Imaging self-sputtering and backscattering from the substrate during pulsed laser deposition of gold

Abstract

The expansion dynamics of the plasma generated during pulsed laser deposition of gold in vacuum has been investigated at the laser fluences of 2.5, 6.0, and 9.0 J cm −2 . A severe distortion of the expansion is observed in the presence of a substrate that is accompanied at 9.0 J cm −2 by the appearance of a secondary plasma front expanding from the substrate surface. Langmuir probe analysis at 9.0 J cm −2 shows that the substrate surface is bombarded by a high transient flux of energetic Au + ions ͑3.0ϫ 10 19 ions cm −2 s −1 ͒ having very large kinetic energies ͑Ͼ400 eV͒. Analysis of the plasma dynamics shows that these observations are consistent with self-sputtering of Au neutrals from the substrate induced by incident Au ions while a fraction of them are backscattered. Self-sputtering is found to be 2 orders of magnitude larger than backscattering. The comparison with experimental data allows concluding that the apparent recoil of the plasma front is caused by collision with self-sputtered neutrals, while the secondary emission is originated by backscattered ions. INTRODUCTIONPulsed laser deposition ͑PLD͒ has become a well established technique for the production of a broad variety of materials, particularly complex oxides.1,2 However, much less attention has been paid to the production of metals despite early attempts to synthesize them in the 1970s. 3 More recently, the interest in complex thin film metal nanostructures such as metal-dielectric or metal multilayers, 4,5 as well as metal nanoparticles, 6-8 has triggered new efforts to use PLD for metal deposition. These metal structures are characterized by the localization and enhancement of the electric and magnetic fields, leading to effects such as giant magnetoresistance, 9 enhanced nonlinear optical properties, 8 or enhanced Raman response, 10 among others. Layer thickness, bulk and interface structure in the case of the multilayers, or size, shape, and arrangement of nanoparticles determine these properties. Thus, practical applications require excellent control of the structure and morphology of the nanostructures.PLD is characterized by a high instantaneous flux of species reaching the substrate, a fraction of them having high kinetic energies.11-14 Both characteristics are expected to have a strong influence on the features of the metal structures produced. 6,15 In addition, the presence of highly energetic species may induce undesired processes such as selfsputtering, also termed resputtering, of a fraction of the material that is being deposited 14,[16][17][18][19] or an increase of surface roughness, 4 among others. Finally, both self-sputtering and backscattering of a fraction of the incident species may have a significant effect on the morphology of the nanoparticles due to their nanometric dimensions. While self-sputtering is considered to be a common process when PLD is performed in vacuum, backscattering has seldom been observed or discussed in the literature, 20 and little effort has been devoted to determine its influence on the properti...

Quantification of self-sputtering and implantation during pulsed laser deposition of gold

“…12,15,16 In addition to the ions detected by the LP and the excited neutrals detected by the ICCD, there is a significant fraction of ground neutrals in the plasma that are not detected by any of the probes used in this work. Ground neutrals are expected to behave similarly to excited neutrals or to have even smaller kinetic energies.…”

“…A narrow band pass filter has been used in order to record only the emission corresponding to the 479.3 and 481.2 nm emission lines of excited gold neutrals and the 2D images are similar to those reported elsewhere. 12 The intensity profiles I͑z͒ of these transient excited-neutrals spatial emission distributions are taken along the expansion direction z and converted into kinetic energy distributions using the following method. First, the emission near the target surface ͑1-2 mm͒ that corresponds to bremsstrahlung is removed from the profiles.…”

“…We neglect secondary self-sputtering from the back substrate due to the reduced kinetic energies ͑Յ30 eV͒ of the species reaching the back substrate. 12 In the present work we have measured the material deposited at point ͑0, 8͒ of the back substrate and thus we must define the integration limits along the plane xy of the front substrate to evaluate numerically Eq. ͑A3͒.…”

“…Furthermore, the importance of self-sputtering has been reported during PLD of Au through the observation of a severe distortion of the plasma expansion caused by self-sputtered Au atoms. 12 In addition, ion implantation in the substrate leading to subsurface formation of Bi and Au NPs has been reported for fluences in the range 0.4-5 J cm −2 and 2.7-9 J cm −2 , respectively, 3,11 the implantation depth scaling with laser fluence. Although both sputtering and implantation have unambiguously been observed in earlier PLD works and related to the high kinetic energy of the species involved, their complete origin is not yet well understood.…”

Diversity in plasma arrival sequence correlated to oxygen deficiencies of oxide thin films grown by PLD

“…In vacuum, the plasma species possesses a large kinetic energy (E K ) up to several hundreds of eV [4−6], which is sufficient to re-sputter the film elements with a preference of lighter composition [7,8]. Meanwhile, the plasma elements can be recoiled or backscattered by the surface of the as-grown film, which is also in preferential for the lighter plasma constituents [9]. These well explain the oxygen deficiency in the oxide thin films grown in vacuum, since oxygen possesses a smaller atomic weight than most of other cation elements [5,6].…”