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Eco‐friendly and low‐cost aluminum gallium nitride (AlGaN) for the epitaxial growth of ultraviolet‐B (UVB) light‐emitting diodes (LEDs) on c‐Sapphire has the possibility of high external‐quantum efficiency (EQE). In this review paper a special growth techniques for 50% relaxed and 4 μm thick AlGaN buffer layer underneath the multi‐quantum wells (MQWs) are challenged to achieve a maximum internal‐quantum efficiency of 50–57% in 310–290 nm band UVB LEDs. The influence of a thin "Valley" layer in p‐type multi‐quantum barrier electron‐blocking layer on 2D hole generation and injection via intraband tunneling was attempted. Finally, the influence of soft polarized Mg‐doped p‐type Al‐graded AlGaN hole injection layer assisted by excimer laser annealing for better hole injection toward the MQWs was investigated and quite high hole concentration of 2 × 1016 cm−3 and resistivity of 24 Ω‐cm at room temperature was achieved. Consequently, the EQE of transparent 310 and 304 nm UVB LEDs, respectively, reached to a confirmed world record values of ≈5% and ≈10% with light powers of 29 and 40 mW on wafer. This EQE value can surpass 21% if flip‐chip, nanoPSS, photonic crystal, and lens with highly reflective p‐electrodes are incorporated in LED. [ABSTRACT FROM AUTHOR]

The emission performance of c‐sapphire‐grown deep‐UV (DUV) light‐emitting diodes (LEDs) at 254 and 258 nm is particularly poor. To overcome this critical limitation, herein, the influence of the Mg‐dopant concentration in a p‐AlGaN hole source layer (HSL) on donor–acceptor pair emission from solid‐state AlGaN‐based DUV LEDs in the 254–258 nm band is investigated. Further, the influence of substrate growth temperature (TS) on the photoluminescence (PL) emission efficiency and external quantum efficiency (EQE) is investigated. When the TS is increased from 1140 to 1170 °C for a p‐AlGaN HSL with a low Mg‐dopant concentration (70 sccm), the maximum power outputs of the fabricated DUV LED at the single peak emissions of 254 and 258 nm are enhanced to 3.4 and 14 mW, respectively. Moreover, the maximum EQE of the 254 and 258 nm‐band DUV LEDs increases by ≈1.2% and 2.2%, respectively, on the wafer during both continuous‐wave and pulsed operation at room temperature. [ABSTRACT FROM AUTHOR]

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed. [ABSTRACT FROM AUTHOR]

Strain plays a crucial role in the performance of ultraviolet (UV) optoelectronic devices. The strain states of AlGaN layers grown on different AlN templates are investigated. Two types of AlN templates are prepared: one is grown solely by metal‐organic chemical vapor deposition (MOCVD), and the other is fabricated with the combination of direct current (DC) sputtering, high‐temperature annealing, and MOCVD growth. The qualities of 4 μm‐thick MOCVD AlN and 1.2 μm‐thick sputter‐based AlN are almost equivalent in terms of dislocation density and surface morphology. However, the strain relaxation ratio of the AlGaN layer is higher for the 1.2 μm sputter‐based AlN than the 4 μm MOCVD AlN, indicating that the strain state depends on the total thickness of the epilayers on the sapphire substrate. In addition, it is found that the curvature of the sample at room temperature is governed mainly by the thickness of the AlGaN layer. As a result, sputter‐based AlN templates allow for the enhanced strain relaxation of the AlGaN layer with a small sample bowing. [ABSTRACT FROM AUTHOR]

The influence of strain relaxation on the optical properties of AlGaN quantum wells (QWs) is investigated. The relaxation ratio is controlled by changing the thickness of an AlGaN buffer layer. The relaxation ratio increases from 20% to 100% with increasing the AlGaN layer thickness from 0.5 to 6.0 μm. Photoluminescence (PL) intensity reaches maximum at a relaxation ratio of 36%, which implies competing effects in radiative and nonradiative recombinations. From the viewpoint of radiative recombination, strain relaxation increases the polarization field in the QW, which decreases the electron–hole overlap. The observed gradual decrease in PL may come from this effect. In terms of nonradiative recombination, the dislocation density is nearly constant up to 4.5 μm from the AlN/AlGaN interface, and decreases at 6.0 μm. Therefore, it is difficult to explain the observed PL intensity by the dislocation‐related nonradiative pathways. Instead, relieved strain can decrease the point‐defect density. Strain relaxation can reduce the vacancy concentration as a result of increased formation energy with the partial relief of compressive strain. The effect of strain on the band structure change is also discussed, where the relieved compressive strain increases the crystal‐field split‐off component in the valence band maximum, resulting in a decreased PL intensity. [ABSTRACT FROM AUTHOR]

AlGaN‐based deep‐ultraviolet light‐emitting diodes (DUV LEDs) have a wide range of applications and a large market is expected. However, the efficiency of DUV LEDs is still much lower than that of blue LEDs due to the quite low light‐extraction efficiency (LEE). We are developing high LEE DUV LEDs by introducing a transparent contact layer and a highly reflective Ni/Al p‐type electrode. In this work, we investigate optimization of the Ni layer thickness of the highly reflective Ni/Al p‐type electrode for AlGaN DUV LEDs. We find that the reflectivity in the UV region becomes higher as the thickness of the Ni layer becomes smaller; however, the external quantum efficiency (EQE) is reduced if the Ni layer is too thin (<0.8 nm). As a result, we find that the most appropriate Ni thickness for the Ni/Al electrode is 0.9 nm. We fabricate 279 nm AlGaN quantum well (QW) DUV LEDs with the optimized electrodes and demonstrate an enhancement in EQE by a factor of 1.8 and a maximum EQE of 9% under bare wafer measurement conditions. We also demonstrate a high output power of 33 mW under an injection current of 100 mA. [ABSTRACT FROM AUTHOR]

