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The fabrication and the optical properties of ultrathin GaN quantum wells on AlN (0001) vicinal surfaces with macrosteps are studied. The employed growth condition spontaneously restricts the GaN thickness to one monolayer (ML) on a planar plane. However, because of a higher density of dangling bonds at macrosteps, 2 ML GaN can be stabilized only at step edges, which creates quantum‐wire‐like 1D nanostructures with a dimension of a few nanometers for the lateral confinement. Exciton (carrier) confinement within the 1D GaN nanostructure is predicted theoretically and is confirmed experimentally by strongly polarized photoluminescence with in‐plane twofold symmetry at a wavelength of ≈250 nm at room temperature. This study demonstrates one path to realize quantum confinement within a fine nanoarchitecture composed of nitride semiconductors and might provide clues to the future creation of novel low‐dimensional nanostructures for the UV–vis spectral range. [ABSTRACT FROM AUTHOR]

We fabricated InGaN quantum wells (QWs) on convex lens-shaped GaN microstructures formed on semipolar ( 1 ¯ 1 ¯ 2 2 ¯ ) GaN substrates. Despite the centrosymmetric lens-like shapes, the overgrown InGaN QWs exhibit an anisotropic emission wavelength distribution. This behavior is distinct from the concentric wavelength distribution found in microlens QWs on the polar (0001) plane. The anisotropic distribution is mainly attributed to the In composition variations, which are influenced by the asymmetry of the ( 1 ¯ 1 ¯ 2 2 ¯ ) crystallographic plane. The obtained results demonstrate that the ( 1 ¯ 1 ¯ 2 2 ¯ ) microlens QWs serve as multi-wavelength light emitters and allow us to investigate unstable planes around the ( 1 ¯ 1 ¯ 2 2 ¯ ) plane, which have remained unexplored. [ABSTRACT FROM AUTHOR]

InGaN/GaN quantum wells (QWs) are homoepitaxially grown on (11 2 ¯ 3) GaN substrates, and atomically smooth surfaces and abrupt InGaN/GaN interfaces are achieved. The optical anisotropy is examined using photo- and electroluminescence spectroscopies. Both methods consistently show in-plane optical polarization along the direction parallel to the projection of the c -axis on the QW plane. This polarization direction is rotated by 90 ° from that of unstrained, bulk InGaN, demonstrating the occurrence of polarization switching in strained, semipolar InGaN QWs. Specifically, the polarization direction is [ 1 ¯ 1 ¯ 22], which offers an advantage in that the {1 1 ¯ 00} m -planes can be used as cavity mirrors in edge-emitting lasers. The previously proposed analytical equation reasonably reproduces the experimental results. [ABSTRACT FROM AUTHOR]

Temperature-dependent electroluminescence measurements are performed for 265-nm AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) grown on AlN substrates. The external quantum efficiency (EQE) increases as the temperature decreases from 293 K to 6 K. Using two assumptions, the internal quantum efficiency (IQE) and current injection efficiency (CIE) are unity at the peak EQE at 6 K and the light extraction efficiency is independent of current and temperature, the current and temperature dependences of the product (IQE × CIE) are derived. The temperature dependence of the EQE cannot be simply explained by the Auger recombination processes. This observation enables the CIE and IQE to be separately extracted by rate equation analysis. The room-temperature EQE of the AlGaN-based DUV LEDs is limited by the CIE and not the IQE. We propose that the relatively low CIE may originate from the nonradiative recombination process outside quantum-well layers. [ABSTRACT FROM AUTHOR]

Broadband ultraviolet (UV) emission is achieved using AlGaN microstructure 2D arrays. 2D arrays of trenches are initially formed on AlN templates on sapphire (0001) substrates. AlGaN‐based quantum wells (QWs) are subsequently regrown on top of the patterned templates. Bunched steps are formed within the trench, inducing variations in the Al composition and AlGaN thickness in the QWs. Regions with and without bunched steps coexist and cause emission wavelength variations. The formation mechanism of bunched steps is attributed to the position‐dependent AlN growth rate due to variations of the source–precursor flow within narrow trenches. [ABSTRACT FROM AUTHOR]

