We have fabricated phototubes with photocathodes consisting of the Mg-doped GaN films. The spectral shapes of the response and the quantum efficiency (QE) strongly depend on the Mg-doping concentration. The calibrated QE of the photocathode is maximized to be 71.9% at a photon energy of 5.4 eV by a Mg-doping concentration of 3.0×1019cm−3. Consequently, a phototube with the GaN-based photocathode is realized to demonstrate a very high QE, more than 50% and sharp cutoff characteristic over three orders of magnitude.

PACS 85.60.HaWe have designed and fabricated new photo-tubes and photomultiplier tubes (PMTs) using In x Ga 1-x N films as photocathodes. Longer spectral cutoffs were controlled with Indium compositions x, from 365 nm to 430 nm. The calibrated maximum quantum efficiency (QE) of 56% at 200 nm was achieved for x = 0.11 Indium composition in the photo-tubes. Sharp cutoffs at around 160 nms were determined by window materials and those at around 400 nms were determined by the band gaps of the photocathodes. Gain and input/output linearity properties were confirmed to be very good for the PMTs. 1 Introduction In these days, photocathode devices detecting in UV and blue spectral region are important for detection of scintillation and Cherenkov radiation in high-energy physics, like Kamiokande, and astrophysics experiments. Besides, researchers in space UV astrophysics and planetary require advanced detectors [1][2][3]. A high quantum efficiency (QE), a sharp cutoff, a low dark current, and large formats are important for these detectors. Because of a wide dynamic range, a low dark current, and being able to operate in single photon counting mode, GaN-based photocathodes are suitable for UV space missions.GaN-based photocathodes are being studied since 70's, some good results have been reported [4][5][6][7]. Recently, we have realized extremely high QE for GaN-based photocathodes [8]. The spectral shapes of the response and the QE strongly depend on the Mg-doping concentration. The calibrated QE of the GaN-based photocathode is maximized to be 71.9% at 230 nm by a Mg-doping concentration of 3.0 x 10 19 cm -3 . Within the frame work of the Spicer's three step model [9], the diffusion length and the escape probability were identified to be 150 nm, and 0.8, respectively.Recently, InGaN photocathodes are reported [1,10,11]. However, the relationship between the In composition of the InGaN films and the sharp cutoff or the QE is not clear. Then, we have designed and fabricated the InGaN photocathodes with high QEs, and have incorporated them into photomultiplier tubes (PMTs) with high gain and wide dynamic ranges.

We have fabricated photomultiplier tubes (PMTs) with photocathodes consisting of In0.11Ga0.89N films. To realize sharp cutoff characteristics of the In0.11Ga0.89N-based photocathodes, a “real time” monitoring system was employed. This system utilized light sources of light-emitting diodes (LEDs), and their peak wavelengths are 285, 375, and 470 nm respectively. With this monitoring system, PMTs with In x Ga1-x N-based photocathodes are realized to demonstrate sharp cutoff characteristics over three orders of magnitude with high quantum efficiencies (QEs).