Rippled area formed by surface plasmon polaritons upon femtosecond laser double-pulse irradiation of silicon: the role of carrier generation and relaxation processes

The formation of laser-induced periodic surface structures (LIPSS, ripples) upon irradiation of silicon with multiple irradiation sequences consisting of femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied numerically using a rate equation system along with a two-temperature model accounting for one- and two-photon absorption and subsequent carrier diffusion and Auger recombination processes. The temporal delay between the individual equal-energy fs-laser pulses was varied between 0 and ～4 ps for quantification of the transient carrier densities in the conduction band of the laser-excited silicon. The results of the numerical analysis reveal the importance of carrier generation and relaxation processes in fs-LIPSS formation on silicon and quantitatively explain the two time constants of the delay-dependent decrease of the low spatial frequency LIPSS (LSFL) area observed experimentally. The role of carrier generation, diffusion and recombination is quantified individually.     

Complementary ODMR and photo-ESR studies of deep donor-acceptor pair recombination processes in ZnS and GaP

The complementary approach to studies of deep donor acceptor pair (DAP) recombination processes is presented. On the basis of supplementary light induced Electron Spin Resonance (photo-ESR) and Optically Detected Magnetic Resonance (ODMR) studies, the nature of radiative recombination transitions is cleared out and the centers participating in these transitions are identified. We also show a new approach to determining the recombination rate for DAP’s of different separations. The latter is possible due to frequency-resolved ODMR experiments. The relevant experimental results for deep DAP transitions in ZnS and GaP are presented.     

Determination of cesium distributions in oxides of MOS structures by photoinjection studies

The photoinjection technique of Berglund and Powell has been used for the determination of cesium profiles in the oxide films of MOS-structures. Measurements of photocurrent as a function of the applied voltage provide information on the distribution of charges in the vicinity (10–200 Å) of both the interfaces of the insulating layer. The charge profiles to be measured were introduced into SiO₂ by ion implantation. Cesium ions, which cause positive surface charge and show great stability under temperature-bias stress, have been implanted before and after oxidation. In the first case, cesium ions were implanted into the silicon surface, which was subsequently oxidized. After oxidation, the cesium ions were found mainly at both the interfaces and only a few of them in bulk SiO₂. In the second case, cesium ions were implanted into SiO₂ at an energy of 5 keV. The cesium distribution measured by the photoinjection technique agrees within 5 Å with that predicted by implantation theory.     

Cesium profiles in silicon and in SiO2-Si double-layers as determined by SIMS measurements

Secondary ion mass spectrometry is used to determine

1. the range distributions of 10–100 keV cesium ions implanted into monocrystalline silicon and
2. the cesium profiles in SiO₂-Si double layers formed by implantation of cesium into silicon surfaces and subsequently converting the implanted layer into SiO₂ through thermal oxidation at 800°C and 1000°C.

In the first case (a) non-Gaussian distributions are observed for the implanted cesium atoms. Their projected ranges show a somewhat stronger energy dependence as predicted from LSS-theory. The values for standard deviations and third central moments are in good agreement with theory at low energies but are larger up to a factor of 1.5 at higher energies. In the second case (b) cesium ions present in the implanted silicon layer are introduced into the growing oxide layer during oxidation and pile up at both oxide interfaces.     

On the morphology of Si/SiO2/Ni nanostructures with swift heavy ion tracks in silicon oxide

Si/SiO₂/Ni nanostructures are fabricated by the irradiation of an oxidized Si surface with swift heavy ions, nanopore etching in the SiO₂ layer, and the electrochemical deposition of nickel in the nanopores with their partial (～50%) or complete filling. Studies of the morphology of the metal in the nanopores shows that the nickel-cluster structure is rather homogeneous and formed by crystallites ～30–50 nm in size. The effects of deposition modes and structure morphology on current transport are analyzed with the use of test temperature dependences of the magnetoresistance. The reproducibility and stability of the magnetoresistance values for the case of homogeneous structure and pore filling with nickel make the Si/SiO₂/Ni system promising for application as the base element for high-sensitivity low-temperature magnetic field sensors.     

Depth-profiling of Cu-Ni sandwich samples by secondary ion mass spectrometry

SIMS depth profiles of copper-nickel thin film targets were measured with argon and nitrogen primary ions. While pronounced cone formation is observed in case of argon irradiation the erosion is much more uniform with nitrogen projectiles, probably due to formation of nitride surface layers.     

Optimization of C<Superscript>5+</Superscript> Balmer-α line intensity at 182 Å from laser-produced carbon plasma

Parametric dependence of the intensity of 182 Å Balmer-α line (C⁵⁺; n = 3 → 2), relevant to xuv soft X-ray lasing schemes, from laser-produced carbon plasma is studied in circular spot focusing geometry using a flat field grating spectrograph. The maximum spectral intensity for this line in space integrated mode occurred at a laser intensity of 1.2 × 10¹³ W cm^?2. At this laser intensity, the space resolved measurements show that the spectral intensity of this line peaks at ～1.5 mm from the target surface indicating the maximum population of C⁵⁺ ions (n = 3), at this distance. From a comparison of spatial intensity variation of this line with that of C⁵⁺ Ly-α (n = 2 → 1) line, it is inferred that n = 3 state of C⁵⁺ ions is predominantly populated through three-body recombination pumping of C⁶⁺ ions of the expanding plasma consistent with quantitative estimates on recombination rates of different processes.     

