A new surface science in situ transmission and reflection electron microscope

Abstract

We describe an ultrahigh vacuum instrument for transmission electron microscopy and reflection electron microscopy for the study of surfaces and thin film growth. The focus of previous experiments was on the high spatial resolution (<3 Å) generally associated with microscopy, at the cost of controlled growth and characterization. We have taken a different approach. It has been shown that most experiments using diffraction and diffraction contrast imaging can be performed well at poorer resolution (∼20 Å), including the imaging of monatomic steps and monolayer coverages. The instrument is designed for best control of growth and vacuum, with sacrifices in optical resolution, which is theoretically ∼2 nm. The instrument is called SHEBA (surface high-energy electron beam apparatus). We can examine a ∼1 cm2 sample in both transmission electron microscopy and reflection electron microscopy, in situ with well-controlled molecular beam epitaxy (MBE) growth capabilities, well characterized vacuum, and surface characterization by Auger spectroscopy and low-energy electron diffraction. Preliminary experiments of cobalt on silicon have shown MBE growth rates ranging from 1.5 monolayers per hour to 18.3 monolayers per hour depending on the temperature of the evaporation source. Using SHEBA for a reactive MBE experiment has provided the first direct evidence of the formation of aluminum nitride by nitridation of basal plane sapphire. The most serious design problems were stray magnetic fields and mechanical vibrations. These have been overcome and allow a spatial resolution of <10 nm. The instrument is expected to be very valuable in studies of film growth.