Amorphous 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 grown with hydrogen incorporated into the structure, the researchers passivated the α-Si surfaces with 1% hydrogen. With the addition of hydrogen, a two-state motion was not observed, which was attributed to the fact that hydrogenation quenches the two-state dynamics by relaxing the surface to lower energy structures. Hydrogen passivation caps the most strained, least-bonded Si atoms to lower the surface free energy, thereby reducing two-state dynamics. Furthermore, the surface showed signs of crystallization including larger clusters, cracks, and highly structured patches. Cracks indicate that the density of the remaining α-Si surface increased, as expected if strain is relieved. The crystalline patches ranged from just a few atoms to hundreds of atoms in surface area, consistent with a Si(111) surface structure, the lowest energy surface structure for Si. Blobs observed on the surface indicated the merging and undercutting of the surface structure as a result of a reaction with hydrogen. Thus hydrogen passivation has major structural and dynamical consequences.

This research provides an improved understanding of the glassy behavior of amorphous silicon. Although two-state dynamics hopping has been predicted by theory, and many researchers have inferred it indirectly from measurements, this is the first time this type of two-state dynamics has been visualized, said the researchers.

Two-state dynamic on α-Si surfaces. (a) Five consecutive images on a surface prepared using a c-Si (100) surface. (b) Single cluster trace (SCT) with a time resolution of 6 min. (c) Selected frames of α-Si surface vapor deposited on c-Si (100) substrate, where the circled cluster shows the two-state dynamics and (d) corresponding SCT. Scale bar is 1.1 nm = 5 at. diam. Reproduced with permission from Phys. Rev. Lett. 106 (23) 2011 (DOI: 10.1103/PhysRevLett. 106.235501). © 2011 American Physical Society.