Two-state dynamics achieved on amorphous silicon surface
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MRS BULLETIN
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VOLUME 36 • SEPTEMBER 2011
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www.mrs.org/bulletin
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morphous silicon (α-Si) is widely used in the semiconductor industry for a range of device applications due to its low cost and because it is much easier to form flexible thin films than crystalline silicon. In an effort to settle the continuing debate over whether α-Si is a glass or simply an amorphous solid, M. Grubele, J. Lyding, G. Scott, and S. Ashtekar from the University of Illinois at Urbana-Champaign have attempted to observe the two-state dynamics of α-Si clusters which is characteristic of glass. As reported in the June 10 issue of Physical Review Letters (DOI: 10.1103/ PhysRevLett.106.235501), the researchers used low energy ion implantation and chemical vapor deposition (CVD) to create an amorphous silicon surface from a Si substrate, generating the two-state dynamics. A scanning tunneling microscope (STM) was utilized to directly observe the hopping between the two states at a temperature of 295 K (see figure). This temperature lies above the tunneling regime and below the glass transition temperature of α-Si as reported at 900 K, a universal observation of glassy behavior.
Since α-Si surfaces are normally structure for Si. Blobs observed on the grown with hydrogen incorporated into surface indicated the merging and unthe structure, the researchers passivated dercutting of the surface structure as a the α-Si surfaces with 1% hydrogen. result of a reaction with hydrogen. Thus With the addition of hydrogen, a twohydrogen passivation has major strucstate motion was not observed, which tural and dynamical consequences. was attributed to the fact that hydrogeThis research provides an improved nation quenches the two-state dynamunderstanding of the glassy behavior ics by relaxing the surface to lower enof amorphous silicon. Although twoergy structures. Hydrogen passivation state dynamics hopping has been precaps the most strained, least-bonded Si atoms to lower the surface free energy, thereby reducing two-state dynamics. Furthermore, the surface showed signs of 0 50 100 150 200 crystallization including Time (min) larger clusters, cracks, and highly structured patches. Cracks indicate that the density of the remaining α-Si surface 0 20 40 60 80 100 120 140 increased, as expected Time (min) if strain is relieved. Two-state dynamic on α-Si surfaces. (a) Five consecutive The crystalline patches images on a surface prepared using a c-Si (100) surface. (b) Single cluster trace (SCT) with a time resolution of 6 ranged from just a few min. (c) Selected frames of α-Si surface vapor deposited on atoms to hundreds of c-Si (100) substrate, where the circled cluster shows the two-state dynamics and (d) corresponding SCT. Scale bar atoms in surface area, is 1.1 nm = 5 at. diam. Reproduced with permission from consistent with a Si(111) Phys. Rev. Lett. 106 (23) 2011 (DOI: 10.1103/PhysRevLett. surface structure, the 106.235501). © 2011 American Physical Society. lowest energy surface State
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Ion bombarded
Two-state dynamics achieved
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