| AVS 56th International Symposium & Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM2-TuM |
| Session: | Complex and Multifunctional Oxides |
| Presenter: | L. Miotti, North Carolina State University |
| Authors: | L. Miotti, North Carolina State University K.P. Bastos, North Carolina State University G. Lucovsky, North Carolina State University D. Nordlund, Stanford Synchrotron Research Lightsource |
| Correspondent: | Click to Email |
The use high dielectric constant materials (high-κ) as gate dielectric in complementary metal-oxide-semiconductor devices based on Si and other higher carrier mobility semiconductors has been focus of intense research. So far, hafnium oxide is among the high-κ dielectrics with higher dielectric constant and higher technologic interest. The most common phase of HfO2 thin films after deposition and processing is monoclinic with a dielectric constant of κ~ 16-20. However, its dielectric constant can be improved by increasing it in its tetragonal (κ~ 30-35) or cubic phase (κ~ 20-25), which are its thermodynamic equilibrium phases at higher temperatures
We report here on the investigation of Ge doping HfO2 films and its role in stabilizing the tetragonal phase of hafnium oxide phase. The films were deposited by remote plasma enhanced chemical vapor deposition on both Si(001) and Ge(001) substrates. Hf-t-butoxide and GeH4 were used as source for Hf and Ge, while O2 as the oxidizing agent. Germanium concentration in the films was determined by Auger electron spectroscopy. Interface reaction was hindered by nitridation of the substrates before the deposition of the films. Conventional x-ray diffraction analysis is not suited to investigate the crystallographic order in very thin films (< 10 nm), as well as those of interest for device applications. Therefore, we probed the local symmetries by x-ray absorption spectroscopy using the beam line 10 in SSRL.
The x-ray absorption spectra for 2 and 5 nm thick HfO2 films deposited on Si with 0, 5, or 15 at.% Ge, as well as the second derivative of these spectra was studied. Based on previous studies of HfO2 films on Si, the shape of the transitions to the Eg and T2g states of the 5 nm thick film without Ge doping corresponds to a monoclinic phase with a different Jahn-Teller distortion than the trigonal. A significant change in these features is observed when the film is doped with 5 at.% Ge. In this case the Eg and T2g transitions correspond to a tetragonal symmetry. Higher germanium doping leads to a mixture of monoclinic and tetragonal phases causing a broadening of the absorption spectra. There is not significant difference in the absorption spectra for the 2 nm thick films with different doping, indicating that the Ge stabilization of a tetragonal phase can only be achieved above a critical thickness.
The mechanism that stabilizes the tetragonal phase of HfO2 using a tetravalent dopant is significantly different from that associated with trivalent impurities as yttrium, which involves oxygen vacancies. This mechanism results in decrease of the c/a ratio with no vacancies involved.