Visible photoluminescence from silicon nanoclusters embedded in silicon nitride films prepared by remote plasma-enhanced chemical vapor deposition

The photoluminescence (PL) of silicon nanoclusters embedded in silicon nitride films grown by remote plasma-enhanced chemical vapor deposition at 200 °C, using mixtures of SiCl₄/H₂/Ar/NH₃ is investigated. It was found that the color and the intensity of the PL of the as-grown samples depend on the H₂ flow rate, and there is an optimum flow for which a maximum luminescence is obtained. A strong improvement of the PL intensity and change in color was obtained with annealing treatments in the range of 500–1000 °C. The changes in the composition, structure and optical properties of the films, as a function of H₂ flow rate and thermal treatments, were studied by means of Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, ellipsometry and ultraviolet–visible transmission measurements. We conclude that the PL can be attributed to quantum confinement effect in silicon nanoclusters embedded in silicon nitride matrix, which is improved when a better passivation of the nanoclusters surface is obtained.     

带状束高功率微波源宽通带收集极

设计了一种适用于带状电子束高功率微波源的宽通带收集极,在有效吸收束-波相互作用后的带状电子束的同时,保证了带状电子束高功率微波源的工作模式矩形波导TM11模式高效率地通过。研究结果表明：在13~27 GHz范围内,功率传输效率大于95%,这一宽通带特性使得该类型的收集极与带状电子束高功率微波源能够更好配合,显著提高了微波源的模拟优化和实验调试效率;TM11模式微波的传输效率对收集极厚度和长度等参数不敏感;该类型收集极结构具有良好的散热能力,在不加外部水冷装置的条件下,仅靠空气自然对流冷却和辐射冷却,可以承受电流3 kA、电压300 kV、脉冲宽度30 ns及重复频率50 Hz带状电子束的连续冲击。     

Removal of oxygen from silicon by electron beam melting

Small amounts of multicrystalline silicon were melted in an electron beam furnace in different experimental conditions in order to investigate the oxygen evaporation behavior during the electron beam melting (EBM) process. The oxygen content level before and after EBM was determined by secondary ion mass spectroscopy. The oxygen content was reduced from 6.177 to 1.629 ppmw when silicon was melted completely at 15 kW with removal efficiency up to 73.6 %. After that, it decreased continually to <0.0517 ppmw when the refining time exceeded 600 s with a removal efficiency of more than 99.08 %. During the melting process, the evaporation rate of silicon is 1.10 × 10^?5 kg/s. The loss of silicon could be reduced up to 1.7 % during oxygen removal process to a desirable figure, indicating EBM is an effective method to remove oxygen from silicon and decrease the loss of silicon.     

Parametric instabilities produced by a relativistic electron beam in a plasma

The parametric instability driven by the primary spectrum of the hydrodynamic two-stream instability produced by a relativistic electron beam in a plasma is investigated. The saturated level of the primary wave electric field is determined by electron trapping in the potential well of the wave or by the quasilinear beam relaxation process. After saturation, the primary wave collapses by way of the oscillating two-stream instability. The cases of the strong and weak primary electric field in comparison with the thermal energy of a plasma are considered. For a strong field the growth rates of the parametric instability and plasma heating due to the latter are found. Ion heating is not significant in comparison with electron heating (approximately as the cube root of the mass ratio). In a weak field the parametric oscillating two-stream spectrum of saturation is found. In the one-dimensional case this spectrum of electric field energy fluctuations varies as k⁻² if the fluctuation field exceeds the threshold pump electric field for the oscillating two-stream instability. For the weak field plasma heating rate is found. Since the energy transfer is via Landau damping, the particle heating is characterized by the formation of high-energy tails on the distribution function.     

Magnetic iron oxide nanopowders produced by CO2 laser evaporation

Magnetic iron oxide nanopowders were produced with the laser evaporation technique under normal process gas pressure. In addition, the generated particles were coated in situ with stearic acid and separated on magnets. The methods and the used laboratory setup are briefly described. Influences of essential process conditions, particularly the use of continuous and pulsed laser radiation as well as the properties of the process gas, were experimentally investigated. The produced nanopowders were analysed with TEM, XRD, and magnetic measurements and confirmed the in-principle suitability of the presented method.     

