EXAFS study of noble gases implanted in highly stressed amorphous carbon films

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Abstract

In this work we report on the study of the local environment of Ar and Kr atoms implanted into an amorphous carbon (a-C) matrix. By intentionally changing the a-C deposition conditions we were able to trap noble gas (NG) atoms under different internal pressure (intrinsic stress) ranging from 1 up to 12 GPa. This enables us to investigate the effects of the internal pressure on the implanted NG atoms subjected to the highly strained environment of the carbon matrix. For this purpose, extended X-ray absorption fine spectroscopy (EXAFS) was performed on the K absorption edge of the implanted Ar (3.2 keV) and Kr (14.3 keV) atoms as a function of the intrinsic stress of the carbon matrix. The analysis of X-ray near edge spectroscopy (XANES) shows an evident increase of the white line for both gases as the stress increases. This result is indicative of clustering of the implanted NG atoms. The analysis of the EXAFS results for Ar atom indicates an increase of the first neighbor distance from 2.4 to about 2.8 Å with increasing compressive stress. The latter figure is comparable to the Ar–Ar (Kr–Kr) inter-atomic distance of compressed solid Ar (Kr). This finding gives further evidence for the formation of small NG clusters in a highly strained a-C matrix.

Introduction

It has been known for a considerable time that noble gases (NG) are not so inert as originally thought. Solid NGs, free NG clusters, and dimers (ArF2,KrF2,XeF2,) have been extensively studied in the past years [1], [2], [3]. Due to its technological appeal, the implantation of NG into a solid matrix has also been the focus of much interest [1], [4]. A better understanding of the interaction between the implanted NG atoms and the host matrix is an important issue, since NG are frequently used for thin film deposition by sputtering, for film densification, in surface sputter-cleaning process, and for depth profiling [4]. This implies that a few at.% of NG always remain unintentionally trapped in the host structure. It is also known that after implantation, the NG tends to agglomerate in the form of clusters in the matrix [5], [6], [7]. A relationship between the pressure exerted by the matrix and the formation of the NG clusters has been proposed [5], [6], [7], [8], although not much is known about the real role of the matrix pressure on the implanted NG atoms. Furthermore, as reported, in some cases only the annealing at low temperatures caused the formation of clusters [8].

In this paper, we investigate the interactions between the NGs argon and krypton with the amorphous carbon (a-C) matrix. By intentionally changing the a-C deposition conditions, the trapped NGs are exposed to increasing internal pressures (or intrinsic stress) varying within an order of magnitude (from 1 up to 12 GPa). Extended near edge X-ray absorption spectroscopy (XANES/EXAFS) was used to probe the local environment changes of the NG due to the internal pressure of the a-C matrix.

Section snippets

Experiment

The a-C films were prepared by ion-beam-assisted deposition (IBAD) using argon and krypton as working gases. X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) and Raman analysis indicate that the material is composed of a highly compressed and dense sp2 network. Some 3 at.% residual NGs are trapped into the film during the deposition and are subjected to the highly strained environment of the a-C matrix. More details about the structural properties of these films can be found

Results

To study the consequences of an increasing strained environment of the carbon matrix on the implanted NGs, X-ray absorption measurements were performed on the K-edge of Kr and Ar as a function of the compressive stress of the a-C films. The intrinsic stress, as a macroscopic property, can be thought as the average internal pressure experienced by the trapped NGs. Fig. 1 shows a clear evolution of the absorption spectra of the implanted Ar atoms as the intrinsic stress of the a-C matrix

Discussion

The appearance of the white line in NGs is understood as a transition between discrete unoccupied Rydberg states of the isolated atom to unoccupied states in the continuum of a solid. Thus, the white line reflects, to a certain extent, the degree of interaction between the NG atoms. The presence of the white line in the absorption spectra of the implanted NG is a strong evidence of the interaction between the wave functions of the NGs. The white line is not observed in the K-edge absorption

Conclusions

In this paper is presented a detailed study of the modifications of the X-ray absorption spectra of implanted NG atoms (Ar and Kr) induced by the increasing internal pressure of the carbon matrix. The increase of the XANES white line with increasing intrinsic stress of the carbon films evidences an increasing interaction between the implanted NG atoms. From the analysis of the EXAFS oscillations, a clear evidence for the induced agglomeration of the NG atoms occurs as the intrinsic stress of

Acknowledgements

This research was partially performed at LNLS National Syncrotron Light Laboratory, Brazil. CNPq, FAPESP and PADCT also supported this work. The authors are in debt to Dr A. Ramos for fruitful discussions and for some FEFF analysis.

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