Highly resistive sputtered ZnO films implanted with copper
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Highly resistive sputtered ZnO films implanted with copper M. K. Puchert, A. Hartmann, and R. N. Lamba) Surface Science and Technology, School of Chemistry, University of New South Wales, Sydney, New South Wales 2052, Australia
J. W. Martin School of Physics, University of New South Wales, Sydney, New South Wales 2052, Australia (Received 31 October 1995; accepted 13 March 1996)
Polycrystalline (0001)-oriented thin films of ZnO (thickness 120 nm) were deposited by rf magnetron sputtering and post-deposition annealed at 500 ±C in oxygen (1 atm). The films were subsequently implanted with copper at doses over the range 1016 to 1017 ionsycm2 . X-ray diffraction (XRD) indicates the compressive intrinsic film stress is largely relieved by the preimplantation anneal, and does not change when implanted or when further annealed after implantation, suggesting that the dominant cause of intrinsic stress is the atomic packing density rather than the crystallographic defect density. Resistivity measurements indicate that annealing of pure ZnO films causes the perpendicular resistivity to increase from 1.3 3 105 V ? cm to 5 3 1010 V ? cm. Copper implantation results in a lower resistivity of the order of 107 V ? cm, but subsequent annealing actually increases resistivity beyond that of annealed nonimplanted ZnO to 3 3 1012 V ? cm. It is proposed that copper increases the resistivity of those annealed films by trapping free electrons with the Cu 3d hole state occurring in CuO (formed predominantly during annealing). In order to check this, the oxidation state of the implanted copper was studied before and after annealing by x-ray photoelectron spectroscopy (XPS) and extended x-ray absorption fine structure (EXAFS). Three oxidation states of copper (Cu0 , Cu1+ , Cu2+ ) are detected in the implanted films, and postimplantation annealing results in oxidation of copper to the Cu2+ state, confirming that the presence of CuO in ZnO is associated with increased resistivity.
I. INTRODUCTION
Polycrystalline thin film zinc oxide is a versatile material which has been successfully used in a wide range of thin film devices, including piezoelectric actuators for microlevers,1 surface acoustic wave (SAW) applications such as mass-loading sensors,2 gas sensors,3 and a number of optical modulator designs.4–6 Our interest is in maximizing the piezoelectric and electrooptic (EO) properties of the film while maintaining the wide optical bandgap (,3.2 eV) for applications in blue light modulators.7 The magnitudes of the anisotropic piezoelectric and EO effects in polycrystalline thin films ZnO are inherently less than in single crystal ZnO, but can be increased by giving the film structure a single crystallographic orientation with large grains. It is also desirable to maximize the ZnO electrical resistivity in order that electromechanical and electro-optic device applications are not limited to high operation frequencies (.1 MHz), and that electric fields can be generated across the film with low dr
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