Epitaxial growth, of thin films

Epitaxial growth of thin films usually involves the formation of strained material as a result of mismatch between the film and substrate and because of the large surface to volume ratio in the film. Surface stress can be a major factor, even when the lattice constants of film and substrate are perfectly matched. Although it appears to be difficult to eliminate the stress totally, it is important to be able to control it and even use it to produce desired qualities. 

MBE (molecular beam epitaxy), which involves epitaxial growth of thin films on either the same material as substrate (homoepitaxial) or a lattice-matched substrate (heteroepitaxial) the heated substrate reacts with a molecular beam of compounds containing the constituent elements of the semiconductor as well as any dopants the resultant film is essentially a single crystal slow growth rates produce films from a few nanometers thick to at most several hundred nanometers that have very high purity and controlled levels of dopants. 

Data obtained from STM images can be useful in providing information on the relative importance of molecule-molecule and molecule-substrate interactions, as well as the types of forces responsible for the packing order at the surface. This is useful in such applications as epitaxial growth of thin films, chromatography, lubrication, and microelectronics fabrication, each of which involves interactions between molecules on a surface and can be investigated by these procedures. 

Historically, EC-ALE has been developed by analogy with atomic layer epitaxy (ALE) [76-82], ALE is a methodology used initially to improve epitaxy in the growth of thin-films by MBE and VPE. The principle of ALE is to use surface limited reactions to form each atomic layer of a deposit. If no more than an atomic layer is ever deposited, the growth will be 2-D, layer by layer, epitaxial. Surface limited reactions are developed for the deposition of each component element, and a cycle is formed with them. With each cycle, a compound monolayer is formed, and the deposit thickness is controlled by the number of cycles. 

The growth of one crystal upon the surface of another is epitaxy. In the broadest sense of this term, epitaxy includes all of adsorption, corrosion and growth of thin films. When a thin film of deposit is laid down, it can be expected that its structure may differ radically from the bulk structure of a thick deposit. Structure of a deposit containing less than one monolayer can be as different from that of a thin film which precedes bulk growth, as the thin film structure may be from that of a heavy deposit. Some remarkable LEED observations of this kind have been made that are still only poorly understood. 

Fig. 2.12 Principle arrangement of the metal evaporator for the epitaxial growth of thin metallic films. The current on the collector, due to metal ions during evaporation, facilitates a constant and reproducible flux with a deviation below 5% (from [2], used with permission)...

Introduction Evaporation of gold onto various supports preceded SAMs by many years. Originally, the interest focused on the epitaxial growth of thin metal films on mica and other flat surfaces. This introduction, therefore, deals with general aspects of evaporated gold films, while the proceeding subchapters refer specifically to studies of SAMs on the various gold surfaces. 

Since 1980, interest in organometallic compounds of Ga, In and T1 has grown, mainly because of their potential use as precursors to semiconducting materials such as GaAs and InP. Volatile compounds can be used in the growth of thin films by MOCVD metal organic chemical vapour deposition) or MOVPE metal organic vapour phase epitaxy)... 

Betts, RA. and Pitt C.W. (1985) Growth of thin-film lithium niobate by molecular beam epitaxy. Electron. Lett., 21, 960-962. 

Group 12 (II B) bis(diethyldithiocarbamate) complexes, [MISaCNEtala] (M = Zn, Cd), were first utilized toward the growth of thin films of ZnS and CdS by O Brien and co-workers in 1989 (1969). Thin films were obtained at pressures of 10 " Torr and between 370 and 420°C. The CdS films were polycrystalline and hexagonal on glass, but thin epitaxial layers were obtained on InP(lOO) and GaAs (100). The quality of ZnS films obtained by this method was lower than those of CdS. 

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. 

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

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