Semipolar III-nitrides have attracted increasing attention because of the reduced piezoelectric field in the active layer. In particular, (11-22) plane is promising due to the small polarization field along the growth axis and the epitaxial matching on m-plane sapphire. The aim of this study was to assess the effect of growth conditions on the structural and electrical properties of Si-doped (11-22) AlGaN. Semipolar AlGaN/AlN layers were grown on m-plane sapphire by metal-organic chemical vapor deposition at different temperatures, V/III ratios, and tetraethylesilane flows. Surface morphology, crystalline quality, and electrical properties of the AlGaN layers were investigated with AFM, XRD, and by Hall measurements, respectively. [ABSTRACT FROM AUTHOR]

SUMMARY Deep-ultraviolet (DUV) light-emitting diodes (LEDs) are in demand for a wide variety of potential applications, such as sterilization, water and air purification, medical uses, and so on. We have demonstrated 222-351 nm AlGaN and quaternary InAlGaN-based DUV-LEDs by developing a low threading dislocation density (TDD) AlN crystal. We achieved an external quantum efficiency (EQE) of about 4% and an output power greater than 30 mW in DUV-LEDs for use in sterilization applications by developing new crystal growth techniques. [ABSTRACT FROM AUTHOR]

The increasing resistance of methicillin‐resistant Staphylococcusaureus to antibiotics is a major challenge faced by mankind in thehistory of medical science and according to United Nations, 700‐000 patients worldwide die every year from an infection with multidrug‐resistant organisms (MROs). Aluminum gallium nitride‐based 228 nm Far‐ultraviolet‐C (Far‐UVC) lightsources can be safely used as a germicidal application in both manned as wellas in unmanned environments against these MROs. Previously, the 228 nm Far‐UVC light‐emitting diode (LED) with emission power of 1 mW was reported by ourgroup, however, the value of external quantum efficiency (EQE) was not reportedusing conventional thick Ni (20 nm)/Au (100 nm) p‐electrode. Herein, animproved Far‐UVC LED on c‐Sapphire is attempted using a special technique in SR4000 type of metal‐organic chemical vapor deposition reactor to control the Al composition in n‐AlGaN buffer and across the 2 inch‐wafer. As a result, the light emission power of 1.8 mW and EQE of 0.32% in 228 nm Far‐UVC LED aresuccessfully achieved using very thin p‐electrode (Ni/Au). However, arelatively high junction temperature of ≈100°C around thejunction of Far‐UVC LED is observed. Finally, some simple heat‐sink modules forheat dissipation of Far‐UVC LED panel with light power of 30 mW are implemented. [ABSTRACT FROM AUTHOR]

Previously, we attempted to the optically-pumped AlGaN ultraviolet (UV) laser diodes (LDs) on AlN template on c-Sapphire substrate, using low-cost epitaxial growth technique in low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE). As a result, stimulated lasing spectra at 274 nm were observed for the first time at room temperature (RT) in the optically pumped LD. However, the electrically-pumped AlGaN-based (270–298 nm)-band UV LD was quite challenging due to both the issue of low injection current density and light confinement factor (Г). In this paper, the injection current density in the electrically pumped AlGaN UV LD at 283 nm emission was remarkably improved up to 200 kA-cm−2 by using undoped Al-graded cladding and Al-graded Mg-doped p-AlGaN hole source layer (HSL). Also, some weak oscillation at 296 nm near to the main spontaneous emission peak of 298 nm-band LD under the 600 mA (24 V) current using pulse width of 50 ns at RT was observed. To further improve the lasing phenomenon, some theoretical studies was conducted, by making the p-side waveguide (p-WG) layer non-doped (Mg = 0) and also increasing the quantum-well (QW) thickness from 2 nm to 4 nm in the multiquantum-wells (MQWs). Consequently, the optical-loss was significantly reduced from 22.7 to 8.7 cm−1 and the Г was enhanced from 1.77% (Experimental LD) to 4.65% (simulated LD). • Optically-pumped AlGaN ultraviolet (UV) laser diodes (LDs) on low-cost AlN template on c-Sapphire substrate in low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) was realized. • Stimulated lasing spectra at 274 nm were observed at room temperature (RT) in the optically pumped UV LD. • Electrically pumped UV LD was also attempted. The injection current density in the electrically pumped AlGaN UV LD at 283 nm emission was remarkably improved up to 200 kA-cm−2. • Some weak oscillation at 296 nm near the main spontaneous emission peak of 298 nm-band UV LD under injection current of 600 mA (24 V) was observed using pulse width of 50 ns at RT. • The optical-loss was significantly reduced from 22.7 to 8.7 cm−1 and light confinement factor (Г) was remarkably enhanced from 1.77% (Experimental UV LD) to 4.65% (simulated UV LD). [ABSTRACT FROM AUTHOR]

Nonradiative recombination (NRR) processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence (PL) spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature. • Investigates NRR processes in UV-B AlGaN MQW by adding below-gap excitation light. • Detects NRR centers by observing PL-intensity change due to below-gap excitation. • Identifies correlation of T -dependent NRR processes with T -dependent PL peak-energy. • Interpret T -dependent NRR processes by a model based on SRH statistics. [ABSTRACT FROM AUTHOR]