Atomic Layer Deposition of titanium, zirconium and hafnium dioxides: growth mechanisms and properties of thin films.

Jaan Aarik
Abstract & Cover

The research described in this thesis allowed comparison of several ALD processes for deposition of TiO2, ZrO2 and HfO2. The processes that were studied included those described earlier as well as new ones (TiOC3H7)4-H2O2, ZrC14-H2O-H2O2 and Hf14-02) reported in the original publications of this thesis for the first time. For the TiOC3H7)4-H2O2 and ZrC14-H2O-H2O2 processes and also for several earlier-known ALD processes the real-time characterization of the reaction mechanisms was performed for the first time. The results of the work and comparison of those with literature data demonstrated that chlorides were the most stable metal precursors from those studied so far. They could be used in wide ranges of deposition temperatures and showed high reactivity in surface reactions.

The dependence of the growth rate on the substrate temperature was demonstrated to be in agreement with the changes in the growth mechanism, when crystallization did not influence the surface roughness and adsorption of precursors. In the opposite cases, the effect of crystallization on the growth rate might even exceed the changes related to variations in the mechanisms of exchange reactions. Most significantly the crystal growth influenced ALD of Tio, films. Tio, of the anatase phase grew markedly faster than the amorphous TiO2 phase did. This led to significant surface roughening of the TiO2 films, which contained mixtures of amorphous and anatase phases. Faster growth of anatase and the increase of the surface area with surface roughening resulted in a dramatic growth-rate increase with the transition from the amorphous to anatase phase. This kind of crystallization-related increase of the growth rate was observed with the increase of the growth temperature as well as film thickness. Thus, a common assumption that the film thickness is proportional to the number of ALD cycles employed does not apply in the cases, when the degree of crystallinity increases and the texture becomes more developed with increasing film thickness. The development of crystallinity and texture with increasing film thickness is, however, a very usual phenomenon in ALD of polycrystalline thin films.

The growth rate of ZrO2 and HfO2 thin films did not depend on crystallization as strongly as the growth rate of TiO2 did. Nevertheless, evidence of this kind of effect was found in original studies of this thesis as well as in publications of other authors. The studies performed also demonstrated that the choice of metal precursors and deposition temperature significantly influenced ALD of HfO2 in the initial stage of the deposition on silicon substrates. A marked delay and the three-dimensional nature of the film growth was observed in the high-temperature chloride processes. Much more uniform growth without measurable delay was obtained for the iodide-based and low-temperature (300°C and lower) chloride-based ALD processes.

Crystallization of ZrO2 and HfO2 films in the ALD processes caused an expected increase of the optical density and dielectric constant. In the case of TiO2, by contrast, inhomogeneous crystallization of non-epitaxial films sometimes resulted even in the decrease of the mean density compared with that of amorphous films. The optical density of epitaxial films was, however, always higher than that of the amorphous phase and reached the values of respective single crystals.

The optical band gap of amorphous TiO2 was found to be wider than the band gaps of crystalline TiO2 phases. On the contrary, the band gaps of amorphous phases of ZrO2 and HfO2 films were comparable to or even narrower than the band gaps determined for crystalline phases of corresponding oxides. Optical studies also revealed that the absorption spectra of monoclinic ZrO2 and HfO2 phases markedly differed from the absorption spectra of amorphous, cubic and tetragonal phases. Although similar differences between the absorption spectra of amorphous and crystalline (monoclinic) HfO2 films have recently been observed in works of other authors, too, and explanations to these differences given, verification of the main reasons for this effect needs further studies.


Source of Information
Thesis document
Institute of Experimental Physics and Technology, Institute of Material Science and Institute of Physics, University of Tartu
(Tartu, Estonia)
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