Anomalous piezoresistance effect in ultrastrained silicon nanowires

In this paper we demonstrate that under ultrahigh strain conditions p-type single crystal silicon nanowires possess an anomalous piezoresistance effect. The measurements were performed on vapor-liquid-solid (VLS) grown Si nanowires, monolithically integrated in a microelectro-mechanical loading module. The special setup enables the application of pure uniaxial tensile strain along the <111> growth direction of individual, 100 nm thick Si nanowires while simultaneously measuring the resistance of the nanowires. For low strain levels (nanowire elongation less than 0.8%), our measurements revealed the expected positive piezoresistance effect, whereas for ultrahigh strain levels a transition to anomalous negative piezoresistance was observed. For the maximum tensile strain of 3.5%, the resistance of the Si nanowires decreased by a factor of 10. Even at these high strain amplitudes, no fatigue failures are observed for several hundred loading cycles. The ability to fabricate single-crystal nanowires that are widely free of structural defects will it make possible to apply high strain without fracturing to other materials as well, therefore in any application where crystallinity and strain are important, the idea of making nanowires should be of a high value.