Sputtering of CaF2 Thin Films

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particles/ nanoclusters has been studied through Elastic Recoil Detection Analysis (ERDA) and ... material ejection due to elastic collisions between.
Sputtering of CaF2 Thin Films Ratnesh K Pandeya, Saif A Khanb, D K Avasthib, Avinash C Pandeya a

Nanotechnology Application Center, University of Allahabad, Allahabad b Inter University accelerator Center, New Delhi

Abstract. In the present work CaF2 thin films of different thicknesses deposited on Si substrate have been irradiated with 120 MeV Ag ions with equilibrium charge state and electronic sputtering yield as well as emission of smaller particles/ nanoclusters has been studied through Elastic Recoil Detection Analysis (ERDA) and Transmission Electron Microscopy (TEM) respectively. The observed results show maximum sputtering yield at smallest thickness and also nanostructure formation for that thickness has been observed through TEM. Keywords: Electronic Sputtering, Nanostructure, ERDA, TEM PACS: 60

TEM11. TEM results show the emission of nanoclusters in this case. In the present work, we have studied the sputtering of CaF2 thin films and investigated it with ERDA and TEM. CaF2 due to its simple structure, chemical stability and easy production, is presently among the best known materials having numerous applications in the field of optoelectronic devices, tunable colour center lasers, radiation dosimeters etc. and can play an important role in the study of ion solid interaction process. To determine the mass removal, online ERDA technique was employed, which was first demonstrated by Avasthi et al12. and had been subsequently used by several groups.

INTRODUCTION Ion beam bombardment on solid surfaces has been an interesting area for experimentalists due to some important modifications in materials induced by ion beams. Ions having energy in the range of few MeV/u are known as Swift Heavy Ions (SHI). The removal of atoms from the surface due to impact of energetic ions is termed as sputtering1. Depending on the projectile energy, different scenario of sputtering occurs, such as material ejection due to elastic collisions between atoms (nuclear sputtering) at keV1 energies, electronic sputtering governed by electronic energy loss at higher energies(>1MeV/u)2, and potential sputtering governed by slow highly charged ions3,4. The nuclear sputtering has been studied in different materials and Sigmund’s theory1 has described it well. In the electronic loss regime, Sigmund’s theory fails to explain the large increase in sputtering yield5. Although several mechanisms such as Coulomb explosion6 and thermal spike7 has been proposed for mass removal due to electronic energy loss, the electronic sputtering is still to be understood and is, therefore, a fundamental research problem. Electronic sputtering in LiF thin films has been discussed by Manvendra et al. Thickness effect8, grain size effect9, and substrate effect10 on electronic sputtering yield in case of LiF thin films has already been reported. Large sputtering yield at smaller thickness8 led to the thought of emission of smaller particles/ nanostructures from the film surface. Recently, CaF2 crystals have been irradiated with swift heavy ions and were investigated by ERDA and

EXPERIMENTAL Electron beam evaporation at room temperature has been used to deposit CaF2 films of different thicknesses (20nm, 50nm and 150nm) on Si(111) under a vacuum of ~10-6 Torr. All the substrates were thoroughly cleaned before deposition. Fused CaF2 pieces of 99.9% purity were used as source material for deposition. The rate of deposition was 0.5 nm/s and the thickness of deposited film was monitored using a quartz crystal monitor. Pristine films were characterized by glancing angle x ray diffraction (GAXRD) to investigate their structural properties. GAXRD studies were performed using a bruker AXS D8 advanced diffractometer with CuKα (1.54 Å) operated at 40 kV voltage and 40 mA current. All the GAXRD patterns were taken in the 2θ range of 25° 60°.

