p型层结构与掺杂对GaInN发光二极管正向电压温度特性的影响

在温度变化时, 如果GaInN发光二极管能够保持相对稳定的工作电压对其实际应用具有重要意义. 本文通过金属有机化学气相沉积生长了一系列包含不同有源区结构、不同p型层结构以及不同掺杂浓度纵向分布的样品, 并对其在不同温度区间内正向电压随温度变化的斜率(dV/dT)进行了研究. 结果表明: 1)有源区中包括插入层设计、量子阱结构以及发光波长等因素的变化对正向电压随温度变化特性影响很小; 2)影响常温区间(300 K± 50 K)正向电压随温度变化斜率的最主要因素为p-AlGaN 电子阻挡层起始生长阶段的掺杂形貌, 具有p-AlGaN陡掺界面的样品电压变化斜率为-1.3 mV·K^-1, 与理论极限值 -1.2 mV·K^-1十分接近; 3) p-GaN主段层的掺Mg浓度对低温区间(<200 K)的正向电压随温度变化斜率有直接影响, 掺Mg浓度越低则dV/dT斜率越大. 以上现象归因于在不同温度区间, p-AlGaN 以及p-GaN 发生Mg受主冻结效应的程度主要取决于各自的掺杂浓度. 因此Mg掺杂浓度纵向分布不同的样品在不同的温度区间具有不同的串联电阻, 最终表现为差异很大的正向电压温度特性.     

F−, Cl−, Li+ and Na+ adsorption on AlN nanotube surface: A DFT study

Adsorption of two anions (F⁻ and Cl⁻) and two cations (Li⁺ and Na⁺) on the surface of aluminum nitride nanotubes (AlNNTs) is investigated by density functional theory. The reactions are site-selective, so that the cations and anions prefer to be adsorbed atop the N and Al atoms of the tube surface, respectively. The adsorption energies of anions (−4.46 eV for F⁻ and −1.12 eV for Cl⁻) are much higher than those of cations (about −0.17 eV for Li⁺ and −0.12 eV for Na⁺) which can be explained using frontier molecular orbital theory. It was found that the adsorption of anions may facilitate the electron emission from the AlNNT surface by reducing the work function due to the charge transfer occurs from the anions to the tube. It has been predicted that in contrast to the cations the adsorption of anions also obviously increases the electrical conductivity of AlNNT.     

Ih和D5h对称的三金属氮化物富勒烯Sc3N@C80的几何结构、电子结构和磁学特性

采用密度泛函理论（density functional theory, DFT）方法对具有Ih和D5h对称的三金属氮化物富勒烯Sc3N@C80的几何结构、电子结构及其磁学特性进行了计算研究.几何结构优化显示掺杂Sc3N之后,C80的结构只是发生了细微的变化,仍然保持了Ih和D5h对称性.能级图和局部态密度图表明Sc原子对能级的变化贡献最大,掺杂之后能隙增加,简并度下降,增强了两种异构体的稳定性.磁学特性分析指出掺杂之后,Sc3N的磁性完全淬灭,两种异构体均没有磁矩,都不能作为磁性材料.     

Direct Synthesis of Porous Nanorod‐Type Graphitic Carbon Nitride/CuO Composite from Cu–Melamine Supramolecular Framework towards Enhanced Photocatalytic Performance

Facile and direct synthesis of porous nanorod‐type graphitic carbon nitride/CuO composite ( CuO‐g‐C₃N₄ ) has been achieved by using a Cu–melamine supramolecular framework as a precursor. The CuO‐g‐C₃N₄ nanocomposite demonstrated improved visible‐light‐driven photocatalytic activities. The results indicate that metal–melamine supramolecular frameworks can be promising precursors for the preparation of efficient g ‐C₃N₄ nanocomposite photocatalysts.     

Deep UV resonance Raman spectroscopic study on electron‐phonon coupling in hexagonal III‐nitride wide bandgap semiconductors

Electron–phonon coupling (EPC) is an important issue in semiconductor physics because of its significant influence on the optical and electrical properties of semiconductors. In this work, the EPC in wide bandgap semiconductors including hexagonal BN and AlN was studied by deep UV resonance Raman spectroscopy. Up to fourth‐order LO phonons are observed in the resonance Raman spectrum of hexagonal AlN. By contrast, only the prominent emission band near the band‐edge and the Raman band attributed to E_2g mode are detected for hexagonal BN with deep UV resonance excitation. The different behavior in resonant Raman scattering between the III‐nitrides reflects their large difference in EPC. The mechanism for EPC in hexagonal BN is the short‐range deformation interaction, while that in hexagonal AlN is mainly associated with the weak long‐range Fröhlich interaction. Copyright © 2016 John Wiley & Sons, Ltd.     

