No abstract

Alternating current bridge, step response, and dc conduction measurements are reported on anodic oxide films on tantalum with counterelectrodes of gold and indium. The step response currents followed a t−n law, where n was from 0.9 to 1.0 depending on conditions. For a linear dielectric response, this corresponds to ε″ (ω) varying as ωn−1, i.e., as either ω−0.1 or independent of frequency, respectively. For low values of applied step voltages with specimens made in certain ways, the magnitude of the step response agreed with extrapolated bridge measurements. Deviations are discussed in terms of electronic space-charge effects. Below room temperature, ε″/T was constant as expected for a superposition of Debye losses with relaxation times determined by a thermal activation process with a flat distribution of activation energies. The slope of log J vs F1/2 plots for electronic conduction corresponded to the Schottky or Poole-Frenkel values depending on whether the Ta electrode was negative or positive.

The structural and electrical properties of oxygen-doped tantalum films, sputtered in a triode system, have been investigated. X-ray diffraction traces from films with a low oxygen content indicate the presence of bcc Ta with a strained lattice. The intensity of the (110) bcc Ta diffraction peak decreases with increasing oxygen content and is interpreted as a decrease in the amount of bcc Ta phase present. Over this same range the appearance and rise in intensity of the β-Ta (200) peak indicates the formation of β-Ta. As the oxygen content is further increased, the structural data indicate a reorientation of the tetragonal β-Ta (200) to the (202) planes with respect to the film surface, an effect which has been previously observed in diode-sputtered tantalum; further increase in oxygen content produces a stretching of the β-Ta lattice by as much as 10% of the value reported by Read and Altman. The room-temperature resistivity increases with increasing oxygen content until a ``plateau'' region is reached at ∼ 210 μΩ cm corresponding to 12 at. % oxygen, and together with the x-ray data the electrical measurements indicate a change of phase from bcc to β-Ta. Above approximately 40 at. % oxygen the resistivity increases rapidly suggesting the formation of tantalum oxide.

A dielectric loss model based on the tunneling of electrons from metal electrodes into traps in the insulator near the interface is described. The temperature dependence of the losses is controlled primarily by the energy distribution of the traps and the frequency dependence is controlled by their spatial distribution. Traps distributed uniformly in both energy and space result in essentially temperature- and frequency-independent losses. The model predicts an inverse dependence of losses on sample thickness. The step-response currents for the tunnel mechanism become nonlinear at high applied fields; however, the detailed behavior is very sensitive to the trap distribution assumed. For a particular trap distribution it is possible to have a polarity-dependent, irreversible step response.

Anodization current efficiency measurements were used to determine the oxygen content of sputtered tantalum films. Films consisting mainly of b.c.c. Ta had less than 5 at. % oxygen while 'normal' j3-Ta contained 10-15 at. % oxygen. Films with a more negative temperature coefficient of resistance had a higher oxygen content.The field strength required to cause oxide growth on 'normal' j3-Ta was lower than for b.c.c. Ta films or for oxygen rich j3-Ta films. The cause of these changes in field strength is not known. The dielectric constants of the oxides grown on all films were the same, but depended on the measurement conditions and history of the oxide.