Conformal ALD of tin-doped indium oxide transparent p-ohmic contacts for micro- and nano-LEDs

Abstract

High-performance micro-LEDs require transparent conductive oxides (TCOs) that exhibit excellent electrical and optical properties while remaining compatible with p-GaN. Various TCO deposition methods, including sputtering and e-beam evaporation, have been explored, each with its advantages and challenges. In this study, plasma-enhanced atomic layer deposition (PEALD) was employed to deposit tin-doped indium oxide (ITO) for micro-LED applications. To prevent plasma-induced damage, a bilayer structure was developed with a thin ALD indium oxide buffer layer beneath the PEALD ITO. The resulting ITO film, with a stoichiometry of In_1.82Sn_0.18O₃ and a thickness of ∼80 nm, exhibited a smooth surface (RMS roughness 0.49 nm), high optical transparency, and excellent conductivity. Post-deposition rapid thermal annealing promoted partial diffusion of the interfacial oxide layer into the ITO and p-GaN, and reduced hydrogen-related defect complexes located near the p-GaN interface. The optimized ITO layer was integrated into micro-LEDs of various sizes (150 × 150 μm² to 20 × 20 μm²), demonstrating uniform electrical performance and low reverse leakage currents of 10⁻⁹ A. Long-term stability is confirmed through a constant voltage stress (CVS) conducted at 3.2 V for 76.5 hours. The conformal deposition capability of ALD enables precise and uniform coatings, highlighting its transformative potential in advancing TCO technologies for next-generation optoelectronic devices and high-efficiency displays.