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Authors

Mizuki Yamanaka, Kenichi Eriguchi, Keitaro Ikejiri, Hiroki Tokunaga

Abstract

Metal-organic chemical vapor deposition (MOCVD) is the most widely method to manufacture GaN power devices and GaN-based LEDs. As the demand for GaN devices increases in the future, high-throughput, low mass production cost equipment will be required.

Epi-wafer manufacturing usually requires periodic cleaning of the internal parts of the reactor. One common method is in-situ cleaning, which chlorine is introduced into the MOCVD reactor and heated for cleaning. The other method, which is Taiyo Nippon Sanso is working on the development, is ex-situ cleaning. In ex-situ cleaning, reactor parts are unloaded from the MOCVD reactor and cleaned in separate equipment. Compared to the in-situ cleaning, we believe that the ex-situ cleaning is suitable for MOCVD mass production performance because the cleaning and the epi growth of the MOCVD can run in parallel. In contrast, conventional ex-situ cleaning has the disadvantages of having to remove the parts from the reactor, which requires manual labor, and exposing the cleaned materials to the air environment, which cannot be ignored of a negative effect on the crystal growth. To eliminate the effects of air exposure, baking after cleaning was thought to be necessary, which was an obstacle to improving throughput.

To realize ex-situ cleaning overcoming these issues, we developed the UR26K-CCD, a new generation mass-production MOCVD. By installing a cassette-to-cassette wafer transport system and a reactor parts transport robot, cleaning and wafer load/unload can be performed without manual handling. This has resulted in a high-throughput, mass-production MOCVD system that reduces operating costs and allows most of the operating time to be used for epitaxial growth. In addition, by integrating a dry-cleaning equipment, the bad effects of air exposure after cleaning can be eliminated. We confirmed this benefit by HEMT growth(1).

Regarding performance for light emitting devices, we demonstrated GaN-based LEDs with wider range of emission wavelength by controlling  In composition of InGaN layer. The UR26K-CCD reactor has a narrow flow width and is suitable for growing InGaN with high In composition(2).

In this presentation, we will demonstrate the performance of HEMTs and LEDs growth by UR26K-CCD with integrating a dry-cleaning equipment and present the productivity and stability of mass production equipment.

(1) Yamanaka et al. (2023). ICNS14 Fukuoka ThP-GR-15.

(2) Ohkawa et al. (2019).  J. Cryst. Growth 512 69–73.

Authors

Shashwat Rathkanthiwar¹, Maki Kushimoto², Hiroshi Amano^2,3, Yudai Shimizu⁴, Kazutada Ikenaga⁴, Mayank Bulsara⁴, Keitaro Ikejiri⁴, Leo J Schowalter^1,3,5

¹ Lit Thinking, Orlando, Florida, USA. ² Graduate School of Engineering, Nagoya University, Nagoya, Japan. ³ Center for Integrated Research of Future Electronics, Institute of Materials Research and System for Sustainability, Nagoya University, Nagoya, Japan. ⁴ Taiyo Nippon Sanso, Innovation Unit, Yokohama, Kanagawa, Japan. ⁵ Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida, USA

Abstract

The unique capability of far-UVC (λ<240 nm) radiation to inactivate infectious agents in indoor settings without endangering the skin and eyes of occupants has accelerated the advancement of far-UVC light sources. Given the continuing progress of achieving shorter wavelengths in LEDs based on nitride semiconductors, it is attractive to think that the well-known advantages of visible, and near UV LEDs (smaller size, lower weight, reduced operating power, optimized wavelength, and longer lifetime) could be duplicated in the far-UVC wavelength regime which would greatly advance this field. Wavelengths as short as 210 nm have been demonstrated and commercial AlGaN LEDs with wavelengths below 240 nm are currently available. However, the efficiency and lifetime of these devices at practical current densities appear to drop dramatically as the Al content is increased. A crucial observation made by several groups is that the apparent concentration of point defects increases during the epitaxial growth of high-Al content AlGaN films and it has been suggested that these defects may be playing a major role in the degradation of short wavelength UVC LEDs.

In this work, we report on the epitaxy development of far-UVC LED structures grown on single-crystal AlN substrates using a low-pressure, resistively heated, horizontal flow MOCVD reactor and intend to compare these results with other epitaxial growth techniques in an attempt to gain a greater understanding of defect incorporation. A step-flow morphology was achieved throughout the heterostructure growth. Reciprocal space mapping revealed that the entire LED structure was fully strained to the substrate indicating pseudomorphic growth. While pseudomorphic, step-flow growth has been demonstrated in vertical MOCVD designs, it was important to demonstrate that horizontal flow reactors were also capable of such high-quality growth.  The Si-doped Al0.75Ga0.25N n-contact layer exhibited a sheet resistance of 390 Ω/□. Next, the optical characteristics of stand-alone, unintentionally doped AlGaN layers, multi-quantum wells, and full LED structures were characterized using temperature- and power-dependent photoluminescence. The peak emission wavelength varied less than 3 nm with temperature (300 to 4K) and about 1 nm with excitation density ranging from 10 to 5000 µJ/cm2. Finally, the study systematically investigated how growth conditions affect the IQE of far-UVC LED structures operating below 240 nm.