Ryoichi Ohno

Performance of a new Schottky CdTe detector for hard X-ray spectroscopy

We report a significant improvement of spectral properties of a cadmium telluride (CdTe) detector. This was accomplished via the use of a high quality CdTe crystal, where high Schottky barrier for the holes on a CdTe surface was formed by using a low work-function metal, indium. With a 2/spl times/2 mm/sup 2/ detector at a thickness of 0.5 mm: the leakage current is measured to be 0.7 nA at room temperature (20/spl deg/C) and below 1 pA at -70/spl deg/C for 400 V bias voltage. The low leakage current allows us to operate the detector at a higher bias voltage than for previous CdTe detectors. The energy resolution we achieved at room temperature is 1.1-2.5 keV FWHM from the energy range of 2 keV to 150 keV at 20/spl deg/C without any charge-loss correction electronics. At -70/spl deg/C, we obtained an energy resolution of 1.0 keV FWHM at 122 keV and 2.1 keV FWHM at 662 keV.

Growth and characterization of CdTe single crystals for radiation detectors

To improve the productivity of CdTe radiation detectors, the crystal growth by traveling heater method (THM) as well as the quality of the fabricated detectors were investigated. In the THM growth, optimization of the solvent volume was found to be essential because it affects the shape of the growth interface. The use of the slightly tilted seed from 〈1 1 1〉B was also effective to limit the generation of twins having different directions. Single-crystal (1 1 1) wafers, larger than 30×30 mm2 were successfully obtained from a grown crystal of 50 mm diameter. Pt/CdTe/Pt detectors of dimensions 4×4×2 mm3, fabricated from the whole crystal ingot, showed an energy resolution (FWHM of 122 keV peak from a 57Co source) between 6% and 8%. Similarly, Pt/CdTe/In detectors of dimensions 2×2×0.5 mm3 showed a resolution better than 3%. These characteristics encourage the practical applications of various types of CdTe detectors.

High-resolution CdTe detector and applications to imaging devices

Using a high quality cadmium telluride (CdTe) wafer, we formed a Schottky junction and operated the detector as a diode (CdTe diode). The low leakage current of the CdTe diode allows us to apply a much higher bias voltage than was possible with the previous CdTe detectors. For a relatively thin detector of /spl sim/0.5 mm thick, the high bias voltage results in a high electric field in the device. Both the improved charge collection efficiency and the low-leakage current lead to an energy resolution of 1.1 keV FWHM at 60 keV for a 2/spl times/2 mm/sup 2/ device and 2 keV for a 10/spl times/10 mm/sup 2/ device at 5/spl deg/C without any charge-loss correction electronics. For astrophysical applications, we have developed a an initial prototype CdTe pixel detector based on the CdTe diode. The detector has 400 pixels with a pixel size of 625/spl times/625 /spl mu/m/sup 2/. Each pixel is gold-stud bonded to a fanout board and routed to a front end ASIC to measure pulse height information for each /spl gamma/-ray photon.

Improvement of the CdTe diode detectors using a guard-ring electrode

Recent results from the Schottky CdTe diode detectors employing a guard-ring (GR) electrode are reported. A cathode electrode, made of platinum, was separated into an active electrode(s) and a surrounding GR. Typical leakage current of a device with active area of 2 /spl times/ 2 mm/sup 2/ and 0.5 mm thickness surrounded by a GR, is 7 and 20 pA at a bias of 100 and 500 V, respectively, operated at 20/spl deg/C. Spectral resolution of this device is 0.93 and 1.2 keV (FWHM) at 59.5 and 122 keV, respectively, operated at 20/spl deg/C with a bias of 800 V. Detailed study of the characteristics of these devices working as a gamma-ray detector is presented.

Quantitative Analysis of Polarization Phenomena in CdTe Radiation Detectors

Polarization phenomena in a Schottky-type CdTe radiation detector were studied. We evaluated the distribution of electric field in a biased CdTe detector by measuring the progressive change of Schottky barrier lowering with time. The parameters of deep acceptors such as detrapping time, concentration, and the depth of the energy level were quantitatively evaluated. In the case of applying the conventional model of charge accumulation, the obtained result shows that the CdTe bulk is never undepleted. We modified the charge accumulation model by taking account of the occupation state of the deep acceptor level. When a modified model is applied, the time that the depletion width in the bulk begins to diminish closely fits the time that the photopeak position begins to shift in radiation measurements. In this paper, we present a distribution of electric field during biasing and a simple method for the evaluation of the parameters of deep acceptors in CdTe bulk.

