Piezoelectric Effect on Piezoresistance of InAs/AlGaSb Heterostructure Nanobeam
Kenji Yamazaki1, Samir Etaki2, Herre S. J. van der Zant2, and Hiroshi Yamaguchi1
1Physical Science Laboratory, 2Delft University of Technology
Nanoelectromechanical systems (NEMS) are attracting much interest as sensors with very high sensitivities and as new-principle devices. Our group has reported mechanical systems using InAs/AlGaSb heterostructures and sensitivity enhancement using quantum effects on electrons in the structures. However, the basic mechanism of piezoresistance (i.e., resistance change due to mechanical strain) in such systems has not been understood well. In this study, we fabricated InAs/AlGaSb NEMS and measured/analyzed its piezoresistance. The results strongly imply that the piezoresistance in such systems is significantly affected by the piezoelectric effect and thus reveale, for the first time, the importance of the piezoelectric effect in mechanical systems using very thin heterostructures.
Figure 1 shows an atomic force microscopy (AFM) image of a fabricated double-clamped nanobeam made of an InAs/AlGaSb heterostructure. The thicknesses of InAs and AlGaSb layers are 15 and 35 nm, respectively. Depending on the depth of wet etching, which makes the nanobeam suspended, the shape of the beam becomes straight or arched. This is because the shear stress due to the lattice mismatch curls the wide suspended supports upward, and the beam linking the two supports thus becomes arched. The piezoresistance measured using an AFM tip for deflecting the beam had large dependence on the beam shape. That is, the gauge factor (GF) derived from the resistance change and strain, which usually reflects material properties, was positive or negative depending on whether the beam was straight or arched, and the magnitude was much larger than for bulk (See Table) . To try to understand these surprising phenomena, we calculated the piezoresistance, including the piezoelectric effect (Fig. 2). The results suggest that the piezoresistance is significantly affected by the piezoelectric effect for heterostructure thin film. That is, additional charges on the beam surface, which are induced by the piezoelectric effect when the structure is strained, change the resistance, and this effect is larger when the conductive layer is thinner and thus the carrier density is lower. Moreover, the opposite signs were also obtained for the arched and straight beams by calculation, which means that our understanding roughly explains the experimental results.
These results enhance our understanding of piezoresistance in NEMS, which could lead to more effective designs for them.
This work was supported in part by Japan Society for the Promotion of Science (JSPS) KAKENHI(16206003) and the Dutch NWO VICI-grant.
 K. Yamazaki, S. Etaki, H. S. J. van der Zant, and H. Yamaguchi, J. Cryst. Growth 301-302 (2007) 897.
Fig. 1. AFM image of fabricated 50-nm-thick arched beam.
Fig. 2. Calculated resistance with and without the piezoelectric effect (PE).
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