Summarizing the examples discussed above, a number of conclusions may be drawn: 1. Due to their high sensitivity surface analysis methods can detect details in the film formation process which are completely overlooked by the more conventional methods. 2. Due to the insufficient understanding of the physical processes involved in some of the methods such as SIMS and ISS - as illustrated by the growth of Pb on Au and of Au on Si respectively - the full information contained in some of the methods cannot be extracted yet at present. 3. The various methods give complementary, partially overlapping information such as TDS, ITD, LEED and AES combined with quartz oscillator measurements about the monolayer saturation coverage or AES and ISS about the composition of the topmost layer. Therefore such complementary methods should be combined in order to obtain as complete a picture of the film formation process as possible. 4. Even in the most weakly interacting system discussed here, Pb on W, the substrate may not be considered to be static in the growth process. In the more strongly interacting systems such as Au on W severe interface changes occur on high index planes at elevated temperature although no alloying takes place. 5. In systems which form compounds or alloys such as Pb on Au or Au on Si a pure monolayer is formed initially before alloy or compound formation begins. Depending upon the stability of the alloy or compound the whole film may consist of it (e.g. in the case of Pb on Au) or it may be confined to the interface between film and substrate, e.g. in the system Au on Si. If the diffusivity of the substrate atoms in the film is large, such as of Si in Au, the alloy may also cover the surface of the growing film. 6. All not-alloying systems discussed here follow either the Frank-van der Merwe (FIG.20b) or the Stranski-Krastanov growth mechanism (FIG.20c) but not the Vollmer-Weber-mechanism (FIG.20a). This is understandable on the basis of the growth criteria [84] for Pd, Au and Pb on W but not for Ag and Au on Si. In the latter systems the specific nature of the semiconductor surface ("dangling bonds") has to be taken into account and the additional compli-cation of the alloy interface layer which makes the application of the growth criteria difficult. In alloying systems the growth criteria have to be generalized to include the free energy of the alloy or compound, the interfacial energy between compound and substrate and, as indicated in the system Pb on Au, the possibility of lowering the surface energy of the compound by an adsorbed layer of the deposit material. In conclusion, it has been demonstrated that surface analysis methods are ideal tools to study the elementary processes involved in epitaxy.

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Although the III-V compounds and their alloys are increasingly Important in the ever-widening range of electronic and electro-optic devices, fundamental thermodynamic and chemical kinetic data for the chemical transport reactions used for their preparation is sparse, with few exceptions. The modified entrainment method developed by Faktor, Garrett and co-workers can be used to study chemical transport reactions and to obtain such data. The theory of the method is outlined and illustrated with examples from the GaAs-HCI, GaAs-HBr, InAs-HCl, GaP-HBr and other III-V systems.

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General relationships for adsorption of species in gaseous solutions are presented. The contribution of the adsorption layer/step flux to the growth rate is compared with the contribution of the direct gas/step flux. The adsorption of Cl, H, GaCl, As, As_2, As_4, Ga and AsGaCl species on GaAs (111)Ga and GaAs (111)AS have been considered. The results of an analogous consideration for Si(111) are presented. For Si(111) and GaAs (111)Ga the amount of vacant adsorption sites is = 1-10%, whereas for GaAs (111)AS it reaches =50%, depending on the gas phase composition and temperature. Surface diffusion is seriously hindered on the faces covered with the dense adsorption layers, the diffusion length going down to several tens of interatomic distances. Thus the possibility of an adlayer - gas-phase reaction is expected to play a considerable role. In the Ga-As-H-Cl system the GaCl and As_4 molecules turn out to be the major Ga- and As- containing species adsorbed on the (111) faces.

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