Surfactants in Semiconductor Epitaxy

Surfactants in Semiconductor Epitaxy Thomas F. Kuech Department of Chemical and Biological Engineering University of Wisconsin -Madison 1415 Engineering Dr. Madison, WI53706 USA Abstract Surfactants have been used to alter the growth behavior and structure of epitaxial films. Surfactants are characterized by a low vapor pressure, low or negligible solubility in the host material and the ability to segregate to the growing surface. The change in surface composition due to this surface segregation can lead to a wide range of phenomena resulting in changes to the chemical and physical structure of the growing film. Thermodynamic and kinetic considerations associated with the influence of surfactant during the growth of lattice mismatched semiconductors are presented. Keywords: Growth models, Surface processes, Surfactant PACS: 81.10.Aj, 81.10.-h, 82.70.Uv

INTRODUCTION The growth of epitaxial films is both a chemical and physical process. Atoms or molecules impinging at the growth front serve to provide the supersaturation required for the formation of the film. These growth nutrients however must undergo several kinetic steps in order to interact and react with the film surface. The film surface additionally may have a different chemical composition and stoichiometry, as well as a differing bonding arrangement or surface reconstruction, than the truncated bulk crystal. All of these factors result in the surface being both chemically and physically distinct and open to modification by the ambient. The surface can therefore be chemically modified and affect changes in growth behavior that can have dramatic effects in the resulting film.

SURFACE STRUCTURE AND THERMODYNAMICS From a thermodynamic point of view, the surface is often treated as distinct from the bulk, possessing its own thermodynamic properties. Perhaps the more common of these properties are the surface free energy and the surface stress. The surface free energy, Gs, is the energy spent in forming the surface and is sometimes referred to as the surface tension, y. The formation of a solid surface can be conceptually thought of as proceeding in two steps: 1) the solid is cleaved such that the new surface is exposed and the atoms are retained in their bulk positions, and 2) the atoms on the surface or at the near-surface regions are allowed to relax themselves to a new equilibrium position. CP916, Perspectives on Inorganic, Organic, and Biological Crystal Growth: From Fundamentals to Applications, 13t International Summer School on Crystal Growth edited by M. Skowronski, J. J. DeYoreo, and C. A. Wang © 2007 American Institute of Physics 978-0-7354-0426-7/07/S23.00

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For liquids, the two steps are thought to generally occur as a single step however even water is now thought to have a slightly different surface structure than the bulk liquid [1]. The surface stress and surface tension of liquids is therefore treated as the same. Solids however have the atoms which are not free to easily move. As a result only small atomic motions without a change in the number of atoms on the surface occur [2]. The surface energy of a solid is a function of the crystallographic direction as is the surface stress, i.e. y=yhu- Surfaces can also possess surface stresses which can be derived from the in-surface changes in bond length and orientation. It is defined in a manner similar to the stress-strain relationships in the bulk.

'-Y8n +

(1)

where Oy is the force per line exerted by the atoms across a line of intersection in direction, /', of a plane whose normal is in t h e / 1 direction and % is the corresponding strain [3]. In liquids, the second term in equation (1) would be zero. These concepts of surface stress and surface energy of a solid are useful in determining the underlying causes for surface rearrangement and the nucleation and growth behavior in epitaxial growth. The surface energy is almost always positive, indicating that surfaces are not energetically favored entities. A solid is always reluctant to form a new surface since it costs energy. Solids at high temperatures in equilibrium with their vapor or liquid phase will form a surface shape which minimizes this energy expenditure. Solids will therefore minimize their surface energy by altering their shape. Most epitaxial growth takes place on a single crystal substrate which has a well defined overall orientation. Most wafers are oriented to a specific low-index crystallographic direction, e.g., (100) or (111), within a certain accuracy. If a wafer has a surface which is exactly a crystallographic plane, it is referred to as a singular surface and is conceptualized as an atomically flat surface. Often, the wafer will be specified to have a polished (100) surface with an intentional misorientation angle towards another major crystal direction, e.g. (100) tilted 2° towards the nearest direction. Such surfaces are often referred to as vicinal surfaces. This intentional or unintentional off-orientation of the wafer increases the structure on the wafer surface. The detailed surface structure is characterized by three separate but related features: terraces, kinks and ledges along with surface vacancies as schematically shown in Fig. 1. Based on the number of possible broken bonds, an atom within a terrace will have the least number of broken bonds and will have the higher number of bonds to adjacent surface atoms. Atoms on ledges will have a higher number of broken bonds than those within the terrace while atoms at kink sites have the fewest bonds to the surface. The surface energy is in part a function of the number of unsatisfied bonds on the surface. The surface energy will therefore be a function of the specific density of these individual surface structures. The kink sites will have the highest per atom energy followed by the ledge sites and finally the terrace sites. It should be noted that the number of kinks, ledges and the terrace width are not completely independent variables but are partially constrained by the geometry of the surface,

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ledge surface vacancy

FIGURE 1. The surface of a semiconductor consists of terraces, ledges and kinks in the bonding arrangements with additional other surface defects such as vacancies and adatoms. These sites can serve as areas of increased chemical reactivity.

