which are environmentally friendly and enable portability and high efficiency. Up to date, great progress has been made on the UVC light-emitting diodes (LEDs) by using active regions of AlGaN multiple quantum wells (MQWs) .[8-14] However, the optical output power of current UVC LEDs drops significantly as the light emission wavelength gets shorter. Those LEDs suffer from poor hole injection efficiency in high-Al-content p-type AlGaN, low internal quantum efficiency (IQE) caused by large-lattice-mismatch heteroepitaxy, and strong quantum-confined Stark effect (QCSE), as well as the absorption by the nontransparent GaN contact layers. [15][16][17] A promising approach that dramatically improves the light output power is electron-beam (e-beam) pumping, especially for the short-wavelength UVC spectral range. [3,[18][19][20][21][22][23][24] This approach allows one to bypass the need for p-type or n-type injection layers and, thus, can largely increase the carrier injection efficiency. This provides a unique advantage over conventional LEDs at UVC range, since the p-type doping for high-Al-content AlGaN is High-output-power electron-beam (e-beam) pumped deep ultraviolet (DUV) light sources, operating at 230-270 nm, are achieved by adjusting the well thickness of binary ultrathin GaN/AlN multiple quantum wells. These structures are fabricated on high-quality thermally annealed AlN templates by metal-organic chemical vapor deposition. Owing to the reduced dislocation density, large electron-hole overlap, and efficient carrier injection by e-beam, the DUV light sources demonstrate high output powers of 24.8, 122.5, and 178.8 mW at central wavelengths of 232, 244, and 267 nm, respectively. Further growth optimization and employing an e-gun with increased beam current lead to a record output power of ≈2.2 W at emission wavelength of ≈260 nm, the key wavelength for water sterilization. This work manifests the practical levels of high-output-power DUV light sources operated by using e-beam pumping method. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.201801763.Solid-state deep ultraviolet (DUV) optoelectronic devices in the spectral range of 200-280 nm, i.e., ultraviolet-C (UVC), have attracted much attention for their wide applications in sterilization, medical treatment, security, solar-blind photodetection, and so on. [1][2][3][4][5][6][7] Currently, Al(Ga)N material system is the most promising candidate for solid-state UVC light sources

Quasi-2D GaN layers inserted in an AlGaN matrix are proposed as a novel active region to develop a high-output-power UV light source. Such a structure is successfully achieved by precise control in molecular beam epitaxy and shows an amazing output power of ≈160 mW at 285 nm with a pulsed electron-beam excitation. This device is promising and competitive in non-line-of-sight communications or the sterilization field.

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure with spin locked to momentum, which is interesting in physics and may also have important applications in spintronics. In this work, by measuring the tunable helicity-dependent photocurrent (HDP), we present an identification of the HDP from the Dirac-like surface states at room temperature. It turns out that the total HDP has two components, one from the Dirac-like surface states, and the other from the surface accumulation layer. These two components have opposite directions. The clear gate tuning of the electron density as well as the HDP signal indicates that the surface band bending and resulted surface accumulation are successfully modulated by the applied ionic liquid gate, which provides a promising way to the study of the Dirac-like surface states and also potential applications in spintronic devices.

Applying shear strain has been considered as a hopeful method to open a band gap of graphene. To study the transport properties of graphene under shear strain, a device was fabricated to apply shear strain, up to 16.7%, to graphene grown by chemical vapor deposition method. A top gate with ionic liquid as the dielectric material was used to tune the carrier density. The conductance of the Dirac point and carrier mobility is found to increase with a comparatively small increasing strain but then decrease with a larger one. Such a behavior might be related to several factors: the wrinkles, the transverse conducting channels, and the grain boundaries of graphene. Our study is helpful to further understand the strain engineering in graphene.

High-temperature (HT) annealing effects on the evolution of strain in AlN films grown on sapphire have been investigated. It is found that there is a significant transition behavior from tensile to compressive strain in AlN before and after HT annealing at an optimal temperature of 1700 °C. Based on a microstructural analysis, it is clarified that the HT annealing will result in the (1) disappearance of grains that account for the tensile stress before HT annealing, (2) generation of a new interface that has little influence on the lattice constant upper/below this interface, and (3) regular 8/9 arrangement of misfit dislocation at the AlN/sapphire interface that relieves almost all stress associated with lattice mismatch. It is thus deduced that the remnant compressive strain in AlN after HT annealing mainly comes from the cooling down process due to thermal mismatch between sapphire and AlN. This understanding of the annealing effect is certainly of great significance in AlN materials science and technology.

