How to reduce polarization mismatch in potential wells/barriers for high output power LEDs

According to Compound Semiconductor, researchers from Rensselaer Polytechnic Institute (RPI), Samsung LED and Pohang University of Science and Technology are jointly developing multi-quantum well (MQW) LEDs. Unlike the traditional use of GaN semiconductor materials as barrier (QB) layers, the barrier material used in this project is InGaN (Applied Physics Letters (APL) 2010, Vol. 107, p063102). The well material is also InGaN, but the difference is that the In component content is higher, resulting in a narrower energy band than the barrier (Figure 1). Its luminous wavelength is 480-443nm, which is in the blue light range of 490-440nm. The

motivation for the above design is to reduce the piezoelectric field caused by the lattice mismatch strain between InGaN and GaN materials. At the same time, spontaneous polarization electric fields are generated in nitride semiconductor materials such as InGaN. The electric field across the LED structure has several negative effects on the performance of the device. For example, the electric field can separate the wave functions of electrons and holes, thereby reducing the chance of the two carriers to recombine and emit light. Other effects include the shift of the emission wavelength under different currents. The above color shift is unfavorable for LEDs with specific wavelengths or LEDs plus phosphor white light devices, and will cause instability in the color rendering index.

Researchers at RPI, led by Professor E Fred Schubert, have been studying polarization effects in LEDs for many years. In a recently published study, they found that the polarization mismatch between the potential well and the barrier and the blue shift in wavelength were reduced when the drive current increased (using a pulse current with a pulse width of 2 microseconds and a duty cycle of 1% to avoid self-heating). The forward voltage was also lower (4.1V at 300mA, compared to 4.6V for InGaN/GaN devices), indicating higher efficiency and lower series resistance. Atomic force microscopy images show that the pit density is also lower, indicating less line dislocation density. The reverse leakage current of the device is also lower.

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). Its light output power (LOP) and external quantum effect (EQE) have also been improved, as shown in Figure 2. In the new structure using InGaN as the barrier material, the external quantum effect also steadily increases with the increase of the forward current and saturates when the current reaches 300mA. In contrast, the external quantum efficiency of the traditional GaN barrier control structure now reaches its peak at about 10mA, and has seriously dropped by about 45% at 300mA.

Simulation calculations show that the polarization electric field of InGaN/InGaN multi-quantum well LED is greatly weakened compared with the control GaN barrier structure. When the forward current is 0-300mA, the former is about 0.8MV/cm, while the latter is about 1.2-1.4MV/cm. In the same current range mentioned above, the emission wavelength of the GaN barrier control device blue-shifts from 465nm to below 445nm. The blue shift of the InGaN barrier LED is relatively small. When the forward current is 0-50mA, the emission wavelength decreases from 448nm to 444nm, and increases slightly to 445nm between 50-300mA.