Characteristic and non-characteristic X-ray yields produced from thick Ti element by sub-relativistic electrons

Measurements are performed to study the electron impact energy dependence of doubly differential bremsstrahlung yields (DDBY) and of characteristic Ti K_α line yields produced from sub-relativistic electrons (10–25 keV) colliding with a thick Ti (Z = 22) target. The emitted radiation is detected by a Si-PIN photo-diode detector with energy resolution (FWHM) of 180 eV at 5.9 keV. The measured data of DDBY are compared with the results predicted by Monte-Carlo (MC) simulations using the general purpose PENELOPE code. A reasonable agreement is found between experimental and simulation results within the experimental uncertainty of measurements of 12%. Characteristic Ti K_α yields are obtained for the considered impact energy range and they are compared with the existing theoretical results. A good agreement is found between the present measurements and the theoretical calculations. Furthermore, data are presented for impact energy dependence of the ratio K_α/(K_α+ K_β) of a thick Ti target under impact of 10–25 keV electrons. The ratio shows a very weak dependence on impact energy in the studied range. The average value of the ratio is found to be 0.881 ± 0.003.     

Determination of the positron diffusion length in Kapton by analysing the positronium emission

Doppler profile spectroscopy and Compton-to-peak ratio analysis have been used to study the positronium (Ps) emission from the Kapton surface as a function of the positron implantation energy E.Two different positions for the sample have been performed in the experiment.In the first case the sample and the Ge-detector are perpendicular to the positron beam. With this geometry the emission of para-positronium (p-Ps) is detected as a narrow central peak.In the second case, by rotating the sample 45° with respect to the beam axis, the emission of p-Ps is detected as a blue-shifted fly away peak. The implantation of the positrons is described by the Makhov profile, where we used the modified median implantation for polymers as given by Algers et al. [J. Algers, P. Sperr, W. Egger, G. Kögel, F.H.J. Maurer, Phys. Rev. B 67 (2003) 125404].Thermalised positrons can diffuse to the surface and may pick up an electron to be emitted as Ps. We found a thermal and or epithermal positron diffusion length L₊ = 5.43 ± 0.71 nm and L₊ = 5.51 ± 0.28 nm correspondingly for both cases, which is much more than the one found by Brusa et al. [R.S. Brusa, A. Dupasquier, E. Galvanetto, A. Zecca, Appl. Phys. A 54 (1992) 233]. The respective efficiency for the emission of Ps by picking up an electron from the surface is found to be f_pu = 0.247 ± 0.012 and f_pu = 0.156 ± 0.003.     

Variation of enhancement effect of Coster-Kronig transition of L3 X-Rays of Ba, La and Ce compounds

The Coster-Kronig (CK) enhancement effect was measured for L₃ subshell X-rays using the experimental Lα X-ray production cross-section, the fraction of Lα X-rays, L₃ subshell fluorescence yields and L₃ subshell photoionisation cross-section. The samples were excited by gamma-rays with 59.5 keV energy from a 75 mCi radioisotope source and L X-rays emitted from samples were counted by a Si(Li) detector with resolution 155 eV at 5.96 keV. Variation of enhancement effect of CK transition of L₃ X-rays of La and Ce compounds was measured to be more than that of Ba. Ba has a partially filled 6s orbital whereas La and Ce have partially filled 5d and 4f orbitals, respectively.     

Preparation and characterization of CdS/Si coaxial nanowires

CdS/Si coaxial nanowires were fabricated via a simple one-step thermal evaporation of CdS powder in mass scale. Their crystallinities, general morphologies and detailed microstructures were characterized by using X-ray diffraction, scanning electron microscope, transmission electron microscope and Raman spectra. The CdS core crystallizes in a hexagonal wurtzite structure with lattice constants of a=0.4140 nm and c=0.6719 nm, and the Si shell is amorphous. Five Raman peaks from the CdS core were observed. They are 1LO at 305 cm⁻¹, 2LO at 601 cm⁻¹, A₁-TO at 212 cm⁻¹, E₁-TO at 234 cm⁻¹, and E₂ at 252 cm⁻¹. Photoluminescence measurements show that the nanowires have two emission bands around 510 and 590 nm, which originate from the intrinsic transitions of CdS cores and the amorphous Si shells, respectively.     

Absolute electron impactK-ionization cross sections of titanium and nickel (≦50 keV)

The absoluteK-ionization cross sections of Ti and Ni by electron impact (impact energy ≦50 keV) was measured detecting the X-rays emitted by thin solid film targets of known mass thickness with a flow proportional counter. The experimental method, especially the correction procedures and the measurements are described, the results compared with calculations in different theoretical approaches. For impact energies aboveE ₀/E _K>1.5 (E _K=K-shell ionization energy) a systematic deviation of about +20% occurs in comparison with the best agreeing calculations of M.R.H. Rudge and S.B. Schwartz. A fit to Drawin's empirical formula reveals that the measurements are approximated better than ±10 % within the range of comparison.     

