Wirkungsquerschnitt und Winkelverteilung der elastischen und unelastischen Elektronenstreuung in Aluminiumschichten

Absolute intensity measurements of the electrons scattered by a polycrystalline Aluminium foil were carried out in the energy range between 25 and 50 keV. The electrons scattered elastically were separated from those scattered inelastically by means of a retarding field. The intensities of the electrons having passed the foil unscattered and of those which were scattered elastically into the Debye-Scherrerrings and into the continuous background can be interpreted by the assumption of reasonable thicknesses of the crystalline Aluminium and the amorphous Aluminium-Oxide. These values agree approximately with the thickness measured by light absorption. Additionally the probability of the inelastic scattering process can be deduced from these measurements. Investigations of the angular distribution were carried out in order to study the influence of the inelastic scattering on the shape of the primary beam, the rings and the continuous background. The results are discussed in detail. Some results are given in particular, concerning the increase of the half width of the rings due to inelastic scattering processes.     

Messung der Elektronenbeugungsintensitäten polykristalliner Aluminiumschichten bei tiefer Temperatur und Vergleich mit der dynamischen Theorie

By means of a retarding field apparatus the electron diffraction intensities of a polycrystalline Aluminium-foil have been measured for different electron energies between 17 and 50 kev. The specimen was kept at 154 °K and the thickness of the crystallites was about 280 Å. It has been shown that the high index reflections behave kinematically, while the first order reflections [e.g. (111), (200)] are much influenced by dynamical extinction effects which can be explained very well by the dynamical two beam approximation. Also the intensities of the higher orders [e.g. (222), (400)] of the strong reflections show marked deviations from the kinematical values. This intensity deficit can be interpreted by multiple scattering processes due to systematic interactions. The experimental results are compared with the second Bethe approximation (introduction of dynamic potentials) as well as with the second Born approximation where double scattering processes within the crystal are taken into account.     

Temperaturabhängigkeit des Streuuntergrundes im Elektroneninterferenzdiagramm polykristalliner Silberschichten

The filtered and unfiltered intensities of electrons (30 to 50 kev) scattered in thin polycrystalline silver foils (300 to 1400 Å) were measured by means of a retarding field apparatus in the temperature range from 170 to 380 °K. Further the absorption coefficients due to the inelastic and the different elastic scattering processes were determined. It is shown, that the diffuse elastic scattering intensity increases with temperature for all values of thickness and electron energy. From the measured absorption coefficients the contributions of the different elastic scattering processes to the total background intensity are calculated. It turns out, that the main contribution is due to multiple scattering processes including both Bragg reflections and thermal diffuse scattering. Furthermore it is shown, that in contrary to the filtered diagram the background intensity of the unfiltered diagram increases with temperature only for larger scattering angles. In the small angle region a reversed temperature behaviour is observed. This effect is caused by that part of inelastically scattered electrons at low angles coming directly from the primary beam and from the low order diffraction rings.     

Unelastische Streuung in der dynamischen Theorie der Elektronenbeugung

A general solution to the fundamental equations of inelastic scattering of fast electrons in crystals is obtained by using a Green function constructed of Bloch waves. Inelastic interband transitions are shown to vanish if they are of a direct type, but not when implying simultaneous Bragg reflexion. These selection rules hold for any type of electron-crystal interaction. The resulting imaginary absorption potentials for the different intraband and interband transitions can be calculated separately. They are reduced to a simple form in the two-beam case. The expression measuring the anomalous absorption contains the Yoshioka potential C ₀ ⁱ g and an additional term which gives a small contribution in the case of crystal electron excitation and no contribution in the case of phonon excitation. Numerical calculations for the scattering of 100 keV electrons in Al, Cu, Ge, Si and MgO are made and compared with experimental data. An analytic expression forYoshioka's C ₀ ⁱ g due to plasmon excitation is derived which vanishes in the random phase approximation.     

Surface properties of electron-hole droplets in semiconductors

The leading gradient terms in the kinetic energy densities of the electrons and holes in Ge and Si type semiconductors are calculated including band structure effects. Band structure effects are found to increase these gradient terms substantially over those for isotropic bands. The surface properties of electron—hole drops in Ge are studied using these gradient terms with several local energy densities, and band structure effects in the gradient terms are shown to increase the surface thickness, surface energy and dipole barrier over isotropic band results. The difference in the chemical potentials of the electrons and holes obtained from these approximate energy functionals indicates that the drop becomes positively charged, which is opposite in sign to that proposed recently by Rice.     

简并态锗在大注入下的自发辐射谱模拟

基于费米狄拉克模型模拟了应变、温度以及掺杂对简并态锗的直接带自发辐射谱的影响.随着温度升高,更多的电子被激发到导带中,使得锗自发辐射谱的峰值强度和积分强度随温度的升高而增大.对自发辐射谱峰值强度的m因子进行计算,结果表明张应变可以显著提高锗自发辐射的温度稳定性.在相同应变水平下,由Γ-hh跃迁引起的自发辐射谱峰值强度大于Γ-lh跃迁引起的自发辐射谱峰值强度,但二者的积分强度几乎相等.此外,计算结果还证明了n型掺杂能显著提高锗的自发辐射强度.以上结果对于研究简并态半导体的自发辐射性质有重要的参考意义.     

