Fundamentals of molecular beam epitaxy Essay Example | Topics and Well Written Essays - 2250 words

Fundamentals of molecular beam epitaxy - Essay Example Molecular beam epitaxy is a process to produce device grade epitaxial films and mutilayers. This is a very advanced and sophisticated process an have evolved gaining clear edge over competing processes like Liquid Phase Epitaxy, Vapor Phase Epitaxy (VPE) etc. This paper provides basic description of Molecular Beam Epitaxy (MBE) process. Different theoretical concepts of this process and essential ingredients of a MBE system are also briefly discussed. Epitaxial growth refers to the situation in which the depositing layer extends the existing crystalline order of the substrate i.e. growth of the new layer does not cause any crystalline discontinuity on the interface between the substrate and the film. This occurs even during industrial processes like welding and cladding; where the initial mode of solidification is planer mode and the growth layer is essentially epitaxial growth of the existing grains. However, there are many grains on which this growth occurs an also this very soon d egenerates into cellular and cellular dendritic growth and thus disrupting the crystalline order of the substrate. However, it is the planer mode of solidification coupled with solute partitioning between liquid and solid phase which form the basic underlying principle of Liquid Phase Epitaxy (LPE) [1- 3]. Similarly, epitaxial films can be grown by placing a substrate in a chamber filled with vapors of constituent atoms / molecules through a process known as Vapor Phase Epitaxy (VPE) [4 – 6]. ... Many thin film technologies such as LPE, VPE, Sputtering, vacuum deposition etc. were developed for producing high quality epitaxial thin films. However, the films produced by these techniques were structurally different from the substrate and hence not useful for device making. Differential vapor pressure of different constituents atoms / molecules was the main problem associated with VPE. GaAs is one such useful film for device making. IN this case vapor pressures of Ga and As differ by two orders of magnitude at about 600 oC. Therefore, these sources will have to be heated at different temperatures to achieve equal vapor pressure and the temperature will have to be controlled very accurately, which is very difficult. [7]. Attempts were made to use different temperatures for different sources [8] and by exploiting angular distribution of the atomic / molecular fluxes [9] for maintaining desired ratio of atomic fluxes. In 1960s it was not possible to perform online monitoring and ch aracterization of the film growth process and the film itself. Even the characterization of the substrate surface condition and vacuum quality level was also not possible. One had to rely on post deposition characterization of the deposited film for the feedback for the subsequent deposition experiments; which was indeed a very slow, laborious and tiring procedure. Development of small mass spectrometers, auger electron spectroscopy and compact electron diffraction instruments made it possible to characterize the films in-situ while it was getting deposited and Molecular Beam Epitaxy (MBE) was discovered as a result of developing a process for surface characterization [10].In-situ characterization of MBE