We report on the fabrication and characterization of vertical geometry transparent Schottky barrier ultraviolet detectors based on n−/n+-GaN structures grown over sapphire substrates. Spectral responsivity measurements were made using illumination through the UV transparent Schottky barrier metal. A responsitivity as high as 0.18 A/W was measured for wavelengths shorter than the absorption edge of GaN. The detector speed was RC limited and the fall time was 118 ns. The 1/f noise is identified to be the main noise contribution. At 300 Hz, we measure the noise equivalent power at less than 4×10−9 W.

Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We initiate the Sejong Open cluster Survey (SOS) -a project dedicated to providing homogeneous photometry of a large number of open clusters in the SAAO Johnson-Cousins' U BV I system. To achieve our main goal, we pay much attention to the observation of standard stars in order to reproduce the SAAO standard system. Many of our targets are relatively small sparse clusters that escaped previous observations. As clusters are considered building blocks of the Galactic disk, their physical properties such as the initial mass function, the pattern of mass segregation, etc. give valuable information on the formation and evolution of the Galactic disk. The spatial distribution of young open clusters will be used to revise the local spiral arm structure of the Galaxy. In addition, the homogeneous data can also be used to test stellar evolutionary theory, especially concerning rare massive stars. In this paper we present the target selection criteria, the observational strategy for accurate photometry, and the adopted calibrations for data analysis such as color-color relations, zero-age main sequence relations, Sp -M V relations, Sp -T eff relations, Sp -color relations, and T eff -BC relations. Finally we provide some data analysis such as the determination of the reddening law, the membership selection criteria, and distance determination.

Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BV I C and JK S photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral type -intrinsic color relation, we obtain the mean interstellarto the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distance to the cluster -3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0 ± 1.0Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of Γ = −0.8 ± 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 × 10 4 M . The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively lowluminosity yellow hypergiants.

We report solar-blind AlxGa1−xN photovoltaic detectors with cutoff wavelengths as short as 290 nm. Mesa geometry devices of different active areas are fabricated and characterized for spectral responsitivity, speed, and noise performance. The responsivity of the devices near the cutoff wavelength is 0.07 A/W. The detector noise is found to be 1/f limited, with a noise equivalent power of 6.6×10−9 W over the total response bandwidth of 100 kHz.

This letter reports on the fabrication and characterization of visible-blind ultraviolet photoconductors using single-crystal AlxGa1−xN layers deposited on basal plane sapphire substrates. With aluminum mole fractions ranging from 5% to 61%, the long-wavelength cutoff can be varied from 350 to 240 nm. Photoresponsitivities as high as several hundred amperes per watt were measured with 10 μm interelectrode spacing.