Improved Photoconductivity of ZnO by Ion Beam Bombardment
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0891-EE10-15.1
Improved Photoconductivity of ZnO by Ion Beam Bombardment I. P. Wellenius, A. Dhawan, and J. F. Muth1 Dept. of Electrical and Computer Engineering, North Carolina State University Raleigh, NC 27695 N. A. Guardala, J. L. Price Positive Ion Accelerator Facility, Naval Surface Warfare Center, Carderock Division West Bethesda, MD 20817 ABSTRACT In this study, single crystal ZnO films are grown by pulsed laser deposition and ion beam processing is used alter the resistivity of the films. A 3He ion beam was chosen with a specific energy to transmutate oxygen into nitrogen. Analytical ion beam techniques were used to monitor the transmutation process, and changes in film characteristics were monitored by making resistance, photoconductivity and luminescence measurements before and after ion beam processing. The amount of nitrogen produced by this method was estimated to be ~ 1014 cm-3, and was too low to be observable as a p-type dopant due to compensation by the naturally n-type material. However, the ion beam processed films improved dramatically in resistivity, defect luminescence was reduced and photoconductivity increases consistent with the improvements with resistivity were observed. These improvements were attributed to ion beam annealing of the crystal resulting in a reduction of point defects. In some films, blistering of the surface occurred and was attributed to the formation of gas bubbles which causes delamination of the film from the substrate. INTRODUCTION Ion beam processing using fast ions has been observed to increase the resistivity of ZnO thin films [1, 2]. There has also been substantial work in seeking alternative doping methods for ZnO films due to the difficulty observed in fabricating p-type material. This difficulty stems from donor-like defects, such as oxygen vacancies and zinc interstitials, which easily form during deposition or processing. As such, it is important to study novel doping methods which do not encourage compensating defect formation [3, 4]. One possible alternative is nuclear transmutation doping (NTD), whereby a nuclear reaction is induced using an energetic ion incident on a lattice atom, producing an atom of a different species in the material. In this study, ion beam processing is used to improve the resistivity of the films using a 6.6 MeV 3He ion beam, which produces a reaction that transmutates oxygen into nitrogen. The resulting films were characterized by resistivity, photoconductivity, scanning electron microscopy, and cathodoluminescence as well as ion beam analysis methods. EXPERIMENTAL DETAILS ZnO thin films were grown on c-plane sapphire substrates by pulsed laser deposition (PLD) using a Neocera Pulsed Energy Deposition system at North Carolina State University 1
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(Raleigh, NC) and subsequently annealed to produce resistive material. The PLD system utilizes a KrF excimer laser, pulsing at 10 Hz with an estimated energy density up to 4 J/cm2. Films were grown at 700 oC, with an oxygen partial pressure of
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