Epitaxy

Epitaxy denotes the growth of a second (bulk) phase on a single crystalline substrate and plays an important role, for example, in semiconductor technology. Several techniques are in use to grow such composite layers apart from simple evaporation. In molecu- 

The growth of a crystalline phase B on top of another one (A) will generally be associated with a mismatch of the lattice constants, and this effect will in turn cause some strain in the overlayer. These strain effects will in turn be responsible for the growth mode in solid-on-solid epitaxy. The essential three growth modes are depicted schematically in Fig. 2.27. 

FIGURE 2.27. Equilibrium growth modes in epitaxy (a) layer-by-layer (Frank-van der Merwe) (b) layer plus island (Stranski-Krastanov) (c) island growth (V olmer-W eber). 

Often the resulting structure is not in thermod)mamic equilibrium but is governed by kinetic effects, in particular by diffusion of the adsorbed atoms across the terraces and over the monoatomic steps [47]. 

1 Epitaxy. There is often a sharp orientation relationship between a singlecrystal substrate and a thin-film deposit, depending on the crystal structures and lattice parameters of the two substances. When such a relationship exists, the deposit is said to be in epitaxy with the substrate. The simplest relationship is parallel orientation, and this is common in semiconductor heterostructures, but more complex relationships are often encountered. 

Oriented ultrathin overgrowth of a polymer on a non-polymeric substrate is the latest combination of materials to show epitaxy. The most recent, remarkable form of this phenomenon is the formation of an array of parallel polymer chains on a substrate by depositing monomers and then polymerising them in situ. The Japanese discoverer of this phenomenon (Sano 1996) has called it polymerisation-induced 

Another recently discovered form of epitaxy is graphoepitaxy (Geis et al. 1979). Here a non-crystalline substrate (often the heat-resistant polymer polyi-mide, with or without a very thin metallic coating) is scored with grooves or pyramidal depressions the crystalline film deposited on such a substrate can have a sharp texture induced by the geometrical patterns. More recently, this has been tried out as an inexpensive way (because there is no need for a monocrystalline substrate) of preparing oriented ZnS films for electroluminescent devices (Kanata et al. 1988). 

Recently, this has been given the name microchannel epitaxy.  

The corresponding relation between the host and guest crystals when evaluating the misfit ratio may be a one-to-one lattice relation in the same direction (a X b to a xb axes), or in different axial directions (aX b axes versus aX 110 axes), or on the basis of one unit cell versus a few unit cell sizes (see Fig. 7.13). Royer s misfit ratio is generally a two-dimensional correspondence, but Hartman [13] extended this relation to the misfit ratio in PBCs (see Section 4.2), which is a one-dimensional correspondence. Royer s epitaxial relations correspond to a relation between the F faces of the host and guest crystals containing more than two PBCs, and an epitaxial relation is not allowed between S faces or K faces. In Hartman s analysis, rela- 

The freedom of the dangling bonds on the crystal surface increases with increasing temperature. As a result, there is a critical temperature below which an epitaxial relation cannot be realized. This temperature is called the epitaxial temperature, and it depends on interface orientation. If the misfit ratio is small, the epitaxial temperature is low if the misfit ratio is large, the epitaxial temperature is high, and an epitaxial relation will not be achieved unless the temperature is higher than the epitaxial temperature. 

Another element controlling the epitaxial relation is the state of the dangling bonds on the surface of the host crystal. When the dangling bonds are no longer active owing to adsorbed molecules, the epitaxial relation is not realized. This is why host surfaces cleaved in air have different epitaxial temperatures from those cleaved in a vacuum. 

In the above discussion, three-dimensional nucleation of a guest crystal on the surface of a host crystal is presumed to be an epitaxial growth mechanism. 

Fundamental to forming high quality structures and devices with thin-films of compound semiconductors is the concept of epitaxy. The definition of epitaxy is variable, but focuses on the formation of single crystal films on single crystal substrates. Homoepitaxy is the formation of a compound on itself. Heteroepitaxy is the formation of a compound on a different compound or element, and is much more prevalent. 

The question is When is it epitaxy Does the deposit have to have the same unit cell, or does it just have to be commensurate, in register with the substrate structure What size single crystal grains need to form to call it epitaxy Is it only nonepitaxial when the deposit is incommensurate with the substrate, or when it is amorphous  

Some materials have a small lattice mismatch with the substrate, less then 1%, and can adopt the same lattice constants at the interface. This, however, still results in some strain, which builds until released, forming slip dislocations etc.. The thickness at which defects occur is of considerable interest and referred to as the critical thickness [14, 15]. Strain can be minimized by adjusting the lattice constants of the 

There are many deposit-substrate combinations where the basic lattice mismatch is very large, such as when a compound is formed on an elemental substrate, but where excessive strain does not necessarily result. Frequently a non one-to-one lattice match can be formed. If a material can match up every two or three substrate surface unit cells, it may still form a reasonable film [16]. In many cases the depositing lattices are rotated from the substrate unit cells, as well. In a strict definition of epitaxy, these may not be considered, however, it is not clear why high quality devices and materials could not be formed. 

