Surface adsorbed or adsorbed layer of first material for the compound has rather stable mono-molecular structure, named as Langmuir-Blogett layer, which can react with the second material forming the monomolecular layer of the compound, with high accuracy. Moreover, photo-chemistry can include the chemical reactions with very high activation energy. However, in this case, most effective effect for the monomoecular thin crystal layer was the improvement of the crystal quality. With the measurement of the mass spectrometer, release of the methyl or ethyl were detected while the TMG or TEG was being absorbed and reacting with arsin. This means adsorbed species may be monomethyl or monomethyl gallium, and dimethyl or diethyl gallium, respectively, or might be both.
In this paper, photo-assisted chemical reactions are explained and the molecular layer epitaxy, which made the crystal growth accurate in the order of single atomic dimentions, is explained, followed by the effect of ultra-violet light irradiations. These methods are also valuable to clarify the surface adsorption phenomena. Some experimental results are explained. (J. Cryst. Soc. Jpn. 28, 133 (1986) ) .
