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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]

A GaAs/AlGaAs‐based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. To realize a high output power, an effective way is to increase the electron density at the active region by using a high doping level. This work aims to optimize a design enabling to use a heavy doping level, by realigning the resonant tunneling which can be seriously degraded due to a band bending from this heavy local doping. With variable quantum well and barrier height, the design owns an improved parameter tuning freedoms, and a saturation of peak gain can be achieved at a relatively high doping concentration of 1.0 × 1017 cm−3, which is twice higher than the recently reported device. Nonequilibrium Green's function method including full carriers scattering effect is used to carry out the design optimization, mainly including the realignment of resonant tunneling and suppression of leakages via high‐energy confined levels. In the experiment, a 14 μm thick active layer for this design is grown and also large mesa size is developed. To ensure a quasi‐continuous wave (quasi‐CW) operation, the heat dissipation is also managed. [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]

Flat and high‐quality semipolar AlN layers are crucial to realizing polarization‐reduced deep‐UV optical devices. We investigate the effects of Ga supply during the growth of semipolar AlN on m‐plane sapphire with the aim to produce better surface morphology. Incorporation of Ga into AlN is negligibly small at high‐temperature growth. The surface roughness significantly decreased with increasing the TMGa flow at a constant V/III ratio. [ABSTRACT FROM AUTHOR]

THz (QCLs) are promising large output power semiconductor‐based THz sources with narrow bandwidths and wide operating frequency ranges. However, the operating temperature of THz QCLs is still limited by the design of its active region. Matyas et al. (2012) theoretically predicted that AlGaAs/GaAs THz QCLs with higher optical gain could be obtained by introducing a variable barrier height structure (Matyas et al., J. Appl. Phys. 2012, 111, 103106). Based on the proposed variable barrier QC structure, we fabricate AlGaAs/GaAs three‐well type THz QCLs with an additional very thin second extraction barrier and a low barrier height emission barrier. With this device, we obtain lasing operation at 3.7 THz at a maximum operating temperature of 145 K. [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]

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 a quite low light‐extraction efficiency (LEE). We improved the LEE of DUV‐LEDs by using a superlattice (SL) hole spreading p‐AlGaN contact layer, a dot matrix Ni/Au electrode and an Al reflector. DUV‐LED samples in which the SL p‐AlGaN contact layer has various period lengths, and forms two different p‐type electrodes, a conventional Ni/Au electrode and a dot matrix Ni/Au electrode with an Al reflector are fabricated. By comparing these LED samples, it is confirmed that contact layers with relatively longer periods are suitable both for vertical hole injection and lateral spreading of the holes. By increasing the Al content and recovering the transparency of the SL p‐AlGaN contact layer, the LEE is increased by up to a factor of 1.6. [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 polarized photoluminescence emitted on the edge of a series of aluminum‐rich (Al,Ga)N‐AlN quantum wells (QWs) grown by molecular beam epitaxy on AlN templates deposited by metal organic chemical vapor deposition on c‐plane sapphire is measured. The contrast and the principal axis of the emission diagrams for 2 nm‐thick (Al,Ga)N QWs grown for aluminum compositions between 40% and 90% are studied. The light is emitted on the edge of the QWs at wavelengths going from 280 nm down to 209 nm. The emission diagram, a change from oblate to prolate with respect to the in‐plane orientation, for an aluminum composition is found to occur around 72%, that is, at an emission wavelength of about 235 nm. The orientations and shapes of the edge‐emission diagrams indicate that the fluctuations of the composition of the (Al,Ga)N confining layer are deep enough for producing intravalence band mixings. This property, that acts in concert with the built‐in strain and quantum‐confined Stark effect, contributes to the anisotropy of the light emission when the aluminum composition reaches 60–70%, that is, for an emission wavelength of 260–235 nm. [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]

Deep‐ultraviolet light‐emitting diodes (DUV‐LEDs) find widespread applications in various industries and are a focus area in environmental and life science research. However, the quantum efficiency of DUV‐LEDs is still low because of the unbalanced hole injection, high operation voltage, and low reflectivity of the p‐electrode. In this study, a smart wide‐bandgap omnidirectional reflector (ODR), which simultaneously acts as an effective hole‐injection electrode, is demonstrated for a flip‐chip DUV‐LED. The smart ODR is composed of p‐type AlGaN (p‐AlGaN)/Ni:AlN/Al with a high reflectivity of 94.2% at 280 nm, in which AlN with a theoretically calculated thickness of 40 nm is Ni‐doped using the pulsed electrical breakdown (PEBD) method. The smart ODR‐mounted AlGaN‐multiquantum‐well‐based DUV‐LED exhibits a remarkable wall‐plug efficiency (4.8%) and high external quantum efficiency (8.5%) at a low operation voltage of 9.75 V owing to the fused layer of Ni3N and Ga vacancies formed in the interfacial region of the p‐AlGaN and AlN layers, through the mutual diffusion of Ni and Ga during PEBD. The interface‐mediated Ohmic contact leads to an effective hole injection without reducing the reflectivity, yielding increased optical output and electric input powers. The proposed smart ODR can provide an innovative route for the manipulation of DUV light. [ABSTRACT FROM AUTHOR]

Influence of Undoped-AlGaN Final Barrier of MQWs on the Performance of Lateral-Type UVB LEDs 17 mW semiconducting aluminium gallium nitride (AlGaN)-based ultraviolet-B (UVB) light-emitting diodes (LEDs) at 300 nm emission grown on AlN template are expected to be useful for production of vitamin D3 in the human body as well as for killing of tomato mosaic virus (ToMV) in tomato. UV-light power is improved in AlGaN UVB LED using optimized undoped-AlGaN final barrier of the quantum well. [Extracted from the article]