Structure and orientation of a Me deposit on S in the initial stage of 3D Me bulk phase formation can be either independent of or influenced by the surface structure of S, which can be modified by 2D Meads overlayer formation and/or 2D Me-S surface alloy phase formation in the UPD range. Epitaxial behavior of 2D and 3D Me phases exists if some or all of their lattice parameters coincide with those of the top layer of S. The epitaxy is determined by a minimum of the Gibbs function at constant temperature and pressure. 

In the case of a weak Me-S interaction, the orientation of 3D Me crystallites deposited in the OPD range on top of a bare foreign substrate S according to the Volmer-Weber mechanism (cf. Fig. 1.1a) only weakly depends on the surface structure of S. The misfit between the lattice parameters of Me and S is adjusted by misfit dislocations. 

In the case of a strong Me-S interaction, the structure and orientation of a Me deposit on top of Me UPD modified S according to the Frank-van der Merwe (cf. Fig. 1.1b) or Stranski-Krastanov mechanisms (cf. Fig. 1.1c) strongly depend on the substrate structure. Independently of crystallographic Me-S lattice misfit, distinct correlations between the epitaxy of a condensed 2D Meads phase and/or 2D Me-S surface alloy phase and the epitaxy of a 3D Me bulk phase can be expected. 

In absence of Me-S lattice misfit, the structure of growing 3D Me layers usually continues that of the condensed and commensurate 2D Meads overlayer and or 2D Me-S surface alloy formed in the UPD range at high For low AEi. The structure of a 3D Me film is usually in complete registry with the structure of the modified substrate surface SQikl) [hkl] II Me(M0 [hkl], where (hkl) and [hkl] are the Miller indices and crystallographic directions, respectively. 

In presence of significant Me-S lattice misfit, the epitaxy of isolated 3D Me crystallites or compact 3D Me films is strongly determined by the structure of internally strained 2D Meads overlayer and/or 2D Me-S surface alloy formed in the UPD range at high F or low AEi. The misfit between the lattice parameters of the 2D Meads phase and/or 2D Me-S surface alloy phase and the 3D Me bulk phase is mainly removed by misfit dislocations. The initial strain disappears after depositing a certain thickness of the 3D Me bulk phase. Usually, a thickness of n Me monolayers where 2 20 is necessary to adjust the 3D Me bulk lattice parameters [4.58, 4.59]. If an incommensurate structure of a 2D Meads overlayer is formed in the UPD range, this structure will also be reflected epitaxially in 3D Me crystallites and ultrathin 3D Me films. 

The primary difficulty inherent in this issue is the small niunber of materials with suitable crystal structures and lattice constants. Some transition metals and ceramics, such as Ni, Cu, Fe, and cBN (Table 5, Ch. 3), are the few isostructural materials with sufficiently similar lattice constants (mismatch 5%). In addition, the extremely high surface energies of diamond (ranging from 5.3 to 9.2 J m for the principle low index planes) and the existence of interfacial misfit and strain energies between diamond films and non-diamond substrates constitute the primary obstacles in forming oriented two-dimensional diamond nuclei. Earlier attempts to grow heteroepitaxial diamond on the transition metals were not successful. The reasons may be related to the high solubility/ mobility of C in/on the metals (for example, Fe, Co, or the 

Heteroepitaxy of diamond on cBN powder has been achieved by Yarbroughl l and others. Heteroepitaxial diamond films, 0.7 to 

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The oriented overgrowth of a crystalline phase on the surface of a substrate that is also crystalline is called epitaxial growth [104]. Usually it is required that the lattices of the two crystalline phases match, and it can be a rather complicated process [105]. Some new applications enlist amorphous substrates or grow new phases on a surface with a rather poor lattice match. 

A. Schmidt, L. K. Chau, A. Back, and N. R. Armstrong, Epitaxial Phthalocyanine Ultrathin Films Grown by Organic Molecular Beam Epitaxy (OMBE), in Phthalo-cyanines, Vol. 4, C. Leznof and A. P. B. Lever, eds., VCH Publications, 1996. 

It has also been shown that sufiBcient surface self-diflfiision can occur so that entire step edges move in a concerted maimer. Although it does not achieve atomic resolution, the low-energy electron microscopy (LEEM) technique allows for the observation of the movement of step edges in real time [H]. LEEM has also been usefiil for studies of epitaxial growth and surface modifications due to chemical reactions. 

