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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.

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.

We investigated the characteristics of epitaxial lateral overgrowth (ELO) AlN templates fabricated on Si substrates which are applicable to AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs). ELO-AlN layers were grown on stripe-patterned thin AlN seed-layer directly deposited on (111)Si substrates by using an ‘NH3 pulsed-flow multi-layer (ML) growth’ technique. A 4 µm-thick ELO-AlN layer with stripe pattern along direction of AlN can be coalesced successfully. We achieved marked reduction of crack and threading dislocation density (TDD) in the ELO AlN layers. In fact, the (10-12) full width at half maximum (FWHM) of the X-ray diffraction ω-scan rocking curves (XRCs) of ELO-AlN template on Si substrate was reduced to be 980 arcsec. These results show that the combination of NH3 pulsed-flow ML growth and ELO method is effective to fabricate high-quality AlN templates on Si substrates. Moreover we fabricated 256-278 nm AlGaN quantum well (QW) DUV LEDs on the low TDD ELO-AlN templates on Si, and achieved single-peak operations under room temperature (RT) cw current injection. The low cost AlGaN based DUV LEDs on Si substrates is expected to be integrated with Si based electric devices on the same chips. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High-efficiency and high-power deep-ultraviolet (DUV) light-emitting diodes (LEDs) with emission wavelength between 200 and 350 nm have a wide range of potential applications. We have developed highly-uniform 260 nm-band AlGaN DUV LEDs fabricated on AlN templates on (0001) sapphire using a 2-inch×3 reactor by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE), for the purpose of realizing commercially-available low-cost DUV LEDs. An NH3 pulsed-flow multilayer (ML) growth method was used for the fabrication of low threading dislocation density (TDD) AlN templates on sapphire substrates. 4 μm-thick low TDD AlN templates were successfully grown on (0001) sapphire substrates. Abnormal hillocks due to polarity inversion were markedly decreased, when the growth temperature of the first NH3 pulsed-flow AlN layer directly grown on sapphire was reduced to be 1100 oC. DUV LEDs on the high quality AlN templates showed good uniformity in the emission wavelength and in the output power, in each 2-inch wafer and among three wafers which were simultaneously grown. The DUV-LEDs were flip-chip bonded and demonstrated output power of 17.1 mW at 500 mA and 40 mW at 1500 mA measured under CW and pulsed operations, respectively. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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. Single-peaked electroluminescence (EL) were obtained for 222– 273 nm AlGaN multi quantum well (MQW) DUV-LEDs. We obtained the maximum output power of 1.1 mW and 4.0 mW for the AlGaN-QW LEDs with wavelengths of 241 nm, 256 nm, respectively, under room temperature (RT) CW operations. The maximum output power of 227 nm and 222 nm AlGaN-QW were 0.15 mW and 0.014 mW, respectively, under RT pulsed operation. The maximum external quantum efficiency (EQE) of the 227 nm and 250 nm AlGaN LEDs were 0.2% and 0.43%, respectively. We also fabricated 280 nm band quaternary InAlGaN-MQW DUV-LEDs with n-type and p-type InAlGaN layers on ML-AlN templates. We demonstrated extremely 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 77 K-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. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report high efficiency blue organic electroluminescent (OEL) devices having a metal-doped electron injection layer at the cathode interface. The typical device structure is an indium-tin oxide (IIO)/arylamine derivative (α-NPD)/blue emitting distyrylarylene derivative/metal-doped organic electron injection layer/Al. The device efficiency is dependent on the work function of the dopant metals and the host organic materials composing the metal-doped layer. A device having Cs-doped bathophenanthroline (Bphen) layer as the cathode interface layer exhibited a high luminous efficiency of 9 lm/W and an external quantum efficiency of 6.5% at luminance of 300 cd/m 2 , which are the highest values reported for blue organic EL devices

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.

It is shown that the dc-superimposed driving mode is more useful for the efficient operation of a novel thermally induced ultrasonic emitter based on nanocrystalline porous silicon (nc-PS) than the conventional simple ac-voltage driving mode. The nc-PS device is composed of a patterned heater electrode, an nc-PS layer and a single crystalline silicon (c-Si) substrate. The almost complete thermally insulating property of nc-PS as a quantum-sized system makes it possible to apply the nc-PS device as an ultrasonic generator by efficient thermo acoustic conversion without any mechanical vibrations. In the dc-superimposed driving mode, the output frequency is the same as the input frequency and a stationary temperature rise is kept constant independent of input peak-to-peak voltage. In addition, power efficiency is significantly increases compared with that in the ac-voltage driving mode without affecting on the temperature rise. The present results suggest the further possibility of the nc-PS device being used as a functional speaker.

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%.

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 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.

