Electron spin relaxation in cubic GaN quantum dots

The spin relaxation time $T_{1}$ in zinc blende GaN quantum dot is investigated for different magnetic field, well width and quantum dot diameter. The spin relaxation caused by the two most important spin relaxation mechanisms in zinc blende semiconductor quantum dots, {i.e.} the electron-phonon scattering in conjunction with the Dresselhaus spin-orbit coupling and the second-order process of the hyperfine interaction combined with the electron-phonon scattering, are systematically studied. The relative importance of the two mechanisms are compared in detail under different conditions. It is found that due to the small spin orbit coupling in GaN, the spin relaxation caused by the second-order process of the hyperfine interaction combined with the electron-phonon scattering plays much more important role than it does in the quantum dot with narrower band gap and larger spin-orbit coupling, such as GaAs and InAs.Comment: 8 pages, 5 figures, PRB 79, 2009, in pres

Coupling of spontaneous emission from GaN/AlN quantum dots into silver surface plasmons

We have demonstrated surface-plasmon induced change in spontaneous emission rate in the ultraviolet regime at ~ 375-380 nm, using AlN/GaN quantum dots (QD). Using time-resolved and continuous-wave photoluminescence measurements, the recombination rate in AlN/GaN QD is shown to be enhanced when spontaneous emission is resonantly coupled to a metal-surface plasmon mode. The exciton recombination process via Ag-surface plasmon modes is observed to be as much as 3-7 times faster than in normal QD spontaneous emission and depends strongly on the emission wavelength and silver thickness.Comment: 11 pages, 4 figure

Modeling and Validation of Denitrification Bioreactors for Agricultural Tile Drainage

Agricultural tile drainage is one of the major causes of increasing nitrate (NO3-) concentrations in surface water bodies thanks to the usage of nitrogen fertilizers and manure. Denitrifying bioreactors are constructed at the edge of agricultural lands in order to remove NO3- from drainage water through labile carbon substrates intended to promote denitrification. This field-scale study is the examination and modeling of NO3- removal performance of a woodchip bioreactor installed in Corvallis, OR. During flow periods, water samples were collected on a weekly basis for lab analysis of nitrate, nitrite and ammonium. NO3- concentrations measured in the influent varied in the range from 2 to 13 mg/L (on average 8 mg/L) while effluent concentrations averaged 6 mg/L (1 - 12 mg/L). Results showed that mean volumetric NO3- removal rate achieved by the bioreactor throughout the study period was 21 g N/ m3 /d and the average percent NO3- reduction was 26% that fell within the range of reported values. These findings further indicated that woodchip bioreactors operating under cold-weather environmental settings are effective means to removal of NO3- loads from agricultural landscapes. The model that integrates simulated drainage discharge into temperature-dependent denitrification rates was validated using observed effluent nitrate with a Nash-Sutcliffe efficiency coefficient (NSE) value of 0.506, indicating the efficacy of the model in line with model evaluation criteria. Based on the univariate analysis conducted, the most sensitive model parameters are influent nitrate, hydraulic retention time and water temperature, respectively. The MIN3P code was also utilized to predict NO3- concentrations along the length of the bioreactor for a simulation interval of a day and nitrate levels in effluent water through the monitoring period. The simulated nitrate concentration profile suggests that nitrate removal occurs primarily within the first few meters of the denitrification bed. The assessment of MIN3P model resulted in a NSE value of 0.043 and a coefficient of determination (R2) of 0.416

Spin orbit coupling in bulk ZnO and GaN

Using group theory and Kane-like $\mathbf{k\cdot p}$ model together with the L\"owdining partition method, we derive the expressions of spin-orbit coupling of electrons and holes, including the linear-$k$ Rashba term due to the intrinsic structure inversion asymmetry and the cubic-$k$ Dresselhaus term due to the bulk inversion asymmetry in wurtzite semiconductors. The coefficients of the electron and hole Dresselhaus terms of ZnO and GaN in wurtzite structure and GaN in zinc-blende structure are calculated using the nearest-neighbor $sp^3$ and $sp^3s^\ast$ tight-binding models separately.Comment: 9 pages, 6 figures, to be published in J. Appl. Phy

Large Bychkov-Rashba spin-orbit coupling in high-mobility GaN/AlGaN heterostructures

