Chemical Beam Epitaxy - Basic Principles

Basic Principles

Chemical Beam Epitaxy was first demonstrated by W.T. Tsang in 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. 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. A comparison of the different processes in the growth chamber for MOCVD, MBE and CBE can be seen in figure 1.