The Effect of the Molecular Mass on the Sputtering of Si, SiC, Ge, and GaAs by Electrosprayed Nanodroplets at Impact Velocities up to 17 km/s

Abstract

Electrosprayed nanodroplets impacting on covalently bonded materials at velocities of a few kilometers per second strongly modify their surfaces by sputtering atoms, amorphizing the region surrounding the impact, and carving craters of comparable size. This article investigates the effects of the projectile's molecular mass on the phenomenology of the impact on Si, SiC, Ge, and GaAs at impact velocities significantly higher than previously studied. An appropriate range of molecular mass is covered by electrospraying the ionic liquids ethylammonium nitrate, EAN, and 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide, EMI-Im, which have molecular masses of 108 and 391 amu, respectively. The beamlets are characterized with the time-of-flight technique to determine the impact velocity, stagnation pressure, and molecular kinetic energy of the projectiles, and to estimate their average diameters. The ranges of these parameters are 7-17 km/s, 40-190 GPa, 50-420 eV, and 10-14 nm. Under these conditions, we find that the molecular mass has a strong effect on sputtering: the sputtering yield for the heavier EMI-Im molecule is always higher than for EAN, with maximum values for Si, SiC, Ge, and GaAs of 4.3, 11.5, 10.9, and 9.4 atoms per EMI-Im molecule, and 1.1, 3.9, 3.3, and 2.9 in the case of EAN. More importantly, droplets of the same diameter (10 nm) and kinetic energy eject significantly different numbers of atoms, with average ratios between the EMI-Im and EAN droplets of 1.8, 1.5, 1.7, and 1.5 for Si, SiC, Ge, and GaAs. Molecular dynamics simulations reproduce the observed enhancement of the sputtering at increasing molecular mass.

Ultrafast physical sputtering of GaN by electrosprayed nanodroplet beams

“…The electrospray source and the sputtering setup used in this research are identical to those described in Ref. [11]. The target is a 100 μm thick epilayer of GaN with a [0001] orientation, grown on a 2ʺ sapphire wafer.…”

Sputtering and amorphization of crystalline semiconductors by Nanodroplet Bombardment

“…The improved efficiency provided by high molecular ionic liquids is not only present when sputtering Si, but also for SiC, Ge and GaAs [45]. MD simulations unveil how heavier molecules transfer more energy to the surface allowing knock-atoms to overcome the surface binding potential, and produce more and more energetic secondary and tertiary knock-on atoms.…”

“…MD simulations unveil how heavier molecules transfer more energy to the surface allowing knock-atoms to overcome the surface binding potential, and produce more and more energetic secondary and tertiary knock-on atoms. This results in a higher fraction of dissipated energy and consequently higher temperatures in the surface, leading ultimately to thermal evaporation [45,46].…”

“…When NB is applied to the surface of GaN with the ionic liquid Emi-Im at high acceleration voltages (up to 33.9 kV), yields of 25 atom/molecule and rates of 1745 nm/min can be obtained [44], surpassing the best rates of RIE systems (ß950 nm/min [8]). A full summary of the maximum sputtering yield and rates for different materials can be found in Table II. www.crt-journal.org (6 of 9) 1600240 Si [45] SiC [45] B 4 C [38] InAs [17] InP [17] Ge [45] GaAs [45] GaSb [17] GaN [44] Once more, the use of molecular dynamics analysis is fundamental to elucidate the mechanisms involved in the sputtering process. By modelling the impact of projectiles with diameters of 5, 10, 20, and 30 nm at velocities between 1 and 6 km/s on a [100] Si slab, three different mechanisms contributing to the sputtering are revealed: collision cascade, thermal sputtering, and hydrodynamic forces [23] (the last one only for projectiles with diameters and velocities larger or equal to 20 nm and 5.5 km/s).…”

Molecular dynamics of nanodroplet impact: The effect of the projectile’s molecular mass on sputtering

“…7 These experiments indicate that liquids with low molecular mass like formamide and ethylammonium nitrate, EAN, have a lower sputtering efficiency than liquids with higher molecular mass such as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF 4 ), 1-ethyl-3-methylimidazolium bis (trifluoro-methylsulfonyl) imide (EMI-Im), triethylsulfoniumbis(trifluoromethylsulfonyl) imide, and trihexyltetrade-cylphosphonium bis(2,4,4-trimethylpentyl) phosphinate. Furthermore, while the impact of the projectiles with higher molecular mass produces micron-sized craters in a range of impact velocities, the surface bombarded by the lower molecular mass projectiles is made only of nanometric indentations at all impact velocities.…”