Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

AlGaN-based materials own direct transition energy bands and wide bandgap and thus can be used in high-efficiency ultraviolet (UV) emitters and detectors. Over the past two decades, AlGaN-based materials and devices experienced rapid development. Deep ultraviolet AlGaN-based light-emitting diodes (LEDs) with improved efficiency of 20.3% (at 275 nm) have been produced. An electron beam (EB)-pumped AlGaN-based UV light source at 238 nm, output power of 100 mW, and power conversion efficiency (PCE) of 40% has also been fabricated. UV stimulated emission from AlGaN multiple-quantum-wells laser diodes (LDs) using electrical pumping at room temperature has also been achieved at a wavelength of 336 nm. Compared with GaN-based blue and green LEDs and LDs, the efficiency of AlGaN-based UV LEDs and LDs is lower. Further optimization and improvements in both structure and fabrication are required to realize high-performance devices. In AlGaN-based UV photodetectors (PDs), gain as high as 104 orders of magnitude has been reported using the separated absorption and multiplication region avalanche photodiode structure but is still far from detecting the weak signal, and thus UV single-photon detectors with high detectivity is challenging. Recently, there has been extensive work in the nonlinear optical properties of AlGaN and AlGaN-based passive devices, such as waveguides and resonators. However, how to minimize the scattering and defect-related absorption needs to be further studied. In this review, first, approaches used to grow an AlGaN epilayer and p-type doping are introduced. Second, progress in AlGaN-based UV LEDs, EB-pumped light sources, LDs, PDs, passive devices, and the nonlinear optical properties are presented. Finally, an overview of potential future trends in AlGaN-based materials and UV devices is given.

AlGaN photonics: recent advances in materials and ultraviolet devices

This paper reports the synthesis and detailed characterization of graphite thin films produced by thermal decomposition of the (0001) face of a 6H-SiC wafer, demonstrating the successful growth of single crystalline films down to approximately one graphene layer. The growth and characterization were carried out in ultrahigh vacuum (UHV) conditions. The growth process and sample quality were monitored by low-energy electron diffraction, and the thickness of the sample was determined by core level x-ray photoelectron spectroscopy. High-resolution angle-resolved photoemission spectroscopy shows constant energy map patterns, which are very sharp and fully momentum-resolved, but nonetheless not resolution limited. We discuss the implications of this observation in connection with scanning electron microscopy data, as well as with previous studies.

Synthesis and characterization of atomically-thin graphite filmson a silicon carbide substrate,

In traditional optoelectronic approaches, control over spin, charge, and light requires the use of both electrical and magnetic fields. In a spin-polarized light-emitting diode (spin-LED), charges are injected, and circularly polarized light is emitted from spin-polarized carrier pairs. Typically, the injection of carriers occurs with the application of an electric field, whereas spin polarization can be achieved using an applied magnetic field or polarized ferromagnetic contacts. We used chiral-induced spin selectivity (CISS) to produce spin-polarized carriers and demonstrate a spin-LED that operates at room temperature without magnetic fields or ferromagnetic contacts. The CISS layer consists of oriented, self-assembled small chiral molecules within a layered organic-inorganic metal-halide hybrid semiconductor framework. The spin-LED achieves ±2.6% circularly polarized electroluminescence at room temperature.

Chiral-induced spin selectivity enables a room-temperature spin light-emitting diode

Lithium niobate (LN), dubbed by many as the silicon of photonics, has recently risen to the forefront of chip-scale nonlinear optics research since its demonstration as an ultralow-loss integrated photonics platform. Due to its significant quadratic nonlinearity (χ(2)), LN inspires many important applications such as second-harmonic generation (SHG), spontaneous parametric downconversion, and optical parametric oscillation. Here, we demonstrate high-efficiency SHG in dual-resonant, periodically poled z-cut LN microrings, where quasi-phase matching is realized by field-assisted domain engineering. Meanwhile, dual-band operation is accessed by optimizing the coupling conditions in fundamental and second-harmonic bands via a single pulley waveguide. As a result, when pumping a periodically poled LN microring in the low power regime at around 1617 nm, an on-chip SHG efficiency of 250,000%/W is achieved, a state-of-the-art value reported among current integrated photonics platforms. An absolute conversion efficiency of 15% is recorded with a low pump power of 115 μW in the waveguide. Such periodically poled LN microrings also present a versatile platform for other cavity-enhanced quasi-phase-matched χ(2) nonlinear optical processes.

Periodically poled thin-film lithium niobate microring resonators with a second-harmonic generation efficiency of 250,000%/W

Development of planar-integrated microresonators with high quality factors (Q’s) is crucial for nonlinear photonics in a robust chip. Compared with silicon and silicon nitride, aluminum nitride (AlN) features intrinsic quadratic and cubic susceptibilities as well as an enormous band gap (∼6.2 eV), making it ideal for nonlinear optical interactions. However, sputtered polycrystalline AlN is susceptible to scattering and defect-related absorption losses, thereby inducing limited Q-factors. Here, we demonstrate single-crystalline AlN epitaxially grown on sapphire as a novel nonlinear platform for broadband chip-scale frequency comb generation. We fabricate an AlN-on-sapphire microring with a high loaded Q-factor of 1.1 × 106 and achieve a pure broadband Kerr comb with observable spectral lines ranging from ∼145 to 275 THz and a low parametric threshold of ∼25 mW. As crystalline AlN exhibits strong Raman gain, we further investigate the influence of stimulated Raman scattering (SRS) on four-wave mixing (FWM) ...

