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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]

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]

We compare the growth evolution of polar-plane-free faceted GaN structures on (11 2 ¯ 2) and ( 1 ¯ 1 ¯ 2 2 ¯ ) planes. The crystal morphologies of the three-dimensional (3D) GaN structures depend on surface orientations. To discuss the underlying mechanism, the temporal developments of the cross-sectional shapes during growth are visualized by periodically inserting AlGaN markers. Quantitative analyses using these markers reveal that as the growth proceeds, the growth rates of the top and inclined facets of the 3D GaN on (11 2 ¯ 2) monotonically decrease, whereas those of the 3D GaN on ( 1 ¯ 1 ¯ 2 2 ¯ ) monotonically increase. The opposite tendencies are attributed to the difference in the surface diffusion of adatoms between the top and inclined facets. Furthermore, it is suggested that the surface bond configuration of each crystallographic plane strongly affects the adatom migration, resulting in distinct 3D GaN morphologies on (11 2 ¯ 2) and ( 1 ¯ 1 ¯ 2 2 ¯ ). [ABSTRACT FROM AUTHOR]

Electrically driven polar-plane-free faceted InGaN light-emitting diodes (LEDs) are demonstrated on semipolar ( 1 ¯ 1 ¯ 2 2 ¯ ) GaN substrates. The doping properties on the ( 1 ¯ 1 ¯ 2 2 ¯ ) plane are investigated using planar LED structures. After establishing the doping conditions, three-dimensional faceted LED structures composed of semipolar and nonpolar planes are fabricated by regrowth on the ( 1 ¯ 1 ¯ 2 2 ¯ ) GaN substrates with dielectric masks. The fabricated LEDs exhibit a clear rectifying property and a polychromatic emission derived from facet-dependent emission colors, which can be tuned by altering the mask geometry. These observations suggest that polar-plane-free LED structures may lead to highly radiative polychromatic light emitters. [ABSTRACT FROM AUTHOR]

We establish a calculation method to determine the critical layer thickness of the lattice relaxation in wurtzite heterostructures with arbitrary pairs of growth planes and slip systems. The calculation, which is based on the force balance model, takes the friction force and thermal assistance for the dislocation motion into account. Experimentally, AlxGa1 − xN/AlN heterostructures are fabricated. The established method well reproduces the crystallographic orientations of experimentally observed dislocation lines. [ABSTRACT FROM AUTHOR]

InGaN quantum wells (QWs) are grown at different temperatures on convex lens‐shaped GaN microstructures formed on the (1¯1¯22¯$\text{}$) plane. Microlens QWs grown at different temperatures have centrosymmetric convex lens shapes, and the wavelength spatial distributions within the structures exhibit similar tendencies. However, lowering the growth temperature broadens the wavelength range of room‐temperature cathodoluminescence spectra from the microlens QWs as ≈70 nm for ≈700 °C, ≈100 nm for ≈660 °C, and ≈150 nm for ≈650 °C. Peak wavelength profiles of the two orthogonal lines along [11¯00$\text{}$] and [112¯3¯$\text{}$] indicate that the broader emission bands are mainly due to the significant spatial distribution of the emission wavelength along the [112¯3¯$\text{}$] direction. Because the QW thickness variations are nearly the same along [11¯00$\text{}$] and [112¯3¯$\text{}$], the observed difference in the emission wavelength distributions along those directions is attributed to the difference in In incorporation, which is determined by the competition among the growth rate, crystal orientation, and growth temperature. [ABSTRACT FROM AUTHOR]

A carbon-containing amorphous film is deposited on metalorganic-vapor-phase-epitaxy-grown AlN or AlGaN templates by flowing propane, aluminum, and nitrogen gases at 1010 °C. The deposited ∼1-nm-thick layers show p-type conductivity with a sheet carrier density of ∼1 × 1013 cm−2. When the film is deposited on Mg-doped Al0.8Ga0.2N prior to metallization, it acts as a hole injection layer into p-type AlGaN and shows nearly a double increase in hole current. The transmittance of the deposited layer is ∼90% in the 200–400-nm wavelength range. Hence, the deposited layer is a promising hole injection layer into high-Al-content p-type AlGaN. [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]

