Molecular beam epitaxy  (Page 3/5)

The gas sources have several advantages. Gas lines can be run into the chamber, which allows the supply to be replenished without opening the chamber. When making alloys, such as Al _x Ga _1-x As, the gases can be premixed for the correct stochiometry or even have their composition gradually changed for making graded structures. For abrupt structures, it is necessary to be able to switch the gas lines with speeds of 1 second or less. However, the gas lines increase the complexity of the process and can be hard on the pumping system.

Substrate choice and preparation

Materials can be grown on substrates of different structure, orientation, and chemistry. In deciding which materials can be grown on a particular substrate, a primary consideration was expected to be lattice mismatch, i.e., differences in spacing between atoms. However, while lattice mismatch can cause strain in the grown layer, considerable accommodation between materials of different sizes can take place during growth. A greater source of strain can be differences in thermal expansion characteristics because the layer is grown at high temperature. On cooling to room temperature, dislocation defects can be formed at the interface or in severe cases, the device may break. Chemical considerations, such as whether the layer's elements will dissolve in the substrate or form compounds with the substrate, must also be considered.

Different orientations of the substrate can also affect growth. Close-packed planes have the lowest surface energy, which allows atoms to desorb from the surface, resulting in slower growth rates. Growth is favored where bonds can be made in several directions at the same time. Therefore, the substrate is often cut off-axis by a 2 - 4° to provide a rougher growth surface. For compound semiconductors, some orientations result in the number of loose bonds changing between layers. This results in changes of surface energy with each layer, which slows growth down.

The greatest cause of defects in the epitaxial layer is defects on the substrate's surface. In general, any dislocations on the substrate are replicated or even multiplied in the epitaxial growth, which is what makes the cleaning of the substrate so important.

Materials grown

MBE is commercially used primarily for GaAs devices. This is partly because the high speed microwave devices made from GaAs required the superior electrical quality of epitaxial layers. Taking place at lower temperature and under better controlled conditions, MBE generally results in layers of better quality than melt-grown.

From solid Ga and As sources, elemental Ga and tetrameric As ₄ are evaporated. For a GaAs substrate, the Ga flux has a sticking coefficient very close to 1 (almost certain to adsorb). The As is much less likely to adsorb, so an excess is usually supplied. Cracker cells are often used on the As ₄ in order to obtain As ₂ instead, which results in faster growth and more efficient utilization of the source beam.

For nominally undoped GaAs grown by MBE, the residual impurities are usually carbon, from substrate contamination and residual gas after the growth chamber is pumped down, and sulphur, from the As source. The most common surface defects are "oval" defects, which seem to form when Ga manages to form metallic droplets during the growth process, which can particularly occur if the substrate was not cleaned properly. These defects can also be reduced by carefully controlling the Ga flux.