Hi, need to submit a 2500 words paper on the topic Basics of Molecular Beam Epitaxy. The problem associated with vapor phase deposition processes was differential vapor pressure of different constituents. For example, the vapor pressure of Ga and As differs by as much as two orders of magnitude at temperatures useful for thin film deposition. Therefore, to achieve equal vapor pressure and hence equal flux of arrival of Ga and As on the substrate by thermal evaporation of separate As and Ga sources will require too stringent control on the temperature to be achieved . Many research groups tried to overcome this limitation by innovative approaches. One remarkable approach by Collins et al  involved using two separate crucibles containing Ga and Sb to deposit films of GaSb onto a glass substrate by evaporating these substances. His basic idea was to exploit decrease in an angular decrease in the flux and hence to place the substrate at an angular location with respect to the individual crucibles where the two atomic fluxes will be equal and hence a stoichiometric film will be produced. However, the films he could produce were polycrystalline. Gϋnter K. G.  tried to arrive at an appropriate vapor ratio by control of the temperatures of the evaporating species and the substrate separately. However, he could also not produce quality ordered films. During those days (1960 s) it was not easy to characterize the condition of the substrate, the quality of the vacuum and the composition and crystallinity of the films. The researchers had to do electron-diffraction after growing the films and this was taking much longer. It was the development of small mass spectrometers, auger electron spectroscopy, and compact electron diffraction instruments which made it .possible to characterize the films in-situ while it was getting deposited.
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