• Semipolar AlN films are grown on 15°-off (0 0 0 1), (1 1 1 ¯ 0 2), and (1 1 2 ¯ 2) AlN substrates. • A high growth pressure realizes pit-free semipolar surfaces, unlike the (0 0 0 1) growth. • The surface pits are not due to threading dislocations in the substrates. • The semipolar surfaces after pit-elimination are atomically smooth. • Strong and narrow near band edge emissions dominate the semipolar AlN emission. Semipolar AlN homoepitaxial films, which are expected to act as underlying layers of highly efficient light emitters, are fabricated on 15°-off (0 0 0 1), (1 1 ¯ 0 2), and (1 1 2 ¯ 2) AlN substrates using the metalorganic vapor phase epitaxy method. In conventional (0 0 0 1) AlN growth, low reactor pressures are preferred to enhance the migration of Al adatoms and to suppress parasitic reactions between trimethylaluminum and ammonia. In contrast, low-pressure growth generates numerous pits on the surface of semipolar AlN grown homoepitaxially, which are derived from defects formed in the initial growth stage. Herein we experimentally demonstrate that higher-pressure growth can drastically decrease the pit density. A higher-pressure growth realizes atomically smooth surfaces, strong near-band-edge emissions with narrow line widths (∼ 1–2 meV), and well-suppressed deep level emissions. The optimal reactor pressure to eliminate pits is 500 Torr in terms of the growth rate and nucleation density. [ABSTRACT FROM AUTHOR]

Spontaneously integrated multicolor InGaN micro‐light‐emitting diodes with a microstripe topography achieve selective electrical operation with four different color emissions: green, bluish‐green, bluish‐purple, and purple. The entire microstripe displays broadband emissions because the stripe topography with a convex lens‐like cross section induces a continuous emission wavelength gradient in the overgrown InGaN active layers. Since each wavelength component is distributed along the stripe direction, multiple narrow p‐electrodes are positioned side by side across the stripe width. This design supports selective current injection into the corresponding local areas. This work represents an important step for realizing broadband visible light emitters where individual wavelength components comprising the broad spectra can be modulated and switched independently. [ABSTRACT FROM AUTHOR]

High‐resolution, low‐excitation photoluminescence (PL) spectroscopy is performed for unintentionally doped, silicon‐doped, and magnesium‐doped homoepitaxial aluminum nitride (AlN) films, using a wavelength‐tunable high‐repetition‐rate laser. The wavelength‐tunable laser is used to distinguish between the luminescence and scattering signals from AlN. Providing the high‐resolution, low‐excitation PL spectra, the current understanding of the deep‐ultraviolet luminescence properties of AlN is reviewed and potential assignments for the unknown luminescence lines and bands are discussed. Although previous studies have led to a consensus on the origins of some emission peaks and bands such as the neutral silicon donor‐bound exciton transition and free exciton transitions involving longitudinal optical phonons, it is shown that many of the emission peaks are still unidentified. The origins of all the emission peaks should be elucidated to enable control of the electronic and optoelectronic properties of AlN. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the tunability of band offsets in gallium arsenide/zinc selenide (ZnSe)(001) heterovalent heterostructures. Nucleation of gallium arsenide on ZnSe; Valence band offsets; Conclusions.

Presents information on a study conducted which examined by way of metalorganic vapor phase epitaxy, the nucleation processes of ZnSe grown on GaAs(001). How the growth of SnSe begins; Information on the ZnSe-GaAs heterovalent heterostructure; Method used to prepare the GaAs surfaces; Examination of the ZnSe surfaces by AFM in air immediately after growth; Methodology of the study; Results of the study.

Presents a study on deep levels in p-ZnSe in connection with the molecular beam epitaxy (MBE). Location of the hole trap level; Observation of electron traps in metalorganic MBE (MOMBE) and metalorganic vapor phase epitaxy (MOVPE)-grown samples; Illustration of findings; Information on results.

Investigates the tunability of band offsets in ZnSe/GaAs heterovalent heterostructures grown by metalorganic vapor phase epitaxy. Control of interface composition by means of zinc or selenium treatment; Structural models responsible for the gallium-rich interface; Tunability of the band offsets at the interface.

