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Latest development of InGaN and Short-Wavelength LD/LED/VCSEL 屠嫚琳 Man-lin Tu.

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Presentation on theme: "Latest development of InGaN and Short-Wavelength LD/LED/VCSEL 屠嫚琳 Man-lin Tu."— Presentation transcript:

1 Latest development of InGaN and Short-Wavelength LD/LED/VCSEL 屠嫚琳 Man-lin Tu

2 2003/05/16 國立彰化師範大學 - 屠嫚琳 2 Introduction Using shorter wavelength blue lasers would decrease the spot size on the disk, creating a four- fold increase in data storage capacity on conventional disks. Much research has been done on high-brightness blue LEDs and LDs for use in full-color displays and full-color indicators with high efficiency, high reliability and high speed.

3 2003/05/16 國立彰化師範大學 - 屠嫚琳 3 Applications Large scale displays (large outdoor television screens) Smaller full-color flat panel display screens (inside trains or subway stations) Full-color scanners Full-color photocopying machines Full-color FAX machines Traffic lights LED white lamps Read/write laser light sources of digital versatile disks (DVD)

4 2003/05/16 國立彰化師範大學 - 屠嫚琳 4 Improved performance of 325-nm emission AlGaN ultraviolet LEDs A. Chitnis et al. report A pulsed-atomic-layer-epitaxy (PALE) growth process was used to improve the material quality of the AlN buffer and the AlN/AlGaN strain-relief layers for reducing the nonradiative recombination. They introduced a 150-Å-thick Mg-doped p-Al 0.42 Ga 0.58 N electron-block layer in order to improve the electron confinement in the active region. It is obtained 40% improvement of external quantum efficiency, and high optical powers of 10.2 mW at a pulsed pump current of 1 A.

5 2003/05/16 國立彰化師範大學 - 屠嫚琳 5 This band arises due to band-to- deep-level recombination in p- AlGaN layers of the structure. They attribute the reduction of this long wavelength emission to a better carrier confinement provided by the p-AlGaN electron-block layer and better structural quality.

6 2003/05/16 國立彰化師範大學 - 屠嫚琳 6 Nitride-Based Cascade Near White Light-Emitting Diodes An InGaN-GaN blue LED structure and an InGaN-GaN green LED structure were grown sequentially onto the same sapphire substrate so as to achieve a nitride-based near white LED. By changing the indium composition in the InGaN well layers, it can achieve different LED output colors range from ultraviolet to red. In order to determine the indium composition in the sample, they grew blue and green MQW separately. In order to avoid thyristor effect, they choose a large 2.1×2.1 mm 2 LED chip size, which was six times larger than that of the normal LED. (next page)

7 2003/05/16 國立彰化師範大學 - 屠嫚琳 7 This specially designed finger cross contact pattern is to minimize current crowding effect due to the large chip size. A novel nitride-based near white LED, which consists of a blue In 0.2 Ga 0.8 N-GaN MQW structure and a green In 0.49 Ga 0.51 N-GaN

8 2003/05/16 國立彰化師範大學 - 屠嫚琳 8 Comparison of ultraviolet LEDs with peak emission at 340 nm grown on GaN substrate and sapphire The UV LED grown on GaN substrate shows about 65% reduction in differential resistance and more than one order of magnitude increase in the output power compared with the LED grown on sapphire. The thermal conductivity of GaN is about five times higher than that of sapphire. A. Yasan et al. demonstrated p-type Al x Ga 1-x N/ GaN superlattices with very high carrier concentrations and low resistivities overcome effective p-type doping, and adding a small amount of indium into ternary AlGaN layers can reduce the defect density.

9 2003/05/16 國立彰化師範大學 - 屠嫚琳 9

10 2003/05/16 國立彰化師範大學 - 屠嫚琳 10 High-Efficiency InGaN-GaN MQW green LEDs with CART and DBR structures DBR and charge asymmetric resonance tunneling (CART) strctures were applied to nitride-based green LEDs to enhance their output efficiency. Without DBR, the output power and external quantum efficiency of the CART LED are about 4 mW and 6.25%, respectively. Add DBR, the output power and external quantum efficiency of the CART LED with DBR structure measured can reach 7.2 mW and 11.25%, respectively.

11 2003/05/16 國立彰化師範大學 - 屠嫚琳 11 CART structure can significantly increase the electron capture rate of the MQW active region through electron tunneling and increase the LED output intensity.

12 2003/05/16 國立彰化師範大學 - 屠嫚琳 12 The other method to increase the output intensity of green LEDs is to include a distributed Bragg reflector (DBR) structure in the backside of LED wafer. They apply SiO 2 -TiO 2 DBR that is formed from a repeated periodic stack of alternating high and low index quarter- wavelength layers.

13 2003/05/16 國立彰化師範大學 - 屠嫚琳 13 Characteristic of InGaN laser diodes in the pure blue region The InGaN multi-quantum well (MQW) LD structure was grown on the GaN structure with ELOG by MOCVD. The threshold current density and voltage of these LDs with an emission wavelength of 460 nm were 3.3 KA/cm 2 and 4.6 V. The estimated lifetime was approximately 3000 h under 50 ℃ continuous-wave operation at an output power of 5 mW.

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