Optimizing Material and Nanostructure Configurations for Efficient Light-Emission Through Inelastic Electron Tunneling

In the Metal-Insulator-Metal (MIM) devices, electrons tunnel the gap along with the bias voltage. When the tunneling electron experiences energy loss, the type of tunneling is called inelastic electron tunneling (IET). The energy loss during IET can be coupled with other kinds of energy, such as photon emission, plasmon resonance, lattice scattering (phonon), etc [1]. This type of nanoscale light source can be used for ultrafast on-chip transduction from electron to photon and optical biosensors that allow point-of-care testing [2]. However, the light source has low light emission efficiency compared to conventional light-emitting diodes or laser diodes.<br/><br/>Therefore, we separated the light emission process into three steps and demonstrated MIM devices with various material and structure configurations for efficiency optimization in each step. 1)-Electrical Characteristic: IET occurs with the bias voltage inside the MIM gap. 2)-Energy Transformation from Electron to Light: Lost energy is transferred into plasmonic energy or photon emission. 3)-Light Outcoupling: Energy of plasmon or photon outcouples to far-field through scattering or transmission, respectively. To achieve high performance in 1)-Electrical Characteristics, both operatable bias voltage range and Fowler-Nordheim tunneling were investigated by demonstrating MIM devices with various materials through thin-film deposition and 2D material transfer techniques. MIM devices that utilize graphene as an electrode have endured a larger bias up to ± 8 V, which may allow light emission with a shorter wavelength and wide range light modulation. To optimize 2)-Energy Transformation from Electron to Light and 3)-Light Outcoupling, we designed and characterized the effects of diverse MIM structures with nanophotonic metasurfaces on plasmon coupling. A nanowire array (NWA) was selected as a basic structure, and it was expanded to the other metasurfaces, including a MIM device with an NWA-patterned top electrode and a fully NWA-patterned MIM device. Localized-Surface Plasmon Resonance (LSPR), Surface Lattice Resonance (SLR), and Fundamental and second-order modes of Gap-Surface Plasmon Resonance (Gap-SPR) were characterized through Finite-difference time-domain (FDTD) simulations and a spectrometer connected with a dark-field microscope, depending on the type of nanostructures. The effects of geometrical parameters on spectral optical characteristics and E/H-field confinements were also analyzed for coupling plasmon resonance in the desired wavelength range. In addition, an inverse design algorithm has been demonstrated, which can also be implemented to increase the outcoupling efficiency by controlling the geometry of metasurfaces [3]. This experimental investigation provides material and structural insights depending on the three light emission steps, enabling the efficiency enhancement of light emission through IET for on-chip light source applications.<br/><br/>This research was supported by the National Research Foundation of Korea (NRF) under the “Korean-Swiss Science and Technology Program” (2019K1A3A1A1406720011), by the NRF grant funded by the Korea government (MSIT) (2023R1A2C2006811), and by the BK21 FOUR (Fostering Outstanding Universities for Research) funded by the Ministry of Education (MOE) of Korea and NRF. The second author Jihye Lee, Ph.D., is currently affiliated with Samsung Electronics.<br/><br/>[1] M. Parzefall and L. Novotny, “Light at the End of the Tunnel”, ACS Photonics 5(11), 4195–4202 (2018).<br/>[2] J. Lee and J.-S. Yeo, “On-Chip Nanoscale Light Source Based on Quantum Tunneling: Enabling Ultrafast Quantum Device and Sensing Applications”, Applied Science and Convergence Technology 30(1), 6-13 (2021).<br/>[3] S. Moon, J. Kang, and J.-S. Yeo, “Review of Generative Models for the Inverse Design of Nanophotonic Metasurfaces”, Applied Science and Convergence Technology 32(6), 141-150 (2023).