KAUST Team Improves High Power InGaN Red LEDs
First over 630nm LEDs at 20 mA with a low operating voltage of 3.3V
Scientists at the King Abdullah University of Science and Technology (KAUST) have been investigating the growth and characterisation of high-power InGaN red LEDs. By precisely controlling stress in InGaN quantum wells, they have developed the first over 630nm LED at 20 mA, with a low operating voltage of 3.3V.
These InGaN-based red LEDs, they say, are superior to InGaP-based ones in the characteristic temperatures and electroluminescence (EL) peak shifts. They report their findings in two recent papers in Applied Physics Letters. ‘633nm InGaN-based red LEDs grown on thick underlying GaN layers with reducing in-plane residual stress' by Daisuke Iida et al; and ‘Effects of size on the electrical and optical properties of InGaN-based red light-emitting diodes' by Zhe Zhuang et al.
InGaN has been used to develop various optical devices with responses over the entire visible spectral range (380-780 nm) by adjusting the amount of indium. But the difficulty of creating devices increases exponentially with indium content, making it particularly difficult to develop InGaN-based red LEDs (620 nm to 780 nm).
This is because InGaN layers with a high indium content suffer from critical issues related to their low-temperature growth. These include a significant lattice mismatch, and the quantum-confined Stark effect (QCSE). Both these issues must be overcome to make high performance InGaN-based LEDs.
In 2012, Kazuhiro Ohkawa's group at Tokyo University, Japan achieved 740-nm InGaN LEDs at 20 mA. However, efficiency was very low. These LEDs were grown on a c-plane sapphire substrate using a high-temperature MOVPE growth technique. The high-temperature growth significantly improved the crystalline quality of the InGaN layers, demonstrating that temperature is a critical factor affecting the quality of the grown InGaN layers. Rapid growth facilitates the use of higher growth temperatures.
Toshiba Ltd (Japan) has reported 629-nm LEDs at 20 mA, 4.4 V. These LEDs were resin-moulded to make output x 1.5. Wall-plug efficiency (WPE) was 1.3 percent, so the output was 1.1 mW. An Osaka University group reported Eu-doped GaN 621-nm LEDs with 1.25 mW at 20 mA, 4.8-6.3 V.
8µm n-GaN layer increases EL intensity
Now using technologies including an enhanced MOVPE method, Ohkawa and colleagues at KAUST, have increased the EL intensity of InGaN-based red LEDs by a factor of 1.3 when the thickness of the underlying n-GaN layer is increased from 2 to 8µm.
As they explain in their paper: “We observed enhanced EL efficiency on varying the thicknesses of the underlying n-GaN layers. A thick underlying GaN layer with lower in-plane stress resulted in reduced surface defects on the red LEDs; this could be attributed to the increased growth temperature of the InGaN red DQWs. The light-output power of the red LEDs was enhanced by using a thicker underlying layer. We obtained a light-output power, a forward voltage, and an EQE of 0.64 mW, 3.3 V, and 1.6 percent at 20 mA, respectively. The reduction of the in-plane compressive stress in the underlying GaN layers was shown to be crucial for enhancing the light-output power of InGaN-based red LEDs on conventional sapphire substrates. “
Ohkawa, professor of Electrical Engineering at KAUST says: “We tried to control stress in the InGaN QWs as the active region of red LEDs. By reducing the stress, we could introduce more indium or increase the growth temperature of the InGaN layers. The increment of growth temperature is the crucial point to realize the high quality of InGaN crystal. Due to the high-quality InGaN, defects originated at the InGaN QWs have been reduced by one fifth compared to defect density 7x108 cm-2 in the conventional structure .“
He continues: “We have achieved the first over 630 nm LED at 20 mA, the operation voltage is as low as 3.3 V. It is 25 percent-50 percent down compared to others, and WPE is as good as 1.0 percent without resin moulding.”
How size influences the properties of InGaN-based red LEDs
Figure 1(a) shows the pattern of the rectangular LED chips. The chips had a constant mesa width of 250µm, while the mesa lengths were 350µm, 450µm, 550µm, and 650µm. Figures 1(b)-1(e) show EL images of the red LED chips with different sizes at 20 mA.
Figure 2. (a) I-V characteristics of an LED device with a mesa length of 650µm. (b) The forward voltage of 10 LED devices with different sizes as a function of 1/active area at 20 mA. (c) Reverse current of ten LED devices with different sizes as a function of the active area at 5 V. The stars represent the average values of 10 devices, and the dashed lines are the linear fit of the average values
In their investigation of the characteristics of the red LEDs, the KAUST team studied temperature dependences of output power and peak wavelength and compared the results of InGaN red LEDs and the conventional InGaP ones.
The characteristic temperature is defined as the temperature dependence of light output and is better for higher values. InGaN red LEDs showed 399 K, which is much larger than 303.8 K for InGaP LEDs. Also, the light peak wavelength shifts with temperature due to the temperature dependence of bandgap and so on. The smaller shift is better in practical usage. InGaN red LEDs showed a small redshift coefficient of 0.066 nm/K. The value is less than half of 0.142 nm/K for InGaP LEDs.
The team investigated the effects of size on electrical and optical properties of InGaN-based red light-emitting diodes (LEDs) by designing rectangular chips with different mesa lengths. They found that larger chips exhibited lower forward voltages because of their lower series resistances. A larger chip helped to realise a longer emission wavelength, narrower full-width at half maximum, and higher external quantum efficiency.
However, temperature-dependent electroluminescence measurements indicated that larger chips are detrimental to applications where high temperature tolerance is required. In contrast, a smaller red LED chip achieved a high characteristic temperature of 399 K and a small redshift tendency of 0.066 nm/K, thus showing potential for temperature tolerant lighting applications.