High Current Injection to a UV-LED grown on a Bulk AlN Substrate
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High Current Injection to a UV-LED grown on a Bulk AlN Substrate Toshio Nishida, Tomoyuki Ban,1 Hisao Saito2, and Toshiki Makimoto NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa, 243-0198, Japan 1 NTT Electronics Atsugi, Kanagawa, 243-0198, Japan 2 NEL Technosupport Atsugi, Kanagawa, 243-0198, Japan
ABSTRACT We applied a bulk AlN substrate to an AlGaN-based ultraviolet light emitting diode (UV-LED) and found that this combination enables high injection current, which shows the LED’s potential for large ultraviolet flux extraction. Heat dissipation is an important issue for LEDs. Bulk AlN substrate has high thermal conductivity, a wurtzite crystal symmetry the same as that of nitride emitters, and transparency in the ultraviolet wavelength range. An UV-LED grown on a bulk AlN substrate shows output power linearity up to high injection current up to 300 mA, whereas a similar device grown on an AlN-template formed on a sapphire substrate only shows linearity up to an injection current of about 150 mA. It also showed very stable emission peak wavelength. For example, the emission peak shift is less than 2 nm in spite of the large injection current of 200 mA. Both findings are attributed to the heat dissipation afforded by the high thermal conductivity of the bulk AlN. This LED still suffers from internal absorption loss caused by the residual color centers in the AlN at present. However, further improvement of bulk AlN substrates will lead to high flux and highly efficient ultraviolet sources.
INTRODUCTION Nitride semiconductors diodes with Al content, such as AlGaN, can provide short band-gap wavelength emission of 200 - 360 nm in the ultraviolet (UV) range. Light sources of such UV range have applications in solid-state lighting utilizing visible phosphors, in spectro-chemical analysis, and in photo-catalytic chemical reaction. An important issue is large
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UV flux extraction, and, therefore, improvement in the dissipation of the heat accompanying the current injection is necessary. AlN has high thermal conductivity of 2.85 W/cm K. This is much higher than that of sapphire (0.23 W/cm K), and even higher than that of GaN (2.1 W/cm K). Furthermore, it has advantages in crystal growth and light extraction efficiency because it has the same crystarographic symmetry as nitride semiconductor devices and is transparent in a wide UV spectral range from 200 to 360 nm. We have already demonstrated highly efficient radiative recombination at the emission wavelength of 353 nm in ultraviolet light emitting diodes (UV-LEDs) having an AlGaN active layer [1,2] grown on a high-quality bulk GaN substrate [3]. The external quantum efficiency was about 1%. The light extraction efficiency of this device was very low because GaN substrate acts as an optically absorptive material at wavelengths shorter than 363 nm, which is the bandgap wavelength of GaN. We have improved the light extraction [4] by introducing a high-quality AlN-template layer formed on a sapphire substrate [5]. This substrate enabled u
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