InGaN/GaN/AiGaN-Based Laser Diodes with an Estimated Lifetime of Longer than 10,000 Hours
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ABSTRACT InGaN multi-quantum-well (MQW) structure laser diodes with A10. 14 Ga 0.86 N/GaN modulation doped strained-layer superlattice cladding layers grown on an epitaxially laterally overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10,000 hours under continuous-wave operation at 20"C. Under operation at a high temperature of 50"C, the lifetime was longer than 1,000 hours. The activation energy of the lifetime was estimated to be 0.5 eV. With the operating current increasing above the threshold, a self-pulsation with a high frequency of 3.5 GHz was observed. The relative intensity noise (RIN) less than -145 dB/Hz was obtained even at the 6% optical feedback using the high-frequency modulation of 600 MHz. The3 19 threshold carrier density of the InGaN MQW-structure LDs was estimated to be 3 x 10 /cm using a carrier lifetime of 1.8 ns. INTRODUCTION GaN and related materials, such as AIGaInN are III-V nitride semiconductors with the wurtzite crystal structure and a direct energy band structure which is suitable for light emitting devices. At present, the main focus of 111-V nitride semiconductor research is the realization of a current-injected laser diode which is expected to be the shortest-wavelength semiconductor laser diode (LD) ever produced. Short-wavelength-emitting devices, such as blue LDs, are currently required for a number of applications, including full-color electroluminescent displays, laser printers, read-write laser sources for high-density information storage on magnetic and optical media, and sources for undersea optical communications. Major developments in wide-gap III-V nitride semiconductors have recently led to the commercial production of high-brightness blue/green light-emitting diodes (LEDs) [11 and to the demonstration of room-temperature (RT) violet laser light emission in InGaN/GaN/AlGaN-based heterostructures under pulsed and continuous-wave (CW) operations [1-6]. The lifetimes of the InGaN multi-quantum-well (MQW)structure LDs have improved to 300-1,150 hours under conditions of RT-CW operation [7,8]. However, further improvements of the LD characteristics, including lifetime, are required to enable the commercialization of short-wavelength LDs. At present, one of the problems is that it is difficult to grow the thick AlGaN cladding layer required for optical confinement, due to the formation of cracks during growth. These cracks are due to the stress introduced in the AlGaN cladding layers caused by lattice mismatch, and the difference in thermal expansion coefficients between the AlGaN cladding layer and GaN layers. In the case of a thin AlGaN layer, the elastic strain is not relieved by the formation of cracks and dislocations, thus the crystal quality of the AlGaN cladding layer improves [9]. The author previously reported the use of the AlGaN/GaN strained-layer superlattices (SLSs) for III-V nitride-based LDs to prevent cracking of the AlGaN during growth [8]. Here, the InGaN MQW-structure LDs, which have GaN/AlGaN modulationdoped strained-laye
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