|Making Silicon Melt in Reverse|
|Wednesday, 24 August 2011 08:59|
A team of MIT scientists, together with scientists at the Advanced Light Source, used an in-situ heating stage at microprobe Beamline 10.3.2 to measure the chemical state of silicon contaminated with copper, nickel, and iron at temperatures nearing 1000°C. Under special cooling conditions, the solid solution decomposes and nanometer- to micron-sized liquid eutectic droplets appear. This material and phenomenon is described in a paper published in Advanced Materials . Team leader Tonio Buonassisi, the SMA Assistant Professor of Mechanical Engineering and Manufacturing, is the senior author, and the lead authors are Steve Hudelson MS ’09, and postdoctoral fellow Bonna Newman PhD ’08.
This finding could lower the cost of solar cell manufacturing by accelerating material purification. During silicon refining or ingot solidification, liquid eutectic droplets are shown to aggregate additional impurities via solid-liquid segregation. “If you can create little liquid droplets inside a block of silicon, they serve like little vacuum cleaners to suck up impurities,” says Buonassisi, leaving cleaner material behind. His group has also demonstrated that this phenomenon can be harnessed to seed microwires over large areas , by spontaneously forming an array of liquid droplets on a three-dimensional wafer surface for vapor-liquid-solid seeding.
Retrograde melting was predicted to occur in silicon by Buonassisi in 2007 . However, it was only with the advent of a high-temperature in-situ stage at ALS microprobe Beamline 10.3.2 that experimental confirmation came within reach, enabled by non-destructive, in-situ X-ray measurements.
Work performed on ALS Beamline 10.3.2
 S. Hudelson, B.K. Newman, S. Bernardis, D.P. Fenning, M.I. Bertoni, M.A. Marcus, S.K. Fakra, B. Lai, and T. Buonassisi, "Retrograde melting and internal liquid gettering in silicon,” Advanced Materials 22, 3948 (2010).
 V. Ganapati, D.P. Fenning, M.I. Bertoni, C.E. Kendrick, A.E. Fecych, J.M. Redwing, and T. Buonassisi, “Seeding of silicon wire growth by out-diffused metal precipitates,” Small 7, 563 (2011).
 T. Buonassisi, M. Heuer, A.A. Istratov, M.D. Pickett, M.A. Marcus, B. Lai, Z. Cai, S.M. Heald, and E.R. Weber, “Transition metal co-precipitation mechanisms in silicon,” Acta Materialia 55, 6119 (2007).
The research was supported by the U.S. Department of Energy, the National Science Foundation, the Clare Booth Luce Foundation, Doug Spreng and the Chesonis Family Foundation, and some equipment was provided by McCrone Microscopes & Accessories.
Press release (MIT news): Silicon can be made to melt in reverse