Andreas Stegmüller

β-Hydrogen Elimination Mechanism in the Absence of Low-Lying Acceptor Orbitals in EH2(t-C4H9) (E = N–Bi)

The β-hydrogen elimination reactions of group 15 alkyl compounds at the example of EH2(t-C4H9) (element E = N-Bi) were investigated and compared to the group 13 example of GaH2(t-C4H9). With the aid of extensive density functional theory based analysis of atomic and electronic structures at the transition state, we can derive three distinct reaction classes. The gallium compound follows the well-known β-hydride route with participation of an empty p orbital at the metal in a concerted, synchronous fashion, exhibiting a low barrier. For compounds with group 15 elements, we find highly nonsynchronous reactions with high reaction barriers. In the case of nitrogen, a proton-like H atom is transferred via attack of the nitrogen nonbonding electron pair. For the heavier homologues (P-Bi), E-Cα bond breaking occurs first and the H atom does not carry charge at the transition state. The reaction barrier in group 15 homologues is thus determined by the E-Cα bond strength down the group. The results enable a rationale for ligand design for precursors involved in chemical vapor-phase deposition processes because a good ligand needs to stabilize the positive charge at Cα.

Surface Chemistry of tert-Butylphosphine (TBP) on Si(001) in the Nucleation Phase of Thin-Film Growth.

We combine density functional theory calculations and scanning tunneling microscopy investigations to identify the relevant chemical species and reactions in the nucleation phase of chemical vapor deposition. tert-Butylphosphine (TBP) was deposited on a silicon substrate under conditions typical for surface functionalization and growth of semiconductor materials. On the activated hydrogen-covered surface H/Si(001) it forms a strong covalent P-Si bond without loss of the tert-butyl group. Calculations show that site preference for multiple adsorption of TBP is influenced by steric repulsion of the adsorbates bulky substituent. STM imaging furthermore revealed an anisotropic distribution of TBP with a preference for adsorption perpendicular to the surface dimer rows. The adsorption patterns found can be understood by a mechanism invoking stabilization of surface hydrogen vacancies through electron donation by an adsorbate. The now improved understanding of nucleation in thin-film growth may help to optimize molecular precursors and experimental conditions and will ultimately lead to higher quality materials.

Interfacial Properties and Growth Dynamics of Semiconductor Interfaces

We present computational results on dynamics and properties of semiconductor materials and interfaces. The adsorption of cyclooctyne on silicon can be shown to proceed barrierless into an on-top structure. Comparing different interfaces of the GaP/Si system, a preference for mixed interfaces (i.e. not purely Si/Ga or Si/P) can be found and understood in terms of the electrostatic potential across the interface and chemical bonding specifics. In further work, the electronic structure of mixed III/V semiconductors will be studied in the way described here for GaAs and used for the prediction of optical properties.

Growth, Structural and Electronic Properties of Functional Semiconductors Studied by First Principles

Ab initio calculations of thermodynamic, electronic and vibrational properties of functional semiconductor materials relevant for silicon photonics were performed. The thermodynamics of hydrogen coverage on Si(001) was investigated as important chemical growth processes depend on surface structure and govern the structural quality of the materials deposited. Exemplarily, the influence of strain and chemical effects on the band gaps was studied by the two optically active alloys Ga(NAsP) and dilute Ga(AsBi). Furthermore, electron-vibron coupling of NTCDA molecules in an interface with the Ag(111) surface was investigated, identifying interfacial dynamical charge transfer to occur and enable the detection of in-plane IR modes. The DFT calculations on electronic structure and dynamics have been found in good agreement with experimental observations.

GaP/Si: Studying Semiconductor Growth Characteristics with Realistic Quantum-Chemical Models

The understanding of microscopic processes and properties is crucial for the development and efficient production of inorganic III/V semiconductor materials. Those materials are grown in chemical vapour deposition procedures where elementary steps have not yet been thoroughly understood. Ab initio calculations are capable to investigate those atomic and electronic properties. Modern implementations of Density Functional Theory were applied to study layered bulk structures, periodic surface properties and adatom transport on Si(001) and GaP-Si(001) materials. By increasing cell sizes and number of atoms to scales that only supercomputing facilities can handle, a realistic chemical environment can be modeled with increased structural degrees of freedom. Bulk supercells were constructed in order to model realistic interfaces between two thin films in the nanometer scale. Supercell models in slab geometry were set up and converged with respect to the volume of vacuum and number of relaxed atoms for an accurate description of slab surfaces. These studies enable a direct comparison to experimental studies on these materials.

From Molecules to Thin Films: GaP Nucleation on Si Substrates

Silicon is prominently used as a substrate for a variety of functionalized materials. Those are tuned towards direct band gap semiconductor materials or for subsequent adsorption and growth of optically active organic compounds.