How does stress in nano-scale improve materials performance? As in nano-oxides in Si1-xGe
Prof. Siu-Wai Chan
Department of Applied Physics & Applied Mathematics
Date & Time
Tuesday, 14 January 2020
Room 7-37, Haking Wong Building, HKU
A mystery: how does Si move into the world of telecommunication in early 200’s?
We explore interface such as SiGe/Si and ways to avoid dislocation generation from lattice misfit and oxidation stress. Dislocations are classic kryptonite to electronic devices. The then perceived straight limitation from classical elastic theory of hetero-epitaxy had almost become the show-stopper.
SWC being the interface dislocation expert was able to lead the IBM team to perform a
series of experiments which demonstrates larger elastic strain can be tolerated in smaller volume without dislocation generation in the active area. The findings and the processes were quickly adopted by IBM with Hughs Telecommunication collaboration and then picked by other chip makers, and has evolved into today’s standard industrial practice. The practice has helped the CMOS FET move to high frequency devices from 100MHz range in late 1990’s to GHz range of today. Essentially Si with Ge has moved from computers into telecommunication devices, enabling the integration of (CMOS FET’s) with heterojunction bipolar transistors (HBT).
To appreciate stress in nano-scale, we will give an overview of the size-dependence lattice-parameter of nano-oxides where a lattice expansion of 0.1 to 0.5% as size decreases to ~5nm for five oxides suggesting a compressive surface stress in each case. The five oxides are MgO, Cu2O, Fe3O4, CeO2 and Co3O4. The finding is different from the tensile surface stress observed in nanoparticles of noble metals notably gold, where surface stress is calculated from the amount of lattice-contraction as crystallite-size decreases. Our investigation is possible because of the mono-dispersed nature of the nano-oxide in each batch. We have also studied the pressure and thermal response of the lattice-parameter of nano-ceria and nano-MgO. Hence, bulk modulus (B) and coefficient of lattice thermal expansion (alpha) were measured. Bulk modulus peaks around 33nm for nano-ceria and 14nm for nano-MgO. In both cases there is a quick decline below the peak. The coefficients of lattice thermal expansion in both cases decrease rapidly after 15nm to ~60% of the bulk values. The findings have a number of implications for the bonding, surface-stress and the amount of elastic energy stored in these nanoparticles.
 J. Materials Research 12, 364-370 (1997)
 Appl. Phys. Letters, 18, 127 (2002)
 Materials Chemistry and Physics 192, 311-316, (2017)
 J. Nanopart Res 19:241(2017)
 J Am Ceram Soc 100:384-392 (2017)
 International Journal of Applied Ceramic Technology 13, 389-394 (2016)
After receiving a doctoral degree from the Massachusetts Institute of Technology in Materials Science and Engineering, Dr. Chan joined Bellcore (then the mini-Bell-Labs) as a principle scientist. She moved to Columbia University and was appointed as tenured professor in 1997. Prof. Chan’s research has covered grain boundaries and interfaces in metals, e.g. gold and aluminum, as well as in superconducting oxides (YBa2Cu3O7-x and Ba1-xKxBiO3) and ionic conductors such as doped CeO2 where oxide boundaries play major roles in determining electrical current flow through supercurrent and ionic-current, respectively. Understanding the boundary problems enables the discovery of solutions to improve/enhance the desired materials properties. Prof. Chan’s recent research concentrates on nano-oxide crystals, the effects/relationship of crystal-size on bond-length, stiffness, thermal expansion, solubility, redox potentials and phase stability. Research results impact applications in catalysis, microelectronics, gas-sensing, biomedical therapies, micro-electro-mechanics and solid state fuel cells. The surface of a nano-crystal covered with surfactants is a special type of interface, probable the simplest and most easily manipulated.
Prof. Chan is a Fellow of the America Physical Society and the American Ceramic Society. Prof. Chan has received Dupont Faculty Award, IBM Faculty Award, BASF Award, Fellowship from John Simon Guggenheim Foundation, Tan Chin Tuan Fellowship from Nanyang Technological University of Singapore, and Diversity award from Columbia University. She was honored as Advanced Fellow (sponsored by USA-National Science Foundation and University of Washington) and Presidential Faculty Fellow from President Bill Clinton.