Toshio Nishida, Hisao Saito, and N. Kobayashi
Physical Science Laboratory
Ultraviolet (UV) light is chemically active and has precise spatial resolution. Therefore, a compact and efficient UV light source will provide a variety of applications in the fields of chemistry, biology, environmental science, and optics. However, present UV light sources are power consumptive and quite large in size. Semiconductor UV light sources, such as light emitting diodes (LEDs), represent a solution to this problem. Nitrides containing aluminium (Al) have a band gap in the wavelength range between 200 nm and 360 nm. Although the potential of nitrides as the UV light source material has been theoretically discussed, practical verification had not been possible due to the difficulty of nitride crystal growth. By employing a sophisticated metalorganic vapor phase epitaxy (MOVPE) technique, we have demonstrated the operation of nitride UV-LED at a wavelength shorter than 350 nm, for the first time.
Atomically flat epitaxial growth and an understanding of polarization field in nitride hetero-structures are indispensable to achieving UV-LED operation. Figure 1 shows an atomic force microscope (AFM) image of a GaN surface prepared by our MOVPE. The surface consists of a regular step array of GaN with unit-cell height, showing atomically flat growth [1]. By utilizing this flat growth mode, we have grown GaN quantum wells that are only 4 and 8 molecular layers thick, and confirmed that extraordinary polarization field as strong as 700 kV/cm [2]. This polarization field makes emission wavelength longer and emission efficiency lower [2, 3]. To suppress the polarization field, we have designed and fabricated an AlGaN-based multi-quantum-well LED. Figure 2 shows its emission spectra under current injection, indicating successful band edge emission at the wavelength of 346 nm [3].
[1] T. Nishida et al., Jpn. J. Appl. Phys. 37 (1998) L459.
[2] T. Nishida et al., Int. Conf. on Blue Laser and Light Emitting Diode 1998.
[3] T. Nishida and N. Kobayashi, Phys. Stat. Sol. A 176 (1999) 45.
Fig. 1. AFM image of GaN surface.
Fig. 2. Emission spectra from AlGaN-UV-LED.
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