Reduction of dislocation density in AlGaN with high AlN molar fraction by using a rugged AlN epilayer
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E3.11.1
Reduction of dislocation density in AlGaN with high AlN molar fraction by using a rugged AlN epilayer Akira ISHIGA1, Takashi ONISHI1, Yuhuai LIU2, Masaya HARAGUCHI3, Noriyuki KUWANO4, Tomohiko SHIBATA5, Mitsuhiro TANAKA5, Hideto MIYAKE1 and Kazumasa HIRAMATSU1 1 Faculty of Eng., Mie University 1515 Kamihama, Tsu, Mie, 514-8507, Japan 2 Satellite Venture Business Lab., Mie University 1515 Kamihama, Tsu, Mie, 514-8507, Japan 3 Interdisciplinary Graduate School of Engineering Sciences, Kyusyu University 6-1 Kasuga-kouen, Kasuga 816-8580, Japan 4 Art, Science and Technology Center for Cooperative Research, Kyusyu University 6-1 Kasuga-kouen, Kasuga 816-8580, Japan 5 NGK Insulators, Ltd. 2-56 Suda-cho, Mizuho-ku, Nagoya, Aichi, 467-8507, Japan ABSTRACT A method for making AlGaN with a high AlN molar fraction and low dislocation density is needed for fabricating deep ultraviolet emitters and detectors. In this study, we reduced the dislocation density in AlGaN over a large surface area by using low-pressure MOVPE on a continuously rugged epitaxial AlN substrate. The AlN molar fraction of the AlxGa1-xN was x = 0.51, and atomic steps in the surface were clearly observed with an atomic force microscope (AFM). In addition, the dislocation density was estimated to be 8.8 × 107 cm-2, which is two orders of magnitude lower than that of AlGaN grown on a flat AlN epitaxial layer. Our results indicate that the dislocation density of AlGaN can be greatly reduced by using a rugged AlN epitaxial substrate with continuously inclined facet. INTRODUCTION AlGaN has a band gap ranging from 3.4 to 6.2 eV, a small refractive index, and a high thermal stability; thus, it is a good candidate as a cladding layer for short wavelength optoelectronic devices, such as the intensely researched ultraviolet light emitting diodes (UV-LED) and ultraviolet laser diodes (UV-LD). AlxGa1-xN with a high AlN molar fraction (x > 0.5) and a low dislocation density is especially needed to improve the luminescence efficiency for deep UV-LEDs with a wavelength of 300 nm or less. 1,2) However, it is extremely difficult to grow a thick AlGaN layer with a high AlN molar fraction on GaN because the AlGaN cracks under the large tensile stress created by the smaller in-plane lattice constant of AlGaN than that of GaN. 3,4)
E3.11.2
Previously, we solved this problem by growing AlxGa1-xN with a wide range of AlN molar fractions 0.2 < x < 0.8 on an AlN template. 5,6) However, the threading dislocatin (TD) density in the AlGaN with high AlN molar fraction was been between 109 and 1010 cm-2. 7) As this dislocation density was too high, the goal in this research was to reduce the dislocation density. In the case of GaN, low dislocation densities have been achieved using, epitaxial lateral overgrowth (ELO). However, in the case of AlGaN, polycrystals tend to deposit on the mask, 10) and this makes it difficult to use ELO with AlGaN. Therefore, we used a maskless lateral growth technique that resulted in inclines on the AlN surface. TDs were bent during the lateral
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