Conduction mechanisms in nanolaminates and mixture films grown by atomic layer deposition
|Indrek Jõgi 1, Kaupo Kukli 1, Mikko Ritala 2, Markku Leskela 2|
1. Department of Physics, University of Tartu, Tähe Street, Tartu 51010, Estonia
Interest in high-permittivity oxide films has grown mainly due to the replacement of silicon oxide in transistors and memory capacitors. Besides single metal oxides, attempts have also been made to tailor useful properties of different oxides, combined as mixed materials of variable stoichiometry, or by deposition of stacked layers of different oxides in the form of nanolaminates. Oxides exhibiting good insulating properties, such as Al2O3, may be combined with oxides of high polarizability, such as TiO2. ALD processes are established, enabling conformal growth of dense oxide layers. However, combination of different materials induces additional amounts of bulk defects as well as multiple interfaces, possibly degrading the electrical properties of these insulators. In addition, different materials exhibit variable stability in contact to semiconductor substrates and metal electrodes, affecting the quality of contact charge barriers.
The Al1-xTixOy mixtures and Al2O3-TiO2-Al2O3 nanolaminates with variable composition and thicknesses were grown to investigate the electrical quality of such films. Several samples were annealed in N2 or O2 at 700 °C to study the effect of thermal post-deposition treatment. At similar capacitance values, films containing TiO2 remained inferior to Al2O3 films in terms of insulating properties. The analysis of current-voltage characteristics revealed strong influence on defects and impurities. However, the modeling of electron emission into layered high-k stacks has first suggested the preferred exploitation of insulating materials with wider band gap even in defect-free cases. Further, the modeling suggested superiority of higher k films (TiO2) when electrodes with higher work function were used, especially when EOT value of stack became smaller than 2 nm. In such films, the leakage in stack containing higher-k film (TiO2) remained smaller even when a discrete impurity level was included to take into account trap assisted currents.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium C, by Indrek Jõgi
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-13 19:04 Revised: 2009-06-07 00:44