Band alignment in Ge/GeO$_{\mathrm{x}}$/HfO$_2$/TiO$_2$ heterojunctions as measured by hard x-ray photoelectron spectroscopy

Hafnium based high-k materials have been widely studied to replaced SiO$_2$ as a gate insulator in field effect transistors. Apart from offering low equivalent oxide thickness, they also offer a favorable band offset with Si. The development in the field of high-k dielectrics has also reduced the significance of Si/SiO$_2$ interface, thus opening new possibilities with high mobility semiconductors such as Ge. It is well known that the leakage current of a gate stack is dependent on the dielectric constant and the tunnel barrier height. Based on the current scaling trend, an oxide with k $\sim$ 40 would be ideal. Among the widely studied oxides TiO$_2$ is known to have a very high dielectric constant. However the poor conduction band offsets with both Si and Ge, makes it completely impractical as a gate oxide material. The problem of poor conduction band offset has been addressed by introducing a suitable interlayer with higher conduction band offset. In our work we investigate the interlayer thickness dependence of band alignment in a germanium based bilayer metal-oxide-semiconductor sandwich with an amorphous HfO$_2$ and TiO$_2$ high k gate dielectric using hard x-ray photoelectron spectroscopy. We see a strong evidence of intermixing at Hf-Ge interface and a deviation from bulk offset for ultra thin HfO$_2$.