Tailored fabrication of a self-rolled-up AlGaN/GaN tubular structure with photoelectrochemical etching.

Group III-nitride semiconductors with tubular structures offer significant potential across various applications, including optics, electronics, and chemical sensors. However, achieving tailored fabrication of these structures remains a challenge. In this study, we present a novel, to the best of our knowledge, method to fabricate micro-sized tubular structures by rolling the layered membrane of group III-nitride alloys utilizing the photoelectrochemical (PEC) etching. To customize the geometry of the tubular structure, we conducted an analytic calculation to predict the strain and deformation for the layered membrane. Based on the calculations, we designed and fabricated an AlGaN/GaN/InGaN/n-GaN/ sapphire structure using metal-organic chemical vapor deposition (MOCVD). Photolithography and PEC etching were employed to selectively etch the sacrificial InGaN layer. We investigated the changes of optical properties of the rolled-up structure by utilizing micro-photoluminescence (µ-PL) and micro-Raman spectroscopy.

Enhancement of Single-Photon Purity and Coherence of III-Nitride Quantum Dot with Polarization-Controlled Quasi-Resonant Excitation.

III-Nitride semiconductor-based quantum dots (QDs) play an essential role in solid-state quantum light sources because of their potential for room-temperature operation. However, undesired background emission from the surroundings deteriorates single-photon purity. Moreover, spectral diffusion causes inhomogeneous broadening and limits the applications of QDs in quantum photonic technologies. To overcome these obstacles, it is demonstrated that directly pumping carriers to the excited state of the QD reduces the number of carriers generated in the vicinities. The polarization-controlled quasi-resonant excitation is applied to InGaN QDs embedded in GaN nanowire. To analyze the different excitation mechanisms, polarization-resolved absorptions are investigated under the above-barrier bandgap, below-barrier bandgap, and quasi-resonant excitation conditions. By employing polarization-controlled quasi-resonant excitation, the linewidth is reduced from 353 to 272 µeV, and the second-order correlation value is improved from 0.470 to 0.231. Therefore, a greater single-photon purity can be obtained at higher temperatures due to decreased linewidth and background emission.

Influence of wafer quality on chip size-dependent efficiency variation in blue and green micro light-emitting diodes.

We propose a key factor associated with both surface recombination velocity and radiative efficiency of an LED to estimate its chip size-dependent radiative efficiencies. The validity of the suggested factor is verified through experimental comparison between various LED wafers. Efficiencies of micro-LEDs from a blue and two green LED wafers are examined by temperature-dependent photoluminescence experiments. Surface recombination velocities are extracted from chip size dependent time-resolved PL results. Possible explanations on the reason why two green wafers show different properties are also given. With the suggested factor, we can provide more accurate prediction on the chip size-dependent efficiency of an LED wafer.

Nanoscale Focus Pinspot for High-Purity Quantum Emitters via Focused-Ion-Beam-Induced Luminescence Quenching.

Epitaxially grown quantum dots (QDs), especially embedded in photonic structures, play an essential role in various quantum photonic systems as on-demand single-photon sources. However, these QDs often suffer from adjacent unwanted emitters, which contribute to the background noise of the QD emission and fundamentally limit the single-photon purity. In this paper, a nanoscale focus pinspot (NFP) technique using focused-ion-beam-induced luminescence quenching enables us to improve single-photon purity from site-controlled QD as a proof-of-concept experiment. The optical quality of the QD emission is not degraded while the signal-to-noise ratio of the QD is improved. Moreover, the QD after the NFP technique reveals the single-photon nature at further elevated temperatures owing to the reduced background noise. As the NFP technique is nondestructive, it retains the apparent physical structures and photonic functions, thereby indicating its promising potential for applying diverse high-purity quantum emitters, particularly integrated in photonic devices and circuits.

AlGaN Deep-Ultraviolet Light-Emitting Diodes with Localized Surface Plasmon Resonance by a High-Density Array of 40 nm Al Nanoparticles.

