High-K Dielectrics–Studies on (Ta2O5)1-x– (TiO2)x, (0 ≤ x ≤ 0.11), Thin Films

Abstract

Recently the increasing demand for miniaturized electronic gadgets has raised the interest in searching the high dielectric constant (K) materials for memory elements. Among the investigated high dielectric constant metal oxides, complex perovskites, such as barium titanate, strontium titanate, etc. and their solid solutions; and binary oxides, such as tantalum pentoxide, titanium oxide, zirconium oxide, etc. and their solid solutions are prominent. Due to the toxicity and compatibility issues of many complex perovskite oxides with the current fabrication procedures, binary oxides have been a preferred choice to use as a gate dielectric in memory devices. Among the binary oxides, tantalum pentoxide (Ta2O5) was found to have high dielectric constant and good compatibility with the existing fabrication procedures of silicon devices and a potential to replace the silicon-based dielectrics, such as SiO2, SiOxNy, etc., in the memory devices. With the addition of TiO2, ZrO2, etc., the dielectric constant of bulk Ta2O5 was found to increase by manifold, and so the potential of miniaturization to improve in that proportion was envisaged. The present study was carried out to search the high dielectric constant and low leakage current thin films of Ta2O5 ¬– TiO2 compositions for memory devices. Thin films of different compositions of (Ta2O5)1−x–(TiO2)x (TTOx) were prepared using mosaic and ceramic targets and the structural, electrical, and optical properties of prepared films were investigated. TTOx thin films with compositions, x = 0, 0.03, 0.06, 0.08, and 0.11, were deposited onto the silicon and quartz substrates by direct current (DC) magnetron sputtering. The as-deposited thin film samples were annealed, in the ambient air, at 500, 600, 700 and 800 ˚C, for 1.5 h. The dielectric constant was found significantly depending on the composition and annealing temperature. Among the prepared compositions, at 1 MHz, the highest dielectric constant 71 was observed for x = 0.06, annealed at 700 ˚C. Using the measured optical transmittance, different optical parameters, viz., refractive index, extinction coefficient, and optical bandgap of the prepared films were obtained and found strongly dependent on the annealing temperature. The observed current–voltage (I–V) characteristics show the decreasing leakage current density with increasing annealing temperature. On annealing at 800 ˚C, the C–V and I–V characteristics of the deposited film compositions were found affected significantly due to the growing interfacial SiO2 layer at the film–substrate (Si) interface. The electrical properties and existing different current conduction mechanisms in different electric field regions were observed depending on the growing interfacial SiO2 layer with increasing annealing temperature. A comparative study of the TTOx films of the same composition prepared following two different routes: first, by the radio frequency (RF) sputtering of the ceramic target; and second, by DC sputtering of mosaic (Ta, Ti) metal target, in the presence of oxygen, was also carried out. The same post-deposition treatment was followed for both types of films deposited from two different routes. The structural, electrical, and optical properties with current conduction mechanisms, for the films prepared from the two different routes, were measured, compared and analyzed. The observations have been described in the present thesis.