A.V. Zotov

Growth of Thin Films

When the adsorbate coverage exceeds the monolayer range, one speaks about thin film growth. The oriented growth of a crystalline film on a single-crystal substrate is referred to as epitaxy, which, in turn, is subdivided into homoepitaxy (when both film and substrate are of the same material) and heteroepitaxy (when film and substrate are different). The film growth is controlled by the interplay of thermodynamics and kinetics. The general trends in film growth are understood within the thermodynamic approach in terms of the relative surface and interface energies. On the other hand, film growth is a non-equilibrium kinetic process, in which the rate-limiting steps affect the net growth mode. In this chapter, the surface phenomena involved in thin film growth and their effect on the growth mode, as well as on the structure and morphology of the grown films, are discussed.

Elementary Processes at Surfaces II. Surface Diffusion

Surface diffusion is the motion of adparticles, such as atoms or molecules, over the surface of a solid substrate. The diffusing particles might be the same chemical species as the substrate (the case referred to as a self-diffusion) or another one (the case of heterodiffusion). In most cases, the adparticle becomes mobile due to thermal activation and its motion is described as a random walk. In the presence of a concentration gradient (in the more general case, of the gradient of the chemical potential), the random walk motion of many particles results in their net diffusion motion in the direction opposite to the direction of the gradient. The diffusion process is affected by many factors, such as interaction between diffusing adspecies, formation of surface phases, presence of defects, etc.

Atomic Structure of Surfaces with Adsorbates

In studies of clean surfaces, the presence of any foreign species is absolutely undesirable. However, a large number of investigations concern surfaces on which a controlled amount of certain foreign atoms or molecules are intentionally added. The foreign species can be added to the surface in different ways, including condensation from vapor phase (adsorption), segregation from the sample bulk, or diffusion along the surface. Taking into account that adsorption is the most widely used technique, the added species is conventionally called the adsorbate. The material of the host surface is called the substrate. In the present chapter, the atomic structure of clean surfaces with adsorbates is discussed. The consideration is limited to adsorbate layers with an effective coverage of up to one atomic layer. Thus, multilayer thin films are beyond the scope of the chapter. Already formed (in most cases, equilibrium) structures are treated, while the dynamic processes involved in their formation will be discussed elsewhere.

Growth and characterization of van der Waals heterostuctures formed by the topological insulator Bi2Se3 and the trivial insulator SnSe2

The formation, structure and electronic properties of SnSe2–Bi2Se3 van der Waals heterostructures were studied. Both heterostructures, SnSe2 on Bi2Se3 and Bi2Se3 on SnSe2, were grown epitaxially with high crystallinity and sharp interfaces. Their electron band structures are of trivial and topological insulators, respectively. The Dirac surface states of Bi2Se3 survive under the SnSe2 overlayer. One triple layer of SnSe2 was found to be an efficient spacer for separating a Bi2Se3 topological-insulator slab into two and creating the corresponding topological surface states.

Atomic Structure of Clean Surfaces

Most studies in surface science start with atomically clean substrate surface. Hence, knowledge of the atomic structure of clean surfaces is of great importance. It appears that the majority of surfaces, especially those of semiconductors, are essentially modified with respect to the corresponding atomic planes in the bulk. In this chapter, the general types of such modifications, called relaxation and reconstruction, are first introduced and then illustrated by several examples. Of course, these examples cannot cover the diversity of atomic structures occurring at clean surfaces of crystals, but they should give an impression of what the surface might look like and what kind of physics might stand behind a particular surface structure.

Elementary Processes at Surfaces I. Adsorption and Desorption

Adsorption/desorption phenomena have already been treated in Chap. 9 devoted to the structure of surfaces with adsorbates. In particular, the basic terms (for example, adsorbate, substrate, physisorption, chemisorption, coverage) have been introduced. However, there the final result of the adsorption/desorption process has been discussed, not the process itself. The latter is the subject of the present chapter, in which adsorption/desorption phenomena are treated within the kinetic approach. It resides in establishing the relationship between the rates of adsorption and desorption of atoms or molecules as a function of the external macroscopic variables, such as vapor pressure, temperature of the substrate and vapor, substrate surface structure, etc. Kinetics is an external manifestation of the atomic-scale dynamic processes occurring at the surface. Analysis of the kinetic data gives an insight into the atomic mechanisms involved in the adsorption and desorption. In particular, adsorption/desorption energetics (the height of energy barriers and the depth of energy wells) are commonly extracted from the experimental kinetic data.

Surface Analysis III. Probing Surfaces with Ions

There is a set of various analytical techniques which employ an ion beam to probe a surface. The most widely used techniques are as follows.

Atomic Manipulations and Nanostructure Formation

Recent progress in material science (and, in particular, in surface science) provides an opportunity for the fabrication of various artificial structures of nanometer size. The main approaches used for such fabrications are atomic manipulations (i.e., building up the structure atom by atom) and self-organization (i.e., spontaneous formation of many structures at once, as a result of certain processes). The growth process and the grown nanostructures themselves present great interest both for science and technology.

Surface Analysis IV. Microscopy

Microscopy techniques are used to produce real space magnified images of a surface showing what it looks like. In general, microscopy information concerns surface crystallography (i.e., how the atoms are arranged at the surface), surface morphology (i.e., the shape and the size of topographic features making the surface), and surface composition (the elements and compounds that the surface is composed of). The operational principles vary greatly from one type of microscopy to another and include electron-beam transmission (transmission electron microscopy) and reflection (reflection electron microscopy, low-energy electron microscopy, scanning electron microscopy), field emission of electrons (field emission microscopy, scanning tunneling microscopy) and ions (field ion microscopy), and scanning the surface by a probing electron beam (scanning electron microscopy) or a probing tip (scanning tunneling microscopy, atomic force microscopy). Most microscopy techniques used in surface science ensure resolution on the nm scale, while field ion microscopy, scanning tunneling microscopy and atomic force microscopy allow acquisition of images with atomic resolution.

Basics of Two-Dimensional Crystallography

In this chapter, the nomenclature used to describe surface structures is developed. An understanding of this nomenclature is very important, as it will be used continuously throughout all the following chapters of the textbook. It should be noted, however, that here only the formal definitions and concepts are given.