Fig. 1: Relaxed atomic structure of H2S bonded to a Ge(001)-(2x1) surface. The green ball and stick represents the germanium atoms, the grey represents the hydrogen atoms and the yellow represents the sulfur atoms.

Fig. 1: Relaxed atomic structure of H2S bonded to a Ge(001)-(2x1) surface. The green ball and stick represents the germanium atoms, the grey represents the hydrogen atoms and the yellow represents the sulfur atoms.

Materials Theory Group > Transport and Atomic Structure in Semiconductor Alloys > Interfaces > Structural and vibrational characteristics of H₂S terminated Ge surface

The germanium surface undergoes extensive surface reconstructions in which their surface atomic geometry differs significantly from that of the bulk [1,2]. The Ge(100) surface reconstructs to form germanium dimers, thereby reducing the number of dangling bonds per surface germanium atom from two to one. Locally, the surface structure of reconstructed Ge(100) is similar to that of Si(100) in that both exhibit dimer rows with similar geometrical spacings, with the lattice constant of germanium being 4% larger.

Termination of the Ge(001) surface by H₂S has been suggested [3] in order to electrically passivate the surface. Sulfur is an atom with flexible chemical bonds and thus is expected to make two bonds with the Ge(001)-(2x1) surface. An experimental study [4] of a Ge (001) surface exposed to H₂S in the gas phase showed 1 monolayer of sulfur coverage with (2x1) surface reconstruction. The amount of sulfur on the germanium surface and the observed periodicity is explained by formation of disulfide bridges between Ge-Ge dimers on the surface. A first-principles molecular dynamics study [4] confirmed that the (2x1) symmetry is preserved after adsorption of the H₂S molecules on the Ge(001)-(2x1) surface and also predict formation of (S-H)-(S-H) inter Ge dimer bridges i.e. disulfide bridges interacting via hydrogen bonding. The computed energy band gap of this atomic configuration is shown to be free of surface states, a very important finding for the potential application of Ge in future high performance integrated circuits. For comparison, using elemental sulfur the surface reconstruction is a (1x1) structure and the computed density of states clearly shows a state in the germanium energy band gap.

Using first-principles density functional theory we calculate the structural and vibrational characteristics of a H₂S-terminated Ge surface. When the fully relaxed structure is obtained, we use the frozen phonon method to obtain the dynamical matrix, whereby the harmonic restoring forces are found upon moving each surface atom by a small displacement from the equilibrium position. A complete calculation of the dynamical matrix results in the vibrational mode frequencies of H₂S molecule on a Germanium(001)-(2x1) surface. The results can then be used as the signature of particular bonding geometries which can be compared to infrared spectroscopy results.

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Mark Harnett
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References

[1] P. W. Loscutoff and S. F. Bent, Annual Review of Physical Chemistry, 57, 467, (2006)
[2] C. B. Duke, Chem. Rev. 96, 1237 (1996)
[3] P. W. Loscutoff and S. F. Bent, Annual Review of Physical Chemistry, 57, 467, (2006)
[4] M. Houssa, D. Nelis, D. Hellin, G. Pourtois, T. Conard, K. Paredis, K. Vanormelingen, A. Vantomme, M. K. Van Bael, J. Mullens, M. Caymax, M. Meuris, and M. M. Heyns, App.Phys. Lett, 90, 222105 (2007)