Enhanced piezoelectric output voltage and Ohmic behavior in Cr-doped ZnO nanorods Nidhi Sinha a, b , Geeta Ray a , Sanjay Godara a , Manoj K. Gupta a , Binay Kumar a, * a Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India b Department of Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India A R T I C L E I N F O Article history: Received 5 March 2014 Received in revised form 15 July 2014 Accepted 18 July 2014 Available online 21 July 2014 Keywords: A. Nanostructures C. Atomic force microscopy C. Transmission electron microscopy (TEM) D. Ferroelectricity D. Electrical properties A B S T R A C T Highly crystalline Cr-doped ZnO nanorods (NRs) were synthesized by solution technique. The size distribution was analyzed by high resolution tunneling electron microscope (HRTEM) and particle size analyzer. In atomic force microscope (AFM) studies, peak to peak 8 mV output voltage was obtained on the application of constant normal force of 25 nN. It showed high dielectric constant (980) with phase transition at 69  C. Polarization vs. electric field (P–E) loops with remnant polarization (6.18 mC/cm 2 ) and coercive field (0.96 kV/cm) were obtained. In I–V studies, Cr-doping was found to reduce the rectifying behavior in the Ag/ZnO Schottky contact which is useful for field effect transistor (FET) and solar cell applications. With these excellent properties, Cr-doped ZnO NRs can be used in nanopiezoelectronics, charge storage and ferroelectric applications. ã 2014 Elsevier Ltd. All rights reserved. 1. Introduction Zinc oxide (ZnO) has a growing technological importance due to its piezoelectric, semiconducting, optical and dielectric properties. Its properties can be coupled to get various optoelectronic applications including blue/ultra violet (UV) light emitting diodes, UV photo-detectors, high electron mobility transistors, spintronics and energy (nano-) generators [1–7]. ZnO nanostructures are known to exhibit strong piezoelectric and pyroelectric properties for nanogenerator and nanoscale biosensor applications [8–11]. Wang and Song have reported the energy generation from ZnO nanowires by utilizing its semiconducting and piezoelectric properties [10]. Huang et al. have reported applications of ZnO in organic hybrid solar cells [2]. For the performance of various ZnO based electronic devices, ZnO-metal contacts play a crucial role. Usually ZnO-metal contacts show Schottkey barrier with non- linear I–V characteristics. Ohmic contact is a challenging require- ment for ZnO-based nanostructures. Ohmic contacts are very useful for electronic and photonic devices, such as field effect transistors (FETs), light emitting diodes (LEDs), solar cells, and sensors [12]. A good Ohmic contact has negligible contact resistance relative to the bulk resistance of the semiconductor, and does not significantly reduce the device performances due to a small voltage drop across the contact. This property of Ohmic contact can be achieved in ZnO by inductively coupled hydrogen and argon plasma treatment [13], focused ion beam treatment [14], annealing or by increasing the effective carrier concentration on the surface during growth process [15]. Increased carrier concentration creates oxygen vacancy on the surface of ZnO. In addition, ZnO has several advantages like easy growth, non- toxicity, coupled optoelectronic properties, etc. However, the low polarization in pure ZnO is a cause of concern and, therefore, the interest of research have been focused on achieving large polarization in ZnO by introducing various dopants like K, Mg, Li, Cr and V in the ZnO lattice [16–21]. ZnO is known to have various possible native point defects present in the lattice as interstitial Zn or O (Zn i or O i ), zinc vacancy (V Zn ), oxygen antisite (O Zn ), zinc antisite (Zn O ) and oxygen vacancy (V O ) [3,7,15]. The enthalphy of oxygen vacancies formation is low so they are more often present in the system. On the other hand, Zn antisites, O antisites, and O interstitials have high energies of formation so they are not present in large concentration under normal conditions. The defects Zn i and Zn O act as shallow donor while oxygen vacancies act as deep donors and zinc vacancies are deep acceptors. Zinc antisites also have a large off-site displacement thus they induce a large local lattice relaxation. The defects acting as donors may spontaneously compensate the deliberately introduced acceptors such as Cr 3+ . These defects directly or indirectly play an important role in controlling the growth process, doping compensation, minority carrier, luminescence efficiency, electrical properties and can originate ferromagnetism in ZnO. Cr is an important transition * Corresponding author. Tel.: +91 9818168001; fax: +91 11 27667061. E-mail addresses: bkumar@physics.du.ac.in, b3kumar69@yahoo.co.in (B. Kumar). http://dx.doi.org/10.1016/j.materresbull.2014.07.032 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved. Materials Research Bulletin 59 (2014) 267–271 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.else vie r.com/locat e/mat resbu