Defect dominated charge transport in amorphous Ta2O5 thin films

Ta2O5 is a candidate for use in metal-oxide-metal (MOM) capacitors in sever al areas of silicon device technology. Understanding and controlling leakag e current is critical for successful implementation of this material. We ha ve 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 method s are similar if one compares films that have the same thickness and electr odes. 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 proce ss involves either tunneling conductivity into states near the Fermi energy or ion motion. The steady state process, seen most commonly in films < 300 Angstrom thick, is dominated by a large number of defects, similar to 10(1 9)-10(20) cm(-3), located near the metal-oxide interfaces. The interior of thick Ta2O5 films has a substantially reduced number of defects. Modest hea ting (300-400 degrees C) of Ta2O5 in contact with a reactive metal electrod e 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 su ccessful integration of Ta2O5 into semiconductor processing requires a bett er understanding of the impact of defects on the electrical characteristics and a better control of the metal-Ta2O5 interface. (C) 2000 American Insti tute of Physics. [S0021-8979(00)03514-3].