Light-emitting diodes with emission wavelengths less than 400 nm have been developed using the AlInGaN material system. For devices operating at shorter wavelengths, alloy compositions with a greater aluminium content are required. The material properties of these materials lie on the border between conventional semiconductors and insulators, which adds a degree of complexity to the development of efficient light-emitting devices. A number of technical developments have enabled the fabrication of LEDs based on group three nitrides (III-nitrides) that emit in the UV part of the spectrum, providing useful tools for a wealth of applications in optoelectronic systems.

Ultraviolet light-emitting diodes based on group three nitrides

Atomic layer deposition (ALD) has recently become the method of choice for the semiconductor industry to conformally process extremely thin insulating layers (high-k oxides) onto large-area silicon substrates. ALD is also a key technology for the surface modification of complex nanostructured materials. After briefly introducing ALD, this Review will focus on the various aspects of nanomaterials and their processing by ALD, including nanopores, nanowires and -tubes, nanopatterning and nanolaminates as well as low-temperature ALD for organic nanostructures and biomaterials. Finally, selected examples will be given of device applications, illustrating recent innovative approaches of how ALD can be used in nanotechnology.

https://www-old.mpi-halle.mpg.de/mpi/publi/pdf/7999_07.pdf

Synthesis and Surface Engineering of Complex Nanostructures by Atomic Layer Deposition

The field of AlGaInN ultraviolet UV light-emitting diodes (LEDs) is reviewed, with a summary of the state-of-the-art in device performance and enumeration of applications. Performance-limiting factors for high-efficiency UV LEDs are identified and recent advances in the development of deep UV emitters are presented.

https://iopscience.iop.org/article/10.1088/0268-1242/26/1/014036/pdf

Advances in group III-nitride-based deep UV light-emitting diode technology

The high-color-rendering (HCR) potential of the light sources consisting of 350-nm ultraviolet light-emitting diodes (UV-LED) and three-basal-color phosphors was simulated and experimentally confirmed. By using an AlGaN-based UV-LED as an excitation source at the 350-nm wavelength, we measured fluorescence spectra of three-basal-color phosphors, and simulated the optimum phosphor mixture based on the Commission Internationale de l’Eclairage standard light sources D75, D65, D55, D50, C, and A, with HCR indexes (Ra) over 92. We confirmed the potential of HCR of this light source consisting of 350-nm UV-LED and three-basal-color phosphors, by obtaining Ra=86∼89 of the incandescent type of standard light source A experimentally. Such high Ra are due to the highly monochromatic UV-LED emission and the wide spectral separation between the UV excitation (350 nm) and visible fluorescences. This combination also provides a diffusive, nonglare light source with highly steady color independent of the emission orient...

High-color-rendering light sources consisting of a 350-nm ultraviolet light-emitting diode and three-basal-color phosphors

In order to realize 250–350-nm-band high-efficiency deep ultraviolet (UV) emitting devices using group-III-nitride materials, it is necessary to obtain high-efficiency UV emission from wide-band-gap (In)AlGaN. The use of the In-segregation effect, which has already been used for InGaN blue emitting devices, is quite effective for achieving high-efficiency deep UV emission. We have demonstrated high-efficiency UV emission from quaternary InAlGaN-based quantum wells in the wavelength range between 290 and 375 nm at room temperature (RT) using the In-segregation effect. Emission fluctuations in the submicron region due to In segregation were clearly observed for quaternary InAlGaN epitaxial layers. An internal quantum efficiency as high as 15% was estimated for a quaternary InAlGaN-based single quantum well at RT. Such high-efficiency UV emission can even be obtained on high threading-dislocation density buffer layers. A comparison of electroluminescence is made between light-emitting diodes (LEDs) with InAlGaN, AlGaN, and GaN active regions fabricated on SiC substrates with emission wavelengths between 340 and 360 nm. The emission intensity from the quaternary InAlGaN UV-LED was more than one order of magnitude higher than that from the AlGaN or GaN UV-LEDs under RT cw operation. We therefore fabricated 310–350-nm-band deep UV-LEDs with quaternary InAlGaN active regions. We achieved submilliwatt output power under RT pulsed operation for 308–314-nm LEDs. We also demonstrated a high output power of 7.4 mW from a 352-nm quaternary InAlGaN-based LED fabricated on a GaN substrate under RT cw operation. The maximum external quantum efficiency (EQE) of the 352-nm InAlGaN-based LED was higher than that obtained for an AlGaN-based LED with the same geometry. From these results, the advantages of the use of quaternary InAlGaN in 350-nm-band UV emitters were revealed.

