Abstract
Optimisation of femtosecond pulsed laser deposition parameters for the fabrication of silicon thin films is discussed. Substrate temperature, gas pressure and gas type are used to better understand the deposition process and optimise it for the fabrication of high-quality thin films designed for optical and optoelectronic applications.
Highlights
Femtosecond pulsed laser deposition technique [1] uses a train of focused femtosecond laser pulses to generate plasma ablation from a target material; this plasma is deposited onto the surface of a substrate material, and the growth of a thin film occurs over time
The fit has been limited to 6 nm diameters and above for particles deposited at 20 mTorr and 4 nm and above for 40 and 60 mTorr. This is because below this diameter, the resolution and contrast ratio of the particles with respect to the copper grid are too low for an accurate assessment of particle size. These results are in good agreement with observations by Amoruso et al [10] in vacuum, where a similar exponential character was identified for the relative yield of particle sizes
One foreseeable consequence of the addition of a background gas is the agglomeration of ablated particles. This is not observed for the present study because the effective temperature of the ablated material is too high, and the strong positive ionisation of many of the particles would inhibit the formation of aggregates
Summary
Femtosecond pulsed laser deposition (fs-PLD) technique [1] uses a train of focused femtosecond laser pulses to generate plasma ablation from a target material; this plasma is deposited onto the surface of a substrate material, and the growth of a thin film occurs over time. We take silicon as an example of a target material; should a regular continuous wave laser be focused onto its surface, with an arbitrary energy just above its bandgap, one would observe the excitation of electrons to the conduction band through an indirect process involving phonons. This is because silicon has an indirect bandgap; one must use a wavelength of approximately 360 nm (3.43 eV) to trigger direct electronic excitation of silicon. The absorption of the initial part of the femtosecond laser pulse gives rise to the formation of an electron-hole plasma in a relatively cold lattice of ions, and the rest of the pulse is absorbed through nonlinear mechanisms in the top surface of the material
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