Characteristics of AlGaN Quantum Well Light Emitting Diodes under Large Current Operation
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AlGaN-based ultraviolet light emitting diodes (LED) are characterized. The devices consist of an Alo.00Gao.94N (2 nm) / Alo.12Gao.88N single quantum well active layer sandwiched by Alo.3Gao.7N carrier blocking layers and cladding layers of an Alo.i6Gao.84N (1.5 nm)/Al0.2Gao.8N (1.5 nm) short-period alloy superlattice grown on bulk GaN substrate. Efficient conversion to UV light is confirmed, which indicates a high-quality active layer, effective suppression of carrier overflow, and transparency of conductive cladding layers. Radiative recombination dominantly occurs in the quantum well at injection current density lower than 0.25 kA/cm2. Further dense injection leads to enhancement of the radiative recombination in the barrier layer. This clarifies the importance of the design of the active layer for highpower application. 1Aluminium gallium nitride (AlGaN) alloy is optically direct band gap material and it covers a wide wavelength range of 200-360 nm within ultraviolet (UV) range. Therefore, AlGaN-based light emitting diodes (LEDs) are expected to be the next-generation of ultraviolet (UV) light sources. Such a semiconductor UV light source will replace fluorescent bulbs for lighting, electroluminescence panels and cathode tubes for displays, and gas tubes for spectroscopic excitation applications, such as used in medicine.We report enhanced light output of GaN-based light-emitting diodes (LEDs) with vertically aligned ZnO nanorod arrays. The ZnO nanorod arrays were prepared on the top layer of GaN LEDs using catalyst-free metalorganic vapor phase epitaxy. Compared to conventional GaN LEDs, light output of GaN LEDs with the ZnO nanorod arrays increased up to 50% and 100% at applied currents of 20 and 50mA, respectively. The source of the enhanced light output is also discussed.
Nanorod
Wide-bandgap semiconductor
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High-efficient light emitting diodes (LEDs) emitting red, amber, green, blue and ultraviolet light have been obtained through the use of an InGaN active layers. The localized energy states caused by In composition fluctuation in the InGaN active layer seem to be related to the high efficiency of the InGaN-based emitting devices in spite of having a large number of threading dislocations (TDs). InGaN single-quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The characteristics of both LEDs was almost same. These results indicate that the dislocation doesn't affect the efficiency practically. Recently, the development of high-power light source using GaN-based LEDs has become active. In such high-power LEDs, the density of forward current is much higher than that of past LEDs. Therefore, an advantage of carrier localization in InGaN active layer becomes small, because of band filling under high injection level. This means that reducing the density of TDs becomes important, just like GaN-based laser diodes. Also, we show recent results of GaN-based LEDs.
Wide-bandgap semiconductor
Active layer
Quantum Efficiency
Indium gallium nitride
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Highly efficient GaInN-GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) were successfully developed by the low-temperature AlN buffer layer method for metal-organic vapor phase epitaxy (MOVPE). The light-emitting layer of the GaInN-GaN MQW drastically enhances the performance of GaN-based LEDs in terms of the efficiency and spectrums. Flip-chip (FC) type MQW LEDs have been newly developed to increase efficiency in extracting light from the GaN-based crystal to the outside. The luminous intensities of FC type blue and green LEDs are typically 6 and 14 cd, respectively, at 20 mA. The output power of the FC-type LEDs was 14 mW at 20 mA, which was approximately two times higher than that of the conventional face-up type blue LEDs. The external quantum efficiency of blue FC-type LEDs was as high as 20% at 20 mA. New multicolor package was developed using these high performance nitride-based LEDs and commercial AlGaInP-based red LEDs, the color range of which is the largest among other flat panel display devices.
Quantum Efficiency
Luminous efficacy
Wide-bandgap semiconductor
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The use of III-nitride tunnel junction (TJ) diodes in light-emitting diodes (LEDs) and laser diodes (LDs) has attracted much attention. However, there are concerns about using III-nitride tunnel junctions, especially in commercial devices, primarily due to the excess voltage needed to allow for tunneling. On the other hand, ultra-violet (UV) LEDs suffer from low light extraction efficiency (LEE). The use of tunnel junctions in UV LEDs can potentially double the LEE by allowing the use of highly reflective mirrors and eliminating the need for absorption p-GaN.
