An atomic force microscope was used to characterize the deformation behavior of amorphous silicon nanostructures subjected to monotonic and cyclic loading. The sample geometry was specially designed using finite element modeling for the purpose of these tests and the samples were grown by the oblique angle vapor deposition technique. These nanostructures are either in the form of a slanted nanorod or in the shape of an elbow. These structures were isolated from each other and were grown on tungsten nano-pillars arranged in either square or triangular pattern on Si(100) substrate. Various samples with different dimensions and rise angles were grown and tested in bending using the atomic force microscope.
The slanted nanorod specimens were tested in bending using the tip of an atomic force microscope cantilever and the mechanical response (force-displacement curve) was also measured. For small stresses/deflections the slanted nanorod specimens exhibited an elastic response consistent with the bulk amorphous silicon behavior. Since, samples with various dimensions and geometries were available; the Young’s modulus was calculated from the slope of the experimentally observed stiffness versus the geometrical factor. This was done to reduce the uncertainty from just one sample. The Young’s modulus for the amorphous silicon measured from the slope of the nanorod stiffness versus the geometric factor was found to be (94 ± 10) GPa.
When the slanted nanorods were tested beyond the elastic limit in order to investigate the plastic properties of the amorphous silicon, the interface between the amorphous silicon and the tungsten failed before reaching the yield point and the eventual plastic deformation (if any) due to the high stress concentration in the region of the joint between the nanorod and the tungsten pillar. With this observation, it was concluded that nanostructures of alternative geometry where the stress is concentrated in the region other than the interface are needed for the characterization of plastic deformation and the failure properties of the nanostructures.
Finite element analysis of different test structures showed that in an elbow structure, the stress is concentrated around the turning point of the elbow, a region different from the interface of the amorphous silicon and the tungsten. The elbow structure was then grown using the oblique angle vapor deposition with swing rotation technique and tested under monotonic and cyclic loading conditions using the atomic force microscope.
When deformed monotonically at room temperature, the amorphous silicon specimens in the form of an elbow exhibited a linear force-displacement response up to a critical force/stress and a nonlinear response at forces larger than the critical force, a phenomenon not observed in bulk silicon. Since the amorphous silicon nanostructures exhibited a nonlinear deformation behavior during monotonic bending tests, a natural quest began whether these structures exhibit fatigue susceptibility, a phenomenon not observed in the bulk silicon. Since no any reliable fatigue testing technique for the nanoscale specimens existed, a method based on a continuous acquisition of force-displacement curves using atomic force microscope was established. With this method, the number of loading-unloading cycles needed for a complete failure of the specimen could be exactly counted by looking at the instability in the force-displacement curve which results in double peaks.
With the established fatigue test method for nanostructures based on the atomic force microscope, the elbow structures made of amorphous silicon were tested with different force amplitudes and the fatigue life curve (Wohler curve) was obtained. The fatigue life of the amorphous silicon specimens was observed to increase by five orders of magnitude with a 50% reduction in the applied force amplitude. By monitoring the stiffness of the elbow in subsequent cyclic loadings, it was verified that this delayed failure is due to progressive damage accumulation during cyclic loading, a surprising conclusion for a nanoscale specimen.
Despite the nonlinear deformation behavior observed from the monotonic loading tests and the progressive damage accumulation observed during the cyclic loadings of the amorphous silicon nanostructures, the scanning electron microscope images of the fracture surfaces could not point out concrete evidence if the failure was of ductile in nature. The finite element analysis of the stress distribution on the elbow structure due to a static end load revealed that, both principal and the shear stress is located around the turning point of elbow. Also, the magnitudes of the maximum principal stress (responsible for brittle failure) and the maximum shear stress (responsible for ductile failure) are relatively similar.