Nanoindentation induced piezoelectricity in SrTiO 3 single crystals I. Bdikin, a Budhendra Singh, a,⇑ J. Suresh Kumar, b M.P.F. Grac ßa, b A.M. Balbashov, c J. Gra ´cio a and A.L. Kholkin d a TEMA-NRD, Mechanical Engineering Department, Aveiro Institute of Nanotechnology (AIN), University of Aveiro, 3810-193 Aveiro, Portugal b Department of Physics & I3N, University of Aveiro, 3810-193 Aveiro, Portugal c Moscow Power Engineering Institute, Moscow 105835, Russia d Department of Materials and Ceramics Engineering & CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Received 23 August 2013; revised 1 November 2013; accepted 2 November 2013 Available online 9 November 2013 Nanoindentation-induced polar properties in (0 0 1)-oriented SrTiO 3 single crystals were analysed using piezoresponse force microscopy and local Raman spectroscopy measurements. The area directly beneath the indenter showed a strong piezoelectric response, together with an enhanced response in the area along the direction tangential to the residual indent. Stress-induced stable polarization states near the crack areas were also observed. Local poling performed on the strained areas suggests a possibility of polarization reversal and the stability of field-induced polar states. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Perovskite; Nanoindentation; Atomic force microscopy (AFM); Raman spectroscopy; Piezoelectricity Pure SrTiO 3 (STO), a prototype model for perovskite oxide, is a well-known incipient ferroelectric material and it exhibits various multifunctional proper- ties, being a semiconductor [1,2], a high-mobility metal [3] and even a superconductor [4], depending on the oxy- gen vacancy concentration. Recently, it has been dem- onstrated that high-quality epitaxial STO films exhibit room-temperature ferroelectricity when subjected to a compressive or tensile strain [5–11]. Moreover, ferroelec- tricity was also observed in stress-free STO films, being attributed to the intrinsic defects such as Sr- or O-vacan- cies and their disorder (relaxor ferroelectricity) [12]. It should be noted that polar properties in STO can also be induced by the surface [13] and by bending (flexoelec- tricity) [14]. The precise nature of induced polar behav- ior in STO crystals was rationalized in terms of mechanical stress and its gradient [11,15,16], non-stoi- chiometry [17,18], disorder and even ionic processes [19], but the nature of the polarization behavior is still unclear. In this work, we used the well-established meth- od of nanoindentation to show that the polar properties in STO can also be induced by the residual stresses, and in some cases polarization can be reversed by the applied electric field. A similar experiment was performed by Ferna ´ndez et al. [20], who investigated the local polari- zation state and switching behavior of thin films under external mechanical loading where the nanoindentation technique had been used to impose local mechanical stress. They observed an enhanced piezoelectric activity due to stress-induced domain-wall movement. Further, they showed how a variation in residual stress state, asymmetric distribution of charged defects and asym- metric lattice distortion produced by the inhomoge- neous indentation stress field can change the local switching behavior. This property allows for a simple but effective control of polarization behavior at the sur- face of incipient ferroelectrics and can be used, for example, to create novel heterostructures. High-quality (0 0 1)-oriented single crystals of SrTiO 3 were grown by a floating zone technique with radiation heating [21]. The growth was realized with the special URN-2-ZM furnace (Moscow Power Engineering Insti- tute, Russia) at a crystallization speed 10 mm h À1 in the ambient atmosphere. To relieve the thermal stress, the crystals were annealed at 1250 °C and slowly cooled 1359-6462/$ - see front matter Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2013.11.003 ⇑ Corresponding author. Tel.: +351 923049519, +351 234370830; e-mail: bksingh@ua.pt Available online at www.sciencedirect.com ScienceDirect Scripta Materialia 74 (2014) 76–79 www.elsevier.com/locate/scriptamat