| Summary: | 碩士 === 國立臺灣大學 === 電子工程學研究所 === 97 === In this thesis, the heteroepitaxy and optical properties of GaAs grown on planar Si wafer and Si nanostructure by gas-source molecular beam epitaxy (GSMBE) have been investigated. First, the GaAs on planar Si wafer contains high density of structural defects such as misfit dislocations, threading dislocations, stacking faults, microtwins, and antiphase boundaries, which are mostly around the GaAs/Si interface. However, the density of structural defects is significantly reduced with the increment of epilayer thickness. Additionally, the residual tensile strain in GaAs epilayer, resulting from the mismatch of thermal expansion coefficient between GaAs and Si, is also observed.
On the growth of GaAs in Si nano-trenches, the low temperature deposition is incapable of filling GaAs into Si nano-trenches due to the nucleation of GaAs on the trench sidewall and shallow-trench-isolation oxide, which could further develop to thick polycrystalline GaAs layer and seal the trench opening. With orient-beam epitaxy and high temperature one-step growth, high-quality GaAs is selectively grown in 40-nm-wide Si trenches successfully. To our knowledge, it is the first accomplishment of GaAs grown in 40-nm Si trenches by GSMBE. Furthermore, a metamorphic GaAs in 80-nm Si trench was investigated in detail by cross-sectional transmission electron microscopy. The results indicate that no threading dislocations and antiphase boundaries exist in epitaxial GaAs; only microtwins and 60° misfit dislocations are generated, which can efficiently relieve the lattice misfit strain. The disappearance of threading dislocations could be attributed to the large lattice misfit stress and the sidewall image force, which could drive the threading dislocations to glide to the sidewall. The disappearance of antiphase boundaries is probably due to single nucleus growth in the trench.
Finally, the optical properties of GaAs in Si nano-trenches were studied by Raman spectroscopy and cathodoluminescence spectroscopy at room temperature. Irrespective to trench orientation along [110] or [100], strong TO phonon mode is observed in Raman spectra, indicating that the breakdown of Raman selection rule due to the presence of microtwins and surface faceting. Surface optical (SO) phonon mode is also observed in this study. With the decrement of trench width, SO phonon peak slightly shifts to TO phonon and its intensity increases as well. The cathodoluminescence result reveals a strong band-to-band emission of GaAs in Si nano-trenches, indicative of a significant reduction of non-radiative centers and the good crystalline quality.
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