Mechanisms in the Formation of High Quality Schottky Contacts to n-type ZnO
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1035-L10-06
Mechanisms in the Formation of High Quality Schottky Contacts to n-type ZnO Martin Allen1, Holger von Wenckstern2, Marius Grundmann2, Stuart Hatfield3, Paul Jefferson3, Philip King3, Timothy Veal3, Chris McConville3, and Steven Durbin1 1 Electrical and Computer Engineering, University of Canterbury, Christchurch, 8043, New Zealand 2 Institut für Experimentelle Physik II, Universität Leipzig, Leipzig, 04103, Germany 3 Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom
ABSTRACT Pt, Ir, Ni, Pd, and silver oxide Schottky contacts were fabricated on the Zn-polar surface of hydrothermally grown bulk ZnO. A relationship was observed between the barrier height of the contact and the free energy of formation of the ‘metal’ oxide. This is consistent with the dominating influence of oxygen vacancies (VO) which tend to pin the ZnO Fermi level close to the VO (+2,0) defect level at approximately EC - 0.7 eV, where EC is the conduction band minimum. Valence band x-ray photoemission spectroscopy and the current - voltage characteristics of planar Schottky diodes, measured on similar Zn-polar surfaces, showed the existence of a vacuum activated surface accumulation layer. This is possibly a consequence of the observed OH termination of the Zn-polar surface. The surface accumulation layer is compensated in atmospheric conditions by the presence of acceptor-like adsorbates, such as O2 and H2O. The formation of high quality Schottky contacts to ZnO should therefore involve the reduction of near surface oxygen vacancies and the removal of H or OH from the surface. INTRODUCTION Although Schottky contacts to ZnO have been investigated for over forty years, the mechanisms involved in contact formation are still not clearly understood. Most published barrier heights have been in the 0.6 – 0.8 eV range, regardless of the Schottky metal used, with ideality factors typically in excess of 1.5 [1]. This is indicative of interfaces whose properties are determined by the defect nature of the semiconductor rather than the work function of the metal. This situation has not changed significantly with the availability of high quality bulk ZnO grown using either the vapor phase, pressurized melt, or hydrothermal techniques. The repeatable fabrication of Schottky contacts to hydrothermally grown bulk ZnO has proved particularly challenging with only a few reports of high quality contacts. This is surprising considering the high crystal quality, low surface roughness, and low carrier concentration of this material [2]. In this paper, we show how the relative performance of different ‘metal’ Schottky contacts to hydrothermally grown bulk ZnO can be explained by a model of oxygen vacancy formation, which is partly intrinsic and partly due to chemical bonding at the metal - ZnO interface.
EXPERIMENT The main experiment involved the fabrication of arrays of planar Pt, Ir, Ni, and silver oxide Schottky contacts on the Zn-polar surface of a single, hydrothermally grown, bulk ZnO wafer purchased from Tokyo Denpa Co.
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