Formation Mechanisms And Raman Scattering Of Some Semi-conductor Nanostructures

Recently, with the development of nanotechnology, the semiconductor materials used as Si-based optoelectronic integrated material have attracted an increasing in-terest. The interfacial and surface layer of semiconductor nanostructured materials paly a crucial role in their formation mechanism and new function character. To further clearly disclose the related physical problem, we carried out a series of theorical and experimential investigation to reveal the fabrication, formation mecha-nism, and phonon character. The obtained main results are as follows:1. Ordered amorphous silicon nanoisland arrays and reflection spectral de-pendence on nanoisland geometrical parameters. We report an anodic porous alu-mina mask technique for fabricating orderly hemispherical silicon nanoisland arrays with island diameters of 30-120 nm. Reflection spectral measurements show that the phase transition of incident light can periodically be transferred with changes in geometrical parameters of the ordered arrays. Thus the redshift of the reflection peak with decreasing both sizes of the nanoislands and incident angle of light wave can be explained well on the basis of a modified rigorous vector coupled-wave the-ory.2. Surface-polarization-induced formation of amorphous foliaceous SiO2 helical nanobelts. Amorphous foliaceous SiO2 helical nanobelts with equidistant alternat-ing bright and dark stripes are synthesized using thermal evaporation. A very thin polar crystalline layer spontaneously formed on the nanobelt surface stabilizes the twist and the internal shear stress imbalance induces the periodic reconstruction. The periodic bright and dark stripes disappear slowly with growth time because the polar crystalline SiO2 layers are covered slowly by an amorphous layer. The polar surface-driven mechanism, which can adequately explain the appearance and disap-pearance of the periodic fringes, is verified theoretically.3. Raman investigation of oxidation mechanism of silicon nanowires. The Raman inverstigation disclose that the strain induced by oxidation mechanism cuase disintegration of the perfect Si nanowires into smaller crystalline particles encapsu-lated by amorphous Si, when the nanowire diameter is smaller than 4 nm. By con-sidering the morphological transformation, we adopt the phonon confinement models on wires and dots to explain very well the Raman spectra acquired from Si NWs with different diameters. This morphological transformation of Si nanowire plays a crucial role in their electrics and optical behavior.4. The Raman scattering of SiO2 films embedded with Ge nanocrystals. The Ge-Ge optical phonon peak at 300 cm-1 acquired from amorphous SiO2 films em-bedded with Ge nanocrystals by Raman scattering is sensitive to the Si content. When the Si concentration is high, a thin GeSi interfacial layer forms around the Ge nanocrystals. A tensile stress is produced to partially offset the compressive stress imposed by the SiO2 matrix on the Ge nanocrystals, consequently downshifting the frequency of the optical phonon and increasing its linewidth. The phonon confinement and compressive effects can interpret effectively the Ge-Ge optical phonon behavior. Meanwhile, the low-frequency Raman scattering also discloses that the peak position and linewidth of Ge nanocrystal embedded SiO2 films is size-independent. Theoretical calculation based on a modified three-region model discloses that the acoustic impedance of the interfacial GeSiO layer is responsible for the size-independent behavior.5. Twinning Ge0.54Si0.46 nanocrystal growth mechanism in amorphous SiO2 films. Ge0.54Si0.46 alloy nanocrystals with different twinning structures are synthe-sized. The local strain induced by rapid cooling enables neighboring NCs to coa-lesce quickly. Because of insufficient time to form individual structures, a leading twinning interface forms inevitably in the interior of the NCs. The twinning NCs with large surface free energies reconstruct for energy optimization at high tem-perature. Consequently, the twinning layer thickness shrinks slowly, finally trans-forming into untwined stable NCs with the lowest surface free energy. When the Si conten is high (x>0.62), Raman scattering discloses a new 430 cm-1 Si-Si op-tical phonon mode emergence in SixGe1-x nanocrystal embedded SiO2 films. Ex-perimentally examining and theoretically analyzing disclose that this new mode is intimately related with the existence of local silicon cluster nanostructures inside SixGe1-x nanocrystals, therefore, which can be used as a fingerprint to identify the existence of local silicon cluster nanostructures inside SixGe1-x nanocrystals with high silicon content.6. Longitudinal optical phonon-plasmon coupling in luminescent 3C-SiC nanocrystal films. As the NC size increases, the LO phonon intensity increases in the Raman spectra of the solid films and is even larger than that of the transverse optical mode. The Raman spectra cannot be fitted by using only the phonon con-finement model. When further considering the coupling between the LO phonon and plasmon induced by the surface deformation potential in the glycerol layer, good agreement is achieved between the experiments and theory.