Application of XAFS spectroscopy to studying the microstructure and electronic structure of quantum dots

Variations in the microscopic structural parameters of semiconductor nanoclusters (interatomic distances, coordination numbers, and types of neighboring atoms) under variation of the preparation conditions of Ge/Si, GaN/AlN, and InAs/AlAs heterostructures are determined by the EXAFS-and XANES spectroscopy methods. The effect of preparation conditions on interphase diffusion and structure parameters in semiconductor nanoclusters is revealed. The effect of the temperature of synthesis at different stages, the substrate orientation and pretreatment, the composition of the molecular beam (in particular, the presence of Ge⁺ cations) on the local composition of nanostructures is studied. Relations between the size and shape of nanoparticles and their local spatial characteristics (interatomic distances, stoichiometric composition, and phase boundary characteristics) are examined.     

Binding of electron states in multilayer strained Ge/Si heterostructures with type-II quantum dots

Mechanical strains in a multilayer Ge/Si(001) heterostructure with vertically aligned Ge nanoclusters (quantum dots) are calculated using an interatomic potential based on the Keating valence-force-field model. It is found that the nonuniform spatial elastic strain distribution in this medium gives rise to a three-dimensional potential well for electrons in the strained Si layers near Ge nanoclusters. The depth of the potential well reaches 100 meV, and its spatial dimensions are determined by the diameter of the Ge nanoclusters. For a structure consisting of four Ge islands 23 nm in diameter arranged one above another, the electron binding energies in this well and the spatial electron density distribution are determined. The ground state has an s-like symmetry and is characterized by an electron binding energy of ～95 and ～60 meV for the elemental composition of Ge in the nanoclusters c = 1 and c = 0.7, respectively. The existence of bound electron states in the conduction band of strained Si must lead to a relaxation of the selection rules that determine the low efficiency of the radiative recombination in indirect-gap semiconductors. This explains the high value of the oscillator strength observed for the interband transitions in multilayer Ge/Si(001) structures with vertical correlation of the arrangement of Ge nanoclusters.     

Deformation of the Interatomic Bonds in the Upper Layers of the Ge(111) Surface with c(2 × 8), 7 × 7, and 5 × 5 Structures

Journal of Experimental and Theoretical Physics - The deformation of the interatomic bonds in the upper atomic layers of the Ge(111) surface with c(2 × 8), 7 × 7, and 5 × 5...     

Theoretical analysis of structure, thermodynamic properties, and optical properties of Ge-doped amorphous SiO2

Different stable geometric configurations of Ge doped amorphous SiO₂ (a-SiO₂) system, originating from one, two, or three Si atoms in various places of the a-SiO₂ substituted by Ge atoms randomly have been investigated using interatomic potentials in this work. The most stable structures have been identified and corresponding evolutional rules obtained. The structural growth pattern for Ge-doped a-SiO₂ system is that Ge atoms tend to spread far away from each other and keep away from the center. Furthermore, the thermodynamic properties including speci?c heat, Debye temperature, vibrational entropy, and so on are calculated from the structure with 16 Si atoms of the constructed a-SiO₂ cell replaced by Ge atoms and with the biggest Ge-Ge distance. It can be seen that entropy of Ge doped system with larger specific heat is higher than that of the pure system with smaller specific heat. At last, optical properties including optical absorption spectrum and electron energy loss function of nGe-doped a-SiO₂ (n=0-3, 8) system is also obtained.     

Normal modes in the vicinity of the brillouin zone center of tightly bound units in Bi12GeO20 and Bi12SiO20

From Raman data and a group-theoretical analysis, we deduce that in Bi₁₂GeO₂₀ and Bi₁₂SiO₂₀ there are four weakly bound oxygen atoms and a tightly bound unit MO₄, where M = Ge or Si. Some normal modes of the tightly bound units are identified in Raman and in infrared reflection data. The kinds of interatomic binding forces in these crystals are determined. The results are compared with structure data determined by X-ray scattering.     

Liquid–liquid phase transition and anomalous diffusion in simulated liquid GeO2

We perform molecular dynamics (MD) simulation of diffusion in liquid GeO₂ at the temperatures ranged from 3000 to 5000 K and densities ranged from 3.65 to 7.90 g/cm³. Simulations were done in a model containing 3000 particles with the new interatomic potentials for liquid and amorphous GeO₂, which have weak Coulomb interaction and Morse-type short-range interaction. We found a liquid–liquid phase transition in simulated liquid GeO₂ from a tetrahedral to an octahedral network structure upon compression. Moreover, such phase transition accompanied with an anomalous diffusion of particles in liquid GeO₂ that the diffusion constant of both Ge and O particles strongly increases with increasing density (e.g. with increasing pressure) and it shows a maximum at the density around 4.95 g/cm³. The possible relation between anomalous diffusion of particles and structural phase transition in the system has been discussed.     

