There are different techniques to grow nanostructures in a controlled way. Direct use of conventional lithography techniques such as electron beam lithography [5] or scanning tunneling microscope (STM) related nanolithography [6] is quite slow when used to define nanoscale features. Recently, self-assembled techniques have attracted considerable interest for nanoscale device applications because these techniques offer the potential to fabricate nanoscale elements such as quantum dots, quantum wires and electronic device configuration without direct use of conventional lithography techniques. A number of self-assembly techniques have been reported for fabricating nanoscale structures. Molecular beam epitaxy (MBE) is one of the most important techniques working under UHV (1x10-10 mbar) condition for growing self-assembled nano-structures with precise control on growth. The most important feature of MBE grown structures is their crystalline quality and very low impurity level which is an important requirement of modern microelectronics.
Epitaxial growth can occur in different modes [7,8] depending on the surface free energies of the overlayer and the substrate and their lattice misfit. Three established growth modes are (a) Frank - van der Merwe (FM) or layer-by-layer [9], (b) Volmer-Weber (VW) or island [10] and Stranski-Krastanov (SK) or layer-plus-island growth [11]. If the materials of the film and the substrate are the same, it is called homoepitaxial growth whereas in heteroepitaxial growth the materials of the film and the substrate are different [12]. The heteroepitaxial growth can, in general, be categorized as crystal growth under stress and usually produces a strained epitaxial layer due to lattice mismatch between the overlayer and the substrate. The strained layers grow pseudomorphically upto a critical thickness beyond which there is a strain relaxation by introducing dislocations [13]. An initial layer-by-layer growth can be discontinued by island growth (SK mode). Islands can then undergo a shape transition as a mechanism for strain relaxation producing self-assembled quantum dot or quantum wire structures [14,15].
The growth of Ge nanostructures on atomically clean Si surfaces is a classic example of Stranski-Krastanov (SK) growth. Clean silicon surfaces are reconstructed surfaces which are usually prepared by removing the native oxide layer by thermal treatment. Si(100) surfaces show a (2x1) surface reconstruction whereas Si(111) surfaces show a (7x7) surface reconstruction. Due to 4.2% lattice mismatch between Ge and Si, a pseudomorphic Ge layer grows with a strain. As the strain energy builds up, the competition between the surface energy and the strain energy eventually causes the film to undergo a 3-D island growth at coverages usually beyond 3 ML (monolayer) [16]. 3-D island formation leads to a partial relaxation of strain. During growth the islands undergo a rich sequence of shape changes. On the Si(100) surfaces they are initially square or rectangular shaped, called hut cluster [17]. There are also other shapes like pyramid and dome [18]. Self-assembled epitaxial nanostructures can be obtained via SK or VW growth modes.
The study of metal-semiconductor systems has been of great interest for decades in view of their technological applications. Generally speaking, metal/semiconductor nanostructures fall under three different broad categories: (i) single electron tunneling (SET) devices (ii) nanoscale Schottky barriers and (iii) nanoscale Ohmic contacts. The first category relies on conditions where the size of the dot structures is reduced to a few nanometers. The second two categories rely on a confined metal/semiconductor type interface based on the work functions and other electronic properties, like Fermi level pinning, barrier height and barrier width. Ag/Si(111) is one of the most extensively studied system because it is a non-reactive metal semiconductor system [19,20]. The growth of Ag on Si(111) surfaces has assumed a renewed importance as a recently observed novel growth morphology could not be explained within the three accepted growth modes. A low temperature (LT) growth followed by room temperature (RT) annealing leads to 3D plateaulike Ag islands with a strongly preferred height two atomic layers [21]. The islands increase their number density and lateral extension with coverage with no change in height, eventually forming a percolated network of the same preferred height. This growth mode appears to be influenced by an electronic growth mechanism proposed by Zhang and coworkers [22,23]. This behaviour also has been observed for the RT growth of Ag on Si(111) surfaces, where Ag continues to grow with an even layer height preference at higher coverages [24].
Quantum size effect (QSE) due to spatial confinement of electrons in thin films appears to play an important role in this growth mechanism. This introduces discrete quantum well states (QWSs) inside the metal films. In this respect, Ag films are interesting due to the nearly free electron character of s-p bands over a large region of the Brillouin zone (BZ) [25,26,27]. RT growth of Ag on Si(111) surfaces showing an even layer height preference could be a consequence of QWSs. A detailed study of Ag growth on Si(100) at RT and followed by annealing is presented in this thesis. Various aspects of preferred island heights, loss of this preference in thicker films due to formation of screw dislocations and a counter intuitive strain behaviour in the annealed films due to thermal expansion mismatch between Ag and Si are presented in details.
Single electron tunneling (SET) phenomena have been observed on Ag islands grown on Si(111) surfaces. When an electron is transferred into the island there is a rearrangement of charge resulting a change in electronic potential. If this change in potential is greater than the thermal energy, k_BT, it may result in a gap in the energy spectrum at the Fermi energy, leading to the phenomenon of Coulomb blockade. The I-V measurements have been carried out on island structures on a 60 monolayer (ML) thick Ag film on Si(111) surfaces at 100 K sample temperature using scanning tunneling spectroscopy (STS) technique. This shows the SET effect on individual Ag dots. The effect of quantum capacitance on the Coulomb staircase is explained.
Ion beams are used extensively in characterization of epitaxial layers and their modification, e.g. introducing dopants. It would be possible to arrange self-assembled nanodots on the surface of a substrate where a patterned defect suturcture is produced by ion-irradiation. Ion-irradiation effects are extensively used in the conventional electronic device fabrication and characterization in semiconductor industries. In nanoscience and nanodevice fabrication the role of ion beam would be significant. Already, ion-beam sculpting has been used to produce nanopores which are capable of registering a single DNA molecule in aqueous solution [28]. On the other hand one of the fundamental problems in materials science is to understand the effects of ion-solid interaction. During the interaction of energetic ions with solids, the solid surfaces as well as the bulk get modified. The evolution of surface topography during ion-irradiation is determined by different surface relaxation mechanisms. It is important to identify these mechanisms. In this context the effect of ion bombardment on solid surfaces is an essential concept for understanding the evolution of topography in the different length scales. Sputtering yields due to ion-irradiation play an important role for making the surface rough. Under certain conditions ion-irradiation can make the surface smooth as well. Ion-beam induced surface diffusion parallel to the surface is responsible for surface smoothing [29]. In this thesis we present a smoothing phenomenon in ion-solid interaction where smoothing takes place in nanometric length scales (~50 nm). Relevance of this phenomenon to nanostructures would be appreciated from the fact that the electronic transport through the nanostructures depend on the surface and interface roughness and one would like to reduce this roughness. Ion-beam induced nanoscale smoothing is expected to find applications in nanodevice fabrication and modifications.
Ion-beam irradiated smoothed surfaces have been found to be self-affine fractal [30]. A recent theoretical work predicts that the adhesion of the elastic films on a solid surface depends on the fractal dimension of the substrate surface; this in turn would affect the morphology of the film [31]. So the growth of self-assembled nanostrucrues on fractal surfaces are expected to show unusual surface features. We have used ion-beam techniques to prepare different fractal surfaces and studied growth of thin films on them. We have done a detailed study of growth morphology with the determination of fractal dimensions of the bare surfaces and the surfaces with deposited overlayers.
