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Chemical beam epitaxy
Chemical beam epitaxy (CBE) forms an important class of deposition techniques for semiconductor layer systems, especially III-V semiconductor systems. This form of epitaxial growth is performed in an ultrahigh vacuum system. The reactants are in the form of molecular beams of reactive gases, typically as the hydride or a metalorganic. The term CBE is often used interchangeably with metal-organic molecular beam epitaxy (MOMBE). The nomenclature does differentiate between the two (slightly different) processes, however. When used in the strictest sense, CBE refers to the technique in which both components are obtained from gaseous sources, while MOMBE refers to the technique in which the group III component is obtained from a gaseous source and the group V component from a solid source.
== Basic principles ==
Chemical Beam Epitaxy was first demonstrated by W.T. Tsang in 1984.〔W.T. Tsang, “Chemical beam epitaxy of InP and GaAs”. Appl. Phys. Lett. 45, 1234 (1984).〕 This technique was then described as a hybrid of metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) that exploited the advantages of both the techniques. In this initial work, InP and GaAs were grown using gaseous group III and V alkyls. While group III elements were derived from the pyrolysis of the alkyls on the surface, the group V elements were obtained from the decomposition of the alkyls by bringing in contact with heated Tantalum (Ta) or Molybdenum (Mo) at 950-1200 °C.
Typical pressure in the gas reactor is between 102 Torr and 1 atm for MOCVD. Here, the transport of gas occurs by viscous flow and chemicals reach the surface by diffusion. In contrast, gas pressures of less than 10−4 Torr are used in CBE. The gas transport now occurs as molecular beam due to the much longer mean-free paths, and the process evolves to a chemical beam deposition.〔W.T. Tsang, “CBE of Ga0.47In0.53As/InP quantum wells and heterostructure devices”. J. Cryst. Growth. 81, 261 (1987).〕 It is also worth noting here that MBE employs atomic beams (such as aluminium (Al) and Gallium (Ga)) and molecular beams (such as As4 and P4) that are evaporated at high temperatures from solid elemental sources, while the sources for CBE are in vapor phase at room temperatures.〔W.T. Tsang, “From Chemical Vapor Epitaxy to Chemical Beam Epitaxy”. J. Cryst. Growth. 95, 121 (1989).〕 A comparison of the different processes in the growth chamber for MOCVD, MBE and CBE can be seen in figure 1.
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