Characterization of Tin Oxide Grown by Molecular Beam Epitaxy
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Characterization of Tin Oxide Grown by Molecular Beam Epitaxy G. Medina1, P.A. Stampe2, R.J. Kennedy2, R.J. Reeves3, G.T. Dang4, A. Hyland4, M.W. Allen4, M.J. Wahila5, L.F.J. Piper5, and S. M. Durbin1,6,7 1 Department of Electrical Engineering, University at Buffalo, Buffalo, NY 14260, USA 2 Department of Physics, Florida A&M University, Tallahassee, FL 32307, USA 3 Department of Physics, University of Canterbury, Christchurch 8140, New Zealand 4 Department of Electrical and Computer Engineering, University of Canterbury, Christchurch 8140, New Zealand 5 Department of Physics, Binghamton University, Binghamton, NY 13902, USA 6 Department of Physics, University at Buffalo, Buffalo, NY 14260, USA 7 Department of Electrical and Computer Engineering, Western Michigan University, Kalamazoo, MI 49008, USA ABSTRACT We describe the characteristics of a series of thin film tin oxide films grown by plasmaassisted molecular beam epitaxy on r-plane sapphire substrates over a range of flux and substrate temperature conditions. A mixture of both SnO2 and SnO are detected in several films, with the amount depending on growth conditions, most particularly the substrate temperature. Electrical measurements were not possible on all samples due to roughness related issues with contacting, but at least one film exhibited p-type characteristics depending on measurement conditions, and one sample exhibited significant persistent photoconductivity upon ultraviolet excitation in a metal-semiconductor-metal device structure. INTRODUCTION Tin oxide is a particularly interesting semiconductor, in that two different compounds are readily grown in thin film form: SnO2, with a band gap energy of approximately 3.5 eV, and metastable SnO, with a somewhat narrower, less well characterized band gap energy. SnO2 has many similarities with ZnO, another ultraviolet band gap semiconductor, including a strong tendency to be n-type as grown, as well as a surface electron accumulation layer [1,2]. Both of these characteristics are related to a charge neutrality level near the conduction band edge, and together can interfere with both pn junction formation and Schottky based device fabrication. In contrast, the narrower band gap compound SnO is intrinsically p-type. Due to the metastable nature of this material it must be grown at lower temperature, above which mixed phase or purely SnO2 may result. SnO2 is easily grown by a variety of techniques including RF sputtering [3], pulsed laser deposition [4], and chemical vapor deposition [5]. There are fewer reports of SnO growth, although it can be achieved by careful control of deposition conditions [6]. SnO2 has also been grown by molecular beam epitaxy (MBE) [7-9]. With the goal of achieving predominantly SnO single crystal films with an epitaxial relationship with the substrate for subsequent device applications, we report in this work preliminary results on the growth by plasma assisted MBE (PA-MBE) and characterization of tin oxide films.
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EXPERIMENT Films were grown in a custom-built, user-modified
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