Low Temperature Vibrational Properties of Amorphous Silicon

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ABSTRACT We present internal friction and shear modulus measurements of amorphous silicon (a-Si) and germanium (a-Ge) films. The temperature independent plateau in internal friction below 10 K, common to all amorphous solids, also exists in these films. However, its magnitude which depends critically on the deposition method is smaller than found for all other amorphous solids. In particular, hydrogenated a-Si with about 1 at. % H prepared by hot-wire chemical-vapordeposition leads to an internal friction nearly three orders of magnitude smaller than observed for all other amorphous solids. The internal friction increases after the hydrogen is removed by effusion. INTRODUCTION In spite of the rapid development of amorphous silicon (a-Si) and amorphous germanium (a-Ge) technology in solar cells, thin film transistors, flat panel displays, and many other applications, many fundamental questions concerning these tetrahedrally bonded amorphous semiconductors still remain unanswered. An example is the low-energy vibrational excitations in these solids, which are the subject of this investigation. In crystalline solids, lattice vibrations are described as collective excitations, i.e. waves. In amorphous solids, however, localized excitations, in addition to waves, are needed to describe lattice vibrations over a wide temperature range. A broad distribution of localized excitations of energies less than - 10-4 eV dominate the thermal, elastic, and dielectric properties at temperatures below a few degrees Kelvin [1]. These excitations were first observed in 1971 [2]. Shortly after this discovery model calculations based on the proposal that these low-energy excitations are caused by tunneling of atoms or groups of atoms between nearly degenerate equilibrium positions gave a good account of the experiments [1, 3]. Although the concept of two-level tunneling states (TLS) has been successful in accounting for low-temperature properties of amorphous solids, the actual atomic configurations that possess multiple-energy minima have been elusive. It is a great puzzle why all amorphous solids share low energy vibrational excitations of rather similar number density. This ubiquity of the tunneling states that dominate the low temperature elastic properties of amorphous solids is illustrated in Fig. I, that shows the internal friction (Q-') for representative amorphous solids compared to crystalline silicon (c-Si). Q-' of nearly perfect crystals is small and is determined by residual lattice defects and clamping losses, as shown for c-Si [4]. In contrast, Q` for amorphous solids (even including amorphous metals) is in the range of 10' to 10.', and nearly temperature independent below 10 K (the plateau). This independence of temperature is a direct consequence of the constant density of states (more precisely, of their spectral density, P) postulated as part of the tunneling model. According to the tunneling model, [1, 3] the internal friction in the plateau region is given by,

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Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materia