Tokuaki Nihashi

Efficient GaN photocathodes for low-level ultraviolet signal detection

We report on the properties of GaN-based photocathodes for low light ultraviolet (UV) signal detection. Cesiated Mg-doped p-type GaN layers with 1-/spl mu/m thickness were used as photocathode materials. Quantum efficiency (QE) as measured on a completed device showed values as high as 30% at 200 nm. A UV/visible rejection ratio of three orders of magnitude at 500 nm was observed. A net increase in the QE was also observed with increasing conductivity of the material.

Improved spectrometric performance of CdTe radiation detectors in a p-i-n design

CdTe radiation detectors were fabricated using a p-i-n design and a significant improvement in the spectral properties was obtained during room temperature operation. An iodine doped n-CdTe layer was grown on the Te faces of the (111) oriented high resistivity CdTe crystals at the low substrate temperature of 150 °C. An aluminum electrode was evaporated on the n-CdTe side for the n-type contact, while a gold electrode on the opposite side acted as the p-type contact. Very low leakage currents, typically 60 pA/mm2, were attained at room temperature (25 °C) for an applied reverse bias of 250 V. Detectors exhibited excellent spectral responses with an energy resolution of 1.42, 1.7, and 4.2 keV FWHM at 59.5, 122, and 662 keV γ peaks, respectively.

Progress in the fabrication of GaN photocathodes

Currently, photo-cathodes hold the highest promise in the near term (next few years) of being able to detect low light level UV signals at high QE while being nearly blind to visible wavelengths. We briefly discuss the requirements for UV detection for astronomical applications, and then we describe our work on producing GaN based photo-cathodes. The p-type GaN films were grown on sapphire at Northwestern University. The films were then converted into opaque photo-cathodes inside photo-tubes at Hamamatsu. Hamamatsu tested detective quantum efficiencies (DQE) of these detectors to be as high as 30% at 200 nm. The ratio of peak DQE at 200 nm to the minimum DQE at 500 nm was measured to be about 6 X 103. We found a dramatic increase in the DQE at 200 nm versus the conductivity, with the break point being near 0.13 1/(Ohm-cm). Based on this dramatic increase, we believe that further improvement in photo-cathode quantum efficiencies can be achieved by increasing the conductivity. We have recently achieved more than an order of magnitude increase in conductivity by co-doping techniques. Improvements in the solar blindness of the devices depend both on characteristics of the film and its surface properties. A detailed discussion of decreasing the visible response and producing a sharper wave-length cutoff is beyond the scope of this work, but we briefly discuss the attributes that most likely affect the wavelength dependence of the photo-cathode response.

Development of UV-photocathodes using GaN film on Si substrate

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.

Fabrication of CdTe detectors in a new p-i-n design for gamma-ray spectroscopy

CdTe and related compound semiconductors are useful for high energy flux detection at room temperature. We propose a new fabrication technique for CdTe detectors in p-i-n design that is suitable for hard x-ray and gamma-ray spectroscopy. Using a high resistivity single crystal CdTe substrate, an iodine doped n-CdTe layer is grown homoepitaxially on one face of the crystal using the hydrogen plasma-radicals- assisted metalorganic chemical vapor deposition method working at a low substrate temperature of 150 degrees C. An indium electrode is evaporated on the n-CdTe side for an ohmic contact, while a gold electrode is placed on the opposite side which acts as a p-type contact. The p-i-n detector thus fabricated exhibited low leakage current at room temperature operation, below 0.5 nA at an applied bias of 350 V for a 2 X 2 mm2 detector of thickness of 1 mm. Leakage current further decreased to 16 pA at 350 V while cooling the detector down to -15 degrees C. Spectral responses of the detector showed improved energy resolution for different radioisotopes of energies in the range form few tens of keV to several hundred keV and stable operation when operated at high applied biases or slightly cooling the detector. Performance of the different detectors thus fabricated will be presented.

MOCVD growth of GaAs, AlGaAs and its application to transmission photocathodes

Abstract An innovative MOCVD system has been developed to grow GaAs, AlGaAs and their heterostructures for the application to a transmission photocathode. In order to characterize the MOCVD system and investigate the interface abruptness, quantum wells with three different GaAs well thicknesses have been made. The well thicknesses are 20, 40 and 80 A respectively and separated by 500 A Al 0.5 Ga 0.5 As barrier layers. The transition layer thickness between the GaAs and Al 0.5 Ga 0.5 As was estimated to be as thin as 1 to 2 monolayers from photoluminescence measurements. By applying this crystal to the transmission photocathodes, a photoemission sensitivity of 420 μA/lm has been obtained in a sealed-off phototube.