InGaN/GaN/AlGaN-Based Leds and Laser Diodes
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ABSTRACT InGaN quantum-well-structure blue LEDs were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The output power of both LEDs was as high as 6 mW at a current of 20 mA. The LED on sapphire had a considerable amount of leakage current in comparison with that on ELOG. These results indicate that In composition fluctuation is not caused by threading dislocations (TDs), free carriers are captured by radiative recombination centers before they are captured by nonradiative recombination centers in InGaN, and that the dislocations form the leakage current pathway in InGaN. Red LED with an emission peak wavelength of 650 nm was fabricated by increasing the In composition and thickness of InGaN well layer. When the laser diodes (LD) was formed on the GaN layer above the SiO2 mask region, the threshold current density was as low as 3 kAcm-2. When the LD was formed on the window region, the threshold current density was as high as 6 to 9 kAcm-2. There is a possibility that a leakage current due to a large number of TDs caused the high threshold current density on the window region. InGaN multi-quantum-well (MQW) structure LDs grown on the ELOG substrate showed an output power as high as 420 mW under RT-CW operation. The longest lifetime of 9,800 hours at a constant output power of 2 mW was achieved. The InGaN MQW LDs were fabricated on a GaN substrate. The fundamental transverse mode was observed up to an output power of 80 mW. INTRODUCTION All of III-V nitride-based light-emitting diodes (LEDs) [1-3] and laser diodes (LDs) [4,5] use an InGaN active layer instead of a GaN active layer because of the difficulty in fabricating highly efficient light-emitting devices using a GaN active layer. Highly efficient UV/blue/green/amber InGaN quantum-well structure LEDs have been fabricated directly on a sapphire substrate in spite of a high dislocation density of 1-10 x 1010 cm-2 originating from a large lattice mismatch between GaN and the sapphire substrate [1-3]. Due to the high efficiency of the LEDs grown on the sapphire substrates, the dislocations in InGaN do not appear to work as a nonradiative recombination center [6]. Epitaxially laterally overgrown GaN (ELOG) on sapphire was developed recently to reduce the number of threading dislocations in the GaN epitaxial layers [7,8]. Using the ELOG, the number of threading dislocations was reduced
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significantly in the GaN grown on the SiO2 mask. Thus, there is a great interest in fabricating LEDs and LDs using the ELOG substrate which has a small number of dislocations in order to study the role of the dislocations in the InGaN-based LEDs and LDs. For the study, blue InGaN single-quantum-well (SQW) structure LEDs were fabricated on the ELOG and sapphire substrates [9]. The LDs were formed on the SiO2 mask and windo
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