1752 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 4, AUGUST 2009 Electromigration of Mobile Defects in CdTe Eduard Belas, Roman Grill, Marek Bugár, Jan Procházka, Pavel Hlídek, Petr Praus, Jan Franc, and Pavel Höschl Abstract—Electromigration of mobile charged defects in ex- ternal electric field is investigated at various temperatures and biases in conductive undoped and semiinsulating In-doped CdTe, respectively. A set of electric contacts as potential probes arranged linearly along the sample was used for the detection of the drift of the local resistance modulation. The observed modulation drifting along the sample always from the positive toward negative contact after step-like bias polarity reversion points to the migration of positively charged point defects. Mobility and diffusion coeffi- cient of mobile defects at 100 and 600 , respectively are determined. Electromigration of point defects is also tested by low temperature photoluminescence and a model explaining migration of charged defects is suggested. Index Terms—Annealing, CdTe, electromigration. I. INTRODUCTION C dTe/(CdZn)Te is a promising material in the field of room temperature x-ray and gamma-ray detectors, electro-op- tical devices, solar cells and substrates for narrow gap (HgCd)Te epitaxy. Low concentration of mobile impurities (Li, Na, Ag, K, Cu,) in as-grown material is one of the key parameters at the production of CdTe for high-performance applications. Mo- bile impurities disturb self-compensation of charged defects in detector-grade material and locally change the resistivity. In case of substrate-grade material, impurities diffuse into the epi- layer during epitaxial growth and deteriorate the quality of the final (HgCd)Te films [1]. There are several methods for purifica- tion of as-grown material. Standard traveling heater method was used for purification of CdTe and determination of the segrega- tion coefficients of many elements in CdTe [2], [3]. Annealing in liquid metals and salts [4], [5] or annealing under Cd vapor overpressure [6] were presented as alternative methods for CdTe purification. Electromigration (drift and diffusion of the charged defects in external electric field) could be another potential method for purification of CdTe. The ion electromigration was studied as early as the sixties [7] and a detailed overview was presented in [8], [9]. Electromigration is often used for controllable doping of semiconductors and for preparation of p-n junctions [9], [10]. However, few experimental data have been published about electromigration of charged defects in CdTe. Electromi- gration of Cu ions in CdTe/CdS structure was investigated in Manuscript received November 14, 2008; revised February 05, 2009. Current version published August 12, 2009. This work is part of the research plan MSM 0021620834 financed by the Ministry of Education of the Czech Republic and is partly supported by the Grant Agency of Charles University under Contract 215/ 2006 B-FYZ and by the Grant Agency of the Czech Republic under Contract 202/08/0644. The authors are with the Institute of Physics, Charles University, Prague CZ-121 16, Czech Republic (e-mail: belas@karlov.mff.cuni.cz). Digital Object Identifier 10.1109/TNS.2009.2022266 TABLE I THE CONCENTRATION OF IMPURITIES IN AS-GROWN UNDOPED AND IN-DOPED SAMPLES DETERMINED BY GDMS [11]. It was found that the application of low bias (1 V) at a temperature of 150 resulted in Cu migration and the forma- tion/dissociation of ( - ) A-centers. Electromigration of Ag ions in p-CdTe at room temperature (RT) was observed in [12] in contrast to [13], where such effect was not found. Ag electromigration in CdTe was also investigated using electron beam induced current (EBIC) method in [14], where a signif- icant increase of the depth of the p-n junction was observed after application of the 3 V on the surface of the sample, which was immersed in the solution at room temperature. The goal of this work is an investigation of the electromigra- tion of mobile defects in CdTe. To the best of our knowledge, the technique of measurement of electromigration developed in this manuscript has yet been reported in pertinent literature. II. EXPERIMENTAL Undoped and In-doped CdTe single crystals with In doping density in the melt were prepared in our laboratory by Vertical Gradient Freeze and Bridgman method, respectively. Undoped crystal was conductive p-type with the hole concentration and resistivity at RT. In-doped high resistive crystal was n-type with the resistivity . Both crys- tals contained small ( ) Te inclusions with a density of approximately 5 determined by infrared (IR) mi- croscopy. The concentration of foreign impurities in both crystals de- termined by glow discharged mass spectroscopy (GDMS), is shown in Table I, where Cu or Na and Al were identified in un- doped CdTe as possible dominant acceptors and donors, respec- tively. 0018-9499/$26.00 © 2009 IEEE Authorized licensed use limited to: Charles University. Downloaded on September 9, 2009 at 08:16 from IEEE Xplore. Restrictions apply.