Charge Transport in Anodized Ta 2 O 5 Thin Films. C. D. W. Jones, R. M. Fleming, D. V. Lang, M. L. Steigerwald, D. W. Murphy, B. Vyas, G. B. Alers, Y.-H. Wong, R. B. van Dover, J. R. Kwo, A. M. Sergent Bell Laboratories, Lucent Technologies 600 Mountain Ave., Murray Hill, NJ 07974 INTRODUCTION Ta 2 O 5 has been actively investigated for more than fifty years and is an established dielectric material in discrete capacitors made from anodized, porous tantalum substrates. A comprehensive review of the anodization literature to 1975 has been published (1). More recently, the potential new use of Ta 2 O 5 as a thin film dielectric material in silicon devices has promoted an even greater interest in this material (2). However, successful integration of Ta 2 O 5 into silicon devices requires a more detailed understanding of the charge transport mechanisms in this material than is currently available. This is the focus of the current work. Comparisons can also be made to the properties of other anodized films (HfO 2 , ZrO 2 , etc.). EXPERIMENTAL Amorphous Ta 2 O 5 thin films were synthesized by anodization in 0.01M citric acid. The film thicknesses were measured by ellipsometry and varied between 100 Å and 1000 Å. The film properties were studied in a metal- oxide-metal geometry. For most of the studies, 200 μm diameter circular top metal electrodes were applied using e-beam evaporation and a shadow mask. Electrical characteristics of the Ta 2 O 5 films were measured as a function of electric field, time, temperature (100-500 K), incident light energy (photoconductivity) and electrode material. RESULTS AND DISCUSSION We have identified two types of charge transport. The first is a steady state current originating from a large number of defects (10 19 – 10 20 cm -3 ) located near the metal/oxide interface. The second is a transient current that decays as a power-law in time. Steady state current occurs by means of a defect band located about 1.2 eV below the conduction band. Transport within the defect band is temperature and field dependent and can be qualitatively described using the model of phonon assisted tunneling (3). The defect band is associated with oxygen vacancies since modest heating (250 - 400 °C) of the anodized film increases both the steady state conductivity and the number of defect states observed in the gap. Similar increases can be observed if one deposits reactive metals such as Al, Ti or Ta as a top electrode. Using photoconductivity, we have shown that the defects responsible for the steady state conductivity are located within a few hundred angstroms of the metal/oxide interface and are reduced in the bulk of the film. Steady state currents are eliminated in thick films (> 300 Å) where the defect band does not span the width of the film (Figure 1). It is also absent in certain biases, if the energy difference between the metal electrode and the defect band is too large for thermal activation (Figure 1). Transient conductivity is best observed in the absence of the steady state conductivity. The power-law current decay is a well-known phenomenon in metal-oxide films, and has been observed before in Ta 2 O 5 (1, 4, 5). However, the mechanism of transient conductivity is not well understood. Ionic mechanisms have been proposed, and are supported by our observation that transient conductivity is insensitive to above band gap light. However the ionic models do not explain the weak temperature dependence of the transient behavior below room temperature. It is also not clear whether or not the mechanism of transient conductivity is coupled to the same type of defect that gives rise to steady state conductivity. REFERENCES 1. W. D. Westwood, N. Waterhouse, and P. S. Wilcox, Tantalum Thin Films, Academic Press, New York (1975). 2. C. Channeliere, J. L. Autran, R. A. B. Devine, and B. Balland, Materials Science and Engineering, R22, 269 (1998). 3. S. Makram-Ebeid and M. Lannoo, Phys. Rev. B, 25, 6406 (1982). 4. F. C. Aris and T. J. Lewis, J. Phys. D, 6, 1067 (1973). 5. D. A. Vermilyea, J. Electrochem. Soc., 104, 427 (1957). Figure 1: Thickness dependence of Ta 2 O 5 leakage current |J| (A/cm 2 ) plotted versus the applied electric field to the top electrode E (MV/cm) for a series of anodized films (Pt top contacts, Ta bottom contacts). For E < 0 the current is transient for all films until breakdown because of the large energy difference between the defect band and the Pt electrode. For E > 0 the current shows transient behavior for the thickest films and steady state behavior for the thinnest films. -6 -4 -2 0 2 4 6 10 -10 10 -8 10 -6 10 -4 10 -2 10 0 10 2 110 Å 210 Å 440 Å 620 Å 680 Å 1200 Å |J| (A/cm 2 ) E (MV/cm)