ALTERNATIVE TRANSPARENT CONTACTS FOR III-NITRIDE OPTOELECTRONIC DEVICES
Abstract: Conventional p-contacts of LEDs are made of opaque metal layers which blocks the transmission of the emitted light beneath it. In addition, due to the difference of work functions between p layer and the contact material, nonhomogeneous current density, which is called as current crowding, occurs. Current crowding causes a local overheating and catastrophic results on the material. These mentioned two effects mitigate the efficiency and brightness of the LED. To overcome these problems, current spreading layers, which act as transparent contacts as well, are utilized. One of the most commonly used and commercially available transparent contact is indium tin oxide (ITO). However, ITO based contacts have some limitations including complex fabrication and post-annealing processes. Also, volatile indium prices, high mechanical rigidity and low chemical stability remain as other problems to be solved. Hence, investigation of alternative transparent conducting materials is needed. Having high transmittance and low sheet resistance values, silver or copper nanowire random networks are the most promising candidate materials for utilizing as a current spreading layer in LEDs, among alternative transparent conducting materials. In this study, various transparency and sheet resistance values are determined for different Ag nanowire diameters and Ag nanowire network densities, then these Ag nanowire networks are formed on top of a conductive gold (Au) layer evaporated on mica substrates. The work functions of sputtered ITO, Ag nanowires with different wire diameters and Ag nanowire networks with different densities are determined by Kelvin Probe Force Microscopy (KPFM) and Ultraviolet Photoelectron Spectroscopy (UPS). Experimentally defined work functions are used in the electronic band structure calculations of p+-GaN/Ag nanowire and p+-GaN/ITO contacts, while calculation of p+-GaN/traditional metal contacts (Au/Ni) is made according to defined work functions in the literature. In addition to workfunction studies, surface passivation for copper nanowire networks was carried out to prevent oxidation when exposed to air. Atomic layer deposition of very high quality, conformal, uniform and very thin films oxides on copper nanowire random networks was studied for passivation.