Han study thin LED substrate effectively enhance the effectiveness of green LED light

The efficiency of Korea and Egypt researchers used wafer thinning techniques to enhance the nitride semiconductor green LED. Participate in the organization including South Korea, Egypt Chonnam National University Beni Sue Vee J University and Korea photonics technology research institute.

Thinning effect is to reduce the residual nitride semiconductor structure in the compressive stress (residual compressive stress), the stress impact effect on electric field pressure reduction in the LED architecture. The stress components produced by the different thermal expansion coefficients between gallium nitride and sapphire will cause the formation of compressive stress after the epitaxial growth process is cooled.

This effect will reduce the possibility of electron hole recombination to generate photons. This problem is more serious in high indium content (more than 20%) of InGaN (indium gallium nitride) alloy, which is the same as the green LED. At present, the efficiency of blue InGaN LED is 50%, and the efficiency of green LED with higher indium content is usually less than 10%.

The LED architecture using MOCVD growth in 2 inches of C on sapphire substrate. The thickness of the substrate is 430 m, and the traditional technology is integrated into the LED chip of 240 m x of 600 m.

LED epitaxial structure

The substrate thinning technique by grinding and polishing to achieve the soft. These processes are used to minimize the damage during thinning. After thinning, the wafer is cut into a single, N type GaN contact layer for wafer warpage and residual stress measurements show that when the wafer is thinned to 200 m and between 80 m, the warpage increases and the stress decreases.

20mA injection current electroluminescence spectra show that with substrate thinning the current density increased. At the same time, the peak positions of 200 m and 80 m thick substrates were shifted from 520.1nm (2.38eV) to 515.7nm (2.40eV) respectively. The researchers explained: "these findings clearly show that the mechanical stress caused by wafer warpage will change the piezoelectric field in the active region of InGaN/GaN MQW and modify the band value. However, the blue peak wavelength and the energy drift are attributed to the enhancement of the band gap, the band gap is due to the improvement of the InGaN/GaN MQW piezoelectric field reduction. "

Substrate thinning but also enhance the internal quantum efficiency (IQE) and optical output power, but will not reduce the voltage and current behavior. 20mA, the substrate thickness from 200 m to 80 m, optical output power from 7.8mW to 11.5mW. It is proved that the performance of piezoelectric electric field can be reduced. In the case of 20mA, the forward voltage of different substrate thickness (200 m, 170 m, 140 m, m, 110 m, 80) is almost constant at.

Common substrate thickness and the peak external quantum efficiency (EQE) increased from 16.3% to 24%. The researchers compared the performance of their green LED EQE with that of the current best semi pole self-supporting GaN substrate: 20.4% in the (20-21) direction, the direction of the 18.9% in (11-22). The use of semi polar substrates is another way to reduce the piezoelectric field of GaN LED. But the substrate is very expensive.

Researchers introduced South Korea EtaMax (DOSA-IQE) IQE room temperature measurement system of the company, the injection current of at least 10mA, maximum IQE 80 m thickness of the substrate is 92% 20mA, with the thickness of the substrate from 200 m to 80 m, IQE increased from 58.2% to 68.9%.

By comparing EQE and IQE, the researchers determined the light extraction efficiency of thin substrate is higher. The improvement of the extraction rate is attributed to the decrease of the photon absorbed by the sapphire substrate, and the increase of the marginal escape ability of the light from the device.

Finally, photoelectric conversion efficiency with the same substrate thinning technology (WPE) increased from 11.5% to 17.1%.