Ultrasoft X-ray Emission Research Articles

Structure and optical properties of silicon nanopowders

Nanopowders obtained in the work by evaporation of silicon ingot with a powerful electron beam with the energy of electrons ∼1.4 MeV in the atmosphere of Ar and N 2 is a perspective object as for the fundamental scientific investigations as for the practical application. These materials were investigated by transmission electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, ultrasoft X-ray emission spectroscopy (USXES) and X-ray absorption near-edge structure (XANES). Results of investigations demonstrated that for a mean size of the particles of about 3–4 nm they consist of the ordered nanocrystalline silicon, amorphous silicon and silicon oxide. In this case photoluminescence can be observed in the visible spectral range with a peak at 2.1 eV. USXES spectra show that the particles of the powder consist of the phase with the ordered crystalline structure, amorphous silicon and silicon oxide. According to XANES data, the particles of nanopowders are covered with oxide layer with a thickness that is essentially greater than the natural silicon oxide.

ELECTRON STRUCTURE INVESTIGATIONS OF InGaP/GaAs(100) HETEROSTRUCTURES WITH InP QUANTUM DOTS

Ultrasoft X-ray emission spectra (USXES) and X-ray absorption near-edge structure (XANES) spectra with the use of synchrotron radiation in the range of P L2,3-edges were obtained for the first time for nanostructures with InP quantum dots grown on GaAs 〈100〉 substrates by vapor-phase epitaxy from metal–organic compounds. These spectra represent local partial density of states in the valence and conduction bands. The additional XANES peak is detected; its intensity depends on the number of monolayers forming quantum dots. Assumptions are made on the band-to-band origin of luminescence spectra in the studied nanostructures.

Structure of Al2O3 thin layers synthesized on the silicon surface by atomic layer deposition

The distribution of the phase and chemical composition at an Al2O3/Si interface is studied by depth-resolved ultrasoft x-ray emission spectroscopy. The interface is formed by atomic layer deposition of Al2O3 films of various thicknesses (from several to several nanometers to several hundreds of nanometers) on the Si(100) surface (c-Si) or on a 50-nm-thick SiO2 buffer layer on Si. L 2,3 bands of Al and Si are used for analysis. It is found that the properties of coatings and Al2O3/Si interfaces substantially depend on the thickness of the Al2O3 layer, which is explained by the complicated character of the process kinetics. At a small thickness of coatings (up to 10–30 nm), the Al2O3 layer contains inclusions of oxidized Si atoms, whose concentration increases as the interface is approached. As the thickness increases, a layer containing inclusions of metallic Al clusters forms. A thin interlayer of Si atoms occurring in an unconventional chemical state is found. When the SiO2 buffer layer is used (Al2O3/SiO2/Si), the structure of the interface and the coating becomes more perfect. The Al2O3 layer does not contain inclusions of metallic aluminum, does not vary with the sample thickness, and has a distinguished boundary with silicon.

Investigations of the electron energy structure and phase composition of porous silicon with different porosity

Energy structure of the valence and conduction bands together with phase composition of porous silicon (por-Si) obtained at different etching times were investigated using ultrasoft X-ray emission spectroscopy (USXES) and X-ray absorption near-edge structure (XANES) techniques. USXES and XANES spectra represent local partial density of states in valence and conduction band, correspondingly. XANES data were obtained with the use of synchrotron radiation at synchrotron BESSY II. Porous silicon layers were obtained on different types of single-crystalline silicon (1 0 0) and (1 1 1) substrates by standard electrochemical etching in HF solution with etching times from 1 to 10 min. USXES and XANES investigations show formation of Si nanocrystals covered by amorphous phase together with nonstoichiometric silicon sub-oxide SiO x ( x ∼ 1.3) in the surface layers. XANES investigations show that the increase of sample porosity leads to the shift of conduction band edge to the higher energy range that corresponds to increase of the band gap width in porous silicon up to ∼1.4 eV.

