High-quality epitaxial layers are directly related to internal quantum efficiency. The methods used to design such epitaxial layers are reviewed in this article. The ultraviolet C (UVC) light-emitting diode (LED) epitaxial layer structure exhibits electron leakage; therefore, many research groups have proposed the design of blocking layers and carrier transportation to generate high electron–hole recombination rates. This also aids in increasing the internal quantum efficiency. The cap layer, p-GaN, exhibits high absorption in deep UV radiation; thus, a small thickness is usually chosen. Flip chip design is more popular for such devices in the UV band, and the main factors for consideration are light extraction and heat transportation. However, the choice of encapsulation materials is important, because unsuitable encapsulation materials will be degraded by ultraviolet light irradiation. A suitable package design can account for light extraction and heat transportation. Finally, an atomic layer deposition Al2O3 film has been proposed as a mesa passivation layer. It can provide a low reverse current leakage. Moreover, it can help increase the quantum efficiency, enhance the moisture resistance, and improve reliability. UVC LED applications can be used in sterilization, water purification, air purification, and medical and military fields.

This paper is going to review the state-of-the-art of the high-speed 850/940-nm vertical cavity surface emitting laser (VCSEL), discussing the structural design, mode control and the related data transmission performance. InGaAs/AlGaAs multiple quantum well (MQW) was used to increase the differential gain and photon density in VCSEL. The multiple oxide layers and oxide-confined aperture were well designed in VCSEL to decrease the parasitic capacitance and generate single mode (SM) VCSEL. The maximal modulation bandwidth of 30 GHz was achieved with well-designed VCSEL structure. At the end of the paper, other applications of the near-infrared VCSELs are discussed.

In recent years, the process requirements of nano-devices have led to the gradual reduction in the scale of semiconductor devices, and the consequent non-negligible sidewall defects caused by etching. Since plasma-enhanced chemical vapor deposition can no longer provide sufficient step coverage, the characteristics of atomic layer deposition ALD technology are used to solve this problem. ALD utilizes self-limiting interactions between the precursor gas and the substrate surface. When the reactive gas forms a single layer of chemical adsorbed on the substrate surface, no reaction occurs between them and the growth thickness can be controlled. At the Å level, it can provide good step coverage. In this study, recent research on the ALD passivation on micro-light-emitting diodes and vertical cavity surface emitting lasers was reviewed and compared. Several passivation methods were demonstrated to lead to enhanced light efficiency, reduced leakage, and improved reliability.

Two-dimensional (2D) monolayer molybdenum disulfide (MoS 2 ) semiconductors are an emerging material with interesting device applications. MoS 2 crystals grown on a substrate often have random orientations due to weak van der Waals (vdW) interaction with the substrate. This leads to multiple grain boundaries when random orientated crystals coalesce. Understanding the conditions and mechanism to grow 2D crystals with an aligned orientation is crucial for high-quality single-crystal growth. Here, we study the introduction of oxidation etching in chemical vapor deposition to grow aligned MoS 2 crystals and elucidate the mechanism of the guided growth by a sapphire lattice through vdW interaction. Under proper oxygen flow conditions, single crystals are found to grow in two preferential orientations with triangle crystal edges aligned to the [112̅ 0] or [11̅ 00] direction of the sapphire substrate. These two orientations correspond to a superlattice of (3×3) MoS 2 on (2×2) sapphire and (5×5) MoS 2 on (3×3) sapphire and occur in Mo oxide-and sulfur-rich growth environments, respectively. This aligned orientation growth is realized by a carefully balanced etching and growth competition, which acts as a selection mechanism to grow energetically stable structures while etching less stable structures away. The commeasure of MoS 2 crystals with the sapphire lattice in the superlattice increases the bonding of MoS 2 to the sapphire lattice, thereby becoming the preferred stable structure for nucleation orientations. This study demonstrates the important role of etching−growth competition in the substrate lattice-guided 2D material growth and paves the way for the future development of vdW single-crystal epitaxy.

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