Low-energy ion-beam induced effects in Al(1 0 0) surface studied using Rutherford backscattering and channeling

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Abstract

The results of low-energy ion spectroscopy can be compromised by the damage that the ions cause in surface layers of target single crystals, so it is important to characterize the impact of the probe ion on the target. In this investigation Al(1 0 0) single crystal surfaces have been irradiated at room temperature with 1 keV He+ and Ar+ ions typical of low-energy ion scattering spectroscopy (LEIS) and sample sputter cleaning, respectively, and the effects have been studied as a function of ion dose and annealing. Rutherford backscattering and channeling measurements were used to probe the depth distribution and annealing characteristics of the dechanneling yield induced by low-energy ion bombardment. Analysis of the Al surface peak area for channeled MeV He ions, as well as the dechanneling yield from well below the surface, indicated an unusual increase in the depth distribution of the backscattered ion yield from Al with increasing low-energy He+ ion dose. Thermal annealing of the crystal following irradiation with the highest He+ dose of 4×1017 ions/cm2 restored the dechanneled ion yield to its lower starting value, with the relatively low activation energy of 0.05 eV. The increase in backscattered ions is attributed to implanted He atoms located in the [1 0 0] channels. The Ar+ ion induced effects in the channeling spectrum are not as significant as those for He+ ions, even at the highest Ar+ dose of ∼1017 ions/cm2.

Introduction

Ion-solid interactions at surfaces have been the subject of numerous studies over many years and the topic continues to be an active field of research in basic science and technology. The use of ions with a wide range of energies (eV to MeV) forms the basis for many analytical techniques to characterize surface structure and composition for a wide range of materials [1], [2], [3], [4], [5], [6]. Ion irradiation is also a versatile method for surface or near-surface modification of materials in the electronics industry and, more generally, for production and manipulation of the properties of solid surfaces for novel applications [7], [8]. The interaction of low-energy ions with solid surfaces is of special interest since low-energy ion spectroscopies have been used for several decades to elucidate surface structure and atom diffusion at interfaces [4], [5], [6]. Low-energy ions are also used for technical applications such as near-surface doping, surface cleaning, concentration-depth profiling, surface texturing and ion-lithography [7], [8], [9], [10]. Recently, ion-beam methods have been recognized for changing the chemical activity of solid surfaces and for controlling the growth and microstructure during thin-film deposition [11], [12]. Apart from the technical applications, the interactions of low-energy inert gas ions with solid surfaces in analytical spectroscopies may compromise the techniques by the damage that they cause in surface layers of the materials under investigation. The damage produced may limit our understanding of the properties being measured and may affect the depth resolution of the technique. Therefore, improved understanding of ion-induced damage and methods to control or remove of such damage will enhance our ability to prepare solid surfaces on the atomic scale with desired properties to meet the required applications.

The damage induced by the low-energy ion bombardment on semiconductor surfaces is well studied [9]. Much less effort has been made to characterize the ion-beam-induced effects on metal surfaces and the damage induced at low energies is not completely understood. Ion bombardment can make the metal single crystal surfaces partially disordered with increasing ion dose, but they never become completely amorphous as seen for semiconductor surfaces [13], [14]. Further difficulties arise in determining the type and concentration of surface defects as well as the composition and structure of surface and subsurface layers. The high defect-annealing rates, even at room temperature for metal surfaces in contrast to semiconductors, also complicate the analysis procedure. Low-energy electron diffraction (LEED) has been employed in the past for damage analysis of single crystal metal surfaces. Based on LEED investigations it was reported that the surface structure was not completely destroyed and the results were explained on the basis of surface roughness with the ion bombardment on the metal surface [13], [15], [16]. However, since LEED only measures the local atomic structure when long-range order is present, information about local atomic structure on surfaces damaged by ion-beam irradiation is generally not accessible with LEED, and further studies using other techniques are needed. In this work our efforts are aimed at determining the physical processes involved in the damage induced during low-energy He+ and Ar+ ion irradiation on Al(1 0 0) single crystal surfaces.

In the present investigation, Rutherford backscattering and channeling (RBS/c) techniques have been employed to study the irradiation-induced damage in Al(1 0 0) single crystals. Rutherford backscattering spectroscopy is a quantitative tool for composition-depth profile analysis. When combined with channeling techniques, RBS provides the capability to probe local structural changes since channeling effects are mainly determined by local structure along the rows of atoms defining the channel. Channeling techniques have been widely employed to extract the fundamental information about ion irradiation effects in solids. The sample was irradiated using keV He+ ions, typical of low-energy ion scattering spectroscopy (LEIS), and keV Ar+ ions, typical of sputtering to clean surfaces or measure concentration-depth profiles. The LEIS technique provides extreme surface sensitivity for determining the structure and composition of the top most surface layers [5].

The conclusions of this work are based on an analysis of the He+ ions backscattered from Al surface atoms, as well as the dechanneling yield of He+ ions from well below the surface. The results show an unusual increase in the depth distribution of the backscattered ions with increasing low-energy He+ ion dose. Thermal annealing of the irradiated crystal restored the dechanneled ion yield to its lower starting value. Much smaller changes in backscattered ion yield were observed for Ar+ ion bombardment on the Al(1 0 0) crystal, even at the highest dose. The measurements are compared to simulations of the irradiation-induced damage using Stopping and Range of Ions in Matter (SRIM-2000) code [17].

Section snippets

Experiment

The aluminum single crystals used in these experiments were cut and polished to within 0.5° of the (1 0 0) crystallographic plane, as measured using X-ray diffraction. The mechanical damage from polishing was removed by etching the crystal for 15 s in an aqueous solution containing hydrochloric acid (1.5%), hydrofluoric acid (1.5%) and nitric acid (2.5%). The crystal was then mounted in the ultra high vacuum (UHV) chamber that has the facilities for in situ surface preparation and

Clean Al(1 0 0) surface

We first measured the RBS/channeling spectra of the virgin Al(1 0 0) crystal in random and aligned scattering geometries in order to evaluate the crystalline quality. The spectra were collected immediately after the surface preparation of the sample in UHV chamber. Fig. 1 shows the RBS/channeling spectra for the Al(1 0 0) crystal collected using incident 1 MeV He+ ions and a scattering angle of 105°. The upper spectrum (open circles) represents the random yield collected with the ion-beam incident

Conclusions

The room temperature ion-bombardment effects of 1 keV He+ and Ar+ ions typical of LEIS and sputtering and have been studied as a function of ion dose and annealing temperature in Al(1 0 0) metal single crystal surfaces. A large amount of He is implanted into the Al lattice throughout the range of the ions. The concentration of implanted He increases with increasing ion dose and interferes with channeling analysis using MeV He+ ions, resulting in dechannleing effects. Thermal annealing of the Al(1 0 

Acknowledgements

The authors are pleased to acknowledge the technical assistance of Norm Williams. This work is supported by National Science Foundation, NSF Grant DMR-0077534.

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