substrate orientation and vicinality. At higher temperatures, such as those used in the growth of epitaxial materials, the surface will have a more complex structure since entropy can play a role. As the temperature is increased, there is a driving force to increase the amount of disorder on the surface through the generation of atom vacancies, surface ledges and kinks. The addition of these high energy structures increases surface energy but this increase is offset by the increase in the surface entropy. These energy considerations can lead to complicated surface structures. This truncated crystal model does provide an idealized view of the surface structure on an atomic scale. The very energetic dangling bonds left on the surface, in this construction, would however prefer to be part of a covalent bond if possible. This covalent bond could be formed through an interaction within an adsorbed species. In the absence of adsorbed reactive species, the dangling bonds on the surface will often reform, bonding to adjacent atoms on the surface. This reformation process results in a new surface arrangement of atoms. This rearrangement away from the truncated crystal arrangement is referred to as surface reconstruction. Surface reconstruction on a semiconductor surface possesses both a short range and long range structure. The nature of the structure will depend on the temperature and surface chemical composition, as in the case of compound semiconductors. The reconstruction of a surface allows the surface energy to be decreased through the reformation of the 'broken' bonds. Since the bonds are not at the angles and length found in the bulk, the energy expended in the creation of the surface is not fully recovered through the reconstruction process and hence the energy of the surface is still positive. The reconstructed surface of semiconductors will possess an altered chemical reactivity and therefore affects the process of epitaxial growth. The presence of a surface energy based on the surface structure and surface stress leads to a variety of phenomena in the growth of thin films, particularly in the case of strained layer growth. Changes in the chemical composition of the surface can influence the surface energy and stress altering the kinetic processes and eventually the equilibrium state of nature of the solid surface. Surfactants can play such a role.

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STRAIN AND GROWTH MODES The growth behavior in epitaxial systems is determined by the surface transport and the surface energetics of the materials system of interest. The atomic level motion and the energetics of the growth front will dictate the physical arrangement of these deposited atoms. The simple models of epitaxial growth predict different growth structures, derived from the chemical nature of the deposited atom with the substrate serving as the template for the atomic arrangement of these species. There are generally two classes of epitaxial behavior based on whether the deposited atoms build a layer of the same structure and chemical composition as the substrate (homoepitaxial growth) or the growing layer is different in chemical composition and perhaps physical structure from the substrate (heteroepitaxial growth). Since epitaxial growth of a semiconductor requires that the deposited atoms assume an orientation which is directly related to the underlying substrate, there will, in both cases, be a known geometric relationship between the deposited layer and the underlying crystalline substrate. Homoepitaxial Growth There are two dominant forms of growth behavior seen in homoepitaxial growth: layer-by-layer growth and step-flow growth. These growth modes are related to the transport and incorporation kinetics of the adsorbed atom on the growth surface. Since there is no difference in the physical and chemical properties of the deposited film and the substrate during homoepitaxy, this growth can proceed in a well-controlled manner. A common homoepitaxial growth mode is the layer-by-layer growth mode which is referred to as Frank-Van der Merwe growth. In this growth mode, a new layer of the crystal consisting of a monolayer is nucleated at various points over the crystal terrace; adatoms diffuse to and attach to these island edges increasing the island size. Eventually the growing islands merge and coalesce into a new layer of the crystal whereupon the process is repeated with the formation of new monolayer islands. This growth mode relies on the presence of sufficient surface mobility, i.e. a sufficiently high temperature, for the adatoms such that they can move to an island edge. Both a high flux of atoms to the growth front and a low growth temperature, leading to a slow surface diffusion of adatoms across the growing crystal, can lead to the shift from the monolayer-by-monolayer growth to the simultaneous growth of several layers of the crystal. In these cases, adatoms will encounter other adatoms on the growth front. Some of these atoms will bind together and result in the formation of a new layer of the crystal over several layers at once as depicted in Fig. 2a for the growth of GaAs. This multilayer growth can lead to a rough surface with many atomic levels. This rough growth can become part of the internal structure of the heteroepitaxial materials. A related growth mode is found on vicinal surfaces. In this case, the surface has structurally required ledges, or steps, and terraces due to the substrate misorientation. In the step-flow mode, atoms deposited on the growth front diffuse to naturally occurring step edges. Since these step edges and step kinks, shown in Fig. 1, provide several atoms for the migrating atom to attach, the diffusing atoms will naturally bind

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there and be incorporated into the growing film. At high growth temperatures, the atoms have sufficient mobility to migrate across the surface terraces before encountering other adatoms. The epitaxial growth then proceeds with the step flowing across the growth front, leaving a very smooth atomically flat surface with a terracelike structure. A smooth growth surface results in very sharp and planar internal interfaces. Most epitaxial growth therefore is performed under growth conditions which yield step-flow growth behavior. In the case of a high flux or low surface mobility, the step-flow growth will develop into layer-by-layer growth with multiple layers being developed simultaneously during the deposition. The characteristic terrace structure of layer-by-layer growth, interrupted by monatomic steps, is seen in Fig. 2a for the growth of GaAs on a singular surface exhibiting the layer-by-layer growth [4] and for step-flow growth in Fig. 2b for the hydride vapor phase epitaxy of GaN [5]. a)

b) 40

20

3 pm

20

40 ym

FIGURE 2. Layer-by-layer growth is observed in the growth of GaAs by metalorganic vapor phase epitaxy on a singular surface [4], and step-flow growth on GaN grown by hydride vapor phase epitaxy [5].

Heteroepitaxial Growth The growth of strained layer epitaxial materials has been traditionally discussed in terms of two basic concepts: growth mode and pseudomorphic thickness. The chemical bonding and structural differences between the growing epitaxial film and the substrate can determine the growth behavior leading to large-scale morphological features and defect structure. For the cases discussed here, we will only consider the behavior of materials that are initially elastically constrained to the substrate, i.e. the in-plane lattice parameter of the epitaxial film is strained to be the same as the substrate. This physical constraint leads to the development of strain in the epitaxial film. The total amount of stored elastic energy, Estrain, is proportional to the lattice parameter difference squared leading to the in-plane strain of the film, ea, and the film thickness, h, as indicated in eqn. (2):

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Estra