Effect of PbO layers with nickel oxide and vanadium oxide additions on the thermal oxidation of InP

IR and ultrasoft x-ray emission spectroscopy data indicate that the thermal oxidation of PbO/InP structures with additions of transition-metal (nickel and vanadium) oxides leads to the formation of multicomponent layers consisting of nickel, vanadium, lead, and indium phosphates. These oxide additions to PbO nanolayers accelerate the thermal oxidation of InP. A small amount of vanadium oxide added to PbO gives rise to a significant contribution of a catalytic mechanism to the accelerated thermal oxidation of InP. The principal distinction between the PbO/InP and (PbO + NiO)/InP structures is that the thermal oxidation of the latter leads to the formation of an InxNiyPz transition layer, which influences the growth rate and the composition of the resulting layer.

Ultrasoft x-ray reflection and emission spectroscopic analysis of Al2O3/Si structure synthesized by the atomic layer deposition method

X-ray reflection spectroscopy and x-ray emission spectroscopy using electron excitation were used to characterize the 30 nm-thick Al2O3 film grown on a silicon substrate by atomic layer deposition (ALD). According to our investigations the film was amorphous and inhomogeneous in depth. The interfacial layer between film and substrate that was a mixed layer containing Al2O3, a large amount of the SiO2 and elemental Si was evidenced. The concentration of SiO2 increases as one approaches the Si(100) substrate interface because of the interdiffusion of the oxygen and silicon, followed by silicon oxidation. Copyright © 2006 John Wiley & Sons, Ltd.

Electronic structure of diamond/graphite composite nanoparticles

The electronic structure of samples produced by nanodiamond annealing has been examined using ultra-soft X-ray emission and X-ray absorption spectroscopy. Analysis of spectra of diamond/graphite composites showed that carbon atoms constituting the nanoparticles are at least in three states: diamond-like state, graphitic-like state and interface carbon, characterized by high electron localization. Comparison between theoretical spectra of the models and experimental spectra suggested the latter states correspond to three-coordinated carbon atoms from diamond surface.

A study of the local electronic and atomic structure in a-SixC1−x amorphous alloys using ultrasoft X-ray emission spectroscopy

X-ray spectroscopy has been used to obtain data on the local electronic and atomic structure of a-SixC1−x :H(Er) alloys produced by plasma-enhanced chemical vapor deposition (PECVD) with various relative amounts of silane and methane in the gas mixture (x=0.3–0.9). It is shown that the alloys contain silicon and carbon atoms in different coordination environments. Silicon is observed as elementary amorphous silicon and silicon carbide, and the relative amounts of these phases in the films depend on the composition of the gas mixture. Carbon atoms can form bonds with silicon in a coordination close to that found in crystalline silicon carbide, with a noticeable amount of C-H bonds also appearing. In addition, carbon can form an elementary carbon phase with various coordination numbers characteristic of graphite and diamond in the film.

Properties of an Al2O3/Si interface

The phase chemical composition of an Al2O3/Si interface formed upon molecular deposition of a 100-nm-thick Al2O3 layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L 2, 3 emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al2O3 layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al2O3 molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO2, or SiOx (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO2 is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al2O3 layer and differ significantly from the properties of the interfaces of Al2O3 thin layers.

ChandraObservations of X‐Ray–weak Narrow‐Line Seyfert 1 Galaxies

We present Chandra observations of 17 optically-selected, X-ray weak narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified by Williams et al. (2002) in the Sloan Digital Sky Survey Early Data Release, but were not found in the ROSAT All-Sky Survey (RASS) despite having optical properties similar to RASS-detected NLS1s. All objects in this sample were detected by Chandra and exhibit a range of 0.5-2 keV photon indices Gamma=1.1-3.4. One object was not detected in the soft band, but has a best-fit Gamma=0.25 over the full 0.5-8 keV range. These photon indices extend to values far below what are normally observed in NLS1s. A composite X-ray spectrum of the hardest objects in this sample does not show any signs of absorption, although the data do not prohibit one or two of the objects from being highly absorbed. We also find a strong correlation between Gamma and L_1keV; this may be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such variations are seemingly in conflict with the current paradigm that NLS1s accrete near the Eddington limit. Most importantly, this sample shows that strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in fact, a substantial number may exhibit weak and/or low-Gamma X-ray emission.