Solid State Physics, Proceedings of the 55th DAE Solid State Physics Symposium 2010 AIP Conf. Proc. 1349, 651-652 (2011); doi: 10.1063/1.3606026 © 2011 American Institute of Physics 978-0-7354-0905-7/$30.00

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The sputtering was performed with 120 MeV Ag+25 ions at normal incidence. For SHI induced surface and/or near surface modifications of materials, the charge state of the incident ions must be equilibrium charge state, i.e., of the order of effective charge defined in the electronic energy loss theory. To reach equilibrium charge state, all the projectiles from the accelerator having +9 charge states were passed through a thin carbon foil. The charge state +25, being the most probable in charge state distribution, is selected by dipole magnet for the experiment. The ion beam was collimated to a size of 1x 2 mm2 by using a double slit placed before the chamber. During irradiation, the layer thickness and stoichiometry of the sample were continuously monitored by ERDA in reflection geometry using a large area position sensitive detector telescope (LAPSDT) fixed at 45° port of a high vacuum chamber in Material science beam line of Inter University Accelerator Center (IUAC) New Delhi. Typical two dimensional recoil spectrum for 150nm thin CaF2film is shown in Fig.1

Fig. 2 x ray diffraction spectra of 150 nm CaF2thin film

TEM images obtained are shown in Fig. 3 which clearly indicates the formation of one dimensional nanostructure for 20 nm thickness film while for higher thicknesses only cluster emission is seen.

Fig. 3 TEM micrograph for (a) 20nm CaF2, (b) 50nm CaF2, (c) 150 nm CaF2

The formation of nano structure for 20 nm film was correlated with the higher sputter yield observed for that thickness which has been explained on the basis of thermal spike model.

REFERENCES 1

P. Sigmund, Phys. Rev. 184, 383 (1969). R. E. Johnson and B. U. R. Sundquist, Phys. Today 45(3), 28 (1992). 3 T. Neidhart, F. Pichler, F. Aumayr, M. Schmid, H. P. Winter, and P. Varga, Phys. Rev. Lett. 74, 5280 (1995). 4 F. Aumayr, P. Varga, and H. P. Winter, Int. J. Mass. Spectrom. 192, 415(1999). 5 J. Chaumont, H. Bernas, A. Kusnetsov, C. Clerc, and L. Damoulin, Nucl. Instrum. Methods Phys. Res. B 129, 436 (1997). 6 R. L. Fleicher, P. B. Price, and R. M. Walker, Nuclear Tracks in Solids (University of California Press, California, 1975). 7 Z. G. Wang, C. Dufour, E. Paumier, and M. Toulemonde, J. Phys.: Condens. Matter 6, 6733 (1994). 8 M. Kumar, S A Khan. P. Rajput, F. Singh, A. Tripathi, D K Avasthi and A C Pandey, Journal of Appl. Phys. 102, 083510 (2007). 9 M. Kumar et al., Influence of grain size on electronic sputtering in fluoride thin films, Nucl. Instr. And Meth. B(2007). 10 M. Kumar et al., Substrate effect on electronic sputtering yield in polycrystalline fluoride thin films, Appl. Surf. Sci. (2009). 11 W. Aaaman et al., Swift heavy ion induced sputtering of calcium fluoride crystals investigated by ERDA and TEM. 12 D. K. Avasthi et al. Nucl. Instr. Meth. Phys. Res. B 142, 171 (1998). 2

Fig. 1. Primary ERDA spectrum of 150 nm thin CaF2 film which shows well separated bands of the different elements Si, F, O, C and Ca, present in the film and substrate.

To see the emission of smaller particles during sputtering TEM grids were mounted on specially designed catchers along with the films of each thickness. The TEM grid was mounted on the catcher in a way so that it was in a positioned at an angle of about 20°from the incident beam direction, to get the maximum sputtering yield. The whole area of the samples was scanned with the ion beam during this part of experiment.

RESULTS AND DISCUSSION Figure 2 shows x ray diffraction spectra of CaF2 thin films having thickness of 150 nm. It is evident that films are polycrystalline in nature showing reflections at 2θ angles of 28°, 47° and 55° corresponding to (111), (220) and (311) planes, respectively.

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