We report on the fabrication and characterization of AlGaN-based deep ultraviolet light-emitting diodes (LEDs) with the emission wavelength ranging from 255 to 280 nm depending on the Al composition of the active region. The LEDs were flip-chip bonded and achieved external quantum efficiencies of over 3% for all investigated wavelengths. Under cw operation, an output power of more than 1 mW at 10 mA was demonstrated. A moth-eye structure was fabricated on the back side of the sapphire substrate, and on-wafer output power measurement indicated a 1.5-fold improvement of light extraction.

This paper reviews the progress of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs), mainly focusing in the work of the authors’ group. The background to the development of the current device structure on sapphire is described and the reason for using a (0001) sapphire with a miscut angle of 1.0° relative to the m-axis is clarified. Our LEDs incorporate uneven quantum wells (QWs) grown on an AlN template with dense macrosteps. Due to the low threading dislocation density of AlGaN and AlN templates of about 5 × 108/cm2, the number of nonradiative recombination centers is decreased. In addition, the uneven QW show high external quantum efficiency (EQE) and wall-plug efficiency, which are considered to be boosted by the increased internal quantum efficiency (IQE) by enhancing carrier localization adjacent to macrosteps. The achieved LED performance is considered to be sufficient for practical applications. The advantage of the uneven QW is discussed in terms of the EQE and IQE. A DUV-LED die with an output of over 100 mW at 280–300 nm is considered feasible by applying techniques including the encapsulation. In addition, the fundamental achievements of various groups are reviewed for the future improvements of AlGaN-based DUV-LEDs. Finally, the applications of DUV-LEDs are described from an industrial viewpoint. The demonstrations of W/cm2-class irradiation modules are shown for UV curing.

AlGaN-based LEDs (λ < 300 nm) fabricated on n-AlGaN templates with a threading dislocation density larger than 5 × 108/cm2, which were grown on (0001) sapphire with a 1.0° miscut relative to the m-plane, showed external quantum efficiencies (EQEs) of 3.5, 3.9, 6.1, and 6.0% at 266, 271, 283, and 298 nm, respectively. These EQE values reveal significantly high internal quantum efficiencies (IQEs). This performance was obtained using an uneven multiple quantum well (MQW) grown on the AlGaN template with macrosteps having height larger than the well thickness. The electroluminescence spectra of the fabricated LEDs using this MQW structure shifts to a longer wavelength compared with those on sapphire with a miscut angle of 0.3° relative to the m-plane. Furthermore, the LEDs with this MQW show no deleterious effect on the lifetime, broader electroluminescence spectral widths, and higher output powers when using sapphire with a miscut of 1.0°.

We report on the fabrication and characterization of high efficiency ultraviolet (UV) light emitting diodes (LEDs) with emission wavelength ranging from 255 to 355 nm. Epi-layers of UV LEDs were grown on AlGaN templates with sapphire substrates. Flip-chip configuration without removing sapphire is used for characterization of the UV LEDs. External quantum efficiencies (EQEs) over 3% were obtained for all the investigated wavelengths with maximum value reaching 5.1% for 280 nm LED. Under RT DC operation at a current of 500 mA, output powers of 38, 77, and 64 mW were measured for 257, 280, and 354 nm, respectively. By using enhanced light extraction technologies, such as, moth-eye structure on the back side of the sapphire substrate, we expect to improve these values by up to 50%.

Deep ultraviolet (DUV) light-emitting diodes (LEDs) on patterned sapphire substrates (PSSs) have been clearly demonstrated. AlN templates grown on PSSs had average threading dislocation densities (TDDs) of as low as 5×107 cm-2. Flip-chip DUV LEDs fabricated on PSSs demonstrated a significantly high performance. The 266 nm LED exhibited an output power of 5.3 mW and an external quantum efficiency (EQE) of 1.9% at 60 mA DC, and the 278 nm LED had 8.4 mW output and an EQE of 3.4%. Moreover, the 70% lifetime was more than 700 h at 20 mA.