Field ion images

The PLAP applies a short duration (100 ps 10 ns) laser pulse to the apex of the specimen. The heat generated is sufficient to promote the field evaporation at the standing voltage of the specimen. The specimens need only to be sufficiently conductive to permit field ion imaging. The peak temperature in the PLAP is only 300 K for a period of a few nanoseconds, which is not sufficiently high for surface diffusion on semiconductor materials, and so the spatial resolution is not downgraded. 

The basic assumption used for the interpretation of the field ion images, viz, the overlapping mechanism, was independently confirmed by the recent atom-probe experiments which have supplied evidence for the formation of a quasichemical complex between an inert gas atom and a metal... 

There are basically two kinds of experiments which can be used for studying the mechanisms of the field ionization process near a field ion emitter surface and also the field ion image formation process. They are the measurement of field ion current as functions of tip voltage, tip temperature, and other experimental parameters, and the measurement of the ion energy distribution. 

After the above discussion the field ion image contrast can now be easily understood. According to the calculations of Fonash, Sharma and Schrenk,140 who considered carefully the variation in the surface potential right above a surface atom, and an atom between two surface atoms, an image intensity variation of about 10% can be expected on a surface plane. This small variation obviously cannot explain why the field images... 

Fig. 3.6 (a) Stereographic projection map of a (001) oriented cubic crystal. The facet sizes reflect approximately those of the field ion image of a fee crystal. 

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