Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin ﬁlms I.F. Al-Hamarneh a , B.N. Bulos b , M.M. Abdul-Gader Jafar b, * a Department of Applied Science, Prince Abdullah Bin Ghazi Faculty of Science and IT, Al-Balqa Applied University, Salt 19117, Jordan b Department of Physics, Faculty of Science, University of Jordan, Amman 11942, Jordan article info Article history: Received 7 June 2008 Received in revised form 23 October 2008 Available online 25 December 2008 PACS: 61.05.cp 61.43.Dq 73.61.Jc 84.37.+q 81.40.Ef Keywords: Amorphous semiconductors Crystallization X-ray diffraction Dielectric properties, Relaxation, Electric modulus Chalcogenides Scanning electron microscopy Defects Microcrystallinity abstract The effect of post-deposition isothermal annealing (30 °C 6 T A 6 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin ﬁlms deposited at the substrate temperatures T S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) ﬁlms (T S , T A < 50 °C) was mainly capacitive, with no loss peaks being observed in their Z 00 (x)–x curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z 00 (x)–Z 0 (x) diagrams of illuminated polycrystalline Se samples (50 °C 6 T S , T A 6 70 °C) exhibited almost full semicircles, whereas their Z 00 (x)–x curves revealed prominent loss peaks at well-deﬁned frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersec- tions of these semicircles with the Z 0 -axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental ﬁndings were explained in terms of a signiﬁcant increase in electrical conductivity of selenium ﬁlms due to thermally-induced crystalliza- tion at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction In its solid-state phase, selenium (Se) can coexist in one of its main structural allotropic modiﬁcations (e.g., amorphous, crystal- line hexagonal and monoclinic structures), which generally com- posed of different chain-like and/or ring-like molecular microstructures [1–4]. Solid ﬁlms of amorphous selenium (a-Se) have important applications in digital X-ray medical diagnostic imaging systems [5–7] and in other solid-state devices (like scho- ttky diodes, photovoltaic heterojunctions, and solar-cell devices [8–12]). Undoped a-Se exhibits p-type electrical conduction, pos- sesses very high dark room-temperature DC resistivity (>10 12 X cm) and has an optical band-gap energy E g  2 eV at 300 K [1]. Undoped a-Se usually suffers from thermal instability and aging ef- fects due to phase transformation and structural relaxations even in normal ambient conditions as it has a low glass-transformation temperature, T g (40 °C), which is much higher in the case of slightly doped a-Se material [13–17]. Like other chalcogenides, un- doped a-Se is known to contain a high density of bulk/surface traps and localized states due to native (intrinsic) charged defects origi- nating from its lattice disorder and normal broken dangling bonds [1,2,17–20]. This leads to bad electrical performance of devices using a-Se ﬁlms because of unavoidable electron and/or hole trap- ping effects [21–23]. Thermal heating of undoped a-Se ﬁlms at temperatures above 40 °C normally induces prominent structural changes in them [24–26] and transforms them to polycrystalline ones with a hexag- onal-like microstructure and a higher electrical conductance [25]. A recent report [27], however, argued that polycrystalline Se ﬁlms can be deposited at 30 °C and further post-deposition annealing at temperatures far beyond 100 °C did not improve their crystallinity. The literature is wealthy in papers on DC electrical and optical properties of undoped and doped a-Se ﬁlms, but not much work has been reported on the AC behavior of undoped Se ﬁlms sub- jected to thermal heat treatment at temperatures near and beyond the T g -region of a-Se [26]. In the present paper, we shall report 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.11.006 * Corresponding author. Tel.: +962 6 5355000-22042; fax: +962 6 5348932. E-mail address: mmjafar@ju.edu.jo (M.M. Abdul-Gader Jafar). Journal of Non-Crystalline Solids 355 (2009) 305–310 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol