Kenji Tsubaki

Deep-ultraviolet light-emitting diodes with external quantum efficiency higher than 20% at 275 nm achieved by improving light-extraction efficiency

Enhancing the light-extraction efficiency is the key issue for realizing highly efficient AlGaN-based ultraviolet light-emitting diodes (UV-LEDs). We introduced several features to improve the light extraction: a transparent AlGaN:Mg contact layer, a Rh mirror electrode, an AlN template on a patterned sapphire substrate, and encapsulation resin. The combination of the AlGaN:Mg contact layer and the Rh mirror electrode significantly improved the output power and the external quantum efficiency (EQE) of UV-LEDs. By introducing the aforementioned features, a maximum EQE of >20% at an emission wavelength of 275 nm and a 20-mA direct current was achieved.

Realization of 256–278 nm AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Si Substrates Using Epitaxial Lateral Overgrowth AlN Templates

We demonstrated 256–278 nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) on Si substrates by using epitaxial lateral overgrowth (ELO) AlN templates. A 4-µm-thick ELO-AlN layer grown in a striped pattern along the direction can be coalesced successfully. Low-threading-dislocation-density AlN templates were achieved on Si wafers by a combination of the ELO and NH3 pulsed-flow multilayer growth methods. Single-peaked AlGaN LEDs with wavelengths shorter than 280 nm were achieved by fabricating them on ELO-AlN templates on Si. These low-cost AlGaN-based DUV LEDs on Si substrates are expected to be integrated on the same chips with Si-based electrical circuits.

222-282 nm AlGaN and InAlGaN based deep-UV LEDs fabricated on high-quality AlN template

We demonstrate 222-282 nm AlGaN and InAlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on low threading dislocation density (TDD) AlN template. Low TDD AlN templates were realized by using ammonia (NH3) pulse-flow multilayer (ML) growth technique. The edge- and screw-type dislocation densities of AlN layer were reduced to 7.5×108 and 3.8×107, respectively. We obtained significant increase of an AlGaN quantum well (QW) emission (by more than 50 times) by fabricating them on a low TDD ML-AlN template. We fabricated AlGaN multi (M)QW DUV-LEDs with emission range of 222-273 nm on ML-AlN templates. Single-peaked electroluminescence (EL) was obtained for AlGaN DUV-LEDs. We obtained the maximum output power of 1.1, 2.4 and 3.3 mW for the AlGaN LEDs with wavelengths of 241, 253 and 273 nm, respectively, under RT CW operation. The maximum output power of 227 and 222 nm AlGaN-QW were 0.15mW and 0.014mW, respectively, under RT pulsed operation. The maximum external quantum efficiency (EQE) of the 227 and 250 nm AlGaN LEDs were 0.2% and 0.43 %, respectively. We also fabricated 280 nm-band quaternary InAlGaN-MQW DUV-LEDs with p-type InAlGaN layers on low TDD ML-AlN templates. We obtained significant increase of photoluminescence (PL) intensity by introducing Si-doped InAlGaN buffer and barrier layers and undoped InAlGaN interlayer. We then demonstrated high internal quantum efficiency (IQE) of 284 nm InAlGaN-QW emission, which was confirmed by the fact that the ratio of the integrated intensity of the RT-PL against the 77K-PL was 86%. The maximum output power and EQE of the 282 nm InAlGaN LED were 10.6 mW and 1.2%, respectively, under RT CW operation.

Development of 260 nm band deep-ultraviolet light emitting diodes on Si substrates

Deep-ultraviolet (DUV) light-emitting diodes (LEDs) have a wide range of potential applications, such as sterilization, water purification, and medicine. In recent years, the external quantum efficiency (EQE) and the performance of AlGaNbased DUV LEDs on sapphire substrates have increased markedly due to improvements in the crystalline-quality of high Al-content AlGaN layers, and the optimization of LED structures. On the other hand, DUV LEDs fabricated on Si substrates are very promising as a low-cost DUV light-source in the near future. However, AlN layers on Si have suffered from cracking induced by the large mismatch in lattice constants and thermal expansion coefficients between AlN and Si. In this paper, DUV LEDs on Si were realized by a combination of a reduction in the number of cracks and of the threading dislocation density (TDD) of AlN templates by using the epitaxial lateral overgrowth (ELO) method. The ELO-AlN templates were successfully coalesced on trench-patterned substrates, with the stripes running along the <1-100> direction of AlN. The density of cracks was greatly reduced in 4- μm-thick ELO-AlN templates, because voids formed by the ELO process relaxed the tensile stress in the AlN layer. Furthermore, the AlN templates showed low-TDD. The full-width-at-half-maximum values of the (0002) and (10-12) X-ray rocking curves were 780 and 980 arcsec, respectively. DUV LEDs fabricated on these high-quality ELO-AlN/Si substrates showed single peak emission at 256- 278 nm in electroluminescence measurements. It is expected that we will be able to realize low-cost DUV LEDs on Si substrates by using ELO-AlN templates.

Over 10% EQE AlGaN deep-UV LED using transparent p-AlGaN contact layer (Conference Presentation)

AlGaN deep ultraviolet light-emitting diodes (DUV-LEDs) are attracting much attention for a wide variety of applications, however, the efficiency of DUV-LED is still low suppressed by low light-extraction efficiency (LEE). Transparent contact layer is considered to be necessary in order to obtain high LEE in AlGaN DUV LEDs. In this work, we demonstrate over 10% external quantum efficiency (EQE) in an AlGaN DUV-LED by using transparent p-AlGaN contact layer and highly reflective p-type electrode. We fabricated AlGaN quantum well (QW) DUV LEDs with transparent p-AlGaN contact layers on AlN/sapphire templates. EQEs were compared between LEDs with Ni/Al highly reflective electrode and with conventional Ni/Au electrode. The transparency of the p-AlGaN contact layer was confirmed to be more than 97 %. The maximum EQE for 261 nm LEDs with Ni/Al and Ni/Au electrodes were approximately 2 and 3.3%, respectively. We confirmed that the LEE was increased by about 1.7 times. We also fabricated flip-chip (FC) UVC LED module with transparent p-AlGaN contact layer and reflective electrode. The FC LED module was encapsulated to increase LEE. The emission wavelengths were 276 nm. The EQE value under the forward current of 120 mA was increased from 2.7 to 8.6% by increasing an LEE. The output power of approximately 60 mW was obtained under the forward current of 150 mA. The EQE value was maximally increased up to 10.8%. LEE was estimated to be increased from 8.6 % to 25.5 % by introducing LEE enhancement structure.

AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes

222–351 nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated, which have been achieved by the development of crystal growth techniques for wide-bandgap AlN and AlGaN. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN quantum well (QW) emissions by introducing low-threading-dislocation density (TDD) AlN grown by an NH3 pulsed-flow multilayer growth method. Electron Injection efficiency (EIE) of the DUV LED was significantly increased by introducing multi-quantum barrier (MQB). Light extraction efficiency (LEE) was also improved by using a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) was increased up to 10.8% for a 276 nm DUV LED.

Development of highly-uniform 270-nm deep-ultraviolet light-emitting diodes

Development of high-quality and highly-uniform AlN/sapphire templates by using a NH3 pulsed-flow method enabled the fabrication of highly-uniform 270-nm AlGaN-based deep-ultraviolet light-emitting diodes with the external quantum efficiency of over 2%.