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]

Multi-wavelength visible light emitters play a crucial role in current solid-state lighting. Although they can be realized by combining semiconductor light-emitting diodes (LEDs) and phosphors or by assembling multiple LED chips with different wavelengths, these design approaches suffer from phosphor-related issues or complex assembly processes. These challenges are significant drawbacks for emerging applications such as visible light communication and micro-LED displays. Herein we present a platform for tailored emission wavelength integration on a single chip utilizing epitaxial growth on flexibly-designed three-dimensional topographies. This approach spontaneously arranges the local emission wavelengths of InGaN-based LED structures through the local In composition variations. As a result, we demonstrate monolithic integration of three different emission colors (violet, blue, and green) on a single chip. Furthermore, we achieve flexible spectral control via independent electrical control of each component. Our integration scheme opens the possibility for tailored spectral control in an arbitrary spectral range through monolithic multi-wavelength LEDs. [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]

Multi-wavelength visible light emitters play a crucial role in current solid-state lighting. Although they can be realized by combining semiconductor light-emitting diodes (LEDs) and phosphors or by assembling multiple LED chips with different wavelengths, these design approaches suffer from phosphor-related issues or complex assembly processes. These challenges are significant drawbacks for emerging applications such as visible light communication and micro-LED displays. Herein we present a platform for tailored emission wavelength integration on a single chip utilizing epitaxial growth on flexibly-designed three-dimensional topographies. This approach spontaneously arranges the local emission wavelengths of InGaN-based LED structures through the local In composition variations. As a result, we demonstrate monolithic integration of three different emission colors (violet, blue, and green) on a single chip. Furthermore, we achieve flexible spectral control via independent electrical control of each component. Our integration scheme opens the possibility for tailored spectral control in an arbitrary spectral range through monolithic multi-wavelength LEDs. [ABSTRACT FROM AUTHOR]

The crystal growth and optical properties of semipolar ( [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]

Optical spectroscopy is performed for c-plane homoepitaxial aluminum nitride (AlN) films. The temperature dependence of the polarization-resolved photoluminescence spectra reveals the exciton fine structure. The experimental results demonstrate that the electron-hole exchange interaction energy (j) in AlN is j=6.8 meV, which is the largest value for typical III-V and II-VI compound semiconductors. We propose the effective interatomic distance as the criterion of the electron-hole exchange interaction energy, revealing a universal rule. This study should encourage potential applications of excitonic optoelectronic devices in nitride semiconductors similar to those using II-VI compound semiconductors. [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]

Metal–organic vapor phase epitaxy of GaN on sapphire (0001) substrates without using low‐temperature (LT) buffer layers is demonstrated. The growth of GaN is achieved by pretreatment of sapphire with trimethylaluminum (TMA) at a high temperature (1050 °C) in a H2 + N2 gas mixture. The TMA pretreatment forms AlN, which acts as nucleation seeds for the subsequent growth of GaN at the same temperature. When AlN created by the TMA pretreatment is three‐dimensional, similar to conventional LT buffer layers, the GaN layers exhibit good structural properties such as atomically smooth surfaces and narrow X‐ray diffraction line widths, comparable to those of GaN on LT buffer layers. In addition, the growth evolution of GaN on TMA‐pretreated sapphire is similar to that on GaN or AlN LT buffer layers. These similarities between the TMA pretreatment and conventional LT buffer‐layer technologies might offer an opportunity to further generalize the heteroepitaxy growth model of GaN. [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 investigated the effect of growth parameters for obtaining high-quality AlN grown directly on sapphire substrates by a hybridized method, derived from simultaneous source supply and conventional migration-enhanced epitaxy. At an optimal growth temperature of 1200°C, AlN was atomically smooth and pit-free, while below and above 1200°C, AlN was rough and with pits, respectively. Surface morphologies also depended on the V/III ratio. Rough surfaces became atomically smooth but then pits appeared, as the V/III ratio increased. The crystallinity revealed by X-ray diffraction changed accordingly. The 600-nm-thick AlN grown under the optimal conditions showed X-ray line widths of as narrow as ∼43 and ∼250arcsec for (0002) and (101¯2) diffractions, respectively. [Copyright &y& Elsevier]

AlN layers were grown directly on sapphire (0001) substrates using three different growth sequences based on metal-organic vapor phase epitaxy with an emphasis on initial nucleation processes. These three methods were simultaneous, alternating supply of aluminum and nitrogen sources, and a combination of the two. In all the methods, nucleation was initiated by three-dimensional (3D) islands with a typical diameter of ∼20 nm. Enhanced migration by the alternating source supply caused highly 3D AlN ridge structures at the sapphire molecular steps. These ridge structures prevented a flattened AlN surface and, in addition, moderated lattice relaxation, suggesting the importance of controlling the initial nucleation in determining the film’s properties. In fact, the hybridized method, derived from the simultaneous and alternating supply methods, was able to control the initial nucleation, and provided the best film quality; the 600-nm-thick AlN grown by this method had an atomically flat surface free of pits and particles, and the x-ray diffraction line widths were ∼45 and ∼250 arcsec for the (0002) and (1012) planes, respectively. [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]

