The observation of the low-frequency acoustic phonon torsional modes in nanocrystalline silicon

Si and Ge nanocrystals (nc-Si and nc-Ge) with average sizes in the range of 6 and 6.3 nm, embedded in SiO 2 /GeO 2 matrix, were fabricated and their acoustic-phonon vibrational properties were investigated using two different approaches by considering the elastic continuum model and fixed boundary condition. The breathing and quadrupolar modes are found in the spectra. The presence of medium significantly affects the phonon peaks and results into the broadening of the modes which is more in the case of elastically similar materials. The phonon line width is found to depend inversely on the size, similar to that reported experimentally. Using first and second-order optical modes, the electron-phonon coupling strengths have been estimated. The result shows that e-p coupling strength is more in the case of elastically dissimilar materials. V

Section: Methodssupporting

confidence: 90%

Section: Theoretical Considerationssupporting

confidence: 84%

Section: Introductionmentioning

confidence: 99%

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Low-frequency Raman scattering from Si/Ge nanocrystals in different matrixes caused by acoustic phonon quantization

Mankad

Gupta

Jha

et al. 2012

“…Up to now, many experiments have shown that the lowfrequency vibration characters of nc-Si and nc-Ge are strongly dependent upon the sample preparation conditions which directly determine the surface structure of nanoparticles. [6][7][8][9][10] Our recent work also disclosed that the low-frequency vibrations of nc-Ge, nc-Si, and nc-Ge 1−x Si x in silicon oxide depend on the coupling between nanocrystals and the matrix. 11 However, the influence of the matrix on the vibration properties of nanocrystals is sophisticated and still far from clearness.…”

Section: Introductionmentioning

confidence: 86%

Local vibration at the surface of a Ge nanocrystal embedded in a silicon oxide matrix

Yang

et al. 2006

Self Cite

A low-frequency Raman vibration mode, whose peak position and linewidth are independent of the sizes of Ge nanocrystals and the polarization configuration of incident excitation light, was observed in silicon oxide films embedded with Ge nanocrystals which were prepared using magnetron cosputtering of SiO 2-Ge-Si targets. The peak position of the Raman mode is sensitive to the content of Si in the matrix. After the sample is annealed above a special temperature that increases with the content of Si, the Raman mode disappears. Microstructural observations and spectral analyses disclose that this low-frequency Raman mode arises from a local structure which is positioned at the surfaces of Ge nanocrystals and consists of Ge, Si, and O atoms. High-temperature annealing leads to the removal of Ge atoms from the local structure. As a result, the local vibration mode vanishes.

“…In the previous work, 12 we have observed the acousticphonon torsional mode of nc-Si in the porous structure formed with C + -implanted Si wafer and then anodization. We have also reported the surface-quasifree vibrations of perfectly spherical nc-Si in Er-doped Si nanostructures.…”

Section: Introductionmentioning

confidence: 95%

Low-frequency Raman scattering of Ge and Si nanocrystals in silica matrix

Yang

et al. 2005

Self Cite

Ge and Si nanocrystals ͑nc-Ge and nc-Si͒ with average sizes in the range of 2-7 nm, embedded in silica matrix, were fabricated for investigating their acoustic-phonon vibrational properties. The freely elastic sphere theory was found to be unsuitable for explaining the low-frequency phonon vibration character of both nc-Ge and nc-Si in our current experiments. We have assigned the observed low-frequency Raman peaks to "LA-like mode" and "TA-like mode" in terms of their polarization and depolarization behaviors. In addition, it is revealed that the lattice contraction phenomenon exists in nc-Ge and nc-Si with sizes smaller than 4 nm, which leads to a contrary effect against matrix traction on the phonon vibrational frequencies.