Effect of size on BDT (brittle to ductile transition) temperature in single crystal silicon
Silicon based micro- and nanometer scale devices operating at various temperatures are ubiquitous today. However, thermo-mechanical properties of silicon at the small scale and their underlying mechanisms remain elusive. The brittle-to-ductile transition (BDT) is one such property relevant to these devises. Materials can be brittle or ductile depending on temperature. The BDT occurs over a small temperature range. For bulk silicon, the BDT is about 545°C. It is speculated that the BDT temperature of silicon may decrease with size at the nanoscale. However, recent experimental and computational studies have provided inconclusive evidence, and are often contradictory. Potential reasons for the controversy might originate from the lack of an in situ methodology that allows variation of both temperature and sample size. We resolved this controversy by carrying out in situ thermo-mechanical bending tests on single crystal silicon samples with concurrent control of these two key parameters. We showed unambiguously that the BDT temperature reduces with sample size. For example, the BDT temperature decreases to 293°C for a sample size 720 nm. We developed a mechanism-based model to interpret the experimental observations.
Currently we are addressing the following questions: (1) What is the mechanism by which silicon reduces the BDT temperature with decreasing size? (2) What is the role of size in determining the yield strength of silicon under bending? What is the underlying mechanism? We are addressing these questions in collaboration with University of Vienna and Max Planck Institute, Düsseldorf , Germany.