Herman M A and Sitter H 1996 Moiecuiar Beam Epitaxy Fundamentais and Current Status (Berlin Springer)... 

Another example of epitaxy is tin growdi on the (100) surfaces of InSb or CdTe a = 6.49 A) [14]. At room temperature, elemental tin is metallic and adopts a bet crystal structure ( white tin ) with a lattice constant of 5.83 A. However, upon deposition on either of the two above-mentioned surfaces, tin is transfonned into the diamond structure ( grey tin ) with a = 6.49 A and essentially no misfit at the interface. Furtliennore, since grey tin is a semiconductor, then a novel heterojunction material can be fabricated. It is evident that epitaxial growth can be exploited to synthesize materials with novel physical and chemical properties. 

The following two sections will focus on epitaxial growth from a surface science perspective with the aim of revealing the fundamentals of tliin-film growth. As will be discussed below, surface science studies of thin-film deposition have contributed greatly to an atomic-level understanding of nucleation and growth. 

MgO films have been grown on a Mo(lOO) substrate by depositing Mg onto a clean Mo(lOO) sample in O2 ambient at 300 K [39, 40]. LEED results indicated that MgO grows epitaxially at an opthnum O2 pressure of... 

Figure A3.10.13 Ball model illustration of an epitaxial MgO overlayer on Mo(lOO) [38].

Foord J S, Davies G J and Tsang W S 1997 Chemical Beam Epitaxy and Related Techniques (New York Wiley)... 

Panish M B and Temkin H 1993 Gas Source Molecular Beam Epitaxy (New York Springer)... 

Stringfellow G B 1989 Organometallic Vapor-Phase Epitaxy (San Diego, CA Academic)... 

Brune H 1998 Microscopic view of epitaxial metal growth nucleation and aggregation Surf. Sc . Rep. 31 121... 

Although the structure of the surface that produces the diffraction pattern must be periodic in two dimensions, it need not be the same substance as the bulk material. Thus LEED is a particularly sensitive tool for studying the structures and properties of thin layers adsorbed epitaxially on the surfaces of crystals. 

Figure Bl.19.27. AFM topographic images (7x7 pm ) of 20 epitaxial Ag films on mica prepared at five substrate temperatures (75, 135, 200, 275, and 350 °C) and four film thicknesses (50, 110, 200, and 300 mn)...

Aspens D E and Dietz N 1998 Optical approaches for controlling epitaxial growth Appl. Surf. Sc/. 130-132 367-76... 

Figure Bl.24.11. The backscattering yield from an Si sample tiiat has been implanted with Si atoms to fonn an amorphous layer. Upon annealing this amorphous layer reerystallizes epitaxially leading to a shift in the amorphous/single-erystal interfaee towards the surfaee. The aligned speetra have a step between the amorphous and erystal substrate whieh shifts towards the surfaee as the amorphous layer epitaxially reerystallizes on the Si.

Chaimelling only requires a goniometer to inelude the effeet in the battery of MeV ion beam analysis teelmiques. It is not as eonnnonly used as tire eonventional baekseattering measurements beeause the lattiee loeation of implanted atoms and the aimealing eharaeteristies of ion implanted materials is now reasonably well established [18]. Chaimelling is used to analyse epitaxial layers, but even then transmission eleetron mieroseopy is used to eharaeterize the defeets. 

Csepregi L, Kennedy E F, Gallagher T J, Mayer J W and Sigmon T W 1978 Substrate orientation dependence of the epitaxial regrowth rate from Si-implanted amorphous Si J. Appi. Phys. 49 3906... 

The growth of a well ordered fullerene monolayer, by means of molecular beam epitaxy, has been used for the controlled nucleation of single crystalline thin films. The quality and stability of molecular thin films has been shown... 

Sakurai M, Tada FI, Saiki K, Koma A, Funasaka FI and Kishimoto Y 1993 Epitaxial growth of Cgg and Cyq films... 

Koma A 1992 Van Der Waals epitaxy—a new epitaxial growth method for a highly lattice mismatched system Thin Soiid Fiims 216 72-6... 

Dura J A, Pippenger P M, Flalas N J, Xiong X Z, Chow P C and Moss S C 1993 Epitaxial integration of single crystal Cgq Appi. Rhys. Lett. 63 3443-5... 

Fischer J E, Werwa E and Fleiney P A 1993 Pseudo epitaxial Cgq films prepared by a hot wall method Appi. Rhys. A 56 193-6... 

III-V compound semiconductors with precisely controlled compositions and gaps can be prepared from several material systems. Representative III-V compounds are shown in tire gap-lattice constant plots of figure C2.16.3. The points representing binary semiconductors such as GaAs or InP are joined by lines indicating ternary and quaternary alloys. The special nature of tire binary compounds arises from tlieir availability as tire substrate material needed for epitaxial growtli of device stmctures. 

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