Abstract
Light-emitting diodes (LEDs) in the wavelength region of 535–570 nm are still inefficient, which is known as the “green gap” problem. Light in this range causes maximum luminous sensation in the human eye and is therefore advantageous for many potential uses. Here, we demonstrate a high-brightness InGaN LED with a normal voltage in the “green gap” range based on hybrid multi-quantum wells (MQWs). A yellow-green LED device is successfully fabricated and has a dominant wavelength, light output power, luminous efficiency and forward voltage of 560 nm, 2.14 mW, 19.58 lm/W and 3.39 V, respectively. To investigate the light emitting mechanism, a comparative analysis of the hybrid MQW LED and a conventional LED is conducted. The results show a 2.4-fold enhancement of the 540-nm light output power at a 20-mA injection current by the new structure due to the stronger localization effect, and such enhancement becomes larger at longer wavelengths. Our experimental data suggest that the hybrid MQW structure can effectively push the efficient InGaN LED emission toward longer wavelengths, connecting to the lower limit of the AlGaInP LEDs’ spectral range, thus enabling completion of the LED product line covering the entire visible spectrum with sufficient luminous efficacy.
Highlights
Growth conditions of InGaN have been optimized to reduce defects in the InGaN active region
Because only c-plane GaN is used in the industrial manufacture of Light-emitting diode (LED), control of the quantum-confined Stark effect (QCSE) of LEDs on c-plane sapphire substrates is significant, and this control has been explored by quantum well (QW) band engineering approaches, including the staggered QW structure[18], triangular-shaped QW19, type-II InGaN QW20, an embedded AlGaN layer[21], and so on
We prove that the harmful effects of the crystal quality deterioration and the QCSE are reduced by the stronger localization effect in this structure
Summary
The InGaN/GaN MQW LED samples in this work were grown by the metal-organic chemical-vapor deposition (MOCVD) technique. The temperature was ramped to 1,050 °C for annealing and growth of a 3-μ m-thick Si-doped GaN layer. The band-tail model[24] was adapted to fit the TDPL peak energy vs temperature curves. The density of localized state (DOS) is assumed to have a Gaussian-shaped distribution such that the temperature (T) dependence of the PL peak energy (E(T)) curve can be fitted by the band-tail model. The LED epi-wafers were annealed in N2 at 700 °C to activate the Mg-doping to form p-type GaN. The two comparative samples were measured as LED chips using a standard industrial LED testing machine to obtain the normalized light output power, I-V and EL spectra. The output power was measured in a calibrated integrating sphere
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