Investigation of optical polarization characteristics of ultraviolet-C AlGaN multiple quantum wells by angle-resolved cathodoluminescence

Abstract

AlGaN-based ultraviolet-C (UV-C) light-emitting diodes (LEDs) face challenges related to their extremely low external quantum efficiency, which is predominantly attributed to the remarkably inadequate transverse magnetic (TM) light extraction efficiency (LEE). In this study, we employ angle-resolved cathodoluminescence (ARCL) spectroscopy to assess the optical polarization of (0001)-oriented AlGaN multiple quantum well (MQW) structures in UV-C LEDs, in conjunction with a focused ion beam and scanning electron microscopy (FIB/SEM) system to etch samples with various inclination angles (θ) of sidewall. This technique effectively distinguishes the spatial distribution of TM- and transverse electric (TE)-polarized photons contributing to the luminescence of the MQW structure. CL spectroscopy confirms that UV-C LEDs with a θ of 35° exhibit the highest CL signal compared to samples with other θ. Furthermore, we establish a model using finite difference time domain (FDTD) simulation to validate the mechanism of the outcomes. The complementary contribution of TM and TE photons at different specific angles are distinguished by ARCL and confirmed by simulation. At angles near the sidewall, the CL is dominated by the TM photons, which mainly contribute to the increased LEE and the decreased degree of polarization (DOP) to make the spatial distribution of CL more uniform. Additionally, this method allows us to analyze the polarization of light without the need for polarizers, enabling the differentiation of TE and TM modes. This distinction provides flexibility for selecting different emission mode based on various application requirements. The presented approach not only opens up new opportunities for enhanced UV-C light extraction but also provides valuable insights for future endeavors in device fabrication and epitaxial film growth.