Well-aligned ZnO nanocolumns grown by reactive electron beam evaporation

We report on the reactive electron beam evaporative growth of well-aligned ZnO nanocolumns on Si (001) wafers in the environment of NH₃/H₂ gas mixture by using polycrystalline ZnO ceramic target as source material. The growth was carried out at low temperatures (400-450 °C) without employing any metal catalysts. Field emission scanning electron microscopy (FESEM) revealed that nanocolumns with uniform distributions in their diameters, lengths, and densities were grown vertically from the substrates and terminated by smooth hexagonal (0001) facets with no terrace-like steps emerged, which should render potential applications such as inherent resonance cavities in fabricating ultraviolet-laser arrays. X-ray diffraction measurements revealed that ZnO nanocolumns were highly c-axis oriented, which is well consistent with the FESEM observations. More importantly, photoluminescence investigations of the nanocolumns demonstrated the strong excitonic emission and extremely weak deep level emission, indicating the high crystalloid and optical quality of the nanocolumns.     

Changes in the silicon microhardness induced by a low-intensity electron beam

It is shown that nonequilibrium point defects are of primary importance in the changes in the silicon microhardness induced by a low-intensity (I ～ 10⁵ cm^?2 s^?1) electron beam. It is found that the necessary condition for softening under low-intensity electron irradiation is the presence of an oxide layer on the surface. The thickness of the surface layer in which anomalous changes in the microhardness are observed is determined by the layer-by-layer etching technique.     

Microstructure of metals and alloys in the heat-affected zone produced by a high-power ion beam

The microstructure of heat-affected zones produced by high-power ion beams in Al, Cu, Ni, and AD1 and Monel alloys are investigated by optical and scanning electron microscopy and X-ray diffraction analysis. It is ascertained that the microstructure is divided into characteristic layers differing in terms of grain sizes and phase components. It is shown that the formation of the structural-phase states of each layer occurs in both solid and liquid phases under pulsed ion-beam treatment and depends on the irradiation parameters and the thermophysical characteristics of a target.     

Determination of the profile of energy release from a high-power electron beam in an aerogel

Optical methods are used to investigate the dynamics of the interaction of a high-current electron beam with an aerogel (a highly porous transparent dielectric with a low density ρ=0.36 g/cm³). The measured profile of the glow of the aerogel and the pattern of its expansion are compared with the results of a numerical simulation. The influence of the space charge on the profile of the energy absorption from the high-current relativistic electron beam is discussed. Zh. Tekh. Fiz. 67, 26–32 (November 1997)     

Optical constants of aluminum films prepared by electron beam evaporation

The metal aluminum （Al） is widely used because it has high reflectivity from the ultraviolet to the infrared band. But the new deposited Al films is exposed to the atmosphere, it forms transparent Al2O3 films on its surface at once. In this letter, the Al films is deposited on the quartz substrate by electron beam evaporation. The effect of Al films oxidation on refractive index and extinction coefficient is investigated by spectroscopic ellipsometry （SE）. The optical constants of Al films change with the increase of oxidation time. The two parameters become stable when these films are exposed in air more than 2 days.     

Visible photoluminescence from Ge quantum dots

Spectroscopic analyses on stacked Ge quantum dots (QDs) on Si (1 0 0) substrates are presented. Strong and visible photoluminescence around 620 nm from stacked Ge QDs is observed. The luminescence is intense and clearly visible to the naked eye at both room temperature and low temperature. We have investigated the temperature dependence of the luminescence, as well as the composition of Ge dots via transmission electron microscopy and the Raman spectroscopy. Possible causes of the visible luminescence are also speculated in this report.     

“Instantaneous” spectrum of passively ionized electrons in a dielectric irradiated by a high-power electron beam

A technique is developed for theoretical analysis, and the “instantaneous energy spectrum,” that is, the energy spectrum of passively ionized electrons in a dielectric irradiated by an electron beam of moderate or high density which arises prior to electron-phonon relaxation is calculated. The source of the beam is usually a high-current electron accelerator. The computation algorithm is designed so as to make it possible to fully take into account and make use of the following data: first, the actual (measured) spectrum of the electron beam; second, the complete electron spectrum of the dielectric, including the spectrum of the density of occupied states, which is continuous within the valence bands and discrete in the region of the low-lying quasiatomic levels; and the spectrum of the density of unoccupied states, which begins in the conduction band and, in a continuous process, turns into the spectrum of quasi-free electrons in the crystal; third, the differential ionization cross-sections of the atoms due to electron impact, which the present report refines so as to take in into account the actual spectrum of the density of the final states; the latter spectrum is continuous within the valence bands and discrete in the region in which the law of dispersion of a band electron turns into the law of dispersion of a quasi-free electron. The results of the theoretical analysis are compared with the data of a numerical experiment carried out in the present report by the Monte-Carlo method using the same initial data. The agreement between the theoretical results and the data from the numerical experiment demonstrates the stability and reliability of the algorithm used to calculate the instantaneous spectrum. The precision of the calculation is determined solely by the initial data. The instantaneous spectrum which is obtained is the initial distribution function for all the kinetic equations describing subsequent relaxation of the electrons in the irradiated dielectric. Tomsk Polytechical Institute. Tomsk State University. Institute of High-Current Electronics, Russian Academy of Sciences (Siberian Division). Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 136–144, November, 1996.     

