Epitaxial oxide heterostructures on silicon

Silicon-based MOSFETs will soon be limited by the large off- state leakage current due to tunneling through the ~1 nm thick silicon oxynitride gate dielectric layer. One solution is to replace the silicon oxynitride with a high dielectric constant material, such as LaAlO$_3$, which has a relatively large dielectric constant of \~24 and band gap of 5.6 eV. We have recently grown LaAlO$_3$ epitaxially onto silicon via a transition layer consisting of SrTiO$_3$. The thickness of this SrTiO$_3$ layer is kept between 2 and 5 unit cells because of considerations of epitaxial strain and the atomic-scale interactions between the perovskite structure and the silicon substrate. The oxide heterostructures show atomically abrupt interfaces and dielectric constants close to the bulk value of LaAlO$_3$. Frequency and voltage dependent measurements of the complex impedance of the as-grown oxide heterostructures show a pinned Fermi level and a high density of interface states. Annealing at low temperatures in wet oxygen shows that the Fermi level can be unpinned, with a greatly reduced density of interface states.