By employing a single AlGaN layer with low Al composition, high quality and uniformity AlGaN/GaN heterostructures have been successfully grown on Si substrates by metal-organic chemical vapor deposition (MOCVD). The heterostructures exhibit a high electron mobility of 2150 cm2 /Vs with an electron density of 9.3 × 10 12 cm −2 . The sheet resistance is 313 ± 4 Ω/◻ with ±1.3% variation. The high uniformity is attributed to the reduced wafer bow resulting from the balance of the compressive stress induced and consumed during the growth, and the thermal tensile stress induced during the cooling down process. By a combination of theoretical calculations and in situ wafer curvature measurements, we find that the compressive stress consumed by the dislocation relaxation (~1.2 GPa) is comparable to the value of the thermal tensile stress (~1.4 GPa) and we should pay more attention to it during growth of GaN on Si substrates. Our results demonstrate a promising approach to simplifying the growth processes of GaN-on-Si to reduce the wafer bow and lower the cost while maintaining high material quality.Recently, AlGaN/GaN heterostructures grown on Si substrates have attracted much attention for high power, high frequency, and high temperature applications [1][2][3][4] . These can offer several advantages such as large wafer size, high thermal conductivity, low cost, and great potential of the compatibility with existing processing technologies developed for Si integrated circuits [5][6][7][8] . Despite the promising applications, GaN-on-Si technology is facing reproducibility and reliability issues, which are likely to be related to growth processes and crystalline quality (defects and residual stress) 1,9 . Due to the large lattice mismatch and thermal mismatch between GaN and Si substrates, it is challenging to grow high-quality and stress-free GaN-based epilayers. Several complicated stress-control approaches such as patterned Si substrate technology 10 , LT-AlN 11 , AlN/GaN superlattice 12,13 , and compositionally graded AlGaN layer 14,15 have been proposed to achieve crack-free GaN based heterostructures. However, the crystalline quality (defects and residual stress), as well as uniformity issues still remain, especially for growth onto large diameter substrates.For the method with compositionally graded buffers, three step-graded AlGaN (with Al composition of about 75%, 50% and 25%) or multiple step-graded AlGaN buffers with thickness up to 1 μm are generally used [15][16][17] . The main purpose of this method is to slow down the relaxation rate of compressive stress by decreasing the lattice mismatch between the two neighbouring layers. There is thus a larger compressive stress accumulated in the GaN layer during the growth at high temperature. One issue in this case with thick buffers is that the wafer is convexly bowed. As a result, it will significantly affect the wafer uniformity during the subsequent growth. Another issue is that the growth rate of AlGaN ternary alloy is generally lower than that of GaN layer an...

Epitaxial growth of AlN films on c-sapphire using a multilayer structure has been investigated by metalorganic chemical vapor deposition adopting multiple alternation cycles of low-and high-temperature (LT-HT) growth. It is found that the surface morphology and crystal quality can be greatly improved using three alternation cycles with X-ray diffraction ω-scan full width at half maximum values of 311 and 548 arcsec for the (0002) and (10−12) peaks, respectively, which are induced by the alternation of the three-dimensional (3D) and two-dimensional (2D) growth modes caused by the LT-HT process. The first 3D-2D cycle is found to play a major role in threading dislocation reduction, while the second and third cycles mainly account for tensile stress relaxation. CrystEngCommThis journal is

Fabricating single-crystalline gallium nitride (GaN)-based devices on a Si(100) substrate, which is compatible with the mainstream complementary metal-oxide-semiconductor circuits, is a prerequisite for next-generation high-performance electronics and optoelectronics. However, the direct epitaxy of single-crystalline GaN on a Si(100) substrate remains challenging due to the asymmetric surface domains of Si(100), which can lead to polycrystalline GaN with a two-domain structure. Here, by utilizing singlecrystalline graphene as a buffer layer, the epitaxy of a single-crystalline GaN film on a Si(100) substrate is demonstrated. The in situ treatment of graphene with NH 3 can generate sp 3 CN bonds, which then triggers the nucleation of nitrides. The one-atom-thick single-crystalline graphene provides an in-plane driving force to align all GaN domains to form a single crystal. The nucleation mechanisms and domain evolutions are further clarified by surface science exploration and first-principle calculations. This work lays the foundation for the integration of GaN-based devices into Si-based integrated circuits and also broadens the choice for the epitaxy of nitrides on unconventional amorphous or flexible substrates.