Extended fine structure in elastically scattered electron spectra: application to a study of Bi on an a-Si surface

A new surface structure sensitive technique was used to study bismuth adsorption on an a-Si surface. The technique is based on the extended (oscillating) fine structure in the energy dependence of the elastic peak intensity. It was found that the mean interatomic distance at theBi/Si interface increased with Bi coverage. It is noteworthy that, in the submonolayer region, the mean interatomic distance increases linearly with Bi coverage. We thus propose to treat the Bi/Si interface as a “quasi-2D-solid solution”.     

Germanium–vacancy color center in isotopically enriched diamonds synthesized at high pressures

We report on the high-pressure synthesis of novel nano- and microcrystalline diamonds with germanium–vacancy (Ge–V) color centers emitting at 602 nm. The synthesis was carried out in non-metallic growth systems C–H–Ge and C–H–O–Ge enriched with germanium and carbon isotopes. We demonstrate germanium and carbon isotope shifts in the fine structure of the luminescence, which allows us to unambiguously associate the center with the germanium impurity entering into the diamond lattice. We show that there are two ground-state energy levels with the separation of 0.7 meV and two excited-state levels separated by 4.6 meV in the electronic structure of the center and suggest a split-vacancy structure of this center. High-intensity and narrow-line emission of high-pressure synthesized small diamonds with Ge–V centers makes them promising candidates for single-photon emitters.     

Morse势函数和钼单晶等的临界切应力的温度依赖性

综述了以往钼、铁、铝、镁等单晶临界切应力温度依赖性的实验；指出：在滑移系统不变的条件下，在一定的温度范围内，临界切应力和温度呈近似指数关系是相当普遍的规律。Ｍｏｒｓｅ势函数应用和实验的吻合说明原子间力对屈服应力的重要作用。结合近来发展讨论今后的发展，指出金属范性的本质深入到电子结构层次研究的重要意义。     

利用电子结构数据拟合H2分子的电子壳模型势函数参数

电子壳模型势函数在离子晶体的原子级计算机模拟中有广泛应用,其势参数主要通过拟合晶体的实验数据或电子结构数据得到.提出了通过拟合双原子分子的量子化学从头计算电子结构数据来获得该势函数的方法,并由H2分子的电子结构数据建立了H原子间的电子壳模型势函数.此外,还应用该势函数对H+2分子离子进行了计算.该势函数拟合方案更适合于共价键型的分子.     

Random perculation in metal-Ge mixtures

We have measured the electrical resistivity of InGe, PbGe and A?Ge mixtures co-evaporated onto room temperature substrates, and studied their microstructure by transmission electron microscopy. In InGe and PbGe, where we have observed a random distribution of the constituents, the metal-insulator transition occurs at a metal concentration of about 15% vol., in agreement with random continuum percolation theory. In contrast, A?Ge has a highly regular granular structure and a much higher critical concentration (～ 55%), as do most metal-insulator thin film mixtures.     

Theory of reconstruction induced subsurface strain — application to Si(100)

The stress produced by surface layer reconstruction on semiconductors can produce sizable elastic distortions of deeper layers. These are calculated using a standard bulk model of interatomic force constants for the pairing model of 2 × 1 reconstruction on the (100) surface of C, Si and Ge. Kinematic analysis for the predicted geometry with subsurface distortions is shown to give good agreement with LEED intensity data for Si. This reconciles the LEED results, for which the simple pairing model fails completely, with spectroscopic measurements, which are best fit by the pairing model.     

Formation of carbon composite structures on the Ge(110) surfaces

We present our first-principles calculation of the adsorption and diffusion of a carbon adatom on the H-terminated and clean Ge(110) surfaces, which are essential processes in the nucleation and growth of a monolayer graphene on Ge(110) by chemical vapor deposition. On the H-terminated surface, the C adatom spontaneously substitutes H atom(s) to form a monohydride structure (CH) or a dihydride structure (CH₂) and makes direct bonds with the substrate Ge atoms. The resulting diffusion barriers of the C adatom are 2.67 and 6.45 eV parallel to and perpendicular to the zigzag Ge chains of the surface, respectively. On the clean surface, the C adatom embeds into the zigzag Ge chain with nearly no barrier, kicking out a Ge atom out of the chain at the same time. The kicked-out Ge atom, instead of the C adatom, becomes a diffusion species with the barrier less than 0.63 eV. The formation of the C composite structures makes the C adatom difficult to diffuse both on the H-terminated and clean Ge(110) surfaces, which suggests that the nucleation and growth of the graphene islands from C seeds is much suppressed. We propose a growth mechanism of graphene monolayer going round the diffusion of the C adatoms on the Ge(110) surfaces.     

