4.3 Crystal growth-bulk and epitaxial film-part 4  (Page 3/4)

RHEED Gun-Reflection High Energy Electron Diffraction Gun gives a beam of electron which can be made incident on the epitaxially grown film. The diffraction pattern of electrons is studied on the fluorescent screen. This shows a maximum when there is a completed monolayer and a minimum when there is a partial layer as this produces more scattering. Thus RHEED Gun is used for in-situ monitoring of the growth of the epitaxial film mono-layer by mono-layer.

Computer controlled shutters of each furnace allows precise control of the thickness of each layer, down to a single layer of atoms.

Substrate Heating- to obtain high quality epitaxial layer, growth temperature must be relatively high. The substrate wafer must be heated to allow the atoms to move about the surface and reach the proper ordered site. Growth must be at a temperature where growth rate is insensitive of minor temperature variation. Atoms arriving at the substrate surface may undergo absorption to the surface, surface migration, incorporation into the crystal lattice, and thermal desorption. Which of the competing pathways dominates the growth will depend strongly on the temperature of the substrate. At a low temperature, atoms will stick where they land without arranging properly - leading to poor crystal quality. At a high temperature, atoms will desorb (reevaporate) from the surface too readily - leading to low growth rates and poor crystal quality. In the appropriate intermediate temperature range, the atoms will have sufficient energy to move to the proper position on the surface and be incorporated into the growing crystal.

The Inventors of MBE are : J.R. Arthur and Alfred Y. Chuo (Bell Labs, 1960).

MBE is a technique for epitaxial growth of single crystal atomic layer films on single crystal substrate. It gives precise control in chemical composition and doping profiles. To avoid contamination of the epitaxial films, Ultra High Vacuum within MBE chamber is imperative. It requires Very/Ultra high vacuum (10 ^-8 Pa or 10 ^-11 Torr). This implies that Epitaxial film is grown at slow deposition rate (1 micron/hour). This permits epitaxial growth of single atomic layer if desired. Slow deposition rates require proportionally better vacuum. Ultra-pure elements are heated in separate quasi-knudson effusion cells (e.g., Ga and As) until they begin to slowly sublimate. Gaseous elements then condense on the wafer, where they may react with each other (e.g., GaAs). The term “beam” in MBE means the evaporated atoms do not interact with each other or with other vacuum chamber gases until they reach the wafer. Each gas beam may be turned on and off rapidly with a shutter or a valve. Beam intensity (called the flux) is adjusted for precise control of layer composition. A collection of gas molecules moving in the same direction constitute the molecular beam. Simplest way to generate a molecular beam is Effusion cell or Knudsen cell . Oven contains the material to make the beam. Oven is connected to a vacuum system through a hole. The substrate is located with a line-of-sight to the oven aperture. From kinetic theory, the flow through the aperture is simply the molecular impingement rate on the area of the orifice. Impingement rate is: The total flux through the hole. The spatial distribution of molecules from the orifice of a knudsen cell is normally a cosine distribution. The intensity drops off as the square of the distance from the orifice. Intensity is maximum in the direction normal to the orifice and decreases with increasing θ, which causes problems. Use collimator, a barrier with a small hole; it intercepts all of the flow except for that traveling towards the sample.