Variable Energy Positron Annihilation Spectroscopy of GaN Grown on Sapphire Substrates with MOCVD

Depth profiled Doppler broadening of positron annihilation spectroscopy (DBPAS), which is also called the variable energy positron annihilation spectroscopy (VEPAS), is used in characterization of GaN grown on sapphire substrates with metal-organic chemical vapour deposition (MOCVD). The GaN film and the film/substrate interface are investigated. The VEPFIT (variable energy positron fit) software was used for analysing the data,and the positron diffusion length of the sapphire is obtained. The results suggest that there is a highly defected region near the GaN/sapphire interface. This thin dislocated region is generated at the film/substrate interface to relieve the strain. Effects of implantation dose on defect formation, for the GaN/Sapphire samples, which implanted by Al^ ions, are also investigated. Studies on AI implanted GaN films (not including the interface and sapphire) have revealed that there are two different regions of implantation damage. For the low Al^ implantation dose samples, in the region close to the surface, defects are mainly composed of vacancy pairs with small amount of vacancy clusters, and in the interior region of the film the positron traps are vacancy clusters without micro-voids. For the highest dose sample, however, some positron trap centres are in the form of micro-voids in the second region.     

Nanoporous Structure in Low-Dielectric Films with Positronium Annihilation Lifetime Spectroscopy

We investigate nano-porous structures in thin low-dielectric films, i.e. the pore sizes, distributions, and interconnectivity, by using depth profiled positronium annihilation lifetime spectroscopy (PALS). It is found that PALS has good sensitivity to probe both interconnected and closed pores in the range from 0.3nm to 3Onto, even in the film buried beneath a diffusion barrier. A series of low dielectric constant films of organosilicon-silsequioxane with different weight percentages of porogen have been comparatively investigated. The PALS technique can be used to distinguish the open porosity from the closed one, to determine the pore size, and to detect the percolation threshold with the increasing porosity that represents the transition from closed pores to interconnected pores.Furthermore, the pore percolation length can be derived.     

公务员招聘问题的研究

建立了不符合度、失望度、不满意度等概念,将公务员招聘问题转化为求不满意度总和最小的0—1规划模型,很好地解决了在不考虑和考虑应聘人员意愿的两种情形下公务员招聘的问题.充分体现了录取的公平公正性和对人才的充分利用.     

A Fractal Model for Effective Thermal Conductivity of Isotropic Porous Silica Low-k Materials

We establish a new model based on fractal theory and cubic spline interpolation to study the effective thermal conductivity of isotropic porous silica low-k materials. A 3D fractal model is introduced to describe the structure of the silica xerogel and silica hybrid materials （such as methylsilsesquioxane, MSQ）. Combined with fractal structure, a more suitable medium approximation is developed to study the isotropic porous silica xerogel and MSQ materials. Cubic spline interpolation for fitting discrete predictions from the fractal model is used to obtain the continuous function of the effective thermal conductivity versus porosity. Compared with other common models, the effective thermal conductivity predicted by our model presents better agreement with the experimental data for all porosity. These results indicate that the proposed model is valid.     

Positronium diffusion in porous methylsilsesquioxanethin films

Depth profiled positronium (Ps) annihilation lifetime spectroscopy (PALS) is an extremely useful probe of the pore characteristics in nanoporous low-dielectric (low-k) constant thin films. PALS has also been considered as a potential probe to investigate diffusion barrier integrity and the structural changes of porous low-k films during their integration with Cu. Hence, it is essential to understand the diffusion behaviour of positronium/Cu atoms in the films. In this work, based on the fact that porous materials possess characteristics of statistical self-similarity, a fractal model, the Menger sponge model, has been applied to simulate the structure of a promising dielectric, porous methylsilsesquioxane (MSQ) films. The diffusion behaviour of Ps out of the fractal model and into the surrounding vacuum is studied by means of the diffusion equation and traditional advective-diffusive theory. Predictive results from our model show good agreement with measurement data.     