电子束激发原子对金属蒸发的影响

在原子蒸气激光同位素分离工程(AVLIS)中,金属受电子束的加热而熔化,并向真空蒸发,蒸气原子通过电子束的过程中,可能与电子发生非弹性碰撞,被激发到高能级.在膨胀的过程中,高能级的原子通过与原子的非弹性碰撞消激发,将原子内的电子能量转换为蒸气的平动能.为了分析电子束与原子作用对蒸气的密度、速度和温度等物理特性的影响,采样直接模拟蒙特卡罗法(DSMC)模拟钆原子蒸发过程.模拟结果表明,电子束与原子的作用使得原子蒸气的速度增加,密度减小,温度升高 关键词： 金属蒸发 AVLIS DSMC 电子枪     

Visible photoluminescence from a-Si : H/SiO2 superlattices fabricated by UHV evaporation

Hydrogenated amorphous-Si/SiO₂ (a-Si:H/SiO₂) superlattices with different a-Si : H thickness in the range of a few nanometers have been fabricated by ultra high vacuum evaporator (UHV evaporator). The photoluminescence (PL) of our superlattices is observed in the visible spectral region and the peak energy shifts to higher energy as the a-Si : H layer thickness decreases. The temperature dependence of the PL spectra reveals four sub-bands by fitting. Bands at 2.2, 1.9, 1.65 and 1.45 eV are detected and are attributed to E′δ centers, nonbridging-oxygen–hole centers (NBOHC), Si/SiO₂ interface and a-Si : H layer, respectively. We explain the overall blueshift of the PL spectra by the modification of the contribution of these sub-bands.     

Simulation of generation of ultradisperse particles upon irradiation of metals by a high-power electron beam

A model of formation of ultradisperse particles in the plasma torch emerging during evaporation of a metal target by a high-power electron beam is described. A model of heterogeneous media is proposed for describing the plasma torch dynamics taking into account heat conduction, heat transfer and friction between components, relaxation of components to equilibrium, condensation, and evaporation and coagulation of drops as a result of their collisions. Numerical simulation of the generation of ultradisperse particles in the plasma torch formed during irradiation of a metal target by a powerful electron beam is performed. The size distribution of ultradisperse particles is obtained for various regimes of irradiation and cooling.     

Use of an electron accelerator to produce nanopowders by evaporation of initial materials at atmospheric pressure

High-purity nanodispersed powders with extraordinary properties that can be used in various technologies (electronics, catalysis, ceramics, modification of composite rheology, and so on) can be produced by evaporation of various initial materials at atmospheric pressure. In particular, the authors succeeded in producing nanodispersed powders of oxides [including silicon dioxide and oxide (SiO ₂ and SiO) and magnesium (MgO), aluminum (Al ₂ O ₃), and cuprous (Cu ₂ O) oxides], metals [including tantalum (Ta), molybdenum (Mo), aluminum (Al), silver (Ag), and some others] in various atmospheres, semiconductors [including silicon (Si)], nitrides [including aluminum (AlN) and titanium (TiN) nitrides], and other materials. It is important for the developed production process that the main component of the setup (an industrial accelerator) is capable of creating high temperatures for evaporation of any refractory materials. The production process has high efficiency and productivity (reaching several kilograms of oxides per hour). __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 22–26, February, 2007.     

Estimation of the longitudinal electric field produced by a relativistic electron beam in a plasma

An axially symmetric model is used to analytically estimate the longitudinal electric field created in a plasma by an injected electron beam. The relationship can be used to estimate the electron beam energy loss in the range of parameters where theoretical studies rely only on numerical simulations. The results are of interest for creating relativistic electron beams, studying their relaxation in plasma, and developing new methods for charged particle acceleration.     

Study of the acoustic field produced by a 50-MeV electron beam in water

For the first time, a detailed space-time dependence of acoustic pressure produced by a 50-MeV electron beam in water are obtained at the electron accelerator of Moscow State University. Measurements are performed at 100 points taken along one line parallel to the beam axis and spaced 6.5 cm apart from this axis. The 2D (time-distance) patterns display two paths corresponding to signals from two sound sources: a cylindrical acoustic antenna produced by the electron beam and the entrance cap separating the water and air media.