IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 4, AUGUST 2013 2845 High Energy Resolution CdTe Schottky Diode -Ray Detectors L. A. Kosyachenko, T. Aoki, C. P. Lambropoulos, V. A. Gnatyuk, E. V. Grushko, V. M. Sklyarchuk, O. L. Maslyanchuk, O. F. Sklyarchuk, and A. Koike Abstract—Schottky diode X-/ -ray detectors based on semi-in- sulating Cl-doped CdTe crystals have been developed and inves- tigated. Both the Schottky and Ohmic contacts were formed by vacuum deposition of Ni electrodes on the opposite faces of (111) oriented CdTe crystals pretreated by Ar ion bombardment with different conditions. Record-low leakage current in the fabricated Ni/CdTe/Ni structure at high voltages ( 5 nA at 300 K for the area of 10 mm at bias voltage of 1500 V) was achieved. The charge transport mechanisms in the detectors are interpreted on the basis of known theoretical models. The developed detectors have shown the record-high energy resolution in the measured spectra of Cs (0.42% FWHM). From a comparison of the spectra taken with the detector irradiated from the Schottky contact side and from the opposite side with an Ohmic contact, the concentration of uncom- pensated impurities (defects) in the CdTe crystals has been deter- mined. The obtained value has been found to be close to the optimal one determined from the calculation results. Index Terms—CdTe crystals, detection efficiency, Schottky diodes, uncompensated impurities, X- and gamma-ray detectors. I. INTRODUCTION C ADMIUM telluride (CdTe) is a common material for the manufacture of X- and -ray room temperature semicon- ductor detectors, which are widely used in science, industry, and other application areas [1]–[3]. Particular attention is drawn to multielement CdTe X-/ -ray imaging detectors used in medicine, astronomy, security, and different technologies requiring high energy resolution [4], [5]. Recently, a significant improvement in the structural, electrical, and, hence, spectral properties of semi-insulating CdTe single crystals grown by THM have been achieved [6], [7]. It is well known that high resistivity, large mobility, and long lifetime of charge carriers are mandatory requirements for CdTe crystals used in X-/ -ray detectors. However, to provide full Manuscript received November 30, 2012; revised January 30, 2013, February 05, 2013, March 08, 2013; accepted April 17, 2013. Date of publication May 23, 2013; date of current version August 14, 2013. These studies were performed in the framework of the Collaborative Project COCAE (Grant 218000) of the European Community’s Seventh Framework Program (FP7/2007-2013). L. A. Kosyachenko, E. V. Grushko, V. M. Sklyarchuk, O. L. Maslyanchuk, and O. F. Sklyarchuk are with the Yury Fedkovych Chernivtsi National Univer- sity, Chernivtsi 58012, Ukraine (e-mail: lakos@chv.ukrpack.net). T. Aoki is with the Research Institute of Electronics, Shizuoka University, Hamamatsu 422-8017, Japan (e-mail: rtaoki@ipc.shizuoka.ac.jp). C. P. Lambropoulos is with the Technological Educational Institute of Chalkida, Psahna, Evia 34400, Greece (e-mail: lambrop@teihal.gr). V. A. Gnatyuk is with the V. E. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, Kyiv 01601, Ukraine (e-mail: gnatyuk@lycos.com). A. Koike is with the ANSeeN Inc., Hamamatsu 4328011, Japan (e-mail: koike@anseen.com). Digital Object Identifier 10.1109/TNS.2013.2260356 charge collection, a rather high-bias voltage has to be applied to sufficient thin CdTe wafers, and in so doing, the problem of leakage current should be overcome. Dark current is drastically reduced if detectors are developed as barrier structures that has been implemented either as p-i-n structured diodes [8], [9] or Schottky diodes [10], [11] since the 1990s. In the early twenty-first century, very low leakage current (a few nA cm at a bias of several hundred volts), achieved in detectors with a blocking electrode, led to intense development of CdTe-based Schottky diode X-/ -ray detectors with high en- ergy resolution operating at room temperature. In this paper, the electrical and detection properties of the CdTe Schottky diodes with low leakage currents at bias voltages up to 1500 V and the record-high energy resolution are studied. It is shown that high energy resolution is also due to the optimal concentration of un- compensated impurities in the CdTe crystals. II. CHARACTERISTICS OF CDTE CRYSTALS AND NI/CDTE/NI DIODES A. Electrical Characteristics of Crystals For the fabrication of detectors, Cl-doped CdTe single crys- tals, produced by Acrorad Company, Ltd., Okinawa, Japan, were used [6], [7]. The room-temperature resistivity of the semi-insulating -like semiconductor was cm that is close or even higher than the value of a material with intrinsic conductivity cm, if the parameters are set: the bandgap 1.47 eV, the effective masses of electrons and holes , where is the free electron mass [12]. Fig. 1 shows the temperature dependence of the re- sistivity of the CdTe crystal and, for comparison, the similar dependence of an intrinsic semiconductor , where is the electron charge, is the charge carrier concen- tration in intrinsic CdTe, and and are the effective state densities in the conduction and valence bands, respectively ( is the Boltzmann constant). The observed excess of over the resistivity of intrinsic CdTe is attributed to an increase in the contribution of holes (with their much lower mobility than electrons) to the electrical conduction of the material. Really, expressing as , where is the hole concentration in the valence band, and equating the derivative to zero, it is easy to show that the maximum possible semiconductor resistivity is determined by expression [1] (1) 0018-9499 © 2013 IEEE