THM Growth and Characterization of 100 mm Diameter CdTe Single Crystals

The THM growth technology for 100 mm diameter CdTe single crystals has been intensively developed. In consequence of the optimization of growth conditions, we have succeeded in controlling growth interface shape and growing 100 mm diameter CdTe single crystals with 300 mm in length. Concerning the behavior of Te inclusions in the grown crystal, their size and density were investigated by IR transmission microscopy. The size distribution of Te inclusions was divided into two groups, and the density of them was less than 1 × 105 cm−3. Charge transport properties of the grown crystal were investigated by using the “μτ-model” spectral fitting method, and were found to be quite uniform all over the wafer. To investigate the homogeneity of radiation detector performances, Ohmic type and Schottky type detectors with 4 mm × 4 mm × 1 mm were fabricated from the left half and the right half of the 100 mm diameter wafer, respectively. Standard deviations of their energy resolutions for the 122 keV line from 57Co were less than 6 %. This excellent uniformity is essential for the room temperature semiconductor detectors in the major application areas, such as medical imaging, non-destructive inspection and homeland security.

Improvement of the Productivity in the THM Growth of CdTe Single Crystal as Nuclear Radiation Detector

The effect of the THM growth rate on the CdTe crystalline quality and the detector performance was intensively investigated. The maximum growth rate for the single crystal growth was found to be approximately 15 mm/day which was 3 times greater than the conventional one. By optimizing other growth conditions, 90% of every ingot volume has become a single crystal. Te inclusions in the CdTe single crystal grown at various growth rates were also investigated by IR transmission microscopy. There was no correlation between the behavior of Te inclusions and the growth rate. The detector performance was also independent of the growth rate. Taking advantage of the large volume CdTe single crystals, about 700 000 Schottky detectors with 4 mm × 7.5 mm × 1 mm were fabricated for the research and development of the new positron emission tomography (PET) system using CdTe detectors. The average FWHM for the 662 keV line from 137Cs and its standard deviation were 2.24% and 0.48%, respectively. This uniformity was essential for the development of the new PET system.

Effect of He Plasma Treatment on the Rectification Properties of Al/CdTe Schottky Contacts

We investigated the effect of He plasma treatment on the surface composition of CdTe and the electrical properties of Al/CdTe Schottky contacts. The composition of the initial CdTe surface is Te-rich due to Br-methanol etching. When Al Schottky contacts are formed on the CdTe surfaces with the Te-rich layer, the barrier height is low and the rectification property is not good. In this paper, we propose the He plasma treatment method to remove the Te-rich layer. From X-ray photoelectron spectroscopy measurement, it was found that the plasma treatment can remove the Te-rich layer. The rectification property of the Al Schottky contacts on the plasma-treated surfaces is improved, and their barrier heights are estimated to be about 0.65 eV. In γ-ray spectrometry, a high-energy resolution of 1.6 keV full width at half maximum at 59.5 keV was obtained from the plasma-treated Al/CdTe/Pt detector. The results indicate that the plasma treatment of CdTe surfaces significantly improves the energy resolution of Schottky-type Al/CdTe/Pt radiation detectors.

Large-area CdTe diode detector for space application

Abstract The current status of Schottky CdTe diode detectors, especially in view of their space application for hard X-ray and gamma-ray astronomy, are reported. For practical use in space science, a large-area CdTe diode with a size of 21.5×21.5 mm 2 and a thickness of 0.5 mm was developed. A good energy resolution, 2.8 keV (FWHM) at −20°C, and high homogeneity to within 0.2% over the detector were achieved for the spectral performance. This device has successfully passed a series of tests required for its use in space, in view of utilizing Japanese M-V rockets. The tests include the mechanical environment test, vacuum test, long run for weeks and proton-beam radiation. Initial results from a 2×2 segmented electrode large-area device with a guard-ring are also presented.

Large area gamma-ray imaging detector based on high-resolution CdTe diode

We are developing a large array detector composed of 1024 individual CdTe diodes. Each detector has the dimensions of 1.2 mm /spl times/ 5.0 mm and a thickness of 1.2 mm. An edge-on geometry is used for the injection of /spl gamma/-rays, to obtain a cross-section thickness of 5 mm. With this geometry, the distance between the two electrodes can be kept small, and we can therefore apply the high electric field which is necessary to achieve a high energy resolution (by reducing the low energy tail) and also to sustain the long-term stability of the CdTe diode. Signals from each detector element are fed into newly developed low noise ASICs. We use 32 chips for the readout of 1024 elements. In this paper, we report the basic characteristics of the individual detectors and the overall performance of the gamma-camera. Design of the readout electronics system is also described.