Quaternary InAlGaN-based high-efficiency ultraviolet light-emitting diodes

By introducing a GaN-free layer structure, we have transparent light-emitting diodes (LEDs) at the ultraviolet emission wavelength of 348–351 nm, which is shorter than the GaN band gap wavelength of 363 nm. The buffer layer consists of an AlGaN alloy directly grown on an AlN template layer on a sapphire substrate, and a short period alloy superlattice is adopted as p-type cladding and p-type contact layers. The transparency of the epitaxially grown layer structure is confirmed from transmission spectra. The output powers of the device are 1 mW at injection currents of 20 and 7 mW at 220 mA under room temperature continuous wave operation. The highest external quantum efficiency is 1.4%. This value is superior to that of an ultraviolet LED grown on a high-quality bulk-GaN substrate, where the performance was significantly deteriorated by light absorption into the GaN substrate. The results here indicate the importance of a transparent device structure free of GaN to improve the performance of ultraviolet L...

GaN-free transparent ultraviolet light-emitting diodes

We have confirmed the potential of a bulk AlN substrate for high current operation of nitride ultraviolet-light-emitting diodes (UV-LEDs). For the high flux UV extraction from nitride UV-LEDs, transparency and high thermal conductivity of the substrates are important issues. The bulk AlN is one of the best candidates, because it satisfies requirements above, and has the same crytallographic symmetry with those of AlGa(In)N families, which is beneficial to the high-quality crystal growth of the nitride device structures. We formed AlGaN-based UV-LEDs on a bulk AlN substrate and compared its performance with that of a reference device grown on an AlN-template grown on a sapphire substrate. The output power linearly increases with a saturation injection current of 300 mA, which is two times higher than that of the reference device. The emission spectrum under high current injection is much more stable than that of conventional substrate.

AlGaN-based ultraviolet light-emitting diodes grown on bulk AlN substrates

We report the merits of the transparent structure for the nitride semiconductor ultraviolet light emitting diodes. The ultraviolet light emitting diodes consisting of AlGaN-based active layers provide a wide spectral range from 200 to 360 nm in the ultraviolet range. To produce a transparent device structure at the emission wavelength, we used sapphire substrates, AlN-template layers, AlGaN alloy buffer layers, and p-type contact layers consisting of a short period alloy superlattice, all of which are transparent to the emission. We confirmed the transparency of the device structure from the transmission spectra. The output power characteristic is improved by a factor of four due to the transparent structure. We also demonstrate an efficient ultraviolet light emitting diode grown on an AlGaN buffer layer of low dislocation density. The external quantum efficiency is 1.4%, which is superior to that of the ultraviolet light emitting diode grown on bulk GaN substrate.

http://iopscience.iop.org/article/10.1143/JJAP.42.2273/pdf

Highly Transparent Structure for Nitride Ultraviolet Light Emitting Diodes

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.

Tomoyuki Ban

Papers

High-color-rendering light sources consisting of a 350-nm ultraviolet light-emitting diode and three-basal-color phosphors

GaN-free transparent ultraviolet light-emitting diodes

AlGaN-based ultraviolet light-emitting diodes grown on bulk AlN substrates

Highly Transparent Structure for Nitride Ultraviolet Light Emitting Diodes

High current injection to a UV-LED grown on a bulk AlN substrate