We present a remarkable improvement in the efficiency of AlGaN deep-ultraviolet light-emitting diodes (LEDs) enabled by the coupling of localized surface plasmon resonance (LSPR) mediated by a high-density array of Al nanoparticles (NPs). The Al NPs with an average diameter of ∼40 nm were uniformly distributed near the Al0.43Ga0.57N/Al0.50Ga0.50N multiple quantum well active region for coupling 285 nm emission by block copolymer lithography. The internal quantum efficiency is enhanced by 57.7% because of the decreased radiative recombination lifetime by the LSPR. As a consequence, the AlGaN LEDs with an array of Al NPs show 33.3% enhanced electroluminescence with comparable electrical properties to those of reference LEDs without Al NPs.

Three-dimensional hierarchical semi-polar GaN/InGaN MQW coaxial nanowires on a patterned Si nanowire template.

We have demonstrated for the first time the hybrid development of next-generation 3-D hierarchical GaN/InGaN multiple-quantum-well nanowires on a patterned Si nanowire-template. The patterned Si nanowire-template is fabricated using metal-assisted chemical-etching, and the conformal growth of the GaN/InGaN multiple-quantum-well (MQW) coaxial nanowires is conducted using metal-organic-chemical-vapor-deposition by the two-step growth approach of vapor-liquid-solid for the GaN core and vapor-solid for the GaN/InGaN MQW shells. The growth directions of the GaN nanowires are confirmed by transmission electron microscopy and selected area electron diffraction patterns. The emission of the GaN/InGaN MQW nanowire is tuned from 440 nm to 505 nm by increasing the InGaN quantum-well thickness. The carrier dynamics were evaluated by performing temperature-dependent time-resolved photoluminescence measurement, and the radiative lifetime of photogenerated electron-hole pairs was found to range from 30 to 35 ps. A very high IQE of 56% was measured due to the suppressed quantum-confined Stark effect which was enabled by the semi-polar growth facet of the GaN/InGaN MQWs. The demonstration of the growth of the hybrid 3-D hierarchical GaN/InGaN MQW nanowires provides a seamless platform for a broad range of multifunctional optical and electronic applications.

Orthogonally Polarized, Dual-Wavelength Quantum Wire Network Emitters Embedded in Single Microrod.

Semiconductor nanowires are attractive building blocks of optoelectronics due to high efficiency and optical controllability. In particular, the mutual controllability of wavelength and polarization of light is essential for versatile applications such as displays, precise metrology, and bioimaging. We present quantum wire network emitters embedded in a single microrod capable of exhibiting orthogonally polarized dual-wavelength visible light at room temperature. The InGaN/GaN shell layers were grown on a single hexagonal GaN core microrod, spontaneously forming site-selective In-rich InGaN quantum wires on each edge between the nonpolar facets as well as each boundary between the nonpolar and semipolar facets. The orthogonally self-arranged, two sets of six quantum wires formed on the edges and the boundaries showed efficient violet and blue-green color emissions with strong linear polarization parallel and perpendicular to the c-axis at room temperature, respectively. This intriguing emission from a single microrod allows us to mutually manipulate the color and the polarization of light, which would be beneficial for photonic applications with unprecedented controllability and functionality.

Interplay of strain and intermixing effects on direct-bandgap optical transition in strained Ge-on-Si under thermal annealing.

The influence of thermal annealing on the properties of germanium grown on silicon (Ge-on-Si) has been investigated. Depth dependencies of strain and photoluminescence (PL) were compared for as-grown and annealed Ge-on-Si samples to investigate how intermixing affects the optical properties of Ge-on-Si. The tensile strain on thermally annealed Ge-on-Si increases at the deeper region, while the PL wavelength becomes shorter. This unexpected blue-shift is attributed to Si interdiffusion at the interface, which is confirmed by energy dispersive X-ray spectroscopy and micro-Raman experiments. Not only Γ- and L-valley emissions but also Δ2-valley related emission could be found from the PL spectra, showing a possibility of carrier escape from Γ valley. Temperature-dependent PL analysis reveals that the thermal activation energy of Γ-valley emission increases at the proximity of the Ge/Si interface. By comparing the PL peak energies and their activation energies, both SiGe intermixing and shallow defect levels are found to be responsible for the activation energy increase and consequent efficiency reduction at the Ge/Si interface. These results provide an in-depth understanding of the influence of strain and Si intermixing on the direct-bandgap optical transition in thermally annealed Ge-on-Si.

Ultrafast carrier dynamics of conformally grown semi-polar (112[combining macron]2) GaN/InGaN multiple quantum well co-axial nanowires on m-axial GaN core nanowires.

