The effect of different scanning schemes on target and film properties in pulsed laser deposition of bismuth
Introduction
Recent theoretical calculations have shown that semimetallic bismuth in the thin film form could be a very interesting material to be used in multiple quantum well structures (MQWS) for thermoelectric applications [1]. The preparation of Bi films have been widely studied in the past due to the peculiar properties of bismuth in the thin film form [2] and references therein. Films have been synthesised by a large number of methods but few studies are concerned with pulsed laser deposition (PLD) [3], [4], [5], [6]. The production of smooth and dense films is critical for their use in multilayer structures. Moreover, thickness homogeneity is necessary in order to be able to determine reliable thermoelectric performances that are thickness dependent.
PLD is a very challenging technique to prepare the Bi films because both the presence of droplets and inhomogeneous thickness profiles over large scale areas (the distribution is in cosn [4], where n can be very high under vacuum thus rendering the thickness profiles very sharp) are the main drawbacks of PLD.
Many solutions have been proposed to eliminate or to reduce the number of droplets on the film surface (e.g., Ref. [7]) but none is completely satisfactory depending on the nature of the material. The morphology of the target surface, that becomes more and more eroded and rough with increasing time of deposition, plays an important role in determining the quantity and size of deposited droplets. The final state of the target depends on both the nature of the material and the ablation conditions (fluence, laser and/or target scanning, vacuum, …). For instance, increasing the fluence seems to be useful to reduce the droplet emission from metals [7], [8], whereas it must be decreased with superconductors [9], [10] and with Bi, as it has recently been reported by Toth et al. [11]. They showed by ultrafast photography that when ablating with a fluence of 5.5 J/cm2, droplets originating from liquid jets were emitted from a molten Bi target. This droplet emission could drastically be lowered by reducing the fluence from 5.5 to 2.5 J/cm2.
Roughening of the target may also be responsible of displacements of film thickness distributions towards the direction of incidence of the laser beam, as it was pointed out by several authors [10], [12], [13]. Finally, target roughening also generally induces a decrease in deposition rate.
In this paper, we focus on the influence of target morphology on the quality of Bi films prepared by PLD. As we have seen previously, the control of the target surface is of importance for the production of smooth and dense films exhibiting homogeneous film thicknesses. We have investigated the influence of target diameter and way of scanning the laser beam over the target surface at various fluences on (i) the target surface morphology, (ii) the quality of the films deposited and (iii) the thickness profiles.
Section snippets
Experimental procedure
Bismuth films were deposited on glass substrates by PLD in an experimental configuration previously described [14], using a Nd:YAG laser working in its second harmonic mode (λ=532 nm, τ=10 ns) at a repetition rate of 5 Hz. The laser beam impinged at an angle of 60° with regard to the normal onto a target in a vertical position located at a distance of 42 mm from the substrate kept at room temperature and centred with regard to the target. It was directed towards the target by the way of
Simulation of the distribution of the laser beam spots over target surface
The effect, on target morphology and film quality, of scanning the laser beam onto the target was studied by using three different scanning schemes called BAL1, BAL2 and BAL3. Schematic representations of these scannings and their main characteristics are reported in Table 1.
BAL1 consists in a vertical linear up and down motion of the laser beam of 6 and 12 mm length along the centres of targets of 12 and 18 mm in diameter, respectively, at a constant speed of 2 mm/s, onto targets rotating at
Conclusion
High quality bismuth films, with a few amount of droplets, can be deposited by PLD at low deposition temperature. We showed that the enhanced droplet density on the bismuth films as the deposition time increases is not linked directly to the microscopic but rather to the macroscopic state and degradation of the target. In order to depress erosion, two ways were successfully tested : (i) the use of targets of large diameters and (ii) the use of an appropriate laser beam scanning scheme leading
Acknowledgements
The authors would like to thank J.-P. Haeussler for his assistance in microprobe analyses and S. Weber for film thickness measurements.
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