Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity
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Schottky Contact Behaviour as a Function of Metal and ZnO Surface Polarity M.W. Allen1, P. Miller2, J.B. Metson3, R.J. Reeves2, M.M. Alkaisi1, and S.M. Durbin1 1 Dept. of Electrical & Computer Eng, University of Canterbury, Christchurch, 8020, New Zealand 2 Dept. of Physics, University of Canterbury, Christchurch, 8020, New Zealand 3 Dept. of Chemistry, University of Auckland, Auckland, New Zealand
ABSTRACT Hall effect, photoluminescence (PL) and Schottky diode measurements were made on the Zn-polar (0001) , O-polar (0001) and m-plane (1100) faces of hydrothermally grown, bulk ZnO. Several polarity related differences were observed in the PL spectra. The most noticeable was increased emission from free excitons and from a triplet of emissions at 3.3725 - 3.3750 eV on the Zn-polar face. Polarity effects were also observed in the properties of highly rectifying, planar, silver oxide diodes fabricated by RF sputtering using an Ag target and an Ar/O2 plasma. The most significant of these was a consistent 130 meV higher barrier height for silver oxide diodes on the Zn-polar face compared to the O-polar face. These polarity effects are thought to result from the internal compensation of bound spontaneous polarization charges at the Zn-polar and O-polar faces. In addition, Au and Ag Schottky diodes with image-force-controlled ideality factors were achieved on the Zn-polar face of bulk ZnO without any special surface treatments. INTRODUCTION The ability to fabricate high quality ohmic and Schottky contacts is a key step towards the manufacture of ZnO based opto-electronic devices. While good progress has been made in the fabrication of low-resistivity ohmic contacts, the production of Schottky contacts with reproducible properties is proving more challenging [1], and there is a need for further study of the role the ZnO surface plays in determining contact properties. The availability of bulk, single crystal ZnO provides material of consistent quality [2] in which the surface factors influencing the performance of Schottky contacts can be investigated. One such factor is the polarity of the ZnO surface. ZnO has a high ionicity and a wurtzite structure with a lack of inversion symmetry along the c-axis. This produces a large spontaneous polarization (-0.057 Cm-2 [3]) with bound sheet charges of opposite sign occurring at the Zn-polar and O-polar faces. These bound polarization charges may have a significant impact on the morphology, chemical activity and electrical properties of the surface, all of which are known to influence Schottky contact formation. In this paper, we compare the rectifying performance of three common Schottky metals (Au, Ag & Pd), investigate the influence of surface polarity on electrical and optical properties, and report on a reliable and reproducible method of fabricating high quality Schottky contacts using silver oxide.
EXPERIMENT Three double-sided polished, undoped, c-axis wafers and one m-plane wafer were purchased from Tokyo Denpa Co. Ltd. (Japan). These wafers were sliced and
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