Deduction of generalised Kirchhoff's Laws from the basic principles of electromagnetism

The deduction of the so well-renowned and established Laws of Kirchhoff relating to the currents flowing in a network, which are considered to be almost axiomatic in electrical engineering sciences, seems to be preposterous and non-sensical at first sight. However, on a closer examination it will appear that these laws are based on two principles, viz, the steady-state condition under which accumulation of free electrical charge is precluded, and, the experimentalOhm's Law, which propounds a linear relation between the difference of potential across a conductor and the total current flowing through the same. But, there are two things, which are normally overlooked. The conductors inKirchhoff's Laws are “wires”, which are one-dimensional lines, and theOhm's Law is the macroscopicOhm's Law for total currents applied to these lumped resistors. Strictly speaking, the “Laws” are to be deduced from the more fundamental electromagnetic equations for continuous media and the microscopicOhm's Law. It is to be noted that theKirchhoff Nodal Law is but a consequence of the steady state condition derivable from these basic equations, viz, the current-density is divergence-free. For a continuous medium, what the form of theKirchhoff's Laws will be, is difficult to guess unless deduced from the basic equations. Once these are established from the basic equations of electromagnetism for a continuous medium, the usual form ofKirchhoff's Laws will follow as corollary of the general case, as has been shown here.     

Luminescent properties of Eu2+ ions in AlN at low and high intensity of excitation

Luminescent properties of powder AlN∶Eu²⁺ phosphors were studied by excitation of 2-nd harmonic of ruby laser (hv=3·56 eV). It was shown that emission spectra of AlN∶Eu²⁺ phosphors consist of five or two overlapping bands according to the concentration of europium and the intensity of excitation. Decay times of luminescence of Eu²⁺ centres in different peaks lie between 0·9 and 1·5 μsec which is in agreement with the value of decay time of 4f⁶ 5d→47⁷ transition of Eu²⁺ ions. It is evident from the decomposition of emission spectra of Eu²⁺ centres that there exist several types of Eu²⁺ centres in AlN as was already demonstrated in the case of oxygen centres in AlN.     

Exciton optics of a disordered semiconductor

Interband light absorption in a disordered semiconductor is considered with due allowance for exciton effects. Allowance is made for the scattering of the electron and hole in a weak static random field and also for collisions of the second kind experienced by the exciton on interaction with a random field. It is shown that the contribution of these latter to the half width of the absorption line of the exciton spectrum (even at temperatures of the order of the exciton ionization energy) is considerably smaller than the contribution arising from the scattering of the particles forming the exciton.     

On the determination of collision cross sections by nuclear Doppler shift

We report on measurements of the energy loss of ions in matter by “Inverted Doppler Shift Attenuation” (IDSA). This new method is an inversion of the “Doppler Shift Attenuation” (DSA) method for the determination of lifetimes of nuclear states. While in DSA the exact knowledge of the velocity dependent energy loss functiondE/ds is required, it is shown that in IDSA this function or the absolute collision cross section, respectively, can be determined from the Doppler spectrum of an excited nuclear fragment recoiling in matter, whose lifetime is known. No corrections or assumptions concerning the collision processes are necessary. ⁷Li^* fragments (E _γ=478 keV) from the¹⁰B(n, α)⁷Li^* reaction produce an easily observable and analysable Doppler spectrum. Here boron must be a constituent or an implanted impurity of the material to be investigated. The experimental set-up is described. An exact relativistic analysis of the Doppler spectrum is given. The measured collision cross sections turn out (a) to be proportional tov within the range 1.5 · 10⁸≦v≦4.8 · 10⁸ cm/sec, (b) are thus only due to pure “electronic” collisions, and (c) sensitively dependent on the charge distributions of the target atoms.     

Mössbauer study of iron removal in a montmorillonite

⁵⁷Fe Mössbauer spectroscopy in conjunction with atomic absorption spectrometry and X-ray powder differaction analyses have been used to study the iron present in a montmorillonite prior to and after different successive stages of two deferration processes. Fe³⁺ ions occupy mainly octahedrical M(2) sites in the mineral structure; no impurities of iron oxides were detected. The quite efficient deferration by HCl refluxing produced a substantial alteration of lamellar structure of montmorillonite, whilst dithionite/citrate treatment did not induce severe structural changes but had low iron removal efficiency.     

Influence of ion bombardment on the profile of the depth distribution of impurity atoms in Si used for solar cells and diode structures

The chemical composition of the surfaces and surface layers of Si samples used in solar cells are studied using Auger electron spectroscopy (AES). It is shown that the low-energy implantation of Ba and O ions leads to the redistribution of impurity atoms in the surface layer and can prevent impurity atoms adsorbed onto the surface from penetrating inside the sample.     

