Numerical study of femtosecond laser-assisted atom probe tomography
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Numerical study of femtosecond laser-assisted atom probe tomography E.P. Silaeva · N.S. Shcheblanov · T.E. Itina · A. Vella · J. Houard · N. Sévelin-Radiguet · F. Vurpillot · B. Deconihout
Received: 3 October 2011 / Published online: 5 September 2012 © Springer-Verlag 2012
Abstract We investigate the mechanisms of a laser-assisted atom probe tomography technique. In this method, a subwavelength tip is subjected both to a very strong static electric field and to a femtosecond laser pulse. As a result, ions are ejected from the tip one by one. By using femtosecond lasers, one can analyze not only metals but also semiconductors and dielectric materials. To better understand the ejection process, a numerical model is developed based on the drift-diffusion approach. The model accounts for such effects as field penetration, hole and electron movement, and laser absorption. For the given value of the dc field, a substantial band bending and an increase in hole density at the surface of the silicon tip are observed. This bending effect changes silicon absorption coefficient at the surface and significantly increases recombination time of laser-induced carriers.
1 Introduction Atom probe tomography (APT) is a technique that allows a three-dimensional reconstruction of the materials in a small volume (∼ 50 × 50 × 100 nm3 ) at the near-atomic resolution [1, 2]. In this technique, a needle-shaped sample (a tip) E.P. Silaeva () · N.S. Shcheblanov · T.E. Itina Laboratoire Hubert Curien, UMR CNRS 5516, Université Jean Monnet, 18 rue Benoît Lauras, 42000 Saint-Etienne, France e-mail: [email protected] Fax: +33-4-77915781 A. Vella · J. Houard · N. Sévelin-Radiguet · F. Vurpillot · B. Deconihout Groupe de Physique des Matériaux, UMR CNRS 6634, UFR Sciences Site du Madrillet, Avenue de l’Université-BP 12, 76801 Saint Etienne Du Rouvray Cedex, France
is subjected to a high voltage in order to generate a strong electric field at the tip apex. Moreover, a short electrical pulse is used to ionize and evaporate its surface atoms. A position sensitive ion detector measures time of flight of each atom for chemical identification and determines its original position on the tip surface. Laser pulses were not used in the common APT technique that was thus limited to metals and heavily doped semiconductors. The rate of the evaporation process depended on the activation energy of the atoms, dc field at the surface, and the temperature of the tip [3]. In the recently developed laser-assisted APT (La-APT), electrical pulses are replaced by ultrashort laser pulses that irradiate the tip side [4]. The La-APT was shown to improve the mass resolution and to allow the analysis of lowconductivity semiconductors and dielectrics [5, 6]. Despite the recent advances in the development of LaAPT, the analysis of semiconductor and dielectric materials still remains poorly understood. For metallic tips, the role of the laser pulse consists of the absorption of laser energy by conduction electrons and subsequent heating of the lattice [7]. For
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