Elsevier

Surface Science

Volume 541, Issues 1–3, 1 September 2003, Pages 160-172
Surface Science

Ion fractions in Ne+ scattering from a GaAs(1 1 0) surface

https://doi.org/10.1016/S0039-6028(03)00884-7Get rights and content

Abstract

Ion scattering spectrometry with time-of-flight analysis is used to measure the ion fractions in 6 keV Ne+ projectiles scattered from a GaAs(1 1 0) surface for a broad incidence and observation angular range. The neutralization of the Ne+ ions is studied by using a Green’s function formalism to solve the time-dependent collisional process. The interacting system is described by a spin-less Anderson-like Hamiltonian where the valence and localized states of the surface are taken into account. Two important ingredients are incorporated in the theoretical model: the description of the reconstructed surface by using a molecular dynamics density functional theory in the local density approximation to calculate the local an partial density of states; and the calculation of the Hamiltonian interaction terms by considering the orientations of the projectile orbitals with respect to the reference frame provided by the surface. The results allow to infer that the different behaviour observed in the ion fractions of the projectiles scattered from As and Ga atoms can be mainly related to resonant neutralization processes that are strongly determined by the local electronic structure of the surface. The angular dependence of the ion fraction for two different scattering angles is analyzed.

Introduction

Ion surface collisions have been used as a tool for surface analysis for a long time. For many surfaces ion scattering spectrometry with time-of-flight (TOF-ISS) analysis is capable to discriminate the contributions to the backscattering yield coming from different atomic layers and from the different atoms present at the surface. The collisional process corresponds to a dynamical situation in which charge exchange between the projectile and the solid target evolves in time. The neutralization of the scattered ions has been noticed to be strongly dependent on the projectile–target system [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], being this fact originated in the varied characteristics of the electronic structure of both, target and projectile. It has been found that the resonant charge-transfer is an important mechanism of neutralization in large angle collisions [5], [6], [7], [8], [9], [10]. Most of the studies reported in the literature deal with metal surfaces [11], [12], [13], [14], while semiconductor surfaces have been less frequently studied [15], [16], [17].

Among the semiconductor surfaces, the GaAs(1 1 0) surface provides an interesting system to be analyzed because it has similar masses for both atoms, but localized and specific electronic properties in each one. Since the outgoing projectile velocities are similar for scattering from both Ga and As atoms, the differences in the scattered ion fractions must be attributed to differences in the local electronic structure. On the other hand, the similar outgoing velocities require high TOF resolution in order to separate the different contributions, and some overlap due to multiple scattering can therefore be expected. The study as a function of the projectile incident and exit directions reveals specific features that should be typical of compound semiconductors with some ionic character, as is the case for GaAs.

In this work we present measurements of the scattered 6 keV Ne+ ion fractions for a broad range of incident directions, i.e., from grazing to normal incidence, and at different azimuths. In order to detect effects due to different distances of approach in the close encounter, two different scattering angles were used (107° and 147°). The calculation of the ion fractions is based on a detailed description of the interaction parameters for the collisional partners Ne–Ga, and Ne–As, and also a realistic density of states of the GaAs(1 1 0) surface calculated by using the Fireball96 method [18]. The energy and hopping terms of the Hamiltonian are obtained from a Hartree–Fock approximation to the dimeric projectile–target atom interacting system. For trajectories at large angles with respect to the surface, a binary picture considering only the scatter surface atom is expected to describe correctly also the inelastic aspects of the collisional process. However, the natural reference frame is provided by the surface. Then, the relative orientations of the projectile orbitals in the surface frame must be included in the calculations for a correct description of the angular dependence of the charge transfer process. The Green’s functions introduced by Keldysh [19] are used to solve the time dependent evolution of the resonant charge-exchange process. In the description of the interacting system it is included not only the valence states of the surface target atoms, but also the 3d core states that can either promote the projectile energy levels through the hybridizations occurring in the short-distance encounters, or participate in direct quasi-resonant charge-exchange processes.

The ion trajectories with respect to the surface are described by taking into account the general aspects of a classical binary collision, and the closest distance of approach is determined from the interaction energy of the projectile–target atom system. In this form the experimental scattering geometries are well reproduced.

