Tension–compression asymmetry in amorphous silicon

Nat. Mater. 2021

DOI: 10.1038/s41563-021-01017-z

Inorganic materials with covalent or ionic bonds usually show higher compressive strength (σC) than tensile strength (σT). This apparent tensile-compressive asymmetry is due to preexisting flaws in the material and how they behave under local strain. A tensile stress opens a flaw during loading and thus reduces strength of a sample, whereas a compressive stress does opposite.

For this experimental work, Wang, Y., Ding, J., Fan, Z. et al. used Bruker's Hysitron PI 95 TEM PicoIndenter on FIB-prepared small-scale samples inside a transmission electron microscope. The study showed that σC - σT asymmetry can be reversed in amorphous silicon (σT>>σC) if preexisting flaws are eliminated from the sample. The article also claims that such extraordinary asymmetry (σT>σC) can also be found in amorphous Ge and amorphous SiO2.

More specifically, from the experiential and MD simulation works, it was concluded that the shear modulus and a shear transformation were the major contributing factors for lowering the strength in compression. In a small-scale sample where the defect density is sufficiently low, compressive stress decreases the shear modulus and, consequently, the activation barrier for shear events, which lowers the yield stress. A shear transformation occurred under compression is the dominating mechanism confirmed by electrical resistance measurements during mechanical loading of the sample. In amorphous silicon, semiconductive motifs changed to a metal-like structure by shear transformation (densification) resulting in lower electrical resistance under compressive stress.

If you have any questions about the Hysitron PI 95 TEM Picoindenter or any of our other nanomechanical testing solutions, please contact us.

_SUBJECT(S):

• The asymmetrical strength of amorphous silicon and other amorphous materials used in photovoltaic applications
• The relationship between compressive and tensile strength as indicated by how preexisting flaws in the material behave under local strain

_{FEATURED BRUKER TECHNOLOGY:}

_{KEY FINDINGS:}

• Showed that, if preexisting flaws are eliminated from the sample, σC - σT asymmetry can be reversed in amorphous silicon (σT>>σC) and amorphous Ge and amorphous SiO2 (σT>σC)  
  
• Concluded that the major contributing factors for lowering the strength in compression are the shear modulus and a shear transformation
• Found that the compressive stress of amorphous silicon lowerwd its electrical resistance; shear transformation (densification) changed its semiconductive motifs to a metal-like structure

_{KEY TERMS:}

Amorphous Silicon, Compressive Stress, Mechanical Loading, Mechanical Properties, Shear Modulus, Shear Transformation, Tension–Compression Asymmetry, Yield Stress