Assessment of Transparent Conducting Zinc Oxide as a Tunneling Contact to p-GaN
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Assessment of Transparent Conducting Zinc Oxide as a Tunneling Contact to p-GaN S. Musunuru, V. Kumbham, J. Justice, K. Lee, D. Korakakis, L. A. Hornak Lane Department of Computer Science and Electrical Engineering, West Virginia University-26506 ABSTRACT Use of aluminum alloyed zinc oxide (AZO) as a transparent contact to p-GaN has received significant attention for GaN/InGaN light emitting diodes applications. Reports show that AZO as deposited on p-GaN forms a Schottky contact given the large work-function difference between AZO and p-GaN [1]. However, utilization of a thin nickel layer inserted between the AZO and p-GaN can result in an ohmic contact [2], given that Ni forms an ohmic contact to p-GaN upon annealing. Here, we undertake simulation studies of this AZO/Ni interface as a function of Nickel layer. Simulation studies of the transport mechanism in this contact indicate the likelihood of a tunneling junction at the AZO/Ni interface, thus the entire AZO/Ni forming an ohmic contact to p-GaN. Potential to better control injected current density uniformity utilizing such a contact structure is also discussed. INTRODUCTION Gallium Nitride-based semiconductor devices have had significant impact in light emitting diode (Optoelectronic device) applications. Achieving less resistive transparent ohmic contacts to p-GaN continues to be a challenge for fabrication of LED’s given the large workfunction of p-GaN (due to its large bandgap of 3.4eV and electron affinity of 4.1eV) that varies depending on the amount of doping [3]. Several reports have been made earlier on the use of annealed Ni/Au ohmic contacts to pGaN, but each have certain limitations for certain device architectures given the fact that the current crowds at the contact edges due to the highly resistive p-GaN [3]. The annealed Ni/Au contacts were only semitransparent in nature, the transparency being around 60-75% in visible and lower than 60% in UV region [4]. A less resistive and more transparent contact is thus required for light emitting diodes to achieve improved extraction efficiency. Transparent conducting oxides are potential material candidates for such electrodes given they are intermediate between metals and semiconductors, conserving the transparency properties of semiconducting materials and the conductivity of metals [5]. Transparent conducting oxides like indium tin oxide (ITO) aluminium alloyed zinc oxide (AZO) etc are synthesized from semiconductor materials indium oxide (I2O3) and zinc oxide (ZnO) via chemical doping of tin (Sn4+) ions for indium (In3+) and aluminium (Al3+) for zinc (Zn2+) ions, respectively . Degenerate doping of these semiconductor materials creates currentcarrying electrons which give rise to far-infrared absorption (Drude-like absorption) and a transition from a semiconducting state to metallic state, thus making the material highly conducting yet maintaining its fundamental band gap property, and remaining optically transparent in the visible region [5].
Indium tin oxide (ITO) has been widely investigated by s
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