Role of sulfur in vibration spectra and bonding and electronic structure of GeSi surfaces and interfaces

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Hartnett, Mark C.
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University College Cork
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A quantum mechanical density functional theory approach was used to investigate the structural atomic configuration, vibration mode frequencies and electronic structure of surfaces and interfaces using germanium. Initially, we investigated the H2S and H2Opassivated germanium surfaces. A supercell approach is used with the local density (LDA), generalized gradient (GGA) approximations and van der Waals (vdW) interactions. The frozen phonon method was used to calculate the vibrational mode frequencies of these surfaces. The calculated frequencies produce stretch, bond bending and wag modes. The differences between the functionals including vdW terms and the LDA or GGA are less than the differences between LDA and GGA for the vibrational mode frequencies. Some of these modes provide useful vibrational signatures of bonding of both sulfur and oxygen on germanium surfaces, which may be compared with vibrational spectroscopy measurements. A bare germanium surface is bonded to a bare silicon surface to form a Ge-Si interface. As germanium has a 4% larger lattice constant than silicon this implies there are regions on the interface where the germanium and silicon match perfectly (aligned) and are completely mismatched (misaligned). The atomic structure of the GeSi aligned interface shows the original crystal structure and the projected band structure (PBS) shows no interface states in the band gap. The GeSi misaligned structure forms a (2x1) configuration. The electronic PBS shows interface states in the band gap. To remove the interface states seen in the GeSi interface, sulfur with its six valence electrons and its flexible chemical bonds is suggested to improve the interface bonding and remove interface states. The PBS in both the aligned and misaligned GeSSi interfaces shows states around the germanium and silicon interface atomic layers and a charge density localised around the sulfur interface atoms. A sulfur terminated germanium surface results in a (1x1) configuration with surface states present in the band gap. However, a H2S terminated germanium surface results in a (2x1) configuration with symmetric Ge-Ge dimers and pushes the surface states into the bulk region, implying the presence of hydrogen results in no surface states. Including hydrogen on our GeSSi interfaces, the atomic configuration remains the same with the hydrogen molecule in the channels. However, upon looking at the PBS, states are clearly visible in the band gap and when we investigate the charge density contour plots, interface states do exist. Therefore, the presence of hydrogen here does not influence the interfaces.
Germanium , Silicon , Vibrational , Mode , Frequencies , Surfaces , Interfaces , Band structure , Sulfur , Hydrogen , Interface states , Conduction and valence bands , Density functional theory
Harnett, M. C. 2016. Role of sulfur in vibration spectra and bonding and electronic structure of GeSi surfaces and interfaces. PhD Thesis, University College Cork.