Study of field-emitting microstructures using a scanning tunneling microscope

Field emission from broad metal cathodes is known to be strongly enhanced at a small number of emitting sites per cm²compared to the expected Fowler-Nordheim emission from ideal, flat surfaces. We have operated a scanning tunneling microscope (STM) in the field emission regime (typical tip voltage: + 80 V) and measured the local field emission strengths and variations on niobium samples. With a modulation technique, which is an adaptation of the standard work function measurement, maps of the field enhancement factor fl have been obtained. An example of an emission site is presented where STM topograph and fl map are compared with a secondary electron microscope image and with field emission data obtained in a standard way using high-voltage anodes. This demonstrates the capability of a scanning tunneling microscope to localize enhanced field emission sites (with typical fl values of 50 in the present work) with high spatial resolution and to study surfaces down to the limit fi = 1.