seen in the figure), are considered potential candidates.

The MBE Institute has dedicated several years to the epitaxial growth of crystalline rare-earth oxides on silicon substrates. Specifically, we have been employing gadolinium oxide (Gd₂O₃) and neodymium oxide (Nd₂O₃) as model systems. Both of these oxides share the same crystallographic structure, but there's a key difference: the lattice spacing of Gd₂O₃ is smaller than that of silicon, while that of Nd₂O₃ is larger than silicon's. Previously, ternary mixed layers of the form (Gd1-xNd_x)₂O₃ (where x = 0 ...1), were grown directly on silicon. This allowed for controlling the lattice mismatch by adjusting the composition. As a result, valuable insights into how lattice strain influences the dielectric properties of these ternary layers were gained [1].

[1] D. Schwendt: Charakterisierung von binären und ternären Seltene Erden Oxiden, Dissertation, Gottfried-Wilhelm Leibniz Universität Hannover, 2012.