Electronic Sputtering

[Summary by Dr. A. Tripathi and Dr. S. Ghosh (IIT Delhi)]

 

Swift heavy ion irradiation results in many  nonequilibrium processes including the formation of  latent tracks and  removal of atoms from the surface termed as electronic sputtering. The electronic sputtering study  is one of the thrust areas of materials science research at IUAC. Sputtering process depends on a number of parameters including the electronic energy deposited in the system by ion beam, the crystallite grain size, conductivity of the material and properties of the substrate such as its conductivity. Due to the energy confinement in reduced dimensions in case of thin films and nano particles, the sputtering process is enhanced. Electronic sputtering is an important tool to understand the fundamentals of interaction of SHI with the matter and  has been studied in detail in carbon allotropes, ionic crystals and many other materials at IUAC.

 

Electronic sputtering of different allotropes of carbon (diamond, graphite, fullerene, a-C and a-C:H) using using on-line elastic recoil detection analysis (ERDA) and catcher technique shows structure dependent sputtering yield of carbon from these allotropes. Hardest known allotrope diamond does not show any sputtering within the detection limit, whereas the soft polymerlike a-C:H shows highest sputtering yield (5.8x105 atoms/ion). This significant variation of electronic sputtering yield of carbon in different allotropes is discussed from the viewpoint of influence of structure on ion–solid interaction [1]. The electronic sputtering study on fullerene (C60) thin films deposited on Si and glass substrates with Au and Ag ions of different energies. The velocity effect is shown to play an important role as slower ion having same electronic energy deposition (Se) as compared to its high velocity counterpart results in higher sputtering yield. Similarly films deposited on more insulating  glass substrate shows higher sputtering yield as compared to those deposited on Si substrate. However, no charge state effect was observed in the electronic sputtering yield within the detection limit of the set up [2,3]. The angular distribution of sputtering from HOPG at different angles of incident and from amorphous carbon shows that the sputtering from HOPG is not isotropic and has been attributed to the crystal structure and formation of a thermal spike induced pressure pulse [4,5].  

 

The sputtering from LiF thin films has been studied in detail. The sputtering yield of the order of 104 atoms/ion is measured and a reduction in sputter yield is observed with increasing grain size. It is explained that a smaller grain size leads to higher grain boundary scattering, lower mean diffusion length of the electrons leading to enhanced energy deposition inside the grains and increased sputter yield [6]. Similarly, a reduction in sputter yield to 2.2x104 from 2.3x106 atoms/ion, is observed with increase in the film thickness, which is attributed to the larger confinement of energy in the film having  smaller grains and/or  lower thickness [7]. Besides it is also shown that the sputtering yield increases exponentially with increasing lattice strain/crystal imperfections and  materials with higher band gap show a higher sputtering yield. Electronic sputtering in CaF and CsI thin films by same group is also underway. 

 

  The sputtering studies on cupric nitride films deposited on borosilicate glass and Si substrates shows a 75% depletion of N whereas the copper content remains unchanged. The surface shows enhanced  conductivity due to formation of nanodimensional metallic zones under Au ion impact [8]. The entire process is understood on the basis of thermal spike model of ion–solid interaction. Another important physical parameter which controls electronic sputtering yield is the band gap of the materials. It has been established in oxide that with higher band gap, the yield increases.

 

Two exponents for the size distribution of n-atom clusters, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation and it is shown that the observed decay exponents do not support any possibility of a thermodynamic liquid-gas-type phase transition taking place, thus resulting in cluster formation [9]. Electronic sputtering of Ag embedded in silica is also under study.

 

          Now, simulation work on electronic sputtering has also been initiated to explain the large electronic sputtering yield [10].  

 

References: 

  1. S. Ghosh et al, Nucl. Instru. Meth. B219–220 (2004) 973–979.
  2. S. Ghosh et al, Nucl. Instru. Meth. B212 (2003) 431–435.
  3. S. Ghosh et al, Nucl. Instru. Meth. B190 (2002) 169–172.
  4. A. Tripathi et al, Nucl. Instru. Meth. B212, 402, (2003).
  5. A. Tripathi et al, Nucl. Instru. Meth. B266 (2008) 1265.
  6. M. Kumar et al, Nucl. Instru. Meth. B 256 (2007) 328–332.
  7. M. Kumar et al J. Appl. Phys. 102, 083510 (2007).
  8. S. Ghosh et al, Nucl. Instru. Meth. B 248 (2006) 71–76
  9. P.K. Kuiri et al, Phys. Rev. Lett. 100, 245501 (2008).
  10. S. Mookerjee et al, Phys. Rev. B78, 045435 (2008).
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