Leakage currents and conduction mechanisms of Ta2O5 layers on Si obtained by RF sputtering

The leakage current and the conduction mechanisms in Ta2O5 layers on silicon with thickness in the range of 20-80 nm, obtained by reactive sputtering of Ta in an Ar/O-2 mixture have been investigated. Some dielectric and electrical properties, important for the application of the layers as storage capacitors in high-density dynamic random access memories (DRAM) and as a gate dielectric in metal oxide silicon transistors (MOSTs) are also considered. The results show that for as-deposited layers the leakage currents are higher for samples obtained at higher deposition temperature. The effect of postdeposition oxygen annealing depends on the thickness of Ta2O5 layers. For thicker layers (40 nm), the leakage current after annealing increases and the effect is stronger for layers deposited at T-s = 493 K, It has been established that for thinner oxides (25 nm) the annealing strongly improves the leakage currents (the density of leakage current is 10(-7) A/cm(2) at applied fields of about 1 MV/cm, which is low enough to satisfy the demands of 64 and 256 Mbit DRAM). For as-deposited samples the conduction mechanism is Poole Frenkei. After annealing depending on the field strength, different types of conduction mechanisms occur: for electric fields in the range 0.8-1.3 MV/cm, the conduction mechanism is dominated by electrode limited Schottky emission and for higher fields (> 1.5 MV/cm) it is bulk limited Poole Frenkel emission. The results are discussed in terms of bulk traps in the initial Ta2O5 and their modification after oxygen annealing, (C) 2000 Elsevier Science Ltd. All rights reserved.