Pulsed-laser-stimulated field ion emission from metal and semiconductor surfaces: A time-of-flight study of the formation of atomic, molecular, and cluster ions

Abstract

In field ion emission, ions are formed beyond a critical distance from the emitter surface. This distance is imposed by the condition that the energy level of the tunneling atomic electron must line up with a vacant electronic state in the solid. As a result the field-emitted ions exhibit a critical energy deficit in the energy distribution. The critical energy deficit is related to the binding energy and the ionization energy of the emitted atoms, and the work function or the electron affinity of the emitter surface. A time-of-flight measurement of the ion energy distribution and the critical energy deficit of various types of ions from both metal and semiconductor surfaces has been made using the pulsed-laser atom probe. From the result mechanisms of the formation of various types of ions in pulsed-laser-stimulated field ion emission, including novel molecular and cluster ions such as ${\mathrm{D}}_{3}^{+}$, ${\mathrm{N}}_{2}$${\mathrm{H}}^{+}$, ${\mathrm{ArH}}^{+}$, ${\mathrm{RhHe}}^{2+}$, ${\mathrm{PtHe}}^{2+}$, ${\mathrm{PtHe}}_{2}^{2+}$,${\mathrm{Si}}_{1\mathrm{t}\mathrm{o}\phantom{\rule{0ex}{0ex}}11}^{2+}$, ${\mathrm{Si}}_{1\phantom{\rule{0ex}{0ex}}\mathrm{t}\mathrm{o}\phantom{\rule{0ex}{0ex}}6}^{+}$, etc., are investigated. It is found that photoexcitation plays an important role in pulsed-laser field evaporation of silicon. Pulsed-laser field evaporation of Si can be sustained almost indefinitely by a field-gradient- and temperature-pulse-induced surface diffusion of Si atoms from the emitter shank. The most abundant Si-cluster ions are ${\mathrm{Si}}_{4}^{2+}$, ${\mathrm{Si}}_{5}^{2+}$, and ${\mathrm{Si}}_{6}^{2+}$. These clusters and ${\mathrm{Si}}_{13}^{4+}$ are the only small, symmetrically structured atomic units one can remove from a Si crystal. The numbers 4, 5, 6, and 13 are the magic numbers in Si-cluster formation. The critical number is found to be 3 for 2+ ions. The higher ionization energies of heavy metal atoms, data of which are not readily available, can also be derived from a measurement of the critical energy deficit of field-evaporated ions.