Band structure of silicon and germanium thin films based on first principles

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic devices. First principles calculation based on density functional theory(DFT) with the generalized gradient approximation(GGA) are carried out to investigate the energy band gap structure on silicon(Si) and germanium(Ge) nanofilms. Simulation results show that the band gaps in Si(100) and Ge(111) nanofilms become the direct-gap structure in the thickness range less than 7.64 nm and7.25 nm respectively, but the band gaps of Si(111) and Ge(110) nanofilms still keep in an indirect-gap structure and are independent on film thickness, and the band gaps of Si(110) and Ge(100) nanofilms could be transferred into the direct-gap structure in nanofilms with smaller thickness. It is amazing that the band gaps of Si~((1-x)/2)Ge~xSi~((1-x)/2)sandwich structure become the direct-gap structure in a certain area whether(111) or(100) surface. The band structure change of Si and Ge thin films in three orientations is not the same and the physical mechanism is very interesting, where the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects.     

Struktur- und Absorptionspotentiale von KCl und NaCl aus Beugungsaufnahmen im konvergenten Elektronenbündel

By improvement of an earlier published preparation method for producing thin single crystals soluble in water, it is possible to determine structure and absorption potentials of some reflections of KCl and NaCl crystals by a convergent beam diffraction technique. The structure potentials evaluated on the basis of the two-beam dynamical theory are found to depend on the thickness of the crystals used. The structure potentials corrected for relativistic, thermal and many-beam effects are compared with theoretical values. The measured (220)- and (420)-absorption potentials of NaCl are in good agreement with theoretical values. Radiation damage in KCl and NaCl crystals by electron beams is observed.     

低温生长的Ge/Si超晶格界面结构的X射线散射研究

低温下用MBE方法生长了Ge/Si超晶格,X射线反射及横向散射研究表明,Ge亚层上下表面的粗糙度呈反对称,下表面大的粗糙度来源于Ge向Si亚层中扩散形成SiGe混合组分结构,这种组分结构可以用一平均成分的SiGe合金层加以拟合,从而使得各亚层均有一个合理的粗糙度,旋转样品进行的X射线散射研究表明,这种SiGe的混合是各向同性的,这与透射电子显微镜的研究结构相一致.     

Adiabatic electron-phonon interaction and high-temperature thermodynamics of A15 compounds

Inelastic neutron scattering was used to measure the phonon densities of states of the A15 compounds V3Si, V3Ge, and V3Co at temperatures from 10 to 1,273 K. It was found that phonons in V3Si and V3Ge, which are superconducting at low temperatures, exhibit an anomalous stiffening with increasing temperature, whereas phonons in V3Co have a normal softening behavior. First-principles calculations show that this anomalous increase in phonon frequencies at high temperatures originates with an adiabatic electron-phonon coupling mechanism. The anomaly is caused by the thermally induced broadening of sharp peaks in the electronic density of states of V3Si and V3Ge, which tends to decrease the electronic density at the Fermi level. These results show that the adiabatic electron-phonon coupling can influence the phonon thermodynamics at temperatures exceeding 1,000 K.     

Si/Gen/Si(001)异质结薄膜的掠入射荧光X射线吸收精细结构研究

利用掠入射荧光X射线吸收精细结构(XAFS)方法研究了在400℃的温度下分子束外延生长的Si/Ge_n/Si(001)异质结薄膜(n=1，2，4和8个原子层)中Ge原子的局域环境结构.结果表明，在1至2个Ge原子层(ML)生长厚度的异质结薄膜中，Ge原子的第一近邻配位主要是Si原子.随着Ge原子层厚度增加到4ML，Ge原子的最近邻配位壳层中的Ge-Ge配位的平均配位数增加到1.3.当Ge原子层厚度增加到8ML时，第一配位壳层中的Ge-Ge配位占的比例只有55%.这表明在400℃的生长条件下，Ge原子有很强的迁移到Si覆盖层的能力.随着Ge层厚度从1 增加到2，4和8ML，Ge原子迁移到Si覆盖层的量由0.5ML分别增加到1.5，2.0和3.0ML.认为在覆盖Si过程中Ge原子的迁移主要是通过产生Ge原子表面偏析来降低表面能和Ge层的应变能. 关键词： XAFS n/Si(001)异质膜')" href="#">Si/Ge_n/Si(001)异质膜 迁移效应     

Temperaturabhängigkeit der anomalen Elektronenabsorption von Wismut-Einkristallen

Polycrystalline bismuth foils could be transformed into single crystal plates by a tempering process. Using the so prepared single crystals of high atomic number and of known thickness the temperature dependence of a dynamical two-beam system for the diffraction of 80 keV electrons has been investigated between 300 K and 80 K. The measured intensities of the (¯2 20)-reflection and the primary beam were compared with the transparency of the polycrystalline foils. They increase exponentially with decreasing temperature and thickness of the crystal. This experimental behaviour can be explained by a dynamical electron-diffraction-theory.     

