Frequency-dependent interface capacitance of Al-Al2O3-Al tunnel junctions

Four-terminal ac impedance measurements have been used to characterize Al-A l2O3-Al tunnel-junction capacitors over the frequency range of 10 Hz-100 kH z. The insulating barriers are thin enough to assure that the response can be modeled by a frequency-dependent interface capacitance in parallel with a frequency-independent tunnel junction resistor R-o. The data reveal no si gn of loss peaks down to 10 Hz and the impedance curves for a single juncti on, annealed to give different tunnel-junction resistance, collapse onto a single curve when R-o is used as a scaling parameter. The loss mechanism is ascribed to interface traps and is found to give an unusual asymptotic pha se angle response when the real and imaginary parts of the complex capacita nce are plotted against each other. (C) 1999 American Institute of Physics. [S0003-6951(99)05002-0].