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DEFECT AND STRESS CONTROL OF ALGAN AND FABRICATION OF HIGH-EFFICIENCY UV-LED H. AMANO, M. IWAYA, S. NITTA, S. TERAO, R. NAKAMURA, T. UKAI, S. SAITOH, S. KAMIYAMA, C. WETZEL AND I. AKASAKI High-Tech Research Center, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468-8502, Japan

ABSTRACT Defects and stress are the most serious issues for growth of AlGaN. Low-temperature deposited (LT -) AlN interlayer between AlGaN and GaN is found to reduce tensile stress during growth, and at the same time suppress the propagation of dislocations having screw components, by which UV-photodetector showing very-low-dark current has been successfully fabricated. However, additional pure-edge dislocations are generated at the LT -interlayer, which resulted in the poor emission property. In addition to the LT -interlayer, lateral growth at the trenched structure was used, thereby achieving crack-free AlGaN and reduction of the density of all types of dislocations in the AlGaN layer. UV light emitting diodes having AlGaN/GaN multi-quantum well active layer was fabricated on the low dislocation density AlGaN. The LED shows strong and sharp UV-emission from GaN-wells.

INTRODUCTION The establishment of a high-yield growth technology utilizing a low-temperature-deposited (LT-) buffer layer on a sapphire substrate by metalorganic vapor phase epitaxy and the realization of conductivity control by doping with Si in case of n-type, and Mg followed by a dehydrogenation treatment in case of p-type resulted in the vast expansion of nitride research worldwide.1-4 Products and sales of nitride-based visible-light emitting diodes (LEDs) and violet laser diodes show an extremely high-growth rate in the market. It is expected that they will soon occupy the major portion in the optoelectronics industry. Detectors and emitters in the vacuum ultraviolet (VUV)/ultraviolet (UV) region are one of the next targets for nitrides, which will have significant impact on the market as visible-LEDs. In order to establish such VUV/UV optoelectronics, high-quality and well-controlled AlGaN is essential. The crystalline quality of AlGaN on a sapphire substrate covered with an LT -AlN buffer layer is far superior to AlGaN directly grown on a sapphire substrate.5 However, it degrades progressively with increasing AlN molar fraction in the AlGaN. Although the quality was significantly improved when an AlGaN was grown on a high-quality GaN,6 at the same time a crack network originating from the lattice mismatch between AlGaN and GaN was generated with a high density if the thickness of AlGaN exceeded a critical value. In-situ stress monitoring revealed that the stiffness of AlGaN is modulated by Si doping or Mg doping; in other words, tensile stress during growth is increased while the strain keeps constant by Si-doping or Mg-doping.7

EFFECT OF LOW TEMPERATURE DEOPSITED ALN INTERLAYER

Al xGa1-xN

Low-temperature deposited interlayer

GaN

Sapphire (b) 60µm

(c)

(a) Fig. 1 (a) Schematic structure of AlGaN growth using LT interlayer. (b) SEM photograph of 1 µm