Electrical Characterization of Proton Irradiated n-Type ZnO
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Electrical Characterization of Proton Irradiated n-Type ZnO F Danie Auret1, Michael Hayes1, Jackie Nel1, Walter Meyer1, Pieter Johan Janse van Rensburg1, Werner Wesch2, and E Wendler2 1 Physics Department, University of Pretoria, Pretoria, 0002, South Africa 2 Institut für Festkörperphysik, Friedrich-Schiller-Universität, Jena, 0002, Germany
ABSTRACT Ru Schottky barrier diodes (SBD’s) were fabricated on the Zn face of melt grown (MG) n-type ZnO. These diodes were irradiated with 1.8 MeV protons at fluences ranging from 1 × 1013 cm-2 to 2.4 × 1014 cm-2. Capacitance and current (I) deep level transient spectroscopy (DLTS) was used to characterise the irradiation-induced defects. Capacitance DLTS showed that proton irradiation introduced a level, Ep1, at 0.52 eV below the conduction band at an introduction rate of 13±1 cm-1. A defect with a very similar DLTS signature was also present in low concentrations in unirradiated ZnO. I-DLTS revealed that a defect with an energy level at (0.036± 0.004) eV below the conduction band in the irradiated material. This defect is clearly distinguishable from a defect with a level at (0.033± 0.004) eV below the conduction band that was present in the unirradiated sample. It is speculated that these shallow level defects are related to zinc interstitials or complexes involving them or hydrogen. INTRODUCTION ZnO is a wurtzitic semiconductor with a bandgap of 3.4 eV, and it is presently used in many diverse products, such as phosphors, paints, piezoelectric transducers, varistors, and transparent conducting cover layers for the photovoltaic industry. ZnO also has a number of properties [1] that might make it suitable for electro-optical applications. It has recently become the focus of many studies, since a wide range of applications are possible due to its large, direct band gap [2]. Devices such as detectors, lasers and diodes operating in the blue and ultra-violet (UV) spectrum have been reported [3], but are not very efficient yet. Further practical advantages of ZnO include bulk-growth capability, amenability to conventional wet chemistry etching, which is compatible with Si technology [2] (unlike the case for GaN), and convenient cleavage planes. Another important consideration in the development of ZnO devices is the lack of a reliable Schottky barrier (SB) fabrication technology. The latter problem not only precludes the realization of field-effect transistors, but also of capacitance-based characterization techniques, including capacitance-voltage (C-V) measurements, deep-level transient spectroscopy (DLTS) [4] and admittance spectroscopy (A-S) [5]. Rectifying contacts can be in the form of p-n
junctions, metal-insullator-semiconductor (MIS) structures of Schottky barrier diodes (SBDs). As-grown ZnO exhibits intrinsically n-type conduction, to form a p-n junction a suitable p-type dopant is required. Alternative approaches for space charge junctions to ZnO are the formation of a p-n heterojunction on the normally n-type ZnO, or the formation of a MIS structure. A
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