Ab initio study of platinum induced reconstructions on Ge(001)-(1×2) surface with dimerization

We present first principle total energy calculation of Pt induced reconstructions on Ge(001)-(1×2) surface with dimerization. Study was undertaken using localized orbitals basis set DFT using SIESTA to compare pure Ge dimerized Ge(001)-(1×2) surface with 0.5 and 1.0 Pt covered dimerized Ge(001)-(1×2) surface with the possibility of homo (Ge-Ge and Pt-Pt) and hetro (Pt-Ge) dimers. From total energy calculation results we calculated dimer bond lengths, buckling angles and formation energy of dimers on Ge(001)-(1×2) surface. By calculating the formation energy of different configurations we find that Ge-Ge buckled dimerized surface has least (−1.23 eV/dimer) and Pt-Pt symmetric dimerized surface has largest (+0.09 eV/dimer) formation energy with respect to unreconstructed surface. We further calculated the electronic DOS and band structure of Ge dimerized as well as Pt dimerized surface to see the change in semiconducting behavior on dimerization. By comparing the DOS and electronic band structure of homo Ge dimerized surface, we found metallicity of Ge(001)-(1×2) surface results from dimer formation. Also by comparing the electronic band structure of homo Ge dimerized surface with unreconstructed surface we find that less number of bands crossing the Fermi level which is perhaps due to the saturation of one dangling bond per Ge surface atom. By introducing Pt at 0.5 and 1.0 coverage in place of Ge, except for homo Pt buckled dimerized surface having 1.0 coverage of Pt, we find in all other cases increase in number of bands are crossing the Fermi level, indicating strong metallic behavior of Ge(001)-(1×2) surface.     

Diffuse scattering from Ge(111) surface at high temperature

On heating a clean Ge(111) surface above 240°C, Ge(111) 2 × 8 surface structure changes to 1 × 1 one. We have first observed twofold splitting diffuse scattering in RHEED patterns from the Ge(111) 1 × 1 surface at high temperatures. A modulated 2 × 2 structure is proposed as a structural model for the diffuse scattering. The Fermi-surface instability of dangling-bond electrons at the surface is studied as an origin of the formation of the modulated structure.     

利用电子结构数据拟合H2分子的电子壳模型势函数参数

电子壳模型势函数在离子晶体的原子级计算机模拟中有广泛应用,其势参数主要通过拟合晶体的实验数据或电子结构数据得到.提出了通过拟合双原子分子的量子化学从头计算电子结构数据来获得该势函数的方法,并由H₂分子的电子结构数据建立了H原子间的电子壳模型势函数.此外,还应用该势函数对H⁺₂分子离子进行了计算.该势函数拟合方案更适合于共价键型的分子. 关键词： 电子壳模型势 参数拟合 共价键 2分子')" href="#">H₂分子     

Ab initio electronic structure of the Ge(111)-(2 × 1) surface in the presence of surface vacancy. Application to STM data analysis

We present the results of ab initio modeling of Ge(111)-(2 × 1) surface in the presence of atomic vacancy in surface bilayer. We showed that simple crystal structure defect affects surface electronic structure to the extent comparable with the influence of doping atom. We demonstrated the strong difference of surface LDOS structure above surface defects of different kind. We have proved that spatial oscillations of LDOS exist around individual surface vacancy in the same tunneling bias range as in case of donor doping atom on Ge(111)-(2 × 1) surface.     

Synthesis and properties of Si and Ge nanoclusters produced by pulsed laser ablation

We show that conventional pulsed laser ablation (PLA) of Si and Ge targets in inert buffer gases is an efficient method of nanocluster synthesis. From a photoluminescence study of Si and Ge nanoclusters produced by PLA we have demonstrated the possibility of tuning the luminescence band from the near infrared to the near ultraviolet regions. The stabilization of the properties of Si nanoclusters by reactive (H₂ gas) PLA synthesis was proved by photoluminescence measurements. Finally, we report a photoluminescence study of gas-suspended Ge nanoclusters during their preparation. They exhibit a broad luminescence spectrum extended from UV to the blue-green region and modulated by a molecule-like structure. We propose an interpretation of the vibronic structure involving Ge-O-Ge vibrations at the surface of photo-excited clusters. To the best of our knowledge, we report here the first observation of vibrational effects from gas-suspended Ge nanoclusters.     

Calculation of astrophysical reaction rate of 82Ge(n,γ)83Ge

The neutron capture reaction on a neutron-rich near closed-shell nucleus 82Ge may play an important role in the r-process following the fallout from nuclear statistical equilibrium in core-collapse supernovae.By carrying out a DWBA analysis for the experimental angular distribution of 82Ge(d, p)83Ge reaction we obtain the single particle spectroscopic factors, S2,5/2 and S0,1/2 for the ground and first excited states of 83Ge=82Ge(⊕)n, respectively. And then these spectroscopic factors are used to calculate the direct capture cross sections for the 82Ge(n, γ)83Ge reaction at energies of astrophysical interest. The optical potential for neutron scattering on unstable nucleus 82Ge is not known experimentally. We employed a real folding potential which was calculated by using the proper 82Ge density distribution and an effective nucleon-nucleon force DDM3Y.The neutron capture reactions on neutron-rich closed-shell nuclei are expected to be dominated by the direct capture to bound states. We will show that the direct capture rates on these nuclei are sensitive to the structure of the low-lying states.     

Calculation of sound velocity and structure factor for interacting bosons

We have calculated the velocities of the first and second sound and the structure factor for a system of interacting bosons. The two-body interatomic potential used is assumed to be composed of a hard core followed by a sum of both repulsive and attractive gaussians.