High-efficiency green light emission from InGaN/GaN using localized surface plasmon resonance tuned by combination of Ag nanoparticles and dielectric thin film.

We achieved significant enhancements in green light emission (550 nm) from InGaN/GaN quantum wells (QWs) by tuning the localized surface plasmon resonance (LSPR) of self-assembled Ag nanoparticles (NPs) through the application of a SiO2 thin film. The LSPR wavelength of Ag NPs was shifted towards shorter wavelengths by 80 nm using a 5 nm SiO2 layer to separate Ag NPs from GaN surface, thereby aligning it effectively with the green region. This strategic placement of Ag NPs and a 5 nm SiO2 film resulted in significant enhancements of photoluminescence (PL) by 15- and 8.8-fold with 5 and 11 nm GaN cap layers, respectively. The LSPR of Ag NPs on a SiO2 thin film facilitated a longer possible distance for the coupling between surface plasmons (SPs) and excitons in a QW. Traditionally, the distance between SPs-generating metal and a QW has been maintained at 10 nm to achieve substantial enhancements. Remarkably, even with a 25 nm cap layer, Ag NPs on a 5 nm SiO2 film boosted PL by 3.1-fold. The enhancements attributable to Ag NPs on SiO2 films were superior, reaching up to 4.8 times greater than those of Ag NPs on GaN surfaces. Additionally, the PL enhancement factors calculated using the finite differential time domain (FDTD) method aligned closely with experimental results.

Flexible topographical design of light-emitting diodes realizing electrically controllable multi-wavelength spectra.

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.

Microscopic origin of thermal droop in blue-emitting InGaN/GaN quantum wells studied by temperature-dependent microphotoluminescence spectroscopy.

To elucidate the microscopic origin of the thermal droop, a blue-emitting indium gallium nitride (InGaN) quantum well grown on epitaxially laterally overgrown gallium nitride was investigated using temperature-dependent microphotoluminescence spectroscopy. Below 300 K, the sample exhibited a well-known dislocation-tolerant luminescence behavior. However, as temperature increases from 300 K to 500 K, the near band-edge emission at the wing region (with lower threading dislocation densities) was stronger than that at the seed region (with higher threading dislocation densities), indicating that threading dislocations are the microscopic origin of the thermal droop. Considering the carrier diffusion length, edge-type threading dislocations should play a major role in the thermal droop of heteroepitaxially grown InGaN-based LEDs.

Impact of microscopic In fluctuations on the optical properties of InxGa1-xN blue light-emitting diodes assessed by low-energy X-ray fluorescence mapping using synchrotron radiation.

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.

Environmentally friendly method to grow wide-bandgap semiconductor aluminum nitride crystals: Elementary source vapor phase epitaxy.

Aluminum nitride (AlN) has attracted increasing interest as an optoelectronic material in the deep ultraviolet spectral range due to its wide bandgap of 6.0 eV (207 nm wavelength) at room temperature. Because AlN bulk single crystals are ideal device substrates for such applications, the crystal growth of bulky AlN has been extensively studied. Two growth methods seem especially promising: hydride vapor phase epitaxy (HVPE) and sublimation. However, the former requires hazardous gases such as hydrochloric acid and ammonia, while the latter needs extremely high growth temperatures around 2000 °C. Herein we propose a novel vapor-phase-epitaxy-based growth method for AlN that does not use toxic materials; the source precursors are elementary aluminum and nitrogen gas. To prepare our AlN, we constructed a new growth apparatus, which realizes growth of AlN single crystals at a rate of ~18 µm/h at 1550 °C using argon as the source transfer via the simple reaction Al + 1/2N2 → AlN. This growth rate is comparable to that by HVPE, and the growth temperature is much lower than that in sublimation. Thus, this study opens up a novel route to achieve environmentally friendly growth of AlN.

Grain size dependence of surface plasmon enhanced photoluminescence.

Photoluminescence (PL) in the InGaN quantum well based light-emitting diodes (LED) is greatly mediated through the coupling with the Surface Plasmons (SPs) at the interface of the sputtered Ag film. SPs coupled PL is independently tuned through controlling the grain size of the sputtered Ag films. The grain size of ~50 nm exhibits the maximum light extraction efficiency (LEE) at the wavelength of 460 nm. This grain size agrees with the periodic lattice constant of the grating structure in the calculation, where the momentum mismatch between the SPs and the radiative light can be compensated.

Micromirror arrays to assess luminescent nano-objects.

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.