Hexagonal GaN (h-GaN) layers are grown by metalorganic vapor-phase epitaxy on GaAs{11n}A and B (n = 8, 4, 3, 2, 1) substrates using AlAs intermediate layers. The best quality of h-GaN is obtained on (1&1macr;4)B, where the crystallographic relationship is found from a pole figure to be h-GaN{0001}||GaAs(3&3macr;5)B and h-GaN{10&1macr;2}||GaAs(001). We propose a simple model that explains why such a crystallographic relationship is easily realized on {114} resulting in the superior structural and optical properties. Furthermore, from a comparison between the growth on the A and B substrates, it is pointed out that the polarity is a key factor in determining the crystallographic properties. © 2001 American Institute of Physics. [DOI: 10.1063/1.1426275]. [ABSTRACT FROM AUTHOR]

Radiative and nonradiative recombination processes are investigated in the temperature range from 10 to 500 K for AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates. Time‐integrated photoluminescence (PL) spectroscopy under selective excitation conditions demonstrates that the decrease in the radiative recombination efficiency with increasing temperature is one of the causes of the thermal droop in AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes. Time‐resolved PL spectroscopy indicates that not only the decreasing nonradiative recombination lifetime but increasing radiative recombination lifetime with increasing temperature contributes to the thermal droop. The temperature dependence of the radiative recombination lifetime is discussed, revealing that luminescence linewidth is a valuable criterion for designing efficient AlGaN‐based DUV emitters. [ABSTRACT FROM AUTHOR]

Among the III-nitride semiconductors, InxGa1-xN is a key material for visible optical devices such as light-emitting diodes (LEDs), laser diodes, and solar cells. Light emission is achieved via electron-hole recombination within the InxGa1-xN layer. When InxGa1-xN-based blue LEDs were first commercialized, the high probability of electron-hole radiative recombination despite the presence of numerous threading dislocations was a mystery. Extensive studies have proposed that carrier localization in nanoscopic potential fluctuations due, for example, to the immiscibility between InN and GaN or random alloy fluctuations is a key mechanism for the high emission efficiency. In actual LED devices, not only nanoscopic potential fluctuations but also microscopic ones exist within the InxGa1-xN quantum well light-emitting layers. Herein we map the synchrotron radiation microbeam X-ray fluorescence of InxGa1-xN blue LEDs at a sub-micron level. To acquire weak signals of In, Ar, which is in the air and has a fluorescent X-ray energy similar to that of In, is evacuated from the sample chamber by He purge. As a result, we successfully visualize the spatial In distribution of InxGa1-xN layer nondestructively and present good agreement with optical properties. Additionally, we demonstrate that unlike nanoscopic fluctuations, microscopic In compositional fluctuations do not necessarily have positive effects on device performance. Appropriately controlling both nanoscopic and microscopic fluctuations at the same time is necessary to achieve supreme device performance. [ABSTRACT FROM AUTHOR]

Heteroepitaxy of wurtzite semiconductors on corundum substrates is widely used in modern optoelectronic devices, because both crystals belong to the same hexagonal close-packed system. However, the constituent atoms in the wurtzite structure align in an ideal hexagon within the (0001) plane, whereas those in the corundum structure are displaced due to empty octahedral sites. Herein, we demonstrate that this atomic arrangement mismatch at the interface generates low-angle grain boundaries in epilayers, and step bunching of corundum substrates with an even number of molecular layers can eliminate the boundaries. Furthermore, we propose that the weakened epitaxial relationship between epilayers and substrates also eliminates low-angle grain boundaries, which may be useful for practical applications. [ABSTRACT FROM AUTHOR]

