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Laser-MBE : Pulsed Laser Deposition under Ultra-High Vacuum (UHV)

Short introduction

An elegant alternative to Molecular Beam Epitaxy is Pulsed Laser Deposition (PLD), where the energy to evaporate atoms from a target is provided by a nanosecond (or shorter) class IV laser, instead of Joule or electron bombardment heating like in MBE. Any kind of material may be evaporated with ease by PLD, even the most refractory ones, due to the high fluence (density of laser energy per pulse and area). Besides, the targets do not need special care concerning thermal conduction for heat evacuation as the average power is low, nor electrical connection. Thus many targets may be stored in situ or exchange through a load-lock, making PLD a versatile tool well suited to fundamental research.
FIG 1 : Our set of ultra-high vacuum chambers for laser-MBE (the lasers are in an adjacent room).
FIG 2 : Scanning tunneling microscopy image of four atomic layers of Cobalt epitaxially-grown on graphene at room temperature. Under identical conditions Cobalt grows as islands with MBE.
Apart from versatility, PLD also brings specific features to the physics of epitaxial growth. The most obvious aspect is the large density of nucleii, induced by the high instantaneous rate of deposition : a 10ns pulse length results in atoms impinging on the sample over 10μs due to a spread of velocities. Shooting at 10Hz to reach an average deposition rate of 1nm/min yields an instantaneous rate of deposition of 10μm/s. The large density of adatoms at the surface induces the large density of nucleii, which in turn favors layer-by-layer growth of materials, and thus very smooth films.

Contributors

  • MNM : J. Coraux, O. Fruchart, Chi Vo-Van, Zahid Ishaque.

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Corresponding author : O. Fruchart

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