Effect of Cd dopant on electrical and optical properties of ZnO thin films prepared by spray pyrolysis route A.D. Acharya a, ⁎, Shweta Moghe a , Richa Panda b , S.B. Shrivastava a , Mohan Gangrade c , T. Shripathi c , D.M. Phase c , V. Ganesan c a School of Studies in Physics, Vikram University, Ujjain, 456010, MP, India b Acropolis Institute of Technology & Research Bhopal, MP, India c UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore, MP 452001, India abstract article info Article history: Received 24 May 2011 Received in revised form 27 October 2012 Accepted 30 October 2012 Available online 7 November 2012 Keywords: Nanocrystalline films Oxide semiconductors Zinc oxide Thin films Spray pyrolysis Cadmium Cd doped ZnO (Cd:ZnO) thin films on the glass substrate prepared by chemical spray pyrolysis technique have been characterized for their optical and electrical properties. The X-ray diffraction and atomic force mi- croscopy results indicate that the crystalline quality degrade due to higher Cd doping in ZnO. The activation energy was found to be decreased when Cd concentration increased. The absorption edge of Cd:ZnO film was found to be red shifted. The direct modulation of band gap caused by Zn/Cd substitution is responsible for the red shift effect in absorption edge of ZnO. The low temperature conduction has been explained by variable range hoping mechanism, which fits very well in the temperature range from 108 K to 301 K. The interaction between Cd and defects in ZnCdO alloy to understand the important roles of Cd in the formation of native defects has also been tentatively discussed. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The investigation of nanocrystalline thin films of ZnO attracts a lot of research interest due to its enormous potential for a variety of op- toelectronic and electronic device applications. The main advantages of ZnO are its large excitation binding energy, wide band gap energy of 3.37 eV at room temperature and the existence of well developed bulk and epitaxial growth processes [1–7]. It is known that the micro- structural, morphological and physical properties such as resistivity and transmittance depend on the nature of the fabrication technique as well as on the amount and nature of the dopant. Reports in the field of metal doped ZnO point out that small amounts of dopant substituting zinc atoms in ZnO lattice appear to cause a strong donor effect [8]; it will lead to a good quality ZnO based semiconductor [9]. Furthermore, when synthesizing doped ZnO films Bougrine et al. [10,11] found that the electrical resistivity of the films initially decreases with the addition of dopant ion, reaches a minimum value and then in- creases with further increase in dopant content. The authors attribute the decrease in the resistivity to the substitution of dopant ions at Zn 2+ sites. With increasing the doping level more dopant atoms occupy the zinc lattice sites but after a certain level the dopant atoms do not able to occupy more lattice sites and they have tendency to occupy in- terstitial sites where they form neutral defects and become ineffective as dopant impurities. On the whole electrical resistivity seems to be re- lated to dopant concentration. Consequently, the present contribution is mainly focused on the physical properties of the Cd:ZnO films when low amounts of Cd dopant are employed. An important step in order to design ZnO based devices is the reali- zation of band gap engineering to create barrier layers and quantum wells in device heterostructures. Because of the smaller band gap of CdO (2.3 eV) as compared to ZnO (3.32 eV), the Cd incorporation into ZnO serves the purpose of band gap narrowing efficiently, keeping the crystalline structure and lattice parameter close to ZnO. Moreover, the incorporation of Cd into ZnO is very useful for the fabrication of ZnO/ ZnCdO heterojunction and superlattice structures, which are the key elements in ZnO based light emitters and detectors. The physical properties of undoped and doped ZnO films have widely reported, but the certain effects of either some doping or preparation procedure still remain unclear. Unlike the influence of indium and other group III elements in ZnO, the effect of cadmium is less discussed [12]. It would therefore be interesting to investigate the effect of Cd dop- ing on the microstructural and physical properties of the ZnO thin films. Various deposition techniques have been used to grow ZnO thin films, such as sputtering, pulsed laser deposition, and spray pyrolysis [13]. Among these techniques, in our experiment, we used the chemical spray pyrolysis technique to prepare thin film of Cd:ZnO, which is a simple, versatile and economically viable method for the preparation of polycrystalline and amorphous thin films. Using the spray pyrolysis, it is easy to grow uniform films with very high growth rates, in the Thin Solid Films 525 (2012) 49–55 ⁎ Corresponding author. Tel.: +91 9424560140. E-mail address: acharyaphysics2011@gmail.com (A.D. Acharya). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.10.100 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf