Defect dominated charge transport in amorphous Ta2O5 thin films

Ta₂O₅ is a candidate for use in metal-oxide-metal (MOM) capacitors in several areas of silicon device technology. Understanding and controlling leakage current is critical for successful implementation of this material. We have studied thermal and photoconductive charge transport processes in Ta₂O₅ MOM capacitors fabricated by anodization, reactive sputtering, and chemical vapor deposition. We find that the results from each of these three methods are similar if one compares films that have the same thickness and electrodes. Two types of leakage current are identified: (a) a transient current that charges the bulk states of the films and (b) a steady state activated process involving electron transport via a defect band. The transient process involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films <300 Å thick, is dominated by a large number of defects, ∼10¹⁹-10²⁰cm^-3, located near the metal-oxide interfaces. The interior of thick Ta₂O₅ films has a substantially reduced number of defects. Modest heating (300-400 °C) of Ta₂O₅ in contact with a reactive metal electrode such as Al, Ti, or Ta results in interfacial reactions and the diffusion of defects across the thickness of the film. These experiments show that successful integration of Ta₂O₅ into semiconductor processing requires a better understanding of the impact of defects on the electrical characteristics and a better control of the metal-Ta₂O₅ interface.