The work is organized as follows: in 2 Experimental details, 3 Experimental results, the experimental details and results are explained; Sections 4 and 5 are devoted to the theoretical model description and to the discussion of the theoretical results; and in Section 6 the concluding remarks are summarized.

Section snippets

Experimental details

The measurements were carried on a UHV system equipped with facilities for Auger electron spectroscopy and TOF-ISS that has been described before [20], [21]. The Ne+ beam was produced in a radio-frequency source, accelerated to 6 keV, mass-analyzed by a magnet, and finally collimated to a spot of 1 mm diameter with angular divergence of 0.1°. For TOF measurements the ion beam (Ne+) is pulsed in the range from 10 to 50 kHz. The sample is mounted on a manipulator that allows independent and

Ion fractions in back-scattered projectiles: TOF––spectra

Figs. 2(a) and (b) show TOF-spectra acquired along the [1 1 2] crystallographic direction (φ=−64.75°) for the two backscattering angles used in this work, i.e., δ=107° and 147°. Fig. 2(c) shows the same spectra of Fig. 2(a) in energy scale. Along the [1 1 2] azimuthal direction the As and Ga top layer atoms are aligned (Fig. 1(a)), which means that for most of the incoming trajectory ions scattered from As and Ga top layer atoms “see” essentially the same surface, although at a different height. In

The time-dependent Hamiltonian and the ion-fraction calculation

An Anderson-like Hamiltonian in the spin-less approximation is proposed to describe the collision between the Ne+ projectile and the GaAs surface, including the interaction with the valence surface states and the core surface states. The active state α of the Ne+ ion is assumed to be the 2pz orbital (asymptotic energy value equal to −15.3 eV with respect to the Fermi level, EF=−6.3 eV).H=∑kεkn̂k+∑lεln̂l+Eα(t)n̂α+∑k[Tk,α(t)ĉk+ĉα+h.c.]+∑l[Tl,α(t)ĉl+ĉα+h.c.]where the creation and annihilate

Theoretical results and discussion

We first present a calculation similar to the one performed in reference [25]. Here the projectile trajectory is also assumed perpendicular to the surface, but a realistic GaAs density of states (Fig. 6), and the energy loss by the projectile–surface atom collision are introduced. In this calculation the projectile energy level is assumed constant and equal to its asymptotic value (−15.3 eV). The results are compared with the experimental data in Fig. 7 for the two scattering angles analyzed.

Summary and conclusions

In this work experimental and theoretical results of the ion fractions of Ne+ scattered from a GaAs(1 1 0) surface are presented for two scattering angles (107° and 147°). The dependence of the ion fractions with the incident angle for a 6 keV kinetic energy of the incoming ions has been analyzed. The calculation assumes a binary model concerned with the collision with only one target atom, but the inelastic processes of charge-exchange is able to account for the natural reference frame provided

Acknowledgements

This work was supported by CAI+D 2000 no. 6-1-76 from Universidad Nacional del Litoral, and CONICET, Argentina. We acknowledge partial support from CONICET (PIP 423/98), SECYT (PICTs 3-6325 and 3-4220), ICTP/CLAF, and Fundación Antorchas (nos. A13927-17, 14116/86, 14022/111).

References (31)

  • M. Maazouz et al.

    Surf. Sci.

    (1998)
  • R. Brako et al.

    Surf. Sci.

    (1981)
  • J.J.C. Geerlings et al.

    Surf. Sci.

    (1986)
  • E.A. Garcı́a et al.

    Surf. Sci.

    (1995)
  • R. Souda et al.

    Nucl. Instr. and Meth. B

    (1986)
  • O. Grizzi et al.

    Surf. Sci.

    (2000)
  • W. Heiland et al.

    Nucl. Instr. and Meth.

    (1976)
  • P.G. Bolcatto et al.

    Phys. Rev. A

    (1994)
  • R. Souda et al.

    Phys. Rev. B

    (1989)
  • R. Souda et al.

    Phys. Rev. B

    (1995)
  • M. Maazouz et al.

    Phys. Rev. B

    (1997)
  • A. Blandin et al.

    J. Phys. (Parı́s)

    (1976)
  • H. Saho et al.

    Phys. Rev. B

    (1994)
  • E.A. Garcı́a et al.

    Phys. Rev. B

    (1995)
  • R.L. Erickson et al.

    Phys. Rev. Lett.

    (1975)
  • View full text