According to Compound Semiconductor, researchers at Rensselaer Polytechnic Institute (RPI), South Korea's Samsung LED and Pohang University of Science and Technology are jointly developing multi-quantum well (MQW) LEDs, and traditionally using GaN semiconductor materials as a barrier (QB) layer. In contrast, the barrier material used in this project is InGaN (Applied Physics Letters (APL) 2010, Vol. 107, p063102). The potential well material is also InGaN, except that the In composition is higher, resulting in a narrower band than the barrier (Figure 1). Its illuminating wavelength is 480-443 nm in the blue light range of 490-440 nm.
Figure 1. (a) Schematic diagram of an epitaxial structure of an InGaN/InGaN multiple quantum well LED; (b) InGaN/GaN multiple quantum well LED control device
The motivation for the above design is to reduce the piezoelectric field caused by the lattice mismatch strain between the InGaN and GaN materials. At the same time, a spontaneously polarized electric field is generated in a nitride semiconductor material such as InGaN. The electric field across the LED structure has several negative effects on the performance of the device. For example, an electric field can separate the wave functions of electrons and holes, thereby reducing the chance of complex luminescence of the two carriers. Other effects include shifting the wavelength of the illumination at different currents. The above color shift is disadvantageous for LEDs of a specific wavelength or LED plus phosphor white light devices, which may cause instability of the color rendering index.
RPI researchers have been working on the polarization effects of LEDs for many years under the leadership of Professor E Fred Schubert. In the latest published research report, they found that the polarization mismatch between the potential well and the barrier and the blue shift of the wavelength are reduced when the drive current rises (using a pulse width of 2 microseconds and a duty cycle of 1%). Pulse current avoids self-heating). The forward voltage is also lower (4.1V at 300mA, 4.6V vs. InGaN/GaN devices), indicating higher efficiency and lower series resistance. It can be seen from the atomic force microscope image that the pit density is also lower, indicating less line dislocation density. The reverse leakage current of the device is also lower.
Figure 2. InGaN/InGaN and InGaN/GaN multiple quantum well LED light output power (LOP, a) and normalized external quantum effect (EQE, b) as a function of forward voltage.
The LED in this project is a multi-quantum well structure epitaxially grown on a sapphire substrate based on metal organic chemical vapor deposition (MOCVD), and its optical output power (LOP) and external quantum effect (EQE) are also improved. . In the new structure using InGaN as a barrier material, the external quantum effect increases steadily with the increase of the forward current and saturates when the current reaches 300 mA. In contrast, the conventional GaN barrier control structure has an external quantum efficiency that peaks at about 10 mA and has been severely reduced by about 45% at 300 mA.
The simulation results show that the polarization field of the InGaN/InGaN multi-quantum well LED is greatly weakened compared with the control GaN barrier structure. The former is about 0.8 MV/cm when the forward current is 0-300 mA, and about 1.2-1.4 MV/cm for the latter. . In the same current range described above, the luminescence wavelength of the GaN barrier control device was shifted from 465 nm blue to below 445 nm. The blue shift of the InGaN barrier LED is relatively small. The forward current 0-50mA emission wavelength is reduced from 448nm to 444nm, and the 50-300mA is slightly increased to 445nm. (LEDC compilation, MH collation, this article was published in the April 2011 issue of LEDC)
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