Christoph Cobet

Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives

This paper discusses the fundamentals, applications, potential, limitations, and future perspectives of polarized light reflection techniques for the characterization of materials and related systems and devices at the nanoscale. These techniques include spectroscopic ellipsometry, polarimetry, and reflectance anisotropy. We give an overview of the various ellipsometry strategies for the measurement and analysis of nanometric films, metal nanoparticles and nanowires, semiconductor nanocrystals, and submicron periodic structures. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about process control, geometry factors, anisotropy, defects, and quantum confinement effects of nanostructures.

Substrate influence on the optical and structural properties of pulsed laser deposited BiFeO3 epitaxial films

Epitaxial BiFeO3 films pulsed laser deposited on SrTiO3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations.

Dielectric function of zinc-blende AlN from 1 to 20 eV: Band gap and van Hove singularities

The dielectric function (DF) of phase-pure cubic AlN films is determined by ellipsometry. The sharp onset of the imaginary part of the DF defines the direct absorption edge corresponding to a conduction-to-valence band spacing at the center of the Brillouin zone (BZ) of 5.93 eV. Phonon-assisted transitions lead to the pronounced absorption tail below this edge from which the indirect gap of zinc-blende AlN is estimated with 5.3 eV. Transitions due to four additional critical points of the BZ are resolved at higher photon energies. The high-frequency and static dielectric constants are determined with 4.25 and 8.07, respectively.

Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite MgxZn1−xO grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The ...

Structural and optical properties of DNA layers covalently attached to diamond surfaces

Label-free detection of DNA molecules on chemically vapor-deposited diamond surfaces is achieved with spectroscopic ellipsometry in the infrared and vacuum ultraviolet range. This nondestructive method has the potential to yield information on the average orientation of single as well as double-stranded DNA molecules, without restricting the strand length to the persistence length. The orientational analysis based on electronic excitations in combination with information from layer thicknesses provides a deeper understanding of biological layers on diamond. The pi-pi* transition dipole moments, corresponding to a transition at 4.74 eV, originate from the individual bases. They are in a plane perpendicular to the DNA backbone with an associated n-pi* transition at 4.47 eV. For 8-36 bases of single- and double-stranded DNA covalently attached to ultra-nanocrystalline diamond, the ratio between in- and out-of-plane components in the best fit simulations to the ellipsometric spectra yields an average tilt angle of the DNA backbone with respect to the surface plane ranging from 45 degrees to 52 degrees . We comment on the physical meaning of the calculated tilt angles. Additional information is gathered from atomic force microscopy, fluorescence imaging, and wetting experiments. The results reported here are of value in understanding and optimizing the performance of the electronic readout of a diamond-based label-free DNA hybridization sensor.

Dielectric function and optical properties of quaternary AlInGaN alloys

The optical properties of quaternary Alx Iny Ga1-x-yN alloy films with 0.16< x<0.64 and 0.02< y<0.13 are presented. The (0001)-oriented AlInGaN layers were grown by metal-organic vapor phase epitaxy on thick GaN/sapphire templates. High-resolution x-ray diffraction measurements revealed the pseudomorphic growth of the AlInGaN films on the GaN buffer. Rutherford backscattering and wavelength-dispersive x-ray spectroscopy analysis were used in order to determine the composition of the alloys. The ordinary dielectric function (DF) of the AlInGaN samples was determined in the range of 1–10 eV by spectroscopic ellipsometry (SE) at room temperature (synchrotron radiation: BESSY II). The sharp onset of the imaginary part of the DF defines the direct absorption edge of the alloys. At higher photon energies, pronounced peaks are observed in the DF indicating a promising optical quality of the material. These features are correlated to the critical points of the band structure (van Hove singularities). An analytica...

Atomic structure and optical anisotropy of III–V(001) surfaces

The optical anisotropy of materials with isotropic bulk crystal structure depends to a large extent on the surface atomic structure. For instance, data obtained by reflectance anisotropy spectroscopy (RAS) on (001) surfaces of zinc blende semiconductors such as InP and GaAs, have a fingerprint character for the various surface reconstructions. Here we present RAS spectra for GaAs(001) and InP(001) recorded at room temperature and at low temperature. We show that by comparison with a theoretical analysis based on ab initio density functional theory in local-density approximation calculations the origin of characteristic spectral features can be identified and thus RAS spectra utilized to discriminate between different competing structural models. We identify contributions related to electronic transitions between surface states as well as features arising from surface perturbed bulk wave functions. We explain the high sensitivity of RAS to the surface structure and chemistry as due to the surface state rel...

DNA Structures on Silicon and Diamond

In the design of DNA-based hybrid devices, it is essential to have knowledge of the structural, electronic and optical properties of these biomolecular films. Spectroscopic ellipsometry is a powerful technique to probe and asses these properties. In this chapter, we review its application to biomolecular films of single DNA bases and molecules on silicon and diamond surfaces characterized in the spectral range from the near-infrared (NIR) through the visible (Vis) and toward the vacuum ultraviolet (VUV). The reported optical constants of various DNA structures are of great interest, particularly in the development of biosensors.

Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry

The complex dielectric tensor of ZnO in the regime of the excitonic transitions is determined with ellipsometry and analyzed concerning the quantization of the electromagnetic field in terms of coupled polariton-eigenmodes. Negative sections in the real part indicate the significant formation of polaritons for the dipole-allowed excitons of the three upper valence-bands Γ7,Γ9,Γ7. The transverse-longitudinal splittings which separate the upper polariton branch from the lower branch, corresponding to the k-vector of the used light, are deduced precisely for each subband. Mainly for E∥c, additional absorption peaks are observed at the longitudinal B-exciton and closely above. One is considered to be a mixed-mode and the other is seen as a consequence of interference effects in an exciton free surface layer which is also visible in reflectance anisotropy spectroscopy.

Electronic and Optical Properties of Strontium Barium Niobate Single Crystals

Electronic structure of Sr x Ba1- x Nb2O6 crystals in the energy range involving the valence and conduction bands has been studied theoretically and experimentally using the DFT-based ab initio VASP code and the spectroscopic ellipsometry method with synchrotron radiation. The Sr0.61Ba0.39Nb2O6 sample of the congruently melting composition is characterized by the greatest difference of the experimental dielectric function ϵ(E) compared to the non congruently melting compositions of Sr x Ba1- x Nb2O6 (x = 0.40, 0.55, 0.65, 0.75). Electronic band structures of Sr0.60Ba0.40Nb2O6 and LiNbO3 crystals in the energy range of 6–8 eV and the corresponding dielectric functions ϵ(E) in the photon energy range of 3–10 eV have been found to be similar, what is caused by similar octahedral NbO6 complexes in the crystals.