Quantum dynamics of charge state in silicon field evaporation

Elena P Silaeva,Kazuyuki Watanabe,Kazuki Uchida

doi:10.1063/1.4960553

Abstract

The charge state of an ion field-evaporating from a silicon-atom cluster is analyzed using time-dependent density functional theory coupled to molecular dynamics. The final charge state of the ion is shown to increase gradually with increasing external electrostatic field in agreement with the average charge state of silicon ions detected experimentally. When field evaporation is triggered by laser-induced electronic excitations the charge state also increases with increasing intensity of the laser pulse. At the evaporation threshold, the charge state of the evaporating ion does not depend on the electrostatic field due to the strong contribution of laser excitations to the ionization process both at low and high laser energies. A neutral silicon atom escaping the cluster due to its high initial kinetic energy is shown to be eventually ionized by external electrostatic field.

Highlights

Field evaporation is a fundamental physical phenomenon used in Atom Probe Tomography (APT) to image the atomic structure of materials.[1]
2+ ions are observed at high dc fields and 1+ ions prevail at low fields
We studied the charge state of ions in field evaporation process by Time-Dependent DFT (TDDFT)-molecular dynamics (MD) method, reproducing an average charge state of evaporating ions as a function of dc field without laser illumination in a reasonable agreement with experimental observations

Summary

Introduction

Field evaporation is a fundamental physical phenomenon used in Atom Probe Tomography (APT) to image the atomic structure of materials.[1]. Atoms are ionized and removed one by one from the APT tip-shaped samples subjected to a high external electrostatic (dc) field of several volts per angstrom. Chemical composition of MgO, ZnO, GaN, AlN, GaAs and others measured in APT experiments often deviates from the known composition and depends on the experimental conditions.[2–6]. These deviations are most likely related to the nontrivial ionization process and behaviour of the charge state of the emitted atoms, that leads to the difficulties of their proper detection Chemical composition of MgO, ZnO, GaN, AlN, GaAs and others measured in APT experiments often deviates from the known composition and depends on the experimental conditions.[2–6] These deviations are most likely related to the nontrivial ionization process and behaviour of the charge state of the emitted atoms, that leads to the difficulties of their proper detection

Methods

Results

Conclusion