The impact of a face-to-face annealed sputtered AlN/sapphire (FFA Sp-AlN) template with threading-dislocation densities (TDDs) of 2 × 108 cm−2 and an n-type AlGaN (n-AlGaN) underlayer on optical properties of AlGaN multiple quantum wells (MQWs) with an ultraviolet C (UVC) emission is investigated comprehensively. For comparison of the FFA Sp-AlN template with low TDDs, a conventional MOVPE (metalorganic vapor phase epitaxially)-grown AlN/sapphire (MOVPE-AlN) template with TDDs of 1 × 109 cm−2 was prepared. Consequently, cathodoluminescence (CL), temperature-dependent photoluminescence (PL), and time-resolved PL (TR-PL) measurements verified that both the FFA Sp-AlN template and n-AlGaN underlayer are indispensable for obtaining MQWs with high internal quantum efficiencies, which decrease the TDDs and point defect (PD) densities. Our results revealed that 10-period quantum wells (10QWs)/n-AlGaN/AlN grown on the FFA Sp-AlN template exhibit a lower dark spot density in CL panchromatic intensity maps, a higher integrated emission intensity ratio from the temperature-dependent PL (from 15 to 300 K), and a longer nonradiative lifetime from the TR-PL measurements at 300 K compared with those grown on the MOVPE-AlN template. Moreover, we found that the optical properties of 10QWs/AlN in FFA Sp-AlN and MOVPE-AlN templates do not exhibit a significant difference because of the existence of numerous PDs. Our experimental results demonstrate the favorable impact of the FFA Sp-AlN template for low-TDDs and the n-AlGaN underlayer for low-PDs, which holds promise for highly efficient AlGaN deep-ultraviolet light-emitting devices. [ABSTRACT FROM AUTHOR]

Coupling between surface plasmons (SPs) and excitons can be used to enhance the emission efficiencies of light-emitting materials and devices. This approach had been theoretically predicted and, in 2004, was experimentally demonstrated by our group for enhancing the visible emission from InGaN/GaN quantum wells (QWs). Exciton–SP coupling increases the spontaneous emission rates of the excited states, causes a relative reduction in nonradiative relaxation, and ultimately increases the internal quantum efficiencies (IQEs) of such devices. Here, we present a brief history of the increases in emission efficiency that have been achieved and the underlying mechanism thereof. This method has the potential to enable the development of high-efficiency light-emitting diodes (LEDs), eventually leading to the replacement of fluorescent lights with solid-state light sources. After the initial discovery of this phenomenon, many device structures were proposed and reported; however, their emission efficiencies have thus far remained insufficient for practical application. Here, we also present recent progress on device applications and the current problems that must be solved. Finally, we explain the future possibilities regarding the extension of SP-enhanced light emission over a broader wavelength region, from the deep ultraviolet (UV) to the infrared (IR). [ABSTRACT FROM AUTHOR]

We compare metalorganic vapor phase epitaxy of InGaN/GaN heterostructures on semipolar (1122) and (1122) GaN bulk substrates. In incorporation efficiency is higher for (1122) InGaN, which enables higher temperature growth of InGaN and is beneficial for quality improvement. InGaN/GaN quantum wells (QWs) on (1122) show abrupt interfaces, but those on (1122) tend to form three-dimensional nanofacets. Differences in growth temperature and structures of the (1122) and (1122) QWs cause higher internal quantum efficiencies of the (1122) [(1122)] QWs at shorter (longer) wavelengths. [ABSTRACT FROM AUTHOR]