We present low temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is $\alpha_{so}$ $\sim$ 6$\times$ 10$^{-13}$eVm, while the conduction band spin-orbit splitting energy amounts to $\Delta_{so}$ $\sim$ 0.3meV at n$_e$=1$\times10^{16}$m$^{-2}$.Comment: Accepted for publication in PR

ZnO Nanostructure Templates as a Cost-Efficient Mass-Producible Route for the Development of Cellular Networks

The development of artificial surfaces which can regulate or trigger specific functions of living cells, and which are capable of inducing in vivo-like cell behaviors under in vitro conditions has been a long-sought goal over the past twenty years. In this work, an alternative, facile and cost-efficient method for mass-producible cellular templates is presented. The proposed methodology consists of a cost-efficient, two-step, all-wet technique capable of producing ZnO-based nanostructures on predefined patterns on a variety of substrates. ZnO—apart from the fact that it is a biocompatible material—was chosen because of its multifunctional nature which has rendered it a versatile material employed in a wide range of applications. Si, Si3N4, emulated microelectrode arrays and conventional glass cover slips were patterned at the micrometer scale and the patterns were filled with ZnO nanostructures. Using HeLa cells, we demonstrated that the fabricated nanotopographical features could promote guided cellular adhesion on the pre-defined micron-scale patterns only through nanomechanical cues without the need for further surface activation or modification. The basic steps of the micro/nanofabrication are presented and the results from the cell adhesion experiments are discussed, showing the potential of the suggested methodology for creating low-cost templates for engineered cellular networks

Influence of symmetry and Coulomb-correlation effects on the optical properties of nitride quantum dots

The electronic and optical properties of self-assembled InN/GaN quantum dots (QDs) are investigated by means of a tight-binding model combined with configuration interaction calculations. Tight-binding single particle wave functions are used as a basis for computing Coulomb and dipole matrix elements. Within this framework, we analyze multi-exciton emission spectra for two different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure. The impact of the symmetry of the involved electron and hole one-particle states on the optical spectra is discussed in detail. Furthermore we show how the characteristic features of the spectra can be interpreted using a simplified Hamiltonian which provides analytical results for the interacting multi-exciton complexes. We predict a vanishing exciton and biexciton ground state emission for small lens-shaped InN/GaN QDs. For larger systems we report a bright ground state emission but with drastically reduced oscillator strengths caused by the quantum confined Stark effect.Comment: 15 pages, 17 figure

High TC ferromagnetism in diluted magnetic semiconducting GaN:Mn films

Wurtzite GaN:Mn films on sapphire substrates were successfully grown by use of the molecular beam epitaxy (MBE) system. The film has an extremely high Curie temperature of around 940 K, although the Mn concentration is only about 3 ~ 5 %. Magnetization measurements were carried out in magnetic fields parallel to the film surface up to 7 T. The magnetization process shows the coexistence of ferromagnetic and paramagnetic contributions at low temperatures, while the typical ferromagnetic magnetization process is mainly observed at high temperatures because of the decrease of the paramagnetic contributions. The observed transport characteristics show a close relation between the magnetism and the impurity conduction. The double exchange mechanism of the Mn-impurity band is one of the possible models for the high-TC ferromagnetism in GaN:Mn.Comment: 20 pages, 4 figures, submitted to Physica

Ground state of excitons and charged excitons in a quantum well

A variational calculation of the ground state of a neutral exciton and of positively and negatively charged excitons (trions) in single quantum well is presented. We study the dependance of the correlation energy and of the binding energy on the well width and on the hole mass. Our results are are compared with previous theoretical results and with avalaible experimental data.Comment: 8 pages, 5 figures presented to OECS

Dislocation density in GaN determined by photoelectrochemical and hot-wet etching

Defects in GaN layers grown by hydride vapor-phase epitaxy have been investigated by photoelectrochemical ~PEC! etching, and by wet etching in hot H3PO4 acid and molten potassium hydroxide ~KOH!. Threading vertical wires ~i.e., whiskers! and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic-force microscopy, we find the density of ‘‘whisker-like’’ features to be 23109 cm22, the same value found for the etch-pit density on samples etched with both H3PO4 and molten KOH. This value is comparable to the dislocation density obtained in similar samples with tunneling electron microscopy, and is also consistent with the results of Youtsey and co-workers