X-ray spectral analysis of the interface of a thin Al2O3 film prepared on silicon by atomic layer deposition

The extent and phase chemical composition of the interface forming under atomic layer deposition (ALD) of a 6-nm-thick Al2O3 film on the surface of crystalline silicon (c-Si) has been studied by depthresolved, ultrasoft x-ray emission spectroscopy. ALD is shown to produce a layer of mixed Al2O3 and SiO2 oxides about 6–8 nm thick, in which silicon dioxide is present even on the sample surface and its concentration increases as one approaches the interface with the substrate. It is assumed that such a complex structure of the layer is the result of interdiffusion of oxygen into the layer and of silicon from the substrate to the surface over grain boundaries of polycrystalline Al2O3, followed by silicon oxidation. Neither the formation of clusters of metallic aluminum near the boundary with c-Si nor aluminum diffusion into the substrate was revealed. It was established that ALD-deposited Al2O3 layers with a thickness up to 60 nm have similar structure.

Ultrasoft x-ray emission from a linear ECR plasma source

Ultrasoft x-ray emission in the energy range 10–100 eV is observed from a linear electron cyclotron resonance (ECR) plasma system. ECR plasma is produced in a short cylindrical stainless steel (SS) chamber with a central axial magnetic field of 875 G, generated by two coils placed axially to have the first two ECR surfaces inside the experimental chamber. Plasma is produced in a pressure range of 2 × 10−5 mbar with hydrogen. Also, plasma is produced with net microwave power varying from 160 to 800 W at 2.45 GHz frequency from a magnetron. The waveguide from the magnetron to the vacuum vessel supports rectangular TE10 mode and the electromagnetic wave propagation in plasma is in extraordinary (X) mode. The ultrasoft x-ray emission is observed with a vacuum photodiode. The results clearly indicate a decrease in x-ray emission on increasing the working pressure and an increase in emission with an increase in the input microwave power. The experiments also ensure that the presence of a metal target enhances the intensity of soft x-rays generated in the ECR plasma system.

Soft X-ray emission spectroscopy of SiO2/Si structures irradiated with high-energy electrons

SiO2/Si structures implanted with Si+ ions and irradiated with high-energy electrons were studied by means of ultrasoft X-ray emission spectroscopy with variations of the electron excitation energy. It was found that previous Si+ ion implantation blocks the oxidation of the Si substrates irradiated with 20-MeV electrons. This effect is more pronounced for samples prepared on n-type silicon substrates, and it also depends on the post-oxidation treatment of the samples.

X-ray-emission study of the structure of Si:H layers formed by low-energy hydrogen-ion implantation

Amorphous silicon layers formed by implantation of 24-keV hydrogen ions into SiO2/Si and Si with doses of 2.7×1017 and 2.1×1017 cm−2, respectively, were studied using ultrasoft X-ray emission spectroscopy with variations in the energy of excitation electrons. It is ascertained that the surface silicon layer with a thickness as large as 150–200 nm is amorphized as a result of implantation. Implantation of hydrogen ions into silicon coated with an oxide layer brings about the formation of a hydrogenated silicon layer, which is highly stable thermally.

USXES AND OPTICAL PHENOMENA IN Si LOW-DIMENSIONAL STRUCTURES DEPENDENT ON MORPHOLOGY AND SILICON OXIDE COMPOSITION ON Si SURFACE

It has been shown that the intensive and broad "red" photoluminescence band in porous silicon is a nonelementary one and could be decomposed on at least three elementary bands. Photoluminescence, ultrasoft X-ray emission spectroscopy, infrared absorption and atomic force microscopy methods were used to study the reasons for both luminescence band appearance in porous silicon photoluminescence spectra, prepared in different technological conditions. The mechanisms of radiative transition for both elementary bands have been discussed as well.

