Optimized SrVOx/Ag/SrVOx Multilayers As High Transparency Electrodes for UV Optoelectronic Devices

Abstract

GaN-based ultraviolet LEDs are important for their use in various applications such as space-to-space communications, curing, sterilization, etc. For these applications, the development of high-efficiency UV LEDs is needed. However, UV LEDs show relatively lower external quantum efficiencies than visible LEDs. To enhance the performance of UV optoelectronic devices by increasing light extraction, it is essential to develop p-type transparent conducting electrodes (TCEs) with high transparency in the UV region. Sn-doped indium oxide (ITO) is typical material for transparent conducting electrodes (TCEs) because of its low resistivity (< 10-4 Ω·cm) and high transmittance (> 85%) in the visible range. However, ITO has large light absorption below 400 nm wavelength and the poor electrical properties when it is thinner than 50 nm. Thus, there is a need for alternative TCEs that have high transmittance in the UV light region (200 – 400 nm). Extensive studies have been conducted to develop TCEs such as graphene, CNT, metal nanowire, and oxide/metal/oxide (OMO) multilayer. Among them, the OMO structure can be a good candidate for TCEs, because the mid-metal layer acts as a conduction path and the oxide layers can lead to zero reflection conditions.In this study, we used SrVOx (SVO) films to develop UV-transparent multilayer and investigated their structural, optical, and electrical properties as functions of SVO and Ag thickness by means of UV-VIS spectrometer, Hall-measurement, and transmission electron microscopy. Unlike ITO film, SVO film exhibited high transmittance (> 80%) up to 320 nm. It was shown that the SVO film showed larger optical bandgap (4.3 eV) than that of ITO (3.7 eV). For the OMO structures with Ag thickness of 15 nm, the sheet resistance was in the range 2.76 – 3.15 Ω/sq, depending on the SVO thickness. In particular, SVO (25 nm)/Ag (15 nm)/SVO (25 nm) sample exhibited the highest Haacke’s figure of merit (FOM) of 144.5 x 10-3 Ω-1 where the average transmittance of the OMO was 91.5% over the 360 – 400 nm wavelength. For the 25 nm-thick SVO-based OMO films, the transmittance and sheet resistance gradually decreased with increasing Ag thickness from 9 nm to 21 nm. For example, the sheet resistance of SVO/Ag/SVO samples was in the range 1.97 – 6.48 Ω/sq and the transmittance varied from 82.6 to 95.5%. Among these samples, SVO/Ag (15 nm)/SVO sample showed the highest FOM value. Rigorous coupled-wave analysis (RCWA) simulations were performed to understand the measured transmittance behavior and zero reflection condition in the SVO/Ag/SVO structure. These results imply that SVO-based OMO structure could be a potentially important TCEs for the fabrication of high-efficiency UV optoelectronic devices. Figure 1