Semipolar ð1122Þ oriented GaN has been grown on a prestructured r-plane sapphire substrate. By using silicon doped marker layers (MLs) we have been able to monitor the growth evolution of the stripes until coalescence. With that technique we correlated the growth type (direction) with the results of cathodoluminescence (CL) and transmission electron microscopy. Both characterization methods show only a few defects for the major part of the structure and a relatively high defect density for material grown in a-direction at one side of the stripes. It is shown that during coalescence these defects are mainly terminated resulting in a flat, planar ð1122Þ GaN layer with strongly reduced defect density. Additionally, X-ray diffraction (XRD) measurements show the high quality of these layers.

We report on the growth of planar semipolar ͑1011͒ GaN on ͑1123͒ prepatterned sapphire. This is a method that allows the growth of semipolar oriented ͑1011͒ GaN on large scale. Using x-ray diffraction only the peaks of the desired ͑1011͒ plane could be observed. Scanning electron, transmission electron, and atomic force microscopy measurements show an atomically flat surface. Further investigations using photoluminescence spectroscopy show spectra that are dominated by the near band edge emission. The high crystal quality is furthermore confirmed by the small full width at half maximum values of x-ray rocking curve measurements of less than 400 arcsec.

SummaryArsenic mobility, bioavailability, and toxicity in soil-water systems are strongly affected by species distribution. In spite of species-selective analytical tools, results may be biased by postsampling oxidation or reduction of compounds. The scope of this study was to delineate a speciation method for inorganic arsenic integrated with pore-water sampling to prevent problems related to postsampling species transformation. Experiments were performed with exchange cartridges packed with quaternary amine (nitrate-form) to separate anionic arsenate from uncharged arsenite. Standard solutions with varied As(III) : As(V) ratios as well as groundwater and soil-solution samples were studied. Analyses were performed by ICP-MS using either extraction cartridges directly coupled to the instrument or samples obtained by offline speciation tests. Results showed that As(III) passes the packed bed with the rinsing solution, while As(V) is retained on the column and can be released by elution with 0.25 M NH 4 NO 3 .Recoveries between 98% and 116% were insensitive to pH variation (pH 3-8) and competing anions. Groundwater samples from a contaminated site yielded recoveries between 92% and 125% when the sum of As(III) and As(V) was compared to total As. Integration of the speciation scheme with pore-water sampling was tested in a soil-column experiment. Soil material from a fen site with elevated geogenic As contents was subjected to a varying moisture regime. Reducing conditions during prolonged saturation caused a marked increase of As(III) concentrations. Conversely, when the soil was drained, total As levels decreased and were dominated by As(V). Overall, extraction results with standard solutions and natural samples indicate reliable performance of the method in experimental investigations and field studies.