Implant isolation of ZnO epitaxial layers
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Implant isolation of ZnO epitaxial layers S. O. Kucheyev,∗ C. Jagadish,† J. S. Williams,† P. N. K. Deenapanray,† Mitsuaki Yano,‡ Kazuto Koike,‡ Shigehiko Sasa,‡ Masataka Inoue,‡ Ken-ichi Ogata§ ∗ Lawrence Livermore National Laboratory, Livermore, California 94550 † Department of Electronic Materials Engineering, RSPhysSE, The Australian National University, Canberra, ACT 0200, Australia ‡ New Materials Research Center and Bio-Venture Center, Osaka Institute of Technology, Asahi-ku, Ohmiya, Osaka 535-8585, Japan § Bio-Venture Center, Osaka Institute of Technology, Asahi-ku, Ohmiya, Osaka 535-8585, Japan ABSTRACT The formation of highly resistive films of single-crystal ZnO as a result of irradiation with MeV Li, O, and Si ions is demonstrated. Results show that the ion doses necessary for electrical isolation close-to-inversely depend on the number of ion-beam-generated atomic displacements. Results show that an increase in the dose of 2 MeV O ions (up to ∼ 2 orders of magnitude above the threshold isolation dose) and irradiation temperature (up to 350 ◦ C) has a relatively minor effect on the thermal stability of electrical isolation, which is limited to temperatures of ∼ 300 − 400 ◦ C. For the case of multiple-energy implantation with keV Cr, Fe, or Ni ions, the evolution of sheet resistance with annealing temperature is consistent with defect-induced isolation, with a relatively minor effect of Cr, Fe, or Ni impurities on the thermal stability of isolation. Based on these results, the mechanism for electrical isolation in ZnO by ion bombardment is discussed. INTRODUCTION Single-crystal ZnO has recently attracted significant research interest as a potential material for (opto)electronic device applications [1]. In future ZnO-based technology, bombardment with energetic ions can be used for electrical isolation of closely-spaced devices. It is generally believed that irradiation-induced degradation of carrier mobility as well as trapping of carriers at deep centers associated with irradiation-produced damage (defect isolation) or with implanted species (chemical isolation) is the mechanism responsible for electrical isolation of semiconductors [2]. Electrical properties of single-crystal ZnO bombarded with electrons or light ions have previously been studied [3–6]. However, we are not aware of any reports demonstrating that ZnO can be rendered highly resistive, as required for electrical isolation. In this paper, we demonstrate irradiation-induced formation of highly resistive layers in n-type single-crystal ZnO epilayers. We also investigate thermal stability and the effect of Cr, Fe, and Ni impurities on the electrical isolation of ZnO. EXPERIMENT The n-type single-crystal wurtzite ZnO epilayers used in this study were ∼ 0.6 µm thick, epitaxially grown on a-plane sapphire substrates by molecular beam epitaxy at OIT. A further description of growth conditions can be found elsewhere [7]. As-grown epilayers had a room-temperature (RT) free electron concentration of ∼ 1017 cm−3 , an effective Hall mobility of ∼ 80 c
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