Electrical resistance of metallic contacts on silicon and germanium during indentation
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W. C. Oliver Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
R. F. Cook, P. D. Kirchner, M. C. Kroll, and T. R. Dinger IBM Research Division, Thomas J. Watson Research Center, Yorktown Heights, New York 10598
D. R. Clarke Materials Department, University of California at Santa Barbara, Santa Barbara, California 93106 (Received 1 July 1991; accepted 11 December 1991)
The effects of indentation on the electrical resistance of rectifying gold-chromium contacts on silicon and germanium have been studied using nanoindentation techniques. The DC resistance of circuits consisting of positively and negatively biased contacts with silicon and germanium in the intervening gap was measured while indenting either directly in the gap or on the contacts. Previous experiments showed that a large decrease in resistance occurs when an indentation bridges a gap, which was used to support the notion that a transformation from the semiconducting to the metallic state occurs beneath the indenter. The experimental results reported here, however, show that a large portion of the resistance drop is due to decreases in the resistance of the metal-to-semiconductor interface rather than the bulk semiconductor. Experimental evidence supporting this is presented, and a simple explanation for the physical processes involved is developed which still relies on the concept of an indentation-induced, semiconducting-to-metallic phase transformation.
I. INTRODUCTION High pressure experiments have shown that both silicon and germanium undergo phase transformations from the diamond cubic structure to the /3-tin structure at moderately elevated pressures.1"7 The /3-tin phase is metallic, and because of this, the transformation is accompanied by a large drop in electrical resistivity.7 The drop is so large, about 6 - 7 orders of magnitude, that it is frequently used as the primary indicator that the transformation has taken place.1>4'7 The pressures needed to trip the transformation from the semiconducting to the metallic state depend on the nature of the stress state. In silicon, the transformation under pure hydrostatic conditions takes place over the range 11.3-12.5 GPa,5 but this can be reduced to values as low as 8.0 GPa when shear stresses are present.' For germanium, the transformation pressure can be lowered from a value of 10.5 GPa for pure hydrostatic stresses6 to about 6.7 GPa when the stresses are highly deviatoric.6 The values quoted here for the transformation pressures take on special significance when compared to the indentation hardnesses of the materials. While there is some variation in the hardnesses reported for silicon and germanium, presumably due to indentation size effects, values typically fall in the range 10-12 GPa for silicon8^15 and 6 - 8 GPa for germanium. 1316 The signiJ. Mater. Res., Vol. 7, No. 4, Apr 1992
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ficance is that these hardnesses, which represent the mean pressures developed under the indenter, are at least equal
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