Some properties of thin metal films observed by field-ion and field emission microscopy

Abstract

Deposits of gold on tungsten and of iron on iridium were prepared by vapour deposition in ultra-high vacuum in a liquid-nitrogen-cooled helium-ion microscope. A preliminary experimental study was made of the problems involved in the preparation, imaging, identification and field-evaporation of the deposits. The approximate temperature of the substrates, either during or following the depositions, ranged from 78°K to 1500°K and the maximum deposit thickness was about 10 atomic layers, grown in an incident vapour flux of between 0.5 and 5 atoms/Å 2/min. The conditions for mobility, crystallinity and stability of the deposits were investigated. Several types of deposit gave satisfactory helium-ion images and were unambiguously identified by the coordinated use of field emission and field-ion microscopy. Deposition of gold onto heated tungsten gave highly-ordered two-dimensional structures whose field-ion images strongly resembled those of the underlying tungsten substrate, and whose surface atoms were very stable towards field evaporation. Such deposits are compared with those lying within a region of transition thickness suggested by investigators using other techniques. The implications of such ‘transition layers’ are discussed in relation to nucleation and epitaxy.