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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 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]

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]

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]

The crystal growth and optical properties of semipolar ( [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]

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]

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]

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]