Electronic stopping cross sections of elements and compounds for swift Li ions

We report on experimental electronic stopping cross sections of elements and compounds for Li ions. They are measured with the Inverted Doppler Shift Attenuation (IDSA) method with an accuracy of the order of 1%. In cases where comparable data from other authors exist, agreement within the limits of error is obtained. Systematic and pronounced deviations up to about 30% from Lindhard's theory are found. It turns out that Bragg's rule is generally not valid for stopping cross sections in the velocity range 1.5 · 10⁸ cm/sec to 4.8 · 10⁸ cm/sec.     

Collisional configuration-rearrangement in fast neon projectiles

Neon ions in either their ground state or an excited state collide with the molecules of an N₂ gas-target. The collisional configuration-rearrangement is observed from theK- x-ray spectrum. A multistate model allowing for population and depopulation of the excited states with equal strength describes qualitatively the x-ray yield as a function of the target pressure.     

Effects of cerium dopant concentration on structural properties and photocatalytic activity of electrospun Ce-doped TiO2 nanofibers

Electrospun \(\hbox {TiO}_2\) and Ce-doped \(\hbox {TiO}_2\) nanofibers were prepared with 0.5, 2.0 and 8.0 % weight Ce. The structural properties and phase composition were characterized using high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction and X-ray absorption near edge spectroscopy (XANES) at the Ti K-edge. The undoped nanofibers are composed of an assembly of \(\hbox {TiO}_2\) nanoparticles and their crystal structure is a mixture of anatase and rutile phases with an anatase:rutile volume ratio close to 3:1. As Ce is introduced, the nanoparticles decrease in size and the rutile phase volume decreases. Ce \(\hbox {L}_3\) -edge XANES probed the local structure of Ce dopants. At 0.5 % Ce, most Ce ions are incorporated in the \(\hbox {Ce}^{3+}\) charge state but, at 2 % Ce, the majority are \(\hbox {Ce}^{4+}\) . Visible light absorption indicated that \(\hbox {Ce}^{3+}\) act as shallow acceptors that only participate in absorption of wavelengths below 420 nm but \(\hbox {Ce}^{4+}\) impurity states are associated with absorption of wavelengths up to 550 nm. Photocatalytic performance of the nanofibers was assessed by measuring the degradation of adsorbed Rhodamine B in aqueous solution under visible and ultraviolet light. The 0.5 % Ce-doped \(\hbox {TiO}_2\) nanofiber showed the best visible-light photocatalytic activity, which is probably due to the majority presence of \(\hbox {Ce}^{3+}\) . At higher Ce concentration, the photocatalytic reaction rate was lower than undoped nanofibers, indicating that recombination at the \(\hbox {Ce}^{4+}\) sites is rate limiting.     

Strong Enhancement of Electron–Ion Recombination Owing to Free–Bound and Bound–Bound Resonance Transitions

The effect of free–bound and bound–bound resonance nonadiabatic transitions of an electron on electron–ion recombination rates in the plasma of a Ne/Xe and Ar/Xe inert gas mixture has been studied. A kinetic model of recombination has been proposed including energy relaxation in collisions with electrons, resonant electron capture to Rydberg states through three-body collisions of Xe⁺ ions with Ne or Ar atoms and dissociative recombination of NeXe⁺ or ArXe⁺ ions, and n → n' resonance transitions. It has been shown that effective resonance processes occurring in quasimolecular systems sharply increase both the recombination coefficient and the effect of collisions with neutral particles even at quite high degrees of ionization of the plasma.     

The effective Hamiltonian for copper oxides

The concentration dependence of the exchange integral for the subsystem of spin moments of copper ions J(h) = J ? J ₁ × h ? J ₂ × h ² has been calculated for the Emery model within the effective Hamiltonian obtained with due regard to intersite interactions and oxygen configurations with different numbers of holes. It is shown that allowance for the oxygen single-hole states occurring upon doping leads to additional contributions to J(h), whose intensities depend on the intersite correlations of the nearest environment of exchange-coupled copper ions.     

Electronegativity of fractionally charged atoms in the Density Functional Theory

The electronegativity (identified with the negative of the chemical potential) of atoms and ions has been calculated in several isoelectronic series using the Density Functional Theory. Then, the electronegativities of atoms and ions with fractional nuclear charge have been obtained by interpolation in each isoelectronic series. Similar interpolations have been performed, starting with approximate electronegativities obtained by Mulliken's finite difference approximation. Both sets of results have been compared.     

Radiative lifetimes of some energy levels of doubly ionized Xenon

Using 1–5 MeV Xenon ions we have studied the beam-foil spectrum of Xenon between 105 nm and 500 nm. Radiative lifetimes were measured for levels of Xenon III. For those lifetimes which have been measured previously (Andersen et al. [l]) good agreement is obtained. The interpretation of the decay curves and the assignment of a measured value, were done with the help of theoretical lifetimes obtained by Coulomb-approximation calculations [2].