The optical polarization properties in nonpolar (1100) AlxGa1-xN/AlN quantum wells (QWs) are investigated over the full range of Al compositions. The k · p calculation predicts that the lowest energy optical transition in (1100) AlxGa1-xN layers pseudomorphically grown on unstrained AlN is dipole allowed for the electric field vector parallel to the [00011 direction, and that the degree of polarization is hardly affected by the AlxGai1-xN QW width regardless of the Al composition. Experimentally, AlxGa1-xN/AlN QWs (0 ⩽ x ⩽ 0.81) are homoepitaxially grown on AlN substrates. Photoluminescence spectroscopy reveals that the emissions of all QWs, including GaN/AlN QWs, are strongly polarized along the [0001] direction. This result agrees with the theoretical prediction. [ABSTRACT FROM AUTHOR]

Ultraviolet light sources, represented by excimer and mercury lamps, are currently used for various applications, including water purification/sterilization, biotechnology, photolithography and surface modification. However, they have the disadvantages of limited portability, low emission efficiency and the presence of harmful constituents. Finding a compact, efficient and environmentally friendly alternative ultraviolet light source is therefore of considerable technological interest. Aluminium-nitride-based semiconductors show promise as materials for this purpose, but because of difficulties in controlling electronic conductivity, in light-emitting diodes are hampered by low external quantum efficiencies. Here, we use an electron-beam pumping technique, demonstrating an output of 100 mW and a record power efficiency of ∼40% from AlxGa1−xN/AlN quantum wells emitting at ∼240 nm. This achievement is attributed to carrier confinement within the high-quality quantum wells, as well as the appropriate design of sample structures for electron-beam pumping, and may be a milestone in the path to realizing next-generation ultraviolet light sources with great ecological and economic benefits. [ABSTRACT FROM AUTHOR]

We investigate the carrier transport and optical properties of a thick InGaN single quantum well (SQW) where an AlGaN δ-layer is embedded. By way of simulation, it is found that the carrier density distribution in the active region is more uniform in such a QW structure, compared to a double QW (DQW) configuration showing a discontinuity in the hole quasi-Fermi level due to the large effective mass of the holes along with the strong piezoelectric field. Through the photoluminescence (PL) measurements, we have shown that the PL peak energy varies depending sensitively on the δ-layer thickness, providing an extra degree of freedom in the wavelength-tuning control. In particular, such a QW structure is highly desired for long-wavelength emission as the wavelength tuning can be achieved with lower indium composition. The embedded δ-layer also increases the wave function overlap between holes and electrons, thereby shortening the PL lifetime. The results of PL measurements are shown to be consistent with the self-consistent numerical results. A possible application of the proposed QW structure is to the design of long-wavelength light-emitting diodes and laser diodes. [ABSTRACT FROM AUTHOR]

Internal quantum efficiencies (IQEs) as high as 69% were realized at room temperature from AlGaN/AlN quantum wells (QWs) emitting at 247 nm grown by metalorganic vapor phase epitaxy. The extremely high IQEs were achieved by examining the source-supply sequence. QWs fabricated by a continuous source-supply method have longer emission wavelengths (λ) and higher IQEs compared to QWs fabricated by modified migration enhanced epitaxy (MMEE). MMEE is an alternating source-supply method where the NH3 interruption promotes Ga evaporation. Thus, to obtain the same λ, MMEE requires a lower growth temperature than the continuous method, compromising the quality of the AlN and AlGaN layers as well as the IQE of QWs. [ABSTRACT FROM AUTHOR]

We investigate anisotropic lattice relaxation in non-c-plane InGaN/GaN multiple quantum wells (MQWs). Transmission electron microscopy analyses of semipolar [formula] MQWs reveal that lattice relaxation preferentially occurs along the [formula] direction by introducing misfit dislocations (MDs) with a Burgers vector of [formula]. To theoretically describe this anisotropic relaxation phenomenon, we expand the force-balance model, where the competition between the force induced by lattice mismatch and the tension of dislocations determines the motion of dislocations. Furthermore, because MDs are introduced at the interface between the bottom InGaN QW and the underlying GaN, we propose to treat InGaN/GaN MQWs as InGaN single layers with effective In compositions. Applying this structure model to the theoretical calculation of the critical layer thicknesses reproduces well the experimentally observed lattice relaxation. This achievement enables us to design semipolar InGaN/GaN MQW structures without lattice relaxation, thereby realizing higher internal emission quantum efficiencies. [ABSTRACT FROM AUTHOR]

