The study of diffusion of Au into Si substrate has long standing interest in device fabrications. Extensive work has been carried out at elevated temperatures, mainly to enhance the diffusion, from which diffusion dynamics in the micron length scale is well established. Also, a lot of work is on going to understand the role of surface binding and surface structure in the diffusion process. It is known that the presence of a native-oxide layer or the growth of an oxide layer at the interface strongly influences the interdiffusion behavior across a metal-semiconductor interface. It is also known that the oxide growth on Si surface could be hindered under non-UHV conditions by passivating the surface dangling bonds. It has been shown that bromine passivates the dangling bonds of the Si(111) surface and this passivated structure is stable for several days in dry air, while the bromine passivated Si(001) surface is not so stable. The stability of Si surface after passivation with other material is again different. All these suggest that by controlling the passivation, diffusion of Au into Si can be controlled, which can also be used in the formation of control interdiffused layer. However, not much work has been carried out in this direction to study the initial interfacial role in the formation of thin Au-Si diffuse layer at room temperature, which is of immense interest not only to produce control diffused junctions in silicon at very shallow depth from the surface for the newly developed devices but also for the understanding of the morphological stability of the grown low-dimensional structures due to the diffusion even at ambient conditions.
 
The stability or instability of the Au-Si(111) and Au-Si(001) interfaces, at ambient conditions, have been studied and compared. Electron density profile and derived Au interdiffusion, obtained from x-ray reflectivity measurements, show different time-evolution for Si surface pretreated differently. The evolution of interdiffused amount has been fitted with more generalized Fickian-type diffusion, considering stretched exponential growth of a blocking layer at the interface, where the growth-time depends on the type of Si surface passivation. In particular, the growth-time and hence the stability of the HF-treated surface is found large compared to that of the Br-treated one. Correspondingly, prominent interdiffusion of Au into H-passivated Si sample takes place. The presence of Au-Si interdiffused layer is also evident from secondary ion mass spectrometry. Island-like topography, mapped by scanning tunneling microscopy, show different height-variation consistent with the amount of interdiffusion. Large in-plane correlation length, in the sample of prominent interdiffusion, clearly indicates the growth of non-uniform blocking layer at the interface. Differences in the Au-XSi(111) interfaces, for X = O, Br and H; and its similarity with those of the Au-XSi(001) interfaces [published in Phys. Rev. B 75, 205411 (2007)] can be well understood considering atomic size, electronegativity, etc. of X materials and number of dangling bonds, open space, etc. of Si surfaces [published in Defect Diffus. Forum 297-301, 1133 (2010)].
 
Gaussian-shape diffused nanolayer, formed due to atomic diffusion of gold into silicon crystal, shows wave-front-like movement with time when the system is in ambient condition, while it remains almost static as long as it is in ultrahigh vacuum condition. This is a clear evidence of simple atmospheric pressure induced diffusion of atomic gold into the silicon crystal and provides interesting concept of inherent pressure inside a crystal structure. The atmospheric pressure at the surface and its gradual decreasing nature from the surface to inside crystal acts as driving and retarding forces, respectively, which can be used to control the formation and movement of the diffused layer in nanolevel. Such diffusion also depends on the crystal structure and freeness of the diffusing atoms. The latter increases as the thickness and/or coverage of the gold layer decreases [published in Phys. Rev. B 79, 155405 (2009)].
 
The growth and evolution of Ag nanolayers on differently-passivated Si(001) substrates at ambient condition have been studied. Initial compactness and smoothness of Ag nanolayer on the H-passivated Si(001) surface are found better compared to those on the Br-passivated Si(001) surface, which can be understood considering surface free energy and surface mobility of the passivated surfaces. As the time passes, the growth of dewetted three-dimensional (3D) islandlike structures (Volmer-Weber-type mode) from comparatively wetted Ag nanolayer (Stranski-Krastanov-type mode) is evident at ambient conditions. Such evolution of growth is through dewetting (related to the change in the interfacial energy due to the oxide growth), migration, and coalesce of Ag, which can even produce large epitaxial [Ag(001)/Si(001)] 3D islands on H-passivated Si(001) surface. The growth rate, size, number density, and epitaxy/nonepitaxy of 3D islands are different for different passivated surfaces. These differences can be realized considering the growth time of oxide (i.e., instability of passivated surface), in-plane inhomogeneity of interfacial energy (i.e., inhomogeneous nature of passivation), and in-plane diffusion of Ag on the passivated surfaces. [published in Phys. Rev. B 79, 155412 (2009)].