Structural, electronic, and vibrational properties of high-density amorphoussilicon: A first-principles molecular-dynamics study

摘要

We report a first-principles study of the structural, electronic, and dynamical properties ofhigh-density amorphous (HDA) silicon, which was found to be formed by pressurizing low-densityamorphous (LDA) silicon (a normal amorphous Si)T. Morishita, Phys. Rev. Lett. 93, 055503(2004); P. F. McMillan, M. Wilson, D. Daisenberger, and D. Machon, Nature Mater. 4, 680 (2005).Striking structural differences between HDA and LDA are revealed. The LDA structure holds atetrahedral network, while the HDA structure contains a highly distorted tetrahedral network. Thefifth neighboring atom in HDA tends to be located at an interstitial position of a distortedtetrahedron composed of the first four neighboring atoms. Consequently, the coordination number ofHDA is calculated to be 5 unlike that of LDA. The electronic density of state (EDOS) shows thatHDA is metallic, which is consistent with a recent experimental measurement of the electronicresistance of HDA Si. We find from local EDOS that highly distorted tetrahedral configurationsenhance the metallic nature of HDA. The vibrational density of state VDOS also reflects thestructural differences between HDA and LDA. Some of the characteristic vibrational modes of LDAare dematerialized in HDA, indicating the degradation of covalent bonds. The overall profile of theVDOS for HDA is found to be an intermediate between that for LDA and liquid Si under pressure(high-density liquid Si).