Scattering effects and damping of electrons in Pt(1 1 1) and Cu(1 1 1)

In this work an analysis of experimental and theoretical data associated with the scattering and attenuation of electrons in the Pt(1 1 1) and Cu(1 1 1) crystalline samples is presented. The information about the crystalline structure of the first few atomic layers was obtained by the directional elastic peak electron spectroscopy (DEPES) at the primary electron beam energies E_p from 1.5 keV to 2.0 keV. The comparison of the experimental and theoretical DEPES distributions indicates a qualitative agreement between experiment and theory. The relative signal values associated with the intensity maxima were found to be different. The latter effect suggest that the electron attenuation in the crystalline samples can have an anisotropic character. A qualitative analysis of the characteristic pattern around the [1 1 1] direction concerning the calculation of the scattering factors was performed. The collective scattering of electrons by atoms located around the threefold symmetry axis resulting in the so called ring focusing effect is discussed.     

Enhanced field emission from Si doped nanocrystalline AlN thin films

Si doped and undoped nanocrystalline aluminum nitride thin films were deposited on various substrates by direct current sputtering technique. X-ray diffraction analysis confirmed the formation of phase pure hexagonal aluminum nitride with a single peak corresponding to (1 0 0) reflection of AlN with lattice constants, a = 0.3114 nm and c = 0.4986 nm. Energy dispersive analysis of X-rays confirmed the presence of Si in the doped AlN films. Atomic force microscopic studies showed that the average particle size of the film prepared at substrate temperature 200 °C was 9.5 nm, but when 5 at.% Si was incorporated the average particle size increased to ∼21 nm. Field emission study indicated that, with increasing Si doping concentration, the emission characteristics have been improved. The turn-on field (E_to) was 15.0 (±0.7) V/μm, 8.0 (±0.4) V/μm and 7.8 (±0.5) V/μm for undoped, 3 at.% and 5 at.% Si doped AlN films respectively and the maximum current density of 0.27 μA/cm² has been observed for 5 at.% Si doped nanocrystalline AlN film. It was also found that the dielectric properties were highly dependent on Si doping.     

Luminescence characterization and electron beam induced chemical changes on the surface of ZnAl2O4:Mn nanocrystalline phosphor

Luminescence characteristics and surface chemical changes of nanocrystalline Mn²⁺ doped ZnAl₂O₄ powder phosphors are presented. Stable green cathodoluminescence (CL) or photoluminescence (PL) with a maximum at ∼512 nm was observed when the powders were irradiated with a beam of high energy electrons or a monochromatic xenon lamp at room temperature. This green emission can be attributed to the ⁴T₁ → ⁶A₁ transitions of the Mn²⁺ ion. Deconvoluted CL spectra resulted in two additional emission peaks at 539 and 573 nm that may be attributed to vibronic sideband and Mn⁴⁺ emission, respectively. The luminescence decay of the Mn²⁺ 512 nm emission under 457 nm excitation is single exponential with a lifetime of 5.20 ± 0.11 ms. Chemical changes on the surface of the ZnAl₂O₄:Mn²⁺ phosphor during prolonged electron beam exposure were monitored using Auger electron spectroscopy. The X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the possible compounds formed on the surface as a result of the prolonged electron beam exposure. The XPS data suggest that the thermodynamically stable Al₂O₃ layer was formed on the surface and is possibly contributing to the CL stability of ZnAl₂O₄:Mn phosphor.     

Study on the structure and optical property of Zn1−xCuxO sol–gel thin films on quartz substrate

Zn_1−xCu_xO thin films (x=0, 1.0, 3.0, 5.0%) are prepared on quartz substrate by sol–gel method. The structure and morphology of the samples are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results show that Cu ions were effectively penetrated into the ZnO crystal lattices with substitutional and interstitial impurities to form stable solid solutions without changing the polycrystalline wurtzite structure. Two peaks at 420 nm (2.95 eV, violet), 485 nm (2.56 eV, blue) have been observed from the photoluminescence (PL) spectra of the samples. It is concluded that the violet peak may correspond to the exciton emission; the blue emission corresponds to the electron transition from the bottom of the conduction band to the acceptor level of zinc vacancy. The optical test shows that the optical band gap E_g is decreased with the increase amount of Cu doping in ZnO. The band gap decrease from 3.40 eV to 3.25 eV gradually. It is also found that the transmission rate is increased rapidly with the increase of Cu ions concentration.     

