Room Temperature CW Operation of GaN-Based Blue Laser Diodes by GaInN/GaN Optical Guiding Layers
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ABSTRACT GaN-based short wavelength laser diodes are the most promising key device for a digital versatile disk. We have been improving the important points of the laser diodes in terms of optical guiding layers, mirror facets. The continuous wave laser irradiation at room temperature could be achieved successfully by reducing the threshold current to 60 mA (4 kA/cm2). We have tried to apply the multi low temperature buffer layers to the laser diodes for the first time to reduce the crystal defects.
INTRODUCTION Recently, III-V nitride semiconductor lasers have been improved to the grade of a digital versatile disk (DVD) application. The multi-quantum-well (MQW) separated confinement hetero-structure (SCH) laser diodes have been realized to more than 10000 hours lifetime under continuous wave (CW) irradiation at room temperature (RT) [1]. The GaInN/GaN MQW laser diodes are the key devices for the realization of an advanced DVD system, therefore many groups have been developing intensively. Akasaki, et al. reported for the first time all over the world the stimulated emission from AlGaN/GaN/GaInN quantum well device by current injection at RT in 1995 [2]. After that, further progress has been achieved. The problem of high dislocation density was resolved by epitaxial lateral over-growth (ELO) GaN [3,4] or pendeo-epitaxial growth [5]. In this way, the several developments were necessary for the improvements of laser diodes such as reduction of the threshold current and extension of lifetime under CW irradiation. In this paper, we report the improvements of characteristics of the laser diodes by applying the reduction of internal loss at cavity and the multi low temperature (LT) buffer layers, the high power operation and a RT CW irradiation of the MQW-SCH laser diodes.
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EXPERIMENTS One of the schematic structures of GaInN/GaN MQW-SCH lasers is shown in figure 1. A LT buffer layer is grown on a-plane sapphire substrate. An n-GaN layer, an nAl0.07Ga0.93N cladding layer, an n-GaInN/n-GaN optical guiding layer, a GaInN/GaN MQW active layer, a p-GaInN/p-GaN optical guiding layer, a p-Al0.07Ga0.93N cladding layer, and a p-GaN contact layer are followed. The MQW consists of 4 GaInN wells (1 to 4 nm) and GaN barriers (5 to 10 nm). The conventional optical guiding layers consist of GaN. However, we applied GaInN/GaN optical guiding layers, because an optical confinement factor of GaInN optical guiding layer is superior to that of GaN. Those GaN epitaxial layers are grown by metalorganic vapor phase epitaxy (MOVPE). The source gases of Al, Ga, In, and N are trimethylaluminum (TMA), trimethylgallium (TMG), trimethylindium (TMI), and ammonia (NH3), respectively. The dopant source gases of Si and Mg are silane (SiH4) and biscyclopentadienylmagnesium (bis-Cp2Mg). The growth conditions are the same as the previous report [6,7]. Mg-doped p-AlGaN and p-GaN layers are activated by low energy electron beam irradiation after the growth. Dislocation density is evaluated by the method of KOH etch pit [8]. The laser diode is mes
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