Properties of amorphous and crystalline Ta2O5 thin films deposited on Si from a Ta(OC2H5)5 precursor

Abstract

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.

Dielectric function of amorphous tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry

“…Prior to application of sputter-deposited films it is necessary to understand the relation between microstructure, growth conditions, and resulting electrical and optical properties. A through optical study of such microstructure related effects was shown previously for magnetron sputtered boron nitride thin films [18][19][20][21] Tantalum oxide films ͑TOF͒ have been grown by a large variety of physical vapor deposition and chemical vapor deposition ͑CVD͒ processes, including ion beam sputtering, 22,23 magnetron sputtering, 2,7,9,24 ͑low temperature͒ thermal oxidation, 9,25 anodization, 26 ion plating, 1,27 electronbeam evaporation, 4,27 laser ablation, 28,29 low pressure CVD, 14,30 photo-CVD, 31,32 atomic layer deposition, 33 and plasma-enhanced CVD. 12,16,34 A metalorganic solution deposition technique 6 was also used to grow TOF.…”

“…3 Thin films of tantalum oxide are also employed as an ion conductor in electrochromic devices. 4,5 Known as a material with high static dielectric constant and with good electrical insulating properties, tantalum oxide has the potential for microelectronic applications as a capacitor and gate material, thereby replacing silicon dioxide in metal-insulator-metal [6][7][8][9][10][11] or metal-insulator-semiconductor [6][7][8][9][10][12][13][14][15][16][17] structures as well as in large-scale integrated circuits for dynamic random access memories. 6 Sputter-deposited thin films tend to show film porosity, which affects electrical or optical properties.…”

“…Postannealing of the amorphous samples at elevated temperatures above 700°C causes crystallization. 16 Depending on the annealing history, a͒ Electronic mail: franke@engrs.unl.edu either orthorhombic 6,8,15,29 or hexagonal 14 phases can be obtained.…”

On a current mechanism in Ta2O5 thin films

“…Leakage current-voltage characteristics have been explained in most cases by the Frenkel-Poole (FP) mechanism (bulk limited currents) [3][4][5][7][8][9][10][11][12][13] or by the Schottky emission of electrons from electrodes [2,7]. However, these models often faced difficulties when a quantitative comparison with experimental data was performed.…”

“…There are many studies on the leakage current mechanism in thin films grown by various methods, with different electrode materials deposited on different substrates [2][3][4][5][6][7][8][9][10][11][12][13]. Leakage current-voltage characteristics have been explained in most cases by the Frenkel-Poole (FP) mechanism (bulk limited currents) [3][4][5][7][8][9][10][11][12][13] or by the Schottky emission of electrons from electrodes [2,7].…”