Growth of Crack-Free thick AlGaN Layer and its Application to GaN-Based Laser Diode
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Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan 2 High-Tech Research center, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan
ABSTRACT In the field of group-III nitrides, hetero-epitaxial growth has been one of the most important key technologies. A thick layer of AlGaN alloy with higher AlN molar fraction is difficult to grow on sapphire substrate, because the alloy layer is easily cracked. It is thought that one cause of generating cracks is a large lattice mismatch between an AlGaN and a GaN, when AlGaN is grown on the underlying GaN layer. We have achieved crack-free Al0.07Ga0.93N layer with the thickness of more than 1µm using underlying Al0.05Ga0.95N layer. The underlying Al0.05Ga0.95N layer was grown directly on sapphire by using the lowtemperature-deposited buffer layer (LT-buffer layer). Since a lattice mismatch between the underlying Al0.05Ga0.95N layer and upper Al0.07Ga0.93N layer is relatively small, the generation of cracks is thought to be suppressed. This technology is applied to a GaN-based laser diode structure, in which thick nAl0.07Ga0.93N cladding layer grown on the Al0.05Ga0.95N layer, improves optical confinement and single-robe far field pattern in vertical direction.
INTRODUCTION GaN-based semiconductor lasers are promising for light sources of highdensity optical data storage systems, because of their short wavelength of around 400nm. However, we have worked with much effort for a long time in order to obtain a high quality crystal, because of the lack of substrates lattice-matched to this material system. In 1986, on the growth method using a LT-buffer layer[1] on sapphire substrate, which has a large lattice-mismatch of about 16%, GaN film with an excellent flatness and crystallinity was successfully obtained. Further, p-type low resistance crystal was realized by Mg-doping and electronbeam irradiation [2], or anneal process [3]. After such progress, high brightness blue and green light emitting diodes (LEDs) have been realized [4],[5]. More recently, several groups have achieved continuous-wave (CW) operation of violet-blue laser diodes(LD)[6]-[9]. It is thought that the GaN-based LD for the practical use will be commercially available in the near future.
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However, the further improvements concerning with a crystalline quality and device performance are required against the practical use. In general, high quality GaN layer is grown on LT-buffer layer / sapphire substrate. When the AlGaN layers, commonly used as cladding layers in LDs, are grown on the GaN layer, the crack is often observed in the surface. This is thought to be due to the difference of lattice constants between GaN and AlGaN. Therefore, the thickness and AlN molar fractio
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