NPOL > Research > Light Emitters

Light Emitters

What is Light Emitter?


          A light-emitting diode (LED) is a semiconductor light source. LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness. When a light-emitting diode is switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. As devices with higher power capabilities have become available, new application areas emerge constantly. Light-emitting diodes are used in applications as diverse as aviation lighting, digital microscopes, automotive lighting, advertising, general lighting, and traffic signals.


Research Area

Novel DUV Light Emitters

          Nitride semiconductor, for example, AlGaInN-based light-emitting diodes (LEDs) have been getting the limelight as an alternative lighting source due to their very promising properties such as high efficiency, long lifetime, and environmental impact. Recently, AlGaN-based deep ultraviolet (DUV) LEDs are drawing great attention for mercury-free and efficient light sources emitting UV-C for many applications such as germicidal disinfection, sterilization, purification of water and air, protein analysis, medical- and defense-related applications. However, widespread adoption of DUV LEDs for such variety of applications is obstructed by challenges to making UV LEDs: (1) Growing crack-free, thick, doped AlGaN epitaxial layers, (2) Realizing enough doping in high aluminum content AlGaN, (3) Reducing non-radiative recombination and enhancing radiative recombination, and (4) Obtaining high light extraction efficiencies. In our laboratory, we designed high-efficiency DUV LED structure with enhanced internal quantum efficiency and light extraction efficiency.


Visible Light Emitters

  1. Nano/Micro LEDs for Panchromatic, Droop-free White LEDs

          Selectively grown GaN by MOCVD has many different GaN facets in micro/nano-structures, determined by growth mask, and growth conditions. Such poly-facet structure LED includes semi-/non-polar facets as well as polar facets, which emit "panchromatic" light originated from different growth behavior. Well designed array of poly-facets show not only white emission with high CRI but also high efficiency due to reduced internal polarization on semi-/non-polar facets


  2. Efficiency Droop Reduced LEDs

    •  Polarization Engineering

          III-nitride semiconductors have a strong polarization along the c-direction due to their asymmetric crystal structure (wurtzite). At hetero-interfaces of GaN-based LED, therefore, polarization-induced sheet charges exist: positive sheet charges at GaN(spacer)/AlGaN(EBL) interfaces, and negative sheet charges at GaN(QB)/InGaN(QW) interfaces. Strong electric field from polarization-induced charges generally results in degraded electrical optical properties of LEDs such as reduced quantum efficiency, high operating voltage and severe efficiency droop. Polarization-matched LED using quaternary alloy (AlGaInN) is one of the solutions to reduce droop by alleviating polarization-induced electric field in the device. The other way, polarization field can be used on purpose for designing droop-reduced LED.


    •  Graded Superlattice Electron Blocking Layer (GSL-EBL)

          The AlGaN/GaN superlattice (SL) electron blocking layer (EBL) with graded Al composition is a promising solution to reduce efficiency droop. It is found that GaN-based LEDs with AlGaN/GaN graded superlattice (GSL) EBLs show reduced efficiency droop as well as comparable or even lower operation voltage compared to LEDs with conventional bulk EBL, while LEDs with SL-EBL with equal Al composition showed higher operation voltage than LEDs with bulk EBL. These improvements in device properties of LEDs with SL-EBL are attributed to (i) high hole concentration enabled by superlattice-doping effect, (ii) reduced barrier height for hole injection by graded Al compositions.

Related Articles

1. Jun Hyuk Park et al., Applied Physics Letters, 103, 061104 (2013) [Link]