Hard X-ray photoelectron spectroscopy from 5-14.5 keV

Photoemission spectroscopy at high energies can be used to probe bulk electronic states. We used a specially designed high-voltage retarding lens and a commercial Perkin-Elmer PHI 10-360 hemispherical electron analyzer to investigate the core and valence band region of Au, YBa₂Cu₃O_7−δ and highly oriented pyrolytic graphite samples with hard X-rays in the energy range 5-14.5 keV. The overall instrumental resolution obtained at 8 keV was 218 meV. The photo ionization cross-sections for Au 5d and 6s excitations were determined experimentally. In comparison with published calculations for atomic cross-sections neglecting corrections for angular anisotropy, the values we find are twice as large for the 5d and an order of magnitude larger for the 6s (conduction band) level. Our results demonstrate the feasibility of bulk sensitive valence band spectroscopy with high resolution at high brilliance X-ray sources such as the ESRF. The measured cross-sections provide important input for improving current theoretical models.     

SrS:Ce/ZnS:Mn-A di-band phosphor for near-UV and blue LED-converted white-light emitting diodes

The SrS:Ce/ZnS:Mn phosphor blends with various combination viz 75:25, 50:50 and 25:75 were assign to generate the white-light emission using near-UV and blue-light emitting diodes (LED) as an excitation source. The SrS:Ce exhibits strong absorption at 427 nm and the corresponding intense emission occurs at 480 and 540 nm due to electron transition from 5d(²D)−4f(²F_5/2, _7/2) of Ce³⁺ ion as a result of spin-orbit coupling. The ZnS:Mn excited under same wavelength shows broad emission band with λ_max=582 nm originates due to 3d (⁴G−⁶S) level of Mn²⁺. Photoluminescence studies of phosphor blend excited using near-UV to blue light confirms the emitted radiation varies from cool to warm white light in the range 430-600 nm, applicable to LED lightings. The CIE chromaticity coordinate values measured using SrS:Ce/ZnS:Mn phosphor blend-coated 430 nm LED pumped phosphors in the ratio 75:25, 50:50 and 25:75 are found to be (0.235, 0.125), (0.280, 0.190) and (0.285, 0.250), respectively.     

Characterization of electronic structure in dielectric materials by making use of the secondary electron emission

The methodology of characterizing electronic structure in dielectric materials will be presented in detail. Energy distribution of the electrons emitted from dielectric materials by the Auger neutralization of ions is measured and rescaled for Auger self-convolution, which is restructured from the energy distribution of the emitted electrons. The Fourier transform is very effective for obtaining the density of states from the Auger self-convolution. The MgO layer is tested as an example of this new measurement scheme. The density of states in the valence band of the MgO layer is studied by measuring the energy distribution of the emitted electrons for MgO crystal with three different orientations of (111), (100) and (110). The characteristic energy of ?₀ corresponding to the peak density of the states in the band is determined, showing that the (111) orientation has a shallow characteristic energy ?₀ = 7.4 eV, whereas the (110) orientation has a deep characteristic energy ?₀ = 9.6 eV, consistent with the observed coefficient γ of the secondary electron emission for MgO crystal. Electronic structure in new functional nano-films spayed over MgO layer is also characterized. It is therefore demonstrated that secondary electron emission by the Auger neutralization of ions is highly instrumental for the determination of the density of states in the valence band of dielectric materials. This method simultaneously determines the valence band structure and the coefficient γ of the secondary electron emission, which plays the most important role in the electrical breakdown phenomena.     

Characterization of defects introduced in Sb doped Ge by 3 keV Ar sputtering using deep level transient spectroscopy (DLTS) and Laplace-DLTS (LDLTS)

We have used deep level transient spectroscopy (DLTS), and Laplace-DLTS to investigate the defects created in antimony doped germanium (Ge) by sputtering with 3 keV Ar ions. Hole traps at E_V+0.09 eV and E_V+0.31 eV and an electron trap at E_C−0.38 eV (E-center) were observed soon after the sputtering process. Room temperature annealing of the irradiated samples over a period of a month revealed a hole trap at E_V+0.26 eV. Above room temperature annealing studies revealed new hole traps at E_V+0.27 eV, E_V+0.30 eV and E_V+0.40 eV.     

