Dynamics of Plume Propagation and Splitting during Pulsed-Laser Ablation

Abstract

An innovative new approach has been developed for modeling the expansion of laser-generated plumes into low-pressure gases where initially the mean free path may be long enough for interpenetration of the plume and background. The model is based on a combination of multiple elastic scattering and hydrodynamic formulations. Although relatively simple in structure, it gives excellent fits to new experimental data for Si in He and Ar, and provides for the first time a detailed, coherent explanation of the observed splitting of the plume into a fast and slow component. [S0031-9007(97)03820-9] PACS numbers: 79.20.Ds, 52.50.JmPulsed laser deposition (PLD) has become an important technique for depositing a variety of materials [1,2], most notably thin films [3] and superlattices [4] of high-T c superconductors. As a consequence, a very active field of research into laser ablation phenomena underlying PLD has developed [5]. But this is not a new field since it dates back to the earliest days of the laser era when many materials were irradiated with high-powered laser pulses [6][7][8]. Thus, the work reported here has applications far beyond the PLD process itself. More recent work has provided a wealth of new diagnostics with which to study the laser ablation process [9]. It is of crucial importance to know the constitution and dynamical behavior of the plume of ablated material in order to understand how film growth can be optimized by varying the laser parameters, the targetsubstrate distance, and ambient gases introduced into the deposition chamber. In particular, the often observed but little understood phenomenon of "plume splitting" [10] into fast (ϳ vacuum speed) and background-slowed components is of great interest because the fast component may damage the growing film or otherwise affect its microstructure. Also, clustering of film constituents in the gas phase or on the surface may cause problems, but may also provide a technologically important method of producing nanostructures [5,11,12].In this paper, a new modeling approach, combining multiple scattering and hydrodynamical elements, is described and applied to recently obtained experimental data on Si ablated into He and Ar gases. The resulting model is remarkably successful in describing quantitatively the data and resolves long-standing uncertainties about the interpretation of many previous experimental observations.Silicon was selected for the target in the experiments because it is well characterized, is readily obtained as single crystals, and has been thoroughly studied in the laser-annealing regime [13]. Background gases of He and Ar were chosen because their ionization energies are high (25 and 16 eV, respectively), hence avoiding ionization, and because one is lighter and one heavier than Si. KrF laser pulses of 3.0 J͞cm 2 provided a good supply of singly ionized Si for the ion-probe detector while avoiding higher ionization states. Measurements revealed only neutral and singly ionized Si in the plume and only neutral atoms in the background. A de...

Pulsed laser ablation of complex oxides: The role of congruent ablation and preferential scattering for the film stoichiometry

“…25,26 This value can be even shorter for the ablated species located at the plume front, due to the increase of the background gas density consequent to the plume confinement. 27,28 Moreover, the confinement also influences the collisional dynamics of the inner plume species.…”

Influence of spot size on propagation dynamics of laser-produced tin plasma

“…5,10,13,14 In the transition regime a part of the plume penetrated into the ambient gas with vacuum-like expansion and another division of the plume species decelerated after interaction with ambient gas that lead to plume splitting. The plume splitting phenomenon was first observed by Geohegan 5 in laser produced YBa 2 Cu 3 O 7 plasma expansion into oxygen ambient and later Wood et al 14 explained these phenomena based on a combination of multiple elastic scattering and hydrodynamic formulations. In spite of extensive work done on this topic, the propagation dynamics of LPP is not well understood and explained.…”

Energy balance of a laser ablation plume expanding in a background gas

“…A more comprehensive knowledge of plume dynamics in a background gas is clearly desirable, in particular because most of the existing treatments are limited to one or two target materials into a single background gas [6][7][8][9]. The expansion of a silicon ablation plume in helium and argon gases has been modeled quite comprehensively, but the results are difficult to apply to other plume-background gas systems [8].…”

“…PLD of a film by nanosecond lasers is an extremely complex process, which runs through several stages from ini-tial laser irradiation of a solid target, material ejection and transfer through vacuum or a background gas to a substrate [3][4][5]. A more comprehensive knowledge of plume dynamics in a background gas is clearly desirable, in particular because most of the existing treatments are limited to one or two target materials into a single background gas [6][7][8][9]. The expansion of a silicon ablation plume in helium and argon gases has been modeled quite comprehensively, but the results are difficult to apply to other plume-background gas systems [8].…”