Frequency comb generation in the green using silicon nitride microresonators

Optical frequency combs enable precision measurements in fundamental physics and have been applied to a growing number of applications, such as molecular spectroscopy, LIDAR and atmospheric trace‐gas sensing. In recent years, the generation of frequency combs has been demonstrated in integrated microresonators. Extending their spectral range to the visible is generally hindered by strong normal material dispersion and scattering losses. In this paper, we report the first realization of a green‐light frequency comb in integrated high‐Q silicon nitride (SiN) ring microresonators. Third‐order optical non‐linearities are utilized to convert a near‐infrared Kerr frequency comb to a broadband green light comb. The 1‐THz frequency spacing infrared comb covers up to 2/3 of an octave, from 144 to 226 THz (or 1327‐2082 nm), and the simultaneously generated green‐light comb is centered around 570‐580 THz (or 517‐526 nm), with comb lines emitted down to 517 THz (or 580 nm) and up to 597 THz (or 502 nm). The green comb power is estimated to be as high as −9.1 dBm in the bus waveguide, with an on‐chip conversion efficiency of −34 dB. The proposed approach substantiates the feasibility of on‐chip optical frequency comb generation expanding to the green spectral region or even shorter wavelengths.

     

Transmission electron microscopy study of defects in AlN crystals with rough and smooth surface grains

Defects present in (0 0 0 1) textured polycrystalline AlN grown by the sublimation–recombination method were analyzed using transmission electron microscopy (TEM) methods. Grains in the polycrystalline boule had either a smooth or a rough surface. The rough surface grains had mainly edge dislocations, whereas the smooth surface grains had some sub-grain boundaries and were mostly free of dislocations. Dislocations at the grain boundaries were pinned and could not be annihilated.     

Evolution of crystal structure of Zn:N films under high temperature

Zinc‐nitrogen (Zn:N) compound thin film was prepared from a pure metallic Zn target by rf magnetron sputtering at ambient temperature under the mixture of nitrogen and argon gases with the ratio of 1:1. High temperature x‐ray diffraction (HTXRD) measurement under vacuum was used to examine the evolution of structural properties of the Zn:N film. At ambient temperature, the (002), (100), and (101) planes corresponding to Zn structure were observed while at higher temperature, the left shifts corresponding to the increase of lattice constants a and c of Zn were observed. At temperatures of 320 °C, 481 °C and 554 °C, the (222), (321) and (400) planes corresponding to Zn₃N₂ structure were observed with the decrease in the intensity amplitudes of the peaks belonging to the Zn structure. The results indicate the gradual transformation of the Zn₃N₂ phase in the Zn:N films at temperature greater than 320 °C. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Glucose Biosensor Based on Silicon Nitride Nanoparticles

For the first time silicon nitride (Si₃N₄) nanoparticles was used for preparation electrochemical biosensor. GOx immobilized on the Si₃N₄ nanoparticles exhibits facile and direct electrochemistry. The surface coverage and heterogeneous electron transfer rate constant (k_s) of immobilized GOx were 6.3×10^?13 mol cm^?2 and 47.4±0.3 s^?1. The sensitivity, linear concentration range and detection limit of the biosensor for glucose detection were 38.57 µA mM^?1 cm^?2, 25 µM to 8 mM and 6.5 µM, respectively. This biosensor also exhibits good stability, reproducibility and long life time. These indicate Si₃N₄ nanoparticles is good candidate material for construction of third generation biosensor and bioelectronics devices.     

Relations of electronic energies and magnetic moments of tetra-3d metal (Mn, Fe, Co and Ni) nitrides calculated using a plane-wave basis method

Geometrical optimization of tetra-3d metal nitrides (Mn₄N, Fe₄N, Co₄N, and Ni₄N) has been performed and the relations of their energies (E) and their total magnetic moments (M) are obtained by plane-wave-basis density-functional calculations without any assumption of specific spin arrangement. The E vs. M relations obtained for Fe₄N and Mn₄N have a bimodal character. The ground state of Fe₄N is a high-spin state, which would correspond to the ferromagnetic character, while that of Mn₄N is a low-spin state, which would correspond to the observed ferrimagnetic character. Lattice constants and total magnetic moments of these tetra-3d metal nitrides are almost accurately predicted. From the spin-polarized densities of states curves, Co₄N would have the largest spin polarization ratio of 0.88, which suggests Co₄N can be a candidate material for ferromagnetic electrodes for spin-injection.