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Wide-bandgap semiconductor
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A step-stage InGaN/GaN multiquantum-well (MQW) structure can enhance the efficiency of GaN-based light-emitting diodes (LEDs). Compared to dual-stage MQW LEDs, the step-stage MQW LEDs have lower forward voltage and higher light output. The measured light output power of step-stage LEDs operating at 350 mA shows an increase of approximately 23% with an external quantum efficiency (EQE) increase of 6.6%, when compared to dual-stage LEDs.
Quantum Efficiency
Wide-bandgap semiconductor
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Recently, we succeeded in producing for the first time, 1-cd-brightness blue InGaN/AlGaN light-emitting diodes (LEDs) suitable for commercial applications.1,5 In this report, co-doping with both Zn and Si into the InGaN active layer in InGaN/AlGaN LEDs in order to increase the output power of the InGaN/AlGaN LEDs is described.
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We demonstrate indium gallium nitride/gallium nitride/aluminum nitride (AlN/GaN/InGaN) multi-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) to improve light output power. Similar to conventional UV LEDs with AlGaN/InGaN MQWs, UV LEDs with AlN/GaN/InGaN MQWs have forward voltages (V(f)'s) ranging from 3.21 V to 3.29 V at 350 mA. Each emission peak wavelength of AlN/GaN/InGaN MQW UV LEDs presents 350 mA output power greater than that of the corresponding emission peak wavelength of AlGaN/InGaN MQW UV LEDs. The light output power at 350mA of AlN/GaN/InGaN MQWs UV LEDs with 375 nm emission wavelength can reach around 26.7% light output power enhancement in magnitude compared to the AlGaN/InGaN MQWs UV LEDs with same emission wavelength. But 350mA light output power of AlN/GaN/InGaN MQWs UV LEDs with emission wavelength of 395nm could only have light output power enhancement of 2.43% in magnitude compared with the same emission wavelength AlGaN/InGaN MQWs UV LEDs. Moreover, AlN/GaN/InGaN MQWs present better InGaN thickness uniformity, well/barrier interface quality and less large size pits than AlGaN/InGaN MQWs, causing AlN/GaN/InGaN MQW UV LEDs to have less reverse leakage currents at -20 V. Furthermore, AlN/GaN/InGaN MQW UV LEDs have the 2-kV human body mode (HBM) electrostatic discharge (ESD) pass yield of 85%, which is 15% more than the 2-kV HBM ESD pass yield of AlGaN/InGaN MQW UV LEDs of 70%.
Indium gallium nitride
Ultraviolet
Wide-bandgap semiconductor
Aluminium nitride
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Electrical characteristics of In0.05Ga0.95N/Al0.07Ga0.93N and In0.05Ga0.95N/GaN multiple quantum well (MQW) ultraviolet light-emitting diodes (UV-LEDs) at 400 nm wavelength are measured. It is found that for InGaN/AlGaN MQW LEDs, both ideality factor and parallel resistance are similar to those of InGaN/GaN MQW LEDs, while series resistance is two times larger. It is suggested that the Al0.07Ga0.93N barrier layer did not change crystal quality and electrical characteristic of p-n junction either, but brought larger series resistance. As a result, InGaN/AlGaN MQW LEDs suffer more serious thermal dissipation problem although they show higher light output efficiency.
Equivalent series resistance
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Active layer
Ultraviolet
Double heterostructure
Quantum Efficiency
Wide-bandgap semiconductor
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GaN-based blue light emitting diodes(LEDs) have undergone great development in recent years,but the improvement of green LEDs is still in progress.Currently,the external quantum efficiency(EQE) of GaN-based green LEDs is typically30%,which is much lower than that of top-level blue LEDs.The current challenge with regard to GaN-based green LEDs is to grow a high quality In GaN quantum well(QW) with low strain.Many techniques of improving efficiency are discussed,such as inserting Al GaN between the QW and the barrier,employing prestrained layers beneath the QW and growing semipolar QW.The recent progress of GaN-based green LEDs on Si substrate is also reported:high efficiency,high power green LEDs on Si substrate with 45.2% IQE at 35 A/cm2,and the relevant techniques are detailed.
Quantum Efficiency
Wide-bandgap semiconductor
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