Interface of tantalum oxide films on silicon by UV annealing at low temperature
Introduction
We have previously reported the growth of 4–10 nm Ta2O5 films on c-Si by photo-induced chemical vapour deposition (photo-CVD) using 222 nm excimer lamps and a special precursor injection system, which exhibited leakage currents as low as 2.19×10−7 A/cm2 at 1 MV/cm [1], [2], [3]. These layers were nonstoichiometric and their optical and electrical properties were strongly affected by both the O/Ta ratio and their interfacial properties. More recently we have found that UV annealing at low temperatures can significantly improve the electrical properties of these films [4], [5], [6], [7], [8]. In this paper, we investigate the interface changes in the Ta2O5/SiO2/Si system as a function of UV annealing using X-ray photoelectron spectroscopy (XPS), FTIR and TEM. Improvements in both the interfacial and electrical properties of these layers by UV annealing in pure oxygen and other atmospheres are also discussed.
Section snippets
Experimental details
The experiments were performed on n-type 4-in. Si (1 0 0) substrates (resistivity 2–4 Ω cm (i.e. Nd∼1×1015/cm3), which received a standard FSIB clean. Tantalum tetraethoxy dimethylaminoethoxide (Ta(OEt)4(dmae)) (made by Inorgtech. Ltd, UK), dissolved in an anhydrite cyclohexane solvent (10%), was the precursor used. Nitrous oxide (N2O) was introduced as the oxidising agent at a fixed flow rate of 20 sccm. A 60 sccm argon plus 20 sccm N2O gas flow carried the precursor through a showerhead at 110
Results and discussion
Films with thickness approximately 40 nm deposited at a temperature of 350 °C, were UV annealed in 1000 mbar in pure oxygen at 350 °C for 10, 15 and 40 min, respectively. Fig. 1 shows TEM cross-section images of these layers. As can be seen the Ta2O5 films (top layer) are amorphous in both the as-deposited and UV annealed states. This was also confirmed by XRD [7]. Also the interfacial layer remains constant during 10-min annealing and then increases with annealing time up to 40 min. This
Conclusions
UV annealing of photo deposited Ta2O5 films has been performed in 1000 mbar of pure oxygen under various conditions, leading to an increase in thickness of the interfacial layer with Si and a dramatic improvement of the layer electrical properties. This improvement is due to conversion of suboxides in the layer into more stoichiometric Ta2O5 and the formation of SiO2 at the interface of the Ta2O5 with Si. Reactive oxygen radicals formed by the high photon energy of the 222 nm excimer lamps
Acknowledgements
This work was supported by the European Commission under IST Research project No. 10541 tantalum pentoxide photodeposition on silicon (TOPS-II).
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