The very early stages of Pb deposition on Si͑111͒-͑7 3 7͒ surfaces have been investigated in real time by scanning tunneling microscopy. The combination of variable temperature scanning with unusual very long periods of imaging time have enabled us to observe that single Pb atoms are highly mobile within each half ͑7 3 7͒ unit cell. Individual jumps of single atoms between different half cells have been resolved as well as the formation of atom pairs as a result. An activation energy of 0.64 6 0.07 eV has been measured for the diffusion of single atoms between different half cells. PACS numbers: 68.35.Fx, 61.16.Ch, 81.15.Ce The observation of diffusion of individual atoms adsorbed on a surface was first reported by means of the field ion microscope (FIM). Although first suggested by Müller [1] in 1957, the first quantitative study of individual adatoms by FIM did not appear until 1966 by Ehrlich and Hudda [2]. Since this pioneering work, a great deal of information has been gathered in the last 30 years on the processes of adatom surface diffusion on metal surfaces by using this valuable technique [3]. Nevertheless, the experimental study of diffusion of single adatoms on semiconductor surfaces has not known a comparable development.Very recently, the application of scanning tunneling microscopy (STM) to this kind of study is starting to fill the information gap. Some experimental difficulties, however, still hamper this development and the number of STM studies on that subject is quite limited. The main problem being the adequacy between scanning speed of STM imaging and the rate of atomic motions produced by diffusion, only some specific systems have been studied [4][5][6][7][8][9]. The surface diffusion of Pb atoms on Ge (111) substrates is one of the cases that has attracted great interest due to the spectacular results that have been obtained with STM [6].The different phases of Pb chemisorption on Si͑111͒-͑7 3 7͒ have been studied using different techniques including STM [10,11]. The main interest has been directed to the analysis of high coverage and high temperature phases, somehow disregarding the very early stages of Pb chemisorption at room temperature (RT).In this paper we present the first experimental evidence showing the mobility of single Pb adatoms deposited at room temperature on Si͑111͒-͑7 3 7͒ surfaces. Ultralow coverage deposition combined with variable temperature STM and unusual very long periods of observation time have enabled us to analyze the trapping character of the reconstructed cells for highly mobile Pb adatoms, as well as the first stages of the nucleation of Pb atom pairs.The experiments were carried out in an ultrahigh vacuum chamber equipped with STM, LEED-AES, sample transfer and heating capabilities, ion sputtering, and a Pb evaporation cell. The base pressure of the system is ϳ5 3 10 211 Torr. Very small amounts of Pb [ϳ0.01 ML (monolayer)] were evaporated at RT on clean reconstructed Si͑111͒-͑7 3 7͒ at typical rates of 0.05 ML͞min ͑1 ML 7.84 3 10 14 Pb atoms/cm 2 ͒, as m...

Electrical measurements have been carried out on epitaxial FeSi2 layers on silicon substrates, the silicide thickness being either 180 or 350 Å. A direct gap of 0.85 eV was measured by optical absorption. Current-voltage characteristics of mesa-structures Cr/Fe/FeSi2/Si show a p-type semiconductor behavior. Capacitance-voltage and capacitance-temperature data at different frequencies indicate a large response of deep levels or interface states. Admittance spectroscopy yields the activation energy and capture cross section of two levels. Finally an energy-band diagram is proposed.

Semiconducting βFeSi2 has been successfully grown on a Si (111) substrate. It has been proven that under ultrahigh vacuum conditions, the solid phase epitaxy temperature can be lowered to ∼800 K, where only the βFeSi2 phase is stabilized. The disilicide formation was monitored in situ by various surface-sensitive techniques such as low-energy electron diffraction, Auger electron spectroscopy, and ultraviolet photoelectron spectroscopy. The epitaxial relationships were ascertained by transmission electron diffraction and microscopy including high-resolution cross-sectional image. The results show the epitaxy of βFeSi2 (110) and (101) planes parallel to the Si (111) plane. The disilicide-silicon heterojunction displays an atomically abrupt interface.

Initial stages of Pb deposition on Si͑100͒ 2ϫ1 surfaces have been studied by means of scanning tunneling microscopy. Scanning tunneling microscopy results show that, at room temperature and low coverage, long quasi-one-dimensional Pb chains can be formed on this substrate. These chains, composed of Pb dimers, run in a direction perpendicular to the underlying Si dimer rows. The chain length suggests that Pb diffusion on the Si surface is a relevant effect even at room temperature. Some indications of Pb dimers movements are presented.

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