Yu. S. Gordeev

Fullerite C 60 as electron-beam resist for 'dry' nanolithography

The fullerite C60 modification induced by electron irradiation was studied using electron energy loss spectroscopy (EELS). We show that prolonged electron irradiation causes gradual transformation of electron and atomic structures of fullerite C60 films towards respective structures of amorphous carbon. Moreover, strongly modified parts of fullerite C60 films become thermally unevaporable. A possible mechanism of these effects is suggested. Finally, we propose a new original nanolithography technique based on two key ideas: the use of fullerite films as negative electron-beam resist and development of lithographic images by appropriate thermal treatment of these films.

Investigation of the extent, rate, and mechanisms of electron-stimulated modification of the C60 fullerite by EELS

A parameter representing the intensity ratio of the two features in the characteristic electron energy loss spectrum that are most sensitive to electron irradiation is proposed for use in characterizing the extent of electron-stimulated modification of fullerite. By normalizing this parameter, we succeeded in obtaining a universal scale for the extent of modification that can be unambiguously related to the π-electron density. The dose dependence of this parameter is shown to be described by one exponential, thus permitting one to conclude that both the polymerization and amorphization of the fullerite are dominated by one mechanism, namely, the formation of intermolecular chemical bonds, which is stimulated by valence-electron excitation. The rate of the electron-stimulated modification, or the dose susceptibility of the material, is defined through the derivative of this parameter. The dependence of this rate on the incident electron energy is obtained. It is shown that structural changes are mainly due to a swarm of numerous secondary and decelerated electrons rather than to the primary electron.

Effect of electron irradiation on the spectra of elementary excitations in C60 fullerite

Electron irradiation produces changes in the spectra of elementary excitations of C60 fullerite, which are manifested by decrease in the π-plasmon energy, bandgap width, and energies of the HOMO-LUMO and other molecular transitions, smoothening of the corresponding spectral features, and significant growth in the quasicontinuous low-energy background intensity, the latter being indicative of an increase in the conductivity. The observed “red shifts” are related to collectivization of a part of the π electrons, the formation of chemical bonds between adjacent molecules (polymerization), and the corresponding increase in the proportion of sp3-hybridized electrons. Characteristic electron energy loss (EEL) spectra of an intact fullerite sample non-perturbed by the polymerization process were measured. The EEL spectra of fullerite exhibit a multipole structure due to the (σ + π)-plasmon and reveal an exciton feature which is highly sensitive with respect to electron irradiation and can be used to characterize the initial fullerite structure and to indicate the polymerization onset.

Formation and Decomposition of Nitrides under Ion Bombardment

The chemical processes of formation and decomposition of narrow-gap nitrides InN and GaAs1 − xNx under ion bombardment have been investigated by Auger electron spectroscopy. It is shown that, due to chemical instability, a large fraction of InN decomposes with formation of metal clusters under ion bombardment. It is established that bombardment of GaAs with a beam of N2+ and Ar+ ions makes it possible to obtain a chemically homogeneous GaAs1 − xNx solid solution with a high nitrogen content (x = 0.1), whereas implantation of only N2+ ions leads to the formation of a mixture of GaN, GaAsN, and GaAs phases. It is concluded that secondary ion cascades, induced by heavy ions, stimulate nitridation reaction, homogenize the spatial atomic distribution, and shift the dynamic equilibrium to the formation of a single-phase solution.

Investigation of the chemical state of copper in Cu/SiO2 composite films by x-ray photoelectron spectroscopy

The concentration and chemical state of copper in the subsurface region of Cu/SiO2 composite films obtained by simultaneous magnetron sputtering from two sources (Cu and SiO2) are determined by x-ray photoelectron spectroscopy (XPS). It is established that copper in the as-grown film is primarily in the form of unoxidized atoms dispersed in a SiO2 matrix. Annealing of the film results in practically no oxidation, but about 70% of the copper atoms condense into metallic clusters with sizes below 10 Å in the subsurface region and about 50 Å in the bulk of the film. The changes in the binding energy of core electrons, and especially in the energies of Auger electrons, are so large in this situation that photoelectron and Auger spectroscopy are efficient methods for monitoring the chemical state of this composite material.