Abstract: We have studied the optical gain and its suppression mechanism of In x Ga1−x N multiple quantum well structures with different indium compositions. The variable stripe length method and the up-conversion method were employed in this research. It was found that the optical gain spectra became broad and the differential gain decreased with increasing the indium composition. These results indicated that inhomogeneous broadening affected not only spontaneous emission but also stimulated emission. The rise time of photoluminescence under a strong excitation condition was changed from 0.95 ps to 12.4 ps depending on the indium composition. Assuming the rise time reflects the relaxation lifetime from the absorption edge energy to population inversion levels, our calculation suggested that long rise times led to gain suppression in In x Ga1−x N laser diodes with larger indium compositions. [Copyright &y& Elsevier]

GaN/AlN ultrathin quantum wells (QWs) emitting in the deep UV spectral range are fabricated by metalorganic vapor phase epitaxy. The GaN thickness is automatically limited to the monolayer (ML) scale due to the balance between crystallization and evaporation of Ga adatoms. This growth characteristic facilitates the fabrication of highly reproducible GaN ML QWs. The strong quantum confinement within the GaN ML QWs achieves emissions below 250 nm. The photoluminescence intensity at room temperature with respect to that at low temperature, which is closely related to the emission internal quantum efficiency, is drastically improved from 0.1% for a conventional Al0.8Ga0.2N/AlN QWs to 5% for a 1 ML GaN/AlN (0001) QW and 50% for a 2 ML GaN/AlN (11¯02) QW under weak excitation conditions. These higher emission efficiencies are attributed to the suppressed nonradiative recombination in the GaN QWs and the enhanced radiative recombination in the (11¯02) QW. [ABSTRACT FROM AUTHOR]

AlxGa1−xN‐based quantum wells (QWs) are fabricated on AlN with macrosteps, which are formed by using vicinal sapphire and AlN (0001) substrates. The QWs on macrosteps (MS‐QWs) show photoluminescence lifetimes of nearly 2 ns at an emission wavelength of 242 nm at room temperature (RT). This is the longest lifetime so far reported for AlxGa1−xN‐based QWs emitting below 270 nm, indicating the suppression of nonradiative recombination, the dominant recombination process at RT. Compared with planar QWs without macrosteps, the emission internal quantum efficiency estimated by photoluminescence spectroscopy is improved approximately by two orders of magnitude under a weak excitation condition. Consequently, AlxGa1−xN‐based MS‐QWs are promising structures for highly efficient ultraviolet emitters. AlxGa1−xN‐based quantum wells (QWs) fabricated on AlN with macrosteps show nonradiative recombination lifetimes of ≈2 ns at 242 nm wavelength at room temperature. This long lifetime indicates suppressed nonradiative recombination processes. Compared with conventional QWs without macrosteps, QWs on macrosteps have the emission efficiency improved approximately by two orders of magnitude, showing promise as highly efficient ultraviolet emitters. [ABSTRACT FROM AUTHOR]

Semipolar [formula] InGaN/GaN quantum wells (QWs) (n = 1-3) are fabricated on top of GaN microstructures, which consist of semipolar [formula] facets. Semipolar planes are obtained via regrowth of three-dimensional structures on (0001) GaN templates under controlled growth conditions. Compared to QWs on [formula] facets, [formula] ridge QWs show an intense emission at ∼440 nm. Time resolved photoluminescence reveals that the radiative lifetime of excitons in {nn01} InGaN ridge QWs at 13 K is 310 ps, which is comparable to that in {1101} QWs. The estimated internal quantum efficiency at room temperature is as high as 57%. [ABSTRACT FROM AUTHOR]