The initial growth of vapour deposited gold films

Abstract

Gold films are extensively used in the production of both ohmic-and Schottky- Studies of gold films have a long history and the use of evaporated gold films to protect and to gild surfaces emerged more or less in parallel with the development of vacuum techniques. Since then, films of gold have been studied to a greater extent than those of any other metal, because of both the relative simplicity of gold evaporation and the resistance of gold to chemical attack during and after the deposition process. The literature on gold films therefore constitutes a large proportion of the literature on metal films in general. Although the mechanisms of growth of metal films have been the subject of excellent reviews such as those of Lewis and Anderson (1), Robins (2), and Le Lay and Kern (3), these articles have tended to concentrate on growth from the nucleation stage onwards. Much less attention has been paid to the pre-nucleation stage.In the present review, attention will therefore be focussed mainly upon this pre-nucleation stage and specifrcally upon the interaction of evaporated gold with surfaces and the growth of the first few atomic monolayers of films of this metal. Particular attention will be paid to the early stages of growth of gold films on semiconductor surfaces as a large research effort has been concentrated recently in this area. The reason for this is the extensive use of such films in the establishment of electrical contacts of the ohmic and Schottky types.Gold films on silicon and GaAs usually foren Schottky barriers even at monolayer (ML) thicknesses. A detailed knowledge of the mechanisms by which they are formed on these semiconductors is therefore of great importance. The film growth has been found to differ according to whether it occurs on clean ordered surfaces or upon surfaces which are either disordered per se or as .a result of the presence of contaminants.For the study of the growth of gold films especially at monolayer level on clean or ordered surfaces, such as those of clean silicon or GaAs, ultra high vacuum (UHV) techniques and analytical methods such as Auger electron spectroscopy (AES), photoelectron spectroscopy (PES) and electron diffraction have been all-important. Initial film growth is characterized by strong interactions between gold atoms and the substrate surface, leading to intermixing of the metal and the substrate. At low temperatures, typically room temperature, the first monolayer of gold is deposited in a disordered state, and gives rise to the subsequent formation of a mixed compound layer (silicide or alloy) upon which nucleation of gold may occur. At elevated temperatures (typically several hundred Celsius), reconstructions occur within the first monolayer of gold on silicon. Interdiffusion is fast and in thicker films of gold, on GaAs for example, leads to a thick intermixed region. The presence of small controlled amounts of impurities on clean surfaces of silicon and GaAs leads to a marked change in the surface reactions with evaporated gold. With increased impuri...

Room temperature evolution of gold nanodots deposited on silicon

“…The critical thickness has been in this case estimated as the plateau exhibited by the growth rate measured by an oscillating quartz, but no morphological analysis is shown. A lower value of 1 ML for the critical thickness has also been found by Auger spectroscopy studies for gold evaporated on clean Si (111) at room temperature [16]. Experiments of minute amounts of gold deposited on NaCl demonstrated that a considerable surface mobility and coalescence is present also at room temperature.…”

“…The gold films were evaporated in this case at T <50°C on clean silicon and characterized in situ by using electron loss spectroscopy and Auger electron spectroscopy. The interacted regions have been identified as metastable alloys or silicides [16,17]. A critical thickness below which the gold seems inert and not able to create alloying has been identified, and, if it is less than 2 ML, gold and Si do not intermix even at temperatures as high as 800°C [15].…”

Electron transparent gold platelets

“…Careful analysis of such images and their electron diffraction patterns as a function of lead thickness on the gold substrate indicates that a very thin intermetallic layer of the face centred cubic Au 2 Pb Lave phase is formed during the initial stages of bimetal growth at room temperature. It is proposed that this film crystallizes after a super-saturated lead-gold solution is formed due to the diffusion of lead into the gold substrate -discussed generally by Andersson (10). Thicker layers of lead which now grow epitaxially on top of this layer have a body centred cubic structure, resulting from the two-dimensional layer or Frank-Van der Merwe mode of growth (10).…”

“…It is proposed that this film crystallizes after a super-saturated lead-gold solution is formed due to the diffusion of lead into the gold substrate -discussed generally by Andersson (10). Thicker layers of lead which now grow epitaxially on top of this layer have a body centred cubic structure, resulting from the two-dimensional layer or Frank-Van der Merwe mode of growth (10). The normal crystal structure of lead is, in fact, face centred cubic and this study illustrates dramatically how a thin interaction layer can provide the formation upon itself of a crystal structure which would otherwise not be thermodynamically stable.…”

Influence of hydrofluoric acid treatment on electroless deposition of Au clusters

“…It has been found that Si atoms diffuse outwards through the deposited gold layers during the growth process with the subsequent formation of Si oxide on their surface. The process stops after a certain thickness of oxide is formed and on top of it gold atoms agglomerate as solid clusters [ 18 – 21 ]. The optical properties of these gold clusters depend on their shape and morphology.…”