Semiconductor light-emitting diodes (LEDs) with emission wavelengths in the range 250-350nm have a wide range of potential applications. So far, high power AlGaN- and InAlGaN-based LEDs in the ultraviolet (UV) region have been fabricated. However, for the realization of commercially-available low-cost UV-LEDs, the fabrication of structures on large-sized or multiple wafers is desirable. We have now demonstrated the growth of AlN templates using an ammonia (NH3) pulsed-flow multilayer (ML) growth technique and AlGaN based UV-LEDs on (0001) sapphire substrates by low-pressure metalorganic chemical vapour deposition (MOCVD) in a 2″ × 3 reactor system. In the case of the (10-12) X-ray rocking curve (XRC), the full-width-at-half-maximum (FWHM) increased from 510 arcsec to 580 arcsec when scanning from the center to the edge of the wafer. On the other hand, the FWHM of the (0002) XRC was slightly increased from 170 arcsec to 190 arcsec. Furthermore, Emission peaks for UV-LEDs were clearly observed in the range from 268 nm to 284 nm. The output power at 100 mA and maximum output power were 0.2 mW and 0.3 mW, respectively. These results indicate that sub-milliwatt operation in the deep-UV range was successfully achieved. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrated CW milliwatt power operations of 270 nm-band AlGaN multi-quantum well (MQW) deep-ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on epitaxial lateral overgrowth (ELO) AlN templates on sapphire. An initial AlN stripe layer was grown directly on sapphire by using ammonia (NH3) pulse-flow multilayer (ML) growth method. An AlN stripe structure with a width of 5 μm and a spacing of 3 μm was completely coalesced after the growth of an approximately 15 μm-thick ELO-AlN layer grown by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). The edge-type threading dislocation density (TDD) of the wing region of the ELO-AlN layer was 3×108 cm-2, as observed by cross-sectional transmission electron microscope (TEM) image. The maximum output power of 2.7 mW was obtained from an AlGaN MQW LED with emission wavelength at 273 nm fabricated on ELO-AlN template under room temperature (RT) CW operation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have fabricated low threading dislocation density (TDD) AlN templates on sapphire substrates for application to deep-ultraviolet (DUV) light-emitting diodes (LEDs) by employing an epitaxial lateral overgrowth (ELO) process. An AlN stripe structure with a width of 5 μm and a spacing of 3 μm was formed by using inductive coupled plasma (ICP) etching. The AlN stripes were completely coalesced and embedded to create a flat surface after the growth of an approximately 15 μm-thick ELO-AlN layer, grown by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). We systematically investigated the appropriate V/III ratio and growth temperature for the ELO-AlN. The edge-type TDD of the AlN was reduced from 2×109 cm–2 (AlN stripe region) to 3×108 cm–2 (in the wing region of the ELO-AlN), as estimated by cross-sectional transmission electron microscope (TEM) imaging. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quaternary InAlGaN alloys are attracting much attention as candidate materials for realizing deep ultraviolet (DUV) light-emitting diodes (LEDs). In this study, we investigated the effect of Si-doping in the quaternary InAlGaN quantum well (QW) emitting region for obtaining a high efficiency DUV emission. Quaternary InAlGaN multi (M)-QWs with Si-doped InAlGaN buffer and barrier layers were grown on sapphire (0001) / AlN / AlGaN template by low-pressure metal-organic chemical-vapor deposition (LP-MOCVD). We found that the surface roughness of InAlGaN layer was significantly improved by introducing the light-doping of Si. The oxygen impurity concentration in the quaternary InAlGaN layer was significantly reduced by the small amount of Si-doping. We also revealed that the emission efficiency of the InAlGaN QW was markedly improved by inserting thin (3-nm-thick) undoped InAlGaN interlayer between n-AlGaN buffer and n-InAlGaN layers. At last, we demonstrated extremely high intensity PL emission from 280 nm-band InAlGaN QWs at room temperature (RT). The ratio of the integrated intensity of the RT-PL against the 77 K-PL was 86%. This result suggests that the internal quantum efficiency (IQE) of the InAlGaN-QW is considerably high. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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.

Ultrasonic three-dimensional (3-D) image sensing in air is successfully demonstrated by combining a novel thermally induced ultrasonic emission from nanocrystalline porous silicon (nc-PS) device with a condenser microphone array. The nc-PS ultrasonic emitter is composed of a patterned heater electrode, an nc-PS layer, and a single-crystalline silicon (c-Si) substrate. In this device, an ideal probe signal can be generated with little reverberation under impulse operation, since the acoustic output shows a flat frequency response in a wide range due to a quick thermo-acoustic conversion at the device surface without any mechanical vibrations. The 3-D object is detected with a spatial resolution considerably higher than that in the case of conventional techniques based on piezoelectric transducers. It is also shown that the device is available for the detection of several objects with different acoustic reflectance coefficients.