In this work, the structural and electrical properties of amorphous and crystalline Ta2O5 thin films deposited on p-type Si substrates by low-pressure chemical vapor deposition from a Ta(OC2H5)5 precursor have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta2O5 was obtained after postdeposition O2 treatment at 800 °C. As evidenced by x-ray diffraction, a hexagonal structure was obtained in the latter case. Physicochemical analysis of our layers shows that the O2-annealing step leads to the growth of a thin (∼1 nm) interfacial SiO2 layer but was not sufficient to reduce the level of hydrocarbon contamination. The dominant conduction mechanism in amorphous Ta2O5 is clearly due to the Poole–Frenkel effect, whereas the situation remains unclear for crystalline Ta2O5 for which no simple law can be invoked to correctly describe its conduction properties. From capacitance–voltage measurements, the dielectric constant was found to be ∼25 for amorphous samples, but values ranging from 56–59 were found for crystalline layers, suggesting a particularly high anisotropic character of the crystalline phase. Finally, the effects of postdeposition annealing in N2 and forming gas at 425 °C have been investigated for both types of films.

Defect state D (0.8 eV) was experimentally detected in Ta2O5 capacitors with ultrathin (physical thickness <10 nm) Ta2O5 films using zero-bias thermally stimulated current spectroscopy and correlated with leakage current. Defect state D can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing and by using TiN instead of Al for top electrode. We believe that defect D is probably the first ionization level of the oxygen vacancy deep double donor. Other important defects are Si/O-vacancy complex single donors and C/O-vacancy complex single donors.

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Flims of metal oxides, such as Ta2O5, Nb2O5, Al2O3, HfO2, ZrO2 and TiO2 have been fabricated by use of different precursor materials, deposition techniques and annealing techniques. Several analytical methods were applied to study the layers. New data of fundamental properties of these metal oxides are reported and related to practical features that are of importance in device design and manufacturing of advanced, highly integrated devices. This overview may facilitate the choice of an optimal combination of precursor material, deposition technique and corresponding annealing procedure for a specific application of these metal oxide films in microelectronics.

Defect states responsible for leakage current in ultrathin (physical thickness <10 nm) tantalum pentoxide (Ta2O5) films were measured with a novel zero-bias thermally stimulated current technique. It was found that defect states A, whose activation energy was estimated to be about 0.2 eV, can be more efficiently suppressed by using N2O rapid thermal annealing (RTA) instead of using O2 RTA for postdeposition annealing. The leakage current was also smaller for samples with N2O RTA than those with O2 RTA for postdeposition annealing. Hence, defect states A are quite likely to be important in causing leakage current.

Defect states in tantalum pentoxide films grown by low-pressure metal-organic chemical vapor deposition on silicon wafers have been studied with Al/Ta2O5/p+-Si and Al/Ta2O5/n+-Si capacitor structures by the zero-bias thermally stimulated current technique. It was demonstrated that a shallow band of defect states is responsible for leakage current. The shallow band of defect states can be suppressed by low-temperature post metallization annealing, resulting in a reduction of leakage current for both positive gate bias and negative gate bias. However, the reduction in leakage current for positive gate bias is much stronger than that for negative gate bias.

In this letter, the authors will point out that defect states related to oxygen vacancies in tantalum oxide capacitors can be suppressed by titanium doping, resulting in significant leakage current reduction. The theory is that titanium forms an acceptor which can move at high temperature and neutralize other donors. However, defect states which cannot be suppressed by titanium doping were detected. These are explained by H2O-related contamination occurring at low temperature (<400°C) during the cooling down period.

Cross-sectional transmission electron microscopy (XTEM), secondary ion mass spectrometry (SIMS) and capacitance measurements were used to study the effect of post-deposition annealing on Ta2O5/Si structures. A significantly thicker SiO x interfacial layer was formed at the Ta2O5/Si interface, if N2O was used instead of O2 for post-deposition annealing. This indicates that N2O is a stronger oxidizing agent than O2. It is known that the leakage current of Ta2O5 capacitors is greatly reduced if N2O is used instead of O2 for post-deposition annealing. This may also be partially explained by postulating that N2O annealing is more effective in the suppression of oxygen vacancies. Furthermore, the suppression of Si diffusion from the Si substrate into Ta2O5 due to the thicker SiO x interfacial layer can be another factor. The basic reason for the superiority of N2O is that the energy required to produce free O atoms is lower than that for O2. From this point of view, we can also predict that the use of NO will be worse than that of O2 because the energy required to produce free O atoms is higher than that of O2.