A.L. Boers

Small angle multiple reflection of low energy (6keV) noble gas ions from single crystal surfaces as a means to study surface texture and contamination

Abstract The elemental composition of a surface can be derived from the energy spectrum of singly reflected noble gas ions. It is shown that it is also possible to obtain information about certain structural features on the surface from the energy spectra of multiply reflected ions. By carefully studying the multiple reflection energy spectra when the angle of incidence ψ is varied at a given scattering angle θ, many different multiple reflection peaks can be resolved and identified. The ideal conditions for measurements on atomically clean and undisturbed surfaces have been closely approached by working with a low residual gas pressure (10−10 torr) in the collision chamber and small primary ion current densities ( −8 A cm 2 ). For Ar+ and Kr+ on (100)-Cu, peaks have been found resulting from an ideal, flat surface and peaks resulting from surface ridges (steps) and surface vacancies. This technique opens possibilities to study nucleation and early stages of growth of thin films, surface defect formation as a result of ion bombardment, the annealing behaviour of surface defects, thermal vibrations of surface atoms and neutralization processes in multiple collision sequences.

Low-energy ion reflection from metal surfaces

Abstract A review is given of the today available adequate information in the field of low-energy (⩽ 30keV) ion reflection from metal surfaces. Considerable attention is paid to the so-called binary collision approach and its ranges of validity. In addition, the different existing models describing the scattering from a surface, and recent results of experimental verification of them are discussed.

CHARGE-EXCHANGE OF LOW-ENERGY HE IONS AND ATOMS SCATTERED FROM A COPPER SINGLE-CRYSTAL

Abstract Ion yield measurements are presented for 3–10 keV He ions and atoms bombarding a (100) surface of a copper target. The scattering angle is 30°. The difference between the ion and atom scattering results is explained by charge exchange processes occurring during the particle-surface interaction. An accurate description of the experimental results can be given by a collision model which takes into account Auger neutralization as well as ionization and neutralization resulting from a violent collision between an incoming particle and a surface atom. From the atom beam experiments we are able to measure the characteristic velocity for Auger neutralization V c and the ionization probability P i . We measured the energy dependence of V c and P i in the energy range of 3–10 keV. The observed values of V c and P i agree with estimations deduced from ion beam experiments.

STUDY OF LOW-ENERGY NOBLE-GAS ION REFLECTION FROM MONOCRYSTALLINE SURFACES - INFLUENCE OF THERMAL VIBRATIONS OF SURFACE ATOMS .1. COMPUTER-SIMULATION OF ENERGY AND SPATIAL DISTRIBUTIONS

Abstract The influence of target temperature on the energy and spatial distributions of low energy noble gas ions, multiply reflected from a monocrystalline surface, has been studied by computer simulation. The results are obtained by using the chain model; this simplification is a good approximation when the surface is bombarded along low index directions. The simulation model is outlined briefly. The consequences of different target temperatures on the shape of energy spectra of specularly reflected particles and on the shape of spatial distributions if the total scattering angle, resp. angle of incidence is fixed, are discussed qualitatively. Calculations of 6 keV Ar + ions reflected from a vibrating Cu 〈100〉 chain are discussed using these qualitative considerations. The shape.of both the energy and the spatial distributions appears to be distinctly temperature dependent under certain conditions.

MULTIPLE ION SCATTERING

Abstract The energy spectra and the angular distributions of noble gas ions, reflected from a metal surface, yield information about a number of important properties of this surface. A large number of investigations have been carried out in the past decades, not only to get insight into the interaction mechanism but also to develop methods for applying of the knowledge gained. To obtain information about the outermost surface layer, it appears necessary to use noble gas ions as primary particles, and to detect the scattered particles in the charged state only; the fraction of noble gas ions reaching the detector, after reflection from target atoms in the second layer, is very small because of the high probability of neutralization. However, this is only valid if the initial energy of the incoming ions is relatively low, namely ⩽ 10 KeV. Under certain experimental circumstances it appears justifiable, down to a lower limit of about 20 eV, to conceive the interaction of these ions with the target atoms as single collisions. The relation between the initial energy and the post-collision energy is then very simple if the collision is an elastic one; it depends upon the scattering angle and upon the ratio of the two given masses only and not upon the interaction potential. The shift of the peaks in an energy spectrum is caused by inelastic collisions and is relatively small in the considered energy region. The causes of this shift will be discussed. As opposed to single collisions, the post-collision energies after a multiple collision depend largely upon the interaction potential. Attention will be paid to the search for these potentials. Utilization of the multiple collision phenomenon in the study of surface geometry is hampered by the vibrational motion of the surface atoms. As a result, the energy spectra are blurred and a shift of the so-called quasi-single and quasi-double peaks can occur. Under certain conditions a third peak emerges which can give additional information about the surface vibrations. The intensity of the scattered ions depends upon the cross section for scattering. Recently it has been shown that the relation between this cross section and the initial energy has an oscillating character for certain combinations of incident ions and target atoms. This phenomenon has very important consequences, e.g. in the use of single ion scattering as an analytical tool. To investigate surface structures it appears that single and multiple ion scattering, combined with LEED and AES, can provide valuable information.

