Disorder in tetrahedrally bonded amorphous solids

Abstract

We have recently found that the density of the low-energy excitations in amorphous silicon ®lms and, in particular, in hydrogenated amorphous silicon (a-Si : H) ®lms can decrease by over three orders of magnitude, to a level of complete disappearance in certain cases from that observed in all other amorphous solids. This observation breaks down the universality of these excitations in amorphous solids. Extending this study, we have measured the low-temperature internal friction of a variety of a-Si : H ®lms, hydrogenated amorphous germanium ®lms and amorphous carbon ®lms, under diOE erent deposition conditions or post-annealing treatments. We ®nd that the density of the low-energy excitations in these systems is smaller than in all other amorphous solids, and that they vary considerably between diOE erent ®lms. We attribute this property to be a unique feature of their tetrahedral bonding, where the overconstrained structure restricts the tunnelling motion of atoms. We conclude that the low-energy excitations in tetrahedrally bonded amorphous solids are sensitive to the existence of local¯oppy modes caused by structural inhomogeneity.

Low temperature internal friction of diamond-like carbon films

“…While this quantity, Q −1 0 , has previously been shown to be of a nearly universal magnitude from 1.5 × 10 −4 to 1.5 × 10 −3 , called 'glassy range', it has recently been found that it can be reduced several orders of magnitude [4]. The variation of the internal friction plateau only occurs in a-Si and to some extent also in a-Ge [3]. Amorphous a-C belongs to the same system with tetrahedral bonding.…”

“…E-mail: xliu@genah.nrl.navy.mil we have reached the conclusion that tetrahedral bonding is an important factor in reducing atomic tunnelling states (TS) in a-Si and a-Ge films at low temperatures [3]. In certain hydrogenated a-Si film, TS can be made to disappear completely [4], which, we believe, is at least partially related to the low internal stress in the material [5].…”

Microstructure dependence of low-temperature elastic properties in amorphous diamondlike carbon films

“…[11][12][13][14] We have concluded that the four-fold coordinated tetrahedral bonding is indeed an important factor in reducing the number of TLS. 13 In certain hydrogenated a-Si films, the TLS can be made to disappear completely. 11 In some of the other cases, however, we have evidence to show that the overconstraint from tetrahedral bonding leads to an increase of internal strain.…”

“…[8][9][10] Following those pioneering studies, we have used a high sensitivity technique to study the low-temperature internal friction of various types of thin films of a-Si, a-Ge, and also amorphous carbon ͑a-C͒. [11][12][13][14] We have concluded that the four-fold coordinated tetrahedral bonding is indeed an important factor in reducing the number of TLS. 13 In certain hydrogenated a-Si films, the TLS can be made to disappear completely.…”

Two-level systems in evaporated amorphous silicon

“…There are conflicting results in the literature on whether TLS occur in a-Si and a-Ge and it has been debated whether the TLS in these systems are the same as those found in other glasses, such as a-SiO 2 [17][18][19][20][21][22][23]. The TLS density in tetrahedrally bonded materials depends strongly on the preparation technique suggesting that the TLS are due to some microstructural detail of the material [5,24]. For example, it was previously thought that hydrogen played a key role in removing TLS from a-Si [4] but our recent results show that the reduction in TLS in the hydrogenated material was likely the result of increasing T S [9].…”