Nanoscale Energy Generation Characteristics of Piezoelectric Thin Films

Madhu Bhaskaran 1*, Sharath Sriram 1, Simon Ruffell2, and Arnan Mitchell1 'Microplatforms Research Group and Platform Technologies Research Institute, School of Electrical and

Computer Engineering, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia 2Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian

National University, ACT 0200, Australia *Corresponding author: Email madhu.bhaskaran@gmail.com.madhu.bhaskaran@rmit.edu.au

Abstract: The use of nanoindentation to characterize in situ the voltage and current generation of piezoelectric thin films is reported for the first time. Continuous thin films and lithographically patterned nanoislands with limited interaction area have been characterised. The influence of size on energy generation parameters is reported, with the demonstration that nanoislands can exhibit more effective current generation.

1 Introduction: Highly efficient piezoelectric thin films, which can convert small mechanical movements into voltages on the scale of silicon transistor logic, could enable micro- and nano-electronic circuits which can scavenge their power from their environment [1]. Energy harvesting using piezoelectrics has immense potential for integration with electronics, to effectively power low power micro- and nano-devices. The use of thin films has greater potential for large scale processing and readily embraced practical applications. To enable better device designs and identification of optimal film geometry, accurate nanoscale characterization of voltage and current generation, and identification of geometry parameters dominating voltage and current density generated are critical.

This work will report on the in situ electrical measurements carried out during nanoscale mechanical indentation, comparing voltage and current generation from continuous piezoelectric thin films and piezoelectric nano-islands.

2 Experimental Details: Piezoelectric thin film samples were synthesized by R F magnetron sputter deposition of PSZT of composition ( Pbo.92Sro.08)(Zro.65 Tio.35)03 onto metalized silicon substrates. This

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composition has been demonstrated to produce the highest piezoelectric response for a thin film on a silicon substrate reported to date [2,3]. A 200 nm platinum coating with a 20 nm titanium dioxide adhesion layer was used to form the conductive bottom electrode metallization of the silicon substrates. PSZT thin films were sputtered for duration of 2 and 4 hours resulting in thin films of thicknesses 700 nm and 1.2 jJm, respectively. Cross-sectional transmission electron microscopy analysis of PSZT thin film samples revealed the existence of a highly ordered columnar grain structure, as shown in Fig. 1 (a). Piezoelectric nanoislands were defined by a combination of electron beam lithography and lift-off patterning [ Fig. 1 (b)].

A Hysitron NanoECR indenter was used for performing in situ electrical measurements during indentation. Small forces from f.lN to mN were applied using a nanoindenter, which was configured to perform electrical measurements using a conductive tip (doped diamond Berkovich tip). A variety of force curves, with variables of force amplitude and frequency were utilised, with the resulting piezoelectric voltage generation stud ied.

3 Results and Discussions: Forces were applied to the samples under test and resulting voltages were measured. Increasing the force level from 0.1 mN to 1 mN resulted in voltage increase from 4 mV to 10 mV. Forces from 0.1-2.5 mN resulted in voltages of 4-23 mV, with the linear dependence of generated voltage on applied force. Measured values for voltage generation for both the 200 nm and 400 nm islands were very similar ( - 4 and -22 mV for 0.1 and 2.5 m N of force)

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Fig. 1. (a) Cross-section image of nano-structured piezoelectric thin film. (b) Scanning electron micrograph of piezoelectric nanoislands. (c) The proportional increase in the voltage produced as force is increased. (d) Current transients produced under an applied force cycle.

and matched those for the 700 nm PSZT thin films. This demonstrated that the planar geometry had no influence on the voltage generation.

Figure 1(d) shows an example of one set of current transients. The value of current generated varies from 40 pA up to a maximum of -180 pA depending on the sample under study and the force function. This value was dependent on both the magnitude of the force and was significantly influenced by the planar geometry of the piezoelectric being stressed (unlike voltage generation). The larger current densities in restricted geometries such as the nanoislands are probably due to the lack of surrounding material, which is a potential leakage path for generated current.

4 Conclusions: The results from this investigation reinforce the potential for use of piezoelectric thin films for low frequency energy scavenging. Moreover, results

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from area dependence of voltage and transient current generation show promise for use of micro-fabricated piezoelectric thin films structures in energy harvesting devices.

Acknowledgements This work has been supported by an Australian Research Council Discovery Project (DP1092717).

References [1] R. Yang, Y. Qin, C. Li, Guangzhu, and Z. L.

Wang, "Converting biomechanical energy into electricity by a muscle-movement-driven nanogenerator," Nano Lett., vol. 9, pp. 1201-1205, March 2009.

[2] S. Sriram, M. Bhaskaran, A. S. Hal/and, K. T. Short, and B. A. Latella, "Measurement of high piezoelectric response of strontiumdoped lead zirconate titanate thin films using a nanoindenter," J. Appl. Phys., vol. 101, art. 104910, May 2007.

[3] S. Sriram, M. Bhaskaran, and A. Mitchell, "Low-temperature deposition of high-response piezoelectric thin films," Scripta Materialia, vol. 63, pp. 189-191, July 2010.