The relation between crystalline phase, electronic structure, and dielectric properties in high-K gate stacks

As high permittivity dielectrics approach use in metal-oxide-semiconductor field effect transistor (MOSFET) production, an atomic level understanding of their electronic and dielectric properties is being rigorously examined. In our work we illustrate studies leading to such an understanding for the important materials HfO ₂ and ZrO ₂. Valence and conduction band densities of states for HfO ₂/SiO ₂/Si and ZrO ₂/SiO _xN _y/n-Si structures were determined by soft X-ray photoemission and inverse photoemission. First principles calculations were used to help in assigning valence band maxima and conduction band minima. The energies of defect states at the band edges were determined by comparing the theoretical and experimental results. From this information, we are able to show that both of these dielectric materials have high enough barriers for both electron and hole transfer. We show that the crystal structure in ultrathin ZrO ₂ films has considerable effects on permittivity as well as bandgap. The films reported here are predominantly amorphous below a critical thickness (∼5.4 nm) and transform to the tetragonal phase upon annealing, while thicker films appear tetragonal as grown. Finally, bandgaps obtained from combined PES and IPES studies were compared with the optical bandgap derived from ellipsometry measurements. The difference in the bandgap values found in this comparison can be attributed to the final state effects in the excitation processes of the spectroscopies involved. We discuss the interplay of the dielectric's crystal phase, defects and electronic properties, as well as the impact of this understanding on possible tailoring of the film phase for improving band-gap, band-offset and leakage. Finally, we note that lack of sufficient understanding of the dielectric's phase and electronic properties can have negative impact on the ability to correctly determine a film's EOT.