Nonpolar/Semipolar GaN Technology for Violet, Blue, and Green Laser Diodes
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GaN Technology for Violet, Blue, and Green Laser Diodes
Hiroaki Ohta and Kuniyoshi Okamoto Abstract To achieve 520–532 nm green laser diodes (LDs), nonpolar and semipolar nitrides have attracted much attention because their usage leads to the elimination of the quantum-confined Stark effect and higher optical gains in this wavelength region. Since the breakthrough in the homoepitaxial growth technology for them, many nonpolar m-plane devices such as mW-class blue light-emitting diodes, violet 405 nm LDs, blue 460 nm LDs, and blue-green LDs beyond 490 nm have been announced. Advantages such as small blueshift and high slope efficiency (high output power to injected current ratio) have been confirmed for the first time in m-plane LDs beyond the blue region. On the other hand, the semipolar plane is also a candidate for green LDs. The pulsed –– – – operation of semipolar (101 1 ) and (112 2) violet LDs and lasing for a (112 2) LD at 514 nm by optical pumping also have been reported. Such rapid progress in this research field will be reviewed.
Introduction Polar c-Plane Technology Since the development of Group III nitride (AlxGayIn1–x–yN)-based laser diodes (LDs) by Nakamura et al.,1,2 violet (405 nm) LDs have been in commercial mass production for Blu-ray discs. This LD has a stack structure, a laminate structure consisting of layers with various compositions and levels of doping, grown along the c-axis of the wurtzite crystal, where strong spontaneous and strain-induced piezoelectric polarization exist.3,4 This polarization (the internal electric field) causes carrier separation in the quantum wells (QWs) (quantum-confined Stark effect [QCSE])5,6 that leads to the low recombination probability. In particular, QCSE becomes more dominant in In-rich InGaN QWs beyond the blue region. This is considered to be one of the causes of gain suppression that has been experimentally observed in 470 nm c-plane InGaN-based LD.7 At present, the maximum lasing wavelength in c-plane InGaN-based LDs under pulsed operation at room temperature is 498 nm (blue-green).8 324
Nonpolar and Semipolar Nitrides One possible way to eliminate QCSE from occurring beyond the blue region and to achieve pure green LDs is –to make – use of nonpolar [(112–0) and (1010)] and –– –– semipolar planes [(1122), (101 1), (101 3)].9 Recently, the benefit of the usage of nonpolar and semipolar orientations—higher optical gain than c-plane LDs—has been theoretically expected, as will be mentioned later. However, regardless of the theoretical prediction, it took many years to achieve nonpolar or semipolar devices because epitaxial growth of certain orientations on the foreign substrates (e.g., r-plane sapphire, m-plane SiC, or γ-LiAlO2) was extremely difficult. The nitride films grown on such substrates had an extremely high density of threading dislocations (TDs) and stacking faults (SFs).10 The electroluminescence (EL) from m-plane light-emitting diodes (LEDs) on m-SiC was quite dim.11 To improve device performance, the focus has turned to two techniques to
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