超强磁场中核磁共振信号与屏蔽效应的理论研究

研究了超强磁场对于核磁共振样品中双原子分子中电子系统的哈密顿量的影响,并且给出了磁共振信号中由于外磁场与氢原子电子系统相互作用所产生的含有B02的修正项.基于核磁共振原理,获得了利用氢的NMR谱对超强磁场进行精确测量的理论依据,推导出了一种基于核磁共振技术的超强磁场测量公式,从而在理论上证明了对超强磁场进行精确测量是可行的.     

Serverless distributed learning for smart grid analytics

The digitization, informatization, and intelligentization of physical systems require strong support from big data analysis. However, due to restrictions on data security and privacy and concerns about the cost of big data collection, transmission, and storage, it is difficult to do data aggregation in real-world power systems, which directly retards the effective implementation of smart grid analytics. Federated learning, an advanced distributed learning method proposed by Google, seems a promising solution to the above issues. Nevertheless, it relies on a server node to complete model aggregation and the framework is limited to scenarios where data are independent and identically distributed. Thus, we here propose a serverless distributed learning platform based on blockchain to solve the above two issues. In the proposed platform, the task of machine learning is performed according to smart contracts, and encrypted models are aggregated via a mechanism of knowledge distillation. Through this proposed method, a server node is no longer required and the learning ability is no longer limited to independent and identically distributed scenarios. Experiments on a public electrical grid dataset will verify the effectiveness of the proposed approach.     

Rectifying barrier at GaN/SiC hetero-junction studied with positron annihilation spectroscopy

Positron annihilation spectroscopy on GaN films grown on SiC substrate with MBE are presented. It is shown that the GaN/SiC interface is rectifying towards positrons, such that positrons can only travel from SiC to GaN and not vice versa. Potential steps seen by the positron at the GaN/SiC interface are calculated from experimental values of electron and positron work function. This “rectifying” effect has been successfully mimicked by inserting a thin region of very high electric field in the Variable Energy Positron Fit （VEPF） analysis. The built-in electric field is attributed to different positron affinities, dislocation and/or interface defects at the GaN/SiC interface.     

Probing Cu Diffusion Barrier Layers on Porous Low-Dielectric-Constant Films by Posireonium Annihilation Lifetime Spectroscopy

Two kinds of Cu diffusion barrier layers, sealed films and capped films, on nanoporous low-dielectric-constant films are investigated by positronium annihilation lifetime spectroscopy （PALS）. We have found that the minimum thickness of Ta to form an effective diffusion barrier is affected by the pore size. The films with large pores require thick barrier layers to form effective diffusion barriers. In addition, a possible ultra-thin diffusion barrier, i.e. a plasma-induced densification layer, has also been investigated. The PALS data confirm that a porous low-dielectric-constant thin film can be shrunk by exposure to plasma. This shrinkage is confined to a surface layer of collapsed pores and forms a dense layer. The dense layer tends to behave as Ps （positronium） diffusion barriers. Indeed, the controlled thin “skin” layer could prevent Cu diffusion into the underlying dielectrics.     

Dielectric Performance of Porous Methyl Silsesquioxane/Triacetyl-β-Cyclodextrin Thin Films

Porous methyl silsesquioxane thin films with low dielectric constant are successfully synthesized by means of the sol-gel spin-coating method. The precursor solutions are prepared by properly mixing the polymer as a matrix with different contents of triacetyl-β-cyclodextrin （TABCD） as a porogen. The chemical structure, dielectric constants, optical constants and void fraction are investigated by the ellipsometric porosimetry, Fourier transform infrared （FTIR） spectroscopy, and other methods. Influences of TABCD and methyl trimethoxysilane （MTMS） on the dielectric properties are discussed. For those samples with higher porogen loading, orientation and electronic polarizations are lower in low and high frequency regions, respectively, because of a considerably smaller number of polar molecules. The FTIR results suggest that high R value （molar ratio of H2 O to MTMS） is more advantageous for formation of cage structures and the cage/network structural ratio increases with the increasing R value.