The enhanced field-emission properties of screen-printed single-wall carbon-nanotube film by electrostatic field

In this work, we improved the field-emission properties of a screen-printed single-wall carbon-nanotube (SWCNT) film by applying a strong electrostatic field during the drying process after the printing. By applying the strong field, more tips of SWCNTs could emerge from the screen-printed film and turn somewhat toward the erecting direction because of the repulsive force among the SWCNTs. The field-emission properties of the film were thus improved obviously. The improved field emitters sample has low electron emission turn-on field (E_to = 1.22 V/μm), low electron emission threshold field (E_th = 2.32 V/μm) and high brightness with good uniformity and stability. The lowest operating field of the improved sample is below 1.0 V/μm and its optimum current density exceeds 3.5 mA/cm².     

Electrical characterization of deep levels in n-type GaAs after hydrogen plasma treatment

Deep level transient spectroscopy (DLTS) and Laplace-DLTS (L-DLTS) have been used to investigate defects in an n-type GaAs before and after exposure to a dc hydrogen plasma (hydrogenation). DLTS revealed the presence of three prominent electron traps in the material in the temperature range 20-300 K. However, L-DLTS with its higher resolution enabled the splitting of two narrowly spaced emission rates. Consequently four electron traps at, E_C—0.33 eV, E_C—0.36 eV, E_C—0.38 eV and E_C—0.56 eV were observed in the control sample. Following hydrogenation, all these traps were passivated with a new complex (presumably the M3), emerging at E_C—0.58 eV. Isochronal annealing of the passivated material between 50 and 300 °C, revealed the emergence of a secondary defect, not previously observed, at E_C—0.37 eV. Finally, the effect of hydrogen passivation is completely reversed upon annealing at 300 °C, as all the defects originally observed in the reference sample were recovered.     

X-ray converters for radiation treatment of thin films

The energy absorbed in thin films of selected materials bombarded by x rays emitted in the braking of low-energy electrons (E ₀<500 keV) in converters with various atomic numbers (Z=29–73) is calculated by the Monte Carlo method. The program takes into account both of the K-shell ionization mechanisms that lead to emission of characteristic photons as a result of electron impact and as a result of the photoelectric effect, and the characteristic radiation is shown to make a large contribution to the absorbed energy in thin films. Calculations show that the proper choice of material and thickness of the converter affords a two-to fivefold increase in the energy of the x radiation absorbed in thin films of semiconductor materials. Zh. Tekh. Fiz. 68, 99–101 (November 1998)     

Attenuation length and escape depth of excited electrons in solids

Escape probability and mean escape depth λ_e of emitted electrons are determined as a function of attenuation length λ_a of excited electrons, their initial energy E_n, and the height of the surface barrier χ. A variable energy loss parameter permits to apply the proposed model to different kinds of electron emission, including the energy loss free Auger and ESCA electrons. An analytical expression was found correlating the internal energy distribution of excited electrons and the external energy distribution of emitted electrons. By means of this excitation energies of secondary electrons and exoelectrons were determined.     

Effective atomic numbers for CoCuNi alloys using transmission experiments

Effective atomic numbers for CuCoNi alloys against changing Ni contents were measured in the X-ray energy range from 15.746 to 40.930 keV. The gamma rays emitted a ²⁴¹Am point source have been send on absorbers to be used transmission arrangement. The X-rays were counted by a Si(Li) detector with a resolution of 160 eV at 5.9 keV. The compositions of the Ni films were determined to be 0.03, 0.47, 0.62, 1.23, 1.22 and 1.6 by a scanning electron microscopy in CuCoNi alloys prepared against changing Ni contents. CoCuNi alloy films were prepared with an electrodeposition technique. Also, the total effective atomic numbers of each alloy were estimated using mixture rule. The measured values were compared with estimated values for alloys.     

Decomposition of thin titanium deuteride films; thermal desorption kinetics studies combined with microstructure analysis

The thermal evolution of deuterium from thin titanium films, prepared under UHV conditions and deuterated in situ at room temperature, has been studied by means of thermal desorption mass spectrometry (TDMS) and a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The observed Ti film thickness dependent morphology was found to play a crucial role in the titanium deuteride (TiD_y) film formation and its decomposition at elevated temperatures. TDMS heating induced decomposition of fine-grained thin Ti films, of 10-20 nm thickness, proceeds at low temperature (maximum peak temperature T_m about 500 K) and its kinetics is dominated by a low energy desorption (E_D = 0.61 eV) of deuterium from surface and subsurface areas of the Ti film. The origin of this process is discussed as an intermediate decomposition state towards recombinative desorption of molecular deuterium. The TiD_y bulk phase decomposition becomes dominant in the kinetics of deuterium evolution from thicker TiD_y films. The dominant TDMS peak at approx. T_m = 670 K, attributed to this process, is characterized by E_D = 1.49 eV.