Gerald Wagner

Infrared dielectric functions and phonon modes of high-quality ZnO films

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300–1200 cm−1) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry anal...

Infrared dielectric functions and phonon modes of wurtzite MgxZn1-xO (x≤0.2)

Infrared dielectric function spectra and phonon modes with polarization parallel and perpendicular to the c axis of high quality, highly relaxed, and single crystalline wurtzite MgxZn1−xO films with 0⩽x⩽0.2 prepared by pulsed-laser deposition on c-plane sapphire substrates were obtained from infrared spectroscopic ellipsometry (380–1200 cm−1) and Raman scattering studies. A two-mode behavior is found for the modes with E1 symmetry, a lattice mode and an impurity-type mode are obtained for the A1 symmetry phonons. Model dielectric function spectra will become useful for future infrared ellipsometry analysis of complex MgxZn1−xO-based heterostructures.

Band-gap energies, free carrier effects, and phonon modes in strained GaNAs/GaAs and GaNAs/InAs/GaAs superlattice heterostructures measured by spectroscopic ellipsometry

Spectroscopic ellipsometry (SE) is employed to study the optical properties of compressively strained short-period GaAs/InAs/GaNxAs1−x (0%<x<2.4%) superlattice (SL) heterostructures for photon energies from 0.75 to 1.55 eV (NIR-SE), and for wave numbers from 250 to 700 cm−1 (IR-SE). The undoped SL structures were grown on top of undoped GaAs buffer layers deposited on Te-doped (001) GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). The InAs sequences consist of single monolayers. Structure, composition, layer thicknesses, and parallel and perpendicular lattice mismatch of the samples are studied by high-resolution transmission electron microscopy and high-resolution x-ray diffraction investigations. We employ Adachi’s critical-point composite model for data analysis in the near-band-gap spectral region (NIR-SE). For analysis of the IR-SE data we use the harmonic oscillator dielectric function model and the Drude model for free-carrier response. We report the direct band-gap energy E0, and the c...

Temperature-dependent dielectric and electro-optic properties of a ZnO-BaTiO3-ZnO heterostructure grown by pulsed-laser deposition

Temperature-dependent dielectric and electro-optic properties of a ZnO-BaTiO3-ZnO heterostructure grown by pulsed-laser deposition on (0001) sapphire are reported. The wurtzite-structure ZnO layers serve as transparent conducting electrodes. Previously observed coupling effects within the wurtzite-perovskite heterostructure by spectroscopic electro-optic ellipsometry birefringence measurements manifest themselves as a “pinning” of the ferroelectric polarization in the BaTiO3 layer by the cladding ZnO layers. Temperature-controlled electro-optic Raman measurements assign the electro-optic birefringence results to a temperature-driven phase transition resulting from the leakage current within the sample. High-temperature small-signal capacitance measurements exploiting the conductive electrode properties of the cladding layers reveal occurrence of the Curie temperature of the BaTiO3 layer at 384 K.

Correlation of magnetoelectric coupling in multiferroic BaTiO3-BiFeO3 superlattices with oxygen vacancies and antiphase octahedral rotations

Multiferroic (BaTiO3-BiFeO3) × 15 multilayer heterostructures show high magnetoelectric (ME) coefficients αME up to 24 V/cm·Oe at 300 K. This value is much higher than that of a single-phase BiFeO3 reference film (αME = 4.2 V/cm·Oe). We found clear correlation of ME coefficients with increasing oxygen partial pressure during growth. ME coupling is highest for lower density of oxygen vacancy-related defects. Detailed scanning transmission electron microscopy and selected area electron diffraction microstructural investigations at 300 K revealed antiphase rotations of the oxygen octahedra in the BaTiO3 single layers, which are an additional correlated defect structure of the multilayers.

Recrystallization behavior in chiral sculptured thin films from silicon

Chiral sculptured thin films, which contain amorphous silicon screws with a fiberlike fine structure, were grown by ion-beam-assisted glancing-angle deposition at room temperature. The thin films were postgrowth annealed in the temperature range from 400to1000°C. Raman spectroscopy and transmission electron microscopy investigations performed before and after sample annealing reveal a recrystallization of silicon at temperatures above 800°C, with a persistence of the chiral structure geometry and fine structure. The Raman results are further discussed in terms of quantum confinement and coexisting phase effects.

