Charge Transport Properties of CdTe X/γ-Rays Detectors with TiO x Schottky Contacts O. Maslyanchuk, M. Solovan, V. Brus, I. Fodchuk, V. Gnatyuk, T. Aoki, Member, IEEE Abstract– The structure and electrical properties of CdTe Schottky diode X/γ-rays detectors fabricated by DC reactive magnetron sputtering of Ti on the surface of semi-insulating p- like CdTe single crystals have been investigated. At relatively low voltages, the current transport processes in the Ti-TiO x /р- CdTe/MoO x -Mo structure are well described in the scope of the carrier generation in the space-charge region and space-charge limited current models. The charge carrier lifetime, energies of generation-recombination centers and hole traps, and density of discrete trapping centers were determined by comparing the experimental and calculation data. Because of the presence of a thin intermediate insulator layer in the р-CdTe/MoO x -Mo contact, a rapid increase in the reverse current due to the Poole- Frenkel emission is observed. The results obtained from the measurements of the I-V characteristics and model of space- charge limited current incorporating the Poole-Frenkel effect are compared and discussed. I. INTRODUCTION ADMIUM telluride (CdTe) offers two main advantages relative to germanium and silicon: CdTe can operate at room temperature due to its sufficient wide band gap (1.5 eV) and this compound has better photoelectric stopping power due high atomic numbers of cadmium and tellurium (Z Cd = 48, and Z Te = 50). However, development of efficient and high energy resolution CdTe-based radiation detectors has been limited due to the reduced structural quality of bulk crystals and detector structures in general with respect to collection of charge carriers generated inside the material. The possibilities of application of CdTe in diode detectors with a p−n junction or surface-barrier structure for the detection and spectroscopy of γ-photons (as well as α- and β-particles) were demonstrated as early as in the middle of the 1960s [1]. These findings were repeatedly confirmed in the following years [2, 3]. At the end of the 1990s, Takahashi and co-workers [4]-[8] presented the results of extremely low dark currents and favorable conditions for charge collection in a CdTe-based Schottky diode. It allowed to fabricate CdTe-based detectors of high-energy photons (for example, 662 keV and higher) exhibiting a limited energy Manuscript received December 14, 2018. This research was supported by the NATO Science for Peace and Security Programme (Project SENERA SfP- 984705). O. Maslyanchuk, M. Solovan, V. Brus, and I. Fodchuk are with the Yury Fedkovych Chernivtsi National University, Chernivtsi, Ukraine (e-mail: emaslyanchuk@yahoo.com). V. Gnatyuk is with the Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan, on leave from the V.E. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, Prospekt Nauky 41, Kyiv 03028, Ukraine (e-mail: gnatyuk@ua.fm). T. Aoki is with the Research Institute of Electronics, Shizuoka University, Hamamatsu, Japan (e-mail: aoki.toru@shizuoka.ac.jp). resolution without special processing of the electrical signal, generated by the detector [9]. However, in order to avoid trapping of charge carriers generated during the absorption of high-energy photons in the CdTe Schottky detector, a high bias voltage is required. Unfortunately, a rapid growth of the leakage current with increasing bias frequently happens. The origin of the excess charge carriers can be either tunneling of charge carriers through a thin insulating film between the crystal and the metal contact [10] or the injection of minority carriers from the imperfect Ohmic contact on the opposite side of the crystal [11, 12]. High work-function metals such as Au or Pt are widely used to form Ohmic contacts to p-CdTe [4, 13], whereas low work-function metals such as In or Al are usually used for obtaining a high barrier Schottky junction with p-CdTe [14, 15]. Our previous studies confirmed possibility of application Ni for both Schottky and near Ohmic contacts using special treatment of CdTe crystal surface in order to obtain necessary concentration of surface states [16, 17]. For the practical application of CdTe detectors, the time stability of the detectors is one of the most important issues. However, high energy-resolution CdTe radiation detectors with Schottky contact show instability with operating time under bias voltage, which is termed as polarization phenomenon. The instability phenomenon is characterized by the accumulation of negative charges at deep acceptor levels in CdTe bulk and the progressive degradation of energy resolution and a shifting of the photopeak position toward low energy with time after applying the bias voltage [5], [18]-[20]. Polarization phenomena can be avoided either periodically removing the applied bias voltage or changing contact barrier [18]. Therefore, the problem of further improvement of contacts to CdTe and search for new contact materials remains an actual scientific and technical challenge. The aim of this study is to offer some clarification of these issues. In this work we continue to investigate the defect structure of CdTe crystals and CdTe-based barrier structures as well as the peculiarities of charge transport in the р-CdTe- detectors, studied in [11, 21, 22]. Despite the high quality CdTe now being commercially available, the charge carrier transport within the detector remain yet not been clearly identified and are discussed in this paper. II. SAMPLE PREPARATION Commercial (111) oriented CdTe wafers (Acrorad Co., Ltd.) with the area of 5  5 mm 2 and thickness of 0.75 mm were used for the fabrication of detectors. The procedures of the formation of both near-Ohmic and Schottky contacts included chemical etching of the crystals in the K 2 Cr 2 O 7 +HNO 3 +H 2 O solution C 978-1-5386-8494-8/18/$31.00 ©2018 IEEE