The surface plasmon (SP) resonance was used to increase the emission efficiencies toward high efficiency light-emitting diodes (LEDs). We obtained the enhancements of the electroluminescence from the fabricated plasmonic LED device structure by employing the very thin p+-GaN layer. The further enhancements should be achievable by optimization of the metal and device structures. Next important challenge is to extend this method from the visible to the deep UV region. By using Aluminum, we obtained the enhancements of emissions at ~260 nm from AlGaN/AlN quantum wells. We succeeded to control the SP resonance by using the various metal nanostructures. These localized SP resonance spectra in the deep-UV regions presented here would be useful to enhance deep UV emissions of super wide bandgap materials such as AlGaN/AlN. We believe that our approaches based on ultra-deep UV plasmonics would bring high efficiency ultra-deep UV light sources.
The effects of the structure design of AlGaN-based quantum wells (QWs) on the optical properties are discussed. We demonstrate that to achieve efficient emission in the germicidal wavelength range (250 – 280 nm), AlxGa1−xN QWs in an AlyGa1−yN matrix (x < y) is quite effective, compared with those in an AlN matrix: Time-resolved photoluminescence and cathodoluminescence spectroscopies show that the AlyGa1−yN matrix can enhance the radiative recombination process and can prevent misfit dislocations, which act as non-radiative recombination centers, from being induced in the QW interface. As a result, the emission intensity at room temperature is about 2.7 times larger for the AlxGa1−xN QW in the AlyGa1−yN matrix than that in the AlN matrix. We also point out that further reduction of point defects is crucial to achieve an even higher emission efficiency.
We propose to use quantum wires (QWRs) instead of quantum wells (QWs) to improve the internal quantum efficiency of AlGaN UV emitters. Crystal growth of AlGaN on the AlN vicinal (0001) surface with bunched steps creates Al-less AlGaN QWRs at the bunched step edges. Cathodoluminescence maps indicate the formation of the potential minima along the step edges. Photoluminescence spectroscopy reveals that the thermal quenching in the QWRs is suppressed by approximately one order of magnitude, compared with that in conventional (0001) AlGaN/AlN QWs, and the spectra are dominated by the QWR emissions at room temperature. We attribute the superior optical property of the AlGaN QWRs to the enhanced radiative recombination processes.
Various semipolar AlGaN/AlN quantum wells (QWs) with atomically smooth surfaces and abrupt interfaces are fabricated on AlN bulk substrates. While keeping the crystal qualities, we can easily adjust the Al compositions in the semipolar AlGaN QWs by changing growth temperatures, similar to the case of conventional (0001) AlGaN QWs. Due to the small internal electric fields in the semipolar QWs, the energy fluctuations are extremely suppressed and the radiative recombination lifetimes are drastically shortened, compared with the (0001) QWs. The shorter radiative recombination lifetimes realize much stronger emissions from the semipolar QWs.
We describe the optical properties of semi/non-polar InGaN and AlGaN quantum wells. In semipolar (11¯22) InGaN QWs, spatially uniform but spectrally broad emissions are observed. This finding is interpreted with consideration of the exciton migration length shortened by the fast radiative recombination lifetime due to the reduced electric field. Non/semipolar AlGaN QWs are also fabricated. In the semipolar (1¯102) AlGaN QWs, the radiative recombination lifetimes faster than that in the (0001) QW are confirmed experimentally. As a consequence, much stronger emission is achieved from the semipolar AlGaN QWs at room temperature
Faceted three-dimensional (3D) AlGaN/AlN multiple quantum wells (MQWs) with semipolar {1 ̄101} and polar (0001)
planes are fabricated by a regrowth technique based on metalorganic vapor phase epitaxy (MOVPE) on trench-patterned
AlN templates. Similar 3D microfacet structures with different height are formed on top of and at the bottom of the AlN
trench. Cathodoluminescence (CL) spectra are separately acquired at semipolar and (0001) facet QWs at room
temperature (RT). The peak energies of {1 ̄101} facet QWs and (0001) facet QWs on higher 3D structures are 5.42 and
5.43 eV, respectively, while that of (0001) facet QWs on lower 3D structures is 5.23eV. Through structural analyses
using transmission electron microscopy (TEM), the peak energy difference between the {1 ̄101} QWs and the lower
(0001) QWs is ascribed mainly to suppressed internal electric fields in the {1 ̄101} facet QWs. Furthermore, Al spatial
distribution causes the peak energy difference between the (0001) facet QWs.
Monolithic multi-color light-emitting diodes (LEDs) based on micro-structured InGaN/GaN quantum wells are
demonstrated. The microstructure is created through regrowth on SiO2 mask stripes along the [1¯100] direction
and consists of (0001) and {11¯22} facets. The LEDs exhibit polychromatic emission, including white, due to the
additive color mixture of facet-dependent emission colors. Altering the growth conditions and mask geometry
easily controls the apparent emission color. Simulations predict high light extraction efficiencies due to their
three-dimensional structures. Furthermore, we demonstrate that the apparent emission colors can externally be
controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000
K to blue along the Planckian locus. The controllability relies on the facet-dependent polychromatic emissions;
the pulsed current operation with the appropriate duties varies their relative intensities and the consequent
apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to
green variation. The proposed LEDs can be fabricated through a simple process and, therefore, may be a key
device for advanced solid-state lighting.
We measure gain spectra for commercial (Al,In)GaN laser diodes with peak gain wavelengths of 470 nm, 440 nm,
405 nm, and 375 nm, covering the spectral range accessible with electrical pumping. For this systematic study we
employ the Hakki-Paoli method, i.e. the laser diodes are electrically driven and gain is measured below threshold
current densities. The measured gain spectra are reasonable for a 2D carrier system and understandable when
we take into account homogeneous and inhomogeneous broadening. While inhomogeneous broadening is almost
negligible for the near UV laser diode, it becomes the dominant broadening mechanism for the longer wavelength
laser diodes. We compare the gain spectra with two models describing the inhomogeneous broadening. The first
model assumes a constant carrier density, while the second model assumes a constant quasi Fermi level. Both
are in agreement with the experimental gain spectra, but predict very different carrier densities. We see our
measurements as providing a set of standard gain spectra for similar laser diodes covering a wide spectral range
which can be used to develop and calibrate theoretical manybody gain simulations.
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