Miniaturization of device dimensions in the microelectronics has put the materials challenges into new perspectives. The decreasing thickness of SiO₂ providing electrical insulation in CMOS circuits will enhance electron tunneling and dopant diffusion effects. Downscaling the gate length thus requires the replacement of SiO₂ with materials of higher permittivity, such as ZrO₂ or HfO₂, in order to increase the thickness without loss in the capacitance. Precise adjustment of the film thickness can be achieved by atomic layer deposition (ALD), where the growth proceeds via alternate surface reactions between submonomolecular layers of highly reactive metal and oxygen precursors. ALD can thus enable the growth of the films of high density at relatively low temperatures. The physical properties of the gate dielectrics can depend on the process, because of different composition of the films and thermal stability of the precursors. For example, ZrO₂ thin films have been grown from ZrCl₄, ZrI₄, or Zr[OC(CH₃)₃]₄ as metal precursors, while H₂O is used as oxygen precursor. HfO₂ films were grown, for instance, from HfCl₄, HfI₄, or Hf[N(CH₃)(C₂H₅)]₄, and H₂O. Alternative oxygen precursors such as H₂O₂ and O₂ have been considered. Halides and alkylamides provide intensely crystallized films mainly consisting of stable polymorphs, whereas the alkoxides result in the less dense nanocrystalline structures. Residual contamination also depends on the choice of the precursors and deposition parameters. Alkoxide precursors are less resistive to the thermal decomposition, compared to halides. Effective permittivity and leakage current density can depend both on the deposition temperature and the chemistry of the process.