Comparative study of transparent rectifying contacts on semiconducting oxide single crystals and amorphous thin films

We demonstrate fully transparent, highly rectifying contacts (TRC) on amorphous GaInZnO and compare them to TRC fabricated on single crystalline bulk ZnO and heteroepitaxial ZnO thin films. The contacts’ transparency in the visible spectral range exceeds 70%. From numerical simulations, we conclude that thermionic emission is the dominating transport mechanism, however, for several samples with low net doping density diffusion theory must be applied. The detailed investigation of the rectification properties of the TRC using temperature-dependent current-voltage and capacitance-voltage measurements reveals that barrier inhomogeneities govern the IV-characteristics of all diodes irrespective of the sample crystallinity. Assuming a Gaussian barrier height distribution, the extracted mean barrier heights typically range between 1.1 and 1.3 V. The width of the barrier distribution correlates with the mean barrier height and ranges from 110 to 130 mV. By compiling literature data, we found that this correlatio...

The influence of the droplet composition on the vapor-liquid-solid growth of InAs nanowires on GaAs (1¯1¯1¯)B by metal-organic vapor phase epitaxy

The heteroepitaxial growth of InAs nanowires (NWs) on GaAs (1¯1¯1¯)B substrate was investigated by metal-organic vapor phase epitaxy. The vapor-liquid-solid (VLS) growth mechanism was applied with gold as seed material. InAs NW with two types of morphology were observed. The first morphology type exhibited a tapered NW shape. In a distinct region below the alloy particle the shape was influenced by the precursor surface diffusion. The NW growth was attributed to Au-rich liquid alloy particles containing gallium as a result of the initial Au–GaAs interaction. Differential scanning calorimetry measurements revealed the lowest eutectic temperature of the Au–Ga–In liquid alloy for different compositions. For a considerable amount of gallium inside the ternary alloy, the eutectic temperature was found to be below the InAs NW growth temperature window. A second type of morphology with a more columnlike shape was related to a very high indium fraction inside the liquid alloy particle during VLS growth. These NW ...

Discovery and Structure Determination of an Unusual Sulfide Telluride through an Effective Combination of TEM and Synchrotron Microdiffraction.

The structure elucidation of the novel sulfide telluride Pb8Sb8S15Te5 demonstrates a new versatile procedure that exploits the synergism of electron microscopy and synchrotron diffraction methods for accurate structure analyses of side-phases in heterogeneous microcrystalline samples. Suitable crystallites of unknown compounds can be identified by transmission electron microscopy and relocated and centered in a microfocused synchrotron beam by means of X-ray fluorescence scans. The refined structure model is then confirmed by simulating HRTEM images of the same crystallite. Pb8Sb8S15Te5 consists of chains of heterocubane-like units. Cation coordination polyhedra form unusually entwined chains of edge- and face-sharing bicapped trigonal prisms. The structure data are precise enough for bond-valence calculations, which confirm the disordered atom distribution. On this basis, optimization of physical properties becomes feasible.

MgZnO:P homoepitaxy by pulsed laser deposition: pseudomorphic layer-by-layer growth and high electron mobility

Homoepitaxial ZnO thin films doped with phosphorus (0.01% to 1% P) and/or alloyed with magnesium (1% to 4% Mg) show pseudomorphic growth with compressive or tensile strain in dependence on the dopant concentration. The structural quality of the used O-face ZnO(001) substrates was inspected by the rocking curves of the symmetric (002) and the skew-symmetric (101) peaks. Preselection of the substrate batches by the supplier decreased the twist dislocation density and increased the structural homogeneity within the batches considerably. TEM cross sections show increasing density of c-plane defects with increasing phosphorus concentration in the films. ZnO(002) rocking curves of MgZnO:P films on ZnO were as narrow as 27 arcsec with a FWHM of the substrate peak of 23 arcsec. The in-plane lattice match was confirmed for all dopant concentrations by HR-XRD triple axis scans of the (002) and (101) peaks. The results show the balance between tensile strain induced by Mg and compressive strain by P in ZnO. Two-dimensional growth with terrace-like surface structure is most prominent for the Mg-alloyed films without P. High electron mobilities up to 190 cm2/Vs at 300K and up to 800 cm2/Vs at 70 K were found in the homoepitaxial MgZnO:P thin films.