B. K. Meyer

Nitrogen-related local vibrational modes in ZnO:N

We study the influence of nitrogen, a potential acceptor in ZnO, on the lattice dynamics of ZnO. A series of samples grown by chemical vapor deposition (CVD) containing different nitrogen concentrations, as determined by secondary ion mass spectroscopy (SIMS), was investigated. The Raman spectra revealed vibrational modes at 275, 510, 582, 643, and 856 cm−1 in addition to the host phonons of ZnO. The intensity of these additional modes correlates linearly with the nitrogen concentration and can be used as a quantitative measure of nitrogen in ZnO. These modes are interpreted as local vibrational modes. Furthermore, SIMS showed a correlation between the concentration of incorporated nitrogen and unintentional hydrogen, similar to the incorporation of the p-dopant magnesium and hydrogen in GaN during metalorganic CVD.

Behind the weak excitonic emission of ZnO quantum dots: ZnO/Zn(OH)2 core-shell structure

The structure of ZnO quantum dots prepared via the wet chemical method was studied. By introducing an annealing treatment (150 °C–500 °C), we also investigated the effect of the change in the structure of the dots on their luminescence properties. Our studies revealed that the surface of the as-prepared dots is passivated by a thin layer of Zn(OH)2, thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. We present evidence that the weak excitonic transition of ZnO quantum dots is strongly correlated with the presence of the surface shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched.

Modified compensation model of CdTe

The traditional compensation model to explain the high resistivity properties of CdTe is based on the presence of a deep acceptor level of the cadmium vacancy in the middle of the band gap. A new compensation model based on a deep intrinsic donor level is presented. The compensation model is used together with an appropriate segregation model to calculate axial distributions of resistivity which are compared with spatially resolved resistivity measurements. The Te-antisite defect is discussed as a possible origin cause of this intrinsic defect, which is also supported by theoretical calculations.

Tungsten and fluorine co-doping of VO2 films

Abstract Fluorine- and tungsten-doped vanadium dioxide (VO2) is a promising coating material for applications as energy-conserving windows. We prepared VO2 films simultaneously co-doped with fluorine and tungsten and report on the results of optical measurements and photoelectron spectrometry. A comparison is given to single-element doping. Clear evidence for an interaction of fluorine and tungsten in VO2 is found in the switching behaviour at the semiconductor-to-metal phase transition. An explanation is given based on two different effects of fluorine incorporation observed in the ultraviolet photoelectron spectrometry results. Concerning other film properties, the two elements act independently of each other.

Dependence of the electrical and optical behaviour of ITO–silver–ITO multilayers on the silver properties

Abstract ITO–metal–ITO (IMI) multilayers and ITO single layers were prepared by DC-magnetron sputtering with different oxygen concentrations of the sputtering atmosphere during ITO deposition, and different substrate temperatures during a subsequent annealing treatment in vacuum. The intermediate metal layer consisted of 10 or 30 nm silver. Electron concentration and mobility of the Ag partial layer were determined from measurements of the electrical resistivity and by evaluating the complex dielectric function, obtained from optical transmission and reflectance. Microstructure and purity were studied with X-ray diffraction (XRD) and with X-ray photo electron spectroscopy (XPS) depth profiles. With a 10-nm intermediate silver layer an IMI sheet resistance of 4.7 Ω/□. was achieved. The Ag films had almost bulk electron concentrations but reduced electron mobilities. The ITO crystallinity in IMI systems influenced the microstructure and purity of the Ag layer. Minimum Ag resistivity and optimum Ag purity were obtained by ITO deposition under stoichiometric conditions and annealing in vacuum at 300°C. An increase of the ITO lattice constant is indicative that as-deposited IMI multilayers prepared with ITO deposition at high oxygen concentration or under stoichiometric conditions are associated with stress. The presence of the Ag layer impedes the lattice relaxation upon annealing.

Structural properties and bandgap bowing of ZnO1−xSx thin films deposited by reactive sputtering

A series of ZnO1−xSx films with 0⩽x⩽1.0 was deposited by radio-frequency reactive sputtering on different substrates. The structural characterization by x-ray diffraction measurements revealed that the films have wurtzite symmetry and correlated investigations of the layer composition by photoelectron spectroscopy showed that the lattice constant varies linearly with x. The composition dependence of the band gap energy in the ternary system was determined by optical transmission and the optical bowing parameter was found to be about 3eV.

W- and F-doped VO2 films studied by photoelectron spectrometry

Thermochromic tungsten- and fluorine-doped vanadium dioxide films, which are in discussion as intelligent window coatings, were deposited by reactive sputtering. Results of optical measurements and photoelectron spectrometry (XPS, UPS) at low doping levels (≤2.6%) are presented, and together with structural properties they can be well correlated. By applying antireflective coatings the transmittance of films in the visible spectral range may be enhanced to more than 60% with fairly good switching characteristics at room temperature in the case of tungsten doping.

Shallow donors and acceptors in ZnO

In order to realize controlled p-type doping in ZnO the role of extrinsic and intrinsic donors has to be clarified. The extrinsic n-type dopants Al, Ga and In are commonly found in bulk ZnO crystals, but hydrogen also appears in relevant concentrations eventually controlling the residual n-type carrier concentrations in nominally undoped ZnO. The optical properties of excitonic recombinations in bulk, n-type ZnO are investigated by photoluminescence (PL). At liquid helium temperature the neutral donor–bound excitons dominate in the PL spectrum. Two electron satellite (TES) transitions of the donor–bound excitons allow us to determine the donor binding energies ranging from 46 to 73 meV. In the as-grown crystals a shallow donor with an activation energy of 30 meV controls the conductivity. Annealing annihilates this shallow donor which has a bound exciton recombination at 3.3628 eV. Correlated by magnetic resonance experiments we attribute this particular donor to hydrogen. These results are in line with the temperature-dependent Hall-effect measurements. The Al, Ga and In donor–bound exciton recombinations are identified based on doping and diffusion experiments, and using secondary ion mass spectroscopy. We report on the optical properties of the shallow nitrogen acceptor in ZnO incorporated by diffusion, by ion implantation and by in situ doping in epitaxial films.

Determination of the Conduction Band Electron Effective Mass in Hexagonal GaN

The electron effective mass in hexagonal GaN films grown by metal organic vapor phase epitaxy on sapphire substrates is determined by cyclotron resonance experiments. Its value is m p* = 0.22±0.005 m o. Taking polaron effects into account the band edge mass is m b* = 0.20±0.005 m o. From the resonance linewidth a mobility of 3500 cm2/Vs at 6 K is obtained.

Giant internal magnetic fields in Mn doped nanocrystal quantum dots

Abstract We observed a giant splitting of exciton spin sublevels in CdS nanocrystals, each doped on the average by one Mn ion. The splitting, which exists in zero external magnetic field, is caused by the gigantic internal magnetic field of the Mn ion and results from the enhancement of the short range spin–spin interactions in nanocrystal quantum dots. The splitting is seen in the strong magnetic circular dichroism of the CdS band edge transitions. The magnitude of the observed band edge splitting is in good agreement with a theoretical calculation of the effective magnetic field.