Distribution of nitrogen and phosphorus in sediment cores and overlying water of Donghu Lake

In this study, the seasonal, vertical distribution of various phosphorus and nitrogen forms in the sediment and overlying water of Donghu Lake were investigated. The concentration of total nitrogen (TN) in overlying water was high in spring and autumn, but that of NO3--N reached its peak in autumn. From summer to autumn and from winter to spring, the concentration of phosphorus in overlying water decreased, while it increased from autumn to winter. Vertical characteristic forms of phosphorus in sediment cores are total phosphorus (TP), labile phosphorus (LP), Fe-P and Al-P, obviously enriched in the surface layer (0-10 cm), but their concentrations are observably reduced along with the depth of sediment. The research is of important theoretical and practical value to understand the status and to control the developmental trend of eutrophication in Donghu Lake.

Sedimentation rates, nitrogen and phosphorus retentions in the largest urban Lake Donghu, China

Sediment core samples were collected in the largest urban Lake Donghu (Stations I and II) in China, and the activities of Pb-210, Ra-226 and Cs-137 were measured by gamma-ray spectrometry. The sedimentation rates, calculated by 210Pb constant rate of supply (CRS) model, ranged from 0.11 to 0.65 (average 0.39) cm(.)y(-1) at Station I, and from 0.21 to 0.78 (average 0.46) cm(.)y(-1) at Station II. Sedimentation rate calculated by Cs-137 as a time marker was 0.55 cm(.)y(-1) at Station II. Based on the average sedimentation rate, we obtained 769 and 147 t(.)y(-1) for nitrogen and phosphorus retentions in Lake Donghu sediments, respectively.

Budgets and dynamics of nitrogen and phosphorus in a shallow, hypereutrophic lake in China

Budgets and dynamics of nitrogen and phosphorus in Lake Donghu were investigated from Oct. 1997 to Sept. 1999. The water residence time was estimated to be 89 days in 1997-1998 and 124 days in 1998-1999. The total external loadings were 53 g N m(-2) yr(-1) and 3.2 g P m(-2) yr(-1) in 1997-1998, and 42 g N m(-2) yr(-1) and 3.1 g P m(-2) yr(-1) in 1998-1999. On average, about 80% of nitrogen and phosphorus input was from sewage outlets, while the rest was from land runoff and precipitation. Ammonium ion was the most abundant form of inorganic nitrogen in the sewage. The nutrient output was mainly through water outflow and fish catch. The percentages of nutrients in fish were estimated to be 7.8%-11.2% for nitrogen and 47.6%- 49.6% for phosphorus. Lake Donghu has a very high nutrient retention (63% for nitrogen and 79% for phosphorus) mainly due to its closure and long water residence time. Sedimentation is an important nutrient retention mechanism in this lake. Using mass balance method, we estimated that denitrification of Lake Donghu involves about 50% of the retained nitrogen. Lake Donghu is rich in inorganic nitrogen and phosphorus and showed great seasonal variation.

Electronic structures of wurtzite ZnO, BeO, MgO and p-type doping in Zn1-xYxO (Y = Mg, Be)

Using the density function theory within the generalized gradient approximation, the band structures of wurtzite ZnO, BeO and MgO have been calculated. The effective-mass parameters are fitted using the calculated eigenvalues. The Dresselhaus spin-orbit effect appears in the k[1 00] direction, and is zero in the high symmetry direction k[00 1]. The orderings of valence band split by the crystal-field and spin-orbit coupling in wurtzite ZnO, BeO and MgO are identified by analyzing the wave function characters calculated by projecting the wave functions onto p-state in the spherical harmonics. For wurtzite ZnO, the ordering of valence band is Still Gamma(7) > Gamma(9) > Gamma(7) due to the negative spin-orbit coupling splitting energy and the positive crystal-field splitting energy. Thus, the Thomas' conclusion is confirmed. For wurtzite BeO and MgO, although their orderings of valence bands are Gamma(7) > Gamma(9) > Gamma(7) too, the origins of their orderings are different from that of wurtzite ZnO. Zn1-x,YxO (Y = Mg, Be) doped with N and P atoms have been studied using first-principles method. The calculated results show that N atom doped in Zn1-x BexO has more shallow acceptor energy level with increasing the concentration of Be atom. (C) 2008 Elsevier B.V. All rights reserved.

First principle study of Mg, Si and Mn co-doped GaN

Calculations of electronic structures and optical properties of Mg (or Si) and Mn co-doped GaN were carried out by means of first-principle plane-wave pesudopotential (PWP) based on density functional theory - The spin polarized impurity bands of deep energy levels were found for both systems. They are half metallic and suitable for spin injectors. Compared with GaN Mn, GaN Mn-Mg exhibits a significant increase in T-C 1 while the 1.3 eV absorption peak in GaN Mn disappears due to addition of Mg. In addition, a strong absorption peak due to T-4(1) (F) -> T-4(2) (F) transition of Mn4+ were observed near 1.1 eV. Nevertheless, GaN Mn-Si failed to show increase of T-C, and the absorption peak was not observed at the low energy side.

Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N-O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.

Effects of edge dislocations and intentional Si doping on the electron mobility of n-type GaN films

The effects of dislocations and Si doping on the electrical properties of n-type GaN grown by metal organic chemical vapor deposition (MOCVD) are investigated. It is found that both electron mobility and carrier concentration are strongly influenced by edge dislocations. A moderate Si doping during the GaN growth improves the electron mobility, but the best doping effect depends on the dislocation density of the sample. High quality about 4-mu m-thick MOCVD-grown GaN film with a room temperature electron mobility as high as 1005 cm(2)/V s is obtained by optimizing growth conditions. (c) 2006 American Institute of Physics.

INTERFACIAL REACTIONS IN THE CO/SI/GAAS AND SI/CO/GAAS SYSTEMS

The interfacial reactions between thin films of cobalt and silicon and (100)-oriented GaAs substrates in two configurations, Co/Si/GaAs and Si/Co/GaAs, were studied using a variety of techniques including Auger electron spectroscopy, x-ray diffraction, and transmission electron microscopy. The annealing conditions were 200, 300, 400, 600-degrees-C for 30 min, and rapid thermal annealing for 15 s. It was found that Si layer in the Co/Si/GaAs system acts as a barrier at the interface between Co and GaAs when annealed up to 600-degrees-C. The interfacial reaction between Co and Si is faster than that between Co and GaAs in the system of Si/Co/GaAs. The sequence of compound formation for the two metallizations studied (Co/Si/GaAs and Si/Co/GaAs) depends strongly on the sample configuration as well as the layer thickness of Si and Co (Co/Si atomic ratio). From our results, it is promising to utilize Co/Si/GaAs multilayer film structure to make a CoSi2/GaAs contact, and this CoSi2 may offer an alternative to the commonly used W silicides as improved gate metallurgies in self-aligned metal-semiconductor field effect transistor (MESFET) technologies.

Thermodynamic model of helium and hydrogen co-implanted silicon surface layer splitting

A thermodynamic model of the evolution of microcracks in silicon caused by helium and hydrogen co-implantation during annealing was studied. The crack growth rate relies on the amount of helium atoms and hydrogen molecules present. Here, the crack radius was studied as a function of annealing time and temperature, and compared with experimental results. The mean crack radius was found to be proportional to the annealing temperature and the helium and hydrogen implanted fluence. The gas desorption should be considered during annealing process. (C) 2009 Elsevier B.V. All rights reserved.