Electrical characterization of AlxTiyOz mixtures and Al2O3–TiO2–Al2O3 nanolaminates

Abstract

Mixtures and nanolaminates of Al2O3 and TiO2 with different alumina to titania ratios were prepared by atomic layer deposition. The studies were aimed at electrical characterization of metal-insulator-semiconductor capacitors formed by combining an insulating oxide with large band gap (Al2O3) with an oxide with high dielectric permittivity (TiO2). In mixtures, the ratio of Al2O3 and TiO2 growth cycles varied from 10:2 to 5:5 with target layer thickness in the range of 6.2–8.8nm. In Al2O3–TiO2–Al2O3 nanolaminates, the thicknesses of Al2O3 and TiO2 constituent layers were in the ranges of 3–6 and 2–15nm, respectively. Appreciable step coverage on deep trenched substrates with high aspect ratio (1:40) was achieved with short pulse and purge times otherwise suited for the deposition of planar capacitors. The measurements confirmed the model calculations of leakage currents for laminates, mixtures, and pure Al2O3 films predicting the lowest leakage for pure Al2O3 films at all possible equivalent oxide thickness (EOT) values. Inclusion of TiO2 as an oxide of higher permittivity but lower band offsets on Si considerably increased the leakage. Currents in the films became strongly affected by chemical and structural defects induced by the deposition process. The as-deposited films possessed higher EOT values and lower breakdown fields, compared to the model predictions. Flatband voltage shifts and hysteresis width of capacitance-voltage curves were also affected by built-in defects. Postdeposition annealing somewhat improved the dielectric performance of the films deposited.