Integrated High-Q Crystalline AlN Microresonators for Broadband Kerr and Raman Frequency Combs

III-nitride semiconductors have attracted considerable attention in recent years owing to their excellent physical properties and wide applications in solid-state lighting, flat-panel displays, and solar energy and power electronics. Generally, GaN-based devices are heteroepitaxially grown on c-plane sapphire, Si (111), or 6H-SiC substrates. However, it is very difficult to release the GaN-based films from such single-crystalline substrates and transfer them onto other foreign substrates. Consequently, it is difficult to meet the ever-increasing demand for wearable and foldable applications. On the other hand, sp2 -bonded two-dimensional (2D) materials, which exhibit hexagonal in-plane lattice arrangements and weakly bonded layers, can be transferred onto flexible substrates with ease. Hence, flexible III-nitride devices can be implemented through such 2D release layers. In this progress report, the recent advances in the different strategies for the growth of III-nitrides based on 2D materials are reviewed, with a focus on van der Waals epitaxy and transfer printing. Various attempts are presented and discussed herein, including the different kinds of 2D materials (graphene, hexagonal boron nitride, and transition metal dichalcogenides) used as release layers. Finally, current challenges and future perspectives regarding the development of flexible III-nitride devices are discussed.

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

Wurtzite aluminum nitride (AlN) is known to exhibit six Raman-active optic phonons, making it appealing for Raman lasing. Here, we demonstrate continuous-wave Raman lasers with a low threshold and a high slope efficiency in high quality factor AlN-on-sapphire microrings. Stokes radiations around 1.7–1.8 μm and cascaded operation into 1.9–2.0 μm are identified with a telecom pump. Two types of Stokes lights with distinct Raman shifts and polarizations are recorded via selective excitation of corresponding optic phonons, in accordance with the Raman selection rules in AlN. The observed lasing behavior is satisfactorily accounted for by a theoretical analysis. Our results indicate that AlN-on-sapphire should be promising for integrated nonlinear optics.

Integrated continuous-wave aluminum nitride Raman laser

We demonstrate aluminum nitride (AlN) on sapphire as a novel platform for integrated optics. High-confinement AlN microring resonators are realized by adopting a partially etched (pedestal) waveguide to relax the required etching selectivity for exact pattern transfer. A wide taper is employed at the chip end facets to ensure a low fiber-to-chip coupling loss of ~2.8 dB/facet for both transverse-electric (TE) and transverse-magnetic (TM) modes. Furthermore, the intrinsic quality factors (Qint) recorded with a high-resolution linewidth measurement are up to ~2.5 and 1.9 million at telecom band for fundamental TE00 and TM00 modes, corresponding to a low intracavity propagation loss of ~0.14 and 0.2 dB/cm as well as high resonant buildup of 473 and 327, respectively. Such high-Q AlN-on-sapphire microresonators are believed to be very promising for on-chip nonlinear optics.

Aluminum nitride-on-sapphire platform for integrated high-Q microresonators.

Stranski-Krastanov (SK) growth mode is widely adopted for the self-assembled growth of semiconductor quantum dots (QDs), wherein a relatively large critical thickness is essential and a thick wetting layer (WL) is formed beneath the QD layer. In this paper, we report the metal organic vapor phase epitaxy of green InGaN QDs, employing a growth interruption method to decrease the critical thickness and improve the morphology of QDs. The QDs exhibit similar photoluminescence properties with those grown by conventional SK mode, implying the existence of a WL. We experimentally verify that the formation of QDs, whether based on the SK mode or the growth interruption method, conforms to the phase separation theory. However, the density of QDs grown by the interruption method exhibits abnormal dependence on the strain when a quantum well (QW) is inserted beneath the QD layer. Furthermore, the underlying QW not only influences the morphology of the QDs but also plays as a reservoir of electrons, which helps enhance the photoluminescence and the electroluminescence of the QDs. The method of QD growth with improved morphology and luminescence by introducing the QW-QD coupled nanostructure is universally applicable to similar material systems. Furthermore, a 550 nm green light-emitting diode (LED) and a 526 nm superluminescent LED based on the nanostructure are demonstrated.

Hongtao Li

Papers

Integrated High-Q Crystalline AlN Microresonators for Broadband Kerr and Raman Frequency Combs

Van der Waals Epitaxy of III-Nitride Semiconductors Based on 2D Materials for Flexible Applications

Integrated continuous-wave aluminum nitride Raman laser

Aluminum nitride-on-sapphire platform for integrated high-Q microresonators.

Abnormal Stranski-Krastanov Mode Growth of Green InGaN Quantum Dots: Morphology, Optical Properties, and Applications in Light-Emitting Devices.