The effect of thinning is to reduce the residual compressive stress in the nitride semiconductor structure, which has an impact on the reduced piezoelectric field in the LED architecture. A stress device made of a different thermal expansion coefficient between gallium nitride and sapphire causes a compressive stress to be generated after the epitaxial growth process is cooled.
This electric field effect will reduce the likelihood of electrons and holes recombining to produce photons. This problem is exacerbated in indium gallium nitride (InGaN) alloys with high indium composition (more than 20%), which is again required for green LEDs. At present, the efficiency of blue InGaN LEDs is 50%, while the efficiency of green LEDs with higher indium contents is usually less than 10%.
This LED architecture was grown on a 2 inch C-plane sapphire substrate using MOCVD. The substrate has a thickness of 430 μm and is integrated into a 240 μm x 600 μm LED chip using a conventional process.
LED epitaxial architecture
This substrate thinning technique is achieved by using grinding and soft polishing. These processes are used to minimize damage during thinning. After thinning, the chip is cut into individual, wafer warpage and residual stress measurements of the n-type GaN contact layer show that as the wafer is thinned to between 200 μm and 80 μm, warpage increases and stress decreases.
The electroluminescence spectrum of the 20 mA injection current showed that the current density increased as the substrate became thinner. At the same time, the peak positions of the 200 μm and 80 μm thick substrates drifted from 520.1 nm (2.38 eV) to 515.7 nm (2.40 eV), respectively. The researchers explained: "These findings clearly show that the mechanical stress caused by wafer warpage changes the piezoelectric field of the InGaN/GaN MQW active region and corrects the band value. However, the blue peak wavelength and energy The drift is due to the increase in bandgap, which is due to the reduced piezoelectric field of InGaN/GaN MQW."
Thinning of the substrate also increases internal quantum efficiency (IQE) and optical output power without reducing current and voltage behavior. At 20 mA, the substrate thickness is from 200 μm to 80 μm, and the light output power is increased from 7.8 mW to 11.5 mW. This again proves that the piezoelectric field boost performance can be reduced. In the case of 20 mA, the forward voltage of different substrate thicknesses (200 μm, 170 μm, 140 μm, 110 μm, 80 μm) is almost constant at 3.4V.
At the general substrate thickness, the peak external quantum efficiency (EQE) increased from 16.3% to 24%. The researchers compared the EQE performance of their green LEDs with the best half-pole self-supporting GaN substrate data: 20.4% in the (20-21) direction and 18.9% in the (11-22) direction. The use of a half-pole substrate is another way to reduce the piezoelectric field of GaN LEDs. But such a substrate is very expensive.
The researchers used the room temperature IQE measurement system of EtaMax (DOSA-IQE) in Korea. When the current is injected at least 10mA, the maximum IQE of the 80μm substrate thickness is 92%. At 20mA, the substrate thickness decreases from 200μm to 80μm. IQE increased from 58.2% to 68.9%.
By comparing EQE and IQE, the researchers determined that light extraction efficiency was higher for thinner substrates. The increase in light extraction rate is attributed to the reduction of photons absorbed by the sapphire substrate, while the light escaping ability of the sapphire from the device is improved.
Finally, the photoelectric conversion efficiency (WPE) using the same substrate thinning technique increased from 11.5% to 17.1%.
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