Low energy (E0 < 10 keV) atom and ion scattering of neon from a Cu(100) surface; Ionization and neutralization

Abstract The ion fractions η+ of low energy (5–10 keV) neon particles scattered from a Cu(100) surface are measured with a time of flight spectrometer. These fractions are obtained for neutral as well as charged projectiles and for different crystal directions. The scattering angle θ was 30°. For a primary energy E0 of 5 keV neutral projectiles have a value for η+ which is 30 times lower than for charged projectiles; these values are 0.15 and 4.5% respectively. For E0 = 10 keV the values of η+ are about the same (~22%). Energy differences up to 22 eV, depending on E0, are observed between the single scattering peaks in the ion spectra of charged and neutral projectiles but also between the single scattering peak in the spectra of all scattered particles and of ions, with ions as projectiles. A qualitative discussion of these data is given, involving charge transfer processes of noble gas particle and target atom. The data suggest that these neutralization processes can be described more adequately with interatomic neutralization processes along the trajectory than with Auger neutralization by conduction electrons.

THE ION FRACTIONS OF 2-10 KEV LITHIUM, SODIUM AND POTASSIUM SCATTERED FROM A COPPER (100) SURFACE

Abstract The ion fractions of lithium, sodium and potassium scattered from a copper (100) surface have been measured as a function of several experimental parameters. The ion fraction appears to be almost independent of the final energy, i.e. independent of the type of scattering trajectory at or below the surface. This means that the alkali ions can be used very well in several cases when experimental data are to be compared with theoretical scattering models. Typical values of the ion fraction are: 63% for lithium, 76% for sodium and 99% for potassium (values are given for a primary energy of 5 keV). A comparison of the results has been made with a theory based on ideas developed for the adsorption and scattering of very low energy alkali particles. A qualitative agreement has been found for the dependence of the ion fraction on the ionization energy of the alkali atoms and the (slight) variation with the primary energy.

MEASUREMENT OF ENERGY-SPECTRA OF NEUTRAL PARTICLES IN LOW-ENERGY ION-SCATTERING - APPARATUS AND EXPERIMENTS

An apparatus is described for low energy (0.1–10 keV) ion scattering (LEIS) experiments. A time of flight (TOF) spectrometer is incorporated in the system to be able to measure the energy of particles in the neutral state after scattering. The energy resolution ΔE/E of the TOF spectrometer is discussed and found to be 0.5% (FWHM). This is sufficient for our scattering experiments. An electrostatic analyzer (ESA) is used to measure the energy of scattered ions [ΔE/E=0.5% (FWHM)]. Experiments show that in general the ion dose needed to obtain a TOF spectrum (2×1010 ions/cm2) is much smaller than the dose needed for an ESA-spectrum (6×1013 ions/cm2). The ion spectra measured with the TOF spectrometer, by subtracting the neutral yield from the total yield, as well as with the ESA are found to agree quite well. This provides a way to calibrate the TOF spectrometer. The determination of the ion fraction of scattered particles is discussed [10 keV40Ar+ on Cu(100), scattering angle 30°]. It is shown that the TOF spectrometer is able to measure light recoil particles (e.g. hydrogen) from a heavy substrate. In the analysing system is, in addition to the TOF spectrometer, also incorporated a stripping cell to measure the energy of neutral scattered particles. An energy spectrum of neutral scattered particles measured with both methods is shown.

STUDY OF LOW-ENERGY NOBLE-GAS ION REFLECTION FROM MONOCRYSTALLINE SURFACES - INFLUENCE OF THERMAL VIBRATIONS OF SURFACE ATOMS .4. POSSIBILITIES FOR ESTIMATION OF ACTUAL ION-ATOM INTERACTION POTENTIAL

Abstract The angular and energy distributions of low energetic noble gas ions, reflected over 30° from a copper (100) target, are measured and compared with calculations. In the simulations the chain-model approach was applied. The interaction potential is estimated by fitting the calculated results to the experiments. This procedure is achieved by varying the screening length of the used Thomas—Fermi potential in the approximation of Moliere. Information about the actual interaction potential can be obtained in three complementary ways: (1) from the width of the angular distributions of ions, reflected (quasi) singly along a low index direction, (2) from the height ratio of the QD and the QS peak and (3) from the width of the QS peak. The resulting values for the fitted screening lenghts appear to be notably smaller than the theoretical ones. A comparison of the experimental and calculational widths of the distributions as a function of the temperature seems to suggest the presence of correlations between thermal displacements of neighbouring surface atoms.

STUDY OF LOW-ENERGY NOBLE-GAS ION REFLECTION FROM MONOCRYSTALLINE SURFACES - INFLUENCE OF THERMAL VIBRATIONS OF SURFACE ATOMS .3. QUALITATIVE COMPARISON OF EXPERIMENTAL AND CALCULATIONAL RESULTS

Abstract The validity of explaining the thermal effects using the chain model approach is investigated. For this purpose a copper (100) surface was bombarded along a 〈100〉 and a 〈110〉 direction, with low energetic noble gas ions. The comparison is troubled by charge exchange effects, which affect strongly the shape of the experimental energy distributions. At highej target temperatures the calculated temperature dependences agree with the experimental observations. At target temperatures below about 400 K discrepancies arise, which can be ascribed satisfactorily to the appearance of surface defects. The angular distributions depend strongly on the occurrence of charge exchange processes. They can be corrected for these effects using a model proposed earlier by the authors.