Our research on high-k materials, such as hafnia, zirconia, and other transition metal oxides, origins from the fact that these materials have replaced
silica as gate dielectric in the current generation of FETs. Hafnia, especially, is used due to its thermodynamic stability against Si, wide band gap,
and high dielectric constant (k=20-25). The high dielectric constant (or high-k) of hafnia allows maintaining the gate capacitance and therefore the
drain-source saturation current without reducing the oxide-thickness. Thus the use of high-k dielectrics enables continued scaling of the complimentary
metal oxide semiconductor (CMOW) technology known as Moore's Law. Because different phases, orientations, defects, and terminations of surfaces and interfaces
may lead to non-uniformity in the electrical properties of the device, a deeper understanding the microscopic nature of high-k material films and interfaces
with silicon dioxide is critically important to further scaling of CMOS technology. We study many perspectives of high-k materials:
1. Phase transitions in hafnia and zirconia
We use a combination of density functional theory and calorimetric measurements to investigate the martensentic transformation in monoclinic hafnia and zirconia. Using first principles calculations we can find the total energy of cubic, tetragonal and monoclinic hafnia and zirconia. We find that the calculated energy difference between different phases agrees very well with the transformation enthalpy measured during the martensitic phase transition in hafnia. The phonon dispersion relationship and phonon density of states of hafnia and zirconia are presented. Then the Gibbs thermodynamic potentials of the tetragonal phase and the monoclinic phase in both material can be estimated using the total energy and entropy calculated from the phonon spectrum. Our calculation shows that the difference of the Gibbs thermodynamic potentials between two phases vanishes around 1820K and 1500K respectively for hafnia and zirconia, which agree well the experimental transition temperature. Further more we analyze the density of states of phonons at ¬¤ point for both monoclinic hafnia and zirconia and find the theoretical spectrums agrees the raman spectra very well. Finally using transition state theory we identify the minimum energy path (MEP) and transition state for the matensitic transformation. We measure the transition temperature, the volume change and the enthalpy of martensitic transformation in hafnia using DSC and TMA all in good agreement with calculated values.
2. Surface properties of hafnia films
Using density functional theory (DFT) we investigate surface energies of monoclinic and tetragonal hafnia films in search for thermodynamic means of controlling the film microstructure. We report the atomic and electronic structure of these films including the surface energy, work function and electron affinity.
3. Theoretical study of the insulator/insultor interface
We study theoretically the band alignment atthe technologically important SiO2/HfO2 interface using density functional theory. We report several differentatomic level models of this interface along with their energies and electronic properties. We find that the valence band offset increases near linearly with the interfacial oxygen coordination, changing from ?2.0 eV to 1.0 eV. For the fully oxidized interface the Schottky limit is reached. We propose a simple model, which relates the screening properties of the interfacial layer to the band offset. Our results may explain a somewhat confusing picture provided by recent experiments.
