The temperature dependence of diffusion length, lifetime, and diffusivity of the free exciton is measured in a commercial ZnO-substrate as well as in an epitaxial ZnO quantum well using nm-spatially and ps-time-resolved cathodoluminescence spectroscopy. The characteristic temperature dependence of the exciton mobility gives information of the underlying excitonic scattering processes. Since excitons are neutral particles, scattering at ionized impurities should be not effective. On both samples, with decreasing temperature, the diffusion lengths, lifetimes, and diffusivity give rise to a monotonic increase of the excitonic mobility. Two different methods will be discussed and are used for determining the excitonic transport parameters. On the one hand, we are able to perform completely pulsed excitation experiments, and on the other hand, a combination of cw excitation and pulsed excitation in two independent measurements is used.

OLED with non-constant dopant concentration profiles have been processed by means of organic vapour phase deposition (OVPD) and were compared with regard to their luminous current efficiencies. Especially when driven at ultra-high luminance (>10,000 cd/A), OLED with a dopant concentration profile starting with a rather high dopant concentration on the anode side of the emissive layer showed improved luminous current efficiencies compared to their conventional counterparts.

To further investigate this effect, the width and location of the recombination zone have been simulated for all investigated concentration profiles by numerical solution of the semiconductor device equations using experimentally determined doping-dependent charge carrier mobilities. The obtained theoretical results are discussed with regard to the accomplished experiments.

The current and luminous efficacy of a red phosphorescent organic light emitting diode (OLED) with sharp interfaces between each of the organic layers can be increased from 18.8 cd/A and 14.1 lm/W (at 1,000 cd/m2) to 36.5 cd/A (+94%, 18% EQE) and 33.7 lm/W (+139%) by the introduction of a layer cross-fading zone at the hole transport layer (HTL) to emission layer (EL) interface. Layer cross-fading describes a procedure of linearly decreasing the fraction in growth rate of an organic layer during deposition over a certain thickness while simultaneously increasing the fraction in growth rate of the following layer. For OLED processing and layer cross-fading organic vapor phase deposition (OVPD) is used. The typical observation of a roll-off in current efficacy of phosphorescent OLED to higher luminance can be reduced significantly. An interpenetrating network of a prevailing hole and a prevailing electron conducting material is created in the cross-fading zone. This broadens the recombination zone and furthermore lowers the driving voltage. The concept of layer cross-fading to increase the efficacies is suggested to be useful in multi-colored OLED stacks as well.

ZnO as a direct wide-band-semiconductor with its band gap of 3.3 eV at room temperature is a promising optoelectronic material. The main obstacle in the ZnO system is its lack of achieving reproducible p-type conductivity. The main reasons for this are the high residual intrinsic and extrinsic defect concentrations which are still not completely understood.

Homoepitaxial growth of ZnO and thus minimization of intrinsic defects due to lattice mismatch and incorporation of residual substrate species could be a solution to overcome these problems. Despite the availability of ZnO bulk single crystals reports regarding ZnO homoepitaxy are still quite rare. In this paper we report on a successful homoepitaxial growth of ZnO thin films by chemical vapor deposition (CVD).

Atomic force microscopy shows that two-dimensional epitaxial growth was achieved without any additional buffer layer. With a rocking curve full width at half maximum (FWHM) of 17 arcsec the deposited films show a superior crystalline quality compared to its substrate. The optical quality of the epitaxial films has been characterized laterally by cathodoluminescence and spectrally by photoluminescence. Excitonic emissions at 4K are as narrow as 110 μeV. A dependence of the appearance of excitonic emissions from the growth polarity can be shown which is attributed to different incorporation rates of extrinsic defects.

The visible and UV plasma emissions from a metal vapor arc were studied during vacuum arc remelting of 165 mm diameter Inconel 718 electrodes using emission spectroscopy. Internal energy distributions of a number of emitting species were analyzed using Boltzmann statistics. Analysis of the data yielded distribution “temperatures” ranging from 5,400 K to 8,200 K for the neutral atomic arc species, whereas values of 12,200 K to 17,500 K were obtained for singly ionized species. These values illustrate that the arc is not in a state of local thermodynamic equilibrium. The excited state densities of various arc constituents were measured and it was found that highly volatile alloy trace components, such as Mn, were present in the arc in dispro-portionately high concentrations. These data suggest that ionized evaporates play an important role in carrying current between the electrodes.