Mapping piezoelectric-field distribution in gallium nitride with scanning second-harmonic generation microscopy

Taking advantage of the electric field-enhanced second-harmonic generation effect in bulk gallium nitride (GaN) and indium gallium nitride (InGaN) quantum wells, we demonstrated the piezoelectric field distribution mapping in bulk GaN and InGaN multiple-quantum-well (MQW) samples using scanning second-harmonic generation (SHG) microscopy. Scanning SHG microscopy and the accompanying third-harmonic generation (THG) microscopy of the bulk GaN sample were demonstrated using a femtosecond Cr:forsterite laser at a wavelength of 1230 nm. Taking advantage of the off-resonant electric field-enhanced SHG effect and the bandtail state-resonance THG effect, the second- and third-harmonic generation microscopic images obtained revealed the piezoelectric field and bandtail state distributions in a GaN sample. Combined with 720 nm wavelength excited two-photon fluorescence microscopy in the same sample, the increased defect density around the defect area was found to suppress bandedge photoluminescence, to increase yellow luminescence, to increase bandtail state density, and to decrease residue piezoelectric field intensity. Scanning SHG microscopy of the InGaN MQW sample was resonant excited with 800 nm femtosecond pulses from a Ti:sapphire laser in order to suppress SHG contribution from the bulk GaN substrate. Taking advantage of the strong piezoelectric field inside the InGaN quantum well, the wavelength resonant effect, and the electric field-enhanced SHG effect of InGaN quantum wells, resonant scanning SHG microscopy revealed the piezoelectric field distribution inside the wells. Combined with accompanying three-photon fluorescence microscopy from the bulk GaN substrate underneath the quantum wells, the direct correspondence between the piezoelectric field strength inside the quantum well and the substrate quality can be obtained. According to our study, the GaN substrate area with bright bandedge luminescence corresponds to the area with strong SHG signals indicating a higher stained-induced piezoelectric field. These scanning harmonic generation microscopies exhibit superior images of the piezoelectric field and defect state distributions in GaN and InGaN MQWs not available before. Combining with scanning multiphoton fluorescence microscopy, these techniques open new ways for the physical property study of this important material system and can provide interesting details that are not readily available by other microscopic techniques.     

Plasma Treatment for Leakage Reduction in AlGaN/GaN and GaN Schottky Contacts

It has been found that the reverse leakage current of AlGaN/GaN Schottky contacts can be significantly reduced by a CF₄ plasma treatment prior to the Schottky metal evaporation. The data of electrical characterization suggest that the leakage reduction is related to the modification of the semiconductor surface by plasma treatment. The leakage reduction effect was also observed in GaN Schottky contacts. Capacitance-voltage characterization of the GaN Schottky contacts indicates that the Schottky barrier height was slightly increased by the plasma treatment. A two-step surface treatment procedure, consisting of a BCI₃ plasma treatment followed by a brief CF₄ plasma treatment, has been developed as an efficient approach to reduce the reverse leakage of the Schottky contacts, while avoiding side effects related to the CF₄ plasma.     

Nano-fin based mercury-sensor for environmental surveillance

This Nano-Fin-Sensor bases on a lithography-independent technology-process, enabling research on Nano-Sensors without cost-intensive technology-equipment. Background for the sensor described within this paper is the high pollution with mercury of the environment and the lack of cheap, easy to use and portable sensors. The lithography-independent process is based on a "deposition and etch-back" technique defining Nano-Fins. Active sensor-material is a gold-layer, deposited on the fin, increasing resistance being exposed to mercury-vapor due to the process of amalgamation. Regeneration is done by heating-up the gold-layer using the poly-silicon fin as resistance-heating-device driving out the adsorbed mercury. To increase the measurement-accuracy, the sensor is made up of four Nano-Fin-Sensors, connected as Wheatstone-bridge. Two sensors have to be passivated by a mercury diffusion barrier, here a silicon-nitride-layer.     

Risk of Metal Mobilization from Redevelopment Activities in Hyperarid Climates: A Laboratory Experiment and Discussion

The reclamation and redevelopment of abandoned mine sites into parks, golf courses, and residential communities in arid and hyperarid regions has been caused, in part, to rising land costs. A laboratory experiment using three columns was monitored for 273 days to evaluate trace metal and metalloid availability. The sediment from the Three Kids Mine that was used in this study is documented to contain high levels of trace metals and metalloids. The experiment simulated agricultural activities over time to assess whether fertilizers would mobilize trapped contaminants. Results indicate that irrigation and fertilizers can provide conditions for Pb, Mn, Zn, Al, Ba, Cu, and As to become mobile, though on a limited basis. There was an increase in Pb, Mn, Zn, Al, Ba, Cu, and As within the first 30 days followed by a decrease at 90 days. Concentrations of Pb, Mn, and Zn increased at 273 days due to application of fertilizer-fortified waters. This study shows the potential for mobilized trace metals and metalloids to enter the wider environment after developmental activities are finished.     

Anti-Personnel Land Mine Detection and Discrimination Using Acoustic Data

An acoustic-based land mine detection system was field-tested by the University of Mississippi with promising results. This system uses a Laser Doppler Vibrometer (LDV) to measure the velocity of the vibration at the surface of the soil induced by acoustic energy in various frequency bands. In this paper, automated methods for detecting and discriminating Anti-Personnel (AP) mines from clutter objects are presented. Pre-processing methods rely on nonlinear filters realized as Choquet integrals. These filters are robust to the non-Gaussian, impulse type noise in this type of data. Detection follows pre-processing and relies on adaptive thresholding over each frequency band and three-dimensional (3D) connected component analysis. Features are extracted from the 2D frequency slices of the 3D connected components. The features are adaptively aggregated over frequency and used for discriminant analysis. Experiments are performed using anti-personnel mines, clutter objects, and blank areas (background samples with no mines or clutter objects). The algorithm detects 92% of the mines for a wide range of parameters. For some threshold values, 100% of the mines are detected and 92% of the mines are classified as mines with no false alarms.     

Solving the single-machine sequencing problem using integer programming

Various integer programming models have been proposed for sequencing problems. However, little is known about the practical value of these models. This paper reports a comparison of six different integer programming formulations of the single-machine total tardiness problem. We created a set of especially difficult test problems and attempted to solve them with each of the formulations, using CPLEX software. We found that one formulation performs much more effectively than the others. A generic integer programming approach is still not capable of solving problems with hundreds of jobs, so in that respect, it does not compete with state-of-the-art tardiness algorithms. However, the integer programming approach remains viable for problems containing as many as 40 or 50 jobs and may be the better algorithmic choice when convenience in implementation is considered.     

Current developments and future directions in computerized personality assessment.

Contends, on the basis of a review of current examples of computer usage in personality assessment, that there is wide acceptance of automated clerical tasks such as test scoring and administration. The computer is also writing narrative interpretive reports from test results. Three proposed strategies (countdown, adaptive typological, and a strategy borrowed from ability testing) for developing computerized adaptive personality tests are described. (50 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)