354 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 58, NO. 1, FEBRUARY 2011 Transport Properties of CdTe X/ -Ray Detectors With - Junction T. Aoki, V. A. Gnatyuk, L. A. Kosyachenko, O. L. Maslyanchuk, and E. V. Grushko Abstract—Charge transport mechanism in X- and -ray detec- tors based on CdTe diodes with a - junction is studied. Shallow - junctions were formed in semi-insulating -like CdTe crystals by laser-induced doping of a thin semiconductor layer with In atoms and, finally, In/CdTe/Au diode structures were fabricated. The energy diagram was developed to explain the reverse I-V char- acteristics of the diodes particularly increased leakage current. It was shown that the I-V characteristics at low bias voltages were described by the Sah-Noyce-Shockley theory. At higher voltages, an additional increase in leakage current was observed and it was attributed to injection of minority carriers (electrons) from the forward-biased Au/CdTe Schottky contact to the reverse-biased - junction (near the In/CdTe contact) through the CdTe crystal. Spectral properties of In/CdTe/Au diode detectors have also been analyzed. Index Terms—CdTe diodes, charge carrier processes, energy spectrum, laser irradiation, leakage current, minority carrier injection, - junctions, X- and -ray detectors. I. INTRODUCTION R OOM temperature high energy photon semiconductor detectors are often affected by an incomplete collection of photon-generated carriers that limits the detector perfor- mance [1]–[3]. A considerable amount of charge loss in the detectors reduces the energy resolution. In CdTe detectors this is mainly due to low mobility and short lifetime of holes. Poor carrier transport causes a situation that holes can be trapped before reaching the cathode. Other possible reasons of the deterioration of functional detector characteristics can be attributed to structural inhomogeneities and transformation of the semiconductor point defect system during detector opera- tion [1]–[3]. Recent progress in the growth of high resistivity (detector-grade) CdTe semiconductor has already made it possible to obtain high quality single crystal wafers with a reduced number of structure defects and accidental impurities and achieve appropriate electrical characteristics [4]–[6]. The Manuscript received March 24, 2010; revised June 10, 2010 and August 18, 2010; accepted October 11, 2010. Date of publication December 17, 2010; date of current version February 09, 2011. This work was performed in the frame- work of the Collaborative Project COCAE (Grant 218000) of the European Community’s Seventh Framework Programme (FP7/2007-2013). T. Aoki is with the Research Institute of Electronics, Shizuoka University, Hamamatsu 432-8011, Japan (e-mail: rtaoki@ipc.shizuoka.ac.jp). V. A. Gnatyuk is with the Research Institute of Electronics, Shizuoka Univer- sity, Hamamatsu 432-8011, Japan, on leave from the V.E. Lashkaryov Institute of Semiconductor Physics of the National Academy of Sciences of Ukraine, Kyiv 03028, Ukraine (e-mail: gnatyuk@lycos.com). L. A. Kosyachenko, O. L. Maslyanchuk, and E. V. Grushko are with Yuriy Fedkovych Chernivtsi National University, Chernivtsi 58012, Ukraine (e-mail: lakos@chv.ukrpack.net). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2010.2090173 problem to obtain high charge collection with low leakage cur- rent in semi-insulating CdTe-based detectors can be overcome if a barrier contact is formed instead of ohmic one. We have recently shown that using commercially available CdTe crystals (0.5 mm thick) with a blocking contact (Schottky barrier) it is possible to obtain diodes with low leakage current (50–100 nA/cm ) even at high bias voltages (1.5–2 kV) [7], [8]. There are also promising CdTe diodes based on electrical junctions with different types of conductivity ( , or - ) because it is easy to form an extended depletion re- gion and improve the charge collection in a homojunction as well as to achieve the stability of the electrical and spectral characteristics of detectors [3]. However, to obtain a shallow and sharp electrical junction, it needs to provide heavy doping of semi-insulating CdTe that is quite problematic because of self-compensation mechanism [9]. One of the promising techniques to form a thin doped layer with less damage of a semiconductor crystal is based on pulsed laser irradiation. We have studied laser-induced doping of high resistivity CdTe with In impurity, and diodes with a built-in - junction in the surface region of the crystals have been developed [10]–[13]. Using laser-induced doping, we can choose the thickness of an In film, which acts as a dopant source, intensity of laser irradiation, environment under doping procedure (vacuum, gas or liquid) and number of laser pulses. Thus, it is possible to vary parameters of the doping procedure to control the concentration of incorporated In atoms and thickness of the doped CdTe layer, and finally to choose the optimal conditions to form an ultra-shallow and sharp built-in - junction in the surface region of CdTe. In the paper, the charge transport mechanism in X- and -ray detectors based on CdTe diodes with a - junction formed by laser-induced doping is studied and ways of optimization of the detection properties are analyzed. II. SAMPLE PREPARATION For detector fabrication, commercial (111) oriented Cl-com- pensated semi-insulating ( cm) like CdTe single crystals with linear dimensions of 5 5 0.5 mm , produced by Acrorad Corporation, were used [4], [5]. The struc- tured In/CdTe/Au diodes were fabricated by the laser doping technique [10]–[13]. A relatively thick ( nm) In film was deposited in vacuum on the B-face (Te-terminated) of CdTe wafers. The whole surface area of the samples was entirely ir- radiated from the In film side by a KrF excimer laser pulse ( nm, ns) with energy density about 100 mJ/cm in an argon environment of 0.3 MPa at room temperature. The film served as an -type dopant source during laser ir- radiation and also as an electrode after laser-induced doping. 0018-9499/$26.00 © 2010 IEEE