Core electron level structure in C60F18 and C60F36 fluorinated fullerenes

The structure of C1s and F1s core electron levels in C60F18 and C60F36 fluorinated fullerenes has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that C1s levels of carbon atoms not bound to fluorine in these compounds are shifted down by 1.0 and 1.6 eV relative to the C1s level in the usual C60 fullerene, so that the binding energies of the core electron levels in C60F18 and C60F36 amount to Eb (C1s, C-C) = 285.7 and 286.3 eV, respectively. These values are characteristic and can be used for the identification of both homogeneous fluorinated fullerenes and combined materials comprising a mixture of various fluorinated fullerenes with each other and with different carbon-containing based materials.

Standardless XPS method for determining the chemical composition of multiphase compounds and its application to studies of InP plasma oxide nanofilms

A modified version of x-ray photoelectron spectroscopy (XPS) is proposed for analysis of the phase chemical composition of substances. In contrast to the well-known XPS method of Siegbahn, the proposed version is standardless and permits determination of the chemical composition of complex multiphase compounds with high accuracy and reliability. In addition to the chemical composition, the method yields the core-level binding energies of atoms in the chemical phases of a studied compound, which have had to be determined in separate experiments on reference samples. The main idea underlying the proposed approach consists in self-consistent unfolding of photoelectron lines of two or more elements. The binding energies act as fitting parameters in this decomposition. The requirement that the contents of like chemical phases derived from the decomposition of spectra of two or more elements be identical makes the solution of the problem unique. This method was used to study the chemical composition of nanofilms of the InP plasma oxide containing several chemical phases. It is shown that, in order to improve the quality of a film and of the interface, the oxidizable surface should be enriched by phosphorus.

Simulation of Fullerite C60 Polymerization Under Particle Beam Irradiation

ABSTRACT A model has been suggested, and the simulation of the fullerite C60 polymerization by electrons within the energy range 0.15 ÷ 1.5 keV has been carried out. The number of created intermolecular bonds was assumed to be proportional (with a coefficient k) to the number of excited molecules. Polarization, exchange, and muffin-tin size were taken into account for elastic scattering. Ionization of deep atom levels was treated as a binary collision, and the inelastic scattering by valence electrons was described in the framework of the dielectric formalism. The comparison of results of the simulation with experimental data gave the coefficient of proportionality k∼(2 ÷ 5) · 10−3.

Modification of GaAs by medium-energy N 2 + ions

The modification of GaAs with a 2500-eV beam containing N2+ and Ar+ ions is examined with Auger electron spectroscopy. Most implanted nitrogen atoms are found to react with the matrix, substituting arsenic atoms to produce a several-nanometer-thick layer of the single-phase GaAs1−xNx (x=6%) solid solution. The GaN phase is absent. Displaced arsenic atoms and nitrogen atoms unreacted with the matrix are present in the layer and on its surface. The former segregate, whereas the latter form molecules.

Photoemission resonance and its quenching during destruction of the molecular structure of a C60 fullerite under synchrotron radiation

The photoelectron spectra of a C60 fullerene condensate are investigated. Under conditions where the photoionization (HOMO-ɛ 1) and Auger (KVV*) transitions are at resonance, the intensity of molecular lines in the photoelectron spectra increases by a factor of several tens. It is found that even insignificant destruction of the molecular structure of fullurenes under synchrotron radiation leads to quenching of the observed resonance. The quenching of the resonance manifests itself in a decrease in the intensity of the molecular lines in the photoemission spectra. The revealed effect can be used to determine the degree of radiation-induced modification of fullerenes.