Sachie Fujikawa

Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light-emitting diodes

In this paper, recent advances in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) are demonstrated. 220–350-nm-band DUV LEDs have been realized by developing crystal growth techniques for wide-bandgap AlN and AlGaN semiconductors. Significant increases in internal quantum efficiency (IQE) have been achieved for AlGaN DUV emissions by developing low-threading-dislocation-density (TDD) AlN buffer layers grown on sapphire substrates. The electron injection efficiency (EIE) of the LEDs was also significantly increased by introducing a multiquantum barrier (MQB). We also discuss light extraction efficiency (LEE), which is the most important parameter for achieving high-efficiency DUV LEDs. We succeeded in improving LEE by developing a transparent p-AlGaN contact layer. The maximum external quantum efficiency (EQE) obtained was 7% for a 279 nm DUV LED. EQE could be increased by up to several tens of percent through the improvement of LEE by utilizing transparent contact layers and photonic nanostructures in the near future.

Realization of 340-nm-Band High-Output-Power (

We have demonstrated 340-nm-band high-output-power InAlGaN quantum well (QW) ultraviolet (UV) light-emitting diodes (LEDs) under room temperature (RT) continuous wave (CW) operation, which were deposited on sapphire (0001) substrates by low-pressure metal–organic chemical vapor deposition (LP-MOCVD). The high-output-power UV LEDs were achieved by introducing p-type InAlGaN layers in order to obtain a high hole concentration and by optimizing the band lineup to suppress electron overflow. The output power of a UV-LED with p-InAlGaN layers was about 4.7 times larger than that of an equivalent structure containing p-AlGaN. We obtained a significant increase in output power by controlling the barrier height of the electron-blocking layer (EBL) and the depth of the quantum wells. We also obtained a marked increase in UV output power by introducing a low-threading-dislocation-density (TDD) AlN buffer layer. The maximum output power and external quantum efficiency (EQE) of LEDs containing p-InAlGaN layers were 8.4 mW and 0.9%, respectively, at an emission wavelength of 346 nm under room temperature (RT) CW operation.

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We have demonstrated 280?300-nm-band quaternary InAlGaN based deep-ultraviolet (DUV) light-emitting diodes (LEDs) fabricated on Si(111) substrates, grown by low-pressure metal?organic chemical vapor deposition (LP-MOCVD). An ammonia pulsed-flow growth method was used in the initial stages of AlN growth on Si substrates in order to achieve low threading dislocation densities with thin AlN layer. We obtained single-peaked spectra from the DUV LEDs at wavelengths between 284 and 300 nm under room temperature continuous-wave (cw) operation. It is expected that low-cost and large-size DUV LEDs could become available in the near future by increasing light-extraction by removing the Si substrate.

mW) InAlGaN Quantum Well Ultraviolet Light-Emitting Diode with p-Type InAlGaN

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.

284–300 nm Quaternary InAlGaN-Based Deep-Ultraviolet Light-Emitting Diodes on Si(111) Substrates

260 nm-band AlGaN deep-ultraviolet (DUV) light-emitting diodes (LED) was fabricated on a connected-pillar AlN buffer layer grown on a sapphire substrate. Low threading dislocation density (TDD) connected-pillar AlN buffer is considered to be quite effective for obtaining high internal quantum efficiency (IQE) and light-extraction efficiency (LEE) in DUV-LEDs. We fabricated an AlN pillar array structure with accurate hexagonal shape pillars grown on a patterned sapphire substrate (PSS) by using an NH3 pulsed flow multi-layer (ML) growth and an epitaxial lateral overgrowth (ELO). The AlN pillars were connected to be flat surface AlN buffer layer with low TDD, through ELO process. We fabricated AlGaN quantum well (QW) DUV-LED on a connected-pillar AlN buffer. The output power obtained was 5.5 mW for 265 nm LED measured under room temperature cw operation.

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

Deep-ultraviolet (DUV) light-emitting diodes (LEDs) have a wide range of potential applications such as sterilization, water purification, medicine and biochemistry, white light illumination, and so on. DUV-LED fabricating using silicon (Si) substrate is attractive as a low-cost DUV light-source, in near future. However, the realization of the LED on the Si substrate which is shorter than 300nm is not reported till now. Quaternary InAlGaN alloy is attracting attention as candidate material for realizing high-efficiency DUV LEDs due to In incorporation effects [1,2]. In this study, we demonstrated 280nm-band InAlGaN QW DUV LEDs on Si (111) substrates.