Silicon network in a-Si:H films containing ordered inclusions

a-Si:H films with inclusions of (SiH2)n clusters or Si nanocrystals have been grown by magnetron-assisted SiH4 decomposition (dc-MASD). The films were characterized by the microstructural parameter R=0.7–1.0. Ultrasoft X-ray emission spectroscopy was applied to establish the effect of these inclusions on the increasing ordering of Si network. It is shown that, irrespective of the nature of the inclusions, their effect is strongest for films of intrinsic material deposited at high temperatures (up to 400°C).

Determination of the phase composition of surface layers of porous silicon by ultrasoft X-ray spectroscopy and X-ray photoelectron spectroscopy techniques

Abstract The phase composition of the surface layers in porous silicon (por-Si) is determined by analysis of the density of states in the valence band using ultrasoft X-ray emission spectroscopy (USXES) and X-ray photoelectron spectroscopy (XPS) of the core levels. Since porous silicon demonstrates instability of its properties the investigations of the changes in its phase composition under atmosphere exposure (e.g. low-temperature anneals in an oxygen flow) have been made. Phase analysis by USXES is performed with the help of a specially elaborated mathematical technique using expansion of the experimental spectrum by the spectra of some standard samples. c-Si and c-SiO2 as well as disordered phases of these species (by USXES) and low-oxidized defect silicon (by USXES and XPS) have been detected in the surface layers of por-Si. Some changes in the composition were found in the samples annealed in oxygen at various temperatures.

Operation of the ultrasoft x-ray system on NSTX (abstract)

The ultrasoft x-ray imaging system on National Spherical Torus Experiment (NSTX) became operational and provided the first data in the filtered diode slow bow tie configuration. Using different band pass filters on each of three arrays allows an approximate spectroscopic estimate of the plasma impurity content, as well as of the electron temperature. Magnetohydrodynamics (MHD) activity from different plasma regions is also observed. The soft x-ray emission profiles are well behaved until an Internal Reconnection Event occurs. Examples of NSTX MHD phenomena seen in the ultrasoft x-ray emission under different operational regimes will be presented. From a technical point of view, we point out that the industrial PC based data acquisition system was not adversely affected by stray magnetic fields due to its close proximity to the NSTX device. Also, the surface barrier diodes withstood baking to 100 °C relatively well.

Modifications of the structure and electrical parameters of the films of amorphous hydrogenated silicon implanted with Si+ ions

The influence of implantation of Si+ ions with energies of 30, 60, and 120 keV was studied on the dark conductivity, photoconductivity, hydrogen concentration, microstructure parameter, and special features of the ultrasoft X-ray emission spectra of a-Si:H films that were deposited at Ts=300°C by the dc-MASD and rf-PECVD methods and that differed in initial structural characteristics.

X-ray emission study of ion beam mixed Cu/Al films on polyimide

Cu (40 nm)/Al/polyimide/Si was mixed with 80 keV Ar+ and N2+ from 5.0×1015 to 15×1015 ions/cm2. Ultrasoft x-ray emission valence spectra (XES) of Cu, C, N and O excited by electron and photon radiation were used for study of chemical reactions in Cu/Al/PI/Si and PI/Si systems induced by ion beam mixing in dependence of type of ions and dose. It is found that ion beam mixing changes the chemical state of Cu atoms with respect to that of pure metal. These changes depend on the dose of ion beam bombardment and type of ions and are attributed to a formation of CuAl2O4 interfacial layer, which can be responsible for enhanced interfacial adhesion strength. On the other hand, it is shown that the shape of C Kα, N Kα and O Kα XES of ion beam mixed polyimide layer (PI/Si) is modified with ion bombardment. This means that the ion-beam mixing process is able to break the bonding of constituent atoms of irradiated PI layers and can induce the formation of chemically bonded complexes linking atoms in the Cu, Al and PI layers.