High-resolution Schottky CdTe diode for hard X-ray and gamma-ray astronomy

We report a significant improvement of the spectral properties of cadmium t elluride (CdTe) detectors, fabricated in the form of a Schottky CdTe diode. With the use of high quality CdTe wafer, we formed a Schottky junction by evaporating indium on the Te-face and operated the detector as a diode. Thi s allows us to apply much higher bias voltage than was possible with the pr evious CdTe detectors. A 2 mm x 2 mm detector of thickness 0.5 mm, when ope rated at a temperature of 5 degrees C, shows leakage current of only 0.2 an d 0.4 nA for an operating voltage of 400 and 800 V, respectively. We found that, at a high-electric field of several kV cm(-1), the Schottky CdTe diod e has very good energy resolution and stability, suitable for astronomical applications. The broad low-energy tail, often observed in CdTe detectors d ue to the low mobility and short lifetime of holes, was significantly reduc ed by the application of a higher bias voltage which improves the charge co llection efficiency. We achieved very good FWHM energy resolution of 1.1% a nd 0.8% at energies 122 and 511 keV, respectively, without any rise time di scrimination or pulse height correction electronics. For the detection of h ard X-rays and gamma-rays above 100 keV, we have improved the detection eff iciency by stacking a number of thin CdTe diodes. Using individual readout electronics for each layer, we obtained high detection efficiency without s acrificing the energy resolution. In this paper, we report the performance of the new CdTe diode and discuss its proposed applications in future hard X-ray and gamma-ray astronomy missions. (C) 1999 Elsevier Science B.V. All rights reserved.