Surface plasmons resonance is a promising way to improve the luminescence efficiency of light-emitting materials including InGaN/GaN-based quantum wells; however, it has rarely been used for GaN epilayer with metal nanoparticles (NPs). We demonstrated in enhancing the bandgap emission from GaN epilayer by localized surface plasmon resonance (LSPR) in a quadrupole oscillation mode using Ag NPs fabricated on the GaN. The shapes of the Ag NPs fabricated on the GaN substrates gradually changed over time and were eventually destroyed completely. This should be the reason why previous studies of enhancement of GaN-based materials by using Ag NPs have not performed as good results as expected. This problem was solved by employing oxide thin films to protect the Ag NPs from destruction or aggregation and obtain enhanced photoluminescence of the GaN epilayer. The localized and enhanced electric fields have been calculated by the finite difference time domain (FDTD) method to elucidate the enhancement mechanism. However, it has been still difficult to use this method to reproduce the enhanced emissions caused by the quantum interaction between the surface plasmons and excitons. We reproduced the highly efficient emissions attributed to the Purcell effect by evaluating the nanoantenna effect using the FDTD method. This technique confirmed that an oxide thin film between the GaN substrate and Ag NPs played an effective role in the local enhancement of the electromagnetic field and light extraction from the LSPR mode when several types of oxide thin films were considered. [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]

This letter describes integration of GaN with Si using a AuGe alloy as a bonding material. GaN is first grown on GaAs and then GaN/GaAs/AuGe/Si and GaAs/GaN/AuGe/Si structures are fabricated by wafer bonding. For the latter structure, the GaAs substrate is removed by mechanical and chemical etching. From the current-voltage measurements of both structures, it is found that the bonded interfaces do not obstruct the carrier transport. Furthermore, the optical reflection measurements reveal that AuGe works well as a mirror, which is a suitable characteristic for the integration of GaN light-emitting devices with Si. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This letter describes successful growth of single-phase hexagonal GaN (h-GaN) layers on cubic GaAs(001) nominally singular substrates with the assistance of thin AlAs intermediate layers. The crystallographic relationship between h-GaN and GaAs is extracted from a pole figure to be h-GaN[0001]||GaAs[001] in the growth direction and h-GaN[101¯0]||GaAs[11¯0] in the in-plane direction. In a photoluminescence spectrum measured at 20 K, excitonic emission from the h-GaN layer is detected at 3.47 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have observed six-bilayer periodic structures in GaN grown on GaAs(001). The periodicity occurs along the zinc-blende(ZB)-[111]A direction, suggesting that it originates from stacking faults on close-packed planes. GaN grown on GaAs includes both ZB and wurtzite phases as a result of formation of stacking faults and the periodic structures are mostly located between these two crystalline phases. Based on this observation, possible layer stacking sequences are proposed, which are classified as 6H polytypes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reflectance spectroscopy was performed for nonpolar and semipolar bulk aluminum nitride (AlN) substrates under uniaxial stress. The exciton-polariton theory was applied to interpret the reflectivity spectra, while the Bir-Pikus deformation potential theory was used to analyze the stress dependence of exciton transition energies. This approach allowed all the deformation potentials in AlN to be determined without the quasicubic approximation. [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]

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments. [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]

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]

Deep-ultraviolet continuous-wave photoluminescence spectroscopy is performed for nano-polycrystalline diamond (NPD) synthesized by a high pressure high-temperature technique. NPD exhibits clear deep-ultraviolet emissions, which originate from intrinsic excitonic transitions assisted by a momentum-conserving phonon with a photon. Surprisingly, the peak emission energy is about 30 meV higher than that of the single-crystalline diamond. Raman scattering spectroscopy indicates that the energy difference should originate from the excitonic properties of the NPD and not the phonon. Hence, NPD has a large bandgap compared to single-crystalline diamond. [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]

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]

We propose an array of submicrometer mirrors to assess luminescent nano-objects. Micromirror arrays (MMAs) are fabricated on Si (001) wafers via selectively doping Ga using the focused ion beam technique to form p-type etch stop regions, subsequent anisotropic chemical etching, and Al deposition. MMAs provide two benefits: reflection of luminescence from nano-objects within MMAs toward the Si (001) surface normal and nano-object labeling. The former increases the probability of optics collecting luminescence and is demonstrated by simulations based on the ray-tracing and finite-difference time-domain methods as well as by experiments. The latter enables different measurements to be repeatedly performed on a single nano-object located at a certain micromirror. For example, a single InGaN/GaN nanocolumn is assessed by scanning electron microscopy and microphotoluminescence spectroscopy. [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]