The growth of semi-polar (112[combining macron]2) GaN/InGaN multiple-quantum-well (MQW) co-axial heterostructure shells around m-axial GaN core nanowires on a Si substrate using MOCVD is reported for the first time. The core GaN nanowire and GaN/InGaN MQW shells are grown in a two-step growth sequence of vapor-liquid-solid and vapor-solid growth modes. The luminescence and carrier dynamics of GaN/InGaN MQW coaxial nanowires are studied by photoluminescence, cathodoluminescence, and low temperature time-resolved photoluminescence (TRPL). The emission is tuned from 430 nm to 590 nm by increasing the InGaN QW thickness. The non-single exponential decay measured by low-temperature TRPL was attributed to the indium fluctuations in the InGaN QW. The ultrafast radiative lifetime was measured from 14 ps to 26 ps with different emission wavelengths at a very high internal quantum efficiency up to 68%. An ultrafast carrier lifetime was assigned to the growth of the InGaN QW on semi-polar (112[combining macron]2) growth facet and the improved carrier collection efficiency due to the radial growth of the GaN/InGaN MQW shells. Such an ultrafast carrier dynamics of NWs provides a meaningful active medium for high speed optoelectronic applications.

Optical properties of GaN nanowires grown on chemical vapor deposited-graphene.

Optical properties of GaN nanowires (NWs) grown on chemical vapor deposited-graphene transferred on an amorphous support are reported. The growth temperature was optimized to achieve a high NW density with a perfect selectivity with respect to a SiO2 surface. The growth temperature window was found to be rather narrow (815°C ± 5°C). Steady-state and time-resolved photoluminescence from GaN NWs grown on graphene was compared with the results for GaN NWs grown on conventional substrates within the same molecular beam epitaxy reactor showing a comparable optical quality for different substrates. Growth at temperatures above 820 °C led to a strong NW density reduction accompanied with a diameter narrowing. This morphology change leads to a spectral blueshift of the donor-bound exciton emission line due to either surface stress or dielectric confinement. Graphene multi-layered micro-domains were explored as a way to arrange GaN NWs in a hollow hexagonal pattern. The NWs grown on these domains show a luminescence spectral linewidth as low as 0.28 meV (close to the set-up resolution limit).

Three-dimensional GaN dodecagonal ring structures for highly efficient phosphor-free warm white light-emitting diodes.

Warm and natural white light (i.e., with a correlated colour temperature <5000 K) with good colour rendition (i.e., a colour rendering index >75) is in demand as an indoor lighting source of comfortable interior lighting and mood lighting. However, for warm white light, phosphor-converted white light-emitting diodes (WLEDs) require a red phosphor instead of a commercial yellow phosphor (YAG:Ce3+), and suffer from limitations such as unavoidable energy conversion losses, degraded phosphors and high manufacturing costs. Phosphor-free WLEDs based on three-dimensional (3D) indium gallium nitride (InGaN)/gallium nitride (GaN) structures are promising alternatives. Here, we propose a new concept for highly efficient phosphor-free warm WLEDs using 3D core-shell InGaN/GaN dodecagonal ring structures, fabricated by selective area growth and the KOH wet etching method. Electrically driven, phosphor-free warm WLEDs were successfully demonstrated with a low correlated colour temperature (4500 K) and high colour rendering index (Ra = 81). From our findings, we believe that WLEDs based on dodecagonal ring structures become a platform enabling a high-efficiency warm white light-emitting source without the use of phosphors.

Electrically driven, highly efficient three-dimensional GaN-based light emitting diodes fabricated by self-aligned twofold epitaxial lateral overgrowth.

Improvements in the overall efficiency and significant reduction in the efficiency droop are observed in three-dimensional (3D) GaN truncated pyramid structures fabricated with air void and a SiO2 layer. This 3D structure was fabricated using a self-aligned twofold epitaxial lateral overgrowth technique, which improved both the internal quantum efficiency and the light extraction efficiency. In addition, a reduced leakage current was observed due to the effective suppression of threading dislocations. While this study focuses primarily on the blue emission wavelength region, this approach can also be applied to overcome the efficiency degradation problem in the ultraviolet, green, and red emission regions.