Electronic structure of SixGe1−x semiconductor solid solutions

The electronic structure of Si_xGe_1?x solid solutions is studied by means of a semi-ab initio method without involving the virtual crystal approximation. The calculations are performed on large cubic cells of the diamond lattice which contain a total of 64 Si and Ge atoms homogeneously distributed in different compositional ratios. The basis functions and the potentials used in the calculation give good band structures of elemental Si and Ge respectively. The calculated variation of band gap with x is in agreement with optical experiment: the two linear curves of gap vs concentration have different slopes at high and low x values with a crossing-over at about x=0.28. The band gaps are direct for xε (0, 0.28) and indirect for x ≥ 0.28. The density of states (DOS) of the solid solution can be very well approximated by the weighted average of the bulk Si and Ge DOS. There is a very slight charge transfer of about 0.05 electron per atom from Si to Ge.     

Photoconductivity of Si/Ge/SiO<Subscript>x</Subscript> and Si/Ge/Si structures with germanium quantum dots

A nonmonotonic dependence of the lateral photoconductivity (PC) on the interband light intensity is observed in Si/Ge/Si and Si/Ge/SiO_x structures with self-organized germanium quantum dots (QDs): in addition to a stepped increase in PC, a stepped decrease in PC is also observed. The effect of temperature and drive field on these features of the PC for both types of structures with a maximum nominal thickness of the Ge layer (N_Ge) is studied. The results obtained are discussed in the context of percolation theory for nonequilibrium carriers localized in different regions of the structure: electrons in the silicon matrix and holes in QDs.     

Hydrocarbon film formed on the surface of a semiconductor irradiated by an electron beam

The rate of formation of a hydrocarbon film on the surface of a sample subjected to the action of an electron beam is studied at room temperature and in cooling a sample to the liquid nitrogen temperature. The thickness and the optical radiation transmission of such films are measured as functions of the electron beam radiation time, the sample temperature, and the level of vacuum. The film thickness is measured with atomic force microscopy. The absorption of the films is determined by comparing the cathodoluminescence intensities from a pure sample surface and from the surface covered with a film. The experimental results can be used to estimate the film formation rate as a function of the sample temperature and the vacuum and to determine the optical radiation absorption at a wavelength of 300, 360, 550, and 665 nm.     

Temperature dependence of anomalous structure in the derivative absorption spectra of Si:P

Temperature dependence of wavelength-derivative absorption spectra of Si:P has been measured, with emphasis on temperature behaviors of anomalous exciton-absorption structure previously observed nearly at the free-exciton absorption thresholds. Such anomalous structure has been observed even at high temperatures where the principal bound-exciton component almost disappears. This result confirms our previous identification that the structure is related to free-exciton absorption rather than bound-exciton absorption.     

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.     

第一性原理研究[112]硅锗异质结纳米线的电子结构与光学性质

基于密度泛函理论体系下的广义梯度近似,本文利用第一性原理方法着重研究了[112]晶向硅锗异质结纳米线的电子结构与光学性质.能带结构计算表明:随着锗原子数的增加,[112]晶向硅锗纳米线的带隙逐渐减小;对Si_(36)Ge_(24)H_(32)纳米线施加单轴应变,其能量带隙随拉应变的增加而单调减小.光学性质计算则表明:随着锗原子数的增加,[112]硅锗纳米线介电函数的峰位和吸收谱的吸收边均向低能量区移动;而随着拉应变的增大,吸收系数峰值呈现出逐渐减小的趋势,且峰位不断向低能量区移动,上述结果说明锗原子数的增加与施加拉应变均导致[112]硅锗纳米线的吸收谱产生红移.本文的研究为硅锗异质结纳米线光电器件研究与设计提供一定的理论参考.     

A Calorimetric Study Assisted with First Principle Calculations of Specific Heat for Si-Ge Alloys within a Broad Temperature Range

Calorimetric measurements are performed to determine the specific heat of Si-xat.% Ge(where x = 0, 10, 30,50, 70, 90 and 100) alloys within a broad temperature range from 123 to 823 K. The measured specific heat increases dramatically at low temperatures, and the composition dependence of specific heat is evaluated from the experimental results. Meanwhile, the specific heat at constant volume, the thermal expansion, and the bulk modulus of Si and Ge are investigated by the first principle calculations combined with the quasiharmonic approximation. The negative thermal expansion is observed for both Si and Ge. Furthermore, the isobaric specific heat of Si and Ge is calculated correspondingly from OK to their melting points, which is verified by the measured results and accounts for the temperature dependence in a still boarder range.