We developed an UV image intensifier tube with a GaN photocathode in semi-transparent mode. In UV spectroscopy
and low-light-level UV-imaging applications, there are strong demands for improved detectors which have higher
quantum efficiency, low dark current, sharper wavelength cut-off response, and stable and robust characteristics.
III-Nitrides semiconductor is one of the promising candidate materials to meet these demands. We developed a GaN
photocathode which is epitaxially grown by MOCVD method. It has flat and high quantum efficiency from 200 nm to
360 nm. The cathode is incorporated into an image intensifier tube, which shows good gating performance and fine
imaging resolution. With these improved performances, the UV image intensifier tube with GaN photocathode will
expand its application fields to include UV spectroscopy and UV-imaging in low light.
We developed GaN photocathodes for detecting ultraviolet radiation by using Mg-doped GaN. Crack-free, 200
nm thick GaN:Mg layers were grown by metal organic chemical vapor phase epitaxy (MOVPE) on a GaN template
having a structure of undoped GaN/(AlN/GaN) multilayers on Si (111) substrate. The Mg concentration was varied in
the range from 7×1018 to 7×1019 cm-3. The grown film was mounted in a phototube to operate in reflection mode; i.e. the
light was incident from the photoemission side. The photoemission surface was activated by sequential adsorption of
cesium and oxygen to reduce electron affinity, ensuring efficient electron emission. Photoemission spectrum was
measured in the range of 200-600 nm. We found that the quantum efficiency of photoemission was affected by the
crystallinity of GaN:Mg, depending on the concentration of Mg dopant and the growth pressure of GaN:Mg top
photoemissive layer. The lower Mg concentration and higher growth pressure resulted in higher quantum efficiency. The
obtained maximum quantum efficiency was 45% at 200 nm (6.2 eV) and 25% at 350 nm (3.54 eV). The elimination ratio
between visible and UV light was 4 decades and the slope of cutoff was 10 nm per decade.
Recently, progress has been made in the development of projectors used for large screen displays. Projection systems employing Metal Halide lamps as a light source, demonstrate unique and desirable features such as longer life and high luminous efficiency. These features are especially important for data projectors. Compared to conventional lamps, Metal Halide lamps have shorter arc lengths and higher color-rendering characteristics, resulting in overall superior performance. For the Hamamatsu 575 W Metal Halide lamp, we have modified the electrodes and chemical composition of the Metal Halide, as well as the glass envelope, all of which result in improved performance. We have extended the average life to 3000 hours or longer, for a compact, single-bulb, projection-type, 575 W lamp.
The main requirements for an optical detector are low noise and high sensitivity. The old method used was only to cool the photomultiplier tube. This new method of compact cooled- PMT cools only the photocathode. The main noise of the photomultiplier tube is generated from the photocathode. By utilizing this new method of cooling the photocathode, the noise from the photomultiplier tube can be reduced significantly. A small thermo-cooler attached to the photocathode can cool the photomultiplier tube directly. As a result, within five minutes, the photocathode reaches a lower temperature, for example, of 5 degrees C in an ambient temperature of 20 degrees C. The performance of the photomultiplier tube is one tenth of the dark current of ambient temperature. Also, the dark count decreases by a factor of one tenth of the ambient temperature. Equivalent noise inputs wer 2.3 X 10-17 (at 600 nm).
Recently, progress has been made in the development of projectors used for large screen displays. Projection systems employing high luminous efficiency Metal Halide Lamps as a light source demonstrate unique and desirable features. For example, compared to conventional lamps, a Metal Halide Lamp has longer life, higher luminous efficiency, the shortest arc length possible, and higher color-rendering characteristics, resulting in an overall superior performance. Several lamps, including Metal Halide, Xenon, and Halogen lamps, are used in HDTV (High Definition Television). Recently, Metal Halide Lamps have become accepted as the light source of choice. The reason for this is the high brightness, color balance, and long life of the Metal Halide Lamp. The fundamental operating principal for Metal Halide Lamps is almost the same as that of Mercury lamps. A Metal Halide Lamp has very different characteristics in that, the spectrum of illumination can be varied by varying the mixture of Metal Halide generated in the lamp. For these lamps, we have improved the electrode, the chemical composition of the Metal Halide, and the glass envelope. We have achieved a longer average life (greater than 2500 hours) for a compact, single-bulb, projection-type lamp.
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