Joost Vennik

A combined LEIS (low energy ion scattering)—LEED study of the V6O13(001) surface

Abstract The results of a combined LEIS-LEED study of the V 6 O 13 (001) surface as a function of oxygen coverage will be presented. The experiments have been performed on V 6 O 13 layers, grown topotactically on V 2 O 5 (001) single crystal surfaces. The growth is initiated by low energy ion impact in UHV (O 2 partial pressure ⪡ 10 -8 Pa) at a temperature of 750 K, and proceeds spontaneously further into the bulk up to thicknesses allowing to identify the bulk V 6 O 13 structure by means of X-ray diffraction. It is seen that under the given conditions of pressure and temperature, the outermost “vanadyl-oxygen” layer is populated by oxygen diffusing from the V 2 O 5 −V 6 O 13 transition region towards the surface and tends to depletion at low oxygen supply rates (thick V 6 O 13 layers). It has furthermore been shown that the outermost vanadium atoms can be considered to be adsorption sites for atomic oxygen in an on-top position.

A LEIS study of potassium enriched vanadium oxide surfaces

Abstract An important surface segregation of potassium to the V 6 O 13 (001) surface is observed. From combined LEED and angular dependent LEIS experiments, it is found that the most probable K site at the V 6 O 13 (001) surface is an interstitial site close to or within the topmost VO layer. It is furthermore shown that on V 6 O 13 the neutralization processes active in He + LEIS experiments can be described by the Hagstrum formalism. This result allows a semi quantitative estimation of the surface concentration. A model for the surface structure is presented.

Design of an E*B filter with improved velocity-separating capability

An E*B separator is described in which effective use is made of the fringing fields of the magnet. In the particular case described, this results in an increase of the velocity-separating capability of 40% with respect to the Wien filter that would have resulted if the fringing fields were completely shielded. It is also shown that for the normal operating conditions, the filter with fringing fields behaves as an ideal Wien filter without fringing fields but with drift spaces at the entrance and the exit. The effective length of these drift spaces and the length of the equivalent ideal Wien filter are slowly varying functions of a single parameter, the reduced magnetic induction, which depends on the operating conditions of the filter. A numerical example using a 12 in laboratory magnet is given.

A novel algorithm to determine the collision in ion scattering simulations

Abstract An algorithm is presented to determine the collision sequence in ion scattering simulations. It is based on the definition of a zone of influence surrounding each lattice atom. The extent of these regions depends among other things on the type and mass of the central atom. In the proposed model, colliding particles are scattered from one zone of influence to a neighbouring one. Neighbours are defined as lattice atoms having a common zonal interface. Before starting the main simulating program for a particular lattice, this neighbourship can be determined and stored in a linked list once and for all. During the trajectory simulations these data allow to determine unambiguously the sequence of collision partners in an easy and time-efficient way.

The evaluation of interface parameters in MIS structures—I. Theory

The theory relative to the interaction of interface states with majority carriers is reconsidered in the high frequency limit. The admittance of an interface state continuum is examined as a function of surface state energy. n nIt is shown that in the high frequency limit ωτ>1, the energy range of the interface states contributing to the parallel capacitance (CpT) and the conductance (GpT) is different. At fixed interface potential uF, the contribution Cp(u) due to states at potential u is a sharply peaked function, and shifts with increasing frequency from the Fermi level towards the majority carrier band edge. At the same time Gp(u) broadens considerably. Both functions thus can not be used jointly to describe the surface density function if the condition ωτ<1 is not met. n nAn alternative function (dduF)(Gpns) that peaks near to the Fermi level, independently of the frequency range considered, is proposed. The properties of this function give rise to the extended conductance technique (E.C.T.), which enables one to obtain Nss and σn from the experimental data. n nThe main advantage of the E.C.T. is that the continuum of interface states is accurately probed at the Fermi level with a practically frequency independent resolution.