Conduction mechanisms in thin atomic layer deposited films containing TiO2

Indrek Jõgi
Abstract & Cover

The present study was carried out in order to determine the conduction mecha-nisms in thin atomic layer deposited TiO2-containing films. For this purpose, MIM and MIS stacks with thin TiO2 and Al203-TiO2 films were prepared by ALD method. The electrical characteristics of resulting stacks were related to phase, chemical and morphological composition which in turn were influenced by the deposition parameters and electrode materials. The effect of post-deposition annealing on the electrical properties was also studied. The analysis of results was carried out on the basis of a number of samples prepared at the same conditions. As a rule, in the case of Mo/Ti02/A1 stacks, space charge limited currents were assigned as dominant conduction mechanism and the electrodes had only slight effect on the currents. The current densities were determined by the TiO2 layer and depended strongly on deposition parameters. At deposition tempera-tures higher than 200°C, the films grew polycrystalline and possessed extremely high leakage currents. At lower deposition temperatures the films grew domi-nantly amorphous and became more insulating. The decrease in temperature increased the amount of defects which further decreased the leakage currents. At the same time, the long term stability also decreased. Besides the growth temperature, the currents depended also on the precursor chemistry. The annealing procedure used in the present study did not improve the insulating properties of TiO2 films in metal-insulator-stack. In the case of some Mo/Ti02/A1 stacks the interface layer between aluminum electrode and TiO2 became insulating. The insulating layer controlled the leakage currents and complicated the analysis of conduction mechanism. Similar insulating layer appeared also in the case of Si substrate. Quality of the Si substrate, i.e. the presence of SiO2 interfacial layer between TiO2 and Si substrate had a significant effect on the conductivity of the TiO2-based capacitor stacks. Numerical calculations were used to account the effect of interface layers on the conduction mechanism. The calculations indicated that at low voltages, trap assisted tunneling through TiO2 layer was prevailing whereas at higher voltages the tunneling through thin interface layers dominated. The comparison of calculations and experimental results allowed one to evaluate the affmity of our TiO2 films which was between 3.8-4.0 eV. Use of electrodes with higher work-function decreased the current through metal-insulator-metal stacks. In the case of Au electrode, Fowler-Nordheim emission through Au/TiO2 barrier controlled the currents. In the case of Pt electrode, tunneling trough traps apparently reduced the effect of higher barrier at Pt/TiO2 interface and the conductivity of stacks was controlled by Poole-Frenkel effect. The latter result highlights the influence of electrode preparation teclmology on the dominant conduction mechanism. 
The use of Al203 together with TiO2 in mixture and nanolaminate con-figurations resulted in more insulating films. Both experimental results and numerical calculations demonstrated that the insulating properties of Al203-TiO2 mixture and laminate films will still remain inferior to pure Al203 in MIS structures. Nevertheless, the use of electrodes with higher work function electrodes would make TiO2 containing films more attractive due to higher permittivity value. 

Source of Information
Väino Sammelselg
University of Tartu
(Tartu, Estonia)
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