The dielectric loss in amorphous, thin-film oxide insulators produces a real part of the ac conductivity σ′(ω) that scales as ωs with s∼1. Conventional models explain this frequency dependence by hopping or tunneling of charge between neighboring defect sites. These models fail at low temperatures since they predict that σ′ should vanish at T=0. We observe that the ac conductivity of Ta2O5, ZnO, and SiO2 has a nonzero extrapolated value at T=0. We propose that this behavior is consistent with the predictions of a Coulomb glass, an insulator with a random distribution of charged defects.

Ta 2 O 5 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, ∼1019–1020 cm−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.

This communication demonstrates the combined use of two techniques for fabricating transistors that incorporate an organic semiconductor and that can operate at low voltages: anodization for thin (similar to 50 nm), high-capacitance gate dielectrics and microcontact printing on electroless silver for high-resolution (similar to 1 mu m) source/drain electrodes. The techniques are attractive for use with organic active components because (i) they are carried out at or near room temperature in aqueous solutions, (ii) they are compatible with important organic semiconductors and flexible plastic substrates, and (iii) they are suitable for reel-to-reel processing, n- and p-Channel devices formed with the organic semiconductors dihexyl quinquethiophene and copper hexadecafluorophthalocyanine